MCP Server Cards (Experimental Extension)

Status: Experimental. This work is for prototyping and feedback only, and is not an accepted or official MCP extension.

This repository defines a TypeScript source-of-truth and generated JSON Schema for MCP Server Cards — a static metadata document that describes a remote MCP server enough for clients to discover and connect to it before initialization.

It tracks SEP-2127 and is intended to be lifted directly into the main spec when Server Cards graduate (see Graduation plan below).

A prior attempt to land these types directly in the core spec (modelcontextprotocol#2652) is being relaunched here as an experimental extension while the SEP itself is still under review.

What is a Server Card?

A Server Card is a JSON document — hosted at any unreserved URI, with GET <streamable-http-url>/server-card reserved as the recommended location (see discovery.md) — describing:

• The server's identity (name, version, description, optional title / icons / repository / websiteUrl)
• Its remote transport endpoints (URLs, headers, variable templates, supported protocol versions)
• Optional registry-style extension metadata (_meta)

The companion Server document is a strict superset that adds locally-runnable packages — it is the shape the MCP Registry uses for server.json.

Server Cards intentionally omit primitive listings (tools, resources, prompts) — those remain subject to runtime listing via the protocol's standard list operations.

Layout

schema.ts                    # TypeScript source of truth (single file)
schema.json                  # Generated JSON Schema 2020-12 (do not edit by hand)
scripts/
  generate-schema.ts         # Generates schema.json from schema.ts
  validate-examples.ts       # Validates examples/ against the generated schema
examples/
  ServerCard/{valid,invalid} # Example Server Card documents
  Server/{valid,invalid}     # Example Server (registry-shaped) documents

The generated schema.json is checked into the repo so consumers can grab it without running the toolchain.

Working on the schema

npm install
npm run generate         # regenerates schema.json from schema.ts
npm run validate         # runs all examples through ajv against schema.json
npm run check            # asserts schema.json is in sync with schema.ts and tsc passes
npm run format           # prettier write

When you change schema.ts, always run npm run generate and commit the updated schema.json in the same commit. CI runs npm run check and will fail if they drift.

Schema URL conventions

The $schema field on every document MUST be a URL of the form:

https://static.modelcontextprotocol.io/schemas/v1/<name>.schema.json

Schema URLs are versioned by their vN segment rather than by date, so additive revisions of the v1 shape don't bump every published document's $schema. Breaking changes would publish a v2 family.

Graduation plan

When the SEP is accepted and Server Cards graduate from this experimental extension:

1. The contents of schema.ts in this repo move into schema/draft/schema.ts of modelcontextprotocol/modelcontextprotocol. The two MetaObject and Icon definitions inlined here at the bottom of schema.ts already exist in the main spec and are dropped from the migration.
2. The main spec's existing scripts/generate-schemas.ts regenerates schema/draft/schema.json (and downstream docs/specification/draft/schema.mdx) — no per-extension generator is required there.
3. Published documents update their $schema to point at the main spec's hosted schema URL (e.g., https://static.modelcontextprotocol.io/schemas/v1/server-card.schema.json served from modelcontextprotocol/static).
4. This repository is archived with a pointer to the relevant section of schema/draft/schema.ts in the main spec.

The schema.ts in this repo is deliberately structured to be copy-pasted into the main spec's schema/draft/schema.ts with no transformation other than removing the inlined MetaObject / Icon definitions.

References

• SEP-2127: MCP Server Cards (PR)
• SEP-2133: Extensions framework (PR)
• Abandoned core spec PR (superseded by this repo): modelcontextprotocol#2652
• MCP Registry server.json schema