diff --git a/codex-rs/app-server/src/message_processor.rs b/codex-rs/app-server/src/message_processor.rs
index f950661180..f45f9d554e 100644
--- a/codex-rs/app-server/src/message_processor.rs
+++ b/codex-rs/app-server/src/message_processor.rs
@@ -38,6 +38,7 @@ use crate::thread_state::ThreadStateManager;
 use crate::transport::AppServerTransport;
 use crate::transport::RemoteControlHandle;
 use async_trait::async_trait;
+use axum::http::HeaderValue;
 use codex_analytics::AnalyticsEventsClient;
 use codex_analytics::AppServerRpcTransport;
 use codex_app_server_protocol::AttestationGenerateParams;
@@ -181,7 +182,7 @@ impl std::fmt::Debug for AppServerAttestationProvider {
 }
 
 impl AttestationProvider for AppServerAttestationProvider {
-    fn generate_header_value(&self, context: AttestationContext) -> GenerateAttestationFuture<'_> {
+    fn header_for_request(&self, context: AttestationContext) -> GenerateAttestationFuture<'_> {
         let outgoing = self.outgoing.clone();
         let attestation_connection_ids = self.attestation_connection_ids.clone();
         Box::pin(async move {
@@ -195,6 +196,7 @@ impl AttestationProvider for AppServerAttestationProvider {
                 ATTESTATION_GENERATE_TIMEOUT,
             )
             .await
+            .and_then(|value| HeaderValue::from_bytes(value.as_bytes()).ok())
         })
     }
 }
diff --git a/codex-rs/core/src/attestation.rs b/codex-rs/core/src/attestation.rs
index 8e5bdcf538..01d4e72e4e 100644
--- a/codex-rs/core/src/attestation.rs
+++ b/codex-rs/core/src/attestation.rs
@@ -1,9 +1,12 @@
 use std::future::Future;
 use std::pin::Pin;
 
+use http::HeaderValue;
+
 pub(crate) const X_OAI_ATTESTATION_HEADER: &str = "x-oai-attestation";
 
-pub type GenerateAttestationFuture<'a> = Pin<Box<dyn Future<Output = Option<String>> + Send + 'a>>;
+pub type GenerateAttestationFuture<'a> =
+    Pin<Box<dyn Future<Output = Option<HeaderValue>> + Send + 'a>>;
 
 /// Request context that host integrations can use when deciding whether to
 /// generate an attestation header value.
@@ -15,8 +18,7 @@ pub struct AttestationContext {
 /// Host integration boundary for just-in-time attestation header values.
 ///
 /// Implementations own the policy for when attestation should be attempted and
-/// return the opaque string expected by the upstream `x-oai-attestation`
-/// header. Core only forwards valid HTTP header values returned by the host.
+/// return the upstream `x-oai-attestation` header value when one should be sent.
 pub trait AttestationProvider: std::fmt::Debug + Send + Sync {
-    fn generate_header_value(&self, context: AttestationContext) -> GenerateAttestationFuture<'_>;
+    fn header_for_request(&self, context: AttestationContext) -> GenerateAttestationFuture<'_>;
 }
diff --git a/codex-rs/core/src/client.rs b/codex-rs/core/src/client.rs
index 21b69064a7..ab850770ca 100644
--- a/codex-rs/core/src/client.rs
+++ b/codex-rs/core/src/client.rs
@@ -657,11 +657,10 @@ impl ModelClient {
         self.state
             .attestation_provider
             .as_ref()?
-            .generate_header_value(AttestationContext {
+            .header_for_request(AttestationContext {
                 uses_chatgpt_auth: provider.uses_chatgpt_auth,
             })
             .await
-            .and_then(|value| HeaderValue::from_str(&value).ok())
     }
 
     /// Builds request telemetry for unary API calls (e.g., Compact endpoint).
diff --git a/codex-rs/core/src/client_tests.rs b/codex-rs/core/src/client_tests.rs
index 38058e2baa..55b0c227c6 100644
--- a/codex-rs/core/src/client_tests.rs
+++ b/codex-rs/core/src/client_tests.rs
@@ -502,10 +502,7 @@ fn model_client_with_counting_attestation() -> (ModelClient, Arc<AtomicUsize>) {
     }
 
     impl AttestationProvider for CountingAttestationProvider {
-        fn generate_header_value(
-            &self,
-            context: AttestationContext,
-        ) -> GenerateAttestationFuture<'_> {
+        fn header_for_request(&self, context: AttestationContext) -> GenerateAttestationFuture<'_> {
             let calls = self.calls.clone();
             Box::pin(async move {
                 if !context.uses_chatgpt_auth {
@@ -513,7 +510,7 @@ fn model_client_with_counting_attestation() -> (ModelClient, Arc<AtomicUsize>) {
                 }
 
                 let call = calls.fetch_add(1, Ordering::Relaxed) + 1;
-                Some(format!("v1.header-{call}"))
+                Some(http::HeaderValue::from_bytes(format!("v1.header-{call}").as_bytes()).unwrap())
             })
         }
     }