codex - 💡(How to fix) Fix Auto-unwrap FastMCP results [1 comments, 2 participants]

What variant of Codex are you using?

App, CLI

What feature would you like to see?

FastMCP servers will auto-wrap all tool responses in a {"result": ...} JSON structure in order to comply with the MCP spec when returning primitive values (see: https://fastmcp.wiki/en/clients/tools#primitive-type-unwrapping). FastMCP assumes that the MCP client will auto-unwrap these.

Codex does not currently unwrap FastMCP results. I tested both Gemini CLI and the VSCode GitHub Copilot extension, and they both seem to unwrap them.

I think this would be a nice feature to have. With the current behavior, any FastMCP primitive strings are kept JSON-encoded, meaning newlines are escaped. For example:

{"result": "example1\nexample2"}

instead of

example1
example2

I've noticed that with strings with lots of newlines, model performance seems to be slightly worse when in this escaped form rather than as a raw string.

In order to avoid tampering with non-FastMCP servers that happen to return JSON of the form {"result": ...}, FastMCP seems to set "x-fastmcp-wrap-result": true in the schema which can be used to detect FastMCP servers.

The only potential downside I can think of this feature is that it increases risk of prompt injection, so (maybe) this could be an opt-in feature in the config.toml.

Additional information

No response