orchestrator-mcp-server 0.1.0

Capability-routed MCP server: route work to the LLM that is best at it.

orchestrator-mcp

An MCP server that routes each request to the model configured for that kind of work — research to one model, coding to another — and returns every answer through a fixed, validated envelope.

Pointing a capability at your own deployment is a YAML edit. There is no code to change.

Published to PyPI as orchestrator-mcp-server — the shorter name is an empty registered project owned by someone else. The import package is orchestrator_mcp.

How it works

A capability is a LiteLLM model_name alias group. Several deployments can share one name, and litellm.Router already load-balances, retries, cools down, and falls back across them. So the routing engine is the config file:

model_list:
  - model_name: coding                  # capability, not a model
    litellm_params:
      model: anthropic/claude-sonnet-4-5
      api_key: os.environ/ANTHROPIC_API_KEY

  - model_name: coding                  # same capability, your own box
    litellm_params:
      model: openai/qwen-coder
      api_base: http://vllm.internal:8000/v1
      api_key: os.environ/LOCAL_VLLM_KEY

The calling agent states which capability it wants. There is no intent classifier — the caller is already a language model and knows whether it is asking a coding question; paying a second model to guess what the first one already knows buys a cost increase and a new failure mode.

Quick start

Write a config.yaml — start from config.example.yaml — and check that it loads:

ORCHESTRATOR_CONFIG=config.yaml uvx --from orchestrator-mcp-server python -c "from orchestrator_mcp.server import build_server; build_server(); print('config ok')"

A bad config fails here rather than at request time: every deployment must route to a declared capability, every capability must have a deployment behind it, and every fallback must name a real capability.

Because the file is LiteLLM's own config schema, litellm --config config.yaml runs on it unchanged. Keep it out of version control — it holds your endpoints.

Claude Code

claude mcp add orchestrator --env ORCHESTRATOR_CONFIG=$PWD/config.yaml -- uvx orchestrator-mcp-server

Codex

In ~/.codex/config.toml:

[mcp_servers.orchestrator]
command = "uvx"
args = ["orchestrator-mcp-server"]
env = { ORCHESTRATOR_CONFIG = "/absolute/path/to/config.yaml" }

Both speak stdio, and every tool result is returned as structured content and as JSON text, so a client that reads only one of the two still gets the whole envelope.

Provider keys. config.yaml references them as os.environ/NAME, and they are read from the environment the server process gets — which is the client's environment, not your shell's. If a capability comes back auth_failed while the same config works from a terminal, add the key to the client's env block.

From a checkout

uv sync && uv run pytest -q

claude mcp add orchestrator --env ORCHESTRATOR_CONFIG=$PWD/config.yaml -- uv run --directory $PWD orchestrator-mcp-server

Tools

ask

Argument	Notes
capability	Enum, built from your config. Bad values are rejected by the protocol layer.
prompt	Required. Capped by limits.max_prompt_chars.
context	Source material. When set, the model is told to answer only from it and to abstain otherwise.
system	Extra instructions. Applied before the server's own directives, so it cannot disable them. Capped by limits.max_system_chars.
response_schema	JSON Schema ("type": "object"). Switches on structured mode. Capped by limits.max_schema_chars — it is inlined into the prompt verbatim.
temperature	Pinned to 0 whenever response_schema is set.
max_output_tokens	Capped by limits.max_output_tokens.

Every call returns the same envelope:

{
  "ok": true,
  "content": "…",
  "data": null,
  "insufficient_context": false,
  "capability_requested": "coding",
  "model_used": "anthropic/claude-sonnet-4-5",
  "fallback_used": false,
  "finish_reason": "stop",
  "usage": { "prompt_tokens": 10, "completion_tokens": 20, "total_tokens": 30, "cost_usd": 0.0002 },
  "latency_ms": 412,
  "error": null
}

content holds the prose answer and is null in structured mode and on failure; data holds the validated object and is set only in structured mode; error is { code, message } whenever ok is false.

error.code comes from a closed set — invalid_request, no_deployment, upstream_error, rate_limited, context_exceeded, schema_validation_failed, timeout, content_filtered, auth_failed, output_truncated — so callers branch on a value instead of matching substrings. error.message is bounded at 500 characters and never quotes the rejected output back at you.

list_capabilities

What each capability is for, the deployments behind it, and where it falls back.

Guardrails, and their limits

This server sees a prompt and a completion. It has no ground truth, so it cannot verify factual claims, and nothing here should be read as a hallucination detector. What it does enforce:

• Shape is validated, not assumed. Structured replies are checked against your schema locally with jsonschema, regardless of whether the provider claims to enforce response_format. A violation is a failure, not a payload.
• Bounded repair. An invalid structured reply gets limits.schema_repair_attempts retries carrying the validator's complaint, then fails as schema_validation_failed. Never a best-effort half-parsed object.
• An unfinished answer is a failure, not a short answer. A completion cut off by the token limit comes back as output_truncated with content: null, and one the provider filtered as content_filtered. Neither is returned as prose, because a half answer reads exactly like a whole one.
• The error tells you what broke, not what the model wrote. error.message gives the failing path and constraint (schema violation at answer/city: failed the 'maxLength' constraint) and is capped at 500 characters. The rejected value itself goes only back to the model that produced it, in the repair turn.
• request_timeout_s bounds the call. Retries, cross-capability fallback, and repair turns all spend from one budget, so 120 cannot become 360.
• Abstention is typed. With context set, the model is given an explicit way to say the material does not support an answer; it arrives as insufficient_context, not as prose you have to pattern-match.
• The server never ghostwrites. When ok is false, content and data are both null. It will not put a "Sorry, I couldn't…" string where a model's answer goes, because callers cannot tell those apart. Enforced by an assertion on every response and covered by tests.
• Degradation is visible. fallback_used and model_used always ride along, so an answer served by the backup after the primary died never passes as the intended one.
• The caller cannot smuggle a model. There is no free-form model parameter, only the capability enum. Routing stays operator-controlled.
• Boundaries reject early. Unknown capability, oversized prompt or system, empty prompt, and a malformed or oversized response_schema all fail before a provider is called. A nonsensical limits: block fails at startup instead.

Two known gaps. The MCP SDK drops unknown arguments before the handler sees them, so an unrecognized key is ignored at the protocol layer rather than rejected — direct calls into Orchestrator.ask do reject it. And a response_schema containing a pathological pattern can burn CPU on the event loop during validation: the schema is size-capped but not analyzed, so treat schema authorship as a trusted operation.

Tests

uv run pytest -q

72 tests, no network — deployments are stubbed with LiteLLM's mock_response, and the shapes it cannot express (no choices, null content, a truncated or filtered reply) are stubbed as raw ModelResponse objects. Includes the rate-limit-then-fallback path and the cooled-down-group path.

Because all of that is stubbed, it proves the orchestrator's logic and nothing about your providers. For that:

uv run python smoke_live.py

Real calls against your config.yaml, roughly four short ones per capability, so it costs a little money — run it deliberately, not in CI. It checks the things only a live endpoint can answer: whether response_format survives the round trip, whether the model honours the abstention path instead of inventing, and what the provider actually sends as finish_reason when it runs out of room. Name capabilities as arguments to check only some (uv run python smoke_live.py fast).

Contributing

Issues and pull requests are welcome. The bar for a change is a test that fails without it — the suite runs offline, so there is no key to obtain and no cost to pay. Keep config.yaml out of your commits.

If you are adding a capability to your own setup, you do not need a pull request: it is a model_list entry.

Not included

Semantic/embedding routing and RouteLLM-style predictive routing (the caller states its capability); Redis-backed distributed cooldown state (single process — LiteLLM enables it via config when you need a second node); streaming (MCP tool results return whole); sampling/createMessage loops; PII redaction and telemetry callbacks (available as LiteLLM callbacks when a requirement names one).