fix(cursor): preserve reserved-prefix call ids

src/adapters/cursor/call-id.ts

@@ -5,9 +5,10 @@
  * literal newline ("call-<uuid>-<n>\nfc_<uuid>_<n>"). OpenCodex forwards ids
  * verbatim, so that newline leaked into Responses-visible `call_id` values,
  * where line-oriented clients (logging, splitting, validation) break. The codec
- * encodes only ids containing CR/LF into a versioned single-line form and
- * decodes both that form and legacy raw multi-line ids back to the exact
- * upstream bytes before anything is serialized toward Cursor.
+ * encodes ids containing CR/LF into a versioned single-line form. It also
+ * escapes ids already in that form's reserved namespace so encoding remains
+ * injective. Both forms decode back to the exact upstream bytes before
+ * anything is serialized toward Cursor.
  */
 
 const CALL_ID_PREFIX = "ocxc1_";
@@ -19,7 +20,7 @@ function needsEncoding(id: string): boolean {
 
 /** Encode a Cursor wire call id into a single-line Responses-safe id. */
 export function encodeCursorCallId(id: string): string {
-  if (!needsEncoding(id)) return id;
+  if (!needsEncoding(id) && !id.startsWith(CALL_ID_PREFIX)) return id;
   return CALL_ID_PREFIX + Buffer.from(id, "utf8").toString("base64url");
 }
 
@@ -37,6 +38,7 @@ 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) && !decoded.startsWith(CALL_ID_PREFIX)) return id;
     return decoded;
   } catch {
     return id;

tests/cursor-call-id.test.ts

@@ -17,6 +17,30 @@ describe("cursor call-id codec", () => {
     expect(decodeCursorCallId("call_abc123")).toBe("call_abc123");
   });
 
+  test("reserved-prefix ids are escaped and round-trip", () => {
+    for (const id of ["ocxc1_", "ocxc1_Y2FsbF8x", "ocxc1_!!not-base64url!!", "ocxc1_raw\nwire"]) {
+      const encoded = encodeCursorCallId(id);
+      expect(encoded).not.toBe(id);
+      expect(encoded.startsWith("ocxc1_")).toBe(true);
+      expect(encoded).not.toContain("\n");
+      expect(encoded).not.toContain("\r");
+      expect(decodeCursorCallId(encoded)).toBe(id);
+    }
+  });
+
+  test("reserved-prefix ids resembling legacy newline encodings stay opaque", () => {
+    const id = "ocxc1_YQpi";
+    const encoded = encodeCursorCallId(id);
+    expect(encoded).not.toBe(id);
+    expect(decodeCursorCallId(encoded)).toBe(id);
+  });
+
+  test("legacy encoded line breaks remain decodable", () => {
+    expect(decodeCursorCallId("ocxc1_YQpi")).toBe("a\nb");
+    expect(decodeCursorCallId("ocxc1_DQ")).toBe("\r");
+    expect(decodeCursorCallId("ocxc1_DQo")).toBe("\r\n");
+  });
+
   test("newline composite id round-trips through a single-line form", () => {
     const encoded = encodeCursorCallId(COMPOSITE);
     expect(encoded).not.toContain("\n");
@@ -34,15 +58,18 @@ describe("cursor call-id codec", () => {
     expect(decodeCursorCallId("ocxc1_!!not-base64url!!")).toBe("ocxc1_!!not-base64url!!");
   });
 
-  test("tool_call_start ids are single-line at the adapter boundary", () => {
-    const events = mapCursorServerMessage(
-      { type: "tool_call_start", id: COMPOSITE, name: "get_weather" },
-      mapperState(),
-    );
-    expect(events).toHaveLength(1);
-    const event = events[0]!;
-    if (event.type !== "tool_call_start") throw new Error("expected tool_call_start");
-    expect(event.id).not.toContain("\n");
-    expect(decodeCursorCallId(event.id)).toBe(COMPOSITE);
+  test("tool_call_start ids are reversible and single-line at the adapter boundary", () => {
+    for (const id of [COMPOSITE, "ocxc1_YQpi"]) {
+      const events = mapCursorServerMessage(
+        { type: "tool_call_start", id, name: "get_weather" },
+        mapperState(),
+      );
+      expect(events).toHaveLength(1);
+      const event = events[0]!;
+      if (event.type !== "tool_call_start") throw new Error("expected tool_call_start");
+      expect(event.id).not.toContain("\n");
+      expect(event.id).not.toContain("\r");
+      expect(decodeCursorCallId(event.id)).toBe(id);
+    }
   });
 });