0c0eb23625The "Schema defaults and enums" section of docs/client.md links SEP-1034 to https://modelcontextprotocol.io/seps/1034 and SEP-1330 to https://modelcontextprotocol.io/seps/1330, and both of those bare-number URLs are dead. modelcontextprotocol.io serves SEPs at number-plus-slug URLs, which is what every other SEP link in this repo already uses (seps/2575-stateless-mcp, seps/2577-deprecate-roots-sampling-and-logging, seps/2596-feature-lifecycle-and-deprecation-policy, seps/2207-oidc-refresh-token-guidance); these two were the only bare-number ones left. I took the replacement slugs from the site's own sitemap.xml, so the `1034--` double hyphen is what is published rather than a typo here. curl -sIL on the two old URLs returns 404 and 404, and on the two new ones 200 and 200. The edit is in internal/docs/client.src.md with docs/client.md regenerated by `go generate ./internal/docs`, and `go build ./... && go test ./...` pass. Co-authored-by: Guglielmo Colombo <guglielmoc@google.com>
237 lines
11 KiB
Markdown
237 lines
11 KiB
Markdown
# Support for MCP client features
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%toc
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## Roots
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> **Note:** The roots feature is deprecated as of protocol version 2026-07-28
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> ([SEP-2577](https://modelcontextprotocol.io/seps/2577-deprecate-roots-sampling-and-logging)).
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> It remains fully functional during the deprecation window (at least twelve
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> months). The SDK continues to support roots for compatibility. New code
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> should pass paths via tool parameters, resource URIs, or configuration
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> instead.
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MCP allows clients to specify a set of filesystem
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["roots"](https://modelcontextprotocol.io/specification/2025-06-18/client/roots).
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The SDK supports this as follows:
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**Client-side**: The SDK client always has the `roots.listChanged` capability.
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To add roots to a client, use the
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[`Client.AddRoots`](https://pkg.go.dev/github.com/modelcontextprotocol/go-sdk/mcp#Client.AddRoots)
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and
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[`Client.RemoveRoots`](https://pkg.go.dev/github.com/modelcontextprotocol/go-sdk/mcp#Client.RemoveRoots)
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methods. If any servers are already [connected](protocol.md#lifecycle) to the
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client, a call to `AddRoot` or `RemoveRoots` will result in a
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`notifications/roots/list_changed` notification to each connected server.
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**Server-side**: To query roots from the server, use the
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[`ServerSession.ListRoots`](https://pkg.go.dev/github.com/modelcontextprotocol/go-sdk/mcp#ServerSession.ListRoots)
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method. To receive notifications about root changes, set
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[`ServerOptions.RootsListChangedHandler`](https://pkg.go.dev/github.com/modelcontextprotocol/go-sdk/mcp#ServerOptions.RootsListChangedHandler).
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For protocol versions `2026-07-28` and later, `ListRoots` requests are
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delivered via the
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[Multi Round-Trip Requests](#multi-round-trip-requests)
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pattern.
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%include ../../mcp/client_example_test.go roots -
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### Roots list changed
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`Client.AddRoots` and `Client.RemoveRoots` notify every connected server that
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the list changed. Servers observe this through
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[`ServerOptions.RootsListChangedHandler`](https://pkg.go.dev/github.com/modelcontextprotocol/go-sdk/mcp#ServerOptions.RootsListChangedHandler);
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as with the server-side list-changed notifications, it reports only that
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something changed, so read the list back with `ServerSession.ListRoots`.
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%include ../../mcp/client_example_test.go rootslistchanged -
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## Sampling
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> **Note:** The sampling feature is deprecated as of protocol version
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> 2026-07-28
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> ([SEP-2577](https://modelcontextprotocol.io/seps/2577-deprecate-roots-sampling-and-logging)).
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> It remains fully functional during the deprecation window (at least twelve
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> months). The SDK continues to support sampling for compatibility. Servers
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> that need LLM completions should call LLM provider APIs directly.
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[Sampling](https://modelcontextprotocol.io/specification/2025-06-18/client/sampling)
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is a way for servers to leverage the client's AI capabilities. It is
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implemented in the SDK as follows:
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**Client-side**: To add the `sampling` capability to a client, set
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[`ClientOptions.CreateMessageHandler`](https://pkg.go.dev/github.com/modelcontextprotocol/go-sdk/mcp#ClientOptions.CreateMessageHandler).
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This function is invoked whenever the server requests sampling.
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**Server-side**: To use sampling from the server, call
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[`ServerSession.CreateMessage`](https://pkg.go.dev/github.com/modelcontextprotocol/go-sdk/mcp#ServerSession.CreateMessage).
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For protocol versions `2026-07-28` and later, sampling requests are
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delivered via the
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[Multi Round-Trip Requests](#multi-round-trip-requests)
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pattern.
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%include ../../mcp/client_example_test.go sampling -
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## Elicitation
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[Elicitation](https://modelcontextprotocol.io/specification/2025-06-18/client/elicitation)
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allows servers to request user inputs. It is implemented in the SDK as follows:
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**Client-side**: To add the `elicitation` capability to a client, set
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[`ClientOptions.ElicitationHandler`](https://pkg.go.dev/github.com/modelcontextprotocol/go-sdk/mcp#ClientOptions.ElicitationHandler).
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The elicitation handler must return a result that matches the requested schema;
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otherwise, elicitation returns an error. If your handler supports [URL mode
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elicitation](https://modelcontextprotocol.io/specification/2025-11-25/client/elicitation#url-mode-elicitation-requests),
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you must declare that capability explicitly (see [Capabilities](#capabilities))
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**Server-side**: To use elicitation from the server, call
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[`ServerSession.Elicit`](https://pkg.go.dev/github.com/modelcontextprotocol/go-sdk/mcp#ServerSession.Elicit).
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For protocol versions `2026-07-28` and later, elicitation requests are
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delivered via the
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[Multi Round-Trip Requests](#multi-round-trip-requests)
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pattern.
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%include ../../mcp/client_example_test.go elicitation -
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### Schema defaults and enums
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`ElicitParams.RequestedSchema` is a flat schema of primitive fields, which the
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client renders as a form. Two field keywords shape that form.
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A `Default` ([SEP-1034](https://modelcontextprotocol.io/seps/1034--support-default-values-for-all-primitive-types-in)) prefills a
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field. When the user accepts without supplying it, the SDK fills the field in
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from the schema before the result reaches either side's caller — the client
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does so after its elicitation handler returns, and `ServerSession.Elicit` does
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so again on receipt. This is unconditional; there is no opt-in flag. Marking a
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defaulted field `Required` defeats it: accepted content is validated against
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the schema before defaults are applied, so an answer that omits the field is
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rejected rather than defaulted.
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An `Enum` ([SEP-1330](https://modelcontextprotocol.io/seps/1330-elicitation-enum-schema-improvements-and-standards)) restricts a
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field to a fixed set of values, which the client renders as a choice. Enums
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are supported only on `"string"` fields; declaring one on another type is
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rejected. To label the choices, set the legacy `enumNames` keyword through
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`Schema.Extra`, with exactly one name per enum value — a mismatched length is
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rejected.
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%include ../../mcp/client_example_test.go elicitationschema -
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### Completing a URL elicitation
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In URL mode the user finishes out of band, in a browser, so nothing in the
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elicitation result tells the client when they are done. The server signals that
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with
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[`ServerSession.NotifyElicitationComplete`](https://pkg.go.dev/github.com/modelcontextprotocol/go-sdk/mcp#ServerSession.NotifyElicitationComplete),
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passing the same `ElicitationID` the request carried; the client observes it
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through
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[`ClientOptions.ElicitationCompleteHandler`](https://pkg.go.dev/github.com/modelcontextprotocol/go-sdk/mcp#ClientOptions.ElicitationCompleteHandler).
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Send it from whatever endpoint the hosted flow redirects back to.
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The notification matters most when a handler rejects a request with
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[`URLElicitationRequiredError`](https://pkg.go.dev/github.com/modelcontextprotocol/go-sdk/mcp#URLElicitationRequiredError):
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the client parks the original request until a notification names that
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`ElicitationID`, and then retries it automatically. Until one arrives, the
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client waits.
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%include ../../mcp/client_example_test.go elicitationcomplete -
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## Multi Round-Trip Requests
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[SEP-2322](https://github.com/modelcontextprotocol/modelcontextprotocol/pull/2322)
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introduces the MRTR pattern: server-to-client requests for sampling,
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elicitation, and roots are no longer issued as fresh JSON-RPC requests but
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are carried inside the in-flight reply of a `tools/call`, `prompts/get`, or
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`resources/read`. The client must respond by retrying the original request
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with the produced responses.
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The SDK installs `clientMultiRoundTripMiddleware` for every client by
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default. The middleware:
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1. Inspects each `tools/call`/`prompts/get`/`resources/read` reply.
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2. If the result's `NeedsInput()` is true, fans out the `InputRequests` map
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concurrently, calling the configured handler for each (`elicit`,
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`createMessage`/`createMessageWithTools`, or `listRoots`).
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3. Threads the server-supplied opaque `RequestState` back unchanged.
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4. Retries the original request with the responses set, repeating until the
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result no longer needs input.
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The middleware is enabled by default. To opt out, set
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[`ClientOptions.MultiRoundTrip.Disabled = true`](https://pkg.go.dev/github.com/modelcontextprotocol/go-sdk/mcp#MultiRoundTripOptions);
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the client will then surface input-required results directly to the caller
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(the returned `CallToolResult`, `GetPromptResult`, or `ReadResourceResult`
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will report `NeedsInput() == true` and expose the server's `InputRequests`
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and opaque `RequestState`). Your code must fulfil each request and re-issue
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the original call with `InputResponses` set and `RequestState` echoed
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back.
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For legacy (`<= 2025-11-25`) servers, the SDK transparently sends server
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requests on the legacy server-initiated channel; the MRTR machinery is a
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no-op in that direction. For legacy clients talking to MRTR-style servers,
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the server SDK applies the inverse compatibility shim — see the
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[server-side documentation](server.md#multi-round-trip-requests).
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## Capabilities
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Client capabilities are advertised to servers during the initialization
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handshake. [By default](rough_edges.md), the SDK advertises the `logging`
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capability. Additional capabilities are automatically added when server
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features are added (e.g. via `AddTool`), or when handlers are set in the
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`ServerOptions` struct (e.g., setting `CompletionHandler` adds the
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`completions` capability), or may be configured explicitly.
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### Capability inference
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When handlers are set on `ClientOptions` (e.g., `CreateMessageHandler` or
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`ElicitationHandler`), the corresponding capability is automatically added if
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not already present, with a default configuration.
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For elicitation, if the handler is set but no `Capabilities.Elicitation` is
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specified, the client defaults to form elicitation. To enable URL elicitation
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or both modes, [configure `Capabilities.Elicitation`
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explicitly](#explicit-capabilities).
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See the [`ClientCapabilities`
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documentation](https://pkg.go.dev/github.com/modelcontextprotocol/go-sdk/mcp#ClientCapabilities)
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for further details on inference.
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### Explicit capabilities
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To explicitly declare capabilities, or to override the [default inferred
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capability](#capability-inference), set
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[`ClientOptions.Capabilities`](https://pkg.go.dev/github.com/modelcontextprotocol/go-sdk/mcp#ClientOptions.Capabilities).
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This sets the initial client capabilities, before any capabilities are added
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based on configured handlers. If a capability is already present in
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`Capabilities`, adding a handler will not change its configuration.
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This allows you to:
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- **Disable default capabilities**: Pass an empty `&ClientCapabilities{}` to
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disable all defaults, including roots.
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- **Disable listChanged notifications**: Set `ListChanged: false` on a
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capability to prevent the client from sending list-changed notifications
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when roots are added or removed.
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- **Configure elicitation modes**: Specify which elicitation modes (form, URL)
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the client supports.
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```go
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// Configure elicitation modes and disable roots.
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client := mcp.NewClient(impl, &mcp.ClientOptions{
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Capabilities: &mcp.ClientCapabilities{
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Elicitation: &mcp.ElicitationCapabilities{
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Form: &mcp.FormElicitationCapabilities{},
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URL: &mcp.URLElicitationCapabilities{},
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},
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},
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ElicitationHandler: handler,
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})
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```
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### Extensions
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[SEP-2133](https://github.com/modelcontextprotocol/modelcontextprotocol/pull/2133)
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adds an `extensions` map to `ClientCapabilities` and `ServerCapabilities` so
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that optional capabilities outside the core protocol can be declared on the
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wire. Keys are namespaced as `"{vendor-prefix}/{extension-name}"`; values
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are per-extension settings objects.
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