mcp: add Extensions field to capabilities per SEP-2133#794
Merged
maciej-kisiel merged 4 commits intomodelcontextprotocol:mainfrom Feb 17, 2026
Merged
mcp: add Extensions field to capabilities per SEP-2133#794maciej-kisiel merged 4 commits intomodelcontextprotocol:mainfrom
maciej-kisiel merged 4 commits intomodelcontextprotocol:mainfrom
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Implement SEP-2133 which defines a formal `extensions` field for MCP
capability negotiation. This allows clients and servers to advertise
support for formally recognized, composable extensions during
initialization, using the "{vendor-prefix}/{extension-name}" identifier
format (e.g., "io.modelcontextprotocol/oauth-client-credentials").
Changes to mcp/protocol.go:
- Add `Extensions map[string]any` field with `json:"extensions,omitempty"`
tag to both ClientCapabilities and ServerCapabilities, following the
same pattern as the existing Experimental field.
- Add AddExtension(name string, settings map[string]any) helper methods
on both capability structs. These lazily initialize the Extensions map
and normalize nil settings to an empty map[string]any{}, enforcing the
spec requirement that each extension's settings value must be a JSON
object (never null).
- Update both clone() methods to clone the Extensions map via
maps.Clone, preventing shared mutation between the original and cloned
capabilities. Add "maps" import.
Changes to mcp/server_test.go:
- Add "extensions preserved" test case to TestServerCapabilities,
verifying extensions set via AddExtension (including nil settings
normalization) appear in computed capabilities.
- Add "Extensions over wire" test case to TestServerCapabilitiesOverWire,
verifying extensions round-trip through the initialize handshake.
- Add TestAddExtensionNilSafety, verifying that AddExtension with nil
settings produces map[string]any{} (not nil) for both
ServerCapabilities and ClientCapabilities.
Changes to mcp/client_test.go:
- Add "extensions preserved" test case to TestClientCapabilities,
verifying extensions set via AddExtension appear in computed
capabilities.
- Add "Extensions over wire" test case to TestClientCapabilitiesOverWire,
verifying client extensions round-trip through the initialize
handshake.
Fixes modelcontextprotocol#777
anowardear062-svg
approved these changes
Feb 16, 2026
anowardear062-svg
approved these changes
Feb 16, 2026
maciej-kisiel
requested changes
Feb 16, 2026
Contributor
|
Also, please fix lint. |
Move misplaced clone() doc comment to its correct location above the method, and remove TestAddExtensionNilSafety which duplicates coverage already provided by TestServerCapabilities and TestClientCapabilities.
Shallow-copy the Experimental map in both ClientCapabilities.clone and ServerCapabilities.clone, matching the existing pattern for Extensions. Add doc comments clarifying that map values are shallow-copied and that callers should not modify maps after assigning them.
Contributor
Author
|
Addressed all review comments, including the lint error. |
maciej-kisiel
approved these changes
Feb 17, 2026
Contributor
|
Thanks for contributing! |
rafaeljusto
added a commit
to Teamwork/mcp
that referenced
this pull request
Feb 27, 2026
Enable the Apps extension: https://github.com/modelcontextprotocol/ext-apps/blob/main/specification/2026-01-26/apps.mdx Related to: * modelcontextprotocol/go-sdk#794 * #156
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This implements SEP-2133 and resolves #777.
Summary of changes
Extensionsfield toServerCapabilitiesandClientCapabilitiesstructs per SEP-2133AddExtensionhelper methods on both types that normalize nil settings to empty maps.maps.Clonein the existingclone()methodsExperimentalfield.What's tested
go test ./mcp/...passes