diff --git a/packages/plugins/sandbox-providers/kubernetes/src/upload-interceptor.ts b/packages/plugins/sandbox-providers/kubernetes/src/upload-interceptor.ts
index 8e567d0a..54b657c5 100644
--- a/packages/plugins/sandbox-providers/kubernetes/src/upload-interceptor.ts
+++ b/packages/plugins/sandbox-providers/kubernetes/src/upload-interceptor.ts
@@ -123,6 +123,15 @@ export class FastUploadInterceptor {
       };
     }
 
+    const activeUpload = this.findActiveUploadForCommand(command);
+    if (activeUpload) {
+      this.buffers.delete(activeUpload.tempPath);
+      return {
+        action: "error",
+        message: `Fast upload protocol violation for ${activeUpload.upload.targetPath}; retry the upload from the beginning.`,
+      };
+    }
+
     return { action: "passthrough", reason: "no upload pattern" };
   }
 
@@ -133,4 +142,13 @@ export class FastUploadInterceptor {
   get pendingCount(): number {
     return this.buffers.size;
   }
+
+  private findActiveUploadForCommand(command: string): { tempPath: string; upload: BufferedUpload } | null {
+    for (const [tempPath, upload] of this.buffers) {
+      if (command.includes(`'${tempPath}'`)) {
+        return { tempPath, upload };
+      }
+    }
+    return null;
+  }
 }
diff --git a/packages/plugins/sandbox-providers/kubernetes/test/unit/upload-interceptor.test.ts b/packages/plugins/sandbox-providers/kubernetes/test/unit/upload-interceptor.test.ts
index dcfc2ec9..e93ff3fb 100644
--- a/packages/plugins/sandbox-providers/kubernetes/test/unit/upload-interceptor.test.ts
+++ b/packages/plugins/sandbox-providers/kubernetes/test/unit/upload-interceptor.test.ts
@@ -46,7 +46,7 @@ describe("FastUploadInterceptor", () => {
     ).toMatchObject({ action: "passthrough", reason: "finalize without buffered state" });
   });
 
-  it("does not intercept chunks with padding before the end", () => {
+  it("fails fast when an unrecognized command targets an active upload", () => {
     const interceptor = new FastUploadInterceptor();
     const target = "/workspace/file.bin";
 
@@ -56,10 +56,12 @@ describe("FastUploadInterceptor", () => {
       ),
     ).toMatchObject({ action: "ack" });
 
-    expect(
-      interceptor.decide(`printf '%s' 'aGVs=bG8=' >> '${target}.paperclip-upload.b64'`),
-    ).toMatchObject({ action: "passthrough", reason: "no upload pattern" });
-    expect(interceptor.pendingCount).toBe(1);
+    const decision = interceptor.decide(`printf '%s' 'aGVs=bG8=' >> '${target}.paperclip-upload.b64'`);
+    expect(decision).toMatchObject({
+      action: "error",
+      message: expect.stringContaining("Fast upload protocol violation"),
+    });
+    expect(interceptor.pendingCount).toBe(0);
   });
 
   it("fails fast when data arrives after a padded chunk", () => {