From fa5e5b61768233c9376f34aa2692f76efa8550c0 Mon Sep 17 00:00:00 2001
From: breken <support@brekfuz.com>
Date: Fri, 9 Oct 2026 10:39:40 -0700
Subject: [PATCH] :bug: Log errors readably in MCP execute_code console output
 (#11930)

* :bug: Log errors readably in MCP execute_code console output

The console captured during execute_code rendered every object argument
with JSON.stringify. An Error has no enumerable fields, so logging a
caught error printed "{}" and the agent lost the message. A circular
object or a BigInt made JSON.stringify throw, so a console.log call
aborted the executed code with an unrelated serialization error.

Format console arguments in a small pure module: errors by their string
form (name and message), other objects as indented JSON, falling back to
the string form when JSON cannot serialize the value.

AI-assisted-by: claude-opus-5.5
Signed-off-by: breken-ai <312387581+breken-ai@users.noreply.github.com>

* :recycle: Keep console formatting within ExecuteCodeTaskConsole

---------

Signed-off-by: breken-ai <312387581+breken-ai@users.noreply.github.com>
Co-authored-by: breken-ai <312387581+breken-ai@users.noreply.github.com>
---
 .../plugin/src/ExecuteCodeTaskConsole.test.ts | 23 +++++++++++++++++
 .../task-handlers/ExecuteCodeTaskHandler.ts   | 25 ++++++++++++++++---
 2 files changed, 44 insertions(+), 4 deletions(-)
 create mode 100644 mcp/packages/plugin/src/ExecuteCodeTaskConsole.test.ts

diff --git a/mcp/packages/plugin/src/ExecuteCodeTaskConsole.test.ts b/mcp/packages/plugin/src/ExecuteCodeTaskConsole.test.ts
new file mode 100644
index 0000000000..40295aee4a
--- /dev/null
+++ b/mcp/packages/plugin/src/ExecuteCodeTaskConsole.test.ts
@@ -0,0 +1,23 @@
+import assert from "node:assert/strict";
+import test from "node:test";
+import { ExecuteCodeTaskConsole } from "./task-handlers/ExecuteCodeTaskHandler.ts";
+
+test("formats plain objects and primitives", () => {
+    const console = new ExecuteCodeTaskConsole();
+    console.log({ a: 1 }, null, "text", 42, undefined);
+    assert.equal(console.getLog(), '[LOG] {\n  "a": 1\n} null text 42 undefined\n');
+});
+
+test("keeps the name and message of a logged error", () => {
+    const console = new ExecuteCodeTaskConsole();
+    console.error("failed", new TypeError("shape is undefined"));
+    assert.equal(console.getLog(), "[ERROR] failed TypeError: shape is undefined\n");
+});
+
+test("does not throw for values JSON cannot serialize", () => {
+    const console = new ExecuteCodeTaskConsole();
+    const circular: { self?: unknown } = {};
+    circular.self = circular;
+    console.log(circular, { big: 1n });
+    assert.equal(console.getLog(), "[LOG] [object Object] [object Object]\n");
+});
diff --git a/mcp/packages/plugin/src/task-handlers/ExecuteCodeTaskHandler.ts b/mcp/packages/plugin/src/task-handlers/ExecuteCodeTaskHandler.ts
index 47004e5cfe..b2fff14f05 100644
--- a/mcp/packages/plugin/src/task-handlers/ExecuteCodeTaskHandler.ts
+++ b/mcp/packages/plugin/src/task-handlers/ExecuteCodeTaskHandler.ts
@@ -8,7 +8,7 @@ import { PenpotUtils } from "../PenpotUtils.ts";
  * Provides the same interface as the native console object but appends
  * all output to an internal log string that can be retrieved.
  */
-class ExecuteCodeTaskConsole {
+export class ExecuteCodeTaskConsole {
     /**
      * Accumulated log output from all console method calls.
      */
@@ -30,15 +30,32 @@ class ExecuteCodeTaskConsole {
         return this.logOutput;
     }
 
+    /**
+     * Formats one argument of a captured console call.
+     * Errors have no enumerable fields, and values such as circular objects
+     * cannot be serialized to JSON, so fall back to their string form.
+     */
+    private formatLogArgument(arg: unknown): string {
+        if (arg instanceof Error) {
+            return String(arg);
+        }
+        if (typeof arg === "object") {
+            try {
+                return JSON.stringify(arg, null, 2);
+            } catch {
+                return String(arg);
+            }
+        }
+        return String(arg);
+    }
+
     /**
      * Appends a formatted message to the log output.
      * @param level - Log level prefix (e.g., "LOG", "WARN", "ERROR")
      * @param args - Arguments to log, will be stringified and joined
      */
     private appendToLog(level: string, ...args: any[]): void {
-        const message = args
-            .map((arg) => (typeof arg === "object" ? JSON.stringify(arg, null, 2) : String(arg)))
-            .join(" ");
+        const message = args.map((arg) => this.formatLogArgument(arg)).join(" ");
         this.logOutput += `[${level}] ${message}\n`;
     }