fix(mcp): return structured compact results

Tools advertise an object outputSchema, so strict MCP clients reject compact TOON responses that omit structuredContent.

Signed-off-by: wargloom <wargloom@gmail.com>

Author: WarGloom

src/mcp/mcp.c

@@ -258,17 +258,30 @@ char *cbm_mcp_text_result(const char *text, bool is_error) {
     yyjson_mut_arr_add_val(content, item);
     yyjson_mut_obj_add_val(doc, root, "content", content);
 
-    if (!is_error && text) {
+    bool has_structured_content = false;
+    if (text) {
         yyjson_doc *structured_doc = yyjson_read(text, strlen(text), 0);
         if (structured_doc) {
             yyjson_val *structured_root = yyjson_doc_get_root(structured_doc);
             if (yyjson_is_obj(structured_root)) {
                 yyjson_mut_val *structured = yyjson_val_mut_copy(doc, structured_root);
-                yyjson_mut_obj_add_val(doc, root, "structuredContent", structured);
+                if (structured) {
+                    yyjson_mut_obj_add_val(doc, root, "structuredContent", structured);
+                    has_structured_content = true;
+                }
             }
             yyjson_doc_free(structured_doc);
         }
     }
+    if (!has_structured_content) {
+        /* Every advertised MCP tool has an object outputSchema, so even compact
+         * TOON/plain-text and error results must carry a conforming structured
+         * object. Keep the text Content block above for model visibility and
+         * backwards compatibility. */
+        yyjson_mut_val *structured = yyjson_mut_obj(doc);
+        yyjson_mut_obj_add_str(doc, structured, is_error ? "error" : "text", text ? text : "");
+        yyjson_mut_obj_add_val(doc, root, "structuredContent", structured);
+    }
     yyjson_mut_obj_add_bool(doc, root, "isError", is_error);
 
     char *out = yy_doc_to_str(doc);

tests/test_mcp.c

@@ -448,10 +448,10 @@ TEST(mcp_text_result) {
     PASS();
 }
 
-TEST(mcp_text_result_skips_structured_content_for_plain_text) {
+TEST(mcp_text_result_wraps_plain_text_as_structured_content) {
     char *json = cbm_mcp_text_result("plain text", false);
     ASSERT_NOT_NULL(json);
-    ASSERT_NULL(strstr(json, "\"structuredContent\""));
+    ASSERT_NOT_NULL(strstr(json, "\"structuredContent\":{\"text\":\"plain text\"}"));
     ASSERT_NOT_NULL(strstr(json, "\"isError\":false"));
     free(json);
     PASS();
@@ -470,6 +470,7 @@ TEST(mcp_cancel_matches_request_id) {
 TEST(mcp_text_result_error) {
     char *json = cbm_mcp_text_result("something failed", true);
     ASSERT_NOT_NULL(json);
+    ASSERT_NOT_NULL(strstr(json, "\"structuredContent\":{\"error\":\"something failed\"}"));
     ASSERT_NOT_NULL(strstr(json, "\"isError\":true"));
     ASSERT_NOT_NULL(strstr(json, "something failed"));
     free(json);
@@ -756,6 +757,7 @@ TEST(tool_search_graph_includes_node_properties) {
              "\"arguments\":{\"project\":\"test-project\",\"label\":\"Function\","
              "\"name_pattern\":\"HandleRequest\",\"limit\":5}}}");
     ASSERT_NOT_NULL(resp);
+    ASSERT_NOT_NULL(strstr(resp, "\"structuredContent\":{\"text\":"));
     char *inner = extract_text_content(resp);
     ASSERT_NOT_NULL(inner);
     ASSERT_NOT_NULL(strstr(inner, "results[")); /* TOON table header */
@@ -5611,7 +5613,7 @@ SUITE(mcp) {
     RUN_TEST(mcp_ingest_traces_items_disallow_additional_properties_issue731);
     RUN_TEST(mcp_get_architecture_aspects_schema_enum_pr560);
     RUN_TEST(mcp_text_result);
-    RUN_TEST(mcp_text_result_skips_structured_content_for_plain_text);
+    RUN_TEST(mcp_text_result_wraps_plain_text_as_structured_content);
     RUN_TEST(mcp_cancel_matches_request_id);
     RUN_TEST(mcp_text_result_error);