Preserve Tool _meta when marshaling to JSON (#609)

* [tool-meta] Add _meta to tool response output

Needed for openai ui integration

Issue: none

Test plan:
🤞

* [tool-meta] add a ton of tests I guess

* [tool-meta] simpler test

* [tool-meta] undo

* [tool-meta] feedback

Author: jaredly

mcp/tools.go

@@ -613,6 +613,11 @@ func (t Tool) MarshalJSON() ([]byte, error) {
 
 	m["annotations"] = t.Annotations
 
+	// Marshal Meta if present
+	if t.Meta != nil {
+		m["_meta"] = t.Meta
+	}
+
 	return json.Marshal(m)
 }
 

mcp/tools_test.go

@@ -1396,3 +1396,54 @@ func TestNewItemsAPICompatibility(t *testing.T) {
 		})
 	}
 }
+
+// TestToolMetaMarshaling tests that the Meta field is properly marshaled as _meta in JSON output
+func TestToolMetaMarshaling(t *testing.T) {
+	meta := map[string]any{"version": "1.0.0", "author": "test"}
+	// Marshal the tool to JSON
+	data, err := json.Marshal(Tool{
+		Name:        "test-tool",
+		Description: "A test tool with meta data",
+		Meta:        NewMetaFromMap(meta),
+		InputSchema: ToolInputSchema{
+			Type: "object",
+			Properties: map[string]any{
+				"input": map[string]any{
+					"type":        "string",
+					"description": "Test input",
+				},
+			},
+		},
+	})
+	assert.NoError(t, err)
+
+	// Unmarshal to map for comparison
+	var result map[string]any
+	err = json.Unmarshal(data, &result)
+	assert.NoError(t, err)
+
+	// Check if _meta field is present and correct
+	assert.Contains(t, result, "_meta", "Tool with Meta should include _meta field")
+	assert.Equal(t, meta, result["_meta"], "_meta field should match expected value")
+}
+
+func TestToolMetaMarshalingOmitsWhenNil(t *testing.T) {
+	// Marshal a tool without Meta
+	data, err := json.Marshal(Tool{
+		Name:        "test-tool-no-meta",
+		Description: "A test tool without meta data",
+		InputSchema: ToolInputSchema{
+			Type:       "object",
+			Properties: map[string]any{},
+		},
+	})
+	assert.NoError(t, err)
+
+	// Unmarshal to map
+	var result map[string]any
+	err = json.Unmarshal(data, &result)
+	assert.NoError(t, err)
+
+	// Check that _meta field is not present
+	assert.NotContains(t, result, "_meta", "Tool without Meta should not include _meta field")
+}