fix: do not omitempty ReadOnlyHint in ToolAnnotations (#908)

`ReadOnlyHint` is a `bool` with omitempty, so the zero value (false) is
indistinguishable from unset, and **drops out** of marshaled JSON.

The current behavior causes an issue with the OpenAI MCP app submission
because they require explicit hints.

Consumers that explicitly set `ReadOnlyHint: false` on write tools lose
the field on the wire. Removing `omitempty` ensures `false` is always
serialized, which matches the MCP spec default.

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
Co-authored-by: Maciej Kisiel <mkisiel@google.com>
Co-authored-by: guglielmoc <guglielmoc@google.com>

mcp/protocol.go

@@ -1346,6 +1346,13 @@ type Tool struct {
 	Icons []Icon `json:"icons,omitempty"`
 }
 
+// hintomitempty is a compatibility parameter that restores the pre-1.7.0
+// behavior of [ToolAnnotations] JSON marshaling, where false-valued bare bool
+// fields (ReadOnlyHint, IdempotentHint) were omitted from the output.
+// See the documentation for the mcpgodebug package for instructions on how to
+// enable it.
+var hintomitempty = mcpgodebug.Value("hintomitempty")
+
 // Additional properties describing a Tool to clients.
 //
 // NOTE: all properties in ToolAnnotations are hints. They are not
@@ -1368,7 +1375,7 @@ type ToolAnnotations struct {
 	// (This property is meaningful only when ReadOnlyHint == false.)
 	//
 	// Default: false
-	IdempotentHint bool `json:"idempotentHint,omitempty"`
+	IdempotentHint bool `json:"idempotentHint"`
 	// If true, this tool may interact with an "open world" of external entities. If
 	// false, the tool's domain of interaction is closed. For example, the world of
 	// a web search tool is open, whereas that of a memory tool is not.
@@ -1378,11 +1385,30 @@ type ToolAnnotations struct {
 	// If true, the tool does not modify its environment.
 	//
 	// Default: false
-	ReadOnlyHint bool `json:"readOnlyHint,omitempty"`
+	ReadOnlyHint bool `json:"readOnlyHint"`
 	// A human-readable title for the tool.
 	Title string `json:"title,omitempty"`
 }
 
+// MarshalJSON implements [json.Marshaler] for ToolAnnotations.
+//
+// To restore the previous behavior where false-valued ReadOnlyHint and
+// IdempotentHint were omitted, set MCPGODEBUG=hintomitempty=1.
+func (t ToolAnnotations) MarshalJSON() ([]byte, error) {
+	if hintomitempty == "1" {
+		type compat struct {
+			DestructiveHint *bool  `json:"destructiveHint,omitempty"`
+			IdempotentHint  bool   `json:"idempotentHint,omitempty"`
+			OpenWorldHint   *bool  `json:"openWorldHint,omitempty"`
+			ReadOnlyHint    bool   `json:"readOnlyHint,omitempty"`
+			Title           string `json:"title,omitempty"`
+		}
+		return json.Marshal(compat(t))
+	}
+	type nomethod ToolAnnotations
+	return json.Marshal(nomethod(t))
+}
+
 type ToolListChangedParams struct {
 	// This property is reserved by the protocol to allow clients and servers to
 	// attach additional metadata to their responses.

mcp/protocol_test.go

@@ -1194,3 +1194,60 @@ func TestContentUnmarshal(t *testing.T) {
 	var gotpm PromptMessage
 	roundtrip(pm, &gotpm)
 }
+
+func TestToolAnnotations_MarshalJSON(t *testing.T) {
+	boolPtr := func(b bool) *bool { return &b }
+
+	tests := []struct {
+		name string
+		in   ToolAnnotations
+		want string
+	}{
+		{
+			name: "ZeroValue",
+			in:   ToolAnnotations{},
+			want: `{"idempotentHint":false,"readOnlyHint":false}`,
+		},
+		{
+			name: "AllFalse",
+			in: ToolAnnotations{
+				DestructiveHint: boolPtr(false),
+				IdempotentHint:  false,
+				OpenWorldHint:   boolPtr(false),
+				ReadOnlyHint:    false,
+			},
+			want: `{"destructiveHint":false,"idempotentHint":false,"openWorldHint":false,"readOnlyHint":false}`,
+		},
+		{
+			name: "AllTrue",
+			in: ToolAnnotations{
+				DestructiveHint: boolPtr(true),
+				IdempotentHint:  true,
+				OpenWorldHint:   boolPtr(true),
+				ReadOnlyHint:    true,
+				Title:           "my tool",
+			},
+			want: `{"destructiveHint":true,"idempotentHint":true,"openWorldHint":true,"readOnlyHint":true,"title":"my tool"}`,
+		},
+		{
+			name: "MixedValues",
+			in: ToolAnnotations{
+				ReadOnlyHint: true,
+				Title:        "read tool",
+			},
+			want: `{"idempotentHint":false,"readOnlyHint":true,"title":"read tool"}`,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got, err := json.Marshal(tt.in)
+			if err != nil {
+				t.Fatalf("json.Marshal(%v) failed: %v", tt.in, err)
+			}
+			if diff := cmp.Diff(tt.want, string(got)); diff != "" {
+				t.Errorf("json.Marshal() mismatch (-want +got):\n%s", diff)
+			}
+		})
+	}
+}