diff --git a/crates/aether-ai/formats/src/formats/registry.rs b/crates/aether-ai/formats/src/formats/registry.rs
index c845077e9..3f8e2ef4a 100644
--- a/crates/aether-ai/formats/src/formats/registry.rs
+++ b/crates/aether-ai/formats/src/formats/registry.rs
@@ -978,6 +978,7 @@ fn standard_request_root_field_is_audited(source: FormatId, key: &str) -> bool {
         FormatId::OpenAiResponses | FormatId::OpenAiResponsesCompact => matches!(
             key,
             "background"
+                | "client_metadata"
                 | "context_management"
                 | "conversation"
                 | "include"
@@ -1418,6 +1419,7 @@ fn request_extension_key_is_cross_format_safe(
             FormatId::OpenAiChat,
             "openai_responses" | "openai_cli",
             "stream"
+                | "client_metadata"
                 | "store"
                 | "service_tier"
                 | "safety_identifier"
@@ -3781,6 +3783,26 @@ mod tests {
         ));
     }
 
+    #[test]
+    fn pure_openai_responses_to_chat_ignores_client_transport_metadata() {
+        let body = json!({
+            "model": "gpt-source",
+            "input": [{"role": "user", "content": "hello"}],
+            "client_metadata": {
+                "session_id": "session-123",
+                "thread_id": "thread-123"
+            }
+        });
+
+        let converted = convert_request_pure("openai:responses", "openai:chat", &body)
+            .expect("client transport metadata should not block cross-format conversion")
+            .value;
+
+        assert_eq!(converted["model"], "gpt-source");
+        assert_eq!(converted["messages"][0]["content"], "hello");
+        assert!(converted.get("client_metadata").is_none());
+    }
+
     #[test]
     fn pure_cross_format_rejects_unknown_source_root_field() {
         let body = json!({
diff --git a/docs/api/format-conversion-audit.md b/docs/api/format-conversion-audit.md
index c279d4f1f..3cdafba31 100644
--- a/docs/api/format-conversion-audit.md
+++ b/docs/api/format-conversion-audit.md
@@ -9,6 +9,7 @@ Statuses:
 - `native`: emitted as a target-native field without semantic change.
 - `mapped`: converted through canonical/provider-specific mapping.
 - `extension-preserved`: preserved in same-format canonical roundtrip or target-approved extension namespace.
+- `transport-only`: audited client transport metadata intentionally omitted when the target has no compatible transport channel.
 - `unaudited`: rejected because the source field is not in the audited provider schema inventory for cross-format conversion.
 - `unsupported`: rejected because the request/field shape is outside the supported conversion surface, independent of schema drift.
 - `lossy-blocked`: conversion fails closed.
@@ -83,6 +84,7 @@ Provider schema refresh is not a runtime dependency. Same-format runtime paths d
 | `top_p` | generation | `top_p` | native |
 | `top_logprobs` | generation | `top_logprobs` | native |
 | `metadata` | canonical metadata | `metadata` | native |
+| `client_metadata` | Responses client transport metadata | none | transport-only; omitted |
 | `parallel_tool_calls` | canonical bool | `parallel_tool_calls` | native |
 | `text.format` | canonical response format | `response_format` | mapped |
 | `text.verbosity` | Responses extension | `verbosity` | mapped |