diff --git a/src/adapters/cursor/call-id.ts b/src/adapters/cursor/call-id.ts
index a76707d00a..03f7677ed4 100644
--- a/src/adapters/cursor/call-id.ts
+++ b/src/adapters/cursor/call-id.ts
@@ -37,6 +37,7 @@ export function decodeCursorCallId(id: string): string {
     const decoded = Buffer.from(payload, "base64url").toString("utf8");
     // Round-trip guard: only trust payloads our encoder could have produced.
     if (Buffer.from(decoded, "utf8").toString("base64url") !== payload) return id;
+    if (!needsEncoding(decoded)) return id;
     return decoded;
   } catch {
     return id;
diff --git a/tests/cursor-call-id.test.ts b/tests/cursor-call-id.test.ts
index def2ece14c..faa170a793 100644
--- a/tests/cursor-call-id.test.ts
+++ b/tests/cursor-call-id.test.ts
@@ -17,6 +17,12 @@ describe("cursor call-id codec", () => {
     expect(decodeCursorCallId("call_abc123")).toBe("call_abc123");
   });
 
+  test("plain ids in the reserved namespace pass through unchanged", () => {
+    const id = "ocxc1_Y2FsbF8x";
+    expect(encodeCursorCallId(id)).toBe(id);
+    expect(decodeCursorCallId(id)).toBe(id);
+  });
+
   test("newline composite id round-trips through a single-line form", () => {
     const encoded = encodeCursorCallId(COMPOSITE);
     expect(encoded).not.toContain("\n");