Part of the StudioMeyer MCP Stack — Built in Mallorca 🌴 · ⭐ if you use it

mcp-hook-conformance

Test-harness CLI that audits any MCP server for Claude Code v2.1.118 mcp_tool lifecycle-hook readiness.

The third foundation pillar:

• mcp-protocol-conformance says: does the server speak the protocol?
• mcp-server-attestation says: who signed this server?
• mcp-hook-conformance says: is it safe to call this tool from a Stop, PreCompact, or UserPromptSubmit hook?

Install

npm install -g mcp-hook-conformance

Or run without install:

npx mcp-hook-conformance check ./hook-conformance.config.json

Quick start

# 1. Scaffold a config
npx mcp-hook-conformance init --server-name my-mcp-server

# 2. Edit hook-conformance.config.json (point command/args at your server)

# 3. Run the audit
npx mcp-hook-conformance check ./hook-conformance.config.json

Exit codes:

• 0 audit passed (no FAILs)
• 1 audit produced at least one FAIL
• 2 config error (missing file, invalid schema, unknown suite name)

Subcommands

| Subcommand | Args | Description | |---|---|---| | check | <config-path> [--output=human\|json\|junit] [--suite=...] [--tool=<name>] [--tries=<n>] | Run audit; emits report to stdout. | | init | [--server-name=<name>] [--force] | Write example hook-conformance.config.json to cwd. | | explain | <finding-code> | Print full explanation and remediation for a finding (e.g. IDEMP-001). | | version-info | — | Print tool version and supported MCP-spec range. (Alias: version.) |

This is a CLI, not an MCP server. Do NOT run mcp-hook-conformance via mcp-inspector — the package declares mcp.specVersion for the targets it audits, not because it speaks MCP itself. Inspector will EACCES on dist/index.js (which is the library export) by design.

Audit suites

| Suite | What it checks | Weight | |---|---|---| | idempotency | Same input N times produces same output, no UUIDs/timestamps unless annotated. | 30 | | side-effects | Read-only tools must not mutate server state. Requires stateProbe config. | 25 | | latency | p50 < 30s, p95 < 60s (configurable). 5 samples after 1 warmup call. | 20 | | determinism | Response shape stable across calls; no float drift. | 15 | | dsgvo | Tool description mentions data flow / retention / deletion (heuristic). | 10 |

Suite scoring: PASS = 100, WARN = 60, INDETERMINATE = 50, FAIL = 0. Tool score = weighted average across configured suites. Aggregate score = mean of tool scores.

Config format

{
  "$schema": "https://studiomeyer-io.github.io/mcp-hook-conformance/config.schema.json",
  "server": {
    "name": "my-mcp-server",
    "transport": "stdio",
    "command": "npx",
    "args": ["-y", "my-mcp-server"],
    "env": {}
  },
  "suites": ["idempotency", "latency", "determinism", "side-effects", "dsgvo"],
  "tries": 3,
  "thresholds": {
    "latencyP50Ms": 30000,
    "latencyP95Ms": 60000
  },
  "probes": [
    {
      "name": "search_things",
      "args": { "query": "audit-fixed-input" },
      "stateProbe": {
        "tool": "list_things",
        "args": {}
      }
    }
  ],
  "toolFilter": ["search_things", "get_thing"]
}

probes lets you pin specific arguments per tool and (for read-only tools) a stateProbe so the side-effects suite can detect mutations. Without a stateProbe the side-effects suite returns INDETERMINATE for that tool.

Finding codes

| Code | Suite | Severity | Title | |---|---|---|---| | IDEMP-001 | idempotency | FAIL | Tool output diverges across identical calls | | IDEMP-002 | idempotency | WARN | Response contains timestamps without nondeterministic annotation | | IDEMP-003 | idempotency | WARN | Response contains UUID/random IDs without nondeterministic annotation | | LAT-001 | latency | FAIL | Tool exceeded p95 latency budget | | LAT-002 | latency | WARN | Tool p50 latency above 30 seconds | | DET-001 | determinism | FAIL | Floating-point output drift between calls | | DET-002 | determinism | WARN | Response shape varies between calls | | SIDE-001 | side-effects | FAIL | Read-only tool produced state delta | | SIDE-002 | side-effects | INDETERMINATE | No state-probe configured for side-effect detection | | DSGVO-001 | dsgvo | FAIL | Destructive tool lacks deletion / retention documentation | | DSGVO-002 | dsgvo | WARN | Tool description missing data-flow keywords |

Run mcp-hook-conformance explain <code> for full remediation text.

Compatibility

| Component | Version | |---|---| | MCP spec | 2025-06-18 | | Claude Code | >= 2.1.118 (mcp_tool lifecycle hooks) | | Node | >= 20.0.0 | | @modelcontextprotocol/sdk | ^1.29.0 |

Tested transports:

• stdio (primary)
• HTTP streamable (experimental, audit semantics same as stdio)

Reference smoke tests

The tests/smoke/ suite runs the audit against production tenants of mcp-nex and mcp-crm. Opt-in:

RUN_SMOKE_TESTS=1 npm run test:smoke

Snapshot drift triggers a manual review, not a CI fail.

Limitations (read these)

• Side-effect detection is not hermetic. Without a stateProbe we cannot black-box prove a read-only tool is pure. We return INDETERMINATE instead of false-positive FAIL.
• Latency is end-to-end stdio roundtrip. Includes JSON-RPC framing, not pure compute.
• DSGVO suite is keyword heuristic. It catches missing docs reliably; "looks compliant" is not legal advice.
• No auto-fix. This tool reports findings; you fix them.

About StudioMeyer

StudioMeyer is an AI and design studio based in Palma de Mallorca, working with clients worldwide. We build custom websites and AI infrastructure for small and medium businesses. Production stack on Claude Agent SDK, MCP and n8n, with Sentry, Langfuse and LangGraph for observability and an in-house guard layer.

License

MIT © 2026 Matthias Meyer (StudioMeyer)