Brawler MCP Server (Python)

FastMCP wrapper that tags tools with agent tasks so a client can list and run context, completion, evaluation, post, and none tools from one MCP server.

This is not a fork of FastMCP. It depends on FastMCP 4 and subclasses FastMCP. Later language ports (brawler-mcp-server-typescript, and so on) should implement the same wire contract.

Brawler agents attach one MCP server as a resource, then fetch tools by task: context tools build the prompt, completion tools are offered to the model, post and eval tools run after a result. Native Brawler skills stay on the agent; MCP does not offer a skill task.

What this adds on top of FastMCP

Untagged tools from a plain FastMCP server are treated as completion by Brawler. Tools from this package always carry an explicit task. FastMCP tags that are not agent tasks (for example skill, router, training) are ignored when resolving the task; an unknown mcp_server.task value is treated as none.

Related MCP server: Enterprise MCP Gateway and Tool Registry

Install

uv add brawler-mcp-server-python
# or
pip install brawler-mcp-server-python

Requires Python 3.10+ and FastMCP 4.1.3+.

Quick start

from brawler_mcp_server import BrawlerMCP

mcp = BrawlerMCP('Docs')


@mcp.context_tool
def policies() -> str:
    """Injected into the agent system prompt."""
    return 'Always cite sources.'


@mcp.context_tool(as_user_message=True)
def ticket() -> str:
    return 'Ticket #123 is still open.'


@mcp.tool
def search(q: str) -> str:
    """Completion tool offered to the model."""
    return f'Results for {q}'


@mcp.eval_tool
def grounded(answer: str) -> tuple[bool, float, str | None]:
    if not answer.strip():
        return False, 0.0, 'Answer is empty.'
    return True, 1.0, None


@mcp.post_tool
def audit(answer: str) -> None:
    return None


@mcp.none_tool
def ping() -> str:
    """Visible to generic MCP clients; Brawler agents ignore none tools."""
    return 'pong'


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

mcp.run() speaks stdio by default. Use FastMCP HTTP transports when you need a remote endpoint.

Example server shipped in examples/:

uv run brawler-mcp-example

Wire contract

MCP tools/list has no task filter. Identity lives on each tool so any language client can list a subset after listing.

{
  "name": "policies",
  "description": "Injected into the agent system prompt.",
  "_meta": {
    "mcp_server": {
      "task": "context",
      "as_user_message": false
    },
    "fastmcp": {
      "tags": ["context"]
    }
  }
}

_meta.mcp_server

FastMCP also copies the task into _meta.fastmcp.tags. Clients may use either field. If both are missing, treat the tool as completion. If task is set to an unsupported value, treat the tool as none.

Listing only context tools

from fastmcp import Client
from brawler_mcp_server import tools_for_task

async with Client(mcp) as client:
    tools = await client.list_tools()
    context_tools = tools_for_task(tools, 'context')

In-process (no MCP round-trip):

context_tools = await mcp.tools_by_task('context')

Return types

Decorators check the annotation at registration time.

Agent tasks (this package)

Router, training, and skill are not included. Router and training are not function-tool tasks on a Brawler agent. Skills are native Brawler tools, not MCP tasks.

Using with Brawler

Register the server as an MCP resource on an agent the same way as any other MCP connection (MCPClient.from_server / agent.mcp(...) / POST /mcps). The persisted row stays entity_type=mcp with task=completion (the connection). Remote tools carry the task in _meta.mcp_server.

At run time the agent lists tools, keeps completion tools for the model, and calls context / post / eval tools at the matching pipeline stage.

Development

uv sync --all-extras --group lint --group dev
uv run pytest tests/
make test
make typecheck

If this repo is checked out as a submodule of brawler, uv run still uses this package's own environment. Do not run these tests with brawler's FastMCP 3 venv.

CI (.github/workflows/testing.yaml) runs lint, pyright, and pytest on pull requests to main. The release workflow (.github/workflows/release.yaml) runs the same tests, builds the package, creates a GitHub release from changelogs/v<tag>.md, and publishes to PyPI when a version tag is pushed.

Releasing

1. Bump version in pyproject.toml.

2. Add changelogs/v<tag>.md and update changelogs/latest.md to match (tag includes the v prefix, e.g. v0.1.0).

3. Merge to main, then tag and push: git tag v0.1.0 && git push origin v0.1.0.

Before the first PyPI publish, register a trusted publisher on pypi.org for project brawler-mcp-server-python: owner DanielFloresDiaz, repository brawler-mcp-server-python, workflow name release.yaml, environment blank. No API token secret is required.

After merging workflow changes, smoke-test locally with uv build and inspect dist/, then confirm the tag workflow uploads GitHub release assets and the new version appears on PyPI.

License

Apache-2.0