1 files changed, 160 insertions, 2 deletions

diff --git a/libpanto/src/conversation.zig b/libpanto/src/conversation.zig
index 6776412..f145d7f 100644
--- a/libpanto/src/conversation.zig
+++ b/libpanto/src/conversation.zig
@@ -51,11 +51,29 @@ pub const ToolResultBlock = struct {
     }
 };
 
+/// How a `.system` content block combines with the system text collected
+/// before it. `append` adds to the running effective prompt; `replace`
+/// discards everything collected so far and starts fresh from this block.
+pub const SystemMode = enum { append, replace };
+
+/// A system-prompt content block. System prompts remain `.system`-role
+/// messages; this block records the mode that governs how its text folds
+/// into the effective system prompt (see `effectiveSystemBlocks`).
+pub const SystemBlock = struct {
+    text: TextualBlock = .empty,
+    mode: SystemMode = .append,
+
+    pub fn deinit(self: *SystemBlock, alloc: Allocator) void {
+        self.text.deinit(alloc);
+    }
+};
+
 pub const ContentBlock = union(enum) {
     Text: TextualBlock,
     Thinking: ThinkingBlock,
     ToolUse: ToolUseBlock,
     ToolResult: ToolResultBlock,
+    System: SystemBlock,
 
     pub fn deinit(self: *ContentBlock, alloc: Allocator) void {
         switch (self.*) {
@@ -63,6 +81,7 @@ pub const ContentBlock = union(enum) {
             .Thinking => |*b| b.deinit(alloc),
             .ToolUse => |*b| b.deinit(alloc),
             .ToolResult => |*b| b.deinit(alloc),
+            .System => |*b| b.deinit(alloc),
         }
     }
 };
@@ -95,10 +114,30 @@ pub const Conversation = struct {
         };
     }
 
+    /// Append a system message in `append` mode. Adds to the effective
+    /// system prompt. (Back-compatible: same external behavior as before
+    /// the `.System` block existed.)
     pub fn addSystemMessage(self: *Conversation, text: []const u8) !void {
+        return self.appendSystemBlock(text, .append);
+    }
+
+    /// Append a system message in `replace` mode. When the effective
+    /// prompt is rebuilt (see `effectiveSystemBlocks`), this discards all
+    /// prior system text and starts fresh.
+    pub fn replaceSystemMessage(self: *Conversation, text: []const u8) !void {
+        return self.appendSystemBlock(text, .replace);
+    }
+
+    /// Append a `.system`-role message whose single content block is a
+    /// `.System` block carrying `mode`.
+    fn appendSystemBlock(self: *Conversation, text: []const u8, mode: SystemMode) !void {
         const tb = try textualBlockFromSlice(self.allocator, text);
         var content: std.ArrayList(ContentBlock) = .empty;
-        try content.append(self.allocator, .{ .Text = tb });
+        errdefer {
+            for (content.items) |*b| b.deinit(self.allocator);
+            content.deinit(self.allocator);
+        }
+        try content.append(self.allocator, .{ .System = .{ .text = tb, .mode = mode } });
         try self.messages.append(self.allocator, .{
             .role = .system,
             .content = content,
@@ -137,6 +176,50 @@ pub const Conversation = struct {
     }
 };
 
+/// Derive the effective ordered list of system-text blocks from a slice of
+/// messages. This is the single shared rule that governs both provider
+/// serialization and session rebuild.
+///
+/// Walk the messages in order; for each `.system` message's `.System`
+/// block:
+///   - `append`: add the block's text to the running list.
+///   - `replace`: clear the running list, then add this block's text.
+///
+/// The returned slices are **borrowed** from `messages` — valid only as
+/// long as the underlying conversation is unmodified. The caller owns the
+/// returned `ArrayList` itself and must `deinit` it (this frees the slice
+/// storage, not the borrowed text).
+///
+/// Running this walk over a *prefix* of the messages reconstructs the
+/// effective prompt as of that point — the `/tree` faithfulness property.
+pub fn effectiveSystemBlocks(
+    alloc: Allocator,
+    messages: []const Message,
+) !std.ArrayList([]const u8) {
+    var out: std.ArrayList([]const u8) = .empty;
+    errdefer out.deinit(alloc);
+    for (messages) |msg| {
+        if (msg.role != .system) continue;
+        for (msg.content.items) |block| {
+            switch (block) {
+                .System => |sb| {
+                    switch (sb.mode) {
+                        .append => {},
+                        .replace => out.clearRetainingCapacity(),
+                    }
+                    try out.append(alloc, sb.text.items);
+                },
+                // Be tolerant of plain `.Text` blocks on a system message
+                // (e.g. hand-built test conversations): treat them as
+                // append-mode text.
+                .Text => |tb| try out.append(alloc, tb.items),
+                else => {},
+            }
+        }
+    }
+    return out;
+}
+
 test "Conversation - add messages and verify content" {
     const allocator = std.testing.allocator;
 
@@ -154,7 +237,11 @@ test "Conversation - add messages and verify content" {
     try std.testing.expectEqual(MessageRole.system, conv.messages.items[0].role);
     try std.testing.expectEqualStrings(
         "You are a helpful assistant.",
-        conv.messages.items[0].content.items[0].Text.items,
+        conv.messages.items[0].content.items[0].System.text.items,
+    );
+    try std.testing.expectEqual(
+        SystemMode.append,
+        conv.messages.items[0].content.items[0].System.mode,
     );
 
     try std.testing.expectEqual(MessageRole.user, conv.messages.items[1].role);
@@ -202,3 +289,74 @@ test "ContentBlock - Thinking variant" {
     try std.testing.expectEqualStrings("hmm...", conv.messages.items[0].content.items[0].Thinking.text.items);
     try std.testing.expectEqualStrings("answer", conv.messages.items[0].content.items[1].Text.items);
 }
+
+test "System block - addSystemMessage records append mode, replaceSystemMessage records replace mode" {
+    const allocator = std.testing.allocator;
+
+    var conv = Conversation.init(allocator);
+    defer conv.deinit();
+
+    try conv.addSystemMessage("base");
+    try conv.replaceSystemMessage("fresh");
+
+    try std.testing.expectEqual(SystemMode.append, conv.messages.items[0].content.items[0].System.mode);
+    try std.testing.expectEqualStrings("base", conv.messages.items[0].content.items[0].System.text.items);
+    try std.testing.expectEqual(SystemMode.replace, conv.messages.items[1].content.items[0].System.mode);
+    try std.testing.expectEqualStrings("fresh", conv.messages.items[1].content.items[0].System.text.items);
+}
+
+test "effectiveSystemBlocks - append accumulates in order" {
+    const allocator = std.testing.allocator;
+
+    var conv = Conversation.init(allocator);
+    defer conv.deinit();
+
+    try conv.addSystemMessage("a");
+    try conv.addSystemMessage("b");
+    try conv.addUserMessage("hi");
+    try conv.addSystemMessage("c");
+
+    var blocks = try effectiveSystemBlocks(allocator, conv.messages.items);
+    defer blocks.deinit(allocator);
+
+    try std.testing.expectEqual(@as(usize, 3), blocks.items.len);
+    try std.testing.expectEqualStrings("a", blocks.items[0]);
+    try std.testing.expectEqualStrings("b", blocks.items[1]);
+    try std.testing.expectEqualStrings("c", blocks.items[2]);
+}
+
+test "effectiveSystemBlocks - replace wipes everything collected so far" {
+    const allocator = std.testing.allocator;
+
+    var conv = Conversation.init(allocator);
+    defer conv.deinit();
+
+    try conv.addSystemMessage("a");
+    try conv.addSystemMessage("b");
+    try conv.replaceSystemMessage("fresh");
+    try conv.addSystemMessage("after");
+
+    var blocks = try effectiveSystemBlocks(allocator, conv.messages.items);
+    defer blocks.deinit(allocator);
+
+    try std.testing.expectEqual(@as(usize, 2), blocks.items.len);
+    try std.testing.expectEqualStrings("fresh", blocks.items[0]);
+    try std.testing.expectEqualStrings("after", blocks.items[1]);
+}
+
+test "effectiveSystemBlocks - prefix reconstructs prompt as of that point" {
+    const allocator = std.testing.allocator;
+
+    var conv = Conversation.init(allocator);
+    defer conv.deinit();
+
+    try conv.addSystemMessage("a");
+    try conv.replaceSystemMessage("fresh");
+    try conv.addSystemMessage("after");
+
+    // Truncate at position 1 (only the first `addSystemMessage`).
+    var blocks = try effectiveSystemBlocks(allocator, conv.messages.items[0..1]);
+    defer blocks.deinit(allocator);
+    try std.testing.expectEqual(@as(usize, 1), blocks.items.len);
+    try std.testing.expectEqualStrings("a", blocks.items[0]);
+}