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

# Run as an MCP server
mcp-lint-server

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 | | cursor/no-missing-title | 🟡 warning | ❌ | Cursor displays tool and parameter titles in the tool picker |

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/[email protected]
        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.

MCP Server

mcp-lint also ships as a stdio MCP server for Claude Desktop, Claude Code, Cursor, VS Code, and registry scanners such as Glama.

npx -y mcp-lint@latest mcp-lint-server

Client config:

{
  "mcpServers": {
    "mcp-lint": {
      "command": "npm",
      "args": ["exec", "--yes", "--package", "mcp-lint@latest", "--", "mcp-lint-server"]
    }
  }
}

Available MCP tools:

| Tool | Purpose | |------|---------| | mcp_lint_list_rules | List built-in rules, clients, severities, and fixability | | mcp_lint_explain_rule | Explain one rule with examples and auto-fix notes | | mcp_lint_check_tools | Lint MCP tool schemas passed as JSON | | mcp_lint_fix_tools | Return safely auto-fixed tool schemas without writing files | | mcp_lint_preflight_action | Score a proposed agent action and return allow/deny/approval/rewrite |

All server tools are read-only from the host perspective: they return diagnostics, fixed JSON, or policy decisions and never execute the action being evaluated.

Docker

docker build -t mcp-lint-server .
docker run --rm -i mcp-lint-server

The repository includes glama.json and a production Dockerfile so MCP directories can build, start, and introspect the server.

License

MIT © Robert