diff --git a/test/cli/commands.test.ts b/test/cli/commands.test.ts
index 8e068c8..09189a5 100644
--- a/test/cli/commands.test.ts
+++ b/test/cli/commands.test.ts
@@ -511,11 +511,6 @@ describe("files", () => {
         expect(await filesCommand(context(), { collection: "9" })).toBe(1);
         expect(stderr.text).toBe("Collection 9 not found\n");
     });
-
-    it("exits 1 for a collection ID that is not a number", async () => {
-        expect(await filesCommand(context(), { collection: "abc" })).toBe(1);
-        expect(stderr.text).toBe("Invalid collection ID\n");
-    });
 });
 
 describe("get and get-thumb", () => {
@@ -546,12 +541,6 @@ describe("get and get-thumb", () => {
         expect(stderr.text).toBe("File 999 not found\n");
         expect(existsSync(out)).toBe(false);
     });
-
-    it("both exit 1 for a file ID that is not a number", async () => {
-        expect(await getCommand(context(), "abc", {})).toBe(1);
-        expect(await getThumbCommand(context(), "abc", {})).toBe(1);
-        expect(stderr.text).toBe("Invalid file ID\nInvalid file ID\n");
-    });
 });
 
 // A live photo, which Ente stores as one ZIP, is written as its image and its
@@ -1035,3 +1024,130 @@ describe("backup-metadata and the thumbnail helpers refresh first", () => {
         expect(stdout.text).toBe("");
     });
 });
+
+// `files --collection`, `get`, `get-thumb` and `helper fix-missing-thumbnails
+// --file` take an ID only when it is written in decimal digits and is a safe
+// integer. `Number()` alone reads `0x10` as 16, `1e3` as 1000 and an empty
+// string as 0, so the command would act on a file the user never named.
+// Anything else is refused with one line naming it, and exit 1, before any
+// request.
+//
+// These run on a real client, read back from a saved session, over a fake Ente
+// server holding an empty account, which records each request it receives. With
+// a good ID the command goes on to ask the server, and finds nothing there.
+describe("an ID given on the command line", () => {
+    const NOT_IDS = [
+        "0x10",
+        "1e3",
+        "1.5",
+        "-1",
+        "",
+        "abc",
+        // One past Number.MAX_SAFE_INTEGER: from here on a number cannot
+        // hold every integer, so two IDs could read as one.
+        "9007199254740992",
+    ];
+
+    const requests: Request[] = [];
+
+    const emptyAccount = (): CliContext => {
+        const ctx = context();
+        const key = toBase64(new Uint8Array(32));
+        saveSession(ctx.sessionDir, {
+            email: "cli@example.com",
+            userID: USER_ID,
+            token: "token",
+            masterKey: key,
+            secretKey: key,
+            publicKey: key,
+        });
+        return {
+            ...ctx,
+            loadSession: (path) =>
+                loadSession(path, {
+                    fetch: async (url, init) => {
+                        requests.push(new Request(url, init));
+                        return new Response(
+                            JSON.stringify({ collections: [] }),
+                            { headers: { "content-type": "application/json" } },
+                        );
+                    },
+                }),
+        };
+    };
+
+    beforeEach(() => {
+        requests.length = 0;
+    });
+
+    it("files looks up a decimal collection ID", async () => {
+        const ctx = emptyAccount();
+        expect(await filesCommand(ctx, { collection: "1" })).toBe(1);
+        expect(stderr.text).toBe("Collection 1 not found\n");
+        expect(requests).not.toHaveLength(0);
+    });
+
+    it.each(NOT_IDS)(
+        "files refuses --collection %j before any request",
+        async (arg) => {
+            const ctx = emptyAccount();
+            expect(await filesCommand(ctx, { collection: arg })).toBe(1);
+            expect(stderr.text).toBe(`Invalid collection ID: ${arg}\n`);
+            expect(stdout.text).toBe("");
+            expect(requests).toHaveLength(0);
+        },
+    );
+
+    it("get looks up a decimal file ID", async () => {
+        expect(await getCommand(emptyAccount(), "100", {})).toBe(1);
+        expect(stderr.text).toBe("File 100 not found\n");
+        expect(requests).not.toHaveLength(0);
+    });
+
+    it.each(NOT_IDS)("get refuses %j before any request", async (arg) => {
+        expect(await getCommand(emptyAccount(), arg, {})).toBe(1);
+        expect(stderr.text).toBe(`Invalid file ID: ${arg}\n`);
+        expect(stdout.text).toBe("");
+        expect(requests).toHaveLength(0);
+    });
+
+    it("get-thumb looks up a decimal file ID", async () => {
+        expect(await getThumbCommand(emptyAccount(), "100", {})).toBe(1);
+        expect(stderr.text).toBe("File 100 not found\n");
+        expect(requests).not.toHaveLength(0);
+    });
+
+    it.each(NOT_IDS)("get-thumb refuses %j before any request", async (arg) => {
+        expect(await getThumbCommand(emptyAccount(), arg, {})).toBe(1);
+        expect(stderr.text).toBe(`Invalid file ID: ${arg}\n`);
+        expect(stdout.text).toBe("");
+        expect(requests).toHaveLength(0);
+    });
+
+    // The fixer reports each file it was given; on this account neither is
+    // found, which is a failure, so it exits 1.
+    it("fix-missing-thumbnails looks up each decimal --file ID", async () => {
+        const ctx = emptyAccount();
+        const opts = { file: ["100", "200"], json: true };
+        expect(await fixMissingThumbnailsCommand(ctx, opts)).toBe(1);
+        expect(JSON.parse(stdout.text)).toMatchObject([
+            { fileID: 100, reason: "file not found in any collection" },
+            { fileID: 200, reason: "file not found in any collection" },
+        ]);
+        expect(requests).not.toHaveLength(0);
+    });
+
+    // One bad ID fails the whole command, so the good one before it is not
+    // fixed either.
+    it.each(NOT_IDS)(
+        "fix-missing-thumbnails refuses --file 100 %j before any request",
+        async (arg) => {
+            const ctx = emptyAccount();
+            const opts = { file: ["100", arg] };
+            expect(await fixMissingThumbnailsCommand(ctx, opts)).toBe(1);
+            expect(stderr.text).toBe(`Invalid file ID: ${arg}\n`);
+            expect(stdout.text).toBe("");
+            expect(requests).toHaveLength(0);
+        },
+    );
+});