Fix MCP tools/list rejection: wrap list_repos slice in object

The go-sdk infers each tool's outputSchema from its handler result
type, and MCP structured output must be type "object". list_repos
returned []repos.State, yielding outputSchema.type "array", which
Claude Desktop rejects at tools/list — taking the whole server down.

Wrap the slice in listReposOutput{Repos: ...} so the schema is an
object, update the round-trip test, and record the struct-result rule
in AGENT.md §8.1.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

AGENT.md

@@ -367,6 +367,13 @@ obeys the safety rules (§1.4).
     `get_pending_switch`, `list_prs`.
   - Act: `git_status/checkout/commit/push/pull/create_branch`,
     `create_pr`, `merge_and_cleanup_pr`, `set_active_project`, `ack_switch`.
+- **A tool's result type must be a struct, never a bare slice/map/scalar.** The
+  go-sdk infers each tool's `outputSchema` from its handler's result type, and MCP
+  structured output must be a JSON **object** (`type: "object"`). A handler that
+  returns `[]T` yields `outputSchema.type: "array"`, which Claude Desktop rejects
+  at `tools/list` — and one bad tool takes the whole server down. Wrap any
+  collection result in a named output struct (e.g. `list_repos` returns
+  `listReposOutput{ Repos []repos.State }`, not `[]repos.State`). Fixed 2026-09-20.
 - **Destructive tools carry the §1.4 contract into MCP:** they describe exactly
   what they will do and default to the safe variant. The confirmation is the
   human's — surfaced through Claude and/or the app UI — not something the tool

internal/mcp/mcp.go

@@ -21,6 +21,13 @@ type getRepoInput struct {
 	Path string `json:"path" jsonschema:"absolute filesystem path of the repository, exactly as returned by list_repos"`
 }
 
+// listReposOutput wraps the repository list. MCP structured output must be a JSON
+// object, so the SDK-inferred outputSchema has to be type "object" — returning a
+// bare slice yields type "array", which Claude Desktop rejects at tools/list.
+type listReposOutput struct {
+	Repos []repos.State `json:"repos" jsonschema:"the discovered repositories"`
+}
+
 // NewServer builds the MCP server and registers the (currently read-only) tools.
 func NewServer(svc *service.Service, version string) *mcpsdk.Server {
 	s := mcpsdk.NewServer(&mcpsdk.Implementation{
@@ -34,8 +41,8 @@ func NewServer(svc *service.Service, version string) *mcpsdk.Server {
 	mcpsdk.AddTool(s, &mcpsdk.Tool{
 		Name:        "list_repos",
 		Description: "List every Git repository GitManager has discovered, each with its current branch, dirty/clean state, ahead/behind counts, and remote names.",
-	}, func(_ context.Context, _ *mcpsdk.CallToolRequest, _ struct{}) (*mcpsdk.CallToolResult, []repos.State, error) {
-		return nil, svc.ListRepos(), nil
+	}, func(_ context.Context, _ *mcpsdk.CallToolRequest, _ struct{}) (*mcpsdk.CallToolResult, listReposOutput, error) {
+		return nil, listReposOutput{Repos: svc.ListRepos()}, nil
 	})
 
 	// get_repo — details for one already-discovered repository.

internal/mcp/mcp_test.go

@@ -65,8 +65,9 @@ func TestMCPRoundTrip(t *testing.T) {
 	if err != nil {
 		t.Fatalf("list_repos: %v", err)
 	}
-	var states []repos.State
-	decodeResult(t, res, &states)
+	var listOut listReposOutput
+	decodeResult(t, res, &listOut)
+	states := listOut.Repos
 	if len(states) != 1 {
 		t.Fatalf("expected 1 repo, got %d: %+v", len(states), states)
 	}