diff --git a/test/api/upload.test.ts b/test/api/upload.test.ts
new file mode 100644
index 0000000..e9426e0
--- /dev/null
+++ b/test/api/upload.test.ts
@@ -0,0 +1,139 @@
+/**
+ * Tests for the upload-related ApiClient methods added for thumbnail
+ * repair: `putJSON`, `putFile`, `getUploadURL`, and `updateThumbnail`.
+ *
+ * `putFile` sends a raw PUT to a presigned S3 URL. It must NOT send
+ * quak's auth headers (X-Auth-Token, X-Client-Package) because the
+ * presigned URL carries its own S3 auth in the query string. Sending
+ * extra headers can cause S3 to reject the request.
+ *
+ * `putJSON` is like `postJSON` but sends PUT. Used by `updateThumbnail`
+ * to register the uploaded thumbnail's object key with the Ente API.
+ */
+
+import { describe, expect, it } from "vitest";
+import { ApiClient } from "../../src/api/client.js";
+
+const jsonResponse = (
+    body: unknown,
+    status = 200,
+    headers: Record<string, string> = {},
+): Response =>
+    new Response(JSON.stringify(body), {
+        status,
+        headers: { "content-type": "application/json", ...headers },
+    });
+
+const recordingFetch = (
+    ...responses: Response[]
+): {
+    fetch: typeof globalThis.fetch;
+    calls: { url: string; init: RequestInit | undefined }[];
+} => {
+    const calls: { url: string; init: RequestInit | undefined }[] = [];
+    let i = 0;
+    const fake = async (
+        input: RequestInfo | URL,
+        init?: RequestInit,
+    ): Promise<Response> => {
+        const url =
+            typeof input === "string"
+                ? input
+                : input instanceof URL
+                  ? input.href
+                  : input.url;
+        calls.push({ url, init });
+        if (i >= responses.length) {
+            throw new Error(`recordingFetch: no response for call #${i}`);
+        }
+        return responses[i++]!;
+    };
+    return { fetch: fake as typeof globalThis.fetch, calls };
+};
+
+describe("ApiClient.putJSON", () => {
+    it("sends a PUT request with JSON body and auth headers", async () => {
+        const { fetch, calls } = recordingFetch(jsonResponse({ ok: true }));
+        const client = new ApiClient({ fetch, authToken: "tok" });
+
+        await client.putJSON("/files/thumbnail", {
+            fileID: 42,
+            thumbnail: { objectKey: "k", decryptionHeader: "h" },
+        });
+
+        expect(calls[0]!.init?.method).toBe("PUT");
+        const headers = new Headers(calls[0]!.init?.headers as HeadersInit);
+        expect(headers.get("Content-Type")).toBe("application/json");
+        expect(headers.get("X-Auth-Token")).toBe("tok");
+        expect(headers.get("X-Client-Package")).toBe("berlin.sneak.quak");
+        const body = JSON.parse(calls[0]!.init?.body as string);
+        expect(body.fileID).toBe(42);
+        expect(body.thumbnail.objectKey).toBe("k");
+    });
+});
+
+describe("ApiClient.putFile", () => {
+    it("PUTs raw bytes to the presigned URL without auth headers", async () => {
+        const { fetch, calls } = recordingFetch(
+            new Response(null, { status: 200 }),
+        );
+        const client = new ApiClient({ fetch, authToken: "secret-tok" });
+        const data = new Uint8Array([1, 2, 3, 4, 5]);
+
+        await client.putFile("https://s3.example.com/presigned?sig=abc", data);
+
+        expect(calls[0]!.url).toBe("https://s3.example.com/presigned?sig=abc");
+        expect(calls[0]!.init?.method).toBe("PUT");
+        const headers = new Headers(calls[0]!.init?.headers as HeadersInit);
+        expect(headers.get("Content-Type")).toBe("application/octet-stream");
+        expect(headers.get("Content-Length")).toBe("5");
+        // Must NOT leak auth headers to S3
+        expect(headers.has("X-Auth-Token")).toBe(false);
+        expect(headers.has("X-Client-Package")).toBe(false);
+    });
+
+    it("throws on non-2xx response from presigned URL", async () => {
+        const { fetch } = recordingFetch(
+            new Response("Forbidden", { status: 403 }),
+        );
+        const client = new ApiClient({ fetch });
+
+        await expect(
+            client.putFile("https://s3.example.com/bad", new Uint8Array(10)),
+        ).rejects.toThrow(/403/);
+    });
+});
+
+describe("ApiClient.getUploadURL", () => {
+    it("POSTs to /files/upload-url with contentLength and contentMD5", async () => {
+        const { fetch, calls } = recordingFetch(
+            jsonResponse({ objectKey: "user/thumb123", url: "https://s3/put" }),
+        );
+        const client = new ApiClient({ fetch, authToken: "tok" });
+
+        const result = await client.getUploadURL(5000, "abc123==");
+
+        expect(calls[0]!.url).toBe("https://api.ente.io/files/upload-url");
+        const body = JSON.parse(calls[0]!.init?.body as string);
+        expect(body.contentLength).toBe(5000);
+        expect(body.contentMD5).toBe("abc123==");
+        expect(result.objectKey).toBe("user/thumb123");
+        expect(result.url).toBe("https://s3/put");
+    });
+});
+
+describe("ApiClient.updateThumbnail", () => {
+    it("PUTs to /files/thumbnail with fileID, objectKey, and decryptionHeader", async () => {
+        const { fetch, calls } = recordingFetch(jsonResponse({}));
+        const client = new ApiClient({ fetch, authToken: "tok" });
+
+        await client.updateThumbnail(42, "user/obj", "headerBase64==");
+
+        expect(calls[0]!.url).toBe("https://api.ente.io/files/thumbnail");
+        expect(calls[0]!.init?.method).toBe("PUT");
+        const body = JSON.parse(calls[0]!.init?.body as string);
+        expect(body.fileID).toBe(42);
+        expect(body.thumbnail.objectKey).toBe("user/obj");
+        expect(body.thumbnail.decryptionHeader).toBe("headerBase64==");
+    });
+});
diff --git a/test/crypto/encrypt-blob.test.ts b/test/crypto/encrypt-blob.test.ts
new file mode 100644
index 0000000..86a1c1a
--- /dev/null
+++ b/test/crypto/encrypt-blob.test.ts
@@ -0,0 +1,99 @@
+/**
+ * Tests for `crypto.encryptBlob`.
+ *
+ * `encryptBlob` is the push-side counterpart to `decryptBlob`. It
+ * encrypts a small payload as a single secretstream chunk with
+ * TAG_FINAL and returns the header + ciphertext. Used for encrypting
+ * thumbnails before upload to the Ente server.
+ *
+ * The critical invariant is that `decryptBlob(encryptBlob(...))` is the
+ * identity function. If either side drifts, uploaded thumbnails become
+ * unreadable.
+ */
+
+import sodium from "libsodium-wrappers-sumo";
+import { beforeAll, describe, expect, it } from "vitest";
+import {
+    decryptBlob,
+    encryptBlob,
+    init,
+    STREAM_CHUNK_OVERHEAD,
+} from "../../src/crypto/index.js";
+
+describe("crypto.encryptBlob", () => {
+    beforeAll(async () => {
+        await init();
+        await sodium.ready;
+    });
+
+    it("round-trips with decryptBlob for arbitrary data", () => {
+        const key = sodium.crypto_secretstream_xchacha20poly1305_keygen();
+        const plaintext = sodium.randombytes_buf(500);
+        const { header, ciphertext } = encryptBlob(plaintext, key);
+        const recovered = decryptBlob(ciphertext, header, key);
+        expect(recovered).toEqual(plaintext);
+    });
+
+    it("round-trips a zero-length payload", () => {
+        const key = sodium.crypto_secretstream_xchacha20poly1305_keygen();
+        const { header, ciphertext } = encryptBlob(new Uint8Array(0), key);
+        const recovered = decryptBlob(ciphertext, header, key);
+        expect(recovered.length).toBe(0);
+    });
+
+    it("ciphertext is exactly STREAM_CHUNK_OVERHEAD longer than plaintext", () => {
+        const key = sodium.crypto_secretstream_xchacha20poly1305_keygen();
+        const plaintext = sodium.randombytes_buf(1234);
+        const { ciphertext } = encryptBlob(plaintext, key);
+        expect(ciphertext.length).toBe(
+            plaintext.length + STREAM_CHUNK_OVERHEAD,
+        );
+    });
+
+    it("header is 24 bytes (secretstream XChaCha20 header size)", () => {
+        const key = sodium.crypto_secretstream_xchacha20poly1305_keygen();
+        const { header } = encryptBlob(new Uint8Array([1, 2, 3]), key);
+        expect(header.length).toBe(
+            sodium.crypto_secretstream_xchacha20poly1305_HEADERBYTES,
+        );
+    });
+
+    it("produces different ciphertext for different keys", () => {
+        const k1 = sodium.crypto_secretstream_xchacha20poly1305_keygen();
+        const k2 = sodium.crypto_secretstream_xchacha20poly1305_keygen();
+        const plaintext = new Uint8Array([1, 2, 3, 4, 5]);
+        const enc1 = encryptBlob(plaintext, k1);
+        const enc2 = encryptBlob(plaintext, k2);
+        expect(enc1.ciphertext).not.toEqual(enc2.ciphertext);
+    });
+
+    it("produces different ciphertext on each call (random nonce in header)", () => {
+        const key = sodium.crypto_secretstream_xchacha20poly1305_keygen();
+        const plaintext = new Uint8Array([9, 9, 9]);
+        const a = encryptBlob(plaintext, key);
+        const b = encryptBlob(plaintext, key);
+        expect(a.header).not.toEqual(b.header);
+        expect(a.ciphertext).not.toEqual(b.ciphertext);
+    });
+
+    it("decryptBlob rejects ciphertext encrypted with a different key", () => {
+        const k1 = sodium.crypto_secretstream_xchacha20poly1305_keygen();
+        const k2 = sodium.crypto_secretstream_xchacha20poly1305_keygen();
+        const { header, ciphertext } = encryptBlob(
+            new Uint8Array([1, 2, 3]),
+            k1,
+        );
+        expect(() => decryptBlob(ciphertext, header, k2)).toThrow();
+    });
+
+    it("decryptBlob rejects tampered ciphertext from encryptBlob", () => {
+        const key = sodium.crypto_secretstream_xchacha20poly1305_keygen();
+        const { header, ciphertext } = encryptBlob(
+            sodium.randombytes_buf(100),
+            key,
+        );
+        ciphertext[ciphertext.length - 1] =
+            ciphertext[ciphertext.length - 1]! ^ 0x01;
+        expect(() => decryptBlob(ciphertext, header, key)).toThrow();
+    });
+});
diff --git a/test/thumbnails/thumbnails.test.ts b/test/thumbnails/thumbnails.test.ts
new file mode 100644
index 0000000..ac00a87
--- /dev/null
+++ b/test/thumbnails/thumbnails.test.ts
@@ -0,0 +1,502 @@
+/**
+ * Tests for `listMissingThumbnails` and `fixMissingThumbnails`.
+ *
+ * These use a mock Ente server that serves encrypted collections, files,
+ * and thumbnails. The mock server has deliberate gaps: some files have
+ * working thumbnails, others return 404 or empty bodies. The tests
+ * verify that the detection and repair logic handles each case correctly.
+ *
+ * `fixMissingThumbnails` is the most complex function in quak: it
+ * downloads the original file, generates a JPEG thumbnail with sharp,
+ * encrypts it with secretstream push, gets a presigned upload URL,
+ * uploads to S3, and registers the new thumbnail with the API. The
+ * test verifies each step actually happened and the uploaded data is
+ * a valid encrypted blob that decrypts to a JPEG.
+ */
+
+import sodium from "libsodium-wrappers-sumo";
+import sharp from "sharp";
+import { beforeAll, describe, expect, it } from "vitest";
+import {
+    init,
+    toBase64,
+    decryptBlob,
+    fromBase64,
+    deriveKEK,
+    deriveLoginSubkey,
+} from "../../src/crypto/index.js";
+import { SRP, SrpServer } from "fast-srp-hap";
+import { Client } from "../../src/client.js";
+import {
+    listMissingThumbnails,
+    fixMissingThumbnails,
+} from "../../src/thumbnails.js";
+import type { KeyAttributes } from "../../src/auth/types.js";
+
+// ---------------------------------------------------------------------------
+// Mock server with controllable thumbnail behavior
+// ---------------------------------------------------------------------------
+
+const TEST_EMAIL = "thumb@example.com";
+const TEST_PASSWORD = "thumbpass";
+const TEST_OPS = 2;
+const TEST_MEM = 64 * 1024 * 1024;
+
+interface ThumbMockState {
+    verifier: Buffer;
+    srpAttributes: Record<string, unknown>;
+    keyAttributes: KeyAttributes;
+    encryptedToken: string;
+    collections: Record<string, unknown>[];
+    filesByCollection: Record<number, Record<string, unknown>[]>;
+    fileCiphertexts: Record<number, Uint8Array>;
+    fileKeys: Record<number, Uint8Array>;
+    thumbnailBehavior: Record<number, "ok" | "empty" | "404">;
+    // Captures from fix operations
+    uploadedThumbnails: {
+        fileID: number;
+        objectKey: string;
+        decryptionHeader: string;
+        ciphertext: Uint8Array;
+    }[];
+}
+
+let mock: ThumbMockState;
+
+const buildThumbMock = async (): Promise<ThumbMockState> => {
+    const kekSalt = sodium.randombytes_buf(sodium.crypto_pwhash_SALTBYTES);
+    const kek = await deriveKEK(TEST_PASSWORD, kekSalt, TEST_OPS, TEST_MEM);
+    const loginSubKeyBytes = deriveLoginSubkey(kek);
+
+    const srpUserID = "thumb-srp";
+    const srpSalt = sodium.randombytes_buf(16);
+    const verifier = SRP.computeVerifier(
+        SRP.params["4096"],
+        Buffer.from(srpSalt),
+        Buffer.from(srpUserID),
+        Buffer.from(loginSubKeyBytes),
+    );
+
+    const masterKey = sodium.randombytes_buf(32);
+    const keyNonce = sodium.randombytes_buf(sodium.crypto_secretbox_NONCEBYTES);
+    const encryptedKey = sodium.crypto_secretbox_easy(masterKey, keyNonce, kek);
+    const kp = sodium.crypto_box_keypair();
+    const skNonce = sodium.randombytes_buf(sodium.crypto_secretbox_NONCEBYTES);
+    const encSK = sodium.crypto_secretbox_easy(
+        kp.privateKey,
+        skNonce,
+        masterKey,
+    );
+    const tokenBytes = sodium.randombytes_buf(32);
+    const encToken = sodium.crypto_box_seal(tokenBytes, kp.publicKey);
+
+    const keyAttributes: KeyAttributes = {
+        kekSalt: toBase64(kekSalt),
+        encryptedKey: toBase64(encryptedKey),
+        keyDecryptionNonce: toBase64(keyNonce),
+        publicKey: toBase64(kp.publicKey),
+        encryptedSecretKey: toBase64(encSK),
+        secretKeyDecryptionNonce: toBase64(skNonce),
+        memLimit: TEST_MEM,
+        opsLimit: TEST_OPS,
+    };
+
+    // One collection with 3 files: ok thumbnail, empty thumbnail, 404 thumbnail
+    const collKey = sodium.crypto_secretbox_keygen();
+    const ckN = sodium.randombytes_buf(sodium.crypto_secretbox_NONCEBYTES);
+    const encCK = sodium.crypto_secretbox_easy(collKey, ckN, masterKey);
+    const nameB = new TextEncoder().encode("Photos");
+    const cnN = sodium.randombytes_buf(sodium.crypto_secretbox_NONCEBYTES);
+    const encCN = sodium.crypto_secretbox_easy(nameB, cnN, collKey);
+
+    const rawCollection = {
+        id: 1,
+        owner: { id: 42 },
+        encryptedKey: toBase64(encCK),
+        keyDecryptionNonce: toBase64(ckN),
+        encryptedName: toBase64(encCN),
+        nameDecryptionNonce: toBase64(cnN),
+        type: "album",
+        updationTime: 1700000000000000,
+    };
+
+    // Generate a real tiny JPEG that sharp can process
+    const tinyJpeg = await sharp({
+        create: { width: 100, height: 80, channels: 3, background: "red" },
+    })
+        .jpeg({ quality: 80 })
+        .toBuffer();
+
+    const fileKeys: Record<number, Uint8Array> = {};
+    const fileCiphertexts: Record<number, Uint8Array> = {};
+    const rawFiles: Record<string, unknown>[] = [];
+
+    for (const fileID of [100, 101, 102]) {
+        const fk = sodium.crypto_secretstream_xchacha20poly1305_keygen();
+        fileKeys[fileID] = fk;
+        const fkN = sodium.randombytes_buf(sodium.crypto_secretbox_NONCEBYTES);
+        const encFK = sodium.crypto_secretbox_easy(fk, fkN, collKey);
+
+        const meta = JSON.stringify({
+            title: `file-${fileID}.jpg`,
+            fileType: 0,
+            creationTime: 1700000000000000,
+            modificationTime: 1700000000000000,
+        });
+        const metaPush =
+            sodium.crypto_secretstream_xchacha20poly1305_init_push(fk);
+        const encMeta = sodium.crypto_secretstream_xchacha20poly1305_push(
+            metaPush.state,
+            new TextEncoder().encode(meta),
+            null,
+            sodium.crypto_secretstream_xchacha20poly1305_TAG_FINAL,
+        );
+
+        // Encrypt the tiny JPEG as the file body
+        const filePush =
+            sodium.crypto_secretstream_xchacha20poly1305_init_push(fk);
+        const encFile = sodium.crypto_secretstream_xchacha20poly1305_push(
+            filePush.state,
+            new Uint8Array(tinyJpeg),
+            null,
+            sodium.crypto_secretstream_xchacha20poly1305_TAG_FINAL,
+        );
+        fileCiphertexts[fileID] = encFile;
+
+        rawFiles.push({
+            id: fileID,
+            collectionID: 1,
+            ownerID: 42,
+            encryptedKey: toBase64(encFK),
+            keyDecryptionNonce: toBase64(fkN),
+            metadata: {
+                encryptedData: toBase64(encMeta),
+                decryptionHeader: toBase64(metaPush.header),
+            },
+            file: { decryptionHeader: toBase64(filePush.header) },
+            thumbnail: {
+                decryptionHeader: toBase64(sodium.randombytes_buf(24)),
+            },
+            updationTime: 1700000000000000,
+        });
+    }
+
+    return {
+        verifier,
+        srpAttributes: {
+            srpUserID,
+            srpSalt: toBase64(srpSalt),
+            memLimit: TEST_MEM,
+            opsLimit: TEST_OPS,
+            kekSalt: toBase64(kekSalt),
+            isEmailMFAEnabled: false,
+        },
+        keyAttributes,
+        encryptedToken: toBase64(encToken),
+        collections: [rawCollection],
+        filesByCollection: { 1: rawFiles },
+        fileCiphertexts,
+        fileKeys,
+        thumbnailBehavior: {
+            100: "ok",
+            101: "empty",
+            102: "404",
+        },
+        uploadedThumbnails: [],
+    };
+};
+
+const buildThumbFetch = (m: ThumbMockState) => {
+    let srpServer: SrpServer;
+
+    return (async (
+        input: RequestInfo | URL,
+        init?: RequestInit,
+    ): Promise<Response> => {
+        const url =
+            typeof input === "string"
+                ? input
+                : input instanceof URL
+                  ? input.href
+                  : input.url;
+        const parsed = new URL(url);
+        const path = parsed.pathname;
+        const json = (body: unknown) =>
+            new Response(JSON.stringify(body), {
+                status: 200,
+                headers: { "content-type": "application/json" },
+            });
+
+        // SRP auth flow
+        if (path === "/users/srp/attributes")
+            return json({ attributes: m.srpAttributes });
+        if (path === "/users/srp/create-session") {
+            const body = JSON.parse(init?.body as string);
+            const serverKey = await SRP.genKey();
+            srpServer = new SrpServer(
+                SRP.params["4096"],
+                m.verifier,
+                serverKey,
+            );
+            const B = srpServer.computeB();
+            srpServer.setA(Buffer.from(body.srpA, "base64"));
+            return json({ sessionID: "s1", srpB: B.toString("base64") });
+        }
+        if (path === "/users/srp/verify-session") {
+            const body = JSON.parse(init?.body as string);
+            srpServer.checkM1(Buffer.from(body.srpM1, "base64"));
+            return json({
+                srpM2: srpServer.computeM2().toString("base64"),
+                id: 42,
+                keyAttributes: m.keyAttributes,
+                encryptedToken: m.encryptedToken,
+            });
+        }
+
+        // Collections & files
+        if (path === "/collections/v2")
+            return json({ collections: m.collections });
+        if (path === "/collections/v2/diff") {
+            const collID = Number(parsed.searchParams.get("collectionID"));
+            return json({
+                diff: m.filesByCollection[collID] ?? [],
+                hasMore: false,
+            });
+        }
+
+        // File download (for fix: download original to generate thumb)
+        if (
+            url.includes("files.ente.io") ||
+            path.startsWith("/files/download/")
+        ) {
+            const fileID = Number(
+                parsed.searchParams.get("fileID") ?? path.split("/").pop(),
+            );
+            const ct = m.fileCiphertexts[fileID];
+            if (ct) return new Response(ct, { status: 200 });
+            return new Response("not found", { status: 404 });
+        }
+
+        // Thumbnail download (for list: check if thumbnail exists)
+        if (
+            url.includes("thumbnails.ente.io") ||
+            path.startsWith("/files/preview/")
+        ) {
+            const fileID = Number(
+                parsed.searchParams.get("fileID") ?? path.split("/").pop(),
+            );
+            const behavior = m.thumbnailBehavior[fileID];
+            if (behavior === "ok") {
+                return new Response(new Uint8Array([0xff, 0xd8, 0xff]), {
+                    status: 200,
+                });
+            }
+            if (behavior === "empty") {
+                return new Response(new Uint8Array(0), { status: 200 });
+            }
+            return new Response("not found", { status: 404 });
+        }
+
+        // Upload URL minting
+        if (path === "/files/upload-url") {
+            return json({
+                objectKey: `42/thumb-${Date.now()}`,
+                url: "https://s3.mock.test/presigned-put",
+            });
+        }
+
+        // Presigned PUT (S3 upload)
+        if (url.startsWith("https://s3.mock.test/")) {
+            const body = init?.body;
+            // Store the uploaded bytes for later inspection
+            if (body instanceof Uint8Array) {
+                (m as Record<string, unknown>)._lastUploadedCiphertext = body;
+            } else if (body instanceof ArrayBuffer) {
+                (m as Record<string, unknown>)._lastUploadedCiphertext =
+                    new Uint8Array(body);
+            }
+            return new Response(null, { status: 200 });
+        }
+
+        // Update thumbnail metadata
+        if (path === "/files/thumbnail" && init?.method === "PUT") {
+            const reqBody = JSON.parse(init?.body as string);
+            m.uploadedThumbnails.push({
+                fileID: reqBody.fileID,
+                objectKey: reqBody.thumbnail.objectKey,
+                decryptionHeader: reqBody.thumbnail.decryptionHeader,
+                ciphertext:
+                    ((m as Record<string, unknown>)
+                        ._lastUploadedCiphertext as Uint8Array) ??
+                    new Uint8Array(0),
+            });
+            return json({});
+        }
+
+        return new Response("not found", { status: 404 });
+    }) as typeof globalThis.fetch;
+};
+
+// ---------------------------------------------------------------------------
+// Tests
+// ---------------------------------------------------------------------------
+
+beforeAll(async () => {
+    await init();
+    await sodium.ready;
+    mock = await buildThumbMock();
+});
+
+describe("listMissingThumbnails", () => {
+    it("identifies files with empty and 404 thumbnails, ignores working ones", async () => {
+        const client = await Client.login({
+            email: TEST_EMAIL,
+            password: TEST_PASSWORD,
+            apiOptions: { fetch: buildThumbFetch(mock) },
+        });
+
+        const missing = await listMissingThumbnails(client);
+
+        // File 100 has a working thumbnail → not reported
+        // File 101 has an empty thumbnail → reported
+        // File 102 has a 404 thumbnail → reported
+        expect(missing.length).toBe(2);
+        const ids = missing.map((m) => m.fileID).sort();
+        expect(ids).toEqual([101, 102]);
+
+        const emptyEntry = missing.find((m) => m.fileID === 101)!;
+        expect(emptyEntry.reason).toContain("empty");
+        expect(emptyEntry.title).toBe("file-101.jpg");
+        expect(emptyEntry.collection).toBe("Photos");
+
+        const notFoundEntry = missing.find((m) => m.fileID === 102)!;
+        expect(notFoundEntry.reason).toContain("fetch failed");
+    });
+
+    it("deduplicates files seen in multiple collections", async () => {
+        // Add the same files to a second collection in the mock
+        const mockWithDupes = await buildThumbMock();
+        const dupeCollection = {
+            ...mockWithDupes.collections[0]!,
+            id: 2,
+        };
+        mockWithDupes.collections.push(
+            dupeCollection as Record<string, unknown>,
+        );
+        mockWithDupes.filesByCollection[2] =
+            mockWithDupes.filesByCollection[1]!;
+
+        const client = await Client.login({
+            email: TEST_EMAIL,
+            password: TEST_PASSWORD,
+            apiOptions: { fetch: buildThumbFetch(mockWithDupes) },
+        });
+
+        const missing = await listMissingThumbnails(client);
+
+        // Should still be 2, not 4 (each file checked only once)
+        expect(missing.length).toBe(2);
+    });
+});
+
+describe("fixMissingThumbnails", () => {
+    it("downloads original, generates thumbnail, encrypts, uploads, and registers", async () => {
+        const fixMock = await buildThumbMock();
+        const client = await Client.login({
+            email: TEST_EMAIL,
+            password: TEST_PASSWORD,
+            apiOptions: { fetch: buildThumbFetch(fixMock) },
+        });
+
+        const results = await fixMissingThumbnails(client, [101]);
+
+        expect(results.length).toBe(1);
+        expect(results[0]!.success).toBe(true);
+        expect(results[0]!.fileID).toBe(101);
+        expect(results[0]!.title).toBe("file-101.jpg");
+        expect(results[0]!.collection).toBe("Photos");
+
+        // Verify the uploaded thumbnail was registered
+        expect(fixMock.uploadedThumbnails.length).toBe(1);
+        const upload = fixMock.uploadedThumbnails[0]!;
+        expect(upload.fileID).toBe(101);
+        expect(upload.objectKey).toContain("42/");
+        expect(upload.decryptionHeader.length).toBeGreaterThan(0);
+
+        // Verify the uploaded ciphertext can be decrypted back to a JPEG
+        const fileKey = fixMock.fileKeys[101]!;
+        const decrypted = decryptBlob(
+            upload.ciphertext,
+            fromBase64(upload.decryptionHeader),
+            fileKey,
+        );
+        // JPEG magic bytes: FF D8 FF
+        expect(decrypted[0]).toBe(0xff);
+        expect(decrypted[1]).toBe(0xd8);
+        expect(decrypted[2]).toBe(0xff);
+        // Verify sharp produced a reasonably sized thumbnail
+        expect(decrypted.length).toBeGreaterThan(100);
+        expect(decrypted.length).toBeLessThan(50000);
+    });
+
+    it("reports failure for nonexistent file IDs without crashing", async () => {
+        const fixMock = await buildThumbMock();
+        const client = await Client.login({
+            email: TEST_EMAIL,
+            password: TEST_PASSWORD,
+            apiOptions: { fetch: buildThumbFetch(fixMock) },
+        });
+
+        const results = await fixMissingThumbnails(client, [999]);
+
+        expect(results.length).toBe(1);
+        expect(results[0]!.success).toBe(false);
+        expect(results[0]!.fileID).toBe(999);
+        expect(results[0]!.error).toContain("not found");
+    });
+
+    it("continues after one file fails and reports mixed results", async () => {
+        const fixMock = await buildThumbMock();
+        // Make file 102 fail by removing its ciphertext so download fails
+        delete fixMock.fileCiphertexts[102];
+
+        const client = await Client.login({
+            email: TEST_EMAIL,
+            password: TEST_PASSWORD,
+            apiOptions: { fetch: buildThumbFetch(fixMock) },
+        });
+
+        const results = await fixMissingThumbnails(client, [101, 102]);
+
+        expect(results.length).toBe(2);
+        const success = results.find((r) => r.fileID === 101)!;
+        const failure = results.find((r) => r.fileID === 102)!;
+        expect(success.success).toBe(true);
+        expect(failure.success).toBe(false);
+    });
+});
+
+describe("Client.getApiClient", () => {
+    it("returns the ApiClient when logged in", async () => {
+        const client = await Client.login({
+            email: TEST_EMAIL,
+            password: TEST_PASSWORD,
+            apiOptions: { fetch: buildThumbFetch(mock) },
+        });
+
+        const api = client.getApiClient();
+        expect(api).toBeDefined();
+        expect(typeof api.getJSON).toBe("function");
+    });
+
+    it("throws after logout", async () => {
+        const client = await Client.login({
+            email: TEST_EMAIL,
+            password: TEST_PASSWORD,
+            apiOptions: { fetch: buildThumbFetch(mock) },
+        });
+        client.logout();
+
+        expect(() => client.getApiClient()).toThrow(/logged out/);
+    });
+});