fix: normalize MCP tool error formatting

TASKS.md

@@ -526,4 +526,43 @@ Register `review_plan`, `review_code`, `debug_issue`, and `architecture_review`
 
 Status: ✅ Complete
 
-### Task 5.4 - Normalize MCP tool error formatting (NEXT)
+### Task 5.4 - Normalize MCP tool error formatting ✅
+
+Ensure all MCP tool error messages have a single, consistent "Error:" prefix.
+
+Before this change, handlers returned errors prefixed with `"Error: "` and server.js prepended another `"Error: "`, producing duplicates like:
+
+```
+Error: Error: Configuration error: OPENAI_API_KEY is missing.
+```
+
+**Changes made (`src/server.js`):**
+
+Each of the 5 tool registration callbacks now normalizes error text:
+
+```js
+const text = result.ok
+  ? result.answer
+  : String(result.error || "Unknown error").startsWith("Error:")
+    ? result.error
+    : `Error: ${result.error}`;
+```
+
+After fix:
+
+- `"Error: Configuration error: OPENAI_API_KEY is missing."` — single prefix ✅
+- Non-"Error:"-prefixed errors receive one prefix added by the server ✅
+- Success responses unchanged ✅
+
+Smoke tests:
+
+| Test | Result |
+|------|--------|
+| npm test | ✅ 523 passed, no regressions |
+| tools/list | ✅ 5 tools, unchanged |
+| ask_chatgpt (no API key) | ✅ Single "Error:" prefix |
+| review_plan (no API key) | ✅ Single "Error:" prefix |
+
+Status: ✅ Complete
+
+### Task 5.5 - Claude Code MCP configuration and local end-to-end setup (NEXT)