1 files changed, 89 insertions, 0 deletions

diff --git a/src/system_prompt.zig b/src/system_prompt.zig
index b796394..0b449dc 100644
--- a/src/system_prompt.zig
+++ b/src/system_prompt.zig
@@ -37,6 +37,31 @@ pub const default_seed = "You are a helpful assistant.";
 
 const seed_filename = "SYSTEM.md";
 const append_filename = "APPEND_SYSTEM.md";
+const compaction_filename = "COMPACTION.md";
+
+/// Last-resort fallback compaction prompt when no `COMPACTION.md` exists at
+/// any layer. In normal operation the base layer ships a bundled
+/// `agent/COMPACTION.md`, so this is only used if that staged file is
+/// missing or unreadable.
+pub const default_compaction_prompt =
+    \\You are compacting a long conversation to reduce its context size.
+    \\
+    \\You will be given a transcript of an earlier portion of a
+    \\conversation between a user and an AI coding assistant, and possibly a
+    \\previous summary of even older history. Produce a single, dense
+    \\summary that preserves everything needed to continue the work
+    \\coherently:
+    \\
+    \\- the user's goals, decisions, and stated preferences
+    \\- the current state of the task and what remains to be done
+    \\- key facts discovered about the codebase or problem
+    \\- important file paths, identifiers, and code structures
+    \\- unresolved questions, bugs, and their understanding
+    \\
+    \\Write the summary as factual notes, not as a chat reply. Do not
+    \\address the user. Do not include pleasantries. Be comprehensive about
+    \\specifics but omit redundant back-and-forth.
+;
 
 /// A resolved set of system blocks for a fresh session: the seed followed
 /// by zero or more appends, already in emission order. All slices
@@ -185,6 +210,39 @@ pub fn reconcileResumeLayers(
     }
 }
 
+/// Resolve the compaction system prompt across the three config layers.
+/// Like `SYSTEM.md` (not `APPEND_SYSTEM.md`): the highest layer present
+/// wins (project > user > base); falls back to a built-in default if no
+/// `COMPACTION.md` exists anywhere. The result is owned by `arena`.
+pub fn resolveCompaction(
+    arena: Allocator,
+    io: Io,
+    environ_map: *const std.process.Environ.Map,
+    base_dir: []const u8,
+) ![]const u8 {
+    const user_dir = try userLayerDir(arena, environ_map);
+    const project_dir = try projectLayerDir(arena, io);
+    return resolveCompactionLayers(arena, io, base_dir, user_dir, project_dir);
+}
+
+/// Layer-explicit variant of `resolveCompaction` (see `resolveLayers`).
+pub fn resolveCompactionLayers(
+    arena: Allocator,
+    io: Io,
+    base_dir: ?[]const u8,
+    user_dir: ?[]const u8,
+    project_dir: ?[]const u8,
+) ![]const u8 {
+    var prompt: []const u8 = default_compaction_prompt;
+    for ([_]?[]const u8{ base_dir, user_dir, project_dir }) |maybe_dir| {
+        const dir = maybe_dir orelse continue;
+        if (try readLayerFile(arena, io, dir, compaction_filename)) |text| {
+            prompt = text;
+        }
+    }
+    return prompt;
+}
+
 // ---------------------------------------------------------------------------
 // Internals
 // ---------------------------------------------------------------------------
@@ -539,3 +597,34 @@ test "Resolved.blocks - seed leads appends" {
     try testing.expectEqualStrings("U", blocks[2]);
     try testing.expectEqualStrings("B", blocks[3]);
 }
+
+test "resolveCompactionLayers - highest COMPACTION.md wins, default otherwise" {
+    var tmp = testing.tmpDir(.{ .iterate = true });
+    defer tmp.cleanup();
+
+    var arena_state = std.heap.ArenaAllocator.init(testing.allocator);
+    defer arena_state.deinit();
+    const arena = arena_state.allocator();
+
+    var buf: [std.fs.max_path_bytes]u8 = undefined;
+    const root_len = try tmp.dir.realPathFile(testing.io, ".", &buf);
+    const root = buf[0..root_len];
+
+    // No files anywhere -> default.
+    {
+        const p = try resolveCompactionLayers(arena, testing.io, root, null, null);
+        try testing.expectEqualStrings(default_compaction_prompt, p);
+    }
+
+    // base + project; project wins.
+    try tmp.dir.createDirPath(testing.io, "base");
+    try tmp.dir.createDirPath(testing.io, "project");
+    try writeTmpFile(tmp.dir, "base/COMPACTION.md", "base compaction");
+    try writeTmpFile(tmp.dir, "project/COMPACTION.md", "project compaction");
+    const base_dir = try std.fs.path.join(arena, &.{ root, "base" });
+    const project_dir = try std.fs.path.join(arena, &.{ root, "project" });
+    {
+        const p = try resolveCompactionLayers(arena, testing.io, base_dir, null, project_dir);
+        try testing.expectEqualStrings("project compaction", p);
+    }
+}