fix(search_code): error on invalid regex instead of silent empty (#283)

With regex=true, a syntactically invalid pattern (e.g. unclosed group) made
the underlying grep fail, which handle_search_code reported as an empty result
set — indistinguishable from a legitimate no-match. Validate the user pattern
with cbm_regcomp up front and return an explicit "invalid regex" error so
callers can tell a broken pattern from zero matches. regex=false is unaffected.

src/mcp/mcp.c

@@ -3536,6 +3536,9 @@ static char *handle_search_code(cbm_mcp_server_t *srv, const char *args) {
     }
 
     if (!validate_search_args(root_path, file_pattern)) {
+        if (has_path_filter) {
+            cbm_regfree(&path_regex);
+        }
         free(root_path);
         free(pattern);
         free(project);
@@ -3543,6 +3546,27 @@ static char *handle_search_code(cbm_mcp_server_t *srv, const char *args) {
         return cbm_mcp_text_result("path or file_pattern contains invalid characters", true);
     }
 
+    /* issue #283: when regex=true, a syntactically invalid pattern (e.g. an
+     * unclosed group) makes the underlying grep fail, which the handler would
+     * otherwise report as an empty result set — indistinguishable from a
+     * legitimate no-match. Validate the user's regex up front and return an
+     * explicit error so callers can tell "broken pattern" from "no matches". */
+    if (use_regex) {
+        cbm_regex_t probe;
+        if (cbm_regcomp(&probe, pattern, CBM_REG_EXTENDED | CBM_REG_NOSUB) != CBM_REG_OK) {
+            if (has_path_filter) {
+                cbm_regfree(&path_regex);
+            }
+            free(root_path);
+            free(pattern);
+            free(project);
+            free(file_pattern);
+            return cbm_mcp_text_result(
+                "invalid regex pattern (regex=true): check for unbalanced (), [], or {}", true);
+        }
+        cbm_regfree(&probe);
+    }
+
     /* ── Phase 0.5: Multi-word → regex conversion ───────────── */
     /* If pattern contains whitespace and is not already a regex, convert to a
      * regex that matches all words in order: "foo bar baz" → "foo.*bar.*baz".

tests/test_mcp.c

@@ -739,6 +739,39 @@ TEST(search_code_multi_word) {
     PASS();
 }
 
+/* issue #283: search_code with regex=true and a syntactically invalid pattern
+ * must return an explicit error, not an empty result indistinguishable from a
+ * legitimate no-match. */
+TEST(search_code_invalid_regex_errors_issue283) {
+    char tmp[512];
+    cbm_mcp_server_t *srv = setup_snippet_server(tmp, sizeof(tmp));
+    ASSERT_NOT_NULL(srv);
+
+    /* Unclosed group under regex=true → must be flagged as an error. */
+    char *resp =
+        cbm_mcp_server_handle(srv, "{\"jsonrpc\":\"2.0\",\"id\":91,\"method\":\"tools/call\","
+                                   "\"params\":{\"name\":\"search_code\","
+                                   "\"arguments\":{\"pattern\":\"func(\",\"regex\":true,"
+                                   "\"project\":\"test-project\"}}}");
+    ASSERT_NOT_NULL(resp);
+    ASSERT_NOT_NULL(strstr(resp, "\"isError\":true"));
+    ASSERT_NOT_NULL(strstr(resp, "invalid regex"));
+    free(resp);
+
+    /* Same pattern as a literal (regex=false) must NOT error. */
+    resp = cbm_mcp_server_handle(srv, "{\"jsonrpc\":\"2.0\",\"id\":92,\"method\":\"tools/call\","
+                                      "\"params\":{\"name\":\"search_code\","
+                                      "\"arguments\":{\"pattern\":\"func(\",\"regex\":false,"
+                                      "\"project\":\"test-project\"}}}");
+    ASSERT_NOT_NULL(resp);
+    ASSERT_TRUE(strstr(resp, "invalid regex") == NULL);
+    free(resp);
+
+    cleanup_snippet_dir(tmp);
+    cbm_mcp_server_free(srv);
+    PASS();
+}
+
 TEST(tool_detect_changes_no_project) {
     cbm_mcp_server_t *srv = cbm_mcp_server_new(NULL);
 
@@ -1937,6 +1970,7 @@ SUITE(mcp) {
     RUN_TEST(tool_search_code_missing_pattern);
     RUN_TEST(tool_search_code_no_project);
     RUN_TEST(search_code_multi_word);
+    RUN_TEST(search_code_invalid_regex_errors_issue283);
     RUN_TEST(tool_detect_changes_no_project);
     RUN_TEST(tool_manage_adr_no_project);
     RUN_TEST(tool_manage_adr_get_with_existing_adr);