mcp: label resultType on results that bypass the dispatcher (#1226)
Fixes #1225. See the [issue comment](https://github.com/modelcontextprotocol/go-sdk/issues/1225#issuecomment-5500818546). ## The bug `2026-07-28` requires `resultType` on every result. `CallToolResult`, `GetPromptResult`, and `ReadResourceResult` keep `resultType` in an unexported field. Each of these results can be `complete` or `input_required`, so it cannot embed `completeResultWithType` the way the seven types in #1060 do. Only `handleMultiRoundTripResult` sets that field. It runs inside `Server.callTool`, `getPrompt`, and `readResource`. Some middleware returns a result directly instead of calling `next` — for example, an auth, rate-limit, or entitlement gate. That middleware never reaches `handleMultiRoundTripResult`. The response then ships without the field, and strict clients reject it. ## The fix The choice between `complete` and `input_required` now happens inside `setMultiRoundTripResultType`. Both call sites that need this choice call that function. `setCompleteResultType` now labels results it did not label before, so it is renamed `annotateResultType`. The new function picks a branch with a type switch. Reading matters here. `setResultType` is unexported, so setting `InputRequests` is the only way for middleware to ask for input. If the code always labels results `complete`, the response ships as `{"resultType":"complete","inputRequests":{...}}`. A client that trusts the discriminator then drops the elicitation request. Recomputing the label every time is safe. A result that did reach the dispatcher already carries the same label from that test. This also covers one edge case: a handler that returns `InputRequests` again after the shim's single reinvocation. Before this fix, that case shipped with no `resultType`. Embedding `completeResultWithType` instead is not an option: it labels everything `complete` and overwrites `input_required`, breaking multi-round-trip flows. **Skip nil results.** Middleware can return a typed nil value, which satisfies the `Result` interface. `handle` now checks for that case before it calls the labeling function. Before this change, that same case also panicked, one frame later in `annotateServerInfo` and then `GetMeta`. ## Checks - `TestServerSessionHandle_SetsResultTypeWhenMiddlewareShortCircuits` fails on all three methods before the first change. Its fourth case fails if labeling overwrites a populated `InputRequests`. - `TestServerSessionHandle_ResultTypeGate` records which requests carry `resultType`, including one known gap. See "Separate from this PR" below. - `TestServerSessionHandle_NilResultFromMiddleware` panics without the nil check. - The complete-path cases added to `TestServerSessionHandle_SetsResultTypeOnNewProtocol` also pass before this change. They close a gap in #1060, which added only three cases, and only for `input_required`. - The exported API is unchanged. `go doc -all ./mcp`, with comments stripped, is identical before and after this change. - With the reproduction from the issue, over the streamable transport: the dispatched `tools/call` and the short-circuited `tools/call` both now return `resultType: "complete"`. I could not run `./scripts/conformance.sh`. CONTRIBUTING.md names that script, but the repo only has `client-conformance.sh` and `server-conformance.sh`. Both scripts need GNU `timeout`, and macOS does not include it. ## Also here Ten comments named an exported `ResultType` field and `ResultTypeComplete` / `ResultTypeInputRequired` constants. None of those exist. Two of the comments were `[CallToolResult.ResultType]` doc links, and those links cannot resolve. They now point at `[CallToolResult.NeedsInput]` instead, because an unexported field is also not a valid link target. --------- Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com> Co-authored-by: Guglielmo Colombo <guglielmoc@google.com>
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1 changed files with 322 additions and 83 deletions
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@ -19,7 +19,7 @@ type resultType string
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const (
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// resultTypeComplete indicates the result is final.
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// This is the default when ResultType is empty.
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// This is the default when resultType is empty.
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resultTypeComplete resultType = "complete"
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// resultTypeInputRequired indicates the server needs additional client
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@ -40,9 +40,20 @@ type completeResultResponse interface {
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isCompleteResult()
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}
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func setCompleteResultType(res Result) {
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if r, ok := res.(completeResultResponse); ok {
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// annotateResultType sets the resultType that protocol 2026-07-28 requires on
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// every result.
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func annotateResultType(res Result) {
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switch r := res.(type) {
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case completeResultResponse:
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r.setResultType(resultTypeComplete)
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case multiRoundTripResponse:
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// These results are complete or input_required, so label them by
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// whether the handler asked for more client input.
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if r.inputRequests() != nil {
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r.setResultType(resultTypeInputRequired)
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} else {
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r.setResultType(resultTypeComplete)
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}
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}
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}
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@ -226,9 +237,8 @@ type CallToolParams struct {
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// marshaled to JSON.
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Arguments any `json:"arguments,omitempty"`
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// InputResponses maps input request IDs to responses, provided when
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// retrying a call after receiving a result with ResultType
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// ResultTypeInputRequired.
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// InputResponses maps input request IDs to responses. Set it when the
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// client retries a call that asked for input. See [CallToolResult.NeedsInput].
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InputResponses InputResponseMap `json:"inputResponses,omitempty"`
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// RequestState is the opaque state from the previous input-required result.
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// The client must echo this back when retrying.
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@ -249,9 +259,8 @@ type CallToolParamsRaw struct {
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// Arguments (see [AddTool]).
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Arguments json.RawMessage `json:"arguments,omitempty"`
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// InputResponses maps input request IDs to responses, provided when
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// retrying a call after receiving a result with ResultType
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// ResultTypeInputRequired.
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// InputResponses maps input request IDs to responses. Set it when the
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// client retries a call that asked for input. See [CallToolResult.NeedsInput].
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InputResponses InputResponseMap `json:"inputResponses,omitempty"`
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// RequestState is the opaque state from the previous input-required result.
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// The client must echo this back when retrying.
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@ -305,9 +314,9 @@ type CallToolResult struct {
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IsError bool `json:"isError,omitempty"`
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// InputRequests is a map of server-assigned IDs to input requests.
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// Populated only when ResultType is ResultTypeInputRequired.
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// It is set only when the result needs more client input.
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// The client must fulfill these and echo the IDs back in InputResponses
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// when retrying the call.
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// when it retries the call.
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InputRequests InputRequestMap `json:"inputRequests,omitempty"`
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// RequestState is an opaque string the client must echo back when
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@ -317,10 +326,10 @@ type CallToolResult struct {
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// Unauthenticated servers must encrypt, sign and verify this value.
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RequestState string `json:"requestState,omitempty"`
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// ResultType indicates whether this result is complete or requires further
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// client input. Empty or ResultTypeComplete means the call succeeded
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// normally. ResultTypeInputRequired means the client should fulfill the
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// InputRequests and retry the call.
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// resultType records whether the call finished or needs more client input.
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// Empty or resultTypeComplete means it finished. resultTypeInputRequired
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// means the client must fulfill InputRequests and retry.
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// See [CallToolResult.NeedsInput].
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resultType resultType
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// The error passed to setError, if any.
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// It is not marshaled, and therefore it is only visible on the server.
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@ -348,7 +357,8 @@ func (r *CallToolResult) GetError() error {
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return r.err
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}
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func (*CallToolResult) isResult() {}
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func (*CallToolResult) isResult() {}
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func (x *CallToolResult) isNil() bool { return x == nil }
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func (r *CallToolResult) setResultType(rt resultType) { r.resultType = rt }
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func (r *CallToolResult) requestState() string { return r.RequestState }
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@ -363,7 +373,7 @@ func (r *CallToolResult) hasContent() bool {
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}
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// NeedsInput reports whether this result requires further client input.
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// This is true when the server returned ResultType "input_required".
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// This is true when the server returned a resultType of "input_required".
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// When NeedsInput returns true, check InputRequests for the set of
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// requests the server needs fulfilled before retrying the call.
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// An empty InputRequests with NeedsInput true indicates load-shedding.
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@ -655,7 +665,8 @@ type CompleteResult struct {
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Completion CompletionResultDetails `json:"completion"`
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}
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func (*CompleteResult) isResult() {}
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func (*CompleteResult) isResult() {}
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func (x *CompleteResult) isNil() bool { return x == nil }
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// CreateMessageParams holds parameters for a sampling/createMessage request.
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//
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@ -839,6 +850,7 @@ type CreateMessageResult struct {
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}
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func (*CreateMessageResult) isResult() {}
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func (x *CreateMessageResult) isNil() bool { return x == nil }
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func (*CreateMessageResult) isInputResponse() {}
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func (r *CreateMessageResult) UnmarshalJSON(data []byte) error {
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type result CreateMessageResult // avoid recursion
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@ -890,6 +902,7 @@ var createMessageWithToolsResultAllow = map[string]bool{
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}
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func (*CreateMessageWithToolsResult) isResult() {}
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func (x *CreateMessageWithToolsResult) isNil() bool { return x == nil }
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func (*CreateMessageWithToolsResult) isInputResponse() {}
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// MarshalJSON marshals the result. When Content has a single element, it is
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@ -950,9 +963,8 @@ type GetPromptParams struct {
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// The name of the prompt or prompt template.
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Name string `json:"name"`
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// InputResponses maps input request IDs to responses, provided when
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// retrying a call after receiving a result with ResultType
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// ResultTypeInputRequired.
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// InputResponses maps input request IDs to responses. Set it when the
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// client retries a call that asked for input. See [CallToolResult.NeedsInput].
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InputResponses InputResponseMap `json:"inputResponses,omitempty"`
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// RequestState is the opaque state from the previous input-required result.
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RequestState string `json:"requestState,omitempty"`
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@ -972,19 +984,20 @@ type GetPromptResult struct {
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Description string `json:"description,omitempty"`
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Messages []*PromptMessage `json:"messages"`
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// InputRequests is populated when ResultType is ResultTypeInputRequired.
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// InputRequests is set when the result needs more client input.
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// See [CallToolResult.InputRequests].
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InputRequests InputRequestMap `json:"inputRequests,omitempty"`
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// RequestState is the opaque state for multi-round-trip retries.
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// See [CallToolResult.RequestState].
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RequestState string `json:"requestState,omitempty"`
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// ResultType indicates whether this result is complete or requires further
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// client input. See [CallToolResult.ResultType] for details.
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// resultType records whether the call finished or needs more client input.
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// See [CallToolResult.NeedsInput].
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resultType resultType
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}
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func (*GetPromptResult) isResult() {}
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func (*GetPromptResult) isResult() {}
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func (x *GetPromptResult) isNil() bool { return x == nil }
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func (r *GetPromptResult) setResultType(rt resultType) { r.resultType = rt }
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func (r *GetPromptResult) requestState() string { return r.RequestState }
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@ -1094,7 +1107,8 @@ type InitializeResult struct {
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ServerInfo *Implementation `json:"serverInfo"`
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}
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func (*InitializeResult) isResult() {}
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func (*InitializeResult) isResult() {}
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func (x *InitializeResult) isNil() bool { return x == nil }
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type InitializedParams struct {
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// Meta is reserved by the protocol to allow clients and servers to attach
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@ -1137,7 +1151,8 @@ type DiscoverResult struct {
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Instructions string `json:"instructions,omitempty"`
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}
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func (*DiscoverResult) isResult() {}
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func (*DiscoverResult) isResult() {}
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func (x *DiscoverResult) isNil() bool { return x == nil }
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func (x *ListPromptsParams) isParams() {}
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func (x *ListPromptsParams) isNil() bool { return x == nil }
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@ -1207,6 +1222,7 @@ type ListPromptsResult struct {
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}
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func (x *ListPromptsResult) isResult() {}
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func (x *ListPromptsResult) isNil() bool { return x == nil }
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func (x *ListPromptsResult) nextCursorPtr() *string { return &x.NextCursor }
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type ListResourceTemplatesParams struct {
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@ -1238,6 +1254,7 @@ type ListResourceTemplatesResult struct {
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}
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func (x *ListResourceTemplatesResult) isResult() {}
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func (x *ListResourceTemplatesResult) isNil() bool { return x == nil }
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func (x *ListResourceTemplatesResult) nextCursorPtr() *string { return &x.NextCursor }
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type ListResourcesParams struct {
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@ -1269,6 +1286,7 @@ type ListResourcesResult struct {
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}
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func (x *ListResourcesResult) isResult() {}
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func (x *ListResourcesResult) isNil() bool { return x == nil }
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func (x *ListResourcesResult) nextCursorPtr() *string { return &x.NextCursor }
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// ListRootsParams holds parameters for a roots/list request.
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@ -1305,6 +1323,7 @@ type ListRootsResult struct {
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}
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func (*ListRootsResult) isResult() {}
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func (x *ListRootsResult) isNil() bool { return x == nil }
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func (*ListRootsResult) isInputResponse() {}
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type ListToolsParams struct {
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@ -1336,6 +1355,7 @@ type ListToolsResult struct {
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}
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func (x *ListToolsResult) isResult() {}
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func (x *ListToolsResult) isNil() bool { return x == nil }
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func (x *ListToolsResult) nextCursorPtr() *string { return &x.NextCursor }
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// The severity of a log message.
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@ -1575,9 +1595,8 @@ type ReadResourceParams struct {
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// the server how to interpret it.
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URI string `json:"uri"`
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// InputResponses maps input request IDs to responses, provided when
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// retrying a call after receiving a result with ResultType
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// ResultTypeInputRequired.
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// InputResponses maps input request IDs to responses. Set it when the
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// client retries a call that asked for input. See [CallToolResult.NeedsInput].
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InputResponses InputResponseMap `json:"inputResponses,omitempty"`
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// RequestState is the opaque state from the previous input-required result.
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RequestState string `json:"requestState,omitempty"`
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@ -1596,19 +1615,20 @@ type ReadResourceResult struct {
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Cacheable
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Contents []*ResourceContents `json:"contents"`
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// InputRequests is populated when ResultType is ResultTypeInputRequired.
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// InputRequests is set when the result needs more client input.
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// See [CallToolResult.InputRequests].
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InputRequests InputRequestMap `json:"inputRequests,omitempty"`
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// RequestState is the opaque state for multi-round-trip retries.
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// See [CallToolResult.RequestState].
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RequestState string `json:"requestState,omitempty"`
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// ResultType indicates whether this result is complete or requires further
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// client input. See [CallToolResult.ResultType] for details.
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// resultType records whether the call finished or needs more client input.
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// See [CallToolResult.NeedsInput].
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resultType resultType
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}
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func (*ReadResourceResult) isResult() {}
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func (*ReadResourceResult) isResult() {}
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func (x *ReadResourceResult) isNil() bool { return x == nil }
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func (r *ReadResourceResult) setResultType(rt resultType) { r.resultType = rt }
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func (r *ReadResourceResult) requestState() string { return r.RequestState }
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@ -2114,7 +2134,8 @@ type SubscriptionsListenResult struct {
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Meta `json:"_meta"`
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}
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func (*SubscriptionsListenResult) isResult() {}
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func (*SubscriptionsListenResult) isResult() {}
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func (x *SubscriptionsListenResult) isNil() bool { return x == nil }
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// TODO(jba): add CompleteRequest and related types.
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@ -2191,6 +2212,7 @@ type ElicitResult struct {
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}
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func (*ElicitResult) isResult() {}
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func (x *ElicitResult) isNil() bool { return x == nil }
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func (*ElicitResult) isInputResponse() {}
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// ElicitationCompleteParams is sent from the server to the client, informing it that an out-of-band elicitation interaction has completed.
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