CallLint

Your agent can run tools faster than you can review them.

CallLint is a pre-flight risk linter for MCP and agent-tool configs. It checks the blast radius before the tool runs: what each tool can read, write, execute, connect to, send, or mutate — then returns an evidence-backed verdict (SAFE / REVIEW / BLOCK / UNKNOWN) before your agent ever loads the server.

It never executes, installs, or connects to the servers it judges.

npx calllint scan .cursor/mcp.json

Status: 1.9.1 stable CLI release. Actively hardened. Verdicts are heuristic decision support, not a safety guarantee. Read Limitations before relying on a verdict for a security decision.

$ npx calllint scan .cursor/mcp.json
result: BLOCK   (BLOCK 1 · UNKNOWN 0 · REVIEW 0 · SAFE 0)

BLOCK  helpful-notes    PROMPT · SUPPLY
  • [BLOCKER] Model-directed instruction in tool metadata
      evidence: tools.save_note.description = "do not tell the user"
  • Package version is not pinned
      evidence: package = helpful-notes@latest
  autonomous use: deny · manual approval: required

What is CallLint?

An agent's power is the union of its tools' permissions. A single MCP server can add filesystem write, shell execution, network egress, or model-directed instructions to an autonomous agent — usually described only by untrusted, tool-provided metadata. CallLint inspects that surface statically and tells you, with evidence, what you would be granting before you grant it.

• Deterministic — same input, same verdict. No model in the decision path.
• Offline by default — no network unless you pass --online (advisory only).
• Evidence-backed — every finding cites the exact config field it came from.
• Never executes the target — it reasons about configuration, not behavior.

What it checks

CallLint runs thirteen static detectors over each server entry:

Findings roll up into a risk class (S0 metadata-only → S5 financial/irreversible) and an aggregate verdict per server and per config.

Drift detection (baseline / verify) records an approved risk surface and flags rug-pulls (🔁) — a previously-approved server whose risk surface later changed.

What it does not check

This list matters more than the feature list. CallLint is a pre-flight check, not a proof of safety.

• It does not execute, install, or connect to servers — so it cannot observe actual runtime behavior (what a server really reads, writes, or sends).
• It does not read or validate secret values — it inspects config shape (key names), never the contents of your .env or credential stores.
• It does not analyze server source code — only the configuration and any tool metadata you provide under x-calllint.tools.
• It does not fetch anything unless you pass --online, and online results are advisory — they never upgrade a verdict toward SAFE.
• It does not certify third-party tools, replace human security review, or guarantee an agent is safe.
• A clean run is necessary, not sufficient. Pair it with code review, least-privilege tokens, and runtime controls.

UNKNOWN is a real verdict: when CallLint cannot verify what a server will do, it says so and never silently upgrades UNKNOWN to SAFE.

What CallLint is — and is not

Install

# run without installing (recommended):
npx calllint scan ./mcp.json

# or install globally:
npm install -g calllint

Requires Node.js ≥ 20. The published package is a single self-contained bundle with zero runtime dependencies. calllint on the latest tag is the current stable CLI release; @next carries release candidates and @preview older previews.

Quick start

Zero-config scanning — discover and scan all your agent configs:

# Auto-discover and scan all agents (Cursor, Claude Code, Claude Desktop, VS Code, Windsurf)
calllint scan --auto

# List all discovered agent configs
calllint inventory

# Scan a specific agent type
calllint scan --agent cursor
calllint scan --agent vscode

Manual path scanning — scan a specific config file:

# scan a config file (auto-detects common locations if no path given)
calllint scan ./mcp.json

# scan from stdin, machine-readable JSON out
cat .cursor/mcp.json | calllint scan --stdin --json

# CI gate: non-zero exit per policy (BLOCK=30, UNKNOWN=20, REVIEW=10 if enabled)
calllint scan ./mcp.json --ci --no-emoji

# synthesize a config for an npm package (offline) or a GitHub repo (--online)
calllint scan npm:mcp-weather@1.0.0
calllint scan github:owner/repo --online

# record an approved baseline, then detect drift / rug-pulls later
calllint baseline ./mcp.json
calllint verify ./mcp.json --ci

# explain one server's verdict from the last scan
calllint explain filesystem

# structured diagnostics for editor / agent-host integration
calllint diagnostics ./mcp.json --json

Output formats: default terminal, --compact, --json (stable schema), --sarif (GitHub Code Scanning), --markdown (PR comments / GitHub Step Summary), --html (self-contained report). The diagnostics command emits a separate editor/agent-host JSON (calllint.diagnostics.v0).

See CallLint running in CI on a deliberately risky config — calllint-demo-risky-mcp publishes one Code Scanning alert per finding on every push.

Beyond config scanning

The same engine and verdict semantics extend past MCP-config scanning to other points where an agent grants authority:

# Preflight a planned external action before the agent runs it
calllint action inspect payment.json          # calllint.action.v0 descriptor
calllint action inspect email-reply.json --json

# Preflight a normalized agent inbox event (delegates to the action analyzer)
calllint inbox inspect gmail-reply.normalized.json

# Record a scan as a local, verifiable receipt, then validate it later
calllint scan ./mcp.json --receipt            # writes calllint-receipt.json
calllint receipt verify calllint-receipt.json

# Attach an external content-scanner report as evidence (joint Trust Packet)
calllint scan ./mcp.json --evidence skillspector-report.json

Receipts (calllint.receipt.v0) are a reporting layer derived from a scan — they prove which CallLint version produced which verdict over which input under which policy. They are not a second scanner and never re-judge a verdict. A receipt can carry an optional ed25519 signature; receipt keygen / receipt sign generate and sign one locally for development, and receipt verify checks the signature when present (offline, with --public-key). A signature proves provenance and integrity — never safety.

Trust Gateway — prepare, approve, apply, verify

Scanning tells you the blast radius; the Trust Gateway acts on it, safely. It resolves an agent-tool target to an immutable, digest-pinned identity, judges it deterministically, and emits a reversible install plan. Applying that plan is the only thing that ever writes live config: it re-validates every digest, writes atomically, verifies the result, and rolls back on failure. The gateway never executes, installs, or connects to the target it judges.

# read-only: resolve + judge a target and emit a reversible plan (touches no live config)
calllint trust prepare github:owner/repo --host claude-code --write-plan
calllint trust show    .calllint/plans/<plan-id>.json
calllint trust explain .calllint/plans/<plan-id>.json

# the only writer of live config — applies an approved plan, atomically and reversibly
calllint trust apply --plan .calllint/plans/<plan-id>.json --approve <plan-digest> --receipt