diff --git a/.changeset/fix-attempt-completion-error-message.md b/.changeset/fix-attempt-completion-error-message.md
new file mode 100644
index 00000000..fa9defc0
--- /dev/null
+++ b/.changeset/fix-attempt-completion-error-message.md
@@ -0,0 +1,6 @@
+---
+"@perstack/base": patch
+"perstack": patch
+---
+
+fix: return actionable error message when attemptCompletion is blocked by incomplete todos
diff --git a/apps/base/src/tools/attempt-completion.test.ts b/apps/base/src/tools/attempt-completion.test.ts
index 7eca2ae6..a6d42af7 100644
--- a/apps/base/src/tools/attempt-completion.test.ts
+++ b/apps/base/src/tools/attempt-completion.test.ts
@@ -7,28 +7,33 @@ describe("attemptCompletion tool", () => {
     await clearTodo()
   })
 
-  it("returns empty object when no todos exist", async () => {
+  it("returns success when no todos exist", async () => {
     const result = await attemptCompletion()
-    expect(result).toStrictEqual({})
+    expect(result).toStrictEqual({
+      content: [{ type: "text", text: JSON.stringify({}) }],
+    })
   })
 
-  it("returns empty object when all todos are completed", async () => {
+  it("returns success when all todos are completed", async () => {
     await todo({ newTodos: ["Task 1", "Task 2"] })
     await todo({ completedTodos: [0, 1] })
     const result = await attemptCompletion()
-    expect(result).toStrictEqual({})
+    expect(result).toStrictEqual({
+      content: [{ type: "text", text: JSON.stringify({}) }],
+    })
   })
 
-  it("returns remaining todos when incomplete todos exist", async () => {
+  it("returns error with instructions when incomplete todos exist", async () => {
     await todo({ newTodos: ["Task 1", "Task 2", "Task 3"] })
     await todo({ completedTodos: [1] })
     const result = await attemptCompletion()
-    expect(result).toStrictEqual({
-      remainingTodos: [
-        { id: 0, title: "Task 1", completed: false },
-        { id: 2, title: "Task 3", completed: false },
-      ],
-    })
+    expect(result.isError).toBe(true)
+    expect(result.content[0].text).toContain(
+      "Cannot complete: there are incomplete todos remaining.",
+    )
+    expect(result.content[0].text).toContain("[id=0] Task 1")
+    expect(result.content[0].text).toContain("[id=2] Task 3")
+    expect(result.content[0].text).toContain("todo tool")
   })
 
   it("registers tool with correct metadata", () => {
@@ -41,7 +46,7 @@ describe("attemptCompletion tool", () => {
     )
   })
 
-  it("returns correct MCP response format", async () => {
+  it("returns correct MCP response format when no todos", async () => {
     const mockServer = { registerTool: mock() }
     registerAttemptCompletion(mockServer as never)
     const handler = mockServer.registerTool.mock.calls[0][2]
diff --git a/apps/base/src/tools/attempt-completion.ts b/apps/base/src/tools/attempt-completion.ts
index dc693b41..38791388 100644
--- a/apps/base/src/tools/attempt-completion.ts
+++ b/apps/base/src/tools/attempt-completion.ts
@@ -1,4 +1,5 @@
 import type { McpServer } from "@modelcontextprotocol/sdk/server/mcp.js"
+import type { CallToolResult } from "@modelcontextprotocol/sdk/types.js"
 import { z } from "zod"
 import { errorToolResult, successToolResult } from "../lib/tool-result.js"
 import { getRemainingTodos } from "./todo.js"
@@ -7,12 +8,30 @@ export type AttemptCompletionResult =
   | { remainingTodos: { id: number; title: string; completed: boolean }[] }
   | Record<string, never>
 
-export async function attemptCompletion(): Promise<AttemptCompletionResult> {
+export async function attemptCompletion(): Promise<CallToolResult> {
   const remainingTodos = getRemainingTodos()
   if (remainingTodos.length > 0) {
-    return { remainingTodos }
+    const todoList = remainingTodos.map((t) => `  - [id=${t.id}] ${t.title}`).join("\n")
+    return {
+      isError: true,
+      content: [
+        {
+          type: "text",
+          text: [
+            "Cannot complete: there are incomplete todos remaining.",
+            "",
+            "Remaining todos:",
+            todoList,
+            "",
+            "You must either:",
+            "1. Complete the remaining work, then mark each todo as done using the todo tool (e.g., todo({ completedTodos: [id1, id2] }))",
+            "2. If the work for a todo is already done, mark it as completed using the todo tool before calling attemptCompletion again",
+          ].join("\n"),
+        },
+      ],
+    }
   }
-  return {}
+  return successToolResult({})
 }
 
 export function registerAttemptCompletion(server: McpServer) {
@@ -26,7 +45,7 @@ export function registerAttemptCompletion(server: McpServer) {
     },
     async () => {
       try {
-        return successToolResult(await attemptCompletion())
+        return await attemptCompletion()
       } catch (e) {
         if (e instanceof Error) return errorToolResult(e)
         throw e