legion-guard — verify before connect

An agent reads an MCP server's tool descriptions as instructions. The server you approved last week can expose other tools, or the same tools with other descriptions, today. legion-guard checks the tools/list your client actually receives, at connect time.

import legion_guard
session = legion_guard.wrap(session, "com.dexpaprika/dexpaprika")   # after session.initialize(); one line, no config file
tools = await session.list_tools()
print(session.report["verdict"])     # NO_CHANGE_DETECTED | CHANGE_DETECTED | UNVERIFIED

Try it in about a minute (including the virtualenv + install)

python3 -m venv v && . v/bin/activate && pip install legion-guard
python -m legion_guard check https://mcp.dexpaprika.com/streamable-http com.dexpaprika/dexpaprika

It connects to the public DexPaprika MCP server (initialize + tools/list, no tool call), fingerprints the 20 tools it receives and checks them against the latest list signed by the LEGION log. Look for "state": "MATCHES_SIGNED" (or DIFFERS_FROM_SIGNED, UNKNOWN, UNVERIFIED); exit code 0 = no difference, 1 = difference, 2 = connection error.

Low risk to add (tested)

• Observes by default. The default mode is warn: changes are logged (logging.getLogger("legion_guard")) and reported in session.report; no tool is hidden and no call is refused.
• Never breaks your app. If a source is unreachable or fails, it is reported UNAVAILABLE and nothing is blocked, in every mode. Internal errors of the guard are logged, never raised.
• Blocking is opt-in. wrap(session, server_id, mode="block") hides from list_tools() and refuses in call_tool() only the tools for which a source has evidence of a change (changed or new tool). UNKNOWN and UNAVAILABLE never block.

Removable: LEGION is the default, not a lock-in

Sources are pluggable. Each one compares the received list with its own reference:

session = legion_guard.wrap(session, "io.example/server", sources=[legion_guard.PinnedSource(".mcp-tools.lock.json")])  # no LEGION at all

Measured on 2026-10-17: the 20-tool list served by mcp.dexpaprika.com/streamable-http matches the list LEGION signed (MATCHES), also when the head must be a checkpoint cosigned by a chosen witness. There is no independent witness of the LEGION log yet: the only cosigning witness today is run by LEGION itself.

Limits

• LEGION observes only part of the servers; absence from the log is not a verdict (UNKNOWN).
• A change is not an attack. This does not certify that a server is secure, and says nothing about what the tools do.
• First-use pinning trusts the first list it sees.
• With witnesses=, the result is UNAVAILABLE (never blocking) until your witnesses have cosigned the log's current checkpoint. The only witness today is run by LEGION and cosigns hourly, so after each new head there is a window of up to about one hour where this option cannot confirm anything (measured on 2026-10-18).
• Fingerprints cover name, description and inputSchema (JCS RFC 8785), as in the LEGION log.
• Tested with the MCP Python SDK ClientSession; frameworks that build on it are not tested yet.

The 0.1 functions (guard_session, evaluate, approve, python3 -m legion_guard check|approve) are unchanged.