signet-eval

Deterministic policy enforcement for AI agent tool calls. Every action an agent proposes passes through user-defined rules before execution. No LLM in the authorization path. No prompt injection surface. 25ms end-to-end.

Install

cargo install signet-eval

Quick Start

1. Hook into Claude Code — add to ~/.claude/settings.json:

{
  "hooks": {
    "PreToolUse": [{
      "matcher": "",
      "hooks": [{"type": "command", "command": "signet-eval", "timeout": 2000}]
    }]
  }
}

2. Done. Every tool call now passes through policy evaluation. The default policy blocks destructive operations, protects its own configuration, and allows everything else.

3. (Optional) Customize — talk to Claude with the MCP server:

claude mcp add --scope user --transport stdio signet -- signet-eval serve

Then say: "Add a $50 limit for amazon orders" or "Block all rm commands".

Default Policy

Self-protection rules are locked — they cannot be removed, edited, or reordered by the AI agent, even through the MCP management server. This prevents the agent from disabling its own guardrails.

Action	Decision	Locked
Write/Edit/Bash touching .signet/	deny	yes
Write/Edit/Bash touching signet-eval binary	deny	yes
Write/Edit settings.json / settings.local.json	ask	yes
Bash kill/pkill/killall + signet	deny	yes
Edit/Write/NotebookEdit without recent plan	ask
Edit/Write on core/DSL/schema paths	ask
rm, rmdir	deny
git push --force	ask
mkfs, format, dd if=	deny
curl | sh, wget | sh	deny
Everything else	allow

Custom Policy

signet-eval init       # write default policy to ~/.signet/policy.yaml
signet-eval validate   # check policy for errors
signet-eval rules      # show current rules

Edit ~/.signet/policy.yaml:

version: 1
default_action: ALLOW
rules:
  - name: block_rm
    tool_pattern: ".*"
    conditions: ["contains(parameters, 'rm ')"]
    action: DENY
    reason: "File deletion blocked"

  - name: books_limit
    tool_pattern: ".*purchase.*"
    conditions:
      - "param_eq(category, 'books')"
      - "spend_plus_amount_gt('books', amount, 200)"
    action: DENY
    reason: "Books spending limit ($200) exceeded"

  - name: protect_my_config
    tool_pattern: ".*"
    conditions: ["contains(parameters, '/etc/')"]
    action: ASK
    locked: true
    reason: "System config changes require confirmation"

Rules are evaluated in order — first match wins. Multiple conditions on a rule are AND'd. Rules with locked: true cannot be modified through the MCP management server.

Condition Functions

Function	Description	Example
contains(parameters, 'X')	Tool input contains string	contains(parameters, 'rm ')
any_of(parameters, 'X', 'Y')	Any string present	any_of(parameters, 'mkfs', 'format')
param_eq(field, 'value')	Field equals value	param_eq(category, 'books')
param_ne(field, 'value')	Field not equal	param_ne(role, 'admin')
param_gt(field, N)	Field > number	param_gt(amount, 100)
param_lt(field, N)	Field < number	param_lt(amount, 5)
param_contains(field, 'X')	Field contains substring	param_contains(command, 'sudo')
matches(field, 'regex')	Field matches regex	matches(file_path, '\\.env$')
has_credential('name')	Credential exists in vault	has_credential('cc_visa')
spend_gt('cat', N)	Session spend > limit	spend_gt('books', 200)
spend_plus_amount_gt('cat', field, N)	Spend + this amount > limit	spend_plus_amount_gt('books', amount, 200)
not(condition)	Negate condition	not(param_eq(format, 'json'))
or(A || B)	Either condition	or(contains(parameters, '-f') || contains(parameters, '--force'))
has_recent_action('search', N)	Recent allowed action matches in tool name or detail; pipe-delimited OR	has_recent_action('EnterPlanMode|TaskCreate', 500)
true / false	Literal	true

Encrypted Vault

Three-tier encrypted storage with passphrase-derived key hierarchy (Argon2id + AES-256-GCM):

Tier	Encryption	Contents
1	None	Action log, spending ledger
2	Session key	Session state
3	Compartment key	CC numbers, API tokens, secrets

signet-eval setup                      # create vault with passphrase
signet-eval store cc_visa 4111...      # store Tier 3 credential
signet-eval status                     # vault status and spending
signet-eval log                        # recent action log
signet-eval unlock                     # refresh session after timeout

Credentials support scoped access via request_capability: domain restrictions, purpose constraints, per-use amount caps, and one-time tokens that auto-invalidate after a single use.

Spending limits use the vault ledger — each tool call that spends money is logged, and spend_plus_amount_gt() checks cumulative totals before allowing the next purchase.

Self-Protection

signet-eval ships with four locked rules that prevent an AI agent from disabling its own policy enforcement:

1. protect_signet_dir — Denies any Write, Edit, or Bash command touching .signet/ (policy files, vault, HMAC)
2. protect_signet_binary — Denies tampering with the signet-eval binary itself
3. protect_hook_config — Requires user confirmation before modifying settings.json (where the hook is configured)
4. protect_signet_process — Denies kill/pkill/killall commands targeting signet processes

These rules are:

• Locked — MCP tools refuse to remove, edit, or reorder them
• Position-protected — Unlocked rules cannot be reordered above locked rules (first-match-wins)
• Hardcoded in defaults — If the policy file is corrupted or missing, the binary falls back to hardcoded defaults that include self-protection
• HMAC-backed — Direct file edits break the policy signature, triggering fallback to safe defaults

MCP Management Server

Manage policies conversationally through Claude:

claude mcp add --scope user --transport stdio signet -- signet-eval serve

Tool	Purpose
signet_list_rules	Show all rules with locked status
signet_add_rule	Add a new rule (appended after locked rules)
signet_remove_rule	Remove a rule (refuses on locked rules)
signet_edit_rule	Modify rule properties (refuses on locked rules)
signet_reorder_rule	Move a rule (refuses on locked, prevents placing above locked)
signet_set_limit	Set a spending limit for a category
signet_test	Test a tool call against the current policy
signet_validate	Check policy for errors
signet_condition_help	Show available condition functions
signet_status	Vault status, spending totals, credential count
signet_recent_actions	Show recent action log
signet_store_credential	Store a Tier 3 credential
signet_use_credential	Request a credential through capability constraints
signet_list_credentials	List credential names
signet_delete_credential	Delete a credential
signet_sign_policy	HMAC-sign the policy file
signet_reset_session	Clear spending counters

All mutating operations auto-sign the policy when the vault is available.

MCP Proxy

Wrap upstream MCP servers with policy enforcement. The agent connects to the proxy, never directly to servers. Policy is hot-reloaded on every call.

# Configure upstream servers
cat > ~/.signet/proxy.yaml << 'YAML'
servers:
  linear:
    command: npx
    args: ["-y", "mcp-linear"]
    env:
      LINEAR_API_KEY: "your-key"
YAML

# Register proxy with Claude Code
claude mcp add --scope user --transport stdio signet-proxy -- signet-eval proxy

All Commands

Command	Purpose
signet-eval	Hook evaluation (default, 25ms)
signet-eval init	Write default policy with locked self-protection rules
signet-eval rules	Show current policy rules (locked rules tagged)
signet-eval validate	Check policy for errors
signet-eval test '<json>'	Test a tool call against policy
signet-eval setup	Create encrypted vault
signet-eval unlock	Refresh vault session
signet-eval status	Vault status and spending
signet-eval store <name> <value>	Store Tier 3 credential
signet-eval delete <name>	Delete a credential
signet-eval log	Recent action log
signet-eval reset-session	Clear spending counters
signet-eval sign	HMAC-sign policy file
signet-eval serve	MCP management server (17 tools)
signet-eval proxy	MCP proxy for upstream servers

Performance

Metric	Value
Hook eval (end-to-end)	25ms — process spawn, stdin, JSON parse, policy load, eval, response
In-process policy eval	14–63μs — 14μs deny, 21μs ask, 63μs spending check
CLI validate / rules	8ms
Binary size	6.2MB (stripped, LTO)

Architecture

signet-eval is the enforcement layer of the Signet personal sovereign agent stack. The core principle: the authorization layer must not be an LLM. It processes structured data only — regex, comparisons, and vault queries. No natural language, no context window, no persuasion surface. A rule either matches or it doesn't.

Agent proposes action  ->  signet-eval evaluates policy  ->  allow / deny / ask
                           (deterministic, 25ms, no NLP)

Security Model

signet-eval is a seatbelt, not a cage. Understanding what it does and doesn't protect against is critical.

What it protects against

A cooperative agent making mistakes. Claude Code follows the hook protocol — it calls signet-eval before every tool use and respects the allow/deny/ask response. Within that protocol, signet-eval reliably enforces policy:

• Blocks destructive commands (rm, mkfs, piped remote execution)
• Enforces spending limits across tool calls
• Gates credential access with domain, purpose, and amount constraints
• Requires user confirmation for sensitive operations (git push --force)
• Prevents casual policy tampering via locked rules and MCP guards

This is the normal operating mode. The agent isn't trying to escape — it's trying to be helpful and the policy keeps it in bounds.

What it does NOT protect against

An adversarial agent with shell access. signet-eval runs at the same privilege level as the agent it constrains. Any process with the same UID can:

• Use shell indirection to evade string matching (alias, eval, base64 -d | sh, python3 -c "...")
• Create symlinks to protected paths under innocent names
• Read the session key file and forge HMAC signatures
• Replace the signet-eval binary itself
• Modify the hook configuration in settings.json
• Use LD_PRELOAD, ptrace, or process injection

No amount of pattern matching fixes this. Adding rules for ln invites alias. Blocking alias invites eval. Blocking eval invites python3 -c. This is an infinite regress — the fundamental issue is privilege level, not pattern coverage.

Where real containment lives

Protecting against a truly adversarial agent requires OS-level controls that operate above the agent's privilege level:

• Separate user account — run the agent as a restricted user with no write access to policy files
• Container/sandbox — Docker, Flatpak, or bubblewrap with a read-only mount for ~/.signet/
• Stripped $PATH — remove ln, alias, and other indirection tools from the agent's environment
• seccomp / AppArmor / SELinux — mandatory access controls that the agent cannot override
• Virtual environment — run Claude Code in a constrained environment with limited filesystem access

signet-eval is the policy layer within such a setup. It handles the "what should this agent be allowed to do" question with clear, auditable rules. The OS handles the "can this agent circumvent the policy" question. Neither replaces the other.

Defense in depth

The layers work together:

Layer	Protects against	Mechanism
String matching	Obvious mistakes, clear UX	Regex, substring, word-boundary conditions
Locked rules	Casual MCP-based policy tampering	Immutable rules, position protection
HMAC signing	Out-of-band file modification	Cryptographic integrity verification
OS controls	Privilege escalation, shell indirection	Sandboxing, RBAC, separate users

Without OS controls, signet-eval is a speed bump, not a wall. With them, it's the policy engine inside a secure perimeter.

License

MIT