diff --git a/src/cli/client_adapter.c b/src/cli/client_adapter.c
index 83a4042764..cfd2719c78 100644
--- a/src/cli/client_adapter.c
+++ b/src/cli/client_adapter.c
@@ -148,38 +148,36 @@ static void append_typebox_type(adapter_sb_t *sb, yyjson_val *val, const char *d
 
     if (type_str && strcmp(type_str, "string") == 0 && enum_val && yyjson_is_arr(enum_val) &&
         yyjson_arr_size(enum_val) > 0) {
-        sb_append(sb, "Type.Union([");
         size_t enum_idx = 0;
         size_t enum_max = yyjson_arr_size(enum_val);
-        bool first_enum = true;
+        size_t valid_count = 0;
         for (enum_idx = 0; enum_idx < enum_max; enum_idx++) {
-            yyjson_val *enum_item = yyjson_arr_get(enum_val, enum_idx);
-            const char *enum_str = yyjson_get_str(enum_item);
-            if (!enum_str) {
-                continue;
+            if (yyjson_get_str(yyjson_arr_get(enum_val, enum_idx))) {
+                valid_count++;
             }
-            if (!first_enum) {
-                sb_append(sb, ", ");
-            }
-            first_enum = false;
-            sb_append(sb, "Type.Literal(");
-            sb_append_js_string(sb, enum_str);
-            sb_append(sb, ")");
         }
-        if (first_enum) {
-            sb_append(sb, "Type.String()");
-        } else {
+        if (valid_count > 0) {
+            sb_append(sb, "Type.String({ enum: [");
+            bool first_enum = true;
+            for (enum_idx = 0; enum_idx < enum_max; enum_idx++) {
+                const char *enum_str = yyjson_get_str(yyjson_arr_get(enum_val, enum_idx));
+                if (!enum_str) {
+                    continue;
+                }
+                if (!first_enum) {
+                    sb_append(sb, ", ");
+                }
+                first_enum = false;
+                sb_append_js_string(sb, enum_str);
+            }
             sb_append(sb, "]");
             if (include_description && description) {
-                sb_append(sb, ", { description: ");
+                sb_append(sb, ", description: ");
                 sb_append_js_string(sb, description);
-                sb_append(sb, " }");
             }
-            sb_append(sb, ")");
+            sb_append(sb, " })");
             return;
         }
-        sb_append(sb, ")");
-        return;
     }
 
     const char *typebox_name = "Any";
diff --git a/tests/test_agent_clients.c b/tests/test_agent_clients.c
index f1bc3699e1..0d57723a07 100644
--- a/tests/test_agent_clients.c
+++ b/tests/test_agent_clients.c
@@ -1177,6 +1177,10 @@ TEST(client_adapter_pi_emits_parameters_and_execute) {
     /* TypeBox parameters schema is embedded with Type.Object syntax. */
     ASSERT_NOT_NULL(strstr(js, "Type.Object("));
     ASSERT_NOT_NULL(strstr(js, "Type.String("));
+    /* #2787: String enums must use Type.String({ enum: [...] }) without anyOf/const */
+    ASSERT_NOT_NULL(strstr(js, "Type.String({ enum: ["));
+    ASSERT_NULL(strstr(js, "Type.Union([Type.Literal"));
+    ASSERT_NULL(strstr(js, "Type.Literal("));
     /* A numeric registry property must remain numeric in the generated schema. */
     ASSERT_NOT_NULL(strstr(js, "depth: Type.Optional(Type.Integer("));
     /* TypeBox import should be present. */
@@ -1244,6 +1248,23 @@ TEST(client_adapter_pi_wraps_result_and_honors_abort) {
     PASS();
 }
 
+/* #2787: Pi extension must emit string enums as Type.String({ enum: [...] })
+ * rather than Type.Union([Type.Literal(...)]) which TypeBox serializes as
+ * anyOf/const, breaking Gemini / Antigravity function declaration validation. */
+TEST(client_adapter_pi_emits_string_enum_issue2787) {
+    char *js = cbm_client_adapter_pi("/usr/local/bin/codebase-memory-mcp");
+    ASSERT_NOT_NULL(js);
+    ASSERT_NULL(strstr(js, "Type.Union([Type.Literal"));
+    ASSERT_NULL(strstr(js, "Type.Literal("));
+    ASSERT_NOT_NULL(strstr(js, "Type.String({ enum: ["));
+    ASSERT_NOT_NULL(strstr(js, "'full'"));
+    ASSERT_NOT_NULL(strstr(js, "'moderate'"));
+    ASSERT_NOT_NULL(strstr(js, "'fast'"));
+    ASSERT_NOT_NULL(strstr(js, "'cross-repo-intelligence'"));
+    free(js);
+    PASS();
+}
+
 /* #616 rejected `"` in its path template but not `\`, so a Windows home like
  * C:\Users\urs\bin produced `\u` — an invalid unicode escape — and the whole
  * auto-loaded plugin failed to parse. A broken plugin in an auto-load directory
@@ -1405,6 +1426,7 @@ SUITE(agent_clients) {
     RUN_TEST(client_adapter_pi_emits_parameters_and_execute);
     RUN_TEST(client_adapter_pi_sends_arguments_over_stdin_issue1834);
     RUN_TEST(client_adapter_pi_wraps_result_and_honors_abort);
+    RUN_TEST(client_adapter_pi_emits_string_enum_issue2787);
     RUN_TEST(client_adapter_escapes_windows_paths_and_quotes);
     RUN_TEST(client_adapter_opencode_sends_the_required_hook_event);
     RUN_TEST(client_adapter_opencode_covers_lifecycle_read_and_compaction);