Validates the MCP Server Card advertised to AI clients
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The auditor discovers Server Cards from the audited URL's origin and then validates every card it finds:
AI Catalog as a link source — fetches /.well-known/ai-catalog.json and follows entries whose type is application/mcp-server-card+json, using their url (fetched with Accept: application/mcp-server-card+json) or inline data. The catalog itself is not audited: a missing, unreachable, or malformed catalog produces no findings, and broken entry links are only recorded as attempts.
Reserved-location fallback — when the catalog yields no card, tries /.well-known/mcp/server-card.json and then /mcp/server-card, stopping at the first HTTP 200.
Document validation — each card is checked against the Server Card v1 schema rules: $schema, name, version, description, and remotes.
Hosting validation — cards fetched over HTTP are additionally checked for media type, CORS, and HTTPS. Inline cards embedded in a catalog entry have no HTTP response of their own, so hosting rules are skipped for them.
Specification pin — every run emits 37A-McpServerCardAuditor-100 (INFORMATION) naming MCP Server Card v1 (27 Jun 2026) and schema.json commit 92999cb3f5d82e648385cb92ce41ed9aa1386eda. That check does not change overall status.
Any failing rule makes the audit FAIL; otherwise a wrong media type produces a WARNING; otherwise the audit is a SUCCESS.
Every row maps to one checkRule* method and one CheckCode. Hosting rules (412, 413, 300) are skipped for inline cards embedded in an AI Catalog entry. 37A-McpServerCardAuditor-100 is emitted on every run, including missing-card and invalid-URL failures. CheckCode suffixes use status ranges: 100 information, 200 success, 300–399 warning, 400–499 fail.
| CheckCode | Status | When it fires | Recommendation | Why it matters |
|---|---|---|---|---|
37A-McpServerCardAuditor-100 | INFORMATION | Every run. Records that this auditor implements MCP Server Card v1 and the pinned server-card.schema.json commit. | This auditor implements MCP Server Card v1; documents must set $schema to https://static.modelcontextprotocol.io/schemas/v1/server-card.schema.json. Schema pin 92999cb3f5d82e648385cb92ce41ed9aa1386eda. | — |
37A-McpServerCardAuditor-400 | FAIL | No Server Card could be obtained from the AI Catalog entries or from either reserved location. | Publish a Server Card at {origin}/mcp/server-card or {origin}/.well-known/mcp/server-card.json, or list its URL in {origin}/.well-known/ai-catalog.json. | Without a Server Card, AI clients and agents cannot discover the MCP server or learn how to connect to it, so the integration stays invisible to the tools that would use it. |
37A-McpServerCardAuditor-401 | FAIL | The document served at the card URL cannot be parsed as a JSON object. | Serve a JSON Server Card at {cardUrl}. | A card that does not parse is useless to every client: AI tools skip the server as if no card had been published. |
37A-McpServerCardAuditor-402 | FAIL | The card has no $schema field. | Set $schema to https://static.modelcontextprotocol.io/schemas/v1/server-card.schema.json on {nameOrUrl}. | Without the $schema field, clients cannot tell which Server Card format the document follows and strict validators reject it, so the server goes undiscovered. |
37A-McpServerCardAuditor-403 | FAIL | $schema is present but is not the v1 Server Card schema URL (for example a date-versioned or registry schema URL). | Replace {observed} with the v1 server-card.schema.json URL. | Clients that validate against the v1 schema reject a card that declares another schema, so the server is skipped as if the card were invalid. |
37A-McpServerCardAuditor-404 | FAIL | The card has no name field. | Set name in reverse-DNS form (e.g. com.example/weather) on the card at {cardUrl}. | The name is the server's identity in registries and client lists; without it, clients cannot identify or list the server and validators reject the card. |
37A-McpServerCardAuditor-405 | FAIL | name does not match the reverse-DNS namespace/name format with exactly one slash. | Change {name} to namespace/name, e.g. com.example/weather. | Registries and clients key on the reverse-DNS name; a name in another shape is rejected by validators and can collide with or be confused for another server. |
37A-McpServerCardAuditor-406 | FAIL | The card has no version field. | Set version on {nameOrUrl} (e.g. 1.0.0); do not use ranges. | Without a version, clients cannot tell which release of the server they are talking to or detect updates, and validators reject the card. |
37A-McpServerCardAuditor-407 | FAIL | version expresses a range (^1.2.3, ~1.2.3, >=1.2.3, 1.x, 1.*) instead of a single version. | Replace {version} on {nameOrUrl} with a single version such as 1.0.0. | A range does not identify a release, so clients cannot pin or compare versions, and schema-strict clients reject the card. |
37A-McpServerCardAuditor-408 | FAIL | The card has no description field (or it is blank). | Add a 1-100 character description to {nameOrUrl}. | The description is what AI clients and catalogs show people choosing a server; without it the server looks empty in listings and validators reject the card. |
37A-McpServerCardAuditor-409 | FAIL | description is longer than 100 characters. | Shorten the description of {nameOrUrl} from {length} to at most 100 characters. | Catalogs and clients cut off descriptions over the limit, so the text people see is truncated, and schema-strict clients reject the card outright. |
37A-McpServerCardAuditor-410 | FAIL | A remotes[] entry has a type other than streamable-http or sse. | Set remotes[{i}].type on {nameOrUrl} to streamable-http or sse instead of {observed}. | Clients only know how to connect over streamable-http and sse; any other transport type is rejected, so the server cannot be reached through the card. |
37A-McpServerCardAuditor-411 | FAIL | A remotes[] entry has a url that does not start with http://, https://, or a {template-variable} placeholder. | Change remotes[{i}].url on {nameOrUrl} from {observed} to an http(s) URL or a {template} URL. | Clients cannot connect to an endpoint that is not an http(s) or templated URL, so the server is unreachable through the card even when everything else is valid. |
37A-McpServerCardAuditor-412 | FAIL | The HTTP response serving the card has no Access-Control-Allow-Origin header, blocking browser-based clients. | Add Access-Control-Allow-Origin: * on {cardUrl}. | Browser-based AI clients cannot read a card without CORS headers, so the server is invisible to web agents and any tool that runs in a browser. |
37A-McpServerCardAuditor-413 | FAIL | The card URL uses the http scheme in production (localhost is exempt). | Serve {cardUrl} over https instead of http. | A card served over plain HTTP can be read or altered in transit, letting an attacker point clients at a malicious server, and clients that require HTTPS refuse it. |
37A-McpServerCardAuditor-300 | WARNING | The HTTP response serving the card has a Content-Type other than application/mcp-server-card+json. | Serve {cardUrl} with Content-Type application/mcp-server-card+json instead of {observed}. | AI clients that check the media type may ignore or distrust a card served as generic JSON, so some tools skip the server even though the card itself is valid. |
37A-McpServerCardAuditor-200 | SUCCESS | Every rule above passed for a discovered card; one SUCCESS check is added per valid card with the message Server Card "{name}" {version} at "{cardUrl}" is valid. | — | — |
| Status | Description | Test logic |
|---|---|---|
| INFORMATION | Provenance only; does not change overall status | Every run includes 37A-McpServerCardAuditor-100 with MCP Server Card v1 and schema commit 92999cb3… in the message and Check.data. |
| SUCCESS | A Server Card was found, matches the v1 schema rules, and is properly hosted | Card discovered via AI Catalog or a reserved location. Valid JSON object with the v1 $schema URL, a reverse-DNS name, a single (non-range) version, a 1-100 character description, and remotes[] entries limited to streamable-http/sse types with http(s) or templated URLs. Fetched cards are served over HTTPS with Access-Control-Allow-Origin and the application/mcp-server-card+json media type. The specification pin check is still present. |
| WARNING | The card is valid but served with the wrong media type | Content-Type differs from application/mcp-server-card+json on an otherwise fully valid card (300). The specification pin check is still present. |
| FAIL | No card was found, the card violates the v1 schema rules, or hosting is unsafe | No card at any discovery location (400); invalid JSON (401); missing or wrong $schema (402, 403); missing or malformed name (404, 405); missing or range version (406, 407); missing or over-long description (408, 409); invalid remotes[] type or URL (410, 411); missing Access-Control-Allow-Origin (412); card served over plain HTTP (413). The specification pin check is still present. |
Without a Server Card at a discoverable location, AI clients cannot find or auto-configure a connection to the site's MCP server, so the server is invisible to agentic sessions.
A card that violates the v1 schema (wrong $schema, malformed name, range version) may be rejected by validating clients even when the MCP server itself works.
Browser-based clients are blocked entirely when the card endpoint lacks Access-Control-Allow-Origin.
Serving the card without the application/mcp-server-card+json media type can break clients that negotiate content via the Accept header.
Invalid remotes[] transport types or endpoint URLs steer clients toward connections that cannot be established.
Server Cards are consumed before the client connects; a card served over plain HTTP can be tampered with in transit and redirect clients to a rogue endpoint.
A stale or inaccurate card (identity, transport, protocol versions) is a confusion or downgrade vector — keep the card consistent with the server's runtime behavior.
The card is a static document: version bumps, endpoint moves, or transport changes on the server must be mirrored in the card, or clients will act on outdated connection details.
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