fix(anthropic-stream): avoid bare/duplicate message_stop on OpenAI upstream (#898)

crates/hermesllm/src/apis/streaming_shapes/anthropic_streaming_buffer.rs

@@ -1,6 +1,9 @@
-use crate::apis::anthropic::MessagesStreamEvent;
+use crate::apis::anthropic::{
+    MessagesMessageDelta, MessagesStopReason, MessagesStreamEvent, MessagesUsage,
+};
 use crate::apis::streaming_shapes::sse::{SseEvent, SseStreamBufferTrait};
 use crate::providers::streaming_response::ProviderStreamResponseType;
+use log::warn;
 use std::collections::HashSet;
 
 /// SSE Stream Buffer for Anthropic Messages API streaming.
@@ -11,13 +14,24 @@ use std::collections::HashSet;
 ///
 /// When converting from OpenAI to Anthropic format, this buffer injects the required
 /// ContentBlockStart and ContentBlockStop events to maintain proper Anthropic protocol.
+///
+/// Guarantees (Anthropic Messages API contract):
+/// 1. `message_stop` is never emitted unless a matching `message_start` was emitted first.
+/// 2. `message_stop` is emitted at most once per stream (no double-close).
+/// 3. If upstream terminates with no content (empty/filtered/errored response), a
+///    minimal but well-formed envelope is synthesized so the client's state machine
+///    stays consistent.
 pub struct AnthropicMessagesStreamBuffer {
     /// Buffered SSE events ready to be written to wire
     buffered_events: Vec<SseEvent>,
 
-    /// Track if we've seen a message_start event
+    /// Track if we've emitted a message_start event
     message_started: bool,
 
+    /// Track if we've emitted a terminal message_stop event (for idempotency /
+    /// double-close protection).
+    message_stopped: bool,
+
     /// Track content block indices that have received ContentBlockStart events
     content_block_start_indices: HashSet<i32>,
 
@@ -42,6 +56,7 @@ impl AnthropicMessagesStreamBuffer {
         Self {
             buffered_events: Vec::new(),
             message_started: false,
+            message_stopped: false,
             content_block_start_indices: HashSet::new(),
             needs_content_block_stop: false,
             seen_message_delta: false,
@@ -49,6 +64,66 @@ impl AnthropicMessagesStreamBuffer {
         }
     }
 
+    /// Inject a `message_start` event into the buffer if one hasn't been emitted yet.
+    /// This is the single source of truth for opening a message — every handler
+    /// that can legitimately be the first event on the wire must call this before
+    /// pushing its own event.
+    fn ensure_message_started(&mut self) {
+        if self.message_started {
+            return;
+        }
+        let model = self.model.as_deref().unwrap_or("unknown");
+        let message_start = AnthropicMessagesStreamBuffer::create_message_start_event(model);
+        self.buffered_events.push(message_start);
+        self.message_started = true;
+    }
+
+    /// Inject a synthetic `message_delta` with `end_turn` / zero usage.
+    /// Used when we must close a message but upstream never produced a terminal
+    /// event (e.g. `[DONE]` arrives with no prior `finish_reason`).
+    fn push_synthetic_message_delta(&mut self) {
+        let event = MessagesStreamEvent::MessageDelta {
+            delta: MessagesMessageDelta {
+                stop_reason: MessagesStopReason::EndTurn,
+                stop_sequence: None,
+            },
+            usage: MessagesUsage {
+                input_tokens: 0,
+                output_tokens: 0,
+                cache_creation_input_tokens: None,
+                cache_read_input_tokens: None,
+            },
+        };
+        let sse_string: String = event.clone().into();
+        self.buffered_events.push(SseEvent {
+            data: None,
+            event: Some("message_delta".to_string()),
+            raw_line: sse_string.clone(),
+            sse_transformed_lines: sse_string,
+            provider_stream_response: Some(ProviderStreamResponseType::MessagesStreamEvent(event)),
+        });
+        self.seen_message_delta = true;
+    }
+
+    /// Inject a `message_stop` event into the buffer, marking the stream as closed.
+    /// Idempotent — subsequent calls are no-ops.
+    fn push_message_stop(&mut self) {
+        if self.message_stopped {
+            return;
+        }
+        let message_stop = MessagesStreamEvent::MessageStop;
+        let sse_string: String = message_stop.into();
+        self.buffered_events.push(SseEvent {
+            data: None,
+            event: Some("message_stop".to_string()),
+            raw_line: sse_string.clone(),
+            sse_transformed_lines: sse_string,
+            provider_stream_response: None,
+        });
+        self.message_stopped = true;
+        self.seen_message_delta = false;
+    }
+
     /// Check if a content_block_start event has been sent for the given index
     fn has_content_block_start_been_sent(&self, index: i32) -> bool {
         self.content_block_start_indices.contains(&index)
@@ -149,6 +224,27 @@ impl SseStreamBufferTrait for AnthropicMessagesStreamBuffer {
         // We match on a reference first to determine the type, then move the event
         match &event.provider_stream_response {
             Some(ProviderStreamResponseType::MessagesStreamEvent(evt)) => {
+                // If the message has already been closed, drop any trailing events
+                // to avoid emitting data after `message_stop` (protocol violation).
+                // This typically indicates a duplicate `[DONE]` from upstream or a
+                // replay of previously-buffered bytes — worth surfacing so we can
+                // spot misbehaving providers.
+                if self.message_stopped {
+                    warn!(
+                        "anthropic stream buffer: dropping event after message_stop (variant={})",
+                        match evt {
+                            MessagesStreamEvent::MessageStart { .. } => "message_start",
+                            MessagesStreamEvent::ContentBlockStart { .. } => "content_block_start",
+                            MessagesStreamEvent::ContentBlockDelta { .. } => "content_block_delta",
+                            MessagesStreamEvent::ContentBlockStop { .. } => "content_block_stop",
+                            MessagesStreamEvent::MessageDelta { .. } => "message_delta",
+                            MessagesStreamEvent::MessageStop => "message_stop",
+                            MessagesStreamEvent::Ping => "ping",
+                        }
+                    );
+                    return;
+                }
+
                 match evt {
                     MessagesStreamEvent::MessageStart { .. } => {
                         // Add the message_start event
@@ -157,14 +253,7 @@ impl SseStreamBufferTrait for AnthropicMessagesStreamBuffer {
                     }
                     MessagesStreamEvent::ContentBlockStart { index, .. } => {
                         let index = *index as i32;
-                        // Inject message_start if needed
-                        if !self.message_started {
-                            let model = self.model.as_deref().unwrap_or("unknown");
-                            let message_start =
-                                AnthropicMessagesStreamBuffer::create_message_start_event(model);
-                            self.buffered_events.push(message_start);
-                            self.message_started = true;
-                        }
+                        self.ensure_message_started();
 
                         // Add the content_block_start event (from tool calls or other sources)
                         self.buffered_events.push(event);
@@ -173,14 +262,7 @@ impl SseStreamBufferTrait for AnthropicMessagesStreamBuffer {
                     }
                     MessagesStreamEvent::ContentBlockDelta { index, .. } => {
                         let index = *index as i32;
-                        // Inject message_start if needed
-                        if !self.message_started {
-                            let model = self.model.as_deref().unwrap_or("unknown");
-                            let message_start =
-                                AnthropicMessagesStreamBuffer::create_message_start_event(model);
-                            self.buffered_events.push(message_start);
-                            self.message_started = true;
-                        }
+                        self.ensure_message_started();
 
                         // Check if ContentBlockStart was sent for this index
                         if !self.has_content_block_start_been_sent(index) {
@@ -196,6 +278,11 @@ impl SseStreamBufferTrait for AnthropicMessagesStreamBuffer {
                         self.buffered_events.push(event);
                     }
                     MessagesStreamEvent::MessageDelta { usage, .. } => {
+                        // `message_delta` is only meaningful inside an open message.
+                        // Upstream can send it with no prior content (empty completion,
+                        // content filter, etc.), so we must open a message first.
+                        self.ensure_message_started();
+
                         // Inject ContentBlockStop before message_delta
                         if self.needs_content_block_stop {
                             let content_block_stop =
@@ -230,15 +317,52 @@ impl SseStreamBufferTrait for AnthropicMessagesStreamBuffer {
                     }
                     MessagesStreamEvent::ContentBlockStop { .. } => {
                         // ContentBlockStop received from upstream (e.g., Bedrock)
+                        self.ensure_message_started();
                         // Clear the flag so we don't inject another one
                         self.needs_content_block_stop = false;
                         self.buffered_events.push(event);
                     }
                     MessagesStreamEvent::MessageStop => {
-                        // MessageStop received from upstream (e.g., OpenAI via [DONE])
-                        // Clear the flag so we don't inject another one
-                        self.seen_message_delta = false;
+                        // MessageStop received from upstream (e.g., OpenAI via [DONE]).
+                        //
+                        // The Anthropic protocol requires the full envelope
+                        //   message_start → [content blocks] → message_delta → message_stop
+                        // so we must not emit a bare `message_stop`. Synthesize whatever
+                        // is missing to keep the client's state machine consistent.
+                        self.ensure_message_started();
+
+                        if self.needs_content_block_stop {
+                            let content_block_stop =
+                                AnthropicMessagesStreamBuffer::create_content_block_stop_event();
+                            self.buffered_events.push(content_block_stop);
+                            self.needs_content_block_stop = false;
+                        }
+
+                        // If no message_delta has been emitted yet (empty/filtered upstream
+                        // response), synthesize a minimal one carrying `end_turn`.
+                        if !self.seen_message_delta {
+                            // If we also never opened a content block, open and close one
+                            // so clients that expect at least one block are happy.
+                            if self.content_block_start_indices.is_empty() {
+                                let content_block_start =
+                                    AnthropicMessagesStreamBuffer::create_content_block_start_event(
+                                    );
+                                self.buffered_events.push(content_block_start);
+                                self.set_content_block_start_sent(0);
+                                let content_block_stop =
+                                    AnthropicMessagesStreamBuffer::create_content_block_stop_event(
+                                    );
+                                self.buffered_events.push(content_block_stop);
+                            }
+                            self.push_synthetic_message_delta();
+                        }
+
+                        // Push the upstream-provided message_stop and mark closed.
+                        // `push_message_stop` is idempotent but we want to reuse the
+                        // original SseEvent so raw passthrough semantics are preserved.
                         self.buffered_events.push(event);
+                        self.message_stopped = true;
+                        self.seen_message_delta = false;
                     }
                     _ => {
                         // Other Anthropic event types (Ping, etc.), just accumulate
@@ -254,24 +378,23 @@ impl SseStreamBufferTrait for AnthropicMessagesStreamBuffer {
     }
 
     fn to_bytes(&mut self) -> Vec<u8> {
-        // Convert all accumulated events to bytes and clear buffer
+        // Convert all accumulated events to bytes and clear buffer.
+        //
         // NOTE: We do NOT inject ContentBlockStop here because it's injected when we see MessageDelta
         // or MessageStop. Injecting it here causes premature ContentBlockStop in the middle of streaming.
-
-        // Inject MessageStop after MessageDelta if we've seen one
-        // This completes the Anthropic Messages API event sequence
-        if self.seen_message_delta {
-            let message_stop = MessagesStreamEvent::MessageStop;
-            let sse_string: String = message_stop.into();
-            let message_stop_event = SseEvent {
-                data: None,
-                event: Some("message_stop".to_string()),
-                raw_line: sse_string.clone(),
-                sse_transformed_lines: sse_string,
-                provider_stream_response: None,
-            };
-            self.buffered_events.push(message_stop_event);
-            self.seen_message_delta = false;
+        //
+        // Inject a synthetic `message_stop` only when:
+        //   1. A `message_delta` has been seen (otherwise we'd violate the Anthropic
+        //      protocol by emitting `message_stop` without a preceding `message_delta`), AND
+        //   2. We haven't already emitted `message_stop` (either synthetic from a
+        //      previous flush, or real from an upstream `[DONE]`).
+        //
+        // Without the `!message_stopped` guard, a stream whose `finish_reason` chunk
+        // and `[DONE]` marker land in separate HTTP body chunks would receive two
+        // `message_stop` events, triggering Claude Code's "Received message_stop
+        // without a current message" error.
+        if self.seen_message_delta && !self.message_stopped {
+            self.push_message_stop();
         }
 
         let mut buffer = Vec::new();
@@ -615,4 +738,133 @@ data: [DONE]"#;
         println!("✓ Stop reason: tool_use");
         println!("✓ Proper Anthropic tool_use protocol\n");
     }
+
+    /// Regression test for:
+    ///   Claude Code CLI error: "Received message_stop without a current message"
+    ///
+    /// Reproduces the *double-close* scenario: OpenAI's final `finish_reason`
+    /// chunk and the `[DONE]` marker arrive in **separate** HTTP body chunks, so
+    /// `to_bytes()` is called between them. Before the fix, this produced two
+    /// `message_stop` events on the wire (one synthetic, one from `[DONE]`).
+    #[test]
+    fn test_openai_to_anthropic_emits_single_message_stop_across_chunk_boundary() {
+        let client_api = SupportedAPIsFromClient::AnthropicMessagesAPI(AnthropicApi::Messages);
+        let upstream_api = SupportedUpstreamAPIs::OpenAIChatCompletions(OpenAIApi::ChatCompletions);
+        let mut buffer = AnthropicMessagesStreamBuffer::new();
+
+        // --- HTTP chunk 1: content + finish_reason (no [DONE] yet) -----------
+        let chunk_1 = r#"data: {"id":"c1","object":"chat.completion.chunk","created":1,"model":"gpt-4o","choices":[{"index":0,"delta":{"role":"assistant","content":"Hi"},"finish_reason":null}]}
+
+data: {"id":"c1","object":"chat.completion.chunk","created":1,"model":"gpt-4o","choices":[{"index":0,"delta":{},"finish_reason":"stop"}]}"#;
+
+        for raw in SseStreamIter::try_from(chunk_1.as_bytes()).unwrap() {
+            let e = SseEvent::try_from((raw, &client_api, &upstream_api)).unwrap();
+            buffer.add_transformed_event(e);
+        }
+        let out_1 = String::from_utf8(buffer.to_bytes()).unwrap();
+
+        // --- HTTP chunk 2: just the [DONE] marker ----------------------------
+        let chunk_2 = "data: [DONE]";
+        for raw in SseStreamIter::try_from(chunk_2.as_bytes()).unwrap() {
+            let e = SseEvent::try_from((raw, &client_api, &upstream_api)).unwrap();
+            buffer.add_transformed_event(e);
+        }
+        let out_2 = String::from_utf8(buffer.to_bytes()).unwrap();
+
+        let combined = format!("{}{}", out_1, out_2);
+        let start_count = combined.matches("event: message_start").count();
+        let stop_count = combined.matches("event: message_stop").count();
+
+        assert_eq!(
+            start_count, 1,
+            "Must emit exactly one message_start across chunks, got {start_count}. Output:\n{combined}"
+        );
+        assert_eq!(
+            stop_count, 1,
+            "Must emit exactly one message_stop across chunks (no double-close), got {stop_count}. Output:\n{combined}"
+        );
+        // Every message_stop must be preceded by a message_start earlier in the stream.
+        let start_pos = combined.find("event: message_start").unwrap();
+        let stop_pos = combined.find("event: message_stop").unwrap();
+        assert!(
+            start_pos < stop_pos,
+            "message_start must come before message_stop. Output:\n{combined}"
+        );
+    }
+
+    /// Regression test for:
+    ///   "Received message_stop without a current message" on empty upstream responses.
+    ///
+    /// OpenAI returns only `[DONE]` with no content deltas and no `finish_reason`
+    /// (this happens with content filters, truncated upstream streams, and some
+    /// 5xx recoveries). Before the fix, the buffer emitted a bare `message_stop`
+    /// with no preceding `message_start`. After the fix, it synthesizes a
+    /// minimal but well-formed envelope.
+    #[test]
+    fn test_openai_done_only_stream_synthesizes_valid_envelope() {
+        let client_api = SupportedAPIsFromClient::AnthropicMessagesAPI(AnthropicApi::Messages);
+        let upstream_api = SupportedUpstreamAPIs::OpenAIChatCompletions(OpenAIApi::ChatCompletions);
+        let mut buffer = AnthropicMessagesStreamBuffer::new();
+
+        let raw_input = "data: [DONE]";
+        for raw in SseStreamIter::try_from(raw_input.as_bytes()).unwrap() {
+            let e = SseEvent::try_from((raw, &client_api, &upstream_api)).unwrap();
+            buffer.add_transformed_event(e);
+        }
+        let out = String::from_utf8(buffer.to_bytes()).unwrap();
+
+        assert!(
+            out.contains("event: message_start"),
+            "Empty upstream must still produce message_start. Output:\n{out}"
+        );
+        assert!(
+            out.contains("event: message_delta"),
+            "Empty upstream must produce a synthesized message_delta. Output:\n{out}"
+        );
+        assert_eq!(
+            out.matches("event: message_stop").count(),
+            1,
+            "Empty upstream must produce exactly one message_stop. Output:\n{out}"
+        );
+
+        // Protocol ordering: start < delta < stop.
+        let p_start = out.find("event: message_start").unwrap();
+        let p_delta = out.find("event: message_delta").unwrap();
+        let p_stop = out.find("event: message_stop").unwrap();
+        assert!(
+            p_start < p_delta && p_delta < p_stop,
+            "Bad ordering. Output:\n{out}"
+        );
+    }
+
+    /// Regression test: events arriving after `message_stop` (e.g. a stray `[DONE]`
+    /// echo, or late-arriving deltas from a racing upstream) must be dropped
+    /// rather than written after the terminal frame.
+    #[test]
+    fn test_events_after_message_stop_are_dropped() {
+        let client_api = SupportedAPIsFromClient::AnthropicMessagesAPI(AnthropicApi::Messages);
+        let upstream_api = SupportedUpstreamAPIs::OpenAIChatCompletions(OpenAIApi::ChatCompletions);
+        let mut buffer = AnthropicMessagesStreamBuffer::new();
+
+        let first = r#"data: {"id":"c1","object":"chat.completion.chunk","created":1,"model":"gpt-4o","choices":[{"index":0,"delta":{"content":"ok"},"finish_reason":"stop"}]}
+
+data: [DONE]"#;
+        for raw in SseStreamIter::try_from(first.as_bytes()).unwrap() {
+            let e = SseEvent::try_from((raw, &client_api, &upstream_api)).unwrap();
+            buffer.add_transformed_event(e);
+        }
+        let _ = buffer.to_bytes();
+
+        // Simulate a duplicate / late `[DONE]` after the stream was already closed.
+        let late = "data: [DONE]";
+        for raw in SseStreamIter::try_from(late.as_bytes()).unwrap() {
+            let e = SseEvent::try_from((raw, &client_api, &upstream_api)).unwrap();
+            buffer.add_transformed_event(e);
+        }
+        let tail = String::from_utf8(buffer.to_bytes()).unwrap();
+        assert!(
+            tail.is_empty(),
+            "No bytes should be emitted after message_stop, got: {tail:?}"
+        );
+    }
 }