mcp-server-builder

MCP Server Builder

Security Scan

The skill's files and runtime instructions line up with its stated purpose (generating and validating MCP server scaffolds from OpenAPI); it does not request credentials, perform network installs, or include unexpected behaviors.

Purpose & Capability

Name, description, SKILL.md, and the two scripts consistently implement OpenAPI→MCP scaffold generation and manifest validation. No unrelated environment variables, binaries, or cloud credentials are required by the skill itself. The reference server templates do show example env usage (API_BASE, API_TOKEN) for generated code — that is expected for produced scaffolds but is not a requirement of the skill package.

Instruction Scope

The SKILL.md and scripts restrict actions to parsing an OpenAPI spec, producing a tool_manifest.json and a server scaffold, and validating manifests. The scripts read only the specified input (file or stdin) and write outputs to the chosen output directory. There are no instructions to read unrelated system files, probe networks, or exfiltrate data. Note: example templates (references/python-server-template.md) use os.environ to access runtime secrets for the generated server — that is standard for downstream runtime but not used by the generator/validator themselves.

Install Mechanism

There is no install spec and no network/download step; the skill is instruction+script bundle only. All code is included in the skill files. The generator optionally imports PyYAML if parsing YAML input, but PyYAML is not pulled in automatically by this skill (the import is attempted only at runtime and will raise a clear error if YAML is supplied without PyYAML).

Credentials

The skill itself declares no required environment variables or credentials (which is proportional). However, the example server templates demonstrate typical runtime env usage (e.g., API_BASE, API_TOKEN). Users should understand those are for the generated server runtime and are not required to run the generator/validator; they must supply appropriate runtime secrets if they run the scaffolded server.

Persistence & Privilege

The skill does not request persistent presence (always=false) and does not modify other skills or global agent configuration. It writes generated files to an output directory chosen by the user, which is expected behavior for a generator.

Assessment

This skill appears to do what it says: convert OpenAPI specs into MCP tool manifests and starter scaffolds, and validate manifests. Before installing or running: review generated output (tool_manifest.json and scaffold) before deploying or committing it; do not commit API tokens or other secrets that the generated server templates may expect in env vars; if you plan to feed YAML OpenAPI specs, ensure PyYAML is installed where you run the generator; run the validator in --strict mode in CI to catch contract errors; and when deploying a generated MCP server, follow the SKILL.md guidance (host allowlists, redact logs, rate limits) because a single MCP server can increase blast radius if it exposes many tools.

Tier: POWERFUL
Category: Engineering
Domain: AI / API Integration

Overview

Use this skill to design and ship production-ready MCP servers from API contracts instead of hand-written one-off tool wrappers. It focuses on fast scaffolding, schema quality, validation, and safe evolution.

The workflow supports both Python and TypeScript MCP implementations and treats OpenAPI as the source of truth.

Core Capabilities

• Convert OpenAPI paths/operations into MCP tool definitions
• Generate starter server scaffolds (Python or TypeScript)
• Enforce naming, descriptions, and schema consistency
• Validate MCP tool manifests for common production failures
• Apply versioning and backward-compatibility checks
• Separate transport/runtime decisions from tool contract design

When to Use

• You need to expose an internal/external REST API to an LLM agent
• You are replacing brittle browser automation with typed tools
• You want one MCP server shared across teams and assistants
• You need repeatable quality checks before publishing MCP tools
• You want to bootstrap an MCP server from existing OpenAPI specs

Key Workflows

1. OpenAPI to MCP Scaffold

1. Start from a valid OpenAPI spec.
2. Generate tool manifest + starter server code.
3. Review naming and auth strategy.
4. Add endpoint-specific runtime logic.

python3 scripts/openapi_to_mcp.py \
  --input openapi.json \
  --server-name billing-mcp \
  --language python \
  --output-dir ./out \
  --format text

Supports stdin as well:

cat openapi.json | python3 scripts/openapi_to_mcp.py --server-name billing-mcp --language typescript

2. Validate MCP Tool Definitions

Run validator before integration tests:

python3 scripts/mcp_validator.py --input out/tool_manifest.json --strict --format text

Checks include duplicate names, invalid schema shape, missing descriptions, empty required fields, and naming hygiene.

3. Runtime Selection

• Choose Python for fast iteration and data-heavy backends.
• Choose TypeScript for unified JS stacks and tighter frontend/backend contract reuse.
• Keep tool contracts stable even if transport/runtime changes.

4. Auth & Safety Design

• Keep secrets in env, not in tool schemas.
• Prefer explicit allowlists for outbound hosts.
• Return structured errors (code, message, details) for agent recovery.
• Avoid destructive operations without explicit confirmation inputs.

5. Versioning Strategy

• Additive fields only for non-breaking updates.
• Never rename tool names in-place.
• Introduce new tool IDs for breaking behavior changes.
• Maintain changelog of tool contracts per release.

Script Interfaces

• python3 scripts/openapi_to_mcp.py --help
  • Reads OpenAPI from stdin or --input
  • Produces manifest + server scaffold
  • Emits JSON summary or text report
• python3 scripts/mcp_validator.py --help
  • Validates manifests and optional runtime config
  • Returns non-zero exit in strict mode when errors exist

Common Pitfalls

1. Tool names derived directly from raw paths (get__v1__users___id)
2. Missing operation descriptions (agents choose tools poorly)
3. Ambiguous parameter schemas with no required fields
4. Mixing transport errors and domain errors in one opaque message
5. Building tool contracts that expose secret values
6. Breaking clients by changing schema keys without versioning

Best Practices

1. Use operationId as canonical tool name when available.
2. Keep one task intent per tool; avoid mega-tools.
3. Add concise descriptions with action verbs.
4. Validate contracts in CI using strict mode.
5. Keep generated scaffold committed, then customize incrementally.
6. Pair contract changes with changelog entries.

Reference Material

• references/openapi-extraction-guide.md
• references/python-server-template.md
• references/typescript-server-template.md
• references/validation-checklist.md
• README.md

Architecture Decisions

Choose the server approach per constraint:

• Python runtime: faster iteration, data pipelines, backend-heavy teams
• TypeScript runtime: shared types with JS stack, frontend-heavy teams
• Single MCP server: easiest operations, broader blast radius
• Split domain servers: cleaner ownership and safer change boundaries

Contract Quality Gates

Before publishing a manifest:

1. Every tool has clear verb-first name.
2. Every tool description explains intent and expected result.
3. Every required field is explicitly typed.
4. Destructive actions include confirmation parameters.
5. Error payload format is consistent across all tools.
6. Validator returns zero errors in strict mode.

Testing Strategy

• Unit: validate transformation from OpenAPI operation to MCP tool schema.
• Contract: snapshot tool_manifest.json and review diffs in PR.
• Integration: call generated tool handlers against staging API.
• Resilience: simulate 4xx/5xx upstream errors and verify structured responses.

Deployment Practices

• Pin MCP runtime dependencies per environment.
• Roll out server updates behind versioned endpoint/process.
• Keep backward compatibility for one release window minimum.
• Add changelog notes for new/removed/changed tool contracts.

Security Controls

• Keep outbound host allowlist explicit.
• Do not proxy arbitrary URLs from user-provided input.
• Redact secrets and auth headers from logs.
• Rate-limit high-cost tools and add request timeouts.