[BUG] Metadata like maxResultSizeChars is stripped from tool spec

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

Hi 👋

I have an example MCP with FastMCP that does this for demonstrational purposes:

@mcp.tool(meta={"anthropic/maxResultSizeChars": 100_000})
def get_large_text(length: int = DEFAULT_LENGTH) -> str:
     ...

It should return a large text and by default something over 50k characters.

anthropic/maxResultSizeChars. Seems to be ignored however. For Claude Code, this isn't a problem really because it still handles it, but Cowork does throw an error and saves it to a file to work with it, spawns incredibly slow agents to parse things again in chunks etc.

Error: result (50,013 characters) exceeds maximum allowed tokens. Output has been saved to ...

Looking at the tool JSON Claude Code and Cowork seems to have, they show me

{
  "name": "mcp__large-text-test__get_large_text",
  "description": "Return a string of the requested length (default 50,000 characters).",
  "parameters": {
    "type": "object",
    "additionalProperties": false,
    "properties": {
      "length": {
        "type": "integer",
        "description": "Number of characters to return.",
        "default": 50000
      }
    }
  }
}

This doesn't include any of the meta field anymore although the raw spec seems to include it

  async def main():
      async with Client('large_text_server.py') as client:
          result = await client.session.list_tools()
          print(json.dumps(result.model_dump(by_alias=True), indent=2))

outputs the following

{
  "_meta": null,
  "nextCursor": null,
  "tools": [
    {
      "name": "get_large_text",
      "title": null,
      "description": "Return a string of the requested length (default 50,000 characters).\n\nArgs:\n    length: Number of characters to return.",
      "inputSchema": {
        "additionalProperties": false,
        "properties": {
          "length": {
            "default": 50000,
            "type": "integer"
          }
        },
        "type": "object"
      },
      "outputSchema": {
        "properties": {
          "result": {
            "type": "string"
          }
        },
        "required": [
          "result"
        ],
        "type": "object",
        "x-fastmcp-wrap-result": true
      },
      "icons": null,
      "annotations": null,
      "_meta": {
        "anthropic/maxResultSizeChars": 100000,
        "fastmcp": {
          "tags": []
        }
      },
      "execution": null
    }
  ]
}

What Should Happen?

Documented metadata like anthropic/maxResultSizeChars should not be ignored when translating raw tool calls into the internal Claude format

Error Messages/Logs

Steps to Reproduce

1. Create file test_mcp.py

#!/usr/bin/env -S uv run --script
# /// script
# requires-python = ">=3.12"
# dependencies = [
#   "fastmcp>=3.4.4",
# ]
# ///

from fastmcp import FastMCP

mcp = FastMCP("Large Text Test Server")

DEFAULT_LENGTH = 50_000


@mcp.tool(meta={"anthropic/maxResultSizeChars": 100_000})
def get_large_text(length: int = DEFAULT_LENGTH) -> str:
    """
    Return a string of the requested length (default 50,000 characters).

    Args:
        length: Number of characters to return.
    """
    pattern = "0123456789abcdefghijklmnopqrstuvwxyz"
    repeats = (length // len(pattern)) + 1
    return (pattern * repeats)[:length]


if __name__ == "__main__":
    mcp.run()

1. Put this into the MCP config

    "large-text-test": {
      "command": "uv",
      "args": [
        "run",
        "/path/to/test_mcp.py"
      ]
    }

3. Start Cowork
4. Use the MCP
5. Notice how it fails with `Error: result (50,013 characters) exceeds maximum allowed tokens. Output has been saved to ...`

### Claude Model

_No response_

### Is this a regression?

I don't know

### Last Working Version

_No response_

### Claude Code Version

2.1.105

### Platform

Anthropic API

### Operating System

macOS

### Terminal/Shell

iTerm2

### Additional Information

_No response_