deus-witness

MCP proxy and SDK for Deus Witness: every tool call of your agent is checked by an independent witness before it runs, and you get a signed receipt for it.

One line in your MCP client config

{"mcpServers": {"fs-witnessed": {"command": "deus-witness-mcp",
  "args": ["--", "npx", "-y", "@modelcontextprotocol/server-filesystem", "/tmp/sandbox"],
  "env": {"DEUS_WITNESS_KEY": "dwk_...", "DEUS_WITNESS_URL": "https://witness.sipkigroup.com"}}}}

Install: pipx install https://witness.sipkigroup.com/dl/deus_witness-0.2.16-py3-none-any.whl

Behaviour per tools/call: pass -> forwarded; allow -> receipt verified (witness key pinned at first use in ~/.config/deus-witness/witness.json), then present, then the tool runs, then report; deny -> the tool is NOT called, the agent gets Blocked by Deus Witness: <reason> (receipt <rid>); defer -> waits up to ~50 s for the owner, then Held for owner approval at <url>. Retry the same call after approval. If the witness is unreachable the call is blocked (fail closed). tools/list, resources, prompts and notifications pass through untouched. The proxy log (stderr) holds only tool, rid, verdict.

0.2: status: held (held by the advisor for owner review) is waited for like defer, then the agent gets Held by advisor for owner review at <url>. Retry the same call after review.

At start (after the client's initialized) the proxy asks the upstream for tools/list itself (the answer is not shown to the client), sends it as POST /v1/catalog (the server suggests a strict toolmap from it), runs the light doctor (configs only, no network) and sends POST /v1/coverage; once per run, before the first call is judged. If the witness cannot be reached at that moment, every call of the run is blocked (fail closed). An HTTP error there (e.g. a 0.1 server) is only logged.

Env: DEUS_WITNESS_KEY (required), DEUS_WITNESS_URL, DEUS_WITNESS_WAIT (s per long poll, 25), DEUS_WITNESS_POLLS (2), DEUS_WITNESS_VERIFY=0 or --no-verify-aar (testing against a stub that does not sign).

Doctor: where can the agent go around the witness

deus-witness doctor [--json] [--no-network] [--project DIR]

Reads (never writes) the client configs: Claude Desktop (claude_desktop_config.json), Claude Code (~/.claude.json, ~/.claude/settings.json, project .mcp.json, .claude/settings.json, .claude/settings.local.json), Cursor (~/.cursor/mcp.json). Prints coverage: C0|C1|C2|C3 and one bypass: <what> -> <how to close> per hole; rc 0 for C2/C3, 2 for C0/C1.

• C0: no MCP server is behind deus-witness-mcp (a server is behind it when its command is deus-witness-mcp).
• C1: proxy in place, but a server is not behind it, or Claude Code's Bash/Write/Edit/WebFetch are not in permissions.deny, or a pass tool has a network effect (known to the proxy at start from the agent toolmap), or Cursor's built-in agent tools.
• C2: all MCP servers behind the proxy and those built-ins denied. C3: C2 + no /bin/sh + egress to 1.1.1.1:443 and example.com:443 refused (only probed without --no-network; the sandbox recipe gives it).

The level is a claim of the operator made from local files, not a proof; the output says so. --no-network opens no socket.

Claude Code hook (built-in tools)

deus-witness-hook is a Claude Code PreToolUse hook: Bash, Edit, Write, NotebookEdit, Read, WebFetch and mcp__* calls are judged by the witness before they run. Answers allow / deny / ask; fail closed. 0.2.16: a thin client - it sends the call as Claude Code gave it (the command line, paths, cwd, CLAUDE_PROJECT_DIR), the realpath / existence of the paths the witness asks about and SHA-256 fingerprints of project files from a list the witness signs; the witness classifies and decides, the hook verifies the signed receipts. What leaves the machine: https://witness.sipkigroup.com/docs/coverage.html and the privacy notice on https://witness.sipkigroup.com. Setup: https://witness.sipkigroup.com/docs/quickstart.html.

SDK (no MCP)

from deus_witness import Witness
w = Witness("https://witness.sipkigroup.com", "dwk_...")
r = w.act("send_email", {"to": "bob@acme.example", "subject": "hi", "body": "..."})
if r.verdict == "allow" and w.present(r.rid)["result"] == "accept":
    ...                      # do the thing
    w.report(r.rid, True)

0.2: w.catalog(tools) sends a tools/list ([{name, description, inputSchema}]) as POST /v1/catalog; w.coverage() runs the light doctor here and sends POST /v1/coverage (or pass your own report: w.coverage({"level": "C2", "unwitnessed": []})). See examples/agent_loop.py. verify_aar(aar_bytes, witness_pub, expect_request) is a standalone receipt check.

Demo tools

deus-witness-demo-tools is a stdio MCP server with send_email, purchase, delete_resource, publish, read_file; nothing real happens, each call appends a line to $DEUS_DEMO_OUT/effects.jsonl.

Tests

.venv/bin/python -m pytest -q tests (starts the contract stub itself). Against a real server: DW_TEST_URL=http://localhost:8810 DW_TEST_KEY=dwk_... pytest tests.