fix(responses): preserve streamed response IDs for conversation continuation

crates/switchyard-translation/src/codecs/responses/stream.rs

@@ -1148,14 +1148,9 @@ fn fnv1a_64(text: &str) -> u64 {
     })
 }
 
-// Converts any upstream message ID into a Responses-looking response ID.
+// Keep the response ID aligned with buffered responses and stored conversation history.
 fn responses_id(state: &StreamTranslationState) -> String {
-    let Some(id) = target_message_id_or_source_message_id(state) else {
-        return "resp_switchyard".to_string();
-    };
-    if id.starts_with("resp_") {
-        id.to_string()
-    } else {
-        format!("resp_{id}")
-    }
+    target_message_id_or_source_message_id(state)
+        .unwrap_or("resp_switchyard")
+        .to_string()
 }

crates/switchyard-translation/tests/stream_translation.rs

@@ -828,6 +828,22 @@ fn openai_chat_to_responses_uses_served_model_without_losing_source_model() -> T
     Ok(())
 }
 
+// Stored conversation history is keyed by the upstream ID, so clients must see it unchanged.
+#[test]
+fn openai_chat_to_responses_keeps_the_upstream_id() -> TestResult {
+    let mut state =
+        StreamTranslationState::new(WireFormat::OpenAiChat, WireFormat::OpenAiResponses);
+    let events = TranslationEngine::default().translate_event(
+        &mut state,
+        WireFormat::OpenAiChat,
+        WireFormat::OpenAiResponses,
+        &json!({"id": "chatcmpl-test", "choices": [{"index": 0, "delta": {"content": "hi"}}]}),
+    )?;
+
+    assert_eq!(events[0]["response"]["id"], "chatcmpl-test");
+    Ok(())
+}
+
 // Verifies OpenAI Chat finish emits a terminal chunk when the source closes without one.
 #[test]
 fn openai_chat_finish_synthesizes_terminal_chunk_after_incomplete_source() -> TestResult {