diff --git a/crates/trusted-server-core/src/integrations/prebid.rs b/crates/trusted-server-core/src/integrations/prebid.rs
index 903b4f355..a1b40281d 100644
--- a/crates/trusted-server-core/src/integrations/prebid.rs
+++ b/crates/trusted-server-core/src/integrations/prebid.rs
@@ -2229,8 +2229,22 @@ impl AuctionProvider for PrebidAuctionProvider {
     ) -> Result<PlatformPendingRequest, Report<TrustedServerError>> {
         log::info!("Prebid: requesting bids for {} slots", request.slots.len());
 
-        let request_info =
-            RequestInfo::from_request(context.request, context.services.client_info());
+        // `ext.trusted_server.request_host` must be the publisher's own domain
+        // (matching `site.domain`/`publisher.domain` below), not the raw edge
+        // `Host` header of the incoming `/auction` request. On the SSAT proxy
+        // path the browser calls `/auction` against the trusted-server edge
+        // domain (e.g. `ts.example.com`), which is not what PBS's
+        // `trusted_server` verification module expects — see the "Host header
+        // value expected by the receiving service" doc on `SigningParams`.
+        // `scheme` is canonical `"https"` rather than detected from the
+        // incoming request: the publisher origin is always https in
+        // production, and PBS verification is expected to match on that,
+        // not on the edge's local scheme (e.g. http under `fastly compute
+        // serve`).
+        let request_info = RequestInfo {
+            host: request.publisher.domain.clone(),
+            scheme: "https".to_owned(),
+        };
 
         // Create signer and compute signature if request signing is enabled
         let signer_with_signature =
@@ -2672,6 +2686,50 @@ mod tests {
         );
     }
 
+    #[test]
+    fn request_bids_sets_trusted_server_request_host_to_publisher_domain_not_edge_host() {
+        let stub = Arc::new(StubHttpClient::new());
+        stub.push_response(200, br#"{"seatbid":[]}"#.to_vec());
+        let services = build_services_with_http_client(
+            Arc::clone(&stub) as Arc<dyn crate::platform::PlatformHttpClient>
+        );
+        let settings = make_settings();
+        let provider = PrebidAuctionProvider::new(base_config());
+        let auction_request = create_test_auction_request();
+        // The incoming `/auction` call arrives on the trusted-server edge
+        // domain (SSAT proxy pattern), which must NOT leak into
+        // `ext.trusted_server.request_host` — that field must track the
+        // publisher's own domain instead.
+        let http_req = http::Request::builder()
+            .method(http::Method::POST)
+            .uri("https://ts.pub.example/auction")
+            .header(header::HOST, "ts.pub.example")
+            .body(EdgeBody::empty())
+            .expect("should build request");
+        let context = AuctionContext {
+            settings: &settings,
+            request: &http_req,
+            timeout_ms: 500,
+            provider_responses: None,
+            services: &services,
+        };
+
+        futures::executor::block_on(provider.request_bids(&auction_request, &context))
+            .expect("should start request");
+
+        let bodies = stub.recorded_request_bodies();
+        assert_eq!(bodies.len(), 1, "should send one upstream PBS request");
+        let sent: Json =
+            serde_json::from_slice(&bodies[0]).expect("should parse sent OpenRTB request body");
+        let request_host = sent["ext"]["trusted_server"]["request_host"]
+            .as_str()
+            .expect("should set ext.trusted_server.request_host");
+        assert_eq!(
+            request_host, auction_request.publisher.domain,
+            "request_host should be the publisher domain, not the edge Host header"
+        );
+    }
+
     fn create_test_auction_context<'a>(
         settings: &'a Settings,
         request: &'a http::Request<EdgeBody>,
diff --git a/crates/trusted-server-core/src/platform/test_support.rs b/crates/trusted-server-core/src/platform/test_support.rs
index 5299d9818..4235b4ed7 100644
--- a/crates/trusted-server-core/src/platform/test_support.rs
+++ b/crates/trusted-server-core/src/platform/test_support.rs
@@ -473,6 +473,21 @@ impl PlatformHttpClient for StubHttpClient {
             .expect("should lock request_headers")
             .push(headers);
 
+        // Capture the outgoing request body, mirroring `send()`, so tests
+        // exercising the async fan-out path (`request_bids` providers) can
+        // assert on it via `recorded_request_bodies()` too.
+        let (_, body) = request.request.into_parts();
+        let body_bytes = body
+            .into_bytes_bounded(MAX_RECORDED_BODY_BYTES)
+            .await
+            .change_context(PlatformError::HttpClient)
+            .attach("failed to capture StubHttpClient request body")?
+            .to_vec();
+        self.request_bodies
+            .lock()
+            .expect("should lock request bodies")
+            .push(body_bytes);
+
         let response = self
             .responses
             .lock()