mcp-lint

Lint MCP server tool schemas for cross-client compatibility.

MCP servers expose tool schemas consumed by Claude, Cursor, Gemini, VS Code Copilot, Windsurf, Cline, OpenAI Agents SDK, and Continue.dev. Each client has different JSON Schema support and quirks — a schema that works in Claude may silently break in Cursor or OpenAI. mcp-lint catches these issues before your users do.

$ npx mcp-lint check tools.json

search-tool
  ✖ Schema node at "inputSchema.properties.query" has `required: false` which is not valid JSON Schema.
    inputSchema.properties.query  [no-required-false]
  ⚠ Parameter "limit" in tool "search-tool" is missing a description.
    inputSchema.properties.limit  [description-exists]

1 error, 1 warning

Installation

# One-time use (no install)
npx mcp-lint check tools.json

# Global
npm install -g mcp-lint

# Per-project dev dependency
npm install --save-dev mcp-lint

Quick Start

# Check a JSON or YAML file
mcp-lint check tools.json

# See compatibility across all clients at a glance
mcp-lint compat tools.json

# Learn what a rule does
mcp-lint explain openai/no-additional-properties

# Auto-fix what can be fixed
mcp-lint fix tools.json --in-place

# Generate a config file
mcp-lint init

`mcp-lint compat` — Compatibility Matrix

See exactly which tools work with which clients:

$ mcp-lint compat tools.json

                    claude    cursor    gemini    vscode    windsurf  cline     openai    continue
search-tool         ✓         ✗         ✓         ✓         ✓         ✓         ✓         ✓
file-reader         ✓         ✓         ✗         ✓         ✓         ✓         ✓         ✗
database-query      ✓         ✓         ✓         ✓         ✓         ✓         ✗         ✓

2 tools have compatibility issues. Run `mcp-lint check` for details.

mcp-lint compat tools.json --format markdown         # for PRs and GitHub Step Summary
mcp-lint compat tools.json --format json             # machine-readable
mcp-lint compat tools.json --client openai           # single client column
mcp-lint compat --server stdio -- node server.js

`mcp-lint explain` — Rule Documentation

$ mcp-lint explain openai/no-additional-properties

Rule: openai/no-additional-properties
Severity:  error
Clients:   openai
Fixable:   yes

Description
OpenAI Agents SDK strict mode requires `additionalProperties: false` on the root inputSchema.

Why
OpenAI's strict function calling mode rejects schemas that don't explicitly set
`additionalProperties: false` at the root level.

Bad example
{
  "type": "object",
  "properties": { "q": { "type": "string" } }
}

Good example
{
  "type": "object",
  "properties": { "q": { "type": "string" } },
  "additionalProperties": false
}

Auto-fix
Add `"additionalProperties": false` to the root inputSchema.

CLI Reference

`mcp-lint check [input]`

mcp-lint check tools.json

# Output formats
mcp-lint check tools.json --format terminal   # default, colored output
mcp-lint check tools.json --format json       # machine-readable (CI/CD)
mcp-lint check tools.json --format markdown   # for PRs and GitHub summaries

# Filters
mcp-lint check tools.json --clients claude,cursor,openai   # only these clients' rules
mcp-lint check tools.json --severity error                 # errors only
mcp-lint check tools.json --quiet                          # same as --severity error
mcp-lint check tools.json --ignore "debug-tool,internal-tool"

# Config
mcp-lint check tools.json --config path/to/.mcplintrc.json
mcp-lint check tools.json --no-color

# Quality score (0–100 per tool with A-F grades)
mcp-lint check tools.json --score

# Watch mode (re-lints on file change)
mcp-lint check tools.json --watch

# Live servers
mcp-lint check --server stdio -- node my-server.js
mcp-lint check --server stdio -- python my_server.py arg1 arg2
mcp-lint check --server sse --url http://localhost:3000/sse

Exit codes:

Code	Meaning
`0`	No errors (warnings don't count)
`1`	One or more errors found
`2`	Invalid input or configuration error

`mcp-lint diff <before> <after>`

Compare lint results between two versions of your schema — useful in CI to catch regressions.

mcp-lint diff tools-v1.json tools-v2.json
mcp-lint diff tools-v1.json tools-v2.json --format markdown >> $GITHUB_STEP_SUMMARY
mcp-lint diff tools-v1.json tools-v2.json --format json

Exit codes: 0 = no new errors introduced, 1 = new errors found, 2 = error

`mcp-lint fix [input]`

Auto-fixes issues that are safe to fix automatically (no semantic changes).

mcp-lint fix tools.json                            # fixed output to stdout
mcp-lint fix tools.json --output fixed-tools.json  # write to file
mcp-lint fix tools.json --in-place                 # overwrite original
mcp-lint fix tools.json --dry-run                  # show what would be fixed
mcp-lint fix tools.json --rules no-required-false,no-empty-enum  # specific rules only

`mcp-lint init`

Creates a default .mcplintrc.json in the current directory.

mcp-lint init          # fails if file already exists
mcp-lint init --force  # overwrite existing

Rules

Universal (all clients)

Rule	Severity	Fixable	Description
`no-required-false`	🔴 error	✅	`required: false` on properties is not valid JSON Schema
`no-content-encoding`	🔴 error	✅	`contentEncoding` is not supported by MCP clients
`no-empty-enum`	🔴 error	✅	Empty `enum: []` makes a parameter impossible to satisfy
`no-recursive-refs`	🔴 error	❌	Circular `$ref` causes crashes in most clients
`valid-json-schema-subset`	🔴 error	❌	Unsupported keywords: `oneOf`, `anyOf`, `allOf`, `if/then/else`, `not`, `patternProperties`…
`description-exists`	🟡 warning	❌	Tools and parameters should have descriptions
`max-depth`	🟡 warning	❌	Schema nesting deeper than 5 levels hurts LLM comprehension
`no-unsupported-formats`	🟡 warning	✅	Obscure `format` values (`iri`, `json-pointer`, `regex`…) not widely supported

Claude-specific

Rule	Severity	Fixable	Description
`claude/no-type-array`	🟡 warning	✅	`"type": ["string", "null"]` array syntax may not work correctly in Claude

Cursor-specific

Rule	Severity	Fixable	Description
`cursor/no-default-without-type`	🔴 error	✅	Cursor requires explicit `type` when a `default` value is present

Gemini-specific

Rule	Severity	Fixable	Description
`gemini/no-optional-without-default`	🟡 warning	❌	Gemini handles optional params better when `default` is explicit
`gemini/no-nested-objects`	🟡 warning	❌	Gemini has limited support for object properties nested more than 2 levels

VS Code Copilot-specific

Rule	Severity	Fixable	Description
`vscode/max-params`	🟡 warning	❌	VS Code Copilot performance degrades with more than 15 parameters

Windsurf-specific

Rule	Severity	Fixable	Description
`windsurf/no-union-types`	🟡 warning	❌	`anyOf` with more than 2 variants not supported

Cline-specific

Rule	Severity	Fixable	Description
`cline/description-max-length`	🟡 warning	✅	Descriptions over 200 characters are truncated by Cline

OpenAI Agents SDK-specific

Rule	Severity	Fixable	Description
`openai/no-additional-properties`	🔴 error	✅	Strict mode requires `additionalProperties: false` on root inputSchema
`openai/strict-types`	🔴 error	❌	Only `string`, `number`, `boolean`, `object`, `array`, `null` allowed

Continue.dev-specific

Rule	Severity	Fixable	Description
`continue/no-default-values`	🟡 warning	❌	Continue.dev ignores `default` fields — document defaults in descriptions instead

Configuration

mcp-lint auto-discovers .mcplintrc.json starting from the current directory up to your home folder. Override with --config.

{
  "rules": {
    "no-required-false": "error",
    "description-exists": "warning",
    "max-depth": "off",
    "claude/no-type-array": "error"
  },
  "clients": ["claude", "cursor", "gemini", "vscode", "windsurf", "cline", "openai", "continue"],
  "ignore": ["internal-debug-tool"],
  "maxDepth": 5
}

Presets

Use "extends" to start from a built-in preset:

{ "extends": "recommended" }

{
  "extends": "strict",
  "rules": {
    "description-exists": "warning"
  }
}

Preset	Description
`recommended`	All 8 clients enabled, default severities
`strict`	All rules set to `error`, `maxDepth: 3`

Rule severities: "error" | "warning" | "info" | "off"

CI/CD — GitHub Actions

Using the official Action

name: MCP Schema Lint
on: [push, pull_request]

jobs:
  lint:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - uses: robert19001-cmyk/mcp-lint@v0.3.0
        with:
          input: tools.json
          severity: warning
          score: 'true'
          fail_on: error

Action inputs:

Input	Default	Description
`input`	(required)	Path to MCP tools JSON file
`format`	`terminal`	`terminal\|json\|markdown`
`severity`	`info`	Minimum severity to report
`clients`	all	Comma-separated client filter
`fail_on`	`error`	Fail CI when issues at this severity found
`score`	`false`	Show quality score

Manual workflow

jobs:
  lint:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - uses: actions/setup-node@v4
        with: { node-version: 20 }

      # Fail the build on errors
      - run: npx mcp-lint check ./src/tools.json

      # Post compatibility matrix to PR summary
      - run: npx mcp-lint compat ./src/tools.json --format markdown >> $GITHUB_STEP_SUMMARY
        if: always()

      # Diff against previous version
      - run: npx mcp-lint diff tools-before.json tools-after.json --format markdown >> $GITHUB_STEP_SUMMARY
        if: always()

Plugin API

Share and reuse rule sets via npm packages.

Using a plugin

npm install mcp-lint-config-nextjs

{
  "plugins": ["mcp-lint-config-nextjs"],
  "extends": "recommended"
}

Writing a plugin

A plugin is an npm package that exports a rules array of Rule objects:

// mcp-lint-config-myserver/index.ts
import type { Rule } from 'mcp-lint';

const myRule: Rule = {
  id: 'myserver/no-large-params',
  severity: 'warning',
  description: 'My server limits param count to 10',
  clients: ['claude', 'cursor'],
  check(tool) {
    const count = Object.keys(tool.inputSchema.properties ?? {}).length;
    if (count > 10) return [{
      toolName: tool.name,
      ruleId: 'myserver/no-large-params',
      severity: 'warning',
      message: `Too many params (${count})`,
      path: 'inputSchema.properties',
    }];
    return [];
  },
};

export const rules = [myRule];

Programmatic API

import { LintEngine } from 'mcp-lint';
import { allRules } from 'mcp-lint/rules';
import { loadFile } from 'mcp-lint/loaders';
import { applyFixes } from 'mcp-lint/fixer';

// Check
const tools = await loadFile('tools.json');
const engine = new LintEngine(allRules, {
  clients: ['claude', 'cursor', 'openai'],
  ignore: ['debug-tool'],
});
const diagnostics = engine.lint(tools);

// Fix
const fixed = applyFixes(tools, diagnostics);

`mcp-lint preflight` — Runtime Decision Firewall

Lint catches bad schemas. Preflight catches bad actions.

At runtime, your agent is about to call a tool — delete a file, send a request, charge a card. Preflight sits between the agent and the action and answers:

Is it allowed?
How risky is it? (0.0–1.0 deterministic score)
Is it reversible?
Does it need human approval?
Is there a safer alternative?

$ mcp-lint preflight action.json --policy preflight.yml

Preflight Decision
────────────────────────────────────────
Decision:      DENY
Risk score:    1.00
Reversibility: irreversible
Policies:      block-destructive-prod
Reasons:       base_shell, destructive_pattern, sensitive_target, irreversible_operation

Action format

{
  "tool_type": "shell",
  "tool_name": "bash",
  "action": "rm -rf /prod/data",
  "target": "/prod/data",
  "context": { "environment": "prod" }
}

Policy file (YAML)

version: 1
defaults:
  approval_threshold: 0.70
  block_threshold: 0.92

rules:
  - id: block-prod-delete
    when:
      tool_type: shell
      action_matches: ["rm -rf"]
      target_matches: ["/prod", "/etc"]
    effect: deny

  - id: rewrite-tmp-delete
    when:
      tool_type: shell
      action_matches: ["rm -rf ./tmp"]
    effect: allow_with_rewrite
    rewrite:
      tool_type: file_delete
      action: move_to_trash
      target: ./tmp

  - id: approval-payments
    when:
      tool_type: payment
    effect: require_approval

Sample policies in examples/policies/: default.yml, strict.yml, permissive-dev.yml.

SDK — embed in your MCP server or agent runtime

import { preflight, loadPolicy } from 'mcp-lint/preflight';

const policy = await loadPolicy('./preflight.yml');

const decision = preflight(
  {
    tool_type: 'shell',
    tool_name: 'bash',
    action: 'rm -rf /tmp',
    target: '/tmp',
  },
  policy,
);

if (decision.decision === 'deny') {
  throw new Error(`Blocked: ${decision.reasons.join(', ')}`);
}
if (decision.decision === 'require_approval') {
  await askUser(decision);
}
if (decision.decision === 'allow_with_rewrite') {
  return executeSafer(decision.safe_alternative);
}

Exit codes

Code	Meaning
`0`	allow / allow_with_rewrite
`1`	require_approval
`2`	deny / error

Use them directly in shell wrappers or CI gates.

License

MIT © Robert

Name		Name	Last commit message	Last commit date
Latest commit History 36 Commits
.github		.github
demo		demo
examples		examples
src		src
tests		tests
vscode-extension		vscode-extension
.gitignore		.gitignore
CHANGELOG.md		CHANGELOG.md
CONTRIBUTING.md		CONTRIBUTING.md
LICENSE		LICENSE
README.md		README.md
SECURITY.md		SECURITY.md
action.yml		action.yml
package-lock.json		package-lock.json
package.json		package.json
tsconfig.json		tsconfig.json
vitest.config.ts		vitest.config.ts

Folders and files

Latest commit

History

Repository files navigation

mcp-lint

Installation

Quick Start

mcp-lint compat — Compatibility Matrix

mcp-lint explain — Rule Documentation

CLI Reference

mcp-lint check [input]

mcp-lint diff <before> <after>

mcp-lint fix [input]

mcp-lint init

Rules

Universal (all clients)

Claude-specific

Cursor-specific

Gemini-specific

VS Code Copilot-specific

Windsurf-specific

Cline-specific

OpenAI Agents SDK-specific

Continue.dev-specific

Configuration

Presets

CI/CD — GitHub Actions

Using the official Action

Manual workflow

Plugin API

Using a plugin

Writing a plugin

Programmatic API

mcp-lint preflight — Runtime Decision Firewall

Action format

Policy file (YAML)

SDK — embed in your MCP server or agent runtime

Exit codes

License

About

Topics

Resources

License

Contributing

Security policy

Uh oh!

Stars

Watchers

Forks

Releases 3

Packages 0

Uh oh!

Contributors

Uh oh!

Languages

`mcp-lint compat` — Compatibility Matrix

`mcp-lint explain` — Rule Documentation

`mcp-lint check [input]`

`mcp-lint diff <before> <after>`

`mcp-lint fix [input]`

`mcp-lint init`

`mcp-lint preflight` — Runtime Decision Firewall

Packages