protect-mcp-setup
Configure Cedar policy enforcement and Ed25519 signed receipts for Claude Code tool calls. Use when setting up projects that need cryptographic audit trails, policy-gated tool execution, or compliance-ready evidence of agent actions.
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Install
Works with every agent the skills CLI supports.
npx skills add https://github.com/wshobson/agents --skill protect-mcp-setup- 01
Install
Run the command in your project. The CLI asks which agents to install for.
- 02
Describe the task
Ask your agent for something this skill covers. It loads protect-mcp-setup automatically when relevant.
- 03
Review the result
The agent follows the SKILL.md steps, scripts and references bundled with the skill.
Security audits
Independent scans of the skill's instructions and bundled files.
SKILL.md
The instructions your agent reads when the skill is triggered.
protect-mcp — Policy Enforcement + Signed Receipts
Cryptographic governance for every Claude Code tool call. Each invocation is evaluated against a Cedar policy and produces an Ed25519-signed receipt that anyone can verify offline.
Overview
Claude Code runs powerful tools: Bash, Edit, Write, WebFetch. By default
there is no audit trail, no policy enforcement, and no way to prove what was
decided after the fact. protect-mcp closes all three gaps:
- Cedar policies (AWS's open authorization engine) evaluate every tool call before execution. Cedar deny is authoritative.
- Ed25519 receipts record the name of each tool that ran, signed with your key.
- Offline verification via
npx @veritasacta/verify. No server, no account, no trust in the operator.
Problem
AI agents make decisions that affect money, safety, and rights. The Claude Code session log records what happened, but the log is:
- Mutable — anyone with access can edit it
- Unsigned — there is no way to prove integrity
- Operator-bound — verification requires trusting whoever holds the log
For compliance contexts (finance, healthcare, regulated research), this is not sufficient. You need tamper-evident evidence that can be verified by third parties without trusting you.
Solution
Add protect-mcp to your Claude Code project:
# 1. Install the plugin (adds hooks + skill to your project)
claude plugin install wshobson/agents/protect-mcp
# 2. Create ./protect.cedar (see below). The plugin installs the hooks.
# 3. Create the signing key once (protect-mcp 0.7.4 sign does not create it).
# An existing key is never replaced. See references/receipt-format.md to rotate.
if [ ! -e ./protect-mcp.key ]; then
d=$(mktemp -d) && npx protect-mcp@0.7.4 init --dir "$d" && mv "$d/keys/gateway.json" ./protect-mcp.key
fi
echo "/protect-mcp.key" >> .gitignore
# 4. Use Claude Code normally. Every tool call is now policy-evaluated
# and produces a signed receipt in ./receipts/
Hook Configuration
Installing the plugin adds both hooks from hooks/hooks.json. Each hook runs a
script bundled with the plugin:
{
"hooks": {
"PreToolUse": [
{
"matcher": ".*",
"hooks": [
{ "type": "command", "command": "\"${CLAUDE_PLUGIN_ROOT}\"/hooks/evaluate.sh" }
]
}
],
"PostToolUse": [
{
"matcher": ".*",
"hooks": [
{ "type": "command", "command": "\"${CLAUDE_PLUGIN_ROOT}\"/hooks/sign.sh" }
]
}
]
}
}
Claude Code passes the hook event to the command as JSON on stdin and does not
set TOOL_NAME or TOOL_INPUT variables. evaluate.sh reads tool_name and
tool_input from that payload and passes them to protect-mcp as flags; sign.sh reads
tool_name only, because the 0.7.4 signer records nothing else. Set
PROTECT_MCP_POLICY, PROTECT_MCP_RECEIPTS, and PROTECT_MCP_KEY to change the
default paths. When the policy file is missing, the PreToolUse hook prints a
warning to stderr and allows the call.
What each hook does
PreToolUse — Runs BEFORE the tool executes. Evaluates the tool call against
your Cedar policy file. If Cedar returns deny, the hook exits with code 2 and
Claude Code blocks the tool call entirely.
PostToolUse runs AFTER the tool completes. It signs a receipt that names
the tool and appends it to ./receipts/receipts.jsonl. protect-mcp 0.7.4 does
not record the tool input or output.
Cedar Policy File
Create ./protect.cedar at the project root:
// Read-only tools: one rule can name several tools in `when`. Add WebFetch
// with your own URL rule.
permit (principal, action == Action::"MCP::Tool::call", resource) when {
resource == Tool::"Read" || resource == Tool::"Glob" || resource == Tool::"Grep"
};
// Safe commands only; git limited to read subcommands
permit (principal, action == Action::"MCP::Tool::call", resource == Tool::"Bash") when {
context has input && context.input has command &&
(context.input.command like "git status*" || context.input.command like "git diff*" ||
context.input.command like "git log*" || context.input.command like "git show*" ||
context.input.command like "npm*" || context.input.command like "ls*" ||
context.input.command like "cat*" || context.input.command like "echo*" ||
context.input.command like "pwd*" || context.input.command like "test*")
};
// No chaining (`&` also denies `2>&1`), `$` expansion, redirection (`>` or
// `<`, which covers `<(`), file output (`git diff --output`), or rm -rf
forbid (principal, action == Action::"MCP::Tool::call", resource == Tool::"Bash") when {
context has input && context.input has command &&
(context.input.command like "*;*" || context.input.command like "*&*" ||
context.input.command like "*|*" || context.input.command like "*$*" ||
context.input.command like "*`*" || context.input.command like "*>*" ||
context.input.command like "*<*" || context.input.command like "*\n*" ||
context.input.command like "*--output*" || context.input.command like "*rm -rf*")
};
// Writes only inside the project (paths are absolute), never via `..`
permit (principal, action == Action::"MCP::Tool::call", resource) when {
(resource == Tool::"Write" || resource == Tool::"Edit") &&
context has input && context.input has file_path &&
context.input.file_path like "/path/to/project/*"
};
forbid (principal, action == Action::"MCP::Tool::call", resource) when {
(resource == Tool::"Write" || resource == Tool::"Edit") &&
context has input && context.input has file_path &&
(context.input.file_path like "*/../*" || context.input.file_path like "*/..")
};
String matching is best-effort: like checks the raw string, not a
resolved path, and an npm* permit runs arbitrary code, so it is only as
safe as the project's scripts.
Verification
Verify every receipt against the public key in ./protect-mcp.key:
PUB=$(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 0 = every receipt verified
# Exit 1 = a receipt failed (tampered, wrong key, or malformed line)
# Exit 2 = the file could not be read
The plugin's slash commands do the same inside Claude Code. /verify-receipt
takes one receipt in its own file, e.g., from
tail -n 1 ./receipts/receipts.jsonl > receipt.json.
/verify-receipt receipt.json
/audit-chain --last 20
Receipt Format
Each receipt is one line of ./receipts/receipts.jsonl. See
references/receipt-format.md for a sample.
- Ed25519 signatures (RFC 8032) over all fields but
signature - JCS canonicalization (RFC 8785) before signing
- No public key in the receipt, so pass it with
--key - No link to the previous receipt, so a deleted line goes undetected
Why This Matters
| Before | After |
|---|---|
| "Trust me, the agent only read files" | Cryptographically provable: every Read logged and signed |
| "The log shows it happened" | The receipt proves it happened, and no one can edit it |
| "You'd have to audit our system" | Anyone can verify every receipt offline |
| "Logs might be different by now" | Ed25519 signatures lock the record at signing time |
Standards
- Ed25519 — RFC 8032 (digital signatures)
- JCS — RFC 8785 (deterministic JSON canonicalization)
- Cedar — AWS's open authorization policy language
- IETF draft — draft-farley-acta-signed-receipts
Related
- npm: protect-mcp
- Verify CLI: @veritasacta/verify
- Source: github.com/ScopeBlind/scopeblind-gateway
- Protocol: veritasacta.com
- Integrations: Microsoft Agent Governance Toolkit (PR #667), AWS cedar-policy/cedar-for-agents (PR #64)
FAQ
What does the protect-mcp-setup skill do?
Configure Cedar policy enforcement and Ed25519 signed receipts for Claude Code tool calls. Use when setting up projects that need cryptographic audit trails, policy-gated tool execution, or compliance-ready evidence of agent actions.
How do I install protect-mcp-setup?
Run npx skills add https://github.com/wshobson/agents --skill protect-mcp-setup in your project. The skills CLI detects the agents you use and copies the skill into the right folder. For Claude Code you can also place the skill folder at ~/.claude/skills/protect-mcp-setup/ (personal) or .claude/skills/protect-mcp-setup/ (project).
Which AI agents can use protect-mcp-setup?
Any agent that supports the open Agent Skills format (a folder with a SKILL.md file), including Claude Code, Codex, Cursor, GitHub Copilot, Gemini CLI, Windsurf, Cline, OpenCode and more.
Is protect-mcp-setup safe to install?
Security partners have reviewed it: Gen Agent Trust Hub passed; Socket passed; Snyk passed; ZeroLeaks passed. As with any skill, read the SKILL.md and bundled scripts before installing.
Who maintains protect-mcp-setup?
It is published by wshobson in wshobson/agents. It has 6,039 recorded installs.
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