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mcp: label resultType on results that bypass the dispatcher #1226

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7ef0cb3
mcp: label resultType on results that bypass the dispatcher
roncodingenthusiast Sep 1, 2026
3b1b704
mcp: simplify comment wording in resultType labeling code
roncodingenthusiast Sep 2, 2026
4cd8d60
Merge branch 'main' into ronald/gh-1225/stamp-resulttype-on-mrtr-results
guglielmo-san Sep 3, 2026
aa5b8ca
Merge branch 'main' into ronald/gh-1225/stamp-resulttype-on-mrtr-results
guglielmo-san Sep 4, 2026
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16 changes: 11 additions & 5 deletions mcp/mrtr.go
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Original file line number Diff line number Diff line change
Expand Up @@ -53,15 +53,21 @@ func handleMultiRoundTripResult(ss *ServerSession, logger *slog.Logger, res mult
if clientSupportsMultiRoundTrip(ss) {
// For older clients the resultType is left unset. Input requests will be handled
// by serverMultiRoundTripMiddleware client calls and handler reinvocation.
if hasInputRequests {
res.setResultType(resultTypeInputRequired)
} else {
res.setResultType(resultTypeComplete)
}
setMultiRoundTripResultType(res)
}
Comment on lines 53 to 57

@guglielmo-san guglielmo-san Sep 4, 2026

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why do we still need to set this inside mrtr ? Now it should be enough the single call from handle

@roncodingenthusiast roncodingenthusiast Sep 4, 2026

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We cannot remove this call from the code as it is now. The dispatcher reads resultType before handle runs.

  • L1018 keeps "content" in the JSON response as null and not [].
  • L1087 returns early, before the Go check res.Contents == nil gives an error.

Removing the mrtr call breaks a bunch of pre-existing tests: (eg. TestServerConformance/mrtr.txtar, the three TestMultiRoundTrip_ReadResource_* tests...).

To remove it, I would also have to change the two dispatcher lines to read InputRequests and not resultType. I tried this approach and it seemed to work. The result is cleaner, but it changes input_required detection on paths that are not related to #1225 or the current PR.

I am happy to do it in this PR if you want or keep this as is. What do you think?

@guglielmo-san guglielmo-san Sep 8, 2026

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I think it would look cleaner, and add it to this PR.
Thanks!

return nil
}

// setMultiRoundTripResultType labels res complete or input_required by
// whether it carries input requests.
func setMultiRoundTripResultType(res multiRoundTripResponse) {
if res.inputRequests() != nil {
res.setResultType(resultTypeInputRequired)
} else {
res.setResultType(resultTypeComplete)
}
Comment thread
roncodingenthusiast marked this conversation as resolved.
}

func clientSupportsMultiRoundTrip(ss *ServerSession) bool {
protocolVersion := latestProtocolVersion
if iparams := ss.InitializeParams(); iparams != nil {
Expand Down
88 changes: 53 additions & 35 deletions mcp/protocol.go
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Original file line number Diff line number Diff line change
Expand Up @@ -19,7 +19,7 @@ type resultType string

const (
// resultTypeComplete indicates the result is final.
// This is the default when ResultType is empty.
// This is the default when resultType is empty.
resultTypeComplete resultType = "complete"

// resultTypeInputRequired indicates the server needs additional client
Expand All @@ -40,9 +40,16 @@ type completeResultResponse interface {
isCompleteResult()
}

func setCompleteResultType(res Result) {
if r, ok := res.(completeResultResponse); ok {
// annotateResultType sets the resultType that protocol 2026-07-28 requires on
// every result.
func annotateResultType(res Result) {
switch r := res.(type) {
case completeResultResponse:
r.setResultType(resultTypeComplete)
case multiRoundTripResponse:
// A middleware can return one of these without reaching the
// dispatcher. This leaves the field unset.
setMultiRoundTripResultType(r)
}
}

Expand Down Expand Up @@ -226,9 +233,8 @@ type CallToolParams struct {
// marshaled to JSON.
Arguments any `json:"arguments,omitempty"`

// InputResponses maps input request IDs to responses, provided when
// retrying a call after receiving a result with ResultType
// ResultTypeInputRequired.
// InputResponses maps input request IDs to responses. Set it when the
// client retries a call that asked for input. See [CallToolResult.NeedsInput].
InputResponses InputResponseMap `json:"inputResponses,omitempty"`
// RequestState is the opaque state from the previous input-required result.
// The client must echo this back when retrying.
Expand All @@ -249,9 +255,8 @@ type CallToolParamsRaw struct {
// Arguments (see [AddTool]).
Arguments json.RawMessage `json:"arguments,omitempty"`

// InputResponses maps input request IDs to responses, provided when
// retrying a call after receiving a result with ResultType
// ResultTypeInputRequired.
// InputResponses maps input request IDs to responses. Set it when the
// client retries a call that asked for input. See [CallToolResult.NeedsInput].
InputResponses InputResponseMap `json:"inputResponses,omitempty"`
// RequestState is the opaque state from the previous input-required result.
// The client must echo this back when retrying.
Expand Down Expand Up @@ -305,9 +310,9 @@ type CallToolResult struct {
IsError bool `json:"isError,omitempty"`

// InputRequests is a map of server-assigned IDs to input requests.
// Populated only when ResultType is ResultTypeInputRequired.
// It is set only when the result needs more client input.
// The client must fulfill these and echo the IDs back in InputResponses
// when retrying the call.
// when it retries the call.
InputRequests InputRequestMap `json:"inputRequests,omitempty"`

// RequestState is an opaque string the client must echo back when
Expand All @@ -317,10 +322,10 @@ type CallToolResult struct {
// Unauthenticated servers must encrypt, sign and verify this value.
RequestState string `json:"requestState,omitempty"`

// ResultType indicates whether this result is complete or requires further
// client input. Empty or ResultTypeComplete means the call succeeded
// normally. ResultTypeInputRequired means the client should fulfill the
// InputRequests and retry the call.
// resultType records whether the call finished or needs more client input.
// Empty or resultTypeComplete means it finished. resultTypeInputRequired
// means the client must fulfill InputRequests and retry.
// See [CallToolResult.NeedsInput].
resultType resultType
// The error passed to setError, if any.
// It is not marshaled, and therefore it is only visible on the server.
Expand Down Expand Up @@ -348,7 +353,8 @@ func (r *CallToolResult) GetError() error {
return r.err
}

func (*CallToolResult) isResult() {}
func (*CallToolResult) isResult() {}
func (x *CallToolResult) isNil() bool { return x == nil }

func (r *CallToolResult) setResultType(rt resultType) { r.resultType = rt }
func (r *CallToolResult) requestState() string { return r.RequestState }
Expand All @@ -363,7 +369,7 @@ func (r *CallToolResult) hasContent() bool {
}

// NeedsInput reports whether this result requires further client input.
// This is true when the server returned ResultType "input_required".
// This is true when the server returned a resultType of "input_required".
// When NeedsInput returns true, check InputRequests for the set of
// requests the server needs fulfilled before retrying the call.
// An empty InputRequests with NeedsInput true indicates load-shedding.
Expand Down Expand Up @@ -655,7 +661,8 @@ type CompleteResult struct {
Completion CompletionResultDetails `json:"completion"`
}

func (*CompleteResult) isResult() {}
func (*CompleteResult) isResult() {}
func (x *CompleteResult) isNil() bool { return x == nil }

// CreateMessageParams holds parameters for a sampling/createMessage request.
//
Expand Down Expand Up @@ -839,6 +846,7 @@ type CreateMessageResult struct {
}

func (*CreateMessageResult) isResult() {}
func (x *CreateMessageResult) isNil() bool { return x == nil }
func (*CreateMessageResult) isInputResponse() {}
func (r *CreateMessageResult) UnmarshalJSON(data []byte) error {
type result CreateMessageResult // avoid recursion
Expand Down Expand Up @@ -890,6 +898,7 @@ var createMessageWithToolsResultAllow = map[string]bool{
}

func (*CreateMessageWithToolsResult) isResult() {}
func (x *CreateMessageWithToolsResult) isNil() bool { return x == nil }
func (*CreateMessageWithToolsResult) isInputResponse() {}

// MarshalJSON marshals the result. When Content has a single element, it is
Expand Down Expand Up @@ -950,9 +959,8 @@ type GetPromptParams struct {
// The name of the prompt or prompt template.
Name string `json:"name"`

// InputResponses maps input request IDs to responses, provided when
// retrying a call after receiving a result with ResultType
// ResultTypeInputRequired.
// InputResponses maps input request IDs to responses. Set it when the
// client retries a call that asked for input. See [CallToolResult.NeedsInput].
InputResponses InputResponseMap `json:"inputResponses,omitempty"`
// RequestState is the opaque state from the previous input-required result.
RequestState string `json:"requestState,omitempty"`
Expand All @@ -972,19 +980,20 @@ type GetPromptResult struct {
Description string `json:"description,omitempty"`
Messages []*PromptMessage `json:"messages"`

// InputRequests is populated when ResultType is ResultTypeInputRequired.
// InputRequests is set when the result needs more client input.
// See [CallToolResult.InputRequests].
InputRequests InputRequestMap `json:"inputRequests,omitempty"`
// RequestState is the opaque state for multi-round-trip retries.
// See [CallToolResult.RequestState].
RequestState string `json:"requestState,omitempty"`

// ResultType indicates whether this result is complete or requires further
// client input. See [CallToolResult.ResultType] for details.
// resultType records whether the call finished or needs more client input.
// See [CallToolResult.NeedsInput].
resultType resultType
}

func (*GetPromptResult) isResult() {}
func (*GetPromptResult) isResult() {}
func (x *GetPromptResult) isNil() bool { return x == nil }

func (r *GetPromptResult) setResultType(rt resultType) { r.resultType = rt }
func (r *GetPromptResult) requestState() string { return r.RequestState }
Expand Down Expand Up @@ -1094,7 +1103,8 @@ type InitializeResult struct {
ServerInfo *Implementation `json:"serverInfo"`
}

func (*InitializeResult) isResult() {}
func (*InitializeResult) isResult() {}
func (x *InitializeResult) isNil() bool { return x == nil }

type InitializedParams struct {
// Meta is reserved by the protocol to allow clients and servers to attach
Expand Down Expand Up @@ -1137,7 +1147,8 @@ type DiscoverResult struct {
Instructions string `json:"instructions,omitempty"`
}

func (*DiscoverResult) isResult() {}
func (*DiscoverResult) isResult() {}
func (x *DiscoverResult) isNil() bool { return x == nil }

func (x *ListPromptsParams) isParams() {}
func (x *ListPromptsParams) isNil() bool { return x == nil }
Expand Down Expand Up @@ -1207,6 +1218,7 @@ type ListPromptsResult struct {
}

func (x *ListPromptsResult) isResult() {}
func (x *ListPromptsResult) isNil() bool { return x == nil }
func (x *ListPromptsResult) nextCursorPtr() *string { return &x.NextCursor }

type ListResourceTemplatesParams struct {
Expand Down Expand Up @@ -1238,6 +1250,7 @@ type ListResourceTemplatesResult struct {
}

func (x *ListResourceTemplatesResult) isResult() {}
func (x *ListResourceTemplatesResult) isNil() bool { return x == nil }
func (x *ListResourceTemplatesResult) nextCursorPtr() *string { return &x.NextCursor }

type ListResourcesParams struct {
Expand Down Expand Up @@ -1269,6 +1282,7 @@ type ListResourcesResult struct {
}

func (x *ListResourcesResult) isResult() {}
func (x *ListResourcesResult) isNil() bool { return x == nil }
func (x *ListResourcesResult) nextCursorPtr() *string { return &x.NextCursor }

// ListRootsParams holds parameters for a roots/list request.
Expand Down Expand Up @@ -1305,6 +1319,7 @@ type ListRootsResult struct {
}

func (*ListRootsResult) isResult() {}
func (x *ListRootsResult) isNil() bool { return x == nil }
func (*ListRootsResult) isInputResponse() {}

type ListToolsParams struct {
Expand Down Expand Up @@ -1336,6 +1351,7 @@ type ListToolsResult struct {
}

func (x *ListToolsResult) isResult() {}
func (x *ListToolsResult) isNil() bool { return x == nil }
func (x *ListToolsResult) nextCursorPtr() *string { return &x.NextCursor }

// The severity of a log message.
Expand Down Expand Up @@ -1575,9 +1591,8 @@ type ReadResourceParams struct {
// the server how to interpret it.
URI string `json:"uri"`

// InputResponses maps input request IDs to responses, provided when
// retrying a call after receiving a result with ResultType
// ResultTypeInputRequired.
// InputResponses maps input request IDs to responses. Set it when the
// client retries a call that asked for input. See [CallToolResult.NeedsInput].
InputResponses InputResponseMap `json:"inputResponses,omitempty"`
// RequestState is the opaque state from the previous input-required result.
RequestState string `json:"requestState,omitempty"`
Expand All @@ -1596,19 +1611,20 @@ type ReadResourceResult struct {
Cacheable
Contents []*ResourceContents `json:"contents"`

// InputRequests is populated when ResultType is ResultTypeInputRequired.
// InputRequests is set when the result needs more client input.
// See [CallToolResult.InputRequests].
InputRequests InputRequestMap `json:"inputRequests,omitempty"`
// RequestState is the opaque state for multi-round-trip retries.
// See [CallToolResult.RequestState].
RequestState string `json:"requestState,omitempty"`

// ResultType indicates whether this result is complete or requires further
// client input. See [CallToolResult.ResultType] for details.
// resultType records whether the call finished or needs more client input.
// See [CallToolResult.NeedsInput].
resultType resultType
}

func (*ReadResourceResult) isResult() {}
func (*ReadResourceResult) isResult() {}
func (x *ReadResourceResult) isNil() bool { return x == nil }

func (r *ReadResourceResult) setResultType(rt resultType) { r.resultType = rt }
func (r *ReadResourceResult) requestState() string { return r.RequestState }
Expand Down Expand Up @@ -2114,7 +2130,8 @@ type SubscriptionsListenResult struct {
Meta `json:"_meta"`
}

func (*SubscriptionsListenResult) isResult() {}
func (*SubscriptionsListenResult) isResult() {}
func (x *SubscriptionsListenResult) isNil() bool { return x == nil }

// TODO(jba): add CompleteRequest and related types.

Expand Down Expand Up @@ -2191,6 +2208,7 @@ type ElicitResult struct {
}

func (*ElicitResult) isResult() {}
func (x *ElicitResult) isNil() bool { return x == nil }
func (*ElicitResult) isInputResponse() {}

// ElicitationCompleteParams is sent from the server to the client, informing it that an out-of-band elicitation interaction has completed.
Expand Down
6 changes: 4 additions & 2 deletions mcp/server.go
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Original file line number Diff line number Diff line change
Expand Up @@ -2039,8 +2039,10 @@ func (ss *ServerSession) handle(ctx context.Context, req *jsonrpc.Request) (any,
if err != nil {
return nil, err
}
if validatedMeta.usesNewProtocol {
setCompleteResultType(res)
// A middleware can return a typed nil value that satisfies the Result
// interface. Annotating that value panics.
if validatedMeta.usesNewProtocol && res != nil && !res.isNil() {
annotateResultType(res)
annotateServerInfo(res, ss.server.impl)
}
return res, nil
Expand Down
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