mcp: preserve structured content number precision

internal/json/json.go

@@ -44,6 +44,15 @@ func Unmarshal(data []byte, v any) error {
 	return NewDecoder(bytes.NewReader(data)).Decode(v)
 }
 
+func UnmarshalUseNumber(data []byte, v any) error {
+	if err := checkMaxDepth(data, defaultMaxDepth); err != nil {
+		return err
+	}
+	dec := NewDecoder(bytes.NewReader(data))
+	dec.dec.UseNumber()
+	return dec.Decode(v)
+}
+
 // checkMaxDepth scans data once and reports [errMaxDepthExceeded] if the
 // nesting of JSON objects and arrays exceeds maxDepth. It is a lightweight pass which
 // tracks '{' and '[' against '}' and ']' while skipping over the contents of strings.

mcp/protocol.go

@@ -280,6 +280,8 @@ type CallToolResult struct {
 	// result of the tool call. Per SEP-2106, it may marshal to any valid JSON
 	// value (object, array, or primitive) conforming to the tool's
 	// [Tool.OutputSchema].
+	// Numbers received from the wire are represented as [json.Number] to preserve
+	// their exact values.
 	//
 	// When using a [ToolHandlerFor] with structured output, you should not
 	// populate this field. It will be automatically populated with the typed Out
@@ -391,12 +393,18 @@ func (x *CallToolResult) UnmarshalJSON(data []byte) error {
 	type res CallToolResult // avoid recursion
 	var wire struct {
 		res
-		Content    []*wireContent `json:"content"`
-		ResultType resultType     `json:"resultType"`
+		Content           []*wireContent  `json:"content"`
+		StructuredContent json.RawMessage `json:"structuredContent"`
+		ResultType        resultType      `json:"resultType"`
 	}
 	if err := internaljson.Unmarshal(data, &wire); err != nil {
 		return err
 	}
+	if len(wire.StructuredContent) > 0 {
+		if err := internaljson.UnmarshalUseNumber(wire.StructuredContent, &wire.res.StructuredContent); err != nil {
+			return err
+		}
+	}
 	var err error
 	if wire.res.Content, err = contentsFromWire(wire.Content, nil); err != nil {
 		return err

mcp/protocol_test.go

@@ -1285,7 +1285,7 @@ func TestContentUnmarshal(t *testing.T) {
 		Meta:              Meta{"m": true},
 		Content:           content,
 		IsError:           true,
-		StructuredContent: 3.0,
+		StructuredContent: json.Number("3"),
 	}
 	var gotf CallToolResult
 	roundtrip(ctrf, &gotf)

mcp/server_test.go

@@ -677,7 +677,7 @@ func TestAddToolNonObjectOutputSchema(t *testing.T) {
 			name:         "primitive number (map-based schema)",
 			outputSchema: map[string]any{"type": "number"},
 			content:      42.0,
-			want:         42.0,
+			want:         json.Number("42"),
 		},
 		{
 			name:         "primitive string (RawMessage schema)",
@@ -724,6 +724,52 @@ func TestAddToolNonObjectOutputSchema(t *testing.T) {
 	}
 }
 
+func TestCallToolStructuredContentPreservesLargeInteger(t *testing.T) {
+	const want int64 = 9007199254740993
+
+	server := NewServer(testImpl, nil)
+	server.AddTool(&Tool{
+		Name:        "large-integer",
+		InputSchema: &jsonschema.Schema{Type: "object"},
+	}, func(context.Context, *CallToolRequest) (*CallToolResult, error) {
+		return &CallToolResult{StructuredContent: map[string]any{"id": want}}, nil
+	})
+
+	clientTransport, serverTransport := NewInMemoryTransports()
+	serverSession, err := server.Connect(context.Background(), serverTransport, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer serverSession.Close()
+
+	client := NewClient(testImpl, nil)
+	clientSession, err := client.Connect(context.Background(), clientTransport, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer clientSession.Close()
+
+	result, err := clientSession.CallTool(context.Background(), &CallToolParams{Name: "large-integer"})
+	if err != nil {
+		t.Fatal(err)
+	}
+	structured, ok := result.StructuredContent.(map[string]any)
+	if !ok {
+		t.Fatalf("StructuredContent type = %T, want map[string]any", result.StructuredContent)
+	}
+	number, ok := structured["id"].(json.Number)
+	if !ok {
+		t.Fatalf("id type = %T, want json.Number", structured["id"])
+	}
+	got, err := number.Int64()
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got != want {
+		t.Errorf("id = %d, want %d", got, want)
+	}
+}
+
 // TestAddToolGenericNonObjectOutput verifies SEP-2106 for the generic
 // AddTool[In, Out] helper: Out may be a slice or primitive whose inferred
 // JSON Schema has a non-object root.
@@ -816,7 +862,7 @@ func TestAddToolGenericNonObjectOutput(t *testing.T) {
 		if res.IsError {
 			t.Fatalf("unexpected tool error: %v", res.Content)
 		}
-		if diff := cmp.Diff(float64(42), res.StructuredContent); diff != "" {
+		if diff := cmp.Diff(json.Number("42"), res.StructuredContent); diff != "" {
 			t.Errorf("structured content mismatch (-want +got):\n%s", diff)
 		}
 	})

mcp/streamable_test.go

@@ -288,8 +288,14 @@ func TestStreamableConcurrentHandling(t *testing.T) {
 					t.Errorf("CallTool failed: %v", err)
 					return
 				}
-				if got := int(res.StructuredContent.(map[string]any)["Count"].(float64)); got != i {
-					t.Errorf("got count %d, want %d", got, i)
+				number, ok := res.StructuredContent.(map[string]any)["Count"].(json.Number)
+				if !ok {
+					t.Errorf("count type = %T, want json.Number", res.StructuredContent.(map[string]any)["Count"])
+					continue
+				}
+				got, err := number.Int64()
+				if err != nil || got != int64(i) {
+					t.Errorf("got count %d, %v; want %d", got, err, i)
 				}
 			}
 		})