calllint/calllint

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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: 1.7.3 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:

| Detector | Risk symbol | What it flags | |---|---|---| | secretEnvKeys | 🔐 Secrets | Env keys whose names imply credentials (tokens, keys, passwords), incl. docker inline -e KEY | | broadFilesystemPath | 📁 Files | Filesystem roots that grant broad read/write (/, ~, home, drive roots), incl. docker bind-mount host paths | | unknownRemote | 🌐 Network | Remote/HTTP transports to unrecognized or unpinned hosts | | promptPoisoning | 🧠 Prompt | Model-directed instructions hidden in tool names, descriptions, or schemas | | hiddenInstructions | 🧠 Prompt | Hidden/obfuscated content (zero-width, bidi, tag-char, HTML comments) in model-visible metadata | | dangerousCommand | ⚙️ Exec | Shell-out / interpreter / package-runner commands (bash -c, npx, …) | | unverifiedLocalSource | ⚙️ Exec | Local script/binary that is not a recognized package, pinned image, or remote | | externalMutation | ✉️ Action | Tools that send or mutate external state (email, messages, posts) | | messagingSend | ✉️ Action | Tools that send messages/email on your behalf (Slack, Twilio, SMTP, …) | | oauthScope | ✉️ Action | OAuth scopes that are undeclared, broad, or expansive (admin, , repo, …) | | gatewayRuntime | ✉️ Action | Long-running gateway runtimes that proxy many downstream tools under one auth | | financialAction | 💸 Money | Payment / transfer / irreversible financial actions | | unpinnedPackage | 🧩 Supply | Unpinned package specs (@latest, no version) — rug-pull surface |

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.