diff --git a/src/lib/db/batches.ts b/src/lib/db/batches.ts
index 4c24e408e0..bb70071426 100644
--- a/src/lib/db/batches.ts
+++ b/src/lib/db/batches.ts
@@ -1,5 +1,5 @@
 import { getDbInstance, rowToCamel, objToSnake } from "./core";
-import { deleteFile } from "./files";
+import { deleteFile, deleteFileOwnedBy } from "./files";
 import { v4 as uuidv4 } from "uuid";
 import { logger } from "../../../open-sse/utils/logger.ts";
 
@@ -439,6 +439,10 @@ export type DeleteCompletedBatchesScope = { apiKeyId: string } | { allTenants: t
  * sweep must never reach records the key does not own, so unowned batches are
  * only swept by `{ allTenants: true }`.
  *
+ * In key mode the file half is owner-scoped too: only files whose api_key_id is
+ * the caller's are soft-deleted; a referenced file another tenant owns (or an
+ * unowned one) is left intact and is not counted in deletedFiles.
+ *
  * The file soft-deletes, the checkpoint DELETE and the batches DELETE run in one
  * transaction, so a mid-sweep failure rolls everything back — no batch row is
  * left pointing at a file whose content was already nulled.
@@ -484,7 +488,11 @@ export function deleteCompletedBatches(scope: DeleteCompletedBatchesScope): {
     let deletedFiles = 0;
     for (const fid of fileIds) {
       try {
-        if (deleteFile(fid)) deletedFiles++;
+        // Key mode: only the key's OWN files. A batch may reference a file
+        // another tenant (or nobody) owns; a bulk destructive sweep must not
+        // reach it (SEC-C). Instance mode keeps the unconditional soft delete.
+        const removed = allTenants ? deleteFile(fid) : deleteFileOwnedBy(fid, apiKeyId as string);
+        if (removed) deletedFiles++;
       } catch (err) {
         log.warn("deleteCompletedBatches: file soft-delete failed", {
           fid,
diff --git a/tests/unit/batches-delete-completed-ownership-wvxc.test.ts b/tests/unit/batches-delete-completed-ownership-wvxc.test.ts
index cc562e5f19..8701342db1 100644
--- a/tests/unit/batches-delete-completed-ownership-wvxc.test.ts
+++ b/tests/unit/batches-delete-completed-ownership-wvxc.test.ts
@@ -189,6 +189,50 @@ describe("deleteCompletedBatches — ownership boundary (GHSA-wvxc-jp3v-5mg5)",
     assert.strictEqual(getFile(unowned.file.id), null, "allTenants soft-deletes its file too");
   });
 
+  it("SEC-C: a key-scoped sweep never nulls a file another key owns, even when its own batch references it", () => {
+    const foreignFile = createFile({
+      bytes: 7,
+      filename: "foreign.jsonl",
+      purpose: "batch",
+      content: Buffer.from("foreign"),
+      apiKeyId: "key-other",
+    });
+    const unownedFile = createFile({
+      bytes: 7,
+      filename: "unowned.jsonl",
+      purpose: "batch",
+      content: Buffer.from("unowned"),
+      apiKeyId: null,
+    });
+    const own = seedCompletedBatch("key-secc", "secc-own");
+    const cross = createBatch({
+      endpoint: "/v1/chat/completions",
+      completionWindow: "24h",
+      inputFileId: foreignFile.id,
+      outputFileId: unownedFile.id,
+      status: "completed",
+      apiKeyId: "key-secc",
+    });
+
+    const result = deleteCompletedBatches({ apiKeyId: "key-secc" });
+
+    assert.strictEqual(result.deletedBatches, 2, "both of the key's completed batches are swept");
+    assert.strictEqual(result.deletedFiles, 1, "only the key's OWN file is soft-deleted");
+    assert.strictEqual(getBatch(own.batch.id), null);
+    assert.strictEqual(getBatch(cross.id), null);
+    assert.strictEqual(getFileContent(own.file.id), null, "own file content nulled");
+    assert.strictEqual(
+      getFileContent(foreignFile.id)?.toString(),
+      "foreign",
+      "another key's file intact"
+    );
+    assert.strictEqual(
+      getFileContent(unownedFile.id)?.toString(),
+      "unowned",
+      "unowned file intact"
+    );
+  });
+
   it("ATOMIC: a failure after the file soft-deletes rolls the file content back", () => {
     const db = getDbInstance();
     const own = seedCompletedBatch("key_atomic_wvxc", "wvxc-atomic");