diff --git a/__test__/warpbuild-mirror.test.ts b/__test__/warpbuild-mirror.test.ts
index 1f75b013..8525a448 100644
--- a/__test__/warpbuild-mirror.test.ts
+++ b/__test__/warpbuild-mirror.test.ts
@@ -172,6 +172,11 @@ describe('warpbuild snapshot cache', () => {
       expect((await lookupSnapshot('123', SHA)).kind).toBe('miss')
     })
 
+    it('maps 409 upload responses to locked (another job is uploading)', async () => {
+      stubFetch(409, {sub_code: 'FVE_GITMIRROR_LOCKED'})
+      expect((await requestUploadURL('123', SHA)).kind).toBe('locked')
+    })
+
     it('maps 403 to disabled (backend kill switch)', async () => {
       stubFetch(403, {sub_code: 'PDE_GITMIRROR_DISABLED'})
       expect((await lookupSnapshot('123', SHA)).kind).toBe('disabled')
diff --git a/dist/index.js b/dist/index.js
index 1cfe740e..241550eb 100644
--- a/dist/index.js
+++ b/dist/index.js
@@ -41699,7 +41699,8 @@ const promises_namespaceObject = __WEBPACK_EXTERNAL_createRequire(import.meta.ur
 
 // Client for backend-core's /api/v1/git-mirrors endpoints, authed by the runner
 // verification token. Contract: 200 = presigned URL; 404 = miss (upload after the
-// stock fetch); 403 = unservable org (skip cache + upload); else = fall back.
+// stock fetch); 403 = unservable org (skip cache + upload); 409 = another job is
+// uploading this snapshot (skip upload); else = fall back.
 const API_TIMEOUT_MS = 10_000;
 function backend_api_baseUrl() {
     return (process.env['WARPBUILD_HOST_URL'] || '').replace(/\/+$/, '');
@@ -41753,6 +41754,10 @@ async function requestUploadURL(repoKey, sha) {
             core_debug(`[wb-cache] upload-url answered 403 (disabled)`);
             return { kind: 'disabled' };
         }
+        if (res.status === 409) {
+            core_debug(`[wb-cache] upload-url answered 409 (locked)`);
+            return { kind: 'locked' };
+        }
         core_debug(`[wb-cache] upload-url answered ${res.status}`);
         return { kind: 'error' };
     }
@@ -41974,9 +41979,11 @@ async function uploadSnapshot(settings) {
         const size = (await external_fs_namespaceObject.promises.stat(tmpTar)).size;
         const upload = await requestUploadURL(repoKey, sha);
         if (upload.kind !== 'ok') {
-            info(upload.kind === 'disabled'
-                ? 'Snapshot cache is disabled by the backend; not uploading'
-                : 'Snapshot cache backend unavailable; not uploading');
+            info(upload.kind === 'locked'
+                ? 'Another job is already uploading this snapshot; skipping'
+                : upload.kind === 'disabled'
+                    ? 'Snapshot cache is disabled by the backend; not uploading'
+                    : 'Snapshot cache backend unavailable; not uploading');
             return;
         }
         const init = {
diff --git a/src/warpbuild/backend-api.ts b/src/warpbuild/backend-api.ts
index d51cd064..5f6ca858 100644
--- a/src/warpbuild/backend-api.ts
+++ b/src/warpbuild/backend-api.ts
@@ -3,7 +3,8 @@ import * as core from '@actions/core'
 
 // Client for backend-core's /api/v1/git-mirrors endpoints, authed by the runner
 // verification token. Contract: 200 = presigned URL; 404 = miss (upload after the
-// stock fetch); 403 = unservable org (skip cache + upload); else = fall back.
+// stock fetch); 403 = unservable org (skip cache + upload); 409 = another job is
+// uploading this snapshot (skip upload); else = fall back.
 
 const API_TIMEOUT_MS = 10_000
 
@@ -22,6 +23,7 @@ export type SnapshotLookup =
 export type UploadURLResult =
   | {kind: 'ok'; url: string}
   | {kind: 'disabled'}
+  | {kind: 'locked'}
   | {kind: 'error'}
 
 function baseUrl(): string {
@@ -89,6 +91,10 @@ export async function requestUploadURL(
       core.debug(`[wb-cache] upload-url answered 403 (disabled)`)
       return {kind: 'disabled'}
     }
+    if (res.status === 409) {
+      core.debug(`[wb-cache] upload-url answered 409 (locked)`)
+      return {kind: 'locked'}
+    }
     core.debug(`[wb-cache] upload-url answered ${res.status}`)
     return {kind: 'error'}
   } catch (error) {
diff --git a/src/warpbuild/mirror-cache.ts b/src/warpbuild/mirror-cache.ts
index 9e604e1b..0bfd32c9 100644
--- a/src/warpbuild/mirror-cache.ts
+++ b/src/warpbuild/mirror-cache.ts
@@ -253,9 +253,11 @@ async function uploadSnapshot(settings: IGitSourceSettings): Promise<void> {
     const upload = await api.requestUploadURL(repoKey, sha)
     if (upload.kind !== 'ok') {
       core.info(
-        upload.kind === 'disabled'
-          ? 'Snapshot cache is disabled by the backend; not uploading'
-          : 'Snapshot cache backend unavailable; not uploading'
+        upload.kind === 'locked'
+          ? 'Another job is already uploading this snapshot; skipping'
+          : upload.kind === 'disabled'
+            ? 'Snapshot cache is disabled by the backend; not uploading'
+            : 'Snapshot cache backend unavailable; not uploading'
       )
       return
     }