modelcontextprotocol/rust-sdk
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feat(capabilities): add extensions field for SEP-1724 (#643)

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Add support for MCP extension capabilities in both ClientCapabilities
and ServerCapabilities structs, as specified in SEP-1724.

Changes:
- Add ExtensionCapabilities type alias (BTreeMap<String, JsonObject>)
- Add 'extensions' field to ClientCapabilities struct
- Add 'extensions' field to ServerCapabilities struct
- Update builder macros and impl blocks for both structs
- Add comprehensive tests for extension capabilities
- Update JSON schema test fixtures

This enables clients to advertise extension support during initialize,
such as:

  {
    "capabilities": {
      "extensions": {
        "io.modelcontextprotocol/ui": {
          "mimeTypes": ["text/html;profile=mcp-app"]
        }
      }
    }
  }

Closes #530
This commit is contained in:
Andrew Harvard 2026-02-03 19:14:22 -05:00 • committed by GitHub
commit 1794fe1548
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4 changed files with 197 additions and 10 deletions
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11
crates/rmcp/tests/test_message_schema/client_json_rpc_message_schema.json
View file

@ -296,6 +296,17 @@
"additionalProperties": true
}
},
"extensions": {
"description": "Optional MCP extensions that the client supports (SEP-1724).\nKeys are extension identifiers (e.g., `\"io.modelcontextprotocol/ui\"`),\nvalues are per-extension settings objects. An empty object indicates\nsupport with no settings.",
"type": [
"object",
"null"
],
"additionalProperties": {
"type": "object",
"additionalProperties": true
}
},
"roots": {
"anyOf": [
{

11
crates/rmcp/tests/test_message_schema/client_json_rpc_message_schema_current.json
View file

@ -296,6 +296,17 @@
"additionalProperties": true
}
},
"extensions": {
"description": "Optional MCP extensions that the client supports (SEP-1724).\nKeys are extension identifiers (e.g., `\"io.modelcontextprotocol/ui\"`),\nvalues are per-extension settings objects. An empty object indicates\nsupport with no settings.",
"type": [
"object",
"null"
],
"additionalProperties": {
"type": "object",
"additionalProperties": true
}
},
"roots": {
"anyOf": [
{

11
crates/rmcp/tests/test_message_schema/server_json_rpc_message_schema.json
View file

@ -2369,6 +2369,17 @@
"additionalProperties": true
}
},
"extensions": {
"description": "Optional MCP extensions that the server supports (SEP-1724).\nKeys are extension identifiers (e.g., `\"io.modelcontextprotocol/apps\"`),\nvalues are per-extension settings objects. An empty object indicates\nsupport with no settings.",
"type": [
"object",
"null"
],
"additionalProperties": {
"type": "object",
"additionalProperties": true
}
},
"logging": {
"type": [
"object",

11
crates/rmcp/tests/test_message_schema/server_json_rpc_message_schema_current.json
View file

@ -2369,6 +2369,17 @@
"additionalProperties": true
}
},
"extensions": {
"description": "Optional MCP extensions that the server supports (SEP-1724).\nKeys are extension identifiers (e.g., `\"io.modelcontextprotocol/apps\"`),\nvalues are per-extension settings objects. An empty object indicates\nsupport with no settings.",
"type": [
"object",
"null"
],
"additionalProperties": {
"type": "object",
"additionalProperties": true
}
},
"logging": {
"type": [
"object",