> ## Documentation Index > Fetch the complete documentation index at: https://docs.userkit.dev/llms.txt > Use this file to discover all available pages before exploring further. # Finish a sign-in with the second factor > The second half of any sign-in that stopped at the first factor. Accepts a TOTP code or a single-use recovery code. The challenge is single use, bound to its environment, and is **not** a session — it carries its own `uk_c2_…` prefix so it can never be spent as one. A wrong code does not consume it; the rate limit is what bounds guessing. The session that lands carries the verification the first factor already earned: a magic link and a social callback both prove the identity before the code is asked for. Rate limited to 20 requests per 5 minutes per IP. ## OpenAPI ````yaml /api-reference/openapi.json post /v1/contact-auth/two-factor openapi: 3.1.0 info: title: UserKit API version: 1.0.0 description: >- The HTTP surface of UserKit. Two planes share one API. The **staff plane** is what a human uses in the panel: users, organizations, roles, members, keys. The **customer plane** is what a developer's own product uses: contacts, identities, hosted or federated sign-in. Every error answers the same envelope — `{"error": {"code", "message"}}`. The `code` is a stable contract to branch on; the `message` is for a person and may change. servers: - url: '{baseUrl}' description: The API host. variables: baseUrl: default: https://api.userkit.dev description: Base URL of the API, no trailing slash. security: - sessionToken: [] tags: - name: Authentication description: >- Public sign-up, sign-in, two-factor and password recovery for staff accounts. - name: Session description: >- The active session: who the caller is, which organization they are in, and signing out. - name: Account description: The caller's own account — profile, password, sessions, avatar. - name: Two-factor description: >- TOTP setup, activation and recovery codes. Returns 501 when two-factor is unavailable on the server. - name: Organizations description: The organizations a user belongs to, and the active one. - name: Environments description: >- The live and test environments seeded with every organization, and their identity settings. - name: Members description: Memberships and invitations. - name: Roles description: Roles and the permission catalogue they draw from. - name: API keys description: Secret keys (`uk_sk_…`) and publishable keys (`uk_pk_…`). - name: Contacts (panel) description: >- The staff view of the customer plane. Reads take `?environment=` as an explicit view parameter. - name: Contacts (server) description: >- The machine surface, authenticated by an API key. The environment is the key's environment and cannot be named by the caller. - name: Customer plane description: >- Called from the developer's own pages with a publishable key: boot, magic link, and the contact's own session. - name: Hosted auth description: >- Sign-up, sign-in, verification and recovery for contacts, when UserKit owns the account. - name: System description: Health and the internal job callback. paths: /v1/contact-auth/two-factor: post: tags: - Hosted auth summary: Finish a sign-in with the second factor description: >- The second half of any sign-in that stopped at the first factor. Accepts a TOTP code or a single-use recovery code. The challenge is single use, bound to its environment, and is **not** a session — it carries its own `uk_c2_…` prefix so it can never be spent as one. A wrong code does not consume it; the rate limit is what bounds guessing. The session that lands carries the verification the first factor already earned: a magic link and a social callback both prove the identity before the code is asked for. Rate limited to 20 requests per 5 minutes per IP. operationId: verifyContactTwoFactor requestBody: required: true content: application/json: schema: type: object required: - publishable_key - challenge_token - code properties: publishable_key: type: string challenge_token: type: string example: uk_c2_… code: type: string description: A six-digit TOTP code, or a recovery code. responses: '200': description: Signed in. content: application/json: schema: type: object properties: contact: $ref: '#/components/schemas/Contact' token: type: string expires_at: type: string format: date-time verified: type: boolean '401': description: >- `invalid_challenge` — unknown, expired, already spent or from another environment. `invalid_code` — the code did not verify. content: application/json: schema: $ref: '#/components/schemas/Error' '403': $ref: '#/components/responses/OriginNotAllowed' '429': $ref: '#/components/responses/RateLimited' '501': $ref: '#/components/responses/TwoFactorUnavailable' security: [] components: schemas: Contact: type: object description: One of the developer's own users — anonymous or identified. properties: id: type: string format: uuid name: type: - string - 'null' email: type: - string - 'null' avatar_url: type: - string - 'null' identified: type: boolean description: Whether this contact is more than a visitor. email_verified: type: boolean description: >- Whether the ADDRESS was proven — by a verification link, a reset link or a magic link. Distinct from a session's `verified` flag. attribution: $ref: '#/components/schemas/Attribution' first_seen_at: type: string format: date-time last_seen_at: type: string format: date-time created_at: type: string format: date-time Error: type: object description: Every error in this API answers this envelope. properties: error: type: object properties: code: type: string description: Stable. Branch on this. message: type: string description: For a person. May change. required: - code - message required: - error example: error: code: forbidden message: your role does not allow this action Attribution: type: object description: >- First touch, captured on the visitor at first load and preserved through a merge. All fields optional; empty means unknown. properties: utm_source: type: - string - 'null' utm_medium: type: - string - 'null' utm_campaign: type: - string - 'null' utm_term: type: - string - 'null' utm_content: type: - string - 'null' referrer: type: - string - 'null' landing_page: type: - string - 'null' responses: OriginNotAllowed: description: >- `origin_not_allowed` — this publishable key does not accept calls from this origin. content: application/json: schema: $ref: '#/components/schemas/Error' RateLimited: description: >- `rate_limited` — too many requests. `Retry-After` carries the window in seconds. The counters are shared across instances. When that store cannot be reached each instance counts on its own instead: the limits get looser, never absent. headers: Retry-After: schema: type: integer description: Seconds until the window resets. content: application/json: schema: $ref: '#/components/schemas/Error' TwoFactorUnavailable: description: '`two_factor_unavailable` — the server cannot keep a TOTP secret safely.' content: application/json: schema: $ref: '#/components/schemas/Error' securitySchemes: sessionToken: type: http scheme: bearer description: >- A staff session token, `uk_st_…`. Minted by sign-up, sign-in or the two-factor exchange. ````