Signed Audit Trails Recipe

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 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 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:

if [ ! -e ./protect-mcp.key ]; then
  d=$(mktemp -d) && npx protect-mcp@0.7.4 init --dir "$d" && mv "$d/keys/gateway.json" ./protect-mcp.key
fi

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:

echo "/protect-mcp.key" >> .gitignore
echo "/receipts/" >> .gitignore

Step 2: Write a Cedar policy

Create ./protect.cedar from the example in 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:

You: Please read the README and summarize it.

Claude: I will read README.md.
  [PreToolUse: Read ./README.md -> allow]
  [Tool: Read executes]
  [PostToolUse: receipt rcpt-a8f3c9d2 signed to ./receipts/]

... summary of README ...

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:

tail -n 1 ./receipts/receipts.jsonl | python3 -m json.tool

The receipt is a signed v2 envelope that names the tool, and it holds no public key. See 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:

PUB=$(node -p 'JSON.parse(require("fs").readFileSync("./protect-mcp.key")).publicKey')
npx @veritasacta/verify@0.9.2 --replay-chain ./receipts/receipts.jsonl --key "$PUB"

Exit codes:

Step 6: Demonstrate tamper detection

Change the newest receipt's decision from allow to deny:

python3 -c "
import json
path = './receipts/receipts.jsonl'
lines = open(path).read().splitlines()
r = json.loads(lines[-1])
r['payload']['decision'] = 'deny'
lines[-1] = json.dumps(r)
open(path, 'w').write('\n'.join(lines) + '\n')
"

npx @veritasacta/verify@0.9.2 --replay-chain ./receipts/receipts.jsonl --key "$PUB"

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, whitespace minimized, strings NFC-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.

Cross-implementation interop

The receipt format has four independent implementations today:

A receipt produced by any of them verifies against @veritasacta/verify. The auditor does not need to trust the operator's tooling choice: the format is the contract.