Generate or edit Python code in a cell with AI could incorrectly produce Markdown code blocks

[harry-xm]

Describe the bug

and when executed it throws an error:

I'm using an OpenAI-compatible provider with glm-4.6 model and have no "custom rules" set.

Will you submit a PR?

• Yes

Environment

{
  "marimo": "0.18.4",
  "editable": false,
  "location": "/Users/<redacted>/.venv/lib/python3.12/site-packages/marimo",
  "OS": "Darwin",
  "OS Version": "25.2.0",
  "Processor": "arm",
  "Python Version": "3.12.11",
  "Locale": "en_US",
  "Binaries": {
    "Browser": "143.0.7499.42",
    "Node": "v24.11.1"
  },
  "Dependencies": {
    "click": "8.3.1",
    "docutils": "0.22.3",
    "itsdangerous": "2.2.0",
    "jedi": "0.19.2",
    "markdown": "3.10",
    "narwhals": "2.13.0",
    "packaging": "25.0",
    "psutil": "7.1.3",
    "pygments": "2.19.2",
    "pymdown-extensions": "10.18",
    "pyyaml": "6.0.3",
    "starlette": "0.50.0",
    "tomlkit": "0.13.3",
    "typing-extensions": "4.15.0",
    "uvicorn": "0.38.0",
    "websockets": "15.0.1"
  },
  "Optional Dependencies": {
    "altair": "6.0.0",
    "loro": "1.10.3",
    "openai": "2.9.0",
    "pandas": "2.3.3",
    "polars": "1.35.2",
    "pyarrow": "22.0.0"
  },
  "Experimental Flags": {}
}

Code to reproduce

Count number of records in df