fix(gateway): support claude_code cross-format Claude messages

crates/aether-provider/transport/src/standard/mod.rs

@@ -6,6 +6,7 @@ use crate::auth::{
     build_claude_passthrough_headers, build_complete_passthrough_headers_with_auth,
     build_openai_passthrough_headers, build_passthrough_headers, ensure_upstream_auth_header,
 };
+use crate::claude_code::build_claude_code_passthrough_headers;
 use crate::headers::force_identity_accept_encoding;
 use crate::rules::{
     apply_local_body_rules, apply_local_body_rules_with_request_headers,
@@ -224,7 +225,23 @@ pub fn build_standard_provider_request_headers(
 ) -> Option<StandardProviderRequestHeaders> {
     let uses_vertex_query_auth =
         uses_vertex_api_key_query_auth(input.transport, input.provider_api_format);
-    let mut headers = if input.same_format {
+    let is_claude_code_messages = input
+        .transport
+        .provider
+        .provider_type
+        .trim()
+        .eq_ignore_ascii_case("claude_code")
+        && aether_ai_formats::normalize_api_format_alias(input.provider_api_format)
+            == "claude:messages";
+    let mut headers = if is_claude_code_messages {
+        build_claude_code_passthrough_headers(
+            input.headers,
+            input.auth_header,
+            input.auth_value,
+            input.extra_headers,
+            input.upstream_is_stream,
+        )
+    } else if input.same_format {
         build_complete_passthrough_headers_with_auth(
             input.headers,
             input.auth_header,
@@ -473,6 +490,41 @@ mod tests {
         );
     }
 
+    #[test]
+    fn builds_claude_code_identity_headers_for_cross_format_messages() {
+        let mut transport = sample_transport("claude:messages");
+        transport.provider.provider_type = "claude_code".to_string();
+        let resolved =
+            build_standard_provider_request_headers(StandardProviderRequestHeadersInput {
+                transport: &transport,
+                provider_api_format: "claude:messages",
+                same_format: false,
+                headers: &HeaderMap::new(),
+                auth_header: "authorization",
+                auth_value: "Bearer oauth-token",
+                extra_headers: &BTreeMap::new(),
+                header_rules: None,
+                provider_request_body: &json!({"model":"claude-opus-4-6"}),
+                original_request_body: &json!({"model":"gpt-source"}),
+                upstream_is_stream: true,
+            })
+            .expect("Claude Code headers should build");
+
+        assert_eq!(resolved.headers.get("x-app"), Some(&"cli".to_string()));
+        assert!(resolved
+            .headers
+            .get("user-agent")
+            .is_some_and(|value| value.starts_with("claude-cli/")));
+        assert!(resolved
+            .headers
+            .get("anthropic-beta")
+            .is_some_and(|value| value.contains("claude-code-20250219")));
+        assert_eq!(
+            resolved.headers.get("authorization"),
+            Some(&"Bearer oauth-token".to_string())
+        );
+    }
+
     #[test]
     fn endpoint_header_rules_do_not_override_protected_authorization() {
         let mut transport = sample_transport("openai:responses");