feat: update type definition of _meta field to text/blob resources.

mcp/types.go

@@ -716,8 +716,9 @@ type ResourceContents interface {
 }
 
 type TextResourceContents struct {
-	// Meta is a metadata object that is reserved by MCP for storing additional information.
-	Meta *Meta `json:"_meta,omitempty"`
+	// Raw per‑resource metadata; pass‑through as defined by MCP. Not the same as mcp.Meta.
+	// Allows _meta to be used for MCP-UI features for example. Does not assume any specific format.
+	Meta map[string]any `json:"_meta,omitempty"`
 	// The URI of this resource.
 	URI string `json:"uri"`
 	// The MIME type of this resource, if known.
@@ -730,8 +731,9 @@ type TextResourceContents struct {
 func (TextResourceContents) isResourceContents() {}
 
 type BlobResourceContents struct {
-	// Meta is a metadata object that is reserved by MCP for storing additional information.
-	Meta *Meta `json:"_meta,omitempty"`
+	// Raw per‑resource metadata; pass‑through as defined by MCP. Not the same as mcp.Meta.
+	// Allows _meta to be used for MCP-UI features for example. Does not assume any specific format.
+	Meta map[string]any `json:"_meta,omitempty"`
 	// The URI of this resource.
 	URI string `json:"uri"`
 	// The MIME type of this resource, if known.

mcp/types_test.go

@@ -138,3 +138,242 @@
 	assert.Equal(t, "A test document", resourceLink.Description)
 	assert.Equal(t, "application/pdf", resourceLink.MIMEType)
 }
+
+func TestResourceContentsMetaField(t *testing.T) {
+	tests := []struct {
+		name         string
+		inputJSON    string
+		expectedType string
+		expectedMeta map[string]any
+	}{
+		{
+			name: "TextResourceContents with empty _meta",
+			inputJSON: `{
+				"uri":"file://empty-meta.txt",
+				"mimeType":"text/plain",
+				"text":"x",
+				"_meta": {}
+			}`,
+			expectedType: "text",
+			expectedMeta: map[string]any{},
+		},
+		{
+			name: "TextResourceContents with _meta field",
+			inputJSON: `{
+				"uri": "file://test.txt",
+				"mimeType": "text/plain",
+				"text": "Hello World",
+				"_meta": {
+					"mcpui.dev/ui-preferred-frame-size": ["800px", "600px"],
+					"mcpui.dev/ui-initial-render-data": {
+						"test": "value"
+					}
+				}
+			}`,
+			expectedType: "text",
+			expectedMeta: map[string]any{
+				"mcpui.dev/ui-preferred-frame-size": []interface{}{"800px", "600px"},
+				"mcpui.dev/ui-initial-render-data": map[string]any{
+					"test": "value",
+				},
+			},
+		},
+		{
+			name: "BlobResourceContents with _meta field",
+			inputJSON: `{
+				"uri": "file://image.png",
+				"mimeType": "image/png",
+				"blob": "iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAADUlEQVR42mP8/5+hHgAHggJ/PchI7wAAAABJRU5ErkJggg==",
+				"_meta": {
+					"width": 100,
+					"height": 100,
+					"format": "PNG"
+				}
+			}`,
+			expectedType: "blob",
+			expectedMeta: map[string]any{
+				"width":  float64(100), // JSON numbers are always float64
+				"height": float64(100),
+				"format": "PNG",
+			},
+		},
+		{
+			name: "TextResourceContents without _meta field",
+			inputJSON: `{
+				"uri": "file://simple.txt",
+				"mimeType": "text/plain",
+				"text": "Simple content"
+			}`,
+			expectedType: "text",
+			expectedMeta: nil,
+		},
+		{
+			name: "BlobResourceContents without _meta field",
+			inputJSON: `{
+				"uri": "file://simple.png",
+				"mimeType": "image/png",
+				"blob": "iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAADUlEQVR42mP8/5+hHgAHggJ/PchI7wAAAABJRU5ErkJggg=="
+			}`,
+			expectedType: "blob",
+			expectedMeta: nil,
+		},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			// Parse the JSON as a generic map first
+			var contentMap map[string]any
+			err := json.Unmarshal([]byte(tc.inputJSON), &contentMap)
+			require.NoError(t, err)
+
+			// Use ParseResourceContents to convert to ResourceContents
+			resourceContent, err := ParseResourceContents(contentMap)
+			require.NoError(t, err)
+			require.NotNil(t, resourceContent)
+
+			// Test based on expected type
+			if tc.expectedType == "text" {
+				textContent, ok := resourceContent.(TextResourceContents)
+				require.True(t, ok, "Expected TextResourceContents")
+
+				// Verify standard fields
+				assert.Equal(t, contentMap["uri"], textContent.URI)
+				assert.Equal(t, contentMap["mimeType"], textContent.MIMEType)
+				assert.Equal(t, contentMap["text"], textContent.Text)
+
+				// Verify _meta field
+				assert.Equal(t, tc.expectedMeta, textContent.Meta)
+
+			} else if tc.expectedType == "blob" {
+				blobContent, ok := resourceContent.(BlobResourceContents)
+				require.True(t, ok, "Expected BlobResourceContents")
+
+				// Verify standard fields
+				assert.Equal(t, contentMap["uri"], blobContent.URI)
+				assert.Equal(t, contentMap["mimeType"], blobContent.MIMEType)
+				assert.Equal(t, contentMap["blob"], blobContent.Blob)
+
+				// Verify _meta field
+				assert.Equal(t, tc.expectedMeta, blobContent.Meta)
+			}
+
+			// Test round-trip marshaling to ensure _meta is preserved
+			marshaledJSON, err := json.Marshal(resourceContent)
+			require.NoError(t, err)
+
+			var marshaledMap map[string]any
+			err = json.Unmarshal(marshaledJSON, &marshaledMap)
+			require.NoError(t, err)
+
+			// Verify _meta field is preserved in marshaled output
+			v, ok := marshaledMap["_meta"]
+			if tc.expectedMeta != nil {
+				// Special case: empty maps are omitted due to omitempty tag
+				if len(tc.expectedMeta) == 0 {
+					assert.False(t, ok, "_meta should be omitted when empty due to omitempty")
+				} else {
+					require.True(t, ok, "_meta should be present")
+					assert.Equal(t, tc.expectedMeta, v)
+				}
+			} else {
+				assert.False(t, ok, "_meta should be omitted when nil")
+			}
+		})
+	}
+}
+
+func TestParseResourceContentsInvalidMeta(t *testing.T) {
+	tests := []struct {
+		name        string
+		inputJSON   string
+		expectedErr string
+	}{
+		{
+			name: "TextResourceContents with invalid _meta (string)",
+			inputJSON: `{
+				"uri": "file://test.txt",
+				"mimeType": "text/plain",
+				"text": "Hello World",
+				"_meta": "invalid_meta_string"
+			}`,
+			expectedErr: "_meta must be an object",
+		},
+		{
+			name: "TextResourceContents with invalid _meta (number)",
+			inputJSON: `{
+				"uri": "file://test.txt",
+				"mimeType": "text/plain",
+				"text": "Hello World",
+				"_meta": 123
+			}`,
+			expectedErr: "_meta must be an object",
+		},
+		{
+			name: "TextResourceContents with invalid _meta (array)",
+			inputJSON: `{
+				"uri": "file://test.txt",
+				"mimeType": "text/plain",
+				"text": "Hello World",
+				"_meta": ["invalid", "array"]
+			}`,
+			expectedErr: "_meta must be an object",
+		},
+		{
+			name: "TextResourceContents with invalid _meta (boolean)",
+			inputJSON: `{
+				"uri": "file://test.txt",
+				"mimeType": "text/plain",
+				"text": "Hello World",
+				"_meta": true
+			}`,
+			expectedErr: "_meta must be an object",
+		},
+		{
+			name: "TextResourceContents with invalid _meta (null)",
+			inputJSON: `{
+				"uri": "file://test.txt",
+				"mimeType": "text/plain",
+				"text": "Hello World",
+				"_meta": null
+			}`,
+			expectedErr: "_meta must be an object",
+		},
+		{
+			name: "BlobResourceContents with invalid _meta (string)",
+			inputJSON: `{
+				"uri": "file://image.png",
+				"mimeType": "image/png",
+				"blob": "iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAADUlEQVR42mP8/5+hHgAHggJ/PchI7wAAAABJRU5ErkJggg==",
+				"_meta": "invalid_meta_string"
+			}`,
+			expectedErr: "_meta must be an object",
+		},
+		{
+			name: "BlobResourceContents with invalid _meta (number)",
+			inputJSON: `{
+				"uri": "file://image.png",
+				"mimeType": "image/png",
+				"blob": "iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAADUlEQVR42mP8/5+hHgAHggJ/PchI7wAAAABJRU5ErkJggg==",
+				"_meta": 456
+			}`,
+			expectedErr: "_meta must be an object",
+		},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			// Parse the JSON as a generic map first
+			var contentMap map[string]any
+			err := json.Unmarshal([]byte(tc.inputJSON), &contentMap)
+			require.NoError(t, err)
+
+			// Use ParseResourceContents to convert to ResourceContents
+			resourceContent, err := ParseResourceContents(contentMap)
+
+			// Expect an error
+			require.Error(t, err)
+			assert.Contains(t, err.Error(), tc.expectedErr)
+			assert.Nil(t, resourceContent)
+		})
+	}
+}

mcp/utils.go

@@ -705,8 +705,15 @@ func ParseResourceContents(contentMap map[string]any) (ResourceContents, error)
 
 	mimeType := ExtractString(contentMap, "mimeType")
 
+	meta := ExtractMap(contentMap, "_meta")
+
+	if _, present := contentMap["_meta"]; present && meta == nil {
+		return nil, fmt.Errorf("_meta must be an object")
+	}
+
 	if text := ExtractString(contentMap, "text"); text != "" {
 		return TextResourceContents{
+			Meta:     meta,
 			URI:      uri,
 			MIMEType: mimeType,
 			Text:     text,
@@ -715,6 +722,7 @@ func ParseResourceContents(contentMap map[string]any) (ResourceContents, error)
 
 	if blob := ExtractString(contentMap, "blob"); blob != "" {
 		return BlobResourceContents{
+			Meta:     meta,
 			URI:      uri,
 			MIMEType: mimeType,
 			Blob:     blob,