fix(translation): strip Codex compaction markers before Responses decode

Signed-off-by: Zengyuan Liu <zengyuanl@nvidia.com>

Author: eric-liu-nvidia

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

@@ -40,6 +40,11 @@ impl FormatCodec for OpenAiResponsesCodec {
 
     fn decode_request(&self, body: &Value, policy: &TranslationPolicy) -> Result<DecodedRequest> {
         let body = crate::util::object(body, "$")?;
+        // Codex marks remote-compact requests with a `compaction_trigger` input
+        // item. It is codex-internal protocol that strict upstream parsers
+        // reject, so drop it before preservation capture and normalization.
+        let sanitized = strip_codex_compaction_markers(body);
+        let body = sanitized.as_ref().unwrap_or(body);
         let mut diagnostics = Vec::new();
         let mut request = LlmRequest {
             model: body
@@ -1108,6 +1113,23 @@ fn pair_tool_calls_with_outputs(items: &mut Vec<Value>) {
     }
 }
 
+// Returns the body without Codex `compaction_trigger` input items, or `None`
+// when there are none (the common case, sparing the clone).
+fn strip_codex_compaction_markers(body: &Map<String, Value>) -> Option<Map<String, Value>> {
+    fn is_marker(item: &Value) -> bool {
+        item.get("type").and_then(Value::as_str) == Some("compaction_trigger")
+    }
+    let input = body.get("input")?.as_array()?;
+    if !input.iter().any(is_marker) {
+        return None;
+    }
+    let mut sanitized = body.clone();
+    if let Some(Value::Array(items)) = sanitized.get_mut("input") {
+        items.retain(|item| !is_marker(item));
+    }
+    Some(sanitized)
+}
+
 // Encodes IR blocks that Responses represents as top-level input items.
 fn encode_responses_special_input(
     block: &ContentBlock,

crates/switchyard-translation/tests/request_translation.rs

@@ -2533,3 +2533,40 @@ fn responses_parallel_tool_calls_pair_with_their_outputs() -> TestResult {
     );
     Ok(())
 }
+
+// Verifies Codex `compaction_trigger` marker items never reach the upstream.
+#[test]
+fn responses_codex_compaction_markers_are_stripped() -> TestResult {
+    let engine = TranslationEngine::default();
+    let policy = TranslationPolicy {
+        preservation: switchyard_translation::PreservationPolicy::Disabled,
+        ..TranslationPolicy::default()
+    };
+    let body = json!({
+        "model": "gpt-5",
+        "input": [
+            {"type": "message", "role": "user", "content": "Continue"},
+            {"type": "compaction_trigger"},
+            {"type": "function_call", "name": "shell", "call_id": "call-a", "arguments": "{}"},
+            {"type": "function_call_output", "call_id": "call-a", "output": "ok"}
+        ]
+    });
+
+    let output = engine
+        .translate_request(
+            WireFormat::OpenAiResponses,
+            WireFormat::OpenAiResponses,
+            &body,
+            &policy,
+        )?
+        .body;
+
+    let types = output["input"]
+        .as_array()
+        .ok_or("input is not an array")?
+        .iter()
+        .filter_map(|item| item["type"].as_str())
+        .collect::<Vec<_>>();
+    assert_eq!(types, vec!["message", "function_call", "function_call_output"]);
+    Ok(())
+}