Recusal

Deterministic governance for Claude agents: an independent verifier that can refuse to certify a tool call before it runs.

Lightweight (zero dependencies) · extensible (a check is just a function that returns a finding) · Claude-native (drops into Claude Code as a hook, MCP tool calls included, and the Claude Agent SDK as a tool gate). The zero-dep core works in any agent loop.

_{Verbatim transcripts, rendered: a live Claude Code session where the repo's own hook refuses rm -rf under --dangerously-skip-permissions, then the offline demo (python examples/claude_refusal.py), no API key.}

A judge recuses themselves from a case they can't impartially decide. The same principle governs autonomous agents: the thing that generates the work must never be the thing that certifies it. Recusal is that independent authority: collect evidence, adjudicate it into PASS / RETRY / FAIL, and let the gate refuse. No model call in the decision path. Same evidence, same verdict, every time, including the "no".

The wedge: don't let the same model grade its own work

The reflex fix for agent safety is another model asking "does this action look OK?", but a judge from the same family shares the builder's blind spots and drifts with it: that is a conflict of interest, not a control. Recusal is an independent, deterministic authority instead: no model in the decision path, a verdict you can replay and audit, and a refusal that holds (a Claude Code deny is honored even under bypassPermissions). "Independent" means the verdict is produced outside the model's decision path; deployment isolation (who owns the config, the file permissions, the runtime) remains the adopter's responsibility.

The published evidence (models faking passing tests, benchmarks gamed by intercepting the evaluator, and the stated limits of Anthropic's own same-family safety layer) is laid out in docs/WHY.md, every source verified in docs/REFERENCES.md.

Builders generate. Recusal certifies. Refusal is a feature.

Architecture

One object model, one pipeline. Checks (or your own evidence) produce Findings; compute_verdict folds them into a Verdict; and the Verdict drives every surface: the gate refuses, the audit log records, the classifier routes.

  data / a proposed agent action / a tool call
          │
     [ checks ]            emit Findings               (recusal.checks, or your own)
          │
   compute_verdict()       fold findings → one Verdict (PASS / RETRY / FAIL)
          │
       Verdict
        │   │   │
        │   │   └─ recusal.classify        route the failure (retry / refuse / ask-human / …)
        │   └───── recusal.audit           tamper-evident, hash-chained record
        └───────── recusal.claude(_code)   allow or refuse the tool call
                   recusal.gates           staged G0-G8 release decision

Zero runtime dependencies, standard library only.

Install

pip install recusal

See it refuse (20 seconds, no API key)

git clone https://github.com/philpaz/recusal && cd recusal
python examples/claude_refusal.py   # a Claude agent stages a write to the WRONG
                                    # customer; the gate refuses before the tool runs
python examples/gallery.py          # the same gate across the OWASP agentic failure modes

Deterministic and offline: same evidence, same verdict, including the no.

Plug it into Claude

Claude Code, drop-in PreToolUse hook

Refuse destructive tool calls before Claude Code runs them, even in auto / bypass mode.

One command:

python -m recusal init          # or: recusal init

scaffolds .claude/hooks/recusal_gate.py (the deny-list starter, edit it, it's yours) and registers the fail-closed launcher in .claude/settings.json, merging with (never clobbering) an existing file; re-running is a no-op, and an existing gate file is never overwritten. --posture allowlist scaffolds the default-deny variant instead. Claude Code asks you to confirm the new hook on the next session: a permission-changing hook is a deliberate step.

Or as a plugin (one gate across every project, no per-project setup):

claude plugin marketplace add philpaz/recusal
claude plugin install recusal-gate@recusal
pip install recusal        # the plugin fails CLOSED without it

The plugin ships the same deny-list shim; if the recusal package is missing it refuses every tool call rather than silently disabling itself. For a policy tailored to one project, prefer python -m recusal init and edit the scaffolded gate.

Prefer to see exactly what it writes? The manual path is the same two pieces. Register a hook in .claude/settings.json:

{ "hooks": { "PreToolUse": [
  { "matcher": ".*", "hooks": [
    { "type": "command", "command": "for p in python3 python py; do \"$p\" -c 'import sys; sys.exit(0 if sys.version_info >= (3, 9) else 1)' 2>/dev/null && { \"$p\" \"$CLAUDE_PROJECT_DIR/.claude/hooks/my_gate.py\"; rc=$?; [ \"$rc\" = 0 ] || { echo 'gate: hook did not run cleanly; failing closed' >&2; exit 2; }; exit 0; }; done; echo 'gate: no python>=3.9; failing closed' >&2; exit 2" } ]}
]}}

The command runs the first python3 → python → py that is >=3.9 and fails closed: Claude Code treats a hook whose command can't launch, or that exits with anything other than 2, as a non-blocking error and lets the tool call proceed, so a bare python3 on a Windows machine (no python3 on PATH), a python that is Python 2, or a hook that raises at import would each silently disable the gate. The loop coerces every one of those into exit 2, the one blocking hook exit code, so a broken or absent interpreter refuses the tool call instead of waving it through.

Windows: shell-form hooks run under Git Bash when it is installed, and Claude Code falls back to PowerShell when it is not - where this POSIX loop is a parse error with a non-blocking exit code, i.e. the gate silently disables (live-verified). recusal init therefore registers a PowerShell-native launcher with an explicit "shell": "powershell" on Windows; the POSIX form above is for macOS/Linux and Windows-with-Git-Bash. For a settings.json shared across operating systems, recusal init --launcher both registers the pair, and recusal doctor validates the registered launcher against the host.

# my_gate.py
from recusal import Finding
from recusal.claude_code import run_pretooluse_hook

def policy(tool_name, tool_input):
    if tool_name == "Bash" and "rm -rf" in tool_input.get("command", ""):
        return [Finding.fail("destructive_bash", severity="CRITICAL", message="refusing rm -rf")]
    return []   # no opinion → defer to Claude Code's normal permission flow

run_pretooluse_hook(policy)

A clean verdict defers (Recusal adds refusals; it never strips Claude Code's own prompts). A non-clean verdict denies, with the reasons. See examples/claude_code_gate.py.

Two paths, one principle: pick by your channel, not by a ranking. The policy above is a deny-list: name the known-bad calls, defer everything else. It drops into a broad, open-ended channel with almost no friction and needs no inventory of your tools, which is why this repo dogfoods it (a general-purpose dev repo runs an unbounded set of legitimate commands). Its boundary is inherent, not a defect: a literal matcher can be obfuscated past, and python script.py runs code no string check ever reads, so a deny-list never earns "cannot be subverted."

The other path is allowlist mode (default-deny): name the affirmatively-safe calls, refuse everything else. It fits a narrow, enumerable, high-stakes channel: nothing runs unless listed, and bare interpreters and shell metacharacters are refused, which closes the documented command-construction and bare-interpreter bypass classes by construction (pinned as tests). Within a correctly registered routed tool channel, an unapproved capability is refused by default rather than inferred safe; what sits outside that channel is named in SECURITY.md. One more honest line: the default-safe tools are nonmutating, not authorized for all data - cat can read a credential file - so add path- and subject-level read rules where confidentiality matters. The trade is friction and maintenance: you enumerate and grow the capability set, and it fails toward refusal until you do.

Neither is "better" in the abstract: a deny-list refusing the unknown would grind a broad channel to a halt, and an allowlist deferring the unknown would defeat the point of a high-stakes one. Choose by the channel. Both ship, and both are pinned as tests.

from recusal.claude_code import allowlist_policy, run_pretooluse_hook

run_pretooluse_hook(allowlist_policy(writable_root="./workspace"))

Don't start from a blank policy. docs/COOKBOOK.md has copy-paste recipes (destructive shell, unscoped SQL, secret-file writes, wrong-subject writes, egress allowlists, injection quarantine, action budgets) that drop straight into the hook above.

Recusal governs this repository exactly this way: a real hook refuses rm -rf, force-pushes, and secret-file writes to its own maintainers. Verbatim, reproducible, CI-locked proof: docs/PROVEN.md.

MCP tools, the same gate

MCP server tools reach Claude Code as ordinary tools: the hooks reference documents that they "appear as regular tools in tool events" (PreToolUse, ...) under the naming pattern mcp__<server>__<tool> (mcp__github__create_issue, mcp__filesystem__write_file). So the .* matcher above already routes every MCP call through the same policy(tool_name, tool_input) seam: no MCP-specific adapter, no extra wiring. The same call-time controls apply to MCP exactly as to Bash: destructive-operation refusal, repository/record scope, write-path confinement, egress and action budgets, the tamper-evident audit record. In allowlist mode the posture is stronger still: an MCP tool is refused unless affirmatively named (allow={"mcp__github__create_issue": vet}), the least-privilege default the MCP spec's own security guidance pushes toward. Pinned as tests.

def policy(tool_name, tool_input):
    if tool_name == "mcp__salesforce__delete_records":
        return [Finding.fail("mcp_destructive_action", severity="CRITICAL",
                             message="bulk Salesforce deletion is not approved")]
    if tool_name == "mcp__github__merge_pull_request":
        repo = tool_input.get("repo")
        if repo not in {"philpaz/recusal"}:
            return [Finding.fail("mcp_repository_scope", severity="CRITICAL",
                                 message=f"repository {repo!r} is outside the approved scope")]
    return []   # defer everything else to Claude Code's normal flow

Runnable: examples/mcp_governance.py (approved-server pinning, destructive-verb refusal, path confinement, allowlist mode). Pinned: tests/test_mcp_governance.py. Recipe: docs/COOKBOOK.md §12. In a custom Agent SDK or MCP-client loop nothing intercepts for you: invoke the gate between the model's proposed MCP call and the client dispatching it (the same gate_tool_use seam as below).

The three MCP boundaries, stated plainly. A call-time policy adjudicates the proposed tool name and arguments; MCP has two more boundaries, and Recusal covers each with its own evidence:

Transport and authorization threats (confused deputy, token passthrough, session hijacking) are the MCP specification's own Security Best Practices layer, complementary to this gate, neither replaces the other. Every source here is verified in docs/REFERENCES.md.

MCP discovery, pin the catalog, refuse the rug pull

The model chooses tools by reading their declared descriptions, so a poisoned declaration steers the agent before any call exists for a call-time policy to see, and the call that follows looks structurally valid. recusal.mcp adds deterministic integrity controls at that boundary the way this library governs every boundary: deterministic evidence, with the human where the judgment is:

recusal mcp pin --claude-config .mcp.json    # review once, pin the approved catalog
recusal mcp verify --claude-config .mcp.json # CI / session start: same catalog, or refuse

--claude-config and --stdio execute the declared server commands to ask them for tools/list; there is no other way to ask a process for its catalog. The first pin is therefore an explicit trust event: review the command/args lines like you review the declarations, then pass --approve-server-launch to record it. After the pin, verify compares each launch specification against the manifest before launching anything, and stdio servers run with a minimal environment by default (--inherit-env is the named opt-out).

Recusal does not judge whether a description is malicious: that is semantic judgment, a human's call at pin time (a deterministic marker screen surfaces the obvious, and pin refuses to write over a flagged catalog until --force records that a human reviewed it). What it detects, deterministically, is unpinned capability and post-approval change: the rug pull, the new tool, the mutated schema. The pin is the confirmed human decision promoted to a deterministic artifact: manifest bytes are reproducible, hashes only (a poisoned description is never embedded anywhere), and the same observed catalog against the same pin yields the same verdict, every time. verify fails closed: a missing manifest, a failed fetch, a wholly empty observation, or a pinned server that can no longer be reached for integrity-checking (e.g. silently swapped to a URL transport) is a refusal, never a clean-looking pass. (A pinned server legitimately removed from the config is recorded as a warning, not refused: a shrunk capability set is not an attack.) The pin also enforces at call time: recusal.mcp.manifest_policy("mcp-manifest.json") drops into the same PreToolUse gate and refuses any mcp__server__tool call that was never pinned (no pin, no MCP), composing with the argument-level rules above. A minimal zero-dependency stdio client collects tools/list; remote/HTTP servers are pinned from a JSON dump you produce with any MCP client (--from, copy-paste recipe: docs/COOKBOOK.md §14; local/.mcp.json servers pin directly, §13). Recusal owns the deterministic adjudication, not the transport, so it inherits neither the HTTP client's dependencies nor its SSRF surface. Collection is never decision: the kernel adjudicates what was observed.

The honest boundary: this is discovery-time and call-time integrity, not a live tap on every message. verify proves the catalog at the moment it runs (wire it into CI and session start); the call-time gate then enforces approved tools only. A server that serves one catalog to verify and a different one to the live session (a client- or time-discriminating server) is a residual this layer names rather than claims to close: run verify against the same endpoint the session uses, close in time. The manifest pins the launch specification as well as the declared catalog: the unexpanded command template, args, cwd, and environment variable names (never values), compared by verify before any process starts, so a rewritten .mcp.json command is refused without the replacement ever executing (pinned by an adversarial test proving the substituted command's marker file is never written). The remaining residuals, named: identity is template-level, so the values of referenced environment variables are not pinned; npx/uvx-style launchers resolve through PATH and fetch what the registry serves (pin package versions in the args); and dump-supplied (--from) servers are pinned as transport: external, their launch being outside recusal's control. Keep protecting .mcp.json and mcp-manifest.json as control-plane files - the default deny-list does.

See the refusal: examples/mcp_manifest_rugpull.py (offline). Pinned as tests: tests/test_mcp_manifest.py, tests/test_mcp_policy_bridge.py, tests/test_mcp_fetch.py, tests/test_mcp_cli.py.

Claude Agent SDK, manual loop

In a manual agent loop, gate each tool call and hand Claude an is_error tool_result on a refusal; it self-corrects:

from recusal.claude import gate_tool_use

allow, refusal = gate_tool_use(tool.id, gather_evidence(tool), tool_name=tool.name)
if not allow:
    results.append(refusal)                          # is_error=True → Claude adapts
else:
    results.append({"type": "tool_result", "tool_use_id": tool.id,
                    "content": execute_tool(tool.name, tool.input)})

Runnable: examples/claude_agent_live.py (real API) and examples/claude_refusal.py (offline, no key). For Managed Agents always_ask, recusal.claude.tool_confirmation is the deterministic decider (the SDK event shape is illustrative; verify it against your Agent SDK version).

Any agent loop, no Claude required

The Claude adapters are conveniences; the zero-dep core is framework-neutral. examples/agent_loop.py gates a plain propose → gate → act loop whose only import is recusal; the same compute_verdict seam drops into LangGraph, the OpenAI Agents SDK, or a homegrown runtime unchanged.

Robustness, across the OWASP Agentic failure modes

python examples/gallery.py runs the gate against the common autonomous-agent failure modes:

  scenario                OWASP                 verdict outcome
  wrong-subject write     ASI03 Identity Abuse  FAIL    REFUSE
  destructive file delete ASI02 Tool Misuse     FAIL    REFUSE
  unscoped SQL mutation   ASI05 Code Execution  FAIL    REFUSE
  data exfiltration       ASI01 Goal Hijack     FAIL    REFUSE
  coverage floor          quality gate          RETRY   BLOCK (retry)
  runaway action volume   ASI08 Cascading       RETRY   BLOCK (retry)
  compliant write         -                     PASS    ALLOW

The tiers are the policy: destructive things REFUSE terminally; recoverable ones BLOCK with a retry; a clean call passes. (Same policies power the demo and the test suite.)

The verdict, directly

from recusal import compute_verdict
from recusal.checks import row_count, null_rate, referential_integrity

verdict = compute_verdict([
    row_count(users, min_rows=1),                                  # CRITICAL if empty
    null_rate(users, "email", max_rate=0.10),                      # ERROR if too sparse
    referential_integrity(orders, users, fk="user_id", pk="id"),   # CRITICAL on orphans
])
if verdict.refused:
    raise RuntimeError(verdict.reasons())

Tamper-evident audit

Pair any verdict with an append-only, hash-chained log: every decision on the record, and an in-place edit or reordering of any entry with a surviving successor is detectable (catching tail-truncation, a tail-suffix rewrite, or a forged append by a write-access attacker needs an external anchor, see recusal.audit):

from recusal import AuditLog, verify

audit = AuditLog(path="audit.jsonl")
audit.append(verdict, action={"tool": "Bash", "command": "rm -rf /"})
ok, problems = verify(audit.entries)   # False if an entry with a later entry was edited/reordered

In the Claude Code hook it is one argument: run_pretooluse_hook(policy, audit=AuditLog("audit.jsonl", resume="tail")) puts every adjudication - defer, allow, and deny - on the chain, with the proposed tool_input bound by SHA-256 fingerprint, never embedded, and an unwritable log failing closed to a deny (the record is part of the control). resume="tail" recovers the chain head from the final record without loading or scanning the full log, and file-backed appends are serialized with an inter-process lock, so hooks for parallel tool calls extend one chain instead of forking it.

Deterministic, stdlib-only, and shaped for OWASP Agentic logging / EU AI Act Article 12 (record-keeping).

Gate your CI

CI is, by construction, not the session that did the work, which makes it the natural place for a recusal verdict. The same kernel runs as a command line with blocking exit codes (PASS 0, RETRY 1, FAIL 2; any operational error exits 2, failing closed):

recusal verdict findings.json --json   # adjudicate any tool's findings; nonzero blocks the job
recusal audit verify audit.jsonl --expect-head "42:<hash>"   # a missing log is NOT an intact log
recusal doctor                         # "the gate silently isn't installed" fails CI, not prod

Or as a GitHub Action (action.yml, dogfooded by this repo's own CI, including the negative case: a tampered audit log must make the gate refuse):

- uses: actions/setup-python@v6
  with:
    python-version: "3.12"
- uses: philpaz/recusal@v0.4.1
  with:
    findings: reports/findings.json   # RETRY exits 1, FAIL exits 2 → the merge is blocked
    audit-log: reports/audit.jsonl
    doctor-dir: "."

The action ref selects the implementation: the action force-installs the recusal bundled with the selected ref, replacing whatever happens to be on the runner, so pinning the action pins the code (proven in CI on a clean runner and against a deliberately conflicting preinstall). The two escape hatches are explicit inputs that name the provenance trade: version: installs a named PyPI release, and use-installed: "true" keeps a deliberately preinstalled checkout.

Given nothing to adjudicate, the action exits 2 rather than pass vacuously: an evidence set that proves nothing certifies nothing.

Classify and route a failure

A refusal or failure is only useful if you know what to do next. The classifier says what kind of failure it is and where it routes, deterministically, with no model:

from recusal import classify_failure

c = classify_failure("Traceback ... TypeError: 'NoneType' object")
c.failure_class   # "code_bug"
c.route           # "fix-code"

Default taxonomy (extend or replace it): transient → retry · policy_violation → refuse · prompt_injection → quarantine · code_bug → fix-code · data_shape → fix-data · data_missing → fetch-data · spec_ambiguity → ask-human. Unmatched failures fall back to ask-human; it never guesses. classify_verdict(verdict) routes a non-PASS verdict.

Documentation

New here? The quick objections (do I need this? doesn't Claude already do it? is it ready to use?) are answered in the docs/FAQ.md. The plain-terms "so what": docs/WHY.md.

Full documentation index: docs/. Comparison with the landscape: docs/LANDSCAPE.md. The principles and why each helps: CONSTITUTION.md. The contract: docs/EVIDENCE.md. Usage & extending: docs/HOWTO.md · docs/EXTENDING.md. Copy-paste policies: docs/COOKBOOK.md. A full worked configuration: docs/EXAMPLE.md. Proof it governs itself: docs/PROVEN.md.

Development

pip install -e ".[dev]"
pytest -q

Contributing

Contributions are welcome. Recusal is deliberately small, and the bar is keeping it that way (no model in the verdict path, no runtime dependencies, don't grow the kernel). Read CONTRIBUTING.md and the CODE_OF_CONDUCT.md first. Security reports go through SECURITY.md, privately.

Contact

Built by Philip Paz. Messages are open, especially from teams running agents in regulated environments, and doubly so if you wired the gate in and found where it leaks (tell me here).

License

Apache-2.0 © Philip Paz