initial commit, moved libpanto over from the pantograph repo

1 files changed, 2103 insertions, 0 deletions

diff --git a/src/file_system_jsonl_store.zig b/src/file_system_jsonl_store.zig
new file mode 100644
index 0000000..9bc9436
--- /dev/null
+++ b/src/file_system_jsonl_store.zig
@@ -0,0 +1,2103 @@
+//! Session lifecycle: create, open, replay, append.
+//!
+//! Backed by an append-only JSONL file on disk. The on-disk types live in
+//! `session.zig`. This module owns:
+//!
+//!   - Path resolution (sessions dir is supplied by the caller; we own the
+//!     filename and writes within it).
+//!   - The in-memory entry index (`by_id` map + leaf pointer).
+//!   - Deferred file creation: the file is not written until the first
+//!     assistant message persists. Until that point, all entries are
+//!     buffered in memory.
+//!   - Append semantics: once flushed, every completed entry is written
+//!     and synced to disk immediately.
+//!   - Crash recovery: on open, the file is parsed line-by-line; the first
+//!     line that fails to parse causes everything from that line onward to
+//!     be truncated from the file.
+//!   - One-time format migration when a future version reads a v1 file
+//!     (currently a no-op; the hook is in place).
+//!   - Rebuilding a `Conversation` from the entry tree, plus determining
+//!     the active provider/model.
+//!
+//! The library-vs-CLI boundary: callers pass an absolute path to the
+//! per-cwd sessions directory. We compute the per-session filename
+//! ourselves (`<uuidv7>.jsonl`) and lazily mkdir the directory on the
+//! first flush. The CLI owns XDG resolution, encoded-cwd grouping, and
+//! the `--resume` flag plumbing.
+
+const std = @import("std");
+const Allocator = std.mem.Allocator;
+const Io = std.Io;
+
+const session_mod = @import("session.zig");
+const conversation_mod = @import("conversation.zig");
+const session_store_mod = @import("session_store.zig");
+const turn_persist = @import("turn_persist.zig");
+
+pub const SessionHeader = session_mod.SessionHeader;
+pub const SessionEntry = session_mod.SessionEntry;
+pub const MessageEntry = session_mod.MessageEntry;
+pub const StoredMessage = session_mod.StoredMessage;
+pub const StoredMessageRole = session_mod.StoredMessageRole;
+pub const StoredSystemMode = session_mod.StoredSystemMode;
+pub const StoredContentBlock = session_mod.StoredContentBlock;
+pub const Usage = session_mod.Usage;
+pub const CURRENT_VERSION = session_mod.CURRENT_VERSION;
+
+// =============================================================================
+// IDs and timestamps
+// =============================================================================
+
+/// Generate a UUIDv7 (RFC 9562 §5.7). Returns a 36-character canonical
+/// hex string with hyphens. Caller owns.
+///
+/// Layout:
+///   - 48 bits: unix_ts_ms (big-endian)
+///   - 4 bits: version (7)
+///   - 12 bits: random
+///   - 2 bits: variant (10)
+///   - 62 bits: random
+pub fn newUuidV7(allocator: Allocator, io: Io) ![]u8 {
+    const ts = Io.Timestamp.now(io, .real);
+    const now_ms: u64 = @intCast(@max(ts.toMilliseconds(), 0));
+
+    var rand_bytes: [10]u8 = undefined;
+    io.random(&rand_bytes);
+
+    var b: [16]u8 = undefined;
+    // Timestamp (48 bits, big-endian).
+    b[0] = @intCast((now_ms >> 40) & 0xFF);
+    b[1] = @intCast((now_ms >> 32) & 0xFF);
+    b[2] = @intCast((now_ms >> 24) & 0xFF);
+    b[3] = @intCast((now_ms >> 16) & 0xFF);
+    b[4] = @intCast((now_ms >> 8) & 0xFF);
+    b[5] = @intCast(now_ms & 0xFF);
+    // Version (4 high bits = 0x7) + 12 bits random.
+    b[6] = 0x70 | (rand_bytes[0] & 0x0F);
+    b[7] = rand_bytes[1];
+    // Variant (2 high bits = 10) + 62 bits random.
+    b[8] = 0x80 | (rand_bytes[2] & 0x3F);
+    b[9] = rand_bytes[3];
+    b[10] = rand_bytes[4];
+    b[11] = rand_bytes[5];
+    b[12] = rand_bytes[6];
+    b[13] = rand_bytes[7];
+    b[14] = rand_bytes[8];
+    b[15] = rand_bytes[9];
+
+    return try std.fmt.allocPrint(
+        allocator,
+        "{x:0>2}{x:0>2}{x:0>2}{x:0>2}-{x:0>2}{x:0>2}-{x:0>2}{x:0>2}-{x:0>2}{x:0>2}-{x:0>2}{x:0>2}{x:0>2}{x:0>2}{x:0>2}{x:0>2}",
+        .{ b[0], b[1], b[2], b[3], b[4], b[5], b[6], b[7], b[8], b[9], b[10], b[11], b[12], b[13], b[14], b[15] },
+    );
+}
+
+/// Generate a fresh 8-character hex entry id. Caller owns.
+fn newEntryIdInto(buf: []u8, io: Io) void {
+    std.debug.assert(buf.len == 8);
+    var bytes: [4]u8 = undefined;
+    io.random(&bytes);
+    _ = std.fmt.bufPrint(buf, "{x:0>2}{x:0>2}{x:0>2}{x:0>2}", .{ bytes[0], bytes[1], bytes[2], bytes[3] }) catch unreachable;
+}
+
+/// Format `now` as an ISO 8601 UTC string with millisecond precision.
+/// Example: `2026-04-25T17:40:15.990Z`. Caller owns.
+pub fn isoTimestamp(allocator: Allocator, io: Io) ![]u8 {
+    const ts = Io.Timestamp.now(io, .real);
+    const ms_total: i64 = ts.toMilliseconds();
+    const seconds_total: i64 = @divTrunc(ms_total, 1000);
+    const ms: u64 = @intCast(@mod(ms_total, 1000));
+
+    const epoch_secs = std.time.epoch.EpochSeconds{ .secs = @intCast(seconds_total) };
+    const epoch_day = epoch_secs.getEpochDay();
+    const day_secs = epoch_secs.getDaySeconds();
+    const year_day = epoch_day.calculateYearDay();
+    const month_day = year_day.calculateMonthDay();
+
+    return try std.fmt.allocPrint(
+        allocator,
+        "{d:0>4}-{d:0>2}-{d:0>2}T{d:0>2}:{d:0>2}:{d:0>2}.{d:0>3}Z",
+        .{
+            @as(u32, year_day.year),
+            month_day.month.numeric(),
+            @as(u32, month_day.day_index) + 1,
+            day_secs.getHoursIntoDay(),
+            day_secs.getMinutesIntoHour(),
+            day_secs.getSecondsIntoMinute(),
+            ms,
+        },
+    );
+}
+
+// =============================================================================
+// FileInfo (per-file listing scan result)
+// =============================================================================
+
+/// Internal scan result for one session file. Carries the file `path` (the
+/// catalog needs it to open/resolve) plus everything the public
+/// `session_store.SessionInfo` needs.
+const FileInfo = struct {
+    path: []u8,
+    id: []u8,
+    created: []u8, // ISO 8601 from header timestamp
+    modified: []u8, // ISO 8601 from last activity, falling back to header
+    message_count: usize,
+    last_user_message: []u8,
+    stamp: ?session_mod.WireStamp, // last-used wire identity
+
+    pub fn deinit(self: FileInfo, alloc: Allocator) void {
+        alloc.free(self.path);
+        alloc.free(self.id);
+        alloc.free(self.created);
+        alloc.free(self.modified);
+        alloc.free(self.last_user_message);
+        if (self.stamp) |s| s.deinit(alloc);
+    }
+};
+
+// =============================================================================
+// SessionFile
+// =============================================================================
+
+pub const Error = error{
+    NoSessionsFound,
+    AmbiguousSessionId,
+    SessionNotFound,
+    InvalidSessionFile,
+} || Allocator.Error || Io.Cancelable;
+
+pub const SessionFile = struct {
+    allocator: Allocator,
+    io: Io,
+
+    /// Absolute path to the per-cwd sessions directory. Lazily created.
+    session_dir: []u8,
+    /// Absolute path to the file we *will* write to (computed at init).
+    /// May not yet exist on disk if `flushed = false`.
+    session_file: []u8,
+
+    /// Header. Allocated at init for new sessions; reloaded from the file
+    /// on resume.
+    header: SessionHeader,
+
+    /// Entries indexed in insertion order. The first entry's `parent_id`
+    /// is null; each subsequent entry's `parent_id` points to its parent
+    /// (currently always the previous entry).
+    entries: std.ArrayList(SessionEntry),
+    /// id → entry index in `entries`. Used both for parent-id lookups
+    /// and for collision detection in `newEntryId`.
+    by_id: std.StringHashMap(usize),
+    /// id of the most recently appended entry, or null if no entries yet.
+    /// Borrowed from the entry; do not free.
+    leaf_id: ?[]const u8,
+
+    /// True once the file exists on disk. False during the "buffered"
+    /// pre-assistant phase. See module-level docs.
+    flushed: bool,
+    /// Number of bytes written to `session_file` so far. Used as the
+    /// offset for the next positional write. Only meaningful when
+    /// `flushed = true`.
+    written_bytes: u64,
+
+    // ---------- Construction ----------
+
+    /// Create a new session in memory. Allocates a UUIDv7, computes the
+    /// file path, but does NOT touch the filesystem. The file is created
+    /// on the first assistant-message flush.
+    ///
+    /// `session_dir` is duplicated; the caller retains ownership of the
+    /// passed slice.
+    pub fn init(
+        allocator: Allocator,
+        io: Io,
+        session_dir: []const u8,
+        metadata: ?[]const u8,
+    ) !SessionFile {
+        const id = try newUuidV7(allocator, io);
+        defer allocator.free(id);
+        return initWithId(allocator, io, session_dir, metadata, id);
+    }
+
+    /// Like `init`, but uses a caller-supplied session id (duped here)
+    /// rather than minting a fresh UUIDv7. Used by the catalog when a
+    /// `Session` handle was minted (with its id) before the first append.
+    pub fn initWithId(
+        allocator: Allocator,
+        io: Io,
+        session_dir: []const u8,
+        metadata: ?[]const u8,
+        session_id: []const u8,
+    ) !SessionFile {
+        const dir = try allocator.dupe(u8, session_dir);
+        errdefer allocator.free(dir);
+
+        const id = try allocator.dupe(u8, session_id);
+        errdefer allocator.free(id);
+
+        const timestamp = try isoTimestamp(allocator, io);
+        errdefer allocator.free(timestamp);
+
+        const metadata_copy: ?[]const u8 = if (metadata) |m| try allocator.dupe(u8, m) else null;
+        errdefer if (metadata_copy) |m| allocator.free(m);
+
+        const filename = try std.fmt.allocPrint(allocator, "{s}.jsonl", .{id});
+        defer allocator.free(filename);
+        const file_path = try std.fs.path.join(allocator, &.{ dir, filename });
+        errdefer allocator.free(file_path);
+
+        return .{
+            .allocator = allocator,
+            .io = io,
+            .session_dir = dir,
+            .session_file = file_path,
+            .header = .{
+                .version = CURRENT_VERSION,
+                .id = id,
+                .timestamp = timestamp,
+                .metadata = metadata_copy,
+            },
+            .entries = .empty,
+            .by_id = std.StringHashMap(usize).init(allocator),
+            .leaf_id = null,
+            .flushed = false,
+            .written_bytes = 0,
+        };
+    }
+
+    /// Open and replay an existing session file. Truncates from the first
+    /// corrupted line. Runs format migration if needed and rewrites the
+    /// file once.
+    pub fn open(
+        allocator: Allocator,
+        io: Io,
+        file_path: []const u8,
+    ) !SessionFile {
+        const path_copy = try allocator.dupe(u8, file_path);
+        errdefer allocator.free(path_copy);
+
+        // The session dir is the file's parent directory.
+        const dir_path = std.fs.path.dirname(path_copy) orelse ".";
+        const dir = try allocator.dupe(u8, dir_path);
+        errdefer allocator.free(dir);
+
+        const bytes = try readWholeFile(allocator, io, path_copy);
+        defer allocator.free(bytes);
+
+        // Walk line-by-line. The first failure causes a truncation back to
+        // the start of that line.
+        var entries: std.ArrayList(SessionEntry) = .empty;
+        errdefer {
+            for (entries.items) |e| e.deinit(allocator);
+            entries.deinit(allocator);
+        }
+        var by_id = std.StringHashMap(usize).init(allocator);
+        errdefer by_id.deinit();
+
+        var header_opt: ?SessionHeader = null;
+        errdefer if (header_opt) |h| h.deinit(allocator);
+
+        var cursor: usize = 0;
+        var valid_bytes: u64 = 0; // length of the file prefix that parses cleanly
+        var saw_corruption: bool = false;
+
+        while (cursor < bytes.len) {
+            // Find the next newline (or EOF).
+            const rest = bytes[cursor..];
+            const nl_rel = std.mem.indexOfScalar(u8, rest, '\n');
+            const line_end_excl: usize = if (nl_rel) |n| cursor + n else bytes.len;
+            const line = bytes[cursor..line_end_excl];
+            const next_cursor: usize = if (nl_rel != null) line_end_excl + 1 else bytes.len;
+
+            // Allow blank lines silently (just whitespace), but a non-empty
+            // trimmed line that won't parse triggers truncation.
+            const trimmed = std.mem.trim(u8, line, " \t\r");
+            if (trimmed.len == 0) {
+                if (nl_rel == null) break;
+                cursor = next_cursor;
+                valid_bytes = cursor;
+                continue;
+            }
+
+            // If the final line has no trailing newline AND we hit EOF, it
+            // is presumed truncated mid-write. Treat as corruption.
+            if (nl_rel == null) {
+                saw_corruption = true;
+                break;
+            }
+
+            const fe = session_mod.parseLine(allocator, line) catch {
+                saw_corruption = true;
+                break;
+            };
+
+            switch (fe) {
+                .header => |h| {
+                    if (header_opt != null) {
+                        // Two headers — treat as corruption from this line on.
+                        h.deinit(allocator);
+                        saw_corruption = true;
+                        break;
+                    }
+                    if (entries.items.len != 0) {
+                        // Header arrived after entries — malformed.
+                        h.deinit(allocator);
+                        saw_corruption = true;
+                        break;
+                    }
+                    header_opt = h;
+                },
+                .entry => |e| {
+                    const idx = entries.items.len;
+                    entries.append(allocator, e) catch |err| {
+                        e.deinit(allocator);
+                        return err;
+                    };
+                    by_id.put(e.base().id, idx) catch |err| {
+                        // Rolling back the append is awkward; in practice
+                        // OOM here is fatal anyway.
+                        return err;
+                    };
+                },
+            }
+
+            cursor = next_cursor;
+            valid_bytes = cursor;
+        }
+
+        // No header at all — refuse to load.
+        const header = header_opt orelse return error.InvalidSessionFile;
+
+        // Truncate the file if anything beyond `valid_bytes` is corrupt.
+        if (saw_corruption and valid_bytes < bytes.len) {
+            try truncateFileTo(io, path_copy, valid_bytes);
+        }
+
+        // Drop assistant tool_use blocks left dangling (no matching
+        // tool_result) by an interrupted prior turn. If anything changed,
+        // rebuild the id index and rewrite the file once.
+        if (elideDanglingToolUses(allocator, &entries)) {
+            by_id.clearRetainingCapacity();
+            for (entries.items, 0..) |e, i| {
+                try by_id.put(e.base().id, i);
+            }
+            try rewriteFile(allocator, io, path_copy, header, entries.items);
+        }
+
+        const leaf_id: ?[]const u8 = if (entries.items.len > 0)
+            entries.items[entries.items.len - 1].base().id
+        else
+            null;
+
+        // Compute final file length on disk so future appends use the
+        // correct offset.
+        const stat = try statFileForLength(io, path_copy);
+
+        return .{
+            .allocator = allocator,
+            .io = io,
+            .session_dir = dir,
+            .session_file = path_copy,
+            .header = header,
+            .entries = entries,
+            .by_id = by_id,
+            .leaf_id = leaf_id,
+            .flushed = true,
+            .written_bytes = stat,
+        };
+    }
+
+    pub fn deinit(self: *SessionFile) void {
+        self.header.deinit(self.allocator);
+        for (self.entries.items) |e| e.deinit(self.allocator);
+        self.entries.deinit(self.allocator);
+        self.by_id.deinit();
+        self.allocator.free(self.session_dir);
+        self.allocator.free(self.session_file);
+    }
+
+    // ---------- Accessors ----------
+
+    pub fn getSessionFile(self: *const SessionFile) []const u8 {
+        return self.session_file;
+    }
+
+    pub fn isFlushed(self: *const SessionFile) bool {
+        return self.flushed;
+    }
+
+    // ---------- Appending ----------
+
+    /// Append a single message, returning its entry id. A thin wrapper over
+    /// `appendMessagesAtomic` (which handles `len == 1`); used by the tests.
+    /// `msg` is consumed (ownership transferred).
+    pub fn appendMessage(
+        self: *SessionFile,
+        msg: StoredMessage,
+        // Wire-format provider identity for the entry. Null on system
+        // messages. Borrowed; duplicated into the entry.
+        stamp: ?session_mod.WireStamp,
+    ) ![]const u8 {
+        var msgs = [_]StoredMessage{msg};
+        const stamps = [_]?session_mod.WireStamp{stamp};
+        try self.appendMessagesAtomic(&msgs, &stamps);
+        return self.leaf_id.?;
+    }
+
+    /// Returns a freshly allocated 8-character hex id, guaranteed not to
+    /// collide with any existing entry id in this session.
+    fn newEntryId(self: *SessionFile) ![]u8 {
+        const max_tries = 100;
+        var i: usize = 0;
+        while (i < max_tries) : (i += 1) {
+            const buf = try self.allocator.alloc(u8, 8);
+            errdefer self.allocator.free(buf);
+            newEntryIdInto(buf[0..8], self.io);
+            if (!self.by_id.contains(buf)) {
+                return buf;
+            }
+            self.allocator.free(buf);
+        }
+        // Fall back to a UUID prefix if 100 retries all collided. With 4
+        // random bytes per id and a session with <<2^16 entries, the
+        // probability of getting here is effectively zero, but we want a
+        // hard guarantee.
+        const long = try newUuidV7(self.allocator, self.io);
+        defer self.allocator.free(long);
+        const buf = try self.allocator.alloc(u8, 8);
+        @memcpy(buf, long[0..8]);
+        return buf;
+    }
+
+    // ---------- Persistence ----------
+
+    /// Write the header + all currently-buffered entries + `new_entries`
+    /// to the file as a single batch. Creates the directory and file.
+    fn flushBufferedMany(self: *SessionFile, new_entries: []const SessionEntry) !void {
+        try mkdirP(self.io, self.session_dir);
+
+        const file = try Io.Dir.cwd().createFile(self.io, self.session_file, .{
+            .truncate = true,
+            .read = false,
+        });
+        defer file.close(self.io);
+
+        var offset: u64 = 0;
+        const header_line = try session_mod.serializeHeader(self.allocator, self.header);
+        defer self.allocator.free(header_line);
+        try file.writePositionalAll(self.io, header_line, offset);
+        offset += header_line.len;
+        try file.writePositionalAll(self.io, "\n", offset);
+        offset += 1;
+
+        for (self.entries.items) |e| {
+            const line = try session_mod.serializeEntry(self.allocator, e);
+            defer self.allocator.free(line);
+            try file.writePositionalAll(self.io, line, offset);
+            offset += line.len;
+            try file.writePositionalAll(self.io, "\n", offset);
+            offset += 1;
+        }
+
+        for (new_entries) |entry| {
+            const line = try session_mod.serializeEntry(self.allocator, entry);
+            defer self.allocator.free(line);
+            try file.writePositionalAll(self.io, line, offset);
+            offset += line.len;
+            try file.writePositionalAll(self.io, "\n", offset);
+            offset += 1;
+        }
+
+        file.sync(self.io) catch {};
+        self.flushed = true;
+        self.written_bytes = offset;
+    }
+
+    pub fn appendMessagesAtomic(
+        self: *SessionFile,
+        messages: []StoredMessage,
+        stamps: []const ?session_mod.WireStamp,
+    ) !void {
+        std.debug.assert(messages.len == stamps.len);
+        if (messages.len == 0) return;
+
+        const base_len = self.entries.items.len;
+        try self.entries.ensureUnusedCapacity(self.allocator, messages.len);
+        try self.by_id.ensureUnusedCapacity(@intCast(messages.len));
+
+        var entries = try self.allocator.alloc(SessionEntry, messages.len);
+        defer self.allocator.free(entries);
+        var built: usize = 0;
+        errdefer {
+            for (entries[0..built]) |*e| e.deinit(self.allocator);
+        }
+
+        var prev_leaf = self.leaf_id;
+        for (messages, 0..) |msg, i| {
+            const msg_local = msg;
+            const id_buf = try self.newEntryId();
+            errdefer self.allocator.free(id_buf);
+            const timestamp = try isoTimestamp(self.allocator, self.io);
+            errdefer self.allocator.free(timestamp);
+            const parent_id_copy: ?[]const u8 = if (prev_leaf) |l| try self.allocator.dupe(u8, l) else null;
+            errdefer if (parent_id_copy) |p| self.allocator.free(p);
+            const stamp_copy: ?session_mod.WireStamp = if (stamps[i]) |st| try st.dupe(self.allocator) else null;
+            errdefer if (stamp_copy) |st| st.deinit(self.allocator);
+            entries[i] = .{ .message = .{
+                .base = .{ .id = id_buf, .parent_id = parent_id_copy, .timestamp = timestamp },
+                .stamp = stamp_copy,
+                .message = msg_local,
+            } };
+            built += 1;
+            prev_leaf = entries[i].base().id;
+        }
+
+        if (self.flushed) {
+            try self.persistEntries(entries);
+        } else {
+            // File is created on the first assistant message (see module docs).
+            for (messages) |m| if (m.role == .assistant) {
+                try self.flushBufferedMany(entries);
+                break;
+            };
+        }
+
+        for (entries, 0..) |entry, i| {
+            self.entries.appendAssumeCapacity(entry);
+            self.by_id.putAssumeCapacity(entry.base().id, base_len + i);
+        }
+        self.leaf_id = entries[entries.len - 1].base().id;
+        built = 0;
+    }
+
+    fn persistEntries(self: *SessionFile, entries: []const SessionEntry) !void {
+        const file = try Io.Dir.cwd().openFile(self.io, self.session_file, .{
+            .mode = .write_only,
+        });
+        defer file.close(self.io);
+
+        var offset = self.written_bytes;
+        for (entries) |entry| {
+            const line = try session_mod.serializeEntry(self.allocator, entry);
+            defer self.allocator.free(line);
+            try file.writePositionalAll(self.io, line, offset);
+            offset += line.len;
+            try file.writePositionalAll(self.io, "\n", offset);
+            offset += 1;
+        }
+        file.sync(self.io) catch {};
+        self.written_bytes = offset;
+    }
+
+    // =============================================================================
+    // Conversation rebuild
+    // =============================================================================
+
+    /// Build a fresh `Conversation` from the entry log. Caller owns the
+    /// returned conversation (call `deinit`).
+    pub fn rebuildConversation(self: *const SessionFile) !conversation_mod.Conversation {
+        var conv = conversation_mod.Conversation.init(self.allocator);
+        errdefer conv.deinit();
+
+        for (self.entries.items) |entry| {
+            switch (entry) {
+                .message => |me| try appendMessageToConv(&conv, self.allocator, me.message, me.stamp),
+            }
+        }
+        return conv;
+    }
+};
+
+/// Best-effort extraction of plain prompt text from a user `StoredMessage`.
+/// Used to populate `SessionInfo.last_user_message`. Returns null if the
+/// message carries no plain text block. Caller owns the returned slice.
+fn extractUserText(alloc: Allocator, msg: StoredMessage) !?[]u8 {
+    for (msg.content) |block| {
+        if (block == .text) {
+            return try alloc.dupe(u8, block.text.text);
+        }
+    }
+    return null;
+}
+
+fn appendMessageToConv(
+    conv: *conversation_mod.Conversation,
+    allocator: Allocator,
+    disk_msg: StoredMessage,
+    stamp: ?session_mod.WireStamp,
+) !void {
+    var content: std.ArrayList(conversation_mod.ContentBlock) = .empty;
+    errdefer {
+        for (content.items) |*b| {
+            var mut = b.*;
+            mut.deinit(allocator);
+        }
+        content.deinit(allocator);
+    }
+    try content.ensureTotalCapacity(allocator, disk_msg.content.len);
+    const sys_mode: conversation_mod.SystemMode = switch (disk_msg.mode) {
+        .append => .append,
+        .replace => .replace,
+    };
+    for (disk_msg.content) |db| {
+        var block = try session_mod.diskContentBlockToInternal(allocator, db);
+        // System-role text blocks become `.System` blocks carrying the
+        // message's recorded mode, so the append/replace derivation works
+        // on the rebuilt conversation exactly as it did when written.
+        if (disk_msg.role == .system and block == .Text) {
+            const tb = block.Text;
+            block = .{ .System = .{ .text = tb, .mode = sys_mode } };
+        }
+        if (block == .Thinking and stamp != null) {
+            block.Thinking.signature_origin = try conversation_mod.SignatureOrigin.init(
+                allocator,
+                stamp.?.api_style,
+                stamp.?.base_url,
+                stamp.?.model,
+            );
+        }
+        content.appendAssumeCapacity(block);
+    }
+    const role: conversation_mod.MessageRole = switch (disk_msg.role) {
+        .system => .system,
+        .user => .user,
+        .assistant => .assistant,
+    };
+    // Reconstruct the per-message producing identity from the wire stamp, so
+    // a later in-session compaction preserves it (rather than re-stamping the
+    // restated turn with the compaction model). Borrowed `stamp` slices are
+    // duped into the conversation allocator.
+    const identity: ?session_store_mod.WireIdentity = if (stamp) |st|
+        try conversation_mod.dupeWireIdentity(allocator, .{
+            .api_style = st.api_style,
+            .base_url = st.base_url,
+            .model = st.model,
+            .reasoning = st.reasoning,
+            .thinking = st.thinking,
+            .effort = st.effort,
+            .thinking_budget_tokens = st.thinking_budget_tokens,
+            .thinking_interleaved = st.thinking_interleaved,
+        })
+    else
+        null;
+    errdefer if (identity) |id| conversation_mod.freeWireIdentity(allocator, id);
+    // Carry the recorded usage forward so compaction can size the retention
+    // window after a session is reopened (it's null for user/system).
+    try conv.messages.append(allocator, .{
+        .role = role,
+        .content = content,
+        .usage = disk_msg.usage,
+        .identity = identity,
+    });
+}
+
+// =============================================================================
+// Entry repair on load
+// =============================================================================
+
+fn elideDanglingToolUses(allocator: Allocator, entries: *std.ArrayList(SessionEntry)) bool {
+    var needed: std.StringHashMap(void) = .init(allocator);
+    defer needed.deinit();
+    var changed = false;
+
+    var i = entries.items.len;
+    while (i > 0) {
+        i -= 1;
+        const entry = &entries.items[i];
+        if (entry.* != .message) continue;
+        const msg = &entry.message.message;
+
+        if (msg.role == .user) {
+            for (msg.content) |block| {
+                if (block == .tool_result) {
+                    needed.put(block.tool_result.tool_use_id, {}) catch {};
+                }
+            }
+            continue;
+        }
+
+        if (msg.role != .assistant) continue;
+        var kept: std.ArrayList(StoredContentBlock) = .empty;
+        defer kept.deinit(allocator);
+        var removed = false;
+        for (msg.content) |block| {
+            if (block == .tool_use and !needed.contains(block.tool_use.id)) {
+                block.deinit(allocator);
+                removed = true;
+                continue;
+            }
+            kept.append(allocator, block) catch unreachable;
+        }
+        if (!removed) continue;
+        allocator.free(msg.content);
+        msg.content = kept.toOwnedSlice(allocator) catch unreachable;
+        changed = true;
+    }
+    return changed;
+}
+
+// =============================================================================
+// File utilities
+// =============================================================================
+
+fn readWholeFile(allocator: Allocator, io: Io, path: []const u8) ![]u8 {
+    const file = Io.Dir.cwd().openFile(io, path, .{ .mode = .read_only }) catch |err| switch (err) {
+        error.FileNotFound => return error.InvalidSessionFile,
+        else => return err,
+    };
+    defer file.close(io);
+
+    const len = file.length(io) catch {
+        // Fall back to a streaming read of a reasonable upper bound.
+        // Sessions over ~10 MB are out of scope for phase 4.
+        var list: std.ArrayList(u8) = .empty;
+        defer list.deinit(allocator);
+        var chunk: [4096]u8 = undefined;
+        while (true) {
+            const n = file.readStreaming(io, &.{&chunk}) catch break;
+            if (n == 0) break;
+            try list.appendSlice(allocator, chunk[0..n]);
+        }
+        return try list.toOwnedSlice(allocator);
+    };
+
+    const buf = try allocator.alloc(u8, @intCast(len));
+    errdefer allocator.free(buf);
+    _ = try file.readPositionalAll(io, buf, 0);
+    return buf;
+}
+
+fn statFileForLength(io: Io, path: []const u8) !u64 {
+    const file = try Io.Dir.cwd().openFile(io, path, .{ .mode = .read_only });
+    defer file.close(io);
+    return try file.length(io);
+}
+
+fn truncateFileTo(io: Io, path: []const u8, new_length: u64) !void {
+    const file = try Io.Dir.cwd().openFile(io, path, .{ .mode = .write_only });
+    defer file.close(io);
+    try file.setLength(io, new_length);
+    file.sync(io) catch {};
+}
+
+/// Write a fresh file containing `header` followed by `entries`. Truncates
+/// any existing content. Used after a migration rewrites the format.
+fn rewriteFile(
+    allocator: Allocator,
+    io: Io,
+    path: []const u8,
+    header: SessionHeader,
+    entries: []const SessionEntry,
+) !void {
+    const file = try Io.Dir.cwd().createFile(io, path, .{
+        .truncate = true,
+        .read = false,
+    });
+    defer file.close(io);
+
+    var offset: u64 = 0;
+    const header_line = try session_mod.serializeHeader(allocator, header);
+    defer allocator.free(header_line);
+    try file.writePositionalAll(io, header_line, offset);
+    offset += header_line.len;
+    try file.writePositionalAll(io, "\n", offset);
+    offset += 1;
+    for (entries) |e| {
+        const line = try session_mod.serializeEntry(allocator, e);
+        defer allocator.free(line);
+        try file.writePositionalAll(io, line, offset);
+        offset += line.len;
+        try file.writePositionalAll(io, "\n", offset);
+        offset += 1;
+    }
+    file.sync(io) catch {};
+}
+
+fn mkdirP(io: Io, path: []const u8) !void {
+    Io.Dir.cwd().createDirPath(io, path) catch |err| switch (err) {
+        error.PathAlreadyExists => {},
+        else => return err,
+    };
+}
+
+// =============================================================================
+// Listing
+// =============================================================================
+
+/// List sessions in `session_dir`. Returns a slice of `SessionInfo`s
+/// sorted by `modified` descending (most recent first). Caller owns the
+/// slice and each `SessionInfo`.
+///
+/// If the directory does not exist, returns an empty slice (no error).
+///
+/// If `on_progress` is non-null, it is invoked after each file is parsed.
+pub fn listSessions(
+    allocator: Allocator,
+    io: Io,
+    session_dir: []const u8,
+    on_progress: ?*const fn (loaded: usize, total: usize) void,
+) ![]FileInfo {
+    var dir = Io.Dir.cwd().openDir(io, session_dir, .{}) catch |err| switch (err) {
+        error.FileNotFound => return try allocator.alloc(FileInfo, 0),
+        else => return err,
+    };
+    defer dir.close(io);
+
+    var names: std.ArrayList([]u8) = .empty;
+    defer {
+        for (names.items) |n| allocator.free(n);
+        names.deinit(allocator);
+    }
+
+    var it = dir.iterate();
+    while (try it.next(io)) |entry| {
+        if (entry.kind != .file) continue;
+        if (!std.mem.endsWith(u8, entry.name, ".jsonl")) continue;
+        const copy = try allocator.dupe(u8, entry.name);
+        errdefer allocator.free(copy);
+        try names.append(allocator, copy);
+    }
+
+    var infos: std.ArrayList(FileInfo) = .empty;
+    errdefer {
+        for (infos.items) |i| i.deinit(allocator);
+        infos.deinit(allocator);
+    }
+    try infos.ensureTotalCapacity(allocator, names.items.len);
+
+    var loaded: usize = 0;
+    for (names.items) |name| {
+        const full = try std.fs.path.join(allocator, &.{ session_dir, name });
+        defer allocator.free(full);
+        const info_opt = buildFileInfo(allocator, io, full) catch null;
+        if (info_opt) |info| {
+            infos.appendAssumeCapacity(info);
+        }
+        loaded += 1;
+        if (on_progress) |cb| cb(loaded, names.items.len);
+    }
+
+    const slice = try infos.toOwnedSlice(allocator);
+    std.sort.pdq(FileInfo, slice, {}, fileInfoNewerFirst);
+    return slice;
+}
+
+fn fileInfoNewerFirst(_: void, a: FileInfo, b: FileInfo) bool {
+    return std.mem.order(u8, a.modified, b.modified) == .gt;
+}
+
+fn buildFileInfo(
+    allocator: Allocator,
+    io: Io,
+    file_path: []const u8,
+) !?FileInfo {
+    const bytes = readWholeFile(allocator, io, file_path) catch return null;
+    defer allocator.free(bytes);
+
+    var header_opt: ?SessionHeader = null;
+    defer if (header_opt) |h| h.deinit(allocator);
+
+    var message_count: usize = 0;
+    var last_activity: ?[]u8 = null;
+    defer if (last_activity) |la| allocator.free(la);
+    var last_user: ?[]u8 = null;
+    defer if (last_user) |lu| allocator.free(lu);
+    var last_stamp: ?session_mod.WireStamp = null;
+    defer if (last_stamp) |st| st.deinit(allocator);
+
+    var lines = std.mem.splitScalar(u8, bytes, '\n');
+    while (lines.next()) |line| {
+        const trimmed = std.mem.trim(u8, line, " \t\r");
+        if (trimmed.len == 0) continue;
+        const fe = session_mod.parseLine(allocator, trimmed) catch break;
+        switch (fe) {
+            .header => |h| {
+                if (header_opt != null) {
+                    h.deinit(allocator);
+                } else {
+                    header_opt = h;
+                }
+            },
+            .entry => |e| {
+                defer e.deinit(allocator);
+                switch (e) {
+                    .message => |m| {
+                        if (m.message.role == .user or m.message.role == .assistant) {
+                            message_count += 1;
+                            if (last_activity) |la| allocator.free(la);
+                            last_activity = try allocator.dupe(u8, m.base.timestamp);
+                        }
+                        if (m.stamp) |st| {
+                            if (last_stamp) |old| old.deinit(allocator);
+                            last_stamp = try st.dupe(allocator);
+                        }
+                        if (m.message.role == .user) {
+                            if (try extractUserText(allocator, m.message)) |ut| {
+                                if (last_user) |lu| allocator.free(lu);
+                                last_user = ut;
+                            }
+                        }
+                    },
+                }
+            },
+        }
+    }
+
+    const header = header_opt orelse return null;
+
+    const path = try allocator.dupe(u8, file_path);
+    errdefer allocator.free(path);
+    const id = try allocator.dupe(u8, header.id);
+    errdefer allocator.free(id);
+    const created = try allocator.dupe(u8, header.timestamp);
+    errdefer allocator.free(created);
+    const modified = if (last_activity) |la| blk: {
+        last_activity = null;
+        break :blk la;
+    } else try allocator.dupe(u8, header.timestamp);
+    errdefer allocator.free(modified);
+    const last_user_message = if (last_user) |lu| blk: {
+        last_user = null;
+        break :blk lu;
+    } else try allocator.dupe(u8, "");
+    errdefer allocator.free(last_user_message);
+    const stamp_out = if (last_stamp) |st| blk: {
+        last_stamp = null;
+        break :blk st;
+    } else null;
+
+    return .{
+        .path = path,
+        .id = id,
+        .created = created,
+        .modified = modified,
+        .message_count = message_count,
+        .last_user_message = last_user_message,
+        .stamp = stamp_out,
+    };
+}
+
+// =============================================================================
+// Recent / resume helpers
+// =============================================================================
+
+/// Resolve a (possibly abbreviated) session id to a session file path
+/// within `session_dir`. Errors if no match or ambiguous prefix.
+pub fn resolveSessionId(
+    allocator: Allocator,
+    io: Io,
+    session_dir: []const u8,
+    id_or_prefix: []const u8,
+) ![]u8 {
+    var dir = Io.Dir.cwd().openDir(io, session_dir, .{}) catch |err| switch (err) {
+        error.FileNotFound => return error.SessionNotFound,
+        else => return err,
+    };
+    defer dir.close(io);
+
+    var match: ?[]u8 = null;
+    errdefer if (match) |m| allocator.free(m);
+
+    var it = dir.iterate();
+    while (try it.next(io)) |entry| {
+        if (entry.kind != .file) continue;
+        if (!std.mem.endsWith(u8, entry.name, ".jsonl")) continue;
+        // Strip `.jsonl` for the prefix match.
+        const stem = entry.name[0 .. entry.name.len - ".jsonl".len];
+        if (!std.mem.startsWith(u8, stem, id_or_prefix)) continue;
+        if (match != null) return error.AmbiguousSessionId;
+        match = try allocator.dupe(u8, entry.name);
+    }
+
+    const name = match orelse return error.SessionNotFound;
+    defer allocator.free(name);
+    match = null;
+    return try std.fs.path.join(allocator, &.{ session_dir, name });
+}
+
+// =============================================================================
+// FileSystemJSONLStore — the directory-backed catalog (SessionStore impl)
+// =============================================================================
+
+/// A directory-backed `SessionStore`: each session is one `<id>.jsonl` file
+/// under `dir`. The catalog mints `Session` handles, lists/resolves files,
+/// loads conversations, and routes appends to the right `SessionFile`.
+///
+/// Open `SessionFile`s are cached by id for the catalog's lifetime so the
+/// buffered-until-first-assistant write discipline survives across the
+/// separate user-prompt and assistant-turn appends of a single turn.
+///
+/// `dir` is the already-resolved sessions directory (the panto CLI derives
+/// the per-cwd grouping; the store itself is cwd-agnostic). Optional
+/// `metadata` is stamped into new session headers for display/provenance only.
+pub const FileSystemJSONLStore = struct {
+    allocator: Allocator,
+    io: Io,
+    dir: []u8, // owned: the sessions directory
+    metadata: ?[]u8, // owned: recorded in new session headers
+    open: std.StringHashMap(*SessionFile),
+
+    pub fn init(allocator: Allocator, io: Io, dir: []const u8) !FileSystemJSONLStore {
+        return initWithMetadata(allocator, io, dir, null);
+    }
+
+    pub fn initWithMetadata(allocator: Allocator, io: Io, dir: []const u8, metadata: ?[]const u8) !FileSystemJSONLStore {
+        const dir_copy = try allocator.dupe(u8, dir);
+        errdefer allocator.free(dir_copy);
+        const metadata_copy: ?[]u8 = if (metadata) |m| try allocator.dupe(u8, m) else null;
+        return .{
+            .allocator = allocator,
+            .io = io,
+            .dir = dir_copy,
+            .metadata = metadata_copy,
+            .open = std.StringHashMap(*SessionFile).init(allocator),
+        };
+    }
+
+    pub fn deinit(self: *FileSystemJSONLStore) void {
+        var it = self.open.iterator();
+        while (it.next()) |e| {
+            e.value_ptr.*.deinit();
+            self.allocator.destroy(e.value_ptr.*);
+            self.allocator.free(e.key_ptr.*);
+        }
+        self.open.deinit();
+        self.allocator.free(self.dir);
+        if (self.metadata) |m| self.allocator.free(m);
+    }
+
+    /// Borrow (opening if needed) the `SessionFile` for `id`, caching it.
+    /// Returns null if the file does not exist on disk and `create_missing`
+    /// is false.
+    fn fileFor(self: *FileSystemJSONLStore, id: []const u8, create_missing: bool) !?*SessionFile {
+        if (self.open.get(id)) |sf| return sf;
+        // Locate the file by exact id.
+        const path = try std.fs.path.join(self.allocator, &.{ self.dir, id });
+        defer self.allocator.free(path);
+        const full = try std.fmt.allocPrint(self.allocator, "{s}.jsonl", .{path});
+        defer self.allocator.free(full);
+
+        const exists = blk: {
+            Io.Dir.cwd().access(self.io, full, .{}) catch break :blk false;
+            break :blk true;
+        };
+        if (!exists and !create_missing) return null;
+
+        const sf = try self.allocator.create(SessionFile);
+        errdefer self.allocator.destroy(sf);
+        sf.* = if (exists)
+            try SessionFile.open(self.allocator, self.io, full)
+        else
+            try SessionFile.initWithId(self.allocator, self.io, self.dir, self.metadata, id);
+        errdefer sf.deinit();
+
+        const key = try self.allocator.dupe(u8, id);
+        errdefer self.allocator.free(key);
+        try self.open.put(key, sf);
+        return sf;
+    }
+
+    fn infoFromFileInfo(self: *FileSystemJSONLStore, fi: FileInfo) !session_store_mod.SessionInfo {
+        const id = try self.allocator.dupe(u8, fi.id);
+        errdefer self.allocator.free(id);
+        const created = try self.allocator.dupe(u8, fi.created);
+        errdefer self.allocator.free(created);
+        const modified = try self.allocator.dupe(u8, fi.modified);
+        errdefer self.allocator.free(modified);
+        const last_user = try self.allocator.dupe(u8, fi.last_user_message);
+        errdefer self.allocator.free(last_user);
+        const base_url = try self.allocator.dupe(u8, if (fi.stamp) |s| s.base_url else "");
+        errdefer self.allocator.free(base_url);
+        const model = try self.allocator.dupe(u8, if (fi.stamp) |s| s.model else "");
+        return .{
+            .id = id,
+            .created = created,
+            .modified = modified,
+            .message_count = fi.message_count,
+            .last_user_message = last_user,
+            .api_style = if (fi.stamp) |s| s.api_style else .openai_chat,
+            .base_url = base_url,
+            .model = model,
+            .reasoning = if (fi.stamp) |s| s.reasoning else .default,
+        };
+    }
+
+    // ---------- vtable ----------
+
+    fn createVT(ctx: *anyopaque) session_store_mod.Session {
+        const self: *FileSystemJSONLStore = @ptrCast(@alignCast(ctx));
+        // Mint a fresh id; nothing hits disk until the first append.
+        const id = newUuidV7(self.allocator, self.io) catch "";
+        // The SessionFile is created lazily on first append via fileFor.
+        const info: session_store_mod.SessionInfo = .{
+            .id = id,
+            .created = self.allocator.dupe(u8, "") catch "",
+            .modified = self.allocator.dupe(u8, "") catch "",
+            .message_count = 0,
+            .last_user_message = self.allocator.dupe(u8, "") catch "",
+            .api_style = .openai_chat,
+            .base_url = self.allocator.dupe(u8, "") catch "",
+            .model = self.allocator.dupe(u8, "") catch "",
+            .reasoning = .default,
+        };
+        return .{ .info = info, .store = self.store() };
+    }
+
+    fn listVT(ctx: *anyopaque) anyerror![]session_store_mod.SessionInfo {
+        const self: *FileSystemJSONLStore = @ptrCast(@alignCast(ctx));
+        const fis = try listSessions(self.allocator, self.io, self.dir, null);
+        defer {
+            for (fis) |fi| fi.deinit(self.allocator);
+            self.allocator.free(fis);
+        }
+        var out = try self.allocator.alloc(session_store_mod.SessionInfo, fis.len);
+        var built: usize = 0;
+        errdefer {
+            for (out[0..built]) |i| i.deinit(self.allocator);
+            self.allocator.free(out);
+        }
+        for (fis, 0..) |fi, i| {
+            out[i] = try self.infoFromFileInfo(fi);
+            built += 1;
+        }
+        return out;
+    }
+
+    fn freeSessionInfosVT(ctx: *anyopaque, infos: []session_store_mod.SessionInfo) void {
+        const self: *FileSystemJSONLStore = @ptrCast(@alignCast(ctx));
+        for (infos) |i| i.deinit(self.allocator);
+        self.allocator.free(infos);
+    }
+
+    fn resolveVT(ctx: *anyopaque, id_or_prefix: []const u8) anyerror!?session_store_mod.Session {
+        const self: *FileSystemJSONLStore = @ptrCast(@alignCast(ctx));
+        const path = resolveSessionId(self.allocator, self.io, self.dir, id_or_prefix) catch |err| switch (err) {
+            error.SessionNotFound => return null,
+            else => return err,
+        };
+        defer self.allocator.free(path);
+        return try self.sessionFromPath(path);
+    }
+
+    fn latestVT(ctx: *anyopaque) anyerror!?session_store_mod.Session {
+        const self: *FileSystemJSONLStore = @ptrCast(@alignCast(ctx));
+        // Most-recently-*modified* wins, matching the `list()` sort order —
+        // not newest-created (lexicographic UUIDv7 filename).
+        const fis = try listSessions(self.allocator, self.io, self.dir, null);
+        defer {
+            for (fis) |fi| fi.deinit(self.allocator);
+            self.allocator.free(fis);
+        }
+        if (fis.len == 0) return null;
+        const info = try self.infoFromFileInfo(fis[0]);
+        return .{ .info = info, .store = self.store() };
+    }
+
+    fn sessionFromPath(self: *FileSystemJSONLStore, path: []const u8) !?session_store_mod.Session {
+        const fi = (try buildFileInfo(self.allocator, self.io, path)) orelse return null;
+        defer fi.deinit(self.allocator);
+        const info = try self.infoFromFileInfo(fi);
+        return .{ .info = info, .store = self.store() };
+    }
+
+    fn loadVT(ctx: *anyopaque, id: []const u8) anyerror!?conversation_mod.Conversation {
+        const self: *FileSystemJSONLStore = @ptrCast(@alignCast(ctx));
+        const sf = (try self.fileFor(id, false)) orelse return null;
+        return try sf.rebuildConversation();
+    }
+
+    fn appendMessagesVT(
+        ctx: *anyopaque,
+        session_id: []const u8,
+        messages: []session_store_mod.PersistentMessage,
+    ) anyerror!void {
+        const self: *FileSystemJSONLStore = @ptrCast(@alignCast(ctx));
+        if (messages.len == 0) return;
+        const sf = (try self.fileFor(session_id, true)).?;
+
+        // Convert each rich PersistentMessage to a StoredMessage + wire stamp.
+        // The FS store deliberately ignores the `conversation` and
+        // `tools_available` provenance fields.
+        var stored = try self.allocator.alloc(StoredMessage, messages.len);
+        var stamps = try self.allocator.alloc(?session_mod.WireStamp, messages.len);
+        defer self.allocator.free(stored);
+        defer self.allocator.free(stamps);
+        var built: usize = 0;
+        errdefer for (stored[0..built]) |sm| sm.deinit(self.allocator);
+
+        for (messages, 0..) |pm, i| {
+            stored[i] = try persistentToStored(self.allocator, pm);
+            // System entries carry no wire stamp; user/assistant do.
+            stamps[i] = if (pm.message.role == .system) null else .{
+                .api_style = pm.identity.api_style,
+                .base_url = pm.identity.base_url,
+                .model = pm.identity.model,
+                .reasoning = pm.identity.reasoning,
+                .thinking = pm.identity.thinking,
+                .effort = pm.identity.effort,
+                .thinking_budget_tokens = pm.identity.thinking_budget_tokens,
+                .thinking_interleaved = pm.identity.thinking_interleaved,
+            };
+            built += 1;
+        }
+
+        // appendMessagesAtomic consumes the StoredMessages; it dupes stamps.
+        try sf.appendMessagesAtomic(stored, stamps);
+    }
+
+    const store_vtable: session_store_mod.SessionStore.VTable = .{
+        .create = createVT,
+        .list = listVT,
+        .freeSessionInfos = freeSessionInfosVT,
+        .resolve = resolveVT,
+        .latest = latestVT,
+        .load = loadVT,
+        .appendMessages = appendMessagesVT,
+    };
+
+    /// Wrap this catalog as a neutral `SessionStore`. The handle borrows
+    /// `self`; `self` must outlive it.
+    pub fn store(self: *FileSystemJSONLStore) session_store_mod.SessionStore {
+        return .{ .ptr = self, .vtable = &store_vtable };
+    }
+};
+
+/// Convert a rich in-memory `PersistentMessage` to the on-disk
+/// `StoredMessage`. Strings are duplicated; the source is untouched.
+fn persistentToStored(
+    alloc: Allocator,
+    pm: session_store_mod.PersistentMessage,
+) !StoredMessage {
+    const msg = pm.message;
+    const blocks = try alloc.alloc(session_mod.StoredContentBlock, msg.content.items.len);
+    var allocated: usize = 0;
+    errdefer {
+        for (blocks[0..allocated]) |b| b.deinit(alloc);
+        alloc.free(blocks);
+    }
+    for (msg.content.items) |block| {
+        blocks[allocated] = try session_mod.contentBlockToDisk(alloc, block);
+        allocated += 1;
+    }
+    const mode: session_mod.StoredSystemMode = blk: {
+        for (msg.content.items) |block| {
+            if (block == .System and block.System.mode == .replace) break :blk .replace;
+        }
+        break :blk .append;
+    };
+    const stop_reason: ?[]const u8 = if (msg.role == .assistant) try alloc.dupe(u8, "stop") else null;
+    errdefer if (stop_reason) |s| alloc.free(s);
+    const metadata: ?[]const u8 = if (msg.metadata) |m| try alloc.dupe(u8, m) else null;
+    const role: session_mod.StoredMessageRole = switch (msg.role) {
+        .system => .system,
+        .user => .user,
+        .assistant => .assistant,
+    };
+    return .{
+        .role = role,
+        .content = blocks,
+        .mode = mode,
+        .stop_reason = stop_reason,
+        .usage = pm.usage,
+        .metadata = metadata,
+    };
+}
+
+// =============================================================================
+// Tests
+// =============================================================================
+
+const testing = std.testing;
+
+/// Borrowed wire stamps for tests (no allocation; the manager dupes them).
+fn oaStamp() session_mod.WireStamp {
+    return .{ .api_style = .openai_chat, .base_url = "https://api.openai.com/v1", .model = "gpt-4o" };
+}
+fn anStamp() session_mod.WireStamp {
+    return .{ .api_style = .anthropic_messages, .base_url = "https://api.anthropic.com", .model = "claude-sonnet-4-20250514" };
+}
+
+/// Test helper: append a single-text user message (see the `addUserMessage`
+/// signature change to a block slice).
+fn addUserText(conv: *conversation_mod.Conversation, text: []const u8) !void {
+    const tb = try conversation_mod.textualBlockFromSlice(conv.allocator, text);
+    var block: conversation_mod.ContentBlock = .{ .Text = tb };
+    errdefer block.deinit(conv.allocator);
+    try conv.addUserMessage(&.{block});
+}
+
+test "newUuidV7: produces 36-char hyphenated string with version 7" {
+    const io = testing.io;
+    const id = try newUuidV7(testing.allocator, io);
+    defer testing.allocator.free(id);
+    try testing.expectEqual(@as(usize, 36), id.len);
+    // Position 14 is the version nibble — should be '7'.
+    try testing.expectEqual(@as(u8, '7'), id[14]);
+    // Hyphens at canonical positions.
+    try testing.expectEqual(@as(u8, '-'), id[8]);
+    try testing.expectEqual(@as(u8, '-'), id[13]);
+    try testing.expectEqual(@as(u8, '-'), id[18]);
+    try testing.expectEqual(@as(u8, '-'), id[23]);
+}
+
+test "isoTimestamp: well-formed ISO 8601 with millisecond precision" {
+    const ts = try isoTimestamp(testing.allocator, testing.io);
+    defer testing.allocator.free(ts);
+    try testing.expectEqual(@as(usize, 24), ts.len);
+    try testing.expectEqual(@as(u8, '-'), ts[4]);
+    try testing.expectEqual(@as(u8, 'T'), ts[10]);
+    try testing.expectEqual(@as(u8, '.'), ts[19]);
+    try testing.expectEqual(@as(u8, 'Z'), ts[23]);
+}
+
+// ---- In-memory + filesystem tests (use a tmp dir) ----
+
+const TmpSessionDir = struct {
+    parent: std.testing.TmpDir,
+    abs_path: []u8,
+
+    fn init(allocator: Allocator) !TmpSessionDir {
+        var parent = std.testing.tmpDir(.{});
+        errdefer parent.cleanup();
+        var path_buf: [std.fs.max_path_bytes]u8 = undefined;
+        const n = try parent.dir.realPath(testing.io, &path_buf);
+        const abs = try allocator.dupe(u8, path_buf[0..n]);
+        return .{ .parent = parent, .abs_path = abs };
+    }
+
+    fn deinit(self: *TmpSessionDir, allocator: Allocator) void {
+        allocator.free(self.abs_path);
+        self.parent.cleanup();
+    }
+};
+
+test "SessionFile.init: does not create file yet" {
+    const io = testing.io;
+
+    var td = try TmpSessionDir.init(testing.allocator);
+    defer td.deinit(testing.allocator);
+
+    // Use a non-existent subdirectory inside the tmp dir to also exercise
+    // lazy directory creation.
+    const sessions = try std.fs.path.join(testing.allocator, &.{ td.abs_path, "sessions" });
+    defer testing.allocator.free(sessions);
+
+    var mgr = try SessionFile.init(
+        testing.allocator,
+        io,
+        sessions,
+        "{\"cwd\":\"/some/cwd\"}",
+    );
+    defer mgr.deinit();
+
+    try testing.expect(!mgr.isFlushed());
+
+    // The directory should not exist yet.
+    const stat_err = Io.Dir.cwd().openDir(io, sessions, .{});
+    try testing.expectError(error.FileNotFound, stat_err);
+}
+
+test "SessionFile: full flow — buffer, flush on assistant, append, resume" {
+    const io = testing.io;
+
+    var td = try TmpSessionDir.init(testing.allocator);
+    defer td.deinit(testing.allocator);
+
+    const sessions = try std.fs.path.join(testing.allocator, &.{ td.abs_path, "sessions" });
+    defer testing.allocator.free(sessions);
+
+    const session_file: []u8 = blk: {
+        var mgr = try SessionFile.init(
+            testing.allocator,
+            io,
+            sessions,
+            "{\"cwd\":\"/proj/foo\"}",
+        );
+        defer mgr.deinit();
+
+        // System message buffers in memory — nothing on disk yet.
+        const sys_blocks = try testing.allocator.alloc(StoredContentBlock, 1);
+        sys_blocks[0] = .{ .text = .{ .text = try testing.allocator.dupe(u8, "You are helpful.") } };
+        _ = try mgr.appendMessage(
+            .{ .role = .system, .content = sys_blocks },
+            null,
+        );
+        try testing.expect(!mgr.isFlushed());
+
+        // User message — still buffered.
+        const usr_blocks = try testing.allocator.alloc(StoredContentBlock, 1);
+        usr_blocks[0] = .{ .text = .{ .text = try testing.allocator.dupe(u8, "hi there") } };
+        _ = try mgr.appendMessage(
+            .{ .role = .user, .content = usr_blocks },
+            oaStamp(),
+        );
+        try testing.expect(!mgr.isFlushed());
+
+        // Assistant message — first flush: header + all buffered entries.
+        const a_blocks = try testing.allocator.alloc(StoredContentBlock, 1);
+        a_blocks[0] = .{ .text = .{ .text = try testing.allocator.dupe(u8, "hello") } };
+        _ = try mgr.appendMessage(
+            .{
+                .role = .assistant,
+                .content = a_blocks,
+                .stop_reason = try testing.allocator.dupe(u8, "stop"),
+            },
+            oaStamp(),
+        );
+        try testing.expect(mgr.isFlushed());
+
+        // Append another user/assistant round.
+        const u_two = try testing.allocator.alloc(StoredContentBlock, 1);
+        u_two[0] = .{ .text = .{ .text = try testing.allocator.dupe(u8, "what's 2+2?") } };
+        _ = try mgr.appendMessage(.{ .role = .user, .content = u_two }, oaStamp());
+
+        const a2 = try testing.allocator.alloc(StoredContentBlock, 1);
+        a2[0] = .{ .text = .{ .text = try testing.allocator.dupe(u8, "4") } };
+        _ = try mgr.appendMessage(
+            .{ .role = .assistant, .content = a2, .stop_reason = try testing.allocator.dupe(u8, "stop") },
+            oaStamp(),
+        );
+
+        try testing.expectEqual(@as(usize, 5), mgr.entries.items.len);
+
+        break :blk try testing.allocator.dupe(u8, mgr.getSessionFile());
+    };
+    defer testing.allocator.free(session_file);
+
+    // Verify the file exists and is well-formed.
+    {
+        const bytes = try readWholeFile(testing.allocator, io, session_file);
+        defer testing.allocator.free(bytes);
+        // 1 header + 5 entries + trailing \n on each = 6 newlines.
+        var nl_count: usize = 0;
+        for (bytes) |b| if (b == '\n') {
+            nl_count += 1;
+        };
+        try testing.expectEqual(@as(usize, 6), nl_count);
+    }
+
+    // Resume.
+    var resumed = try SessionFile.open(testing.allocator, io, session_file);
+    defer resumed.deinit();
+    try testing.expect(resumed.isFlushed());
+    try testing.expectEqual(@as(usize, 5), resumed.entries.items.len);
+    try testing.expectEqualStrings("{\"cwd\":\"/proj/foo\"}", resumed.header.metadata.?);
+
+    // Continue the conversation.
+    const u_three = try testing.allocator.alloc(StoredContentBlock, 1);
+    u_three[0] = .{ .text = .{ .text = try testing.allocator.dupe(u8, "thanks") } };
+    _ = try resumed.appendMessage(.{ .role = .user, .content = u_three }, oaStamp());
+    try testing.expectEqual(@as(usize, 6), resumed.entries.items.len);
+}
+
+test "SessionFile: assistant message tags the message metadata and the entry leaf id is the assistant entry" {
+    const io = testing.io;
+
+    var td = try TmpSessionDir.init(testing.allocator);
+    defer td.deinit(testing.allocator);
+    const sessions = try std.fs.path.join(testing.allocator, &.{ td.abs_path, "sessions" });
+    defer testing.allocator.free(sessions);
+
+    var mgr = try SessionFile.init(testing.allocator, io, sessions, null);
+    defer mgr.deinit();
+
+    const u_blocks = try testing.allocator.alloc(StoredContentBlock, 1);
+    u_blocks[0] = .{ .text = .{ .text = try testing.allocator.dupe(u8, "ping") } };
+    const user_id = try mgr.appendMessage(.{ .role = .user, .content = u_blocks }, oaStamp());
+
+    const a_blocks = try testing.allocator.alloc(StoredContentBlock, 1);
+    a_blocks[0] = .{ .text = .{ .text = try testing.allocator.dupe(u8, "pong") } };
+    const asst_id = try mgr.appendMessage(.{ .role = .assistant, .content = a_blocks }, null);
+
+    // Leaf is the assistant entry.
+    try testing.expectEqualStrings(asst_id, mgr.leaf_id.?);
+    // Parent of assistant is the user entry.
+    const assistant_entry = mgr.entries.items[mgr.by_id.get(asst_id).?];
+    try testing.expectEqualStrings(user_id, assistant_entry.base().parent_id.?);
+    // User entry's parent is null (no system).
+    const user_entry = mgr.entries.items[mgr.by_id.get(user_id).?];
+    try testing.expect(user_entry.base().parent_id == null);
+}
+
+/// Test helper: the active provider/model stamp — the last entry carrying a
+/// wire stamp, walking leaf→root. Null when no stamped message exists yet.
+fn activeStamp(sf: *const SessionFile) ?session_mod.WireStamp {
+    var i = sf.entries.items.len;
+    while (i > 0) : (i -= 1) {
+        switch (sf.entries.items[i - 1]) {
+            .message => |m| if (m.stamp) |st| return st,
+        }
+    }
+    return null;
+}
+
+test "SessionFile: activeStamp is null before any user message, then tracks the latest user stamp" {
+    const io = testing.io;
+
+    var td = try TmpSessionDir.init(testing.allocator);
+    defer td.deinit(testing.allocator);
+    const sessions = try std.fs.path.join(testing.allocator, &.{ td.abs_path, "sessions" });
+    defer testing.allocator.free(sessions);
+
+    var mgr = try SessionFile.init(testing.allocator, io, sessions, null);
+    defer mgr.deinit();
+
+    // No user messages yet — there is no "active" model on disk yet.
+    try testing.expect(activeStamp(&mgr) == null);
+
+    // Stamp a user message with anthropic.
+    const u_blocks = try testing.allocator.alloc(StoredContentBlock, 1);
+    u_blocks[0] = .{ .text = .{ .text = try testing.allocator.dupe(u8, "hi") } };
+    _ = try mgr.appendMessage(.{ .role = .user, .content = u_blocks }, anStamp());
+
+    {
+        const am = activeStamp(&mgr).?;
+        try testing.expectEqual(session_mod.APIStyle.anthropic_messages, am.api_style);
+        try testing.expectEqualStrings("claude-sonnet-4-20250514", am.model);
+    }
+}
+
+test "SessionFile: rebuildConversation reconstructs system/user/assistant turn" {
+    const io = testing.io;
+
+    var td = try TmpSessionDir.init(testing.allocator);
+    defer td.deinit(testing.allocator);
+    const sessions = try std.fs.path.join(testing.allocator, &.{ td.abs_path, "sessions" });
+    defer testing.allocator.free(sessions);
+
+    var mgr = try SessionFile.init(testing.allocator, io, sessions, null);
+    defer mgr.deinit();
+
+    const sys = try testing.allocator.alloc(StoredContentBlock, 1);
+    sys[0] = .{ .text = .{ .text = try testing.allocator.dupe(u8, "you are helpful") } };
+    _ = try mgr.appendMessage(.{ .role = .system, .content = sys }, null);
+
+    const u = try testing.allocator.alloc(StoredContentBlock, 1);
+    u[0] = .{ .text = .{ .text = try testing.allocator.dupe(u8, "hello") } };
+    _ = try mgr.appendMessage(.{ .role = .user, .content = u }, oaStamp());
+
+    const a = try testing.allocator.alloc(StoredContentBlock, 1);
+    a[0] = .{ .text = .{ .text = try testing.allocator.dupe(u8, "hi!") } };
+    _ = try mgr.appendMessage(.{ .role = .assistant, .content = a }, null);
+
+    var conv = try mgr.rebuildConversation();
+    defer conv.deinit();
+    try testing.expectEqual(@as(usize, 3), conv.messages.items.len);
+    try testing.expectEqual(conversation_mod.MessageRole.system, conv.messages.items[0].role);
+    try testing.expectEqualStrings("you are helpful", conv.messages.items[0].content.items[0].System.text.items);
+    try testing.expectEqual(conversation_mod.MessageRole.user, conv.messages.items[1].role);
+    try testing.expectEqualStrings("hello", conv.messages.items[1].content.items[0].Text.items);
+    try testing.expectEqual(conversation_mod.MessageRole.assistant, conv.messages.items[2].role);
+    try testing.expectEqualStrings("hi!", conv.messages.items[2].content.items[0].Text.items);
+}
+
+test "SessionFile: crash recovery truncates corrupted trailing line" {
+    const io = testing.io;
+
+    var td = try TmpSessionDir.init(testing.allocator);
+    defer td.deinit(testing.allocator);
+    const sessions = try std.fs.path.join(testing.allocator, &.{ td.abs_path, "sessions" });
+    defer testing.allocator.free(sessions);
+
+    // Build a valid session first.
+    const session_file: []u8 = blk: {
+        var mgr = try SessionFile.init(testing.allocator, io, sessions, null);
+        defer mgr.deinit();
+        const u = try testing.allocator.alloc(StoredContentBlock, 1);
+        u[0] = .{ .text = .{ .text = try testing.allocator.dupe(u8, "ping") } };
+        _ = try mgr.appendMessage(.{ .role = .user, .content = u }, oaStamp());
+        const a = try testing.allocator.alloc(StoredContentBlock, 1);
+        a[0] = .{ .text = .{ .text = try testing.allocator.dupe(u8, "pong") } };
+        _ = try mgr.appendMessage(.{ .role = .assistant, .content = a }, null);
+        break :blk try testing.allocator.dupe(u8, mgr.getSessionFile());
+    };
+    defer testing.allocator.free(session_file);
+
+    // Corrupt the file: append a partial JSON line at the end.
+    const garbage = "{\"type\":\"message\",\"id\":\"deadbeef\",\"parent";
+    {
+        const file = try Io.Dir.cwd().openFile(io, session_file, .{ .mode = .write_only });
+        defer file.close(io);
+        const len = try file.length(io);
+        try file.writePositionalAll(io, garbage, len);
+    }
+    // Confirm the file got bigger.
+    {
+        const f = try Io.Dir.cwd().openFile(io, session_file, .{ .mode = .read_only });
+        defer f.close(io);
+        const corrupted_len = try f.length(io);
+        try testing.expect(corrupted_len > garbage.len);
+    }
+
+    // Now resume — the partial line should be truncated.
+    var resumed = try SessionFile.open(testing.allocator, io, session_file);
+    defer resumed.deinit();
+    try testing.expectEqual(@as(usize, 2), resumed.entries.items.len);
+
+    // And the file on disk should match.
+    {
+        const bytes = try readWholeFile(testing.allocator, io, session_file);
+        defer testing.allocator.free(bytes);
+        try testing.expect(!std.mem.endsWith(u8, bytes, "parent"));
+        // Should end with a newline after the assistant entry.
+        try testing.expectEqual(@as(u8, '\n'), bytes[bytes.len - 1]);
+    }
+}
+
+test "listSessions: returns most recent first, with counts" {
+    const io = testing.io;
+
+    var td = try TmpSessionDir.init(testing.allocator);
+    defer td.deinit(testing.allocator);
+    const sessions = try std.fs.path.join(testing.allocator, &.{ td.abs_path, "sessions" });
+    defer testing.allocator.free(sessions);
+
+    // Create two sessions.
+    for (0..2) |i| {
+        var mgr = try SessionFile.init(testing.allocator, io, sessions, null);
+        defer mgr.deinit();
+        const u = try testing.allocator.alloc(StoredContentBlock, 1);
+        u[0] = .{ .text = .{ .text = try testing.allocator.dupe(u8, "u") } };
+        _ = try mgr.appendMessage(.{ .role = .user, .content = u }, oaStamp());
+        const a = try testing.allocator.alloc(StoredContentBlock, 1);
+        a[0] = .{ .text = .{ .text = try testing.allocator.dupe(u8, "a") } };
+        _ = try mgr.appendMessage(.{ .role = .assistant, .content = a }, null);
+        // Small sleep so UUIDv7 timestamps differ.
+        io.sleep(.fromMilliseconds(2), .real) catch {};
+        _ = i;
+    }
+
+    const infos = try listSessions(testing.allocator, io, sessions, null);
+    defer {
+        for (infos) |fi| fi.deinit(testing.allocator);
+        testing.allocator.free(infos);
+    }
+    try testing.expectEqual(@as(usize, 2), infos.len);
+    try testing.expectEqual(@as(usize, 2), infos[0].message_count);
+    try testing.expect(std.mem.order(u8, infos[0].modified, infos[1].modified) != .lt);
+}
+
+test "latest: picks most-recently-modified, not newest-created" {
+    const io = testing.io;
+
+    var td = try TmpSessionDir.init(testing.allocator);
+    defer td.deinit(testing.allocator);
+    const sessions = try std.fs.path.join(testing.allocator, &.{ td.abs_path, "sessions" });
+    defer testing.allocator.free(sessions);
+
+    var catalog = try FileSystemJSONLStore.init(testing.allocator, io, sessions);
+    defer catalog.deinit();
+    const st = catalog.store();
+
+    // Before any sessions exist → null.
+    try testing.expect((try st.latest()) == null);
+
+    // Create session A, then session B (newer id), then append to A again
+    // so A is the most recently *modified*.
+    var first_id: ?[]u8 = null;
+    defer if (first_id) |s| testing.allocator.free(s);
+
+    for (0..2) |i| {
+        var mgr = try SessionFile.init(testing.allocator, io, sessions, null);
+        defer mgr.deinit();
+        const u = try testing.allocator.alloc(StoredContentBlock, 1);
+        u[0] = .{ .text = .{ .text = try testing.allocator.dupe(u8, "u") } };
+        _ = try mgr.appendMessage(.{ .role = .user, .content = u }, oaStamp());
+        const a = try testing.allocator.alloc(StoredContentBlock, 1);
+        a[0] = .{ .text = .{ .text = try testing.allocator.dupe(u8, "a") } };
+        _ = try mgr.appendMessage(.{ .role = .assistant, .content = a }, oaStamp());
+        if (i == 0) first_id = try testing.allocator.dupe(u8, mgr.header.id);
+        io.sleep(.fromMilliseconds(2), .real) catch {};
+    }
+    {
+        const path = try resolveSessionId(testing.allocator, io, sessions, first_id.?);
+        defer testing.allocator.free(path);
+        var mgr = try SessionFile.open(testing.allocator, io, path);
+        defer mgr.deinit();
+        const a = try testing.allocator.alloc(StoredContentBlock, 1);
+        a[0] = .{ .text = .{ .text = try testing.allocator.dupe(u8, "a") } };
+        _ = try mgr.appendMessage(.{ .role = .assistant, .content = a }, oaStamp());
+    }
+
+    var latest = (try st.latest()).?;
+    defer latest.info.deinit(testing.allocator);
+    try testing.expectEqualStrings(first_id.?, latest.info.id);
+}
+
+test "SessionFile: tool-use round-trip — assistant w/ ToolUse, user w/ ToolResult, assistant" {
+    const io = testing.io;
+    var td = try TmpSessionDir.init(testing.allocator);
+    defer td.deinit(testing.allocator);
+    const sessions = try std.fs.path.join(testing.allocator, &.{ td.abs_path, "sessions" });
+    defer testing.allocator.free(sessions);
+
+    const session_file: []u8 = blk: {
+        var mgr = try SessionFile.init(testing.allocator, io, sessions, null);
+        defer mgr.deinit();
+
+        const u = try testing.allocator.alloc(StoredContentBlock, 1);
+        u[0] = .{ .text = .{ .text = try testing.allocator.dupe(u8, "list files") } };
+        _ = try mgr.appendMessage(.{ .role = .user, .content = u }, oaStamp());
+
+        // Assistant emits a ToolUse.
+        const am1 = try testing.allocator.alloc(StoredContentBlock, 2);
+        am1[0] = .{ .text = .{ .text = try testing.allocator.dupe(u8, "checking...") } };
+        am1[1] = .{ .tool_use = .{
+            .id = try testing.allocator.dupe(u8, "tool_abc"),
+            .name = try testing.allocator.dupe(u8, "bash"),
+            .input = try testing.allocator.dupe(u8, "{\"command\":\"ls\"}"),
+        } };
+        _ = try mgr.appendMessage(.{ .role = .assistant, .content = am1 }, null);
+
+        // Tool-result user message.
+        const tr = try testing.allocator.alloc(StoredContentBlock, 1);
+        const trp = try testing.allocator.alloc(session_mod.StoredResultPart, 1);
+        trp[0] = .{ .text = try testing.allocator.dupe(u8, "a.txt\nb.txt") };
+        tr[0] = .{ .tool_result = .{
+            .tool_use_id = try testing.allocator.dupe(u8, "tool_abc"),
+            .parts = trp,
+        } };
+        _ = try mgr.appendMessage(.{ .role = .user, .content = tr }, oaStamp());
+
+        // Final assistant reply.
+        const a2 = try testing.allocator.alloc(StoredContentBlock, 1);
+        a2[0] = .{ .text = .{ .text = try testing.allocator.dupe(u8, "two files: a.txt and b.txt") } };
+        _ = try mgr.appendMessage(.{ .role = .assistant, .content = a2 }, null);
+
+        break :blk try testing.allocator.dupe(u8, mgr.getSessionFile());
+    };
+    defer testing.allocator.free(session_file);
+
+    // Reopen and verify content blocks survive.
+    var resumed = try SessionFile.open(testing.allocator, io, session_file);
+    defer resumed.deinit();
+    const entries = resumed.entries.items;
+    try testing.expectEqual(@as(usize, 4), entries.len);
+
+    // [1] = assistant with ToolUse
+    try testing.expectEqual(StoredMessageRole.assistant, entries[1].message.message.role);
+    try testing.expectEqual(@as(usize, 2), entries[1].message.message.content.len);
+    try testing.expect(entries[1].message.message.content[1] == .tool_use);
+    try testing.expectEqualStrings("bash", entries[1].message.message.content[1].tool_use.name);
+    try testing.expectEqualStrings("{\"command\":\"ls\"}", entries[1].message.message.content[1].tool_use.input);
+
+    // [2] = user with ToolResult, stamped with wire identity.
+    try testing.expectEqual(StoredMessageRole.user, entries[2].message.message.role);
+    try testing.expectEqual(session_mod.APIStyle.openai_chat, entries[2].message.stamp.?.api_style);
+    try testing.expect(entries[2].message.message.content[0] == .tool_result);
+    try testing.expectEqualStrings("tool_abc", entries[2].message.message.content[0].tool_result.tool_use_id);
+    try testing.expectEqualStrings("a.txt\nb.txt", entries[2].message.message.content[0].tool_result.parts[0].text);
+
+    // Conversation rebuild yields the same shape.
+    var conv = try resumed.rebuildConversation();
+    defer conv.deinit();
+    try testing.expectEqual(@as(usize, 4), conv.messages.items.len);
+    try testing.expect(conv.messages.items[1].content.items[1] == .ToolUse);
+    try testing.expect(conv.messages.items[2].content.items[0] == .ToolResult);
+}
+
+test "SessionFile: linear chain — each entry's parent_id is the previous entry's id" {
+    const io = testing.io;
+    var td = try TmpSessionDir.init(testing.allocator);
+    defer td.deinit(testing.allocator);
+    const sessions = try std.fs.path.join(testing.allocator, &.{ td.abs_path, "sessions" });
+    defer testing.allocator.free(sessions);
+
+    var mgr = try SessionFile.init(testing.allocator, io, sessions, null);
+    defer mgr.deinit();
+
+    // Three rounds: sys, user, asst, user, asst.
+    const sys = try testing.allocator.alloc(StoredContentBlock, 1);
+    sys[0] = .{ .text = .{ .text = try testing.allocator.dupe(u8, "sys") } };
+    _ = try mgr.appendMessage(.{ .role = .system, .content = sys }, null);
+
+    const u_one = try testing.allocator.alloc(StoredContentBlock, 1);
+    u_one[0] = .{ .text = .{ .text = try testing.allocator.dupe(u8, "u1") } };
+    _ = try mgr.appendMessage(.{ .role = .user, .content = u_one }, oaStamp());
+
+    const a_one = try testing.allocator.alloc(StoredContentBlock, 1);
+    a_one[0] = .{ .text = .{ .text = try testing.allocator.dupe(u8, "a1") } };
+    _ = try mgr.appendMessage(.{ .role = .assistant, .content = a_one }, null);
+
+    const u_two = try testing.allocator.alloc(StoredContentBlock, 1);
+    u_two[0] = .{ .text = .{ .text = try testing.allocator.dupe(u8, "u2") } };
+    _ = try mgr.appendMessage(.{ .role = .user, .content = u_two }, oaStamp());
+
+    const a_two = try testing.allocator.alloc(StoredContentBlock, 1);
+    a_two[0] = .{ .text = .{ .text = try testing.allocator.dupe(u8, "a2") } };
+    _ = try mgr.appendMessage(.{ .role = .assistant, .content = a_two }, null);
+
+    const entries = mgr.entries.items;
+    try testing.expectEqual(@as(usize, 5), entries.len);
+    try testing.expect(entries[0].base().parent_id == null);
+    for (entries[1..], 1..) |e, i| {
+        try testing.expectEqualStrings(entries[i - 1].base().id, e.base().parent_id.?);
+    }
+}
+
+test "resolveSessionId: unique prefix → match, ambiguous → error" {
+    const io = testing.io;
+
+    var td = try TmpSessionDir.init(testing.allocator);
+    defer td.deinit(testing.allocator);
+    const sessions = try std.fs.path.join(testing.allocator, &.{ td.abs_path, "sessions" });
+    defer testing.allocator.free(sessions);
+
+    // Create one session.
+    var mgr = try SessionFile.init(testing.allocator, io, sessions, null);
+    defer mgr.deinit();
+    const u = try testing.allocator.alloc(StoredContentBlock, 1);
+    u[0] = .{ .text = .{ .text = try testing.allocator.dupe(u8, "u") } };
+    _ = try mgr.appendMessage(.{ .role = .user, .content = u }, oaStamp());
+    const a = try testing.allocator.alloc(StoredContentBlock, 1);
+    a[0] = .{ .text = .{ .text = try testing.allocator.dupe(u8, "a") } };
+    _ = try mgr.appendMessage(.{ .role = .assistant, .content = a }, null);
+
+    const id = mgr.header.id;
+    const prefix = id[0..8];
+
+    const resolved = try resolveSessionId(testing.allocator, io, sessions, prefix);
+    defer testing.allocator.free(resolved);
+    try testing.expectEqualStrings(mgr.getSessionFile(), resolved);
+
+    try testing.expectError(error.SessionNotFound, resolveSessionId(testing.allocator, io, sessions, "ffffffff"));
+}
+
+test "compaction summary round-trips through persist + resume + rebuild" {
+    const io = testing.io;
+
+    var td = try TmpSessionDir.init(testing.allocator);
+    defer td.deinit(testing.allocator);
+    const sessions = try std.fs.path.join(testing.allocator, &.{ td.abs_path, "sessions" });
+    defer testing.allocator.free(sessions);
+
+    const session_file: []u8 = blk: {
+        var mgr = try SessionFile.init(testing.allocator, io, sessions, null);
+        defer mgr.deinit();
+
+        // System + an old turn that will be superseded.
+        const sys = try testing.allocator.alloc(StoredContentBlock, 1);
+        sys[0] = .{ .text = .{ .text = try testing.allocator.dupe(u8, "you are helpful") } };
+        _ = try mgr.appendMessage(.{ .role = .system, .content = sys }, null);
+
+        const uo = try testing.allocator.alloc(StoredContentBlock, 1);
+        uo[0] = .{ .text = .{ .text = try testing.allocator.dupe(u8, "old q") } };
+        _ = try mgr.appendMessage(.{ .role = .user, .content = uo }, oaStamp());
+        const ao = try testing.allocator.alloc(StoredContentBlock, 1);
+        ao[0] = .{ .text = .{ .text = try testing.allocator.dupe(u8, "old a") } };
+        _ = try mgr.appendMessage(.{ .role = .assistant, .content = ao }, null);
+
+        // Compaction: summary message + duplicated kept suffix.
+        const cs = try testing.allocator.alloc(StoredContentBlock, 1);
+        cs[0] = .{ .compaction_summary = .{ .text = try testing.allocator.dupe(u8, "SUMMARY") } };
+        _ = try mgr.appendMessage(.{ .role = .user, .content = cs }, oaStamp());
+
+        const ur = try testing.allocator.alloc(StoredContentBlock, 1);
+        ur[0] = .{ .text = .{ .text = try testing.allocator.dupe(u8, "recent q") } };
+        _ = try mgr.appendMessage(.{ .role = .user, .content = ur }, oaStamp());
+
+        break :blk try testing.allocator.dupe(u8, mgr.getSessionFile());
+    };
+    defer testing.allocator.free(session_file);
+
+    var resumed = try SessionFile.open(testing.allocator, io, session_file);
+    defer resumed.deinit();
+    var conv = try resumed.rebuildConversation();
+    defer conv.deinit();
+
+    // The compaction summary block survived as a CompactionSummary.
+    const anchor = conversation_mod.latestCompactionIndex(conv.messages.items).?;
+    try testing.expectEqualStrings(
+        "SUMMARY",
+        conv.messages.items[anchor].content.items[0].CompactionSummary.text.items,
+    );
+
+    // The active window is [summary, recent q].
+    const window = conversation_mod.activeMessageWindow(conv.messages.items);
+    try testing.expectEqual(@as(usize, 2), window.len);
+    try testing.expectEqualStrings("recent q", window[1].content.items[0].Text.items);
+
+    // System prompt survives (derived independently).
+    var sys_blocks = try conversation_mod.effectiveSystemBlocks(testing.allocator, conv.messages.items);
+    defer sys_blocks.deinit(testing.allocator);
+    try testing.expectEqual(@as(usize, 1), sys_blocks.items.len);
+    try testing.expectEqualStrings("you are helpful", sys_blocks.items[0]);
+}
+
+test "loadConversation: trailing user prompt is split out as dangling, excluded from conversation" {
+    const io = testing.io;
+
+    var td = try TmpSessionDir.init(testing.allocator);
+    defer td.deinit(testing.allocator);
+    const sessions = try std.fs.path.join(testing.allocator, &.{ td.abs_path, "sessions" });
+    defer testing.allocator.free(sessions);
+
+    var mgr = try SessionFile.init(testing.allocator, io, sessions, null);
+    defer mgr.deinit();
+
+    // A completed user/assistant round, then a trailing user prompt with no
+    // following assistant. The dangling-prompt recovery feature was dropped
+    // in R2: the trailing user message simply round-trips into the rebuilt
+    // conversation like any other.
+    const um1 = try testing.allocator.alloc(StoredContentBlock, 1);
+    um1[0] = .{ .text = .{ .text = try testing.allocator.dupe(u8, "hi") } };
+    _ = try mgr.appendMessage(.{ .role = .user, .content = um1 }, oaStamp());
+    const am1 = try testing.allocator.alloc(StoredContentBlock, 1);
+    am1[0] = .{ .text = .{ .text = try testing.allocator.dupe(u8, "hello") } };
+    _ = try mgr.appendMessage(.{ .role = .assistant, .content = am1 }, null);
+    const um2 = try testing.allocator.alloc(StoredContentBlock, 1);
+    um2[0] = .{ .text = .{ .text = try testing.allocator.dupe(u8, "what's 2+2?") } };
+    _ = try mgr.appendMessage(.{ .role = .user, .content = um2 }, oaStamp());
+
+    var conv = try mgr.rebuildConversation();
+    defer conv.deinit();
+    // All three messages are present (dangling recovery dropped).
+    try testing.expectEqual(@as(usize, 3), conv.messages.items.len);
+    try testing.expectEqual(conversation_mod.MessageRole.user, conv.messages.items[2].role);
+}
+
+test "FileSystemJSONLStore catalog: create → append → load round-trips" {
+    const io = testing.io;
+
+    var td = try TmpSessionDir.init(testing.allocator);
+    defer td.deinit(testing.allocator);
+    const sessions = try std.fs.path.join(testing.allocator, &.{ td.abs_path, "sessions" });
+    defer testing.allocator.free(sessions);
+
+    var catalog = try FileSystemJSONLStore.init(testing.allocator, io, sessions);
+    defer catalog.deinit();
+    const store = catalog.store();
+
+    var sess = store.create();
+    defer sess.info.deinit(testing.allocator);
+
+    // Build a user + assistant PersistentMessage batch (borrows in-memory
+    // messages owned here).
+    var conv = conversation_mod.Conversation.init(testing.allocator);
+    defer conv.deinit();
+    try addUserText(&conv, "ping");
+    try conv.addAssistantMessage(&.{}, null);
+
+    const id: session_store_mod.WireIdentity = .{ .api_style = .openai_chat, .base_url = "u", .model = "m" };
+    var batch = [_]session_store_mod.PersistentMessage{
+        .{ .message = conv.messages.items[0], .identity = id },
+        .{ .message = conv.messages.items[1], .identity = id },
+    };
+    try sess.append(&batch);
+
+    // The session's last-used api_style updated after append.
+    try testing.expectEqual(session_store_mod.APIStyle.openai_chat, sess.info.api_style);
+
+    // Load it back by id.
+    var loaded = (try store.load(sess.info.id)).?;
+    defer loaded.deinit();
+    try testing.expectEqual(@as(usize, 2), loaded.messages.items.len);
+    try testing.expectEqual(conversation_mod.MessageRole.user, loaded.messages.items[0].role);
+}
+
+test "FileSystemJSONLStore load restores thinking signature origin from message stamp" {
+    const io = testing.io;
+
+    var td = try TmpSessionDir.init(testing.allocator);
+    defer td.deinit(testing.allocator);
+    const sessions = try std.fs.path.join(testing.allocator, &.{ td.abs_path, "sessions" });
+    defer testing.allocator.free(sessions);
+
+    var catalog = try FileSystemJSONLStore.init(testing.allocator, io, sessions);
+    defer catalog.deinit();
+    const store = catalog.store();
+
+    var sess = store.create();
+    defer sess.info.deinit(testing.allocator);
+
+    var conv = conversation_mod.Conversation.init(testing.allocator);
+    defer conv.deinit();
+    try addUserText(&conv, "ping");
+    try conv.addAssistantMessage(&.{
+        .{ .Thinking = .{
+            .text = try conversation_mod.textualBlockFromSlice(testing.allocator, "thinking..."),
+            .signature = try testing.allocator.dupe(u8, "sig123"),
+        } },
+        .{ .Text = try conversation_mod.textualBlockFromSlice(testing.allocator, "pong") },
+    }, null);
+
+    const id: session_store_mod.WireIdentity = .{
+        .api_style = .anthropic_messages,
+        .base_url = "https://api.anthropic.com",
+        .model = "claude-sonnet-4-20250514",
+    };
+    var batch = [_]session_store_mod.PersistentMessage{
+        .{ .message = conv.messages.items[0], .identity = id },
+        .{ .message = conv.messages.items[1], .identity = id },
+    };
+    try sess.append(&batch);
+
+    var loaded = (try store.load(sess.info.id)).?;
+    defer loaded.deinit();
+    const thinking = loaded.messages.items[1].content.items[0].Thinking;
+    try testing.expect(thinking.signature_origin != null);
+    try testing.expect(thinking.signature_origin.?.matches(
+        .anthropic_messages,
+        "https://api.anthropic.com",
+        "claude-sonnet-4-20250514",
+    ));
+}
+
+test "persistTurn: a message's own identity overrides the uniform persist identity" {
+    // Regression for the compaction re-stamping bug: a kept-verbatim turn
+    // produced by model A must persist with A's wire identity even when the
+    // turn is persisted under model B (the compaction model). persistTurn
+    // stamps the uniform identity only on messages that don't already carry
+    // one; a message with its own identity keeps it.
+    const io = testing.io;
+
+    var td = try TmpSessionDir.init(testing.allocator);
+    defer td.deinit(testing.allocator);
+    const sessions = try std.fs.path.join(testing.allocator, &.{ td.abs_path, "sessions" });
+    defer testing.allocator.free(sessions);
+
+    var catalog = try FileSystemJSONLStore.init(testing.allocator, io, sessions);
+    defer catalog.deinit();
+    const store = catalog.store();
+    var sess = store.create();
+    defer sess.info.deinit(testing.allocator);
+
+    var conv = conversation_mod.Conversation.init(testing.allocator);
+    defer conv.deinit();
+    // User turn carries no identity (will be stamped with the persist id).
+    try addUserText(&conv, "ping");
+    // Assistant turn was produced by model A — stamp its identity directly,
+    // as a live turn / a reloaded turn would have.
+    try conv.addAssistantMessage(&.{
+        .{ .Thinking = .{
+            .text = try conversation_mod.textualBlockFromSlice(testing.allocator, "thinking..."),
+            .signature = try testing.allocator.dupe(u8, "sig123"),
+        } },
+        .{ .Text = try conversation_mod.textualBlockFromSlice(testing.allocator, "pong") },
+    }, null);
+    conv.messages.items[1].identity = try conversation_mod.dupeWireIdentity(testing.allocator, .{
+        .api_style = .anthropic_messages,
+        .base_url = "https://api.anthropic.com",
+        .model = "claude-sonnet-4-20250514",
+    });
+
+    // Persist under model B (a different, "compaction" identity).
+    const persist_id: session_store_mod.WireIdentity = .{
+        .api_style = .openai_chat,
+        .base_url = "https://api.openai.com/v1",
+        .model = "gpt-4o",
+    };
+    try turn_persist.persistTurn(testing.allocator, &sess, &conv, 0, persist_id, &.{});
+
+    // Reload: the user turn took model B's identity; the assistant turn kept
+    // model A's — so its thinking signature replays to A, not B.
+    var loaded = (try store.load(sess.info.id)).?;
+    defer loaded.deinit();
+
+    const asst_origin = loaded.messages.items[1].content.items[0].Thinking.signature_origin.?;
+    try testing.expect(asst_origin.matches(.anthropic_messages, "https://api.anthropic.com", "claude-sonnet-4-20250514"));
+    try testing.expect(!asst_origin.matches(.openai_chat, "https://api.openai.com/v1", "gpt-4o"));
+
+    const user_id = loaded.messages.items[0].identity.?;
+    try testing.expectEqual(session_store_mod.APIStyle.openai_chat, user_id.api_style);
+    try testing.expectEqualStrings("gpt-4o", user_id.model);
+}