Signalint

Signalint is a local MCP server for JavaScript and TypeScript diagnostics. It runs Oxlint, TypeScript, and optionally Biome; caches unchanged checks; clusters repeated issues; and warns when the same diagnostic disappears and repeatedly returns. Loop history is restored from valid .signalint/session.jsonl entries when the MCP server restarts; malformed or crash-truncated lines are skipped.

Listed on:

• 
• mcpservers.org
• 
• Official MCP Registry (API listing)

Diagnostic compression example

When a coding agent requests diagnostics on a project, raw compiler and linter outputs quickly flood the context window with repetitive errors across multiple files. Signalint normalizes issues and clusters them by root cause before returning a bounded, priority-ranked response:

Raw diagnostics (40 issues across 10 files · 9,151 bytes)

[
  {
    "issueId": "ts-01",
    "file": "src/file01.ts",
    "line": 10,
    "col": 5,
    "engine": "tsc",
    "rule": "TS2322",
    "severity": "error",
    "message": "Type 'string' is not assignable to type 'number' in fixture assignment 01.",
    "fixable": false
  },
  // ... 39 more raw normalized issues
]

Clustered response returned to agent (4 clusters · 1,233 bytes · 86.5% reduction)

{
  "schemaVersion": "1.1",
  "status": "issues_found",
  "engines": {
    "oxlint": { "status": "ok" },
    "tsc": { "status": "ok" },
    "biome": { "status": "disabled" }
  },
  "totalIssues": 40,
  "clusters": [
    {
      "clusterId": "c1",
      "rootCauseSummary": "10 TS2322 issues across 10 files",
      "ruleIds": ["TS2322"],
      "issueCount": 10,
      "fileCount": 10,
      "priority": 1,
      "suggestedAction": "Review the shared cause of TS2322 across 10 files",
      "sampleIssueIds": ["ts-01", "ts-02"]
    },
    {
      "clusterId": "c2",
      "rootCauseSummary": "10 no-unused-vars issues across 10 files",
      "ruleIds": ["no-unused-vars"],
      "issueCount": 10,
      "fileCount": 10,
      "priority": 2,
      "suggestedAction": "Review the shared cause of no-unused-vars across 10 files",
      "sampleIssueIds": ["unused-01", "unused-02"]
    },
    {
      "clusterId": "c3",
      "rootCauseSummary": "10 eqeqeq issues across 10 files",
      "ruleIds": ["eqeqeq"],
      "issueCount": 10,
      "fileCount": 10,
      "priority": 5,
      "suggestedAction": "Apply structured fixes for eqeqeq across 10 files",
      "sampleIssueIds": ["eqeqeq-01", "eqeqeq-02"]
    },
    {
      "clusterId": "c4",
      "rootCauseSummary": "10 prefer-const issues across 10 files",
      "ruleIds": ["prefer-const"],
      "issueCount": 10,
      "fileCount": 10,
      "priority": 5,
      "suggestedAction": "Apply structured fixes for prefer-const across 10 files",
      "sampleIssueIds": ["const-01", "const-02"]
    }
  ],
  "truncated": false,
  "loopWarning": null
}

The agent receives a concise summary with priority-ordered clusters and sample issue IDs. When deeper detail is needed for a specific cluster or issue, the agent calls get_issue_detail without re-running the whole-project scan.

Requirements

• Node.js 20.19 or later in the Node 20 line, or Node.js 22.12 or later
• A JavaScript or TypeScript project; TypeScript checks require a tsconfig.json
• pnpm 11.9.0 for source development

Install

Install Signalint in the project it should check:

npm install --save-dev signalint-mcp

Run the setup command from that project root. It detects TypeScript, Oxlint, and Biome configuration, writes signalint.config.json, and offers to update a nearby Claude Code, Cursor, Codex CLI, or Antigravity MCP configuration:

npx signalint-mcp init

If no MCP client can be selected safely, the command prints exact configuration snippets to copy. TypeScript is enabled only when a root tsconfig.json exists; Biome is enabled when its config exists; Oxlint is the fallback when no configured linter is detected. To configure Signalint manually, create signalint.config.json:

{
  "engines": {
    "oxlint": true,
    "tsc": true,
    "biome": false
  },
  "ignore": ["node_modules/**", "dist/**", ".signalint/**"],
  "timeoutsMs": {
    "oxlint": 30000,
    "tsc": 120000,
    "biome": 30000
  }
}

Claude Code setup

Run this from the checked project. Project scope writes a shareable .mcp.json:

claude mcp add --scope project signalint -- npx --no-install signalint-mcp
claude mcp get signalint

On native Windows, wrap npx as required by Claude Code:

claude mcp add --scope project signalint -- cmd /c npx --no-install signalint-mcp
claude mcp get signalint

Restart Claude Code if it was already open. Ask it to call Signalint's ping tool, then call check_project with { "paths": ["."] }.

See the Claude Code MCP documentation for scope and troubleshooting details.

Cursor setup

Create .cursor/mcp.json in the checked project:

{
  "mcpServers": {
    "signalint": {
      "command": "npx",
      "args": ["--no-install", "signalint-mcp"]
    }
  }
}

On native Windows use "command": "cmd" and "args": ["/c", "npx", "--no-install", "signalint-mcp"]. Open Cursor's MCP settings, enable signalint, and call ping followed by check_project.

See the Cursor MCP documentation for configuration locations and status controls.

Codex CLI setup

The ChatGPT desktop app, Codex CLI, and IDE extension share a single configuration file. The quick-add command writes to ~/.codex/config.toml (global) automatically:

codex mcp add signalint -- npx --no-install signalint-mcp

For project-scoped configuration (trusted projects only), add to .codex/config.toml in the project root:

[mcp_servers.signalint]
command = "npx"
args = ["--no-install", "signalint-mcp"]

On native Windows, use cmd and pass npx as an argument:

[mcp_servers.signalint]
command = "cmd"
args = ["/c", "npx", "--no-install", "signalint-mcp"]

See the Codex MCP documentation for all configuration options including cwd, env, and per-tool approval settings.

Setting up with Antigravity

Antigravity uses its own MCP configuration file. The path that has been verified through dogfooding on Windows is: %USERPROFILE%\.gemini\antigravity\mcp_config.json.

The init command can update this file after confirmation. The equivalent Windows configuration is:

{
  "mcpServers": {
    "signalint": {
      "command": "cmd",
      "args": ["/c", "npx", "--no-install", "signalint-mcp"],
      "cwd": "<absolute-path-to-your-project>"
    }
  }
}

On macOS or Linux, use "command": "npx" and "args": ["--no-install", "signalint-mcp"]. Restart or reconnect Antigravity after updating the configuration.

Note on Antigravity product variants: Antigravity has split into separate products (IDE, CLI, SDK). Each variant may use a different config path — the IDE path above is the one confirmed working; other variants may use ~/.gemini/config/mcp_config.json or a project-scoped .agents/mcp_config.json. See antigravity.google/docs/mcp for the authoritative list per product.

Windows troubleshooting

Windows .cmd shims created by npm link can expose a junction path to Node. If signalint-mcp ends with an initialize/EOF error or signalint stats exits with code 0 but prints nothing, bypass the shim with the compiled entrypoint paths:

node C:\absolute\path\to\Signalint\dist\src\index.js
node C:\absolute\path\to\Signalint\dist\src\cli.js stats

Current builds canonicalize linked paths before deciding whether to start, but direct Node invocation remains the reliable fallback for older builds or unusual npm setups.

Configuration

engines.oxlint, engines.tsc, and engines.biome are booleans. Defaults are Oxlint and tsc enabled, Biome disabled. Omitted engine keys retain those defaults. Unknown keys and incorrectly typed values fail with a configuration error.

ignore is an array of project-relative globs. Signalint supports *, **, and ?, normalizes Windows separators, and excludes matching requested paths and diagnostics. Because tsc is a whole-program engine, it still receives the complete tsconfig.json program when invoked; ignored TypeScript paths do not trigger an incremental check_files run and their diagnostics are removed from the response.

Engine-native configuration remains in native files. The v1 cache hash recognizes root .oxlintrc, .oxlintrc.json, oxlint.json, tsconfig.json, biome.json, and biome.jsonc. Changing one invalidates the related engine cache. Other valid sources—including .oxlintrc.jsonc, extended configs, and nested package configs— are not part of v1 cache hashing; clear .signalint/ after changing one of them.

timeoutsMs sets positive-integer subprocess deadlines in milliseconds. Defaults are 30 seconds for Oxlint, 120 seconds for tsc, and 30 seconds for Biome. A timed-out engine and its child processes are terminated. In the schema 1.1 check response, that engine has { "status": "error", "message": "tsc did not complete within 120s" } under engines, while completed engines' diagnostics are preserved.

Known Limitations

• Signalint supports JavaScript and TypeScript projects only.
• The built-in engines are Oxlint, TypeScript, and Biome; v1 does not support arbitrary custom engines.
• Signalint reports whether an issue has a structured fix, but v1 does not apply fixes.
• Signalint is not a SAST or security scanner.
• There is no IDE extension yet; integrations use MCP or the command-line client.
• Loop detection is deliberately limited to lint, type, and test issue signatures; it does not detect general agent-conversation loops.
• The tsc adapter requires one tsconfig.json at the project root. Monorepos must provide a solution-style root config using TypeScript Project References; Signalint does not auto-discover independent package configs.
• check_files treats only the files explicitly passed to that call as relevant to TypeScript cache invalidation. If file A changes but is omitted while unchanged file B is checked, and B depends on A, Signalint can reuse a stale tsc result. Include every changed dependency file or run check_project; dependency-graph-based invalidation is not implemented in v1.

MCP tools

• ping checks that the local server is connected and returns pong.
• check_project accepts optional { "paths": ["."] } and returns clustered diagnostics.
• check_files accepts { "files": ["src/file.ts"] } and uses incremental caching.
• get_issue_detail accepts exactly one clusterId or issueId from the latest successful check and returns its full issues, or a status: "stale" response.
• get_loop_status returns issue signatures currently flagged as oscillating.

Cache and session artifacts are written under .signalint/ and should not be committed.

CLI and package smoke test

Run the same project check without an MCP client:

npx --no-install signalint check .

After MCP checks have accumulated in .signalint/session.jsonl, print the Phase 6 measurement summary:

npx --no-install signalint stats

The report includes average normalized-raw-to-clustered JSON payload reduction, engine-file cache hit rate, average and maximum check latency, and the number of distinct issue signatures that triggered loop warnings. An engine-file lookup counts each enabled engine separately, so one changed TypeScript file can miss once for Oxlint and once for tsc. Latency covers handler work from MCP tool entry through engine/cache work, clustering, and loop evaluation; it excludes the telemetry append and stdio transport. Statistics include the active session log and its rotated .1 backup, with their retained overlap counted once. Clean checks with zero raw payload are excluded from the reduction average, and older checks with missing metrics remain counted without contributing to the unavailable aggregate.

The CLI exits with code 1 when issues are found. Two flags support CI use: --format github prints one GitHub Actions annotation (::error file=...,line=...,col=...::message or ::warning ...) per issue instead of JSON, and --fail-on-priority <N> exits non-zero only if a cluster's priority is at or below N instead of on any issue found.

To exercise an actual MCP check_project call against the installed package, run:

node node_modules/signalint-mcp/examples/check-project.mjs .

GitHub Actions

action.yml at the repository root wraps signalint check as a composite action for CI. It installs Node, installs signalint-mcp from npm, and runs the check with --format github so issues appear as inline annotations on the pull request diff:

- uses: TranQui004/signalint@main
  with:
    fail-on-priority: "3"

fail-on-priority defaults to 5, which fails the job on any issue found, matching signalint check's default behavior without the flag. Lower values only fail the job when a cluster is at least that urgent: priority 1 is an error with no structured fix, and priority increases toward 5 as issues become more fixable or more systemic (see scorePriority in src/cluster/clusterEngine.ts).

Development

pnpm 11.9.0 is the canonical package manager for source development. The repository commits pnpm-lock.yaml, declares pnpm in package.json, and uses pnpm in CI.

pnpm install --frozen-lockfile
pnpm lint
pnpm typecheck
pnpm test
pnpm build

If a global npm shim cannot find npm-cli.js, build directly with node node_modules/typescript/bin/tsc -p tsconfig.json.

Before preparing a release, use npm pack --dry-run and verify the packed tarball in a clean project. Publishing requires explicit release approval.

Security

See SECURITY.md for the current npm audit advisory, its evaluated runtime reachability, and the conditions that require reassessment.

Documentation

• Website — overview, docs, and live examples.
• ARCHITECTURE.md — how the layers fit together and what each module does.
• CONTRIBUTING.md — development setup, verification, and pull requests.
• AGENTS.md — coding standards for this repository.
• SECURITY.md — threat model, trust boundaries, and audit status.
• CHANGELOG.md — notable changes by release.
• docs/history/ — original build plan and pre-launch audit trail.

License

Signalint is available under the MIT License.