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")
+}