diff --git a/backend-java/src/test/java/com/nanri/aiimage/modules/file/service/LocalFileKeyLookupTest.java b/backend-java/src/test/java/com/nanri/aiimage/modules/file/service/LocalFileKeyLookupTest.java
new file mode 100644
index 00000000..d4455ef9
--- /dev/null
+++ b/backend-java/src/test/java/com/nanri/aiimage/modules/file/service/LocalFileKeyLookupTest.java
@@ -0,0 +1,121 @@
+package com.nanri.aiimage.modules.file.service;
+
+import com.nanri.aiimage.config.StorageProperties;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.io.TempDir;
+import org.springframework.mock.web.MockMultipartFile;
+
+import java.io.File;
+import java.nio.file.Path;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNotNull;
+import static org.junit.jupiter.api.Assertions.assertNull;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+/**
+ * task-153：fileKey 兜底查找契约（plan 09）。
+ * 上传记录登记内存索引；进程重启后索引为空，按 fileKey 目录枚举兜底重建；
+ * 找不到时明确返回 null（调用方明确失败），不静默。
+ */
+class LocalFileKeyLookupTest {
+
+    @TempDir
+    Path tempDir;
+
+    private LocalFileStorageService service;
+
+    @BeforeEach
+    void setUp() {
+        StorageProperties properties = new StorageProperties();
+        properties.setLocalTempDir(tempDir.toString());
+        service = new LocalFileStorageService(properties);
+    }
+
+    @Test
+    void fileKeyFoundAfterSave() throws Exception {
+        var vo = service.saveTempFile(new MockMultipartFile("f", "a.xlsx", "application/octet-stream", new byte[]{1, 2}), "upload");
+
+        File found = service.findLocalSourceFile(vo.getFileKey());
+
+        assertNotNull(found, "保存后按 fileKey 必须找到");
+        assertTrue(found.isFile());
+        assertEquals(vo.getFileKey(), found.getName().substring(0, vo.getFileKey().length()));
+    }
+
+    @Test
+    void fileKeyFoundAfterRestart() throws Exception {
+        var vo = service.saveTempFile(new MockMultipartFile("f", "b.xlsx", "application/octet-stream", new byte[]{3}), "upload");
+        // 模拟重启：新实例（内存索引为空）→ 目录枚举兜底
+        StorageProperties properties = new StorageProperties();
+        properties.setLocalTempDir(tempDir.toString());
+        LocalFileStorageService restarted = new LocalFileStorageService(properties);
+
+        File found = restarted.findLocalSourceFile(vo.getFileKey());
+
+        assertNotNull(found, "重启后按 fileKey 目录枚举兜底必须找到");
+    }
+
+    @Test
+    void missingFileKeyFailsExplicitly() {
+        assertNull(service.findLocalSourceFile("0123456789abcdef0123456789abcdef"),
+                "不存在的 fileKey 明确返回 null");
+    }
+
+    @Test
+    void nullAndBlankFileKeyAreSafe() {
+        assertNull(service.findLocalSourceFile(null));
+        assertNull(service.findLocalSourceFile("   "));
+        assertNull(service.findLocalSourceFile(""));
+    }
+
+    @Test
+    void deletedFileKeyNotFindable() throws Exception {
+        var vo = service.saveTempFile(new MockMultipartFile("f", "c.xlsx", "application/octet-stream", new byte[]{4}), "upload");
+        File found = service.findLocalSourceFile(vo.getFileKey());
+        assertNotNull(found);
+        assertTrue(found.delete(), "清理测试文件");
+
+        assertNull(service.findLocalSourceFile(vo.getFileKey()), "文件清理后不可再找到");
+    }
+
+    @Test
+    void indexRecordPersistsForLookup() throws Exception {
+        var vo = service.saveTempFile(new MockMultipartFile("f", "d.xlsx", "application/octet-stream", new byte[]{5}), "upload");
+
+        File found = service.findLocalSourceFile(vo.getFileKey());
+
+        assertNotNull(found);
+        assertEquals(vo.getLocalPath(), found.getAbsolutePath(), "索引命中返回登记路径");
+    }
+
+    @Test
+    @SuppressWarnings("unchecked")
+    void indexRebuiltByDirectoryEnumeration() throws Exception {
+        var vo = service.saveTempFile(new MockMultipartFile("f", "e.xlsx", "application/octet-stream", new byte[]{6}), "upload");
+        // 清空内存索引（模拟索引丢失），目录枚举仍可重建
+        ((java.util.Map<String, String>) org.springframework.test.util.ReflectionTestUtils
+                .getField(service, "sourceFileIndex")).clear();
+
+        File found = service.findLocalSourceFile(vo.getFileKey());
+
+        assertNotNull(found, "索引清空后目录枚举兜底重建");
+    }
+
+    @Test
+    void semanticsFrozen() throws Exception {
+        var vo = service.saveTempFile(new MockMultipartFile("f", "f.xlsx", "application/octet-stream", new byte[]{7}), "upload");
+        File found = service.findLocalSourceFile(vo.getFileKey());
+        File restartFound = new LocalFileStorageService(properties()).findLocalSourceFile(vo.getFileKey());
+
+        assertEquals(found.getAbsolutePath(), restartFound.getAbsolutePath(),
+                "同 fileKey 在重启前后解析到同一文件（语义不变）");
+    }
+
+    private StorageProperties properties() {
+        StorageProperties properties = new StorageProperties();
+        properties.setLocalTempDir(tempDir.toString());
+        return properties;
+    }
+}