fix(translation): reject incomplete upstream streams

crates/switchyard-translation/src/helpers.rs

@@ -18,8 +18,9 @@ use switchyard_protocol::LlmClientError;
 use crate::codecs::stream::encode_response_stream_event;
 use crate::sse;
 use crate::{
-    AggLlmResponse, FormatId, LlmRequest, LlmResponseStream, LlmResponseStreamEvent, Result,
-    StreamCodecRegistry, StreamTranslationState, TranslationEngine, TranslationPolicy, WireFormat,
+    AggLlmResponse, FormatId, LlmRequest, LlmResponseChunk, LlmResponseStream,
+    LlmResponseStreamEvent, Result, StreamCodecRegistry, StreamTranslationState, TranslationEngine,
+    TranslationPolicy, WireFormat,
 };
 
 static DEFAULT_TRANSLATION_POLICY: LazyLock<TranslationPolicy> =
@@ -200,6 +201,8 @@ where
         ..StreamTranslationState::default()
     };
     let mut frame = String::new();
+    let mut saw_terminal = false;
+    let mut saw_error = false;
     let stream = Box::pin(try_stream! {
         futures::pin_mut!(lines);
         while let Some(line) = lines.next().await {
@@ -211,9 +214,18 @@ where
                 frame.clear();
                 match parsed {
                     sse::SseFrame::Empty => {}
-                    sse::SseFrame::Done => break,
+                    sse::SseFrame::Done => {
+                        saw_terminal |= source != WireFormat::AnthropicMessages;
+                        break;
+                    }
                     sse::SseFrame::Data(value) => {
+                        saw_terminal |= sse::is_terminal_event(source, &value);
                         let normalized = codec.decode_event(&mut state, &value);
+                        saw_error |= normalized.iter().any(|chunk| matches!(
+                            chunk,
+                            LlmResponseChunk::DecodeError { .. }
+                                | LlmResponseChunk::StreamError { .. }
+                        ));
                         yield LlmResponseStreamEvent::preserved(
                             source_format.clone(),
                             value,
@@ -233,11 +245,29 @@ where
         if !frame.trim_end().is_empty() {
             let parsed = sse::parse_json_sse_frame(&frame, marker)
                 .map_err(|error| LlmClientError::ResponseTranslation(error.to_string()))?;
-            if let sse::SseFrame::Data(value) = parsed {
-                let normalized = codec.decode_event(&mut state, &value);
-                yield LlmResponseStreamEvent::preserved(source_format, value, normalized);
+            match parsed {
+                sse::SseFrame::Done => {
+                    saw_terminal |= source != WireFormat::AnthropicMessages;
+                }
+                sse::SseFrame::Data(value) => {
+                    saw_terminal |= sse::is_terminal_event(source, &value);
+                    let normalized = codec.decode_event(&mut state, &value);
+                    saw_error |= normalized.iter().any(|chunk| matches!(
+                        chunk,
+                        LlmResponseChunk::DecodeError { .. }
+                            | LlmResponseChunk::StreamError { .. }
+                    ));
+                    yield LlmResponseStreamEvent::preserved(source_format, value, normalized);
+                }
+                sse::SseFrame::Empty => {}
             }
         }
+
+        if !saw_terminal && !saw_error {
+            Err(LlmClientError::ResponseTranslation(format!(
+                "{source} stream ended before a terminal event"
+            )))?;
+        }
     });
     Ok(stream)
 }
@@ -683,13 +713,28 @@ mod tests {
     fn decode_stream_decodes_trailing_frame_without_blank_line() -> Result<(), BoxError> {
         // A non-standard upstream omits the final blank line; the last frame
         // must still be decoded rather than dropped.
-        let sse = b"data: {\"choices\":[{\"delta\":{\"content\":\"tail\"}}]}".to_vec();
+        let sse =
+            b"data: {\"choices\":[{\"delta\":{\"content\":\"tail\"}}]}\n\ndata: [DONE]".to_vec();
         let bytes = stream::once(async move { Ok::<Vec<u8>, LlmClientError>(sse) });
         let chunks = decode_all(bytes, WireFormat::OpenAiChat)?;
         assert_eq!(text_of(&chunks), "tail");
         Ok(())
     }
 
+    #[test]
+    fn decode_stream_rejects_eof_before_a_terminal_event() -> Result<(), BoxError> {
+        let sse = b"data: {\"choices\":[{\"delta\":{\"content\":\"partial\"},\"finish_reason\":null}]}\n\n".to_vec();
+        let bytes = stream::once(async move { Ok::<Vec<u8>, LlmClientError>(sse) });
+        let results = block_on(decode_stream(bytes, WireFormat::OpenAiChat)?.collect::<Vec<_>>());
+
+        assert!(results.first().is_some_and(Result::is_ok));
+        let Some(Err(LlmClientError::ResponseTranslation(message))) = results.last() else {
+            panic!("expected incomplete OpenAI stream to fail");
+        };
+        assert_eq!(message, "openai_chat stream ended before a terminal event");
+        Ok(())
+    }
+
     #[test]
     fn decode_stream_decodes_crlf_delimited_frames() -> Result<(), BoxError> {
         // CRLF framing: blank lines are `\r\n\r\n` and the bare `\r` must not

crates/switchyard-translation/src/sse.rs

@@ -29,6 +29,33 @@ pub(crate) fn done_marker(_format: WireFormat) -> Option<&'static str> {
     Some("[DONE]")
 }
 
+/// Returns whether a provider event explicitly completes its wire-format stream.
+pub(crate) fn is_terminal_event(format: WireFormat, event: &Value) -> bool {
+    match format {
+        WireFormat::OpenAiChat => event
+            .get("choices")
+            .and_then(Value::as_array)
+            .into_iter()
+            .flatten()
+            .any(|choice| {
+                choice
+                    .get("finish_reason")
+                    .and_then(Value::as_str)
+                    .is_some()
+            }),
+        WireFormat::AnthropicMessages => {
+            event.get("type").and_then(Value::as_str) == Some("message_stop")
+        }
+        WireFormat::OpenAiResponses => matches!(
+            event
+                .get("type")
+                .or_else(|| event.get("event"))
+                .and_then(Value::as_str),
+            Some("response.completed" | "response.incomplete")
+        ),
+    }
+}
+
 pub(crate) fn parse_json_sse_frame(
     frame: &str,
     done_marker: Option<&str>,
@@ -125,4 +152,24 @@ mod tests {
     fn anthropic_accepts_optional_done_marker() {
         assert_eq!(done_marker(WireFormat::AnthropicMessages), Some("[DONE]"));
     }
+
+    #[test]
+    fn recognizes_provider_terminal_events() {
+        assert!(is_terminal_event(
+            WireFormat::OpenAiChat,
+            &json!({"choices": [{"finish_reason": "stop"}]})
+        ));
+        assert!(is_terminal_event(
+            WireFormat::AnthropicMessages,
+            &json!({"type": "message_stop"})
+        ));
+        assert!(is_terminal_event(
+            WireFormat::OpenAiResponses,
+            &json!({"type": "response.completed"})
+        ));
+        assert!(!is_terminal_event(
+            WireFormat::OpenAiChat,
+            &json!({"choices": [{"finish_reason": null}]})
+        ));
+    }
 }