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.

Install

pip install deus-witness

(or pipx install deus-witness). Then one entry in your MCP client config (the example of the quickstart and of the dashboard; the bundled demo tools, nothing real happens; after -- put the command of the MCP server you want witnessed):

{"mcpServers": {"witnessed": {"command": "deus-witness-mcp",
  "args": ["--", "deus-witness-demo-tools"],
  "env": {"DEUS_WITNESS_KEY": "dwk_YOUR_KEY", "DEUS_WITNESS_URL": "https://witness.sipkigroup.com", "DEUS_DEMO_OUT": "/Users/you/dw-demo"}}}}

Without installing: uvx deus-witness -- <upstream MCP server command...> (e.g. uvx deus-witness -- npx -y @modelcontextprotocol/server-filesystem /tmp/sandbox) runs the same proxy as deus-witness-mcp -- <upstream...> (this is the launch form of the MCP Registry entry). DEUS_WITNESS_KEY must be set in the environment. Where to get the key: sign up on https://witness.sipkigroup.com, then on the dashboard card Agents create an agent; the key dwk_... is shown once (quickstart step 2: https://witness.sipkigroup.com/dl/docs/quickstart.html). Without a key the proxy does not start (deus-witness-mcp: set DEUS_WITNESS_KEY ...); a key the witness refuses ends every call with Blocked by Deus Witness: BAD_KEY ... (create a new agent and use its key). In both cases nothing runs.

Fallback: a wheel is also served at https://witness.sipkigroup.com/dl/ .

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 the agent gets an error result that says Action NOT executed: waiting for the owner's approval., the receipt id (rid) and how to retry (see "Waiting for approval / timeouts" below). 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>. Action NOT executed. (with the rid and the same retry instruction).

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_HOLD_MAX_S (how long one call may wait for the owner; default 50 = WAIT x POLLS; 0.2.22), DEUS_WITNESS_VERIFY=0 or --no-verify-aar (testing against a stub that does not sign).

Waiting for approval / timeouts

When the witness holds an action for the owner, the proxy does not run it. What the agent sees (an ordinary tool result with isError: true, not a protocol error):

Action NOT executed: waiting for the owner's approval.
Receipt (rid): r_...
The owner can approve it at: https://witness.sipkigroup.com/approve.html?rid=r_...
After the owner has approved, retry the exact same call (same tool, same arguments). Do not change the arguments.

After the owner approved, the repeated call (same tool and arguments) gets the same receipt and runs at once, exactly once. The proxy waits up to DEUS_WITNESS_HOLD_MAX_S seconds (default 50, as before) inside one call; the witness keeps the held action for 15 minutes or more.

Cancel (0.2.22). If the MCP client cancels the call (notifications/cancelled, e.g. because its own tool timeout ran out, or you stopped the task), the proxy stops waiting, tells the witness report executed=false and never forwards that call: an approval that arrives later does not run anything behind the agent's back. The cancelled request gets no answer, as the MCP specification says. The repeated call is judged afresh; if the owner approved meanwhile, it runs once.

Progress (0.2.22). If the call carries _meta.progressToken, the proxy sends notifications/progress (no percentage, message waiting for owner approval) about every 12 s while it waits. Clients that restart their timeout on progress (the MCP specification allows it, it does not require it) then wait longer.

Claude Code, from https://code.claude.com/docs/en/mcp (checked 22.10.2026): "Set a per-server tool execution timeout by adding a timeout field in milliseconds to that server's .mcp.json entry, for example "timeout": 600000 for ten minutes. This overrides the MCP_TOOL_TIMEOUT environment variable for that server only." and "The per-server timeout is a hard wall-clock limit per tool call, and progress notifications from the server don't extend it."

{"mcpServers": {"witnessed": {"command": "deus-witness-mcp", "args": ["--", "your-mcp-server"], "timeout": 700000,
  "env": {"DEUS_WITNESS_KEY": "dwk_YOUR_KEY", "DEUS_WITNESS_HOLD_MAX_S": "600"}}}}

HOLD_MAX_S is capped at 3600; the witness' own limit for a held action (15 minutes by default) also applies: after it the call ends with RECEIPT_EXPIRED. The Cursor and Desktop figures are reports of other users, not measured here. The Claude Code hook (deus-witness-hook) has its own waits (DEUS_WITNESS_HOOK_WAIT, below).

Windows: the proxy reads stdin in a thread and finds npx as npx.cmd; this is new in 0.2.21 and was tested on macOS/Linux and in CI only. If something fails on Windows, please report it.

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. It is 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/dl/docs/coverage.html and the privacy notice on https://witness.sipkigroup.com.

Set-up (full text: https://witness.sipkigroup.com/dl/docs/quickstart.html, section "Claude Code hook"):

1. Create an agent with the claude-code preset on the Agents page and copy its key.
2. Print the recommended mandate and import it on the Mandate page (Import from JSON, then review and sign):

   uvx --from deus-witness deus-witness-hook --print-form /path/to/your/project
   

   (uvx deus-witness-hook alone does not work: the command lives in the package deus-witness.) The form lets Write / Edit and shell redirections work inside the project and the temp directory (delete.allow_irreversible: true: the witness judges a write as an irreversible delete of its path); --no-irreversible prints a read-only form that refuses every write.
3. Put this into <project>/.claude/settings.json (or ~/.claude/settings.json). The key does NOT go into this file: export DEUS_WITNESS_KEY=dwk_... in the shell that starts claude, or env in <project>/.claude/settings.local.json.

{"hooks": {
  "SessionStart": [{"hooks": [{"type": "command", "command": "uvx --from deus-witness deus-witness-hook --session-start", "timeout": 15}]}],
  "PreToolUse": [{"matcher": "Bash|Edit|Write|MultiEdit|NotebookEdit|NotebookRead|Read|Grep|Glob|LS|WebFetch|WebSearch|ReadMcpResourceTool|ListMcpResourcesTool|mcp__.*",
    "hooks": [{"type": "command", "command": "uvx --from deus-witness deus-witness-hook || exit 2", "timeout": 30, "onFailure": "block"}]}]},
 "env": {"DEUS_WITNESS_URL": "https://witness.sipkigroup.com", "DEUS_WITNESS_HOOK_TIMEOUT": "20"}}

After pip install deus-witness / pipx install deus-witness write deus-witness-hook instead of uvx --from deus-witness deus-witness-hook. Keep all three layers: the hook denies itself on every error, || exit 2 blocks when the command is missing or broken, "onFailure": "block" blocks on a timeout or crash (Claude Code 2.1.295 or newer; older versions silently drop it). Keep DEUS_WITNESS_HOOK_TIMEOUT at least 5 s below timeout. 4. Check: uvx --from deus-witness deus-witness-hook --doctor "$PWD" must print OK: fail-closed ... (exit 0); in Claude Code /hooks lists the hook. 5. Add a second layer, Claude Code's own permissions.deny (recommended), into the same settings.json:

{"permissions": {"deny": [
  "Bash(curl *)", "Bash(wget *)", "Bash(nc *)", "Bash(ncat *)", "Bash(telnet *)", "Bash(ssh *)", "Bash(scp *)", "Bash(sftp *)", "Bash(rsync *)",
  "Bash(/usr/bin/curl *)", "Bash(/usr/local/bin/curl *)", "Bash(/opt/homebrew/bin/curl *)", "Bash(curl -K*)", "Bash(curl --config*)",
  "Bash(/usr/bin/wget *)", "Bash(/usr/local/bin/wget *)", "Bash(/opt/homebrew/bin/wget *)",
  "WebFetch(domain:gist.github.com)", "WebFetch(domain:gist.githubusercontent.com)", "WebFetch(domain:raw.githubusercontent.com)",
  "WebFetch(domain:pastebin.com)", "WebFetch(domain:transfer.sh)", "WebFetch(domain:webhook.site)",
  "Read(.env)", "Read(.env.*)", "Read(~/.ssh/**)", "Read(~/.aws/**)", "Read(~/.gnupg/**)",
  "Edit(.claude/**)", "Edit(~/.claude/**)", "Edit(.mcp.json)", "Edit(~/.claude.json)"
]}}

Why: Claude Code evaluates deny rules first and "PreToolUse hook decisions don't bypass permission rules" (code.claude.com/docs/en/permissions, "Extend permissions with hooks"). These rules therefore hold even if the hook is missing, broken or switched off, or the witness answers pass for a call. They close the direct ways out (network tools, hosts where anyone can publish, SSH keys, .env files) and stop the agent from editing the settings that switch the hook off. Limits: a Bash rule "matches the command text Claude writes ... and isn't a security boundary around the program" (env curl, curl -sK or sh -c 'curl ...' are not matched; the common absolute paths are listed above; same page, "What a Bash rule doesn't match"), so this is a second layer behind the witness; for enforcement independent of the command text use Claude Code's sandbox network allowlist. Bash(curl *) also stops harmless curl: the agent reads allowed documentation with WebFetch (your mandate lists the domains). Edit(.claude/**) means you edit your own settings by hand.

Held calls (DEUS_WITNESS_HOOK_ON_DEFER). When the witness holds a call for the owner, the hook answers deny by default (0.2.22; earlier versions asked in the terminal): the text names the receipt and the approval link, the owner approves in Witness (on the phone), then the agent retries the same call. A yes/no question in the terminal would let the person at the keyboard approve in the owner's place, and "yes" would run the call WITHOUT a witness receipt. DEUS_WITNESS_HOOK_ON_DEFER=ask brings the question back (only if you want that, set it explicitly in env); defer works in print mode / Agent SDK only. Any other value means deny. deus-witness-hook --help lists it.

Default mandate domains (0.2.22). --print-form no longer allows github.com, raw.githubusercontent.com or any other host where anyone can publish (a WebFetch GET with a query string to such a host is a way out for data). Kept: localhost, 127.0.0.1, docs.python.org, pypi.org, docs.github.com, developer.mozilla.org, www.npmjs.com, nodejs.org, docs.anthropic.com, code.claude.com. Add a host you need on the Mandate page, knowingly. A mandate you signed earlier keeps its own list until you sign a new one.

Cursor

Cursor reads MCP servers from ~/.cursor/mcp.json (or .cursor/mcp.json in a project); the entry is the same as for Claude Desktop. Put the real MCP server after --:

{"mcpServers": {"witnessed": {"command": "uvx",
  "args": ["deus-witness", "--", "npx", "-y", "@modelcontextprotocol/server-filesystem", "/path/to/sandbox"],
  "env": {"DEUS_WITNESS_KEY": "dwk_YOUR_KEY", "DEUS_WITNESS_URL": "https://witness.sipkigroup.com"}}}}

Cursor's own agent tools (shell, file edits) do not go through MCP and are not covered; deus-witness doctor lists them as a bypass.

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": []})). 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.