diff --git a/libwebauthn/src/lib.rs b/libwebauthn/src/lib.rs
index f6ac9bad..0d2c6c49 100644
--- a/libwebauthn/src/lib.rs
+++ b/libwebauthn/src/lib.rs
@@ -144,6 +144,18 @@ pub enum Transport {
     Hybrid,
 }
 
+impl Transport {
+    /// The WebAuthn authenticator attachment modality this transport implies.
+    /// libwebauthn only drives roaming authenticators, so all map to `"cross-platform"`.
+    pub fn authenticator_attachment(self) -> &'static str {
+        match self {
+            Transport::Usb | Transport::Ble | Transport::Nfc | Transport::Hybrid => {
+                "cross-platform"
+            }
+        }
+    }
+}
+
 #[derive(Debug, Clone)]
 #[cfg_attr(any(test, feature = "virt"), derive(PartialEq))]
 pub enum UvUpdate {
diff --git a/libwebauthn/src/ops/webauthn/get_assertion.rs b/libwebauthn/src/ops/webauthn/get_assertion.rs
index 9f62d545..1a3fbc90 100644
--- a/libwebauthn/src/ops/webauthn/get_assertion.rs
+++ b/libwebauthn/src/ops/webauthn/get_assertion.rs
@@ -30,6 +30,7 @@ use crate::{
         Ctap2PublicKeyCredentialUserEntity,
     },
     webauthn::CtapError,
+    Transport,
 };
 
 use super::timeout::DEFAULT_TIMEOUT;
@@ -478,6 +479,8 @@ pub struct Assertion {
     pub credentials_count: Option<u32>,
     pub user_selected: Option<bool>,
     pub unsigned_extensions_output: Option<GetAssertionResponseUnsignedExtensions>,
+    /// Transport the assertion was produced over, stamped by the channel.
+    pub transport: Option<Transport>,
 }
 
 impl WebAuthnIDLResponse for Assertion {
@@ -522,7 +525,9 @@ impl WebAuthnIDLResponse for Assertion {
                 signature: Base64UrlString::from(self.signature.clone()),
                 user_handle,
             },
-            authenticator_attachment: None,
+            authenticator_attachment: self
+                .transport
+                .map(|t| t.authenticator_attachment().to_string()),
             client_extension_results,
             r#type: "public-key".to_string(),
         })
@@ -1422,6 +1427,7 @@ mod tests {
             credentials_count: None,
             user_selected: None,
             unsigned_extensions_output: None,
+            transport: None,
         }
     }
 
@@ -1480,6 +1486,33 @@ mod tests {
         assert_eq!(model.response.signature.0, vec![0xDE, 0xAD, 0xC0, 0xDE]);
     }
 
+    #[test]
+    fn test_assertion_to_idl_model_populates_attachment() {
+        // libwebauthn drives roaming authenticators, so every known transport
+        // yields authenticatorAttachment "cross-platform" (WebAuthn L3 §5.1).
+        let mut assertion = create_test_assertion();
+        let request = create_test_request();
+
+        for transport in [
+            Transport::Usb,
+            Transport::Ble,
+            Transport::Nfc,
+            Transport::Hybrid,
+        ] {
+            assertion.transport = Some(transport);
+            let model = assertion.to_idl_model(&request).unwrap();
+            assert_eq!(
+                model.authenticator_attachment.as_deref(),
+                Some("cross-platform")
+            );
+        }
+
+        // An unknown transport omits the field.
+        assertion.transport = None;
+        let model = assertion.to_idl_model(&request).unwrap();
+        assert_eq!(model.authenticator_attachment, None);
+    }
+
     #[test]
     fn test_assertion_with_user_handle() {
         use crate::proto::ctap2::Ctap2PublicKeyCredentialUserEntity;
diff --git a/libwebauthn/src/ops/webauthn/large_blob.rs b/libwebauthn/src/ops/webauthn/large_blob.rs
index e85bc389..20552004 100644
--- a/libwebauthn/src/ops/webauthn/large_blob.rs
+++ b/libwebauthn/src/ops/webauthn/large_blob.rs
@@ -1063,6 +1063,10 @@ mod tests {
             .await
             .expect("webauthn_get_assertion should succeed");
         assert_eq!(response.assertions.len(), 1);
+        assert_eq!(
+            response.assertions[0].transport,
+            Some(crate::Transport::Usb)
+        );
         let large_blob = response.assertions[0]
             .unsigned_extensions_output
             .as_ref()
@@ -1184,6 +1188,9 @@ mod tests {
             .await
             .expect("get_assertion should succeed");
         assert_eq!(response.assertions.len(), 2);
+        for a in &response.assertions {
+            assert_eq!(a.transport, Some(crate::Transport::Usb));
+        }
         let blob = |i: usize| {
             response.assertions[i]
                 .unsigned_extensions_output
diff --git a/libwebauthn/src/ops/webauthn/make_credential.rs b/libwebauthn/src/ops/webauthn/make_credential.rs
index 177a6dd7..d54ffd46 100644
--- a/libwebauthn/src/ops/webauthn/make_credential.rs
+++ b/libwebauthn/src/ops/webauthn/make_credential.rs
@@ -185,7 +185,9 @@ impl WebAuthnIDLResponse for MakeCredentialResponse {
                 public_key_algorithm,
                 attestation_object: Base64UrlString::from(attestation_object_bytes),
             },
-            authenticator_attachment: None,
+            authenticator_attachment: self
+                .transport
+                .map(|t| t.authenticator_attachment().to_string()),
             client_extension_results,
             r#type: "public-key".to_string(),
         })
@@ -1814,6 +1816,33 @@ mod tests {
         assert!(model.response.transports.is_empty());
     }
 
+    #[test]
+    fn test_response_to_idl_model_populates_attachment() {
+        // libwebauthn drives roaming authenticators, so every known transport
+        // yields authenticatorAttachment "cross-platform" (WebAuthn L3 §5.1).
+        let mut response = create_test_response();
+        let request = create_test_request();
+
+        for transport in [
+            Transport::Usb,
+            Transport::Ble,
+            Transport::Nfc,
+            Transport::Hybrid,
+        ] {
+            response.transport = Some(transport);
+            let model = response.to_idl_model(&request).unwrap();
+            assert_eq!(
+                model.authenticator_attachment.as_deref(),
+                Some("cross-platform")
+            );
+        }
+
+        // An unknown transport omits the field.
+        response.transport = None;
+        let model = response.to_idl_model(&request).unwrap();
+        assert_eq!(model.authenticator_attachment, None);
+    }
+
     #[test]
     fn test_response_to_idl_model_transports_from_get_info() {
         // The authenticator's getInfo (0x09) transports are folded with the
diff --git a/libwebauthn/src/proto/ctap2/model/get_assertion.rs b/libwebauthn/src/proto/ctap2/model/get_assertion.rs
index de0f746f..707b0292 100644
--- a/libwebauthn/src/proto/ctap2/model/get_assertion.rs
+++ b/libwebauthn/src/proto/ctap2/model/get_assertion.rs
@@ -683,6 +683,7 @@ impl Ctap2GetAssertionResponse {
             credentials_count: self.credentials_count,
             user_selected: self.user_selected,
             unsigned_extensions_output,
+            transport: None,
         }
     }
 }
diff --git a/libwebauthn/src/transport/channel.rs b/libwebauthn/src/transport/channel.rs
index 29601be6..461db570 100644
--- a/libwebauthn/src/transport/channel.rs
+++ b/libwebauthn/src/transport/channel.rs
@@ -73,8 +73,9 @@ pub trait Channel: Send + Sync + Display + Ctap2AuthTokenStore {
     async fn status(&self) -> ChannelStatus;
     async fn close(&mut self);
 
-    /// The transport this channel speaks over, used to populate the registration
-    /// response `transports` member.
+    /// The transport this channel speaks over. Drives the registration response
+    /// `transports` member and the `authenticatorAttachment` of both registration
+    /// and assertion responses.
     fn transport(&self) -> Transport {
         Transport::Usb
     }
diff --git a/libwebauthn/src/webauthn.rs b/libwebauthn/src/webauthn.rs
index 386bd4c4..e3285229 100644
--- a/libwebauthn/src/webauthn.rs
+++ b/libwebauthn/src/webauthn.rs
@@ -218,10 +218,15 @@ where
         };
         trace!(?op, "WebAuthn GetAssertion request");
         let protocol = negotiate_protocol(self, op.is_downgradable()).await?;
-        match protocol {
+        let mut response = match protocol {
             FidoProtocol::FIDO2 => get_assertion_fido2(self, op).await,
             FidoProtocol::U2F => get_assertion_u2f(self, op).await,
+        }?;
+        let transport = self.transport();
+        for assertion in &mut response.assertions {
+            assertion.transport = Some(transport);
         }
+        Ok(response)
     }
 }