feat: grid override file — .codedb/model_grid.toml (#269)

src/runtime/grid.zig

@@ -84,16 +84,124 @@ pub fn tierForRole(role: []const u8) ?ModelTier {
     return null;
 }
 
+// ── Grid override file (.codedb/model_grid.toml) ─────────────────────────────
+
+const MAX_OVERRIDES = 32;
+const OVERRIDE_PATH = ".codedb/model_grid.toml";
+
+pub const GridOverride = struct {
+    role: []const u8,
+    tier: ModelTier,
+    rationale: []const u8,
+};
+
+var g_overrides: [MAX_OVERRIDES]GridOverride = undefined;
+var g_override_count: usize = 0;
+var g_overrides_loaded: bool = false;
+
+/// Parse a minimal TOML-like override file. Supports:
+///   [role_name]
+///   model = "tier_name"
+///   rationale = "reason"
+pub fn parseOverrides(content: []const u8) struct { overrides: [MAX_OVERRIDES]GridOverride, count: usize } {
+    var result: [MAX_OVERRIDES]GridOverride = undefined;
+    var count: usize = 0;
+    var current_role: ?[]const u8 = null;
+    var current_rationale: []const u8 = "";
+
+    var lines = std.mem.splitScalar(u8, content, '\n');
+    while (lines.next()) |raw_line| {
+        const line = std.mem.trim(u8, raw_line, " \t\r");
+        if (line.len == 0 or line[0] == '#') continue;
+
+        // Section header: [role_name]
+        if (line[0] == '[' and line[line.len - 1] == ']') {
+            current_role = line[1 .. line.len - 1];
+            current_rationale = "";
+            continue;
+        }
+
+        if (current_role == null) continue;
+
+        // Key = "value"
+        if (std.mem.indexOf(u8, line, "=")) |eq| {
+            const key = std.mem.trim(u8, line[0..eq], " \t");
+            const raw_val = std.mem.trim(u8, line[eq + 1 ..], " \t");
+            // Strip surrounding quotes
+            const val = if (raw_val.len >= 2 and raw_val[0] == '"' and raw_val[raw_val.len - 1] == '"')
+                raw_val[1 .. raw_val.len - 1]
+            else
+                raw_val;
+
+            if (std.mem.eql(u8, key, "model")) {
+                if (ModelTier.fromString(val)) |tier| {
+                    if (count < MAX_OVERRIDES) {
+                        result[count] = .{
+                            .role = current_role.?,
+                            .tier = tier,
+                            .rationale = current_rationale,
+                        };
+                        count += 1;
+                    }
+                }
+            } else if (std.mem.eql(u8, key, "rationale")) {
+                current_rationale = val;
+                // Update the last entry's rationale if it matches this role
+                if (count > 0 and std.mem.eql(u8, result[count - 1].role, current_role.?)) {
+                    result[count - 1].rationale = val;
+                }
+            }
+        }
+    }
+
+    return .{ .overrides = result, .count = count };
+}
+
+/// Load overrides from the file system. Caches after first load.
+pub fn loadOverrides() void {
+    if (g_overrides_loaded) return;
+    g_overrides_loaded = true;
+
+    const file = std.fs.cwd().openFile(OVERRIDE_PATH, .{}) catch return;
+    defer file.close();
+
+    var buf: [8192]u8 = undefined;
+    const len = file.readAll(&buf) catch return;
+
+    const parsed = parseOverrides(buf[0..len]);
+    g_overrides = parsed.overrides;
+    g_override_count = parsed.count;
+}
+
+/// Reset cached overrides (for testing).
+pub fn resetOverrides() void {
+    g_overrides_loaded = false;
+    g_override_count = 0;
+}
+
+/// Look up override tier for a role.
+pub fn overrideTierForRole(role: []const u8) ?ModelTier {
+    loadOverrides();
+    for (g_overrides[0..g_override_count]) |ov| {
+        if (std.mem.eql(u8, ov.role, role)) return ov.tier;
+    }
+    return null;
+}
+
 /// Resolve the model ID for a given role + mode combination.
-/// Priority: role grid → mode default → fallback (Sonnet).
+/// Priority: override file → role grid → mode default → fallback (Sonnet).
 pub fn resolveModel(role: ?[]const u8, mode: AgentMode) []const u8 {
-    // 1. Check grid for role-specific tier
     if (role) |r| {
+        // 1. Check override file (highest priority)
+        if (overrideTierForRole(r)) |tier| {
+            return tier.toModelId();
+        }
+        // 2. Check grid for role-specific tier
         if (tierForRole(r)) |tier| {
             return tier.toModelId();
         }
     }
-    // 2. Fall back to mode default
+    // 3. Fall back to mode default
     return mode.defaultModel();
 }
 
@@ -123,3 +231,76 @@ test "grid: ModelTier round-trips" {
     try std.testing.expectEqual(ModelTier.opus, ModelTier.fromString("opus").?);
     try std.testing.expectEqual(@as(?ModelTier, null), ModelTier.fromString("gpt4"));
 }
+
+// ── Grid override tests (#269) ──────────────────────────────────────────────
+
+test "grid: parseOverrides basic TOML" {
+    const toml =
+        \\# Generated by evolve_agent
+        \\[orchestrator]
+        \\model = "sonnet"
+        \\rationale = "Haiku failed to decompose complex tasks"
+        \\
+        \\[mutator]
+        \\model = "opus"
+        \\rationale = "Opus produces higher fitness organisms"
+    ;
+    const result = parseOverrides(toml);
+    try std.testing.expectEqual(@as(usize, 2), result.count);
+    try std.testing.expectEqualStrings("orchestrator", result.overrides[0].role);
+    try std.testing.expectEqual(ModelTier.sonnet, result.overrides[0].tier);
+    try std.testing.expectEqualStrings("mutator", result.overrides[1].role);
+    try std.testing.expectEqual(ModelTier.opus, result.overrides[1].tier);
+}
+
+test "grid: parseOverrides skips invalid tier" {
+    const toml =
+        \\[reviewer]
+        \\model = "nonexistent"
+    ;
+    const result = parseOverrides(toml);
+    try std.testing.expectEqual(@as(usize, 0), result.count);
+}
+
+test "grid: parseOverrides handles empty content" {
+    const result = parseOverrides("");
+    try std.testing.expectEqual(@as(usize, 0), result.count);
+}
+
+test "grid: parseOverrides rationale before model" {
+    const toml =
+        \\[fixer]
+        \\rationale = "Better at fixing"
+        \\model = "haiku"
+    ;
+    const result = parseOverrides(toml);
+    try std.testing.expectEqual(@as(usize, 1), result.count);
+    try std.testing.expectEqual(ModelTier.haiku, result.overrides[0].tier);
+}
+
+test "grid: parseOverrides strips quotes" {
+    const toml =
+        \\[finder]
+        \\model = "sonnet"
+    ;
+    const result = parseOverrides(toml);
+    try std.testing.expectEqual(@as(usize, 1), result.count);
+    try std.testing.expectEqual(ModelTier.sonnet, result.overrides[0].tier);
+}
+
+test "grid: parseOverrides no quotes works too" {
+    const toml =
+        \\[finder]
+        \\model = opus
+    ;
+    const result = parseOverrides(toml);
+    try std.testing.expectEqual(@as(usize, 1), result.count);
+    try std.testing.expectEqual(ModelTier.opus, result.overrides[0].tier);
+}
+
+test "grid: resolveModel existing tests still pass with override reset" {
+    resetOverrides();
+    try std.testing.expectEqualStrings("gpt-5.4", resolveModel("orchestrator", .smart));
+    try std.testing.expectEqualStrings("claude-sonnet-4-6", resolveModel("finder", .rush));
+    resetOverrides();
+}