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The Skilled OpenAPI plugin turns an existing REST API into a skilled MCP server without rewriting any controllers. A signed skill bundle (an OpenAPI spec plus an OpenAPI Overlay annotated with skill grouping) is consumed at runtime by the plugin, projected into the FrontMCP SkillRegistry, and exposed through three meta-tools — search_skill, load_skill, execute_action. The per-operation tools stay hidden from tools/list so the LLM never sees a 150-endpoint dump.

Skills, not flat tools

The MCP client sees curated skills. Per-operation tools are hidden behind the skill abstraction.

Signed, hot-swappable bundles

OPA-style JWT-of-hashes signing on every bundle. Atomic apply with rollback. Skills update without a server restart.

Standards-aligned wire format

Bundle is an OpenAPI Overlay (OAI 1.0/1.1) carrying skill annotations. Anticipates SEP-2076 (Skills as MCP primitive).

Five-gate authorization stack

Bundle signature → RFC 8707 audience check → per-skill ABAC → credential allowlist → outbound SSRF + IP blocklist.

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The problem this solves

Wrap your REST API with a flat OpenAPI→MCP generator and you hit a familiar wall:
  • The agent sees 47 (or 470) tool definitions and can’t pick the right one. Claude reliability degrades past ~20 tools; OpenAI GPT Actions caps at 30; Cursor caps at 40.
  • Token cost explodes on every tools/list even when the agent uses two endpoints.
  • Credentials end up in tool input schemas — every MCP client sees them.
  • Schema changes ship through a server redeploy, not your CI pipeline.
The Skilled OpenAPI plugin pivots the unit of discovery from “endpoint” to “skill” — a curated bundle of operations the LLM treats as one named capability — and bakes the security gates the OpenAPI security post describes into the runtime path.

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How it works

The MCP client only ever sees the three meta-tools plus whatever skills the active bundle declares. Per-operation tools are reachable only through execute_action, which evaluates the action’s requiredAuthorities against the caller’s authInfo before the executor builds the outbound request.

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When to use this plugin (and when not to)

ScenarioUse this pluginUse the OpenAPI adapter instead
5–20 hand-curated endpoints, MCP-native server✅
50+ endpoints, multi-service, customer-facing✅
Schema changes ship through CI, not redeploys✅
You need every operation visible in tools/list✅
You need ABAC per operation + signed-bundle origin trust✅
The two are designed to coexist in the same FrontMCP server. See Coexistence for the integration story.

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What ships in v1.2.0

  • Three bundle sources: static (filesystem with optional fs.watch), npm (dynamic import of a published bundle package), saas-pull (HTTPS GET against a configured endpoint with JWT, interval polling, last-good cache fallback).
  • OpenAPI Overlay + custom JSON wire formats, both validated by Zod and cross-referenced for service / authBinding / operationId integrity.
  • Bundle signing via RS256 / Ed25519 JWT-of-hashes, modeled on OPA’s signed-bundle pattern.
  • OpenAPI runtime executor that reuses @frontmcp/adapters/openapi’s buildRequest / parseResponse — not a re-implementation.
  • Layered SSRF defenses: scheme allowlist, host allowlist, post-DNS IPv4/IPv6 blocklist (RFC 1918, link-local incl. AWS/GCP/Azure metadata 169.254.169.254, loopback, ULA), cloud-metadata hostname blocklist.
  • ABAC integration via @frontmcp/auth’s AuthoritiesEngine (RBAC + ABAC + custom evaluators).
  • notifications/skills/list_changed broadcast on every bundle swap, with bundleVersion on every skill so polling clients can detect changes regardless of client list_changed support.
Deferred to v1.2.x: Sigstore Cosign keyless verification, GitHub artifact attestations for npm-source, the webhook push channel, content-sanitizer hooks. See Security for the full list.

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Next steps

Quick Start

Install, point the plugin at a fixture bundle, and call execute_action end-to-end in 5 minutes.

Bundle Format

The wire format the plugin consumes — services, authBindings, skills, operations, signature envelope.

Security

Bundle signing, RFC 8707, SSRF defenses, OWASP MCP Top 10 mapping.

API Reference

Exported types, classes, DI tokens.