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mcp: return nil params for missing optional requests #1056

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880589a
mcp: return nil params for missing optional requests
ychampion Jul 8, 2026
d0e13bf
mcp: initialize missing request params
ychampion Jul 9, 2026
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44 changes: 37 additions & 7 deletions mcp/shared.go
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Original file line number Diff line number Diff line change
Expand Up @@ -269,6 +269,9 @@ func newClientMethodInfo[P paramsPtr[T], R Result, T any](d typedClientMethodHan
r := &ClientRequest[P]{Session: s.(*ClientSession)}
if p != nil {
r.Params = p.(P)
} else {
r.Params = P(new(T))
r.missingParams = true
}
return r
}
Expand All @@ -284,6 +287,9 @@ func newServerMethodInfo[P paramsPtr[T], R Result, T any](d typedServerMethodHan
r := &ServerRequest[P]{Session: s.(*ServerSession), Extra: re}
if p != nil {
r.Params = p.(P)
} else {
r.Params = P(new(T))
r.missingParams = true
}
return r
}
Expand Down Expand Up @@ -560,15 +566,17 @@ type Request interface {

// A ClientRequest is a request to a client.
type ClientRequest[P Params] struct {
Session *ClientSession
Params P
Session *ClientSession
Params P
missingParams bool
}

// A ServerRequest is a request to a server.
type ServerRequest[P Params] struct {
Session *ServerSession
Params P
Extra *RequestExtra
Session *ServerSession
Params P
Extra *RequestExtra
missingParams bool
}

// RequestExtra is extra information included in requests, typically from
Expand Down Expand Up @@ -604,8 +612,30 @@ func (*ServerRequest[P]) isRequest() {}
func (r *ClientRequest[P]) GetSession() Session { return r.Session }
func (r *ServerRequest[P]) GetSession() Session { return r.Session }

func (r *ClientRequest[P]) GetParams() Params { return r.Params }
func (r *ServerRequest[P]) GetParams() Params { return r.Params }
func (r *ClientRequest[P]) GetParams() Params {
if r.missingParams {
return nil
}
return requestParams(r.Params)
}

func (r *ServerRequest[P]) GetParams() Params {
if r.missingParams {
return nil
}
return requestParams(r.Params)
}

func requestParams[P Params](params P) Params {
if any(params) == nil {
return nil
}
value := reflect.ValueOf(params)
if value.Kind() == reflect.Ptr && value.IsNil() {
return nil
}
return params
}
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ychampion marked this conversation as resolved.

func (r *ClientRequest[P]) GetExtra() *RequestExtra { return nil }
func (r *ServerRequest[P]) GetExtra() *RequestExtra { return r.Extra }
Expand Down
77 changes: 77 additions & 0 deletions mcp/shared_test.go
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Original file line number Diff line number Diff line change
Expand Up @@ -5,6 +5,7 @@
package mcp

import (
"context"
"encoding/json"
"errors"
"strings"
Expand Down Expand Up @@ -260,6 +261,82 @@ func TestServerRequest_PerRequestAccessors_Empty(t *testing.T) {
}
}

func TestServerRequestGetParamsMissingOptionalParams(t *testing.T) {
info := newServerMethodInfo(
func(context.Context, *ServerRequest[*InitializedParams]) (*emptyResult, error) {
return &emptyResult{}, nil
},
missingParamsOK,
)
params, err := info.unmarshalParams(nil)
if err != nil {
t.Fatal(err)
}
if params != nil {
t.Fatalf("unmarshalParams returned %T, want nil", params)
}

req := info.newRequest(&ServerSession{}, params, nil)
if got := req.GetParams(); got != nil {
t.Fatalf("GetParams() = %T, want nil", got)
}
typedReq := req.(*ServerRequest[*InitializedParams])
if typedReq.Params == nil {
t.Fatal("Params field is nil, want zero InitializedParams")
}
if typedReq.Params.GetMeta() != nil {
t.Fatalf("Params.GetMeta() = %v, want nil", typedReq.Params.GetMeta())
}
}

func TestClientRequestMissingOptionalParamsUsesZeroValue(t *testing.T) {
info := newClientMethodInfo(
func(context.Context, *ClientRequest[*PingParams]) (*emptyResult, error) {
return &emptyResult{}, nil
},
missingParamsOK,
)
params, err := info.unmarshalParams(nil)
if err != nil {
t.Fatal(err)
}
if params != nil {
t.Fatalf("unmarshalParams returned %T, want nil", params)
}

req := info.newRequest(&ClientSession{}, params, nil)
if got := req.GetParams(); got != nil {
t.Fatalf("GetParams() = %T, want nil", got)
}
typedReq := req.(*ClientRequest[*PingParams])
if typedReq.Params == nil {
t.Fatal("Params field is nil, want zero PingParams")
}
if typedReq.Params.GetMeta() != nil {
t.Fatalf("Params.GetMeta() = %v, want nil", typedReq.Params.GetMeta())
}
}

func TestRequestGetParamsTypedNilCustomParams(t *testing.T) {
type customParams struct{ ParamsBase }

var params *customParams
if got := (&ClientRequest[*customParams]{Params: params}).GetParams(); got != nil {
t.Fatalf("client GetParams() = %T, want nil", got)
}
if got := (&ServerRequest[*customParams]{Params: params}).GetParams(); got != nil {
t.Fatalf("server GetParams() = %T, want nil", got)
}

params = &customParams{}
if got := (&ClientRequest[*customParams]{Params: params}).GetParams(); got != params {
t.Fatalf("client GetParams() = %T, want original params", got)
}
if got := (&ServerRequest[*customParams]{Params: params}).GetParams(); got != params {
t.Fatalf("server GetParams() = %T, want original params", got)
}
}

func TestImplementationDescriptionJSON(t *testing.T) {
impl := &Implementation{
Name: "greeter",
Expand Down