fix(translation): drop empty assistant messages in OpenAI Chat encoding

crates/switchyard-translation/src/codecs/openai_chat/buffered.rs

@@ -205,6 +205,12 @@ impl FormatCodec for OpenAiChatCodec {
         for message in &request.messages {
             messages.extend(encode_message_to_openai(message, &mut diagnostics, policy)?);
         }
+        // Some upstreams (e.g. Moonshot/Kimi) reject an assistant message whose
+        // `content` is an empty string, which normalized turns with no text and
+        // no tool calls (e.g. a replayed reasoning-only turn) would otherwise
+        // produce. Such a message carries no information, so drop it rather than
+        // send it upstream.
+        messages.retain(|message| !is_empty_assistant_message(message));
         body.insert("messages".to_string(), Value::Array(messages));
 
         if !request.tools.is_empty() {
@@ -841,6 +847,16 @@ fn is_empty_text_only(content: &[ContentBlock]) -> bool {
     matches!(content, [ContentBlock::Text { text }] if text.is_empty())
 }
 
+// An assistant message with an empty string `content` and no `tool_calls` — a
+// tool-call-only turn instead encodes `content` as `null` (see
+// `encode_message_without_tool_results_to_openai`), so this only matches a
+// turn that carries no information at all.
+fn is_empty_assistant_message(message: &Value) -> bool {
+    message.get("role").and_then(Value::as_str) == Some("assistant")
+        && message.get("content") == Some(&Value::String(String::new()))
+        && message.get("tool_calls").is_none()
+}
+
 /// Encodes normalized content into OpenAI Chat content JSON.
 pub(crate) fn encode_openai_content(
     content: &[ContentBlock],

crates/switchyard-translation/tests/request_translation.rs

@@ -1588,3 +1588,55 @@ fn anthropic_thinking_is_dropped_from_responses_input() -> TestResult {
     );
     Ok(())
 }
+
+// A replayed assistant turn that carried only thinking blocks (no text, no
+// tool call) would otherwise encode as `{"role": "assistant", "content": ""}`
+// for OpenAI Chat — a shape some upstreams (e.g. Moonshot/Kimi) reject outright
+// ("the message at position N with role 'assistant' must not be empty"). The
+// turn carries no information once its thinking is dropped, so it must not
+// appear in the encoded messages at all.
+#[test]
+fn anthropic_thinking_only_turn_does_not_encode_an_empty_openai_chat_message() -> TestResult {
+    let engine = TranslationEngine::default();
+    let body = json!({
+        "model": "claude-opus-4-20250514",
+        "messages": [
+            {"role": "user", "content": "Think about it."},
+            {
+                "role": "assistant",
+                "content": [
+                    {
+                        "type": "thinking",
+                        "thinking": "Just thinking, nothing to say.",
+                        "signature": "sig-abc"
+                    }
+                ]
+            },
+            {"role": "user", "content": "Continue."}
+        ],
+        "max_tokens": 2048
+    });
+
+    let output = engine
+        .translate_request(
+            WireFormat::AnthropicMessages,
+            WireFormat::OpenAiChat,
+            &body,
+            &TranslationPolicy::default(),
+        )?
+        .body;
+
+    let messages = output["messages"]
+        .as_array()
+        .ok_or("messages is not an array")?;
+    assert!(
+        !messages
+            .iter()
+            .any(|message| message["role"] == "assistant" && message["content"] == ""),
+        "thinking-only turn leaked an empty assistant message: {messages:?}"
+    );
+    assert_eq!(messages.len(), 2);
+    assert_eq!(messages[0]["role"], "user");
+    assert_eq!(messages[1]["role"], "user");
+    Ok(())
+}