diff --git a/clients/passkeys-browser/test/base64url.test.ts b/clients/passkeys-browser/test/base64url.test.ts
index 129eaa2..1333590 100644
--- a/clients/passkeys-browser/test/base64url.test.ts
+++ b/clients/passkeys-browser/test/base64url.test.ts
@@ -47,4 +47,21 @@ describe("base64url", () => {
     expect(ab).toBeInstanceOf(ArrayBuffer);
     expect(ab.byteLength).toBe(4);
   });
+
+  // StrykerJS (PR #39, @bernata) flagged the input-type dispatch in toUint8Array.
+  // Encoding a DataView (an ArrayBufferView that is neither Uint8Array nor
+  // ArrayBuffer) exercises the third branch and kills the mutant that forces the
+  // `instanceof ArrayBuffer` check true.
+  it("encodes an ArrayBufferView (DataView) identically to the same bytes", () => {
+    const buf = new Uint8Array([0xde, 0xad, 0xbe, 0xef]).buffer as ArrayBuffer;
+    expect(b64u.encode(new DataView(buf))).toBe(b64u.encode(new Uint8Array(buf)));
+    expect(b64u.encode(new DataView(buf))).toBe("3q2-7w");
+  });
+
+  it("encodes a Uint8Array view that has a non-zero byteOffset", () => {
+    // Subarray view: byteOffset 1, length 2 — guards the Uint8Array branch
+    // against being skipped in favour of a buffer-from-scratch reconstruction.
+    const full = new Uint8Array([0x00, 0xde, 0xad, 0x00]);
+    expect(b64u.encode(full.subarray(1, 3))).toBe(b64u.encode(new Uint8Array([0xde, 0xad])));
+  });
 });
diff --git a/clients/passkeys-browser/test/ceremonies.test.ts b/clients/passkeys-browser/test/ceremonies.test.ts
index 185806a..d868cef 100644
--- a/clients/passkeys-browser/test/ceremonies.test.ts
+++ b/clients/passkeys-browser/test/ceremonies.test.ts
@@ -186,3 +186,209 @@ describe("PkAuthCeremonyClient.register (end-to-end with stubbed credentials)",
     expect(fakeCreds.create).toHaveBeenCalledOnce();
   });
 });
+
+// Additional ceremony coverage driven by the StrykerJS report (PR #39, @bernata):
+// the suite previously exercised only the register() happy path, leaving
+// authenticate(), the credential create/get helpers, the cancellation branches,
+// and the navigator.credentials guard as 0%-covered (every mutant survived).
+// These ceremonies ARE unit-testable because CeremonyOptions.credentials injects
+// a fake CredentialsContainer — so we drive the full flows here.
+
+function fakeAssertion(rawId: Uint8Array): PublicKeyCredential {
+  return fakeCredential(rawId, {
+    clientDataJSON: new Uint8Array([0x7b]).buffer as ArrayBuffer,
+    authenticatorData: new Uint8Array([0x55]).buffer as ArrayBuffer,
+    signature: new Uint8Array([0x99]).buffer as ArrayBuffer,
+    userHandle: null,
+  } as unknown as AuthenticatorResponse);
+}
+
+/** Records every request body keyed by the URL suffix, and serves canned responses. */
+function ceremonyFetch(responses: Record<string, unknown>) {
+  const bodies: Record<string, Record<string, unknown>> = {};
+  const methods: Record<string, string | undefined> = {};
+  const urls: string[] = [];
+  const fetchImpl = vi.fn(async (url: RequestInfo | URL, init?: RequestInit) => {
+    const u = String(url);
+    urls.push(u);
+    const key = Object.keys(responses).find((suffix) => u.endsWith(suffix));
+    if (!key) throw new Error("unexpected " + u);
+    methods[key] = init?.method;
+    if (init?.body) bodies[key] = JSON.parse(String(init.body));
+    return new Response(JSON.stringify(responses[key]), { status: 200 });
+  });
+  return { fetchImpl, bodies, methods, urls };
+}
+
+describe("PkAuthCeremonyClient.register (start-body contract)", () => {
+  it("defaults displayName to username and label to null, POSTing both ceremony steps", async () => {
+    const { fetchImpl, bodies, methods } = ceremonyFetch({
+      "/registration/start": { challengeId: "ch-1", publicKey: CREATE_OPTIONS_JSON },
+      "/registration/finish": { credential: { credentialId: "cred" } },
+    });
+    const fakeCreds = {
+      create: vi.fn(async () =>
+        fakeCredential(new Uint8Array([7]), {
+          clientDataJSON: new Uint8Array([0]).buffer as ArrayBuffer,
+          attestationObject: new Uint8Array([0]).buffer as ArrayBuffer,
+        } as unknown as AuthenticatorResponse),
+      ),
+      get: vi.fn(),
+    } as unknown as CredentialsContainer;
+
+    const client = new PkAuthCeremonyClient(
+      { apiBase: "https://x", fetch: fetchImpl as unknown as typeof fetch },
+      { credentials: fakeCreds },
+    );
+    // No displayName, no label provided.
+    await client.register({ username: "alice" });
+
+    // displayName ?? username  (kills the `&&` mutant) and label ?? null.
+    expect(bodies["/registration/start"]).toMatchObject({
+      username: "alice",
+      displayName: "alice",
+      label: null,
+      challenge: null,
+    });
+    // label ?? null again on the finish body.
+    expect(bodies["/registration/finish"]).toMatchObject({ username: "alice", label: null });
+    // Both ceremony steps must be POST (kills the "POST" -> "" mutants).
+    expect(methods["/registration/start"]).toBe("POST");
+    expect(methods["/registration/finish"]).toBe("POST");
+  });
+
+  it("passes an explicit displayName and label straight through", async () => {
+    const { fetchImpl, bodies } = ceremonyFetch({
+      "/registration/start": { challengeId: "ch-1", publicKey: CREATE_OPTIONS_JSON },
+      "/registration/finish": { credential: { credentialId: "cred" } },
+    });
+    const fakeCreds = {
+      create: vi.fn(async () =>
+        fakeCredential(new Uint8Array([7]), {
+          clientDataJSON: new Uint8Array([0]).buffer as ArrayBuffer,
+          attestationObject: new Uint8Array([0]).buffer as ArrayBuffer,
+        } as unknown as AuthenticatorResponse),
+      ),
+      get: vi.fn(),
+    } as unknown as CredentialsContainer;
+    const client = new PkAuthCeremonyClient(
+      { apiBase: "https://x", fetch: fetchImpl as unknown as typeof fetch },
+      { credentials: fakeCreds },
+    );
+    await client.register({ username: "bob", displayName: "Bobby", label: "yubikey" });
+    expect(bodies["/registration/start"]).toMatchObject({ displayName: "Bobby", label: "yubikey" });
+    expect(bodies["/registration/finish"]).toMatchObject({ label: "yubikey" });
+  });
+
+  it("rejects when the authenticator returns no credential (create cancelled)", async () => {
+    const { fetchImpl } = ceremonyFetch({
+      "/registration/start": { challengeId: "ch-1", publicKey: CREATE_OPTIONS_JSON },
+    });
+    const fakeCreds = { create: vi.fn(async () => null), get: vi.fn() } as unknown as CredentialsContainer;
+    const client = new PkAuthCeremonyClient(
+      { apiBase: "https://x", fetch: fetchImpl as unknown as typeof fetch },
+      { credentials: fakeCreds },
+    );
+    await expect(client.register({ username: "alice" })).rejects.toThrow(/creation was cancelled/);
+  });
+});
+
+describe("PkAuthCeremonyClient.authenticate (end-to-end with stubbed credentials)", () => {
+  it("walks start -> get -> finish and returns the token", async () => {
+    const { fetchImpl, bodies, methods, urls } = ceremonyFetch({
+      "/authentication/start": { challengeId: "ch-9", publicKey: REQUEST_OPTIONS_JSON },
+      "/authentication/finish": { token: "jwt-token" },
+    });
+    const get = vi.fn(async () => fakeAssertion(new Uint8Array([42])));
+    const fakeCreds = { create: vi.fn(), get } as unknown as CredentialsContainer;
+    const client = new PkAuthCeremonyClient(
+      { apiBase: "https://x", fetch: fetchImpl as unknown as typeof fetch },
+      { credentials: fakeCreds },
+    );
+
+    const result = await client.authenticate({ username: "alice" });
+
+    expect(result.token).toBe("jwt-token");
+    expect(get).toHaveBeenCalledOnce();
+    expect(bodies["/authentication/start"]).toMatchObject({ username: "alice", challenge: null });
+    expect(bodies["/authentication/finish"]).toMatchObject({ challengeId: "ch-9" });
+    expect(methods["/authentication/start"]).toBe("POST");
+    expect(methods["/authentication/finish"]).toBe("POST");
+    expect(urls.some((u) => u.endsWith("/authentication/start"))).toBe(true);
+  });
+
+  it("defaults username to null when omitted", async () => {
+    const { fetchImpl, bodies } = ceremonyFetch({
+      "/authentication/start": { challengeId: "ch-9", publicKey: REQUEST_OPTIONS_JSON },
+      "/authentication/finish": { token: "t" },
+    });
+    const fakeCreds = {
+      create: vi.fn(),
+      get: vi.fn(async () => fakeAssertion(new Uint8Array([1]))),
+    } as unknown as CredentialsContainer;
+    const client = new PkAuthCeremonyClient(
+      { apiBase: "https://x", fetch: fetchImpl as unknown as typeof fetch },
+      { credentials: fakeCreds },
+    );
+    await client.authenticate();
+    // username ?? null  (kills the `&&` mutant): no username -> explicit null.
+    expect(bodies["/authentication/start"]).toMatchObject({ username: null });
+  });
+
+  it("requests conditional mediation only when conditional=true", async () => {
+    const responses = {
+      "/authentication/start": { challengeId: "ch-9", publicKey: REQUEST_OPTIONS_JSON },
+      "/authentication/finish": { token: "t" },
+    };
+    // conditional = true -> mediation set
+    const getCond = vi.fn(async () => fakeAssertion(new Uint8Array([1])));
+    const a = ceremonyFetch(responses);
+    await new PkAuthCeremonyClient(
+      { apiBase: "https://x", fetch: a.fetchImpl as unknown as typeof fetch },
+      { credentials: { create: vi.fn(), get: getCond } as unknown as CredentialsContainer },
+    ).authenticate({ conditional: true });
+    expect(
+      (getCond.mock.calls[0]![0] as CredentialRequestOptions & { mediation?: string }).mediation,
+    ).toBe("conditional");
+
+    // conditional defaults to false -> no mediation
+    const getPlain = vi.fn(async () => fakeAssertion(new Uint8Array([1])));
+    const b = ceremonyFetch(responses);
+    await new PkAuthCeremonyClient(
+      { apiBase: "https://x", fetch: b.fetchImpl as unknown as typeof fetch },
+      { credentials: { create: vi.fn(), get: getPlain } as unknown as CredentialsContainer },
+    ).authenticate();
+    expect(
+      (getPlain.mock.calls[0]![0] as CredentialRequestOptions & { mediation?: string }).mediation,
+    ).toBeUndefined();
+  });
+
+  it("rejects when the authenticator returns no credential (get cancelled)", async () => {
+    const { fetchImpl } = ceremonyFetch({
+      "/authentication/start": { challengeId: "ch-9", publicKey: REQUEST_OPTIONS_JSON },
+    });
+    const fakeCreds = { create: vi.fn(), get: vi.fn(async () => null) } as unknown as CredentialsContainer;
+    const client = new PkAuthCeremonyClient(
+      { apiBase: "https://x", fetch: fetchImpl as unknown as typeof fetch },
+      { credentials: fakeCreds },
+    );
+    await expect(client.authenticate()).rejects.toThrow(/authentication was cancelled/);
+  });
+});
+
+describe("PkAuthCeremonyClient credentials guard", () => {
+  it("throws a clear error when no credentials are injected and navigator lacks them", async () => {
+    // jsdom does not implement navigator.credentials, so the fallback guard
+    // fires. Kills the `if (this.credentials)` and navigator-check mutants.
+    const { fetchImpl } = ceremonyFetch({
+      "/registration/start": { challengeId: "ch-1", publicKey: CREATE_OPTIONS_JSON },
+    });
+    const client = new PkAuthCeremonyClient({
+      apiBase: "https://x",
+      fetch: fetchImpl as unknown as typeof fetch,
+    });
+    await expect(client.register({ username: "alice" })).rejects.toThrow(
+      /navigator\.credentials is not available/,
+    );
+  });
+});