Step-by-step cookbook for setting up cryptographically signed audit trails on Claude Code tool calls. Use when explaining, evaluating, or demonstrating the pattern before committing to the protect-mcp runtime hooks. Covers Cedar policy, Ed25519 receipts, offline verification, tamper detection, CI/CD

Signed Audit Trails for Claude Code Tool Calls Cookbook style walkthrough for cryptographically signed receipts on every Claude Code tool call. This is the teaching skill. For the runtime implementation, install the [ protect mcp ](../../../protect mcp/) plugin. What this gives you Every tool call ( Bash , Edit , Write , WebFetch ) is: 1. Evaluated against a Cedar policy before execution. If the policy denies the call, the tool does not run. 2. Signed as an Ed25519 receipt after execution. Receipts are JCS canonical and verifiable offline by anyone with the public key. An auditor, regulator, or counterparty can verify every receipt later (Step 5). No network call, no vendor lookup, no trust in the operator. When to use the pattern Regulated environments (finance, healthcare, critical infrastructure) where you need tamper evident evidence of agent behavior CI/CD pipelines where you want to prove that a policy gate held for every automated build step Multi party collaboration where a counterparty wants to verify your agent's behavior without trusting your operator Compliance contexts (EU AI Act Article 12, SLSA provenance for agent built software) where standard logging is not sufficient Step 1: Install the hook configuration Install the protect mcp plugin with /plugin install protect mcp . Its hooks run evaluate.sh before each tool call and sign.sh after it. Both scripts read the hook event from stdin, because Claude Code sets no TOOL NAME or TOOL INPUT variables. See [ references/hook wiring.md ](references/hook wiring.md) for the hook configuration and what each script passes to protect mcp. protect mcp 0.7.4 does not create the signing key, and without a key the receipts are unsigned. Create ./protect mcp.key once. The command never replaces an existing key: Give auditors the publicKey value from that file. Do not commit the file, because it also holds the private key. Add the private key and receipt directory to .gitignore : Step 2: Write a Cedar policy Create ./protect.cedar from the example in [ references/cedar policy.md ](references/cedar policy.md). It allows read only tools and a short list of Bash commands, denies shell chaining and destructive commands, and limits writes to the project with .. segments denied. Step 3: Use Claude Code normally Start Claude Code. Every tool call goes through both hooks: A session of 20 tool calls appends 20 receipts to ./receipts/receipts.jsonl . Step 4: Inspect a receipt protect mcp 0.7.4 appends each receipt as one line of ./receipts/receipts.jsonl . Print the newest one: The receipt is a signed v2 envelope that names the tool, and it holds no public key. See [ references/receipt format.md ](references/receipt format.md) for a sample and the signed fields. Step 5: Verify the receipts Pass the publicKey value from ./protect mcp.key to the verifier: Exit codes: Code Meaning 0 Every receipt verified 1 A receipt failed verification (tampered, wrong key, or malformed line) 2 The receipts file could not be read Step 6: Demonstrate tamper detection Change the newest receipt's decision from allow to deny : The verifier exits with code 1 and reports which line failed. The Ed25519 signature no longer matches the JCS canonical bytes of the tampered payload. Restore the field and verification passes again. How the cryptography works Two invariants make receipts verifiable offline across any conformant implementation: 1. JCS canonicalization (RFC 8785) before signing. Keys sorted by UTF 16 code units, whitespace minimized, strings not normalized. Two independent implementations produce byte identical signing payloads for the same receipt content. 2. Ed25519 signatures (RFC 8032) over the canonical bytes. Deterministic, fixed size, no nonce dependency. protect mcp 0.7.4 receipts carry no link to the previous receipt, so a deleted receipt goes undetected. For the formal wire format see [draft farley acta signed receipts](https://datatracker.ietf.org/doc/draft farley acta signed receipts/). Cross implementation interop The receipt format has four independent implementations today: Implementation Language Use case [protect mcp](https://www.npmjs.com/package/protect mcp) TypeScript Claude Code, Cursor, MCP hosts [protect mcp adk](https://pypi.org/project/protect mcp adk/) Python Google Agent Development Kit [sb runtime](https://github.com/ScopeBlind/sb runtime) Rust OS level sandbox (Landlock + seccomp) APS governance hook Python CrewAI, LangChain A receipt produced by any of them verifies against [ @veritasacta/verify ](https://www.npmjs.com/package/@veritasacta/verify). The auditor does not need to trust the operator's tooling choice: the format is the contract. CI/CD integration Verify receipts in CI so a tampered receipt fails the build. [ references/ci cd.md ](references/ci cd.md) has a GitHub Actions workflow that runs on pushes to the default branch. It installs the signing key from a branch limited environment, runs the agent, verifies the receipts, and uploads them. It does not run on pull requests, because that would hand the key to unreviewed code. Composition with SLSA provenance for agent built software When Claude Code builds and releases software (running npm install , npm build , npm publish as tool calls), the receipt chain is the per step build log. SLSA Provenance v1 has an extension point for this: the byproducts field can reference the receipt chain alongside the build attestation. The [agent commit build type](https://refs.arewm.com/agent commit/v0.2) documents the pattern using the ResourceDescriptor shape: The SLSA provenance is signed by the builder identity; the receipt attestation is signed by the supervisor hook identity. Two trust domains, cross referenced at the byproduct layer. See [slsa framework/slsa 1594](https://github.com/slsa framework/slsa/issues/1594) for the composition discussion. Common pitfalls Private key in version control. The generated ./protect mcp.key must not be committed. The examples above add it to .gitignore . If a key is accidentally committed, rotate it immediately. Move the key and ./receipts/receipts.jsonl to an archive, then run the Step 1 command again. Verify the archived receipts with the old public key. Hook payload on stdin. Claude Code sets no $TOOL NAME or $TOOL INPUT variables. A hook command that passes tool "$TOOL NAME" sends an empty tool name, so the policy denies every call. Read the payload from stdin as the plugin scripts do. Receipts directory in CI. If Claude Code runs in CI, upload receipts as an artifact at the end of the job or the receipts are lost at job end. Policy is missing. When ./protect.cedar does not exist, evaluate.sh prints a warning to stderr and allows the call. No call is gated until you create the policy in Step 2. Related in this marketplace [ protect mcp ](../../../protect mcp/) — the runtime hook implementation (use this plugin in production) [ review agent governance ](../../../review agent governance/) — require human approval before review surface actions; composes with protect mcp References [ draft farley acta signed receipts ](https://datatracker.ietf.org/doc/draft farley acta signed receipts/) — IETF draft, receipt wire format [RFC 8032](https://datatracker.ietf.org/doc/html/rfc8032) — Ed25519 [RFC 8785](https://datatracker.ietf.org/doc/html/rfc8785) — JCS [Cedar policy language](https://docs.cedarpolicy.com/) [protect mcp on npm](https://www.npmjs.com/package/protect mcp) [@veritasacta/verify on npm](https://www.npmjs.com/package/@veritasacta/verify) [in toto/attestation 549](https://github.com/in toto/attestation/pull/549) — Decision Receipt predicate proposal [agent commit build type](https://refs.arewm.com/agent commit/v0.2) — SLSA provenance for agent produced commits [Microsoft Agent Governance Toolkit](https://github.com/microsoft/agent governance toolkit) ( examples/protect mcp governed/ ) [AWS Cedar for Agents](https://github.com/cedar policy/cedar for agents)

signed-audit-trails-recipe