mcp: add Extensions field to capabilities per SEP-2133 (#794)
This implements [SEP-2133](https://modelcontextprotocol.io/community/seps/2133-extensions) and resolves #777. ## Summary of changes - Add `Extensions` field to `ServerCapabilities` and `ClientCapabilities` structs per [SEP-2133](https://modelcontextprotocol.io/community/seps/2133-extensions) - Add `AddExtension` helper methods on both types that normalize nil settings to empty maps. - [The spec requires an object, not null](https://modelcontextprotocol.io/community/seps/2133-extensions#negotiation:~:text=an%20empty%20object%20indicates%20no%20settings.) - Include shallow-copy support via `maps.Clone` in the existing `clone()` methods - also include shallow-copy support for `Experimental` field. - Add comprehensive tests: unit tests for nil-safety, capability preservation, and over-the-wire round-trip ## What's tested - [x] `go test ./mcp/...` passes - [x] Server conformance tests pass (30 scenarios, 40 assertions, 0 failures) - [x] Client conformance tests match existing baseline (auth scenarios are known failures)
This commit is contained in:
Yamamoto, Hirotaka
GitHub
parent
1580ac9494
commit
3dd262606e
3 changed files with 124 additions and 2 deletions
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@ -402,6 +402,26 @@ func TestClientCapabilities(t *testing.T) {
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RootsV2: &RootCapabilities{ListChanged: true},
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},
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},
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{
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name: "extensions preserved",
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configureClient: func(s *Client) {},
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clientOpts: func() ClientOptions {
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caps := &ClientCapabilities{
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RootsV2: &RootCapabilities{ListChanged: true},
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}
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caps.AddExtension("io.example/ext1", map[string]any{"key": "value"})
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caps.AddExtension("io.example/ext2", nil)
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return ClientOptions{Capabilities: caps}
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}(),
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wantCapabilities: &ClientCapabilities{
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Extensions: map[string]any{
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"io.example/ext1": map[string]any{"key": "value"},
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"io.example/ext2": map[string]any{},
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},
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Roots: RootCapabilities{ListChanged: true},
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RootsV2: &RootCapabilities{ListChanged: true},
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},
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},
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}
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for _, tc := range testCases {
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@ -469,6 +489,23 @@ func TestClientCapabilitiesOverWire(t *testing.T) {
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},
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wantCapabilities: &ClientCapabilities{},
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},
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{
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name: "Extensions over wire",
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clientOpts: func() *ClientOptions {
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caps := &ClientCapabilities{
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RootsV2: &RootCapabilities{ListChanged: true},
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}
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caps.AddExtension("io.example/ext", map[string]any{"key": "value"})
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return &ClientOptions{Capabilities: caps}
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}(),
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wantCapabilities: &ClientCapabilities{
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Extensions: map[string]any{
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"io.example/ext": map[string]any{"key": "value"},
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},
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Roots: RootCapabilities{ListChanged: true},
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RootsV2: &RootCapabilities{ListChanged: true},
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},
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},
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}
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for _, tc := range testCases {
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@ -13,6 +13,7 @@ package mcp
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import (
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"encoding/json"
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"fmt"
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"maps"
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)
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// Optional annotations for the client. The client can use annotations to inform
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@ -192,7 +193,14 @@ type ClientCapabilities struct {
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// [ClientCapabilities.clone].
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// Experimental reports non-standard capabilities that the client supports.
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// The caller should not modify the map after assigning it.
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Experimental map[string]any `json:"experimental,omitempty"`
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// Extensions reports extensions that the client supports.
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// Keys are extension identifiers in "{vendor-prefix}/{extension-name}" format.
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// Values are per-extension settings objects; use [ClientCapabilities.AddExtension]
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// to ensure nil settings are normalized to empty objects.
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// The caller should not modify the map or its values after assigning it.
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Extensions map[string]any `json:"extensions,omitempty"`
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// Roots describes the client's support for roots.
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//
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// Deprecated: use RootsV2. As described in #607, Roots should have been a
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@ -211,9 +219,26 @@ type ClientCapabilities struct {
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Elicitation *ElicitationCapabilities `json:"elicitation,omitempty"`
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}
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// clone returns a deep copy of the ClientCapabilities.
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// AddExtension adds an extension with the given name and settings.
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// If settings is nil, an empty map is used to ensure valid JSON serialization
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// (the spec requires an object, not null).
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// The settings map should not be modified after the call.
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func (c *ClientCapabilities) AddExtension(name string, settings map[string]any) {
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if c.Extensions == nil {
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c.Extensions = make(map[string]any)
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}
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if settings == nil {
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settings = map[string]any{}
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}
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c.Extensions[name] = settings
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}
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// clone returns a copy of the ClientCapabilities.
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// Values in the Extensions and Experimental maps are shallow-copied.
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func (c *ClientCapabilities) clone() *ClientCapabilities {
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cp := *c
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cp.Experimental = maps.Clone(c.Experimental)
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cp.Extensions = maps.Clone(c.Extensions)
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cp.RootsV2 = shallowClone(c.RootsV2)
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if c.Sampling != nil {
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x := *c.Sampling
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@ -1516,7 +1541,14 @@ type ServerCapabilities struct {
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// [ServerCapabilities.clone].
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// Experimental reports non-standard capabilities that the server supports.
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// The caller should not modify the map after assigning it.
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Experimental map[string]any `json:"experimental,omitempty"`
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// Extensions reports extensions that the server supports.
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// Keys are extension identifiers in "{vendor-prefix}/{extension-name}" format.
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// Values are per-extension settings objects; use [ServerCapabilities.AddExtension]
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// to ensure nil settings are normalized to empty objects.
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// The caller should not modify the map or its values after assigning it.
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Extensions map[string]any `json:"extensions,omitempty"`
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// Completions is present if the server supports argument autocompletion
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// suggestions.
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Completions *CompletionCapabilities `json:"completions,omitempty"`
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@ -1530,9 +1562,26 @@ type ServerCapabilities struct {
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Tools *ToolCapabilities `json:"tools,omitempty"`
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}
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// clone returns a deep copy of the ServerCapabilities.
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// AddExtension adds an extension with the given name and settings.
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// If settings is nil, an empty map is used to ensure valid JSON serialization
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// (the spec requires an object, not null).
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// The settings map should not be modified after the call.
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func (c *ServerCapabilities) AddExtension(name string, settings map[string]any) {
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if c.Extensions == nil {
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c.Extensions = make(map[string]any)
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}
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if settings == nil {
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settings = map[string]any{}
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}
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c.Extensions[name] = settings
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}
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// clone returns a copy of the ServerCapabilities.
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// Values in the Extensions and Experimental maps are shallow-copied.
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func (c *ServerCapabilities) clone() *ServerCapabilities {
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cp := *c
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cp.Experimental = maps.Clone(c.Experimental)
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cp.Extensions = maps.Clone(c.Extensions)
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cp.Completions = shallowClone(c.Completions)
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cp.Logging = shallowClone(c.Logging)
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cp.Prompts = shallowClone(c.Prompts)
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@ -441,6 +441,25 @@ func TestServerCapabilities(t *testing.T) {
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Logging: &LoggingCapabilities{},
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},
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},
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{
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name: "extensions preserved",
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configureServer: func(s *Server) {},
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serverOpts: func() ServerOptions {
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caps := &ServerCapabilities{
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Logging: &LoggingCapabilities{},
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}
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caps.AddExtension("io.example/ext1", map[string]any{"key": "value"})
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caps.AddExtension("io.example/ext2", nil)
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return ServerOptions{Capabilities: caps}
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}(),
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wantCapabilities: &ServerCapabilities{
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Extensions: map[string]any{
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"io.example/ext1": map[string]any{"key": "value"},
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"io.example/ext2": map[string]any{},
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},
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Logging: &LoggingCapabilities{},
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},
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},
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}
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for _, tc := range testCases {
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@ -927,6 +946,23 @@ func TestServerCapabilitiesOverWire(t *testing.T) {
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Tools: &ToolCapabilities{ListChanged: true},
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},
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},
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{
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name: "Extensions over wire",
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serverOpts: func() *ServerOptions {
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caps := &ServerCapabilities{
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Logging: &LoggingCapabilities{},
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}
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caps.AddExtension("io.example/ext", map[string]any{"key": "value"})
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return &ServerOptions{Capabilities: caps}
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}(),
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configureServer: func(s *Server) {},
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wantCapabilities: &ServerCapabilities{
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Extensions: map[string]any{
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"io.example/ext": map[string]any{"key": "value"},
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},
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Logging: &LoggingCapabilities{},
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},
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},
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}
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for _, tc := range testCases {
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