fix(api): reject accepting a BLOCKED task directly

The nested-subtask-completion fix (ad00e18) necessarily added
BLOCKED -> COMPLETED to task.ValidTransition so
executor.Pool.maybeUnblockParent can complete a subtask whose own
children all finished. That widened acceptTask's gate (shared by
POST /api/tasks/{id}/accept and the chatbot MCP accept_task tool,
which validates only via task.ValidTransition) to also permit
accepting ANY BLOCKED task directly -- one still awaiting ask_user,
or with genuinely incomplete subtasks -- bypassing the completion
invariant this session's fix was built to protect. Caught by local
subagent review of that fix. Not live in production: the deployed
claudomator service is still on an earlier commit.

1 files changed, 13 insertions, 1 deletions

diff --git a/internal/api/taskops.go b/internal/api/taskops.go
index 2b133ac..a2668f3 100644
--- a/internal/api/taskops.go
+++ b/internal/api/taskops.go
@@ -36,7 +36,9 @@ type badRequestError struct{ msg string }
 
 func (e *badRequestError) Error() string { return e.msg }
 
-func badRequestf(format string, a ...any) error { return &badRequestError{msg: fmt.Sprintf(format, a...)} }
+func badRequestf(format string, a ...any) error {
+	return &badRequestError{msg: fmt.Sprintf(format, a...)}
+}
 
 // submitTaskSpec describes a task a chatbot wants created and immediately run.
 type submitTaskSpec struct {
@@ -127,6 +129,16 @@ func (s *Server) acceptTask(ctx context.Context, id string, actor event.Actor) e
 	if err != nil {
 		return errTaskNotFound
 	}
+	// BLOCKED -> COMPLETED is a valid task.ValidTransition (added so
+	// internal/executor.Pool's maybeUnblockParent can complete a subtask
+	// whose own children all finished), but that transition must only ever
+	// happen via that internal mechanism -- never via a direct accept call,
+	// which would bypass the subtask-completion invariant (or silently
+	// foreclose an open ask_user question) for a task that isn't actually
+	// done yet.
+	if t.State == task.StateBlocked {
+		return conflictf("task cannot be accepted while BLOCKED (awaiting subtasks or a question)")
+	}
 	if !task.ValidTransition(t.State, task.StateCompleted) {
 		return conflictf("task cannot be accepted from state %s", t.State)
 	}