tool-mcp-bridge

Call locally configured MCP servers over stdio. Set MCP_SERVERS JSON. Does not use xAI mcp_call.

Client-side tool for mixing with xAI server-side tools:

https://docs.x.ai/developers/tools/advanced-usage#mixing-server-side-and-client-side-tools

Local first

Tasks that do not need a third-party network service run entirely in this process. Bind address is 127.0.0.1 only.

Related MCP server: agent-hub

Run

python -m venv .venv
.venv/bin/pip install -e ".[dev]"
.venv/bin/pytest -q
TOOL_TOKEN=local TOOL_PORT=8096 .venv/bin/python -m tool_mcp_bridge.server

docker compose up -d --build
curl -sS http://127.0.0.1:8096/healthz
curl -sS http://127.0.0.1:8096/schema
curl -sS -H 'Authorization: Bearer local' -H 'Content-Type: application/json' \
  -d '{"arguments":{}}' http://127.0.0.1:8096/v1/invoke

xAI client-side schema

GET /schema returns the function-calling descriptor. Register it next to any server-side tools (web_search, x_search, code_execution, …) you still want xAI to execute. When the model calls this function, execution pauses and your loop must POST /v1/invoke.

Registry

MCP_SERVERS is a JSON list. command is the argv, not a shell string. env is optional. When it is omitted the child does not inherit this process environment. When it is set, only that object is added on top of the SDK allow-list: PATH, HOME, LOGNAME, SHELL, TERM, USER.

TOOL_TOKEN stops at the HTTP bearer check. It is not forwarded, and neither is MCP_SERVERS. Put a variable the child needs into env.

[
  {
    "label": "files",
    "transport": "stdio",
    "command": ["python", "-m", "demo"],
    "env": {"DEMO_ROOT": "/data"}
  }
]

A list (list_only true, or no tool_name) returns servers: label, transport, protocolVersion, and tools of name, description, inputSchema. command and env are not in that body. An invoke returns the MCP result (resultType, content, isError, and structuredContent on a finished call). There is no stderr and no returncode. isError true is still a finished call. input_required is returned and not completed.

Security

• no-new-privileges, cap_drop: ALL, read-only root, tmpfs /tmp

• published on loopback only

• bearer TOOL_TOKEN (default local — change before any non-loopback bind)

Copyright 2026 Evgeniy Chistyakov · https://exception.expert

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