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[v1.x backport] Allow servers / clients to advertise extensions in the capability object (#1811)

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Den Delimarsky 2026-03-30 03:43:46 -07:00 • committed by GitHub
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4 changed files with 77 additions and 2 deletions
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5
.changeset/add-extensions-capability.md Normal file
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@ -0,0 +1,5 @@
---
'@modelcontextprotocol/sdk': minor
---
Add `extensions` field to `ClientCapabilities` and `ServerCapabilities` to allow servers and clients to advertise extension support per SEP-2133

8
src/spec.types.ts
View file

@ -383,6 +383,10 @@ export interface ClientCapabilities {
};
};
};
/**
* Extensions that the client supports. Keys are extension identifiers (vendor-prefix/extension-name).
*/
extensions?: { [key: string]: object };
}
/**
@ -461,6 +465,10 @@ export interface ServerCapabilities {
};
};
};
/**
* Extensions that the server supports. Keys are extension identifiers (vendor-prefix/extension-name).
*/
extensions?: { [key: string]: object };
}
/**

12
src/types.ts
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@ -511,7 +511,11 @@ export const ClientCapabilitiesSchema = z.object({
/**
* Present if the client supports task creation.
*/
tasks: ClientTasksCapabilitySchema.optional()
tasks: ClientTasksCapabilitySchema.optional(),
/**
* Extensions that the client supports. Keys are extension identifiers (vendor-prefix/extension-name).
*/
extensions: z.record(z.string(), AssertObjectSchema).optional()
});
export const InitializeRequestParamsSchema = BaseRequestParamsSchema.extend({
@ -589,7 +593,11 @@ export const ServerCapabilitiesSchema = z.object({
/**
* Present if the server supports task creation.
*/
tasks: ServerTasksCapabilitySchema.optional()
tasks: ServerTasksCapabilitySchema.optional(),
/**
* Extensions that the server supports. Keys are extension identifiers (vendor-prefix/extension-name).
*/
extensions: z.record(z.string(), AssertObjectSchema).optional()
});
/**

54
test/server/mcp.test.ts
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@ -198,6 +198,60 @@ describe.each(zodTestMatrix)('$zodVersionLabel', (entry: ZodMatrixEntry) => {
message: 'Completed step 3 of 3'
});
});
/***
* Test: Extensions capability registration
*/
test('should register and advertise server extensions capability', async () => {
const mcpServer = new McpServer({
name: 'test server',
version: '1.0'
});
const client = new Client({
name: 'test client',
version: '1.0'
});
mcpServer.server.registerCapabilities({
extensions: {
'io.modelcontextprotocol/test-extension': { listChanged: true }
}
});
const [clientTransport, serverTransport] = InMemoryTransport.createLinkedPair();
await Promise.all([client.connect(clientTransport), mcpServer.connect(serverTransport)]);
const capabilities = client.getServerCapabilities();
expect(capabilities?.extensions).toBeDefined();
expect(capabilities?.extensions?.['io.modelcontextprotocol/test-extension']).toEqual({ listChanged: true });
});
test('should advertise client extensions capability to server', async () => {
const mcpServer = new McpServer({
name: 'test server',
version: '1.0'
});
const client = new Client(
{
name: 'test client',
version: '1.0'
},
{
capabilities: {
extensions: {
'io.modelcontextprotocol/test-extension': { streaming: true }
}
}
}
);
const [clientTransport, serverTransport] = InMemoryTransport.createLinkedPair();
await Promise.all([client.connect(clientTransport), mcpServer.connect(serverTransport)]);
const capabilities = mcpServer.server.getClientCapabilities();
expect(capabilities?.extensions).toBeDefined();
expect(capabilities?.extensions?.['io.modelcontextprotocol/test-extension']).toEqual({ streaming: true });
});
});
describe('ResourceTemplate', () => {