diff --git a/src/server/responses/responses-field-backfill.ts b/src/server/responses/responses-field-backfill.ts
index 4d67531020..32cf727891 100644
--- a/src/server/responses/responses-field-backfill.ts
+++ b/src/server/responses/responses-field-backfill.ts
@@ -34,6 +34,14 @@ const ITEM_ID_PREFIXES: Readonly<Record<string, string>> = {
   message: "msg_",
   reasoning: "rs_",
   function_call: "fc_",
+  custom_tool_call: "ctc_",
+  // A routed tool_search lowering is restored to `tool_search_call` without an id, so this
+  // backfill is what names it. The generic `item_` fallback is not merely cosmetic here:
+  // `stripInvalidItemIds` in the Responses adapter deletes any id whose prefix does not match
+  // the type, so an `item_`-named tool_search_call silently loses its id on the NEXT turn and
+  // the client sees an item it cannot correlate. The prefixes here must stay a superset of the
+  // ones that serializer enforces.
+  tool_search_call: "tsc_",
   web_search_call: "ws_",
   file_search_call: "fs_",
   code_interpreter_call: "ci_",
diff --git a/tests/responses-field-backfill.test.ts b/tests/responses-field-backfill.test.ts
index f6fb63bae3..f4ac338b69 100644
--- a/tests/responses-field-backfill.test.ts
+++ b/tests/responses-field-backfill.test.ts
@@ -365,6 +365,36 @@ describe("responses-field-backfill", () => {
     expect(result.output[0]!.id).toBe("ig_ocx_0");
   });
 
+  // A routed tool_search lowering is restored as `tool_search_call` with no id, so this
+  // backfill names it. The generic `item_` fallback was not cosmetic: `stripInvalidItemIds`
+  // in the Responses adapter deletes an id whose prefix does not match its type, so the item
+  // silently lost its id on the NEXT turn and the client could no longer correlate it.
+  test("tool_search_call gets the prefix the request serializer accepts", () => {
+    const response = {
+      id: "resp_1",
+      object: "response",
+      status: "completed",
+      output: [{ type: "tool_search_call", call_id: "call_x", execution: "client" }],
+    };
+    const result = JSON.parse(backfillResponsesFieldsJson(JSON.stringify(response))) as {
+      output: { id: string }[];
+    };
+    expect(result.output[0]!.id).toBe("tsc_ocx_0");
+  });
+
+  test("custom_tool_call gets its own prefix too", () => {
+    const response = {
+      id: "resp_1",
+      object: "response",
+      status: "completed",
+      output: [{ type: "custom_tool_call", call_id: "call_y" }],
+    };
+    const result = JSON.parse(backfillResponsesFieldsJson(JSON.stringify(response))) as {
+      output: { id: string }[];
+    };
+    expect(result.output[0]!.id).toBe("ctc_ocx_0");
+  });
+
   // A malformed `output_index` falls back to a counter. While that counter lived in the same
   // numeric namespace as real indexes, a response whose real index reached the counter's base
   // produced the SAME id as a fallback — a duplicate, which is the one thing this backfill