Prism MCP for Python

Consuming Model Context Protocol servers as Prism tools, across an explicit trust boundary. The Python port of particle-academy/prism-mcp.

Zero runtime dependencies. Python 3.10+.

pip install prism-ai-mcp

from prism_mcp import Client, TrustPolicy

docs = Client(
    "docs",
    my_transport,
    trust=TrustPolicy.allowing(["search"], pins={"search": "sha256:3f1c…"}),
)


async def search_docs() -> None:
    tools = await docs.list_tools()
    result = await docs.call_tool(tools[0], {"query": "rate limits"})

    print(result.text)

my_transport is a callable that takes a TransportRequest (method, params, headers) and returns an awaitable yielding the decoded server result. An async def transport satisfies that contract. The package opens no connections itself.

Breaking change in 0.2.0: list_tools() and call_tool() must be awaited, and the transport must return an awaitable. Run search_docs() in your application's event loop. A synchronous embedder must manage that loop itself; the package provides one async client path.

Trust

A tool list is not data the model summarises; it is instructions the model follows. So nothing is offered until you say what you trust.

• TrustPolicy.undeclared() is the default. list_tools() raises server_not_trusted instead of returning the server's tools.
• TrustPolicy.allowing([...]) offers only the named tools. allowing([]) is a declaration too, and offers nothing.
• TrustPolicy.allowing_every_tool() offers every tool, including ones the server adds later.
• Pins. Pass pins={name: digest}. ToolDefinition.digest() covers the tool's name, title, description and input schema, so a server that rewrites a description after you trusted it is refused with tool_definition_changed.

A gate callable (server, tool, arguments) -> bool is asked before every call; returning False raises tool_denied. The default allows every call that trust admitted, and deny_all refuses them all.

Results

Every result passes through a ResultGuard before it reaches you:

• A result over max_bytes (65,536 by default) raises result_too_large instead of being truncated.
• The text is wrapped in an <untrusted-tool-output> tag with a random id per result, so the server's output cannot close the wrapper.
• A filter callable (server, tool, text) -> text runs before framing, if you pass one.

The wrapper makes a prompt injection harder. It does not make one impossible, and the guard does not scan the text for injection strings.

Mirrored parameters

A tool's schema can mark an argument with x-mcp-header, which copies the model's value into an Mcp-Param-* request header. A tool whose annotations break the rules (an invalid header name, two arguments mirrored to the same header) is left out of list_tools() rather than repaired.

Protocol

The client speaks MCP 2026-07-28, the stateless revision. Earlier revisions open with an initialize handshake and a session, which this client does not implement. is_stateless_protocol(version) tells the two apart.

Errors

Every failure is an McpError with a stable code: server_not_trusted, tool_definition_changed, tool_denied, result_too_large, mirrored_parameter_refused, unsupported_protocol_version, protocol_failure, tool_call_failed, server_not_configured.

Parity

prism-parity's mcp-tool-digest corpus pins tool digests against the PHP reference and the TypeScript port, so a pin computed in one language holds in the others.

One case is not in that corpus: a schema containing an integral float such as 1.0 digests differently here than in PHP and TypeScript. For such a tool, compute the pin in Python.

License

MIT. See LICENSE.