Merge pull request #650 from stabey/pr/claude-system-responses-20260620

fix(ai-formats): preserve Claude in-message system guidance in Responses

crates/aether-ai-formats/src/formats/conversion/request.rs

@@ -829,6 +829,56 @@ mod tests {
         }
     }
 
+    #[test]
+    fn claude_request_to_responses_preserves_in_message_system_guidance_as_developer_item() {
+        let body = json!({
+            "model": "claude-sonnet",
+            "system": [{
+                "type": "text",
+                "text": "Be exact."
+            }],
+            "messages": [
+                {"role": "user", "content": "hello"},
+                {
+                    "role": "system",
+                    "content": "x-anthropic-billing-header: internal-billing-marker\nSessionStart hook additional context: follow the house style."
+                },
+                {"role": "assistant", "content": "visible answer"},
+                {"role": "user", "content": "continue"}
+            ],
+            "max_tokens": 128
+        });
+
+        let converted = registry::convert_request(
+            "claude:messages",
+            "openai:responses",
+            &body,
+            &FormatContext::default().with_mapped_model("gpt-5.1"),
+        )
+        .expect("responses request");
+
+        let input = converted["input"].as_array().expect("responses input");
+        assert_eq!(input.len(), 5);
+        assert_eq!(input[0]["role"], "developer");
+        assert_eq!(input[0]["content"][0]["text"], "Be exact.");
+        assert_eq!(input[1]["role"], "user");
+        assert_eq!(input[1]["content"][0]["text"], "hello");
+        assert_eq!(input[2]["role"], "developer");
+        assert_eq!(
+            input[2]["content"][0]["text"],
+            "SessionStart hook additional context: follow the house style."
+        );
+        assert!(!input[2]["content"][0]["text"]
+            .as_str()
+            .expect("developer guidance text")
+            .contains("x-anthropic-billing-header:"));
+        assert_eq!(input[3]["role"], "assistant");
+        assert_eq!(input[3]["content"][0]["text"], "visible answer");
+        assert_eq!(input[4]["role"], "user");
+        assert_eq!(input[4]["content"][0]["text"], "continue");
+        assert!(converted.get("instructions").is_none());
+    }
+
     #[test]
     fn openai_responses_request_normalizer_strips_content_cache_control() {
         let body = json!({

crates/aether-ai-formats/src/formats/openai/responses/request.rs

@@ -16,9 +16,9 @@ use crate::{
         openai_response_format_to_canonical, openai_responses_extension,
         openai_responses_generation_config, openai_responses_input_to_canonical_messages,
         openai_responses_tool_choice_to_canonical, openai_responses_tools_to_canonical,
-        openai_tool_choice_raw_to_responses, CanonicalContentBlock, CanonicalInstruction,
-        CanonicalRequest, CanonicalRole, CanonicalThinkingConfig, CanonicalToolChoice,
-        CanonicalToolDefinition, OPENAI_RESPONSES_EXTENSION_NAMESPACE,
+        openai_tool_choice_raw_to_responses, strip_claude_billing_header, CanonicalContentBlock,
+        CanonicalInstruction, CanonicalRequest, CanonicalRole, CanonicalThinkingConfig,
+        CanonicalToolChoice, CanonicalToolDefinition, OPENAI_RESPONSES_EXTENSION_NAMESPACE,
         OPENAI_RESPONSES_LEGACY_EXTENSION_NAMESPACE,
     },
 };
@@ -305,6 +305,12 @@ fn canonical_messages_to_responses_input(canonical: &CanonicalRequest) -> Option
     let mut next_generated_tool_call_index = 0usize;
     let mut pending_tool_call_ids = VecDeque::new();
     for message in &canonical.messages {
+        let strip_claude_billing_header_from_text =
+            is_claude_messages_request(&canonical.extensions)
+                && matches!(
+                    message.role,
+                    CanonicalRole::System | CanonicalRole::Developer
+                );
         let role = match message.role {
             CanonicalRole::Assistant => "assistant",
             CanonicalRole::Tool | CanonicalRole::User | CanonicalRole::Unknown => "user",
@@ -314,6 +320,11 @@ fn canonical_messages_to_responses_input(canonical: &CanonicalRequest) -> Option
             CanonicalRole::Developer if is_openai_responses_input_message(&message.extensions) => {
                 "developer"
             }
+            CanonicalRole::System | CanonicalRole::Developer
+                if is_claude_messages_request(&canonical.extensions) =>
+            {
+                "developer"
+            }
             CanonicalRole::System | CanonicalRole::Developer => continue,
         };
         let mut content = Vec::new();
@@ -396,7 +407,11 @@ fn canonical_messages_to_responses_input(canonical: &CanonicalRequest) -> Option
                     }
                 }
                 other => {
-                    if let Some(part) = canonical_block_to_responses_input_part(other, role) {
+                    if let Some(part) = canonical_block_to_responses_input_part(
+                        other,
+                        role,
+                        strip_claude_billing_header_from_text,
+                    ) {
                         content.push(part);
                     }
                 }
@@ -592,9 +607,15 @@ fn is_openai_responses_reasoning_history_block(extensions: &BTreeMap<String, Val
 fn canonical_block_to_responses_input_part(
     block: &CanonicalContentBlock,
     role: &str,
+    strip_claude_billing_header_from_text: bool,
 ) -> Option<Value> {
     match block {
         CanonicalContentBlock::Text { text, .. } => {
+            let text = if strip_claude_billing_header_from_text {
+                strip_claude_billing_header(text)
+            } else {
+                text.clone()
+            };
             if text.is_empty() {
                 return None;
             }