mcp: fix CallToolResultFor[T] unmarshaling (#46)

Add an UnmarshalJSON method for that type.

Add a test.

mcp/protocol.go

@@ -129,6 +129,25 @@ type CallToolResultFor[Out any] struct {
 	StructuredContent Out `json:"structuredContent,omitempty"`
 }
 
+// UnmarshalJSON handles the unmarshalling of content into the Content
+// interface.
+func (x *CallToolResultFor[Out]) UnmarshalJSON(data []byte) error {
+	type res CallToolResultFor[Out] // avoid recursion
+	var wire struct {
+		res
+		Content []*wireContent `json:"content"`
+	}
+	if err := json.Unmarshal(data, &wire); err != nil {
+		return err
+	}
+	var err error
+	if wire.res.Content, err = contentsFromWire(wire.Content, nil); err != nil {
+		return err
+	}
+	*x = CallToolResultFor[Out](wire.res)
+	return nil
+}
+
 type CancelledParams struct {
 	// This property is reserved by the protocol to allow clients and servers to
 	// attach additional metadata to their responses.

mcp/protocol_test.go

@@ -8,6 +8,8 @@ import (
 	"encoding/json"
 	"maps"
 	"testing"
+
+	"github.com/google/go-cmp/cmp"
 )
 
 func TestParamsMeta(t *testing.T) {
@@ -67,3 +69,47 @@ func TestParamsMeta(t *testing.T) {
 	p.SetProgressToken(int32(1))
 	p.SetProgressToken(int64(1))
 }
+
+func TestContentUnmarshal(t *testing.T) {
+	// Verify that types with a Content field round-trip properly.
+	roundtrip := func(in, out any) {
+		t.Helper()
+		data, err := json.Marshal(in)
+		if err != nil {
+			t.Fatal(err)
+		}
+		if err := json.Unmarshal(data, out); err != nil {
+			t.Fatal(err)
+		}
+		if diff := cmp.Diff(in, out); diff != "" {
+			t.Errorf("mismatch (-want, +got):\n%s", diff)
+		}
+	}
+
+	content := []Content{&TextContent{Text: "t"}}
+
+	ctr := &CallToolResult{
+		Meta:              Meta{"m": true},
+		Content:           content,
+		IsError:           true,
+		StructuredContent: map[string]any{"s": "x"},
+	}
+	var got CallToolResult
+	roundtrip(ctr, &got)
+
+	ctrf := &CallToolResultFor[int]{
+		Meta:              Meta{"m": true},
+		Content:           content,
+		IsError:           true,
+		StructuredContent: 3,
+	}
+	var gotf CallToolResultFor[int]
+	roundtrip(ctrf, &gotf)
+
+	pm := &PromptMessage{
+		Content: content[0],
+		Role:    "",
+	}
+	var gotpm PromptMessage
+	roundtrip(pm, &gotpm)
+}