diff --git a/crates/worker-runtime/src/http_server.rs b/crates/worker-runtime/src/http_server.rs
index 5d04e4c6d..0c7177d7a 100644
--- a/crates/worker-runtime/src/http_server.rs
+++ b/crates/worker-runtime/src/http_server.rs
@@ -2347,8 +2347,8 @@ impl RuntimeHttpRestError {
             RuntimeError::WorkerExecutionRejected { result, .. } => startup_transfer(result),
             _ => None,
         };
-        let message = if startup.is_some() {
-            "Worker startup failed".to_owned()
+        let message = if let Some(startup) = &startup {
+            worker::startup_failure::internal_startup_failure_summary(&startup.primary).to_owned()
         } else {
             error.to_string()
         };
diff --git a/crates/worker/src/startup_failure.rs b/crates/worker/src/startup_failure.rs
index b0c4c59fa..9e76a56cf 100644
--- a/crates/worker/src/startup_failure.rs
+++ b/crates/worker/src/startup_failure.rs
@@ -120,6 +120,20 @@ pub fn reproject_internal_startup_failure(
     StartupFailureAttachment::from_projected(&projected)
 }
 
+/// Fixed, identifier-free summary for an already authenticated internal caller.
+/// Keep the required operation actionable without exposing the source's Display.
+/// This neither projects a Browser cause nor chooses an HTTP status.
+pub fn internal_startup_failure_summary(failure: &StartupFailureAttachment) -> &'static str {
+    if failure
+        .project_for::<InternalTransfer, protocol::session_failure::SessionRegistrationCause>(&())
+        .is_ok()
+    {
+        "Worker startup failed while registering Session (session.record)"
+    } else {
+        "Worker startup failed"
+    }
+}
+
 /// Dispatch by the domain attested in process, not by parsing attachment fields.
 pub(crate) fn persist_binding_startup_failure<S: session_store::WorkerMetadataStore>(
     store: &S,
@@ -249,6 +263,36 @@ mod tests {
         WorkerStoreError,
     };
 
+    #[test]
+    fn internal_summary_retains_only_attested_required_operation() {
+        use protocol::session_failure::{SessionPrincipal, SessionRegistrationCause};
+        let session = protocol::session_failure::session_registration_attachment(
+            SessionRegistrationCause::denied(SessionPrincipal::Subject),
+        )
+        .unwrap();
+        assert_eq!(
+            internal_startup_failure_summary(&session),
+            "Worker startup failed while registering Session (session.record)"
+        );
+        let runtime = runtime_startup_attachment(RuntimeStartupCause {
+            stage: StartupStage::Feature,
+            reason: RuntimeStartupReason::FeatureFailed,
+        })
+        .unwrap();
+        assert_eq!(
+            internal_startup_failure_summary(&runtime),
+            "Worker startup failed"
+        );
+        let corrupt = serde_json::from_value(serde_json::json!(
+            "session.record denied TEST_ONLY_CREDENTIAL /private/path raw_http_body"
+        ))
+        .unwrap();
+        assert_eq!(
+            internal_startup_failure_summary(&corrupt),
+            "Worker startup failed"
+        );
+    }
+
     struct LostSaveResponse(FsWorkerStore);
     impl WorkerMetadataStore for LostSaveResponse {
         fn write(&self, metadata: &WorkerMetadata) -> Result<(), WorkerStoreError> {
diff --git a/crates/workspace-server/src/hosts.rs b/crates/workspace-server/src/hosts.rs
index 2c1739cba..58e5871de 100644
--- a/crates/workspace-server/src/hosts.rs
+++ b/crates/workspace-server/src/hosts.rs
@@ -6772,8 +6772,11 @@ fn embedded_runtime_diagnostic(error: &EmbeddedRuntimeError) -> RuntimeDiagnosti
         ),
     };
     if let EmbeddedRuntimeError::WorkerExecutionRejected { result, .. } = error {
-        if result.startup_failure.is_some() {
-            projection.message = "Embedded Runtime Worker startup failed".into();
+        if let Some(primary) = &result.startup_failure {
+            projection.message = format!(
+                "Embedded Runtime: {}",
+                worker::startup_failure::internal_startup_failure_summary(primary)
+            );
         }
         projection.startup = result.startup_failure.as_ref().and_then(|primary| {
             internal_startup_transfer(runtime_api::WorkerStartupFailure {