sigma-verify

An MCP server that lets Claude Code agents check their findings against a model other than Claude. It wraps OpenAI, Google Gemini, and models served by Ollama (local and cloud) behind a small set of tools, and tags every result with the provider and model that produced it.

It is built on hateoas-agent and is used by the sigma Claude Code plugin (sigma-review, sigma-build), but it works with any MCP client.

Tools

The server starts with one gateway tool. Calling it unlocks the rest.

Provenance tags

Every result names its source, so it's clear which model said what:

• Each result carries provenance_tag: "|source:external-<provider>-<model>|".

• Agents record a successful check as XVERIFY[<provider>:<model>], for example XVERIFY[gpt-oss:gpt-oss:120b-cloud]: agree(high).

• A failed call returns an XVERIFY-FAIL[<provider>:<model>] string in its sigma field with an error class (auth-error, rate-limit, timeout, network-error, and others). That marks a verification gap. It does not mean the model disagreed.

• cross_verify reports coverage: "partial" whenever any provider fails, so a partial run isn't mistaken for full cross-model agreement.

Related MCP server: Zen MCP Server

Providers

Ollama's free and paid tiers can change. Check ollama.com for current terms.

Local models run on your machine at no API cost. Cloud models run on ollama.com through your local Ollama install. init checks that Ollama is reachable and that local models have been pulled. It can't cheaply check whether you're signed in, so if you aren't, a cloud provider will show as available and then fail its first call with an auth-error.

Any model can be overridden with <PREFIX>_MODEL. The endpoint can be overridden with <PREFIX>_BASE_URL. llama, gemma, gpt-oss, and nemotron can also be routed through OpenRouter with <PREFIX>_PROVIDER=openrouter and OPENROUTER_API_KEY. .env.example lists every variable.

Anthropic is excluded by default

The callers are Claude agents, and Claude checking Claude is not cross-model verification. So anthropic is left out of every default selection: the cross_verify default set, the provider verify_finding and challenge pick when none is named, and the providers init lists as available. It is used only when:

• the caller names it (provider="anthropic", or providers="anthropic,..."), or

• SIGMA_VERIFY_ALLOW_ANTHROPIC=1 is set.

Install

Requires Python 3.11+ and uv.

claude mcp add sigma-verify --scope user -- \
  uvx --from git+https://github.com/coloradored13/sigma-verify sigma-verify

To add API-key providers, pass keys with -e:

claude mcp add sigma-verify --scope user \
  -e OPENAI_API_KEY=sk-... \
  -e GOOGLE_AI_API_KEY=... \
  -- uvx --from git+https://github.com/coloradored13/sigma-verify sigma-verify

Then, in Claude Code, ask an agent to call init, or run /mcp to confirm that the server is connected.

Ollama setup (optional)

Ollama-backed providers need no API key.

ollama serve                 # or start the Ollama app
ollama pull llama3.1:8b      # any local model from the table
ollama signin                # only for cloud models

sigma-verify doesn't start Ollama for you by default. If Ollama isn't running, its providers are reported as unavailable. To have init (and server startup) run ollama serve when Ollama is installed but not running, set SIGMA_VERIFY_AUTOSTART_OLLAMA=1, for example with -e SIGMA_VERIFY_AUTOSTART_OLLAMA=1 on claude mcp add.

With no providers

You don't need any provider configured. With none available, init reports no_providers and lists what each provider needs. sigma-review and sigma-build still run, and findings just stay untagged. An untagged finding is neutral: it hasn't been checked against another model, and nothing counts against it.

Development

uv venv .venv
uv pip install -p .venv -e '.[dev]'
.venv/bin/python -m pytest tests/ -q
.venv/bin/ruff check src tests

The test suite runs offline. A shared fixture fakes the Ollama probe and blocks outbound TCP connections.

License

Apache 2.0. See LICENSE and NOTICE.