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OpenSpec Instructions

These instructions are for AI assistants working in this project.

Always open @/openspec/AGENTS.md when the request:

  • Mentions planning or proposals (words like proposal, spec, change, plan)
  • Introduces new capabilities, breaking changes, architecture shifts, or big performance/security work
  • Sounds ambiguous and you need the authoritative spec before coding

Use @/openspec/AGENTS.md to learn:

  • How to create and apply change proposals
  • Spec format and conventions
  • Project structure and guidelines

Keep this managed block so 'openspec update' can refresh the instructions.

CLAUDE.md

This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.

Project Overview

This is an MCP (Model Context Protocol) server that exposes OpenAPI endpoints as MCP tools. It allows LLMs to discover and interact with REST APIs defined by OpenAPI specifications through the MCP protocol.

Essential Development Commands

Building and Testing

npm run build           # Build TypeScript using esbuild
npm test                # Run all tests with Vitest
npm test -- api-client.test.ts   # Run specific test file
npm run test:watch      # Run tests in watch mode
npm run test:coverage   # Generate coverage report

Development Workflow

npm run dev             # Watch mode with auto-rebuild
npm run inspect-watch   # Debug mode with auto-reload (starts MCP inspector)
npm run typecheck       # TypeScript type checking only (no build)
npm run lint            # Run ESLint
npm run lint:fix        # Auto-fix linting issues

Common Pitfall

Never try to run test scripts from command line using node -e - this will always fail. Write a TypeScript test file, build it, and execute it, or use the test runner with npm test.

Architecture & Key Concepts

Core Components

The codebase has a layered architecture:

OpenAPIServer (server.ts)
    ↓ uses
ToolsManager (tools-manager.ts) → manages tool filtering & lookup
    ↓ uses
OpenAPISpecLoader (openapi-loader.ts) → parses specs & creates tools
    ↓ uses
ApiClient (api-client.ts) → executes HTTP requests
    ↓ uses
AuthProvider (auth-provider.ts) → handles authentication

Tool ID System

Tool IDs uniquely identify API endpoints with format: METHOD::pathPart

Critical implementation detail: Uses double underscores (__) to separate path segments:

  • /api/v1/users → GET::api__v1__users
  • /api/resource-name/items → GET::api__resource-name__items
  • Single hyphens within segments are preserved as-is
  • Path parameters have braces removed: /users/{id} → GET::users__---id

Implementation in src/utils/tool-id.ts:

  • generateToolId() - Creates tool IDs from method + path
  • parseToolId() - Parses tool IDs back to method + path
  • sanitizeForToolId() - Ensures safe characters only

Parameter Location Handling

Critical for Issue #50 fix: The system tracks where parameters should be sent using x-parameter-location metadata:

  • path - URL path parameters (e.g., /users/{id})
  • query - Query string parameters (e.g., ?page=1)
  • header - HTTP header parameters (e.g., Authorization: Bearer token)
  • cookie - Cookie parameters

This metadata is set during tool creation in openapi-loader.ts:546 and consumed during request execution in api-client.ts:142-170.

Tools Loading Modes

Three distinct modes controlled by toolsMode:

  1. "all" (default): Load all endpoint-based tools from OpenAPI spec, applying filters
  2. "dynamic": Load only meta-tools (LIST-API-ENDPOINTS, GET-API-ENDPOINT-SCHEMA, INVOKE-API-ENDPOINT)
  3. "explicit": Load only tools specified in includeTools, ignoring all other filters

Tool Name Abbreviation

Tool names must be ≤64 characters with format [a-z0-9-]+. The abbreviation system (in openapi-loader.ts and utils/abbreviations.ts) applies:

  1. Word splitting (camelCase, underscores, numbers)
  2. Common word removal ("controller", "api", "service", etc.)
  3. Standard abbreviations ("management" → "mgmt", "user" → "usr")
  4. Vowel removal for long words
  5. Hash suffix if needed

Disable with disableAbbreviation: true (may cause errors if names exceed 64 chars).

Authentication System

Two approaches:

Static headers (backward compatible):

const config = { headers: { Authorization: "Bearer token" } }
// Creates StaticAuthProvider internally

Dynamic AuthProvider (for token refresh, expiration handling):

interface AuthProvider {
  getAuthHeaders(): Promise<Record<string, string>>  // Called before each request
  handleAuthError(error: AxiosError): Promise<boolean>  // Called on 401/403, return true to retry
}

Key flow: Before each request → getAuthHeaders() → On 401/403 → handleAuthError() → If returns true, retry once with fresh headers.

OpenAPI Processing

Reference resolution: Handles $ref pointers to components (parameters, schemas), detects circular references.

Schema composition: Supports allOf (merges schemas), oneOf/anyOf (preserves composition), not.

Input schema generation: Merges path/query/header/cookie parameters + request body into unified schema. Handles naming conflicts by prefixing body properties with body_.

Parameter inheritance: Path-level parameters are inherited by all operations, operation-level parameters can override.

Testing Strategy

Tests use Vitest with comprehensive coverage across:

  • Unit tests: Individual functions and classes
  • Integration tests: Component interactions
  • Edge case tests: Boundary conditions and errors
  • Regression tests: Prevent known issues (e.g., Issue #33, Issue #50)

Key test files match source files: api-client.test.ts tests api-client.ts, etc.

Each major feature or bug fix should have dedicated test cases with descriptive names referencing the issue number (e.g., "Issue #50: Header Parameter Support").

Filter System

Filters are applied in order with AND logic:

  1. includeTools (highest priority) - If specified, overrides all others
  2. includeOperations - HTTP methods (get, post, etc.)
  3. includeResources - Resource names extracted from paths
  4. includeTags - OpenAPI tags

All filtering is case-insensitive.

Exception: In explicit mode, only includeTools is used, all others are ignored.

Important File Locations

  • Configuration: src/config.ts - Loads from env vars and CLI args
  • CLI entry: src/cli.ts - Command-line interface
  • Transport: src/transport/StreamableHttpServerTransport.ts - HTTP transport with SSE
  • Examples: examples/ directory - Real-world usage patterns
  • Developer docs: docs/developer-guide.md - Comprehensive architecture guide

Common Development Patterns

Adding New Parameter Location Support

When adding support for new parameter locations (like the header fix for Issue #50):

  1. Update openapi-loader.ts to set x-parameter-location metadata
  2. Update api-client.ts to extract and handle the parameter type
  3. Add comprehensive tests covering GET/POST requests, mixed parameters, and auth header merging
  4. Ensure parameters are removed from the main params object after extraction

Adding New Tool Metadata

When adding metadata to ExtendedTool interface:

  1. Define property in openapi-loader.ts interface
  2. Populate during tool creation in parseOpenAPISpec()
  3. Use in filtering logic in tools-manager.ts
  4. Add tests for filter behavior

Working with OpenAPI References

When processing $ref:

  1. Always handle both reference objects and schema objects
  2. Pass components and visited set for cycle detection
  3. Return empty schema for unresolvable references
  4. Use inlineSchema() helper for recursive resolution

Project Structure Notes

  • src/ - TypeScript source files
  • test/ - Test files (mirror src/ structure)
  • dist/ - Built output (created by build process)
  • bin/mcp-server.js - CLI executable entry point
  • build.js - esbuild configuration for bundling
  • examples/ - Complete runnable examples for different use cases