Resources

Executive Summary

A resource is data your server exposes for the client to read — a file, a config, a report — addressed by URI, listed via resources/list, fetched via resources/read. Register static text with one call, dynamic content with a reader callback, and whole URI families with RFC 6570 templates. Readers return contents built with MCPTextContents / MCPBlobContents.

Static text resources

The one-liner for content that never changes while the process runs:

Server.RegisterTextResource('mcp://my-server/motd', 'motd',
  'text/plain', 'Be excellent to each other.',
  'Message of the day');

URI, name, MIME type, the text itself, and an optional description clients show in listings. URIs are yours to design; the mcp://<server>/ convention keeps them collision-free.

Dynamic resources

Content produced at read time uses a reader callback:

function ConfigReader(const AUri: string;
  const ACtx: TMCPRequestContext): TJSONArray;
begin
  Result := MCPTextContents(AUri, 'application/json', LoadConfigJson);
end;

Server.RegisterResource('mcp://my-server/config', 'config',
  'application/json', ConfigReader, 'Live server configuration');

The reader returns a TJSONArray of contents entries:

• MCPTextContents(Uri, MimeType, Text) — text contents.
• MCPBlobContents(Uri, MimeType, Base64) — binary contents, base64-encoded.

Both return a single-entry array; concatenate entries for multi-part reads. A reader exception becomes a JSON-RPC error on resources/read. Because the library cannot know how fresh a callback's data stays, reads served by a dynamic reader always advertise ttlMs: 0 (revalidate) in their caching hints — static text advertises the server-wide TTL (see Configuration).

Method-pointer overloads (of object) exist for both RegisterResource and RegisterResourceTemplate, mirroring tools and prompts.

Resource templates

A template registers a whole URI family; variables matched from the requested URI are passed to the reader:

function ShoutReader(const AUri: string; AVars: TJSONObject;
  const ACtx: TMCPRequestContext): TJSONArray;
begin
  Result := MCPTextContents(AUri, 'text/plain',
    UpperCase(AVars.Get('text', '')));
end;

Server.RegisterResourceTemplate('mcp://my-server/shout/{text}',
  'shout', 'text/plain', ShoutReader, 'Uppercase echo of {text}');

Templates are listed via resources/templates/list; a resources/read whose URI matches no exact resource is matched against the templates.

Matching is intentionally limited to RFC 6570 level 1 ({var} expressions):

• Variables must be non-empty and separated by literal text; matching uses the complete following literal and may backtrack.
• Exact resources win over template matches.
• Captured values are passed to readers exactly as encoded in the URI — percent-decoding is not performed. Decode in the reader if your variables can carry encoded characters.

Errors

Reading an unregistered URI answers the spec's era-appropriate error: -32602 for stateless-era clients, -32002 in the classic-handshake dialect. Reader exceptions become JSON-RPC internal errors, with messages subject to error redaction.

Resource behaviour (resources/list, resources/read, resources/templates/list, error codes, caching hints) implements spec revision 2026-07-28 — see Spec grounding.