diff --git a/server-rs/crates/spacetime-module/src/external_generation.rs b/server-rs/crates/spacetime-module/src/external_generation.rs
index 07c2cbab4..eba694174 100644
--- a/server-rs/crates/spacetime-module/src/external_generation.rs
+++ b/server-rs/crates/spacetime-module/src/external_generation.rs
@@ -1881,11 +1881,10 @@ fn get_worker_owned_external_generation_job(
 fn ensure_external_generation_job_provider_checkpoint_absent(
     row: &ExternalGenerationJob,
 ) -> Result<(), String> {
-    if row.provider_task_id.is_some() {
-        return Err(
-            "external_generation_job.provider_task_id 已存在，禁止覆盖 provider checkpoint"
-                .to_string(),
-        );
+    // checkpoint 由 provider_kind + provider_task_id 两个字段共同构成，
+    // 只判 task_id 会让部分写入的行绕过 at-most-once submit 保护。
+    if row.provider_kind.is_some() || row.provider_task_id.is_some() {
+        return Err("external_generation_job.provider checkpoint 已存在，禁止覆盖".to_string());
     }
     Ok(())
 }