> ## Documentation Index > Fetch the complete documentation index at: https://docs.hex-api.app/llms.txt > Use this file to discover all available pages before exploring further. # Sign a withdrawal's payout > The withdraw wallet signs omni's transfer hash of the withdrawal `nonce`, for the target chain's bridge `withdraw`. The signature is base58 of 65 bytes, laid out as the chain's bridge takes it: `r ‖ s ‖ v` with `v` 27/28 on EVM; `v ‖ r ‖ s` (`v` 0/1) on TON (1117); `r ‖ s ‖ v` (`v` 0/1) on Stellar, Gonka and Juno. On EVM it signs only when the chain's bridge `verifyAddress()` is the MPC address (else `404 Cant sign by HOT Verify`). Withdrawals to Solana (1001) and TON v1 (1111) are not signed here. A signature is kept for a day, across the service: a retry (or anyone asking for the same one) answers the same one, and the nodes sign it once (see Load protection). A withdrawal no payout can land (to the token contract itself, to the zero address, or a Stellar nonce past the bridge's 60 days) is refused with a `400`: it can only be refunded. ## OpenAPI ````yaml https://mpc.hex-api.app/openapi.json post /withdraw/sign openapi: 3.1.0 info: title: HEX MPC API version: 1.0.0 summary: HOT bridge signatures and relay, the bridge registry and monitor. description: > The public API of `hex-mpc`, a Cloudflare Worker in front of the HOT MPC nodes. It replaces the old HOT backend's bridge endpoints (`api0.herewallet.app`: `process_bridge_deposit`, `auto_deposit_register`, ...) and HOR (`rpc1.hotdao.ai`, `rpc2.hotdao.ai`). ## Which routes to use - **Credit a deposit to NEAR Intents**: `POST /relay/deposit` with the source tx (or the deposit's fields). The service reads the deposit on its chain, signs it and sends `mt_deposit_call` on NEAR, paying the gas. This is what `process_bridge_deposit` did. - **Pay out withdrawals**: `POST /relay/withdraw` with the chain and the receiver; poll `GET /relay/jobs/{chain}/{id}`. - **Clear a paid withdrawal**: `POST /relay/clear`. - **Submit transactions yourself**: the `Signatures` routes return the MPC signatures only. - **Record what your users do** (optional, for support and recovery): `POST /bridge/register`, `/bridge/deposits`, `/bridge/withdrawals`. Every relay call sends only what holds on chain (a deposit that happened, the payout of a withdrawal omni holds, the clear of a paid one), and each at most once, so retrying a call is safe. ## Conventions - **Bodies** are JSON objects, at most 32 KiB (`/intents/solver`: 128 KiB). A body that is not a JSON object is a `422`, a larger one a `413`. - **Big integers.** Nonces and amounts are answered as decimal strings. In requests they may be decimal strings or JSON integers: integers above 2^53 are read exactly (the bridge's nonces are about 1.8e21), but a decimal string is the safer choice for clients whose JSON encoder uses doubles. - **Nonces** are omni's: `unix seconds * 10^12 + n`. - **Encodings.** omni's token ids, receivers and the bridge signatures are base58; NEAR tx hashes are base58. - **Chain ids.** Chain ids are omni's (`v2_1.omni.hot.tg`), not always the chain's own: EVM chains use their EIP-155 id (Ethereum 1, Optimism 10, BNB 56, Polygon 137, Monad 143, X Layer 196, Base 8453, Arbitrum 42161, Avalanche 43114, ...); the others have omni's numbers: Solana 1001, NEAR 1010, Stellar 1100, TON 1117 (TON v1, 1111, is dead), Juno 4444118, Gonka 4444119. - **Errors** are `{ "detail": "..." }` with an HTTP status (FastAPI's format, kept from HOR), the router's own `404`, `405` and `429` included. Clients should branch on the status and show or log the `detail`. A `5xx` (and a `502`/`503` in particular) is worth retrying with backoff; a `4xx` will answer the same until something changes on chain. - **CORS.** Any origin; the request's `Origin` is echoed with `access-control-allow-credentials: true`, and preflights allow `GET, POST, OPTIONS` with any header. - **Times** in answers are milliseconds since the epoch unless named `*_s` (seconds). ## Rate limits Per client IP (an IPv6 client by its /64), over 60 seconds, counted per Cloudflare location: | Class | Limit | Routes | | --- | --- | --- | | `bridge` | 300 / 60 s | `/withdraw/sign`, `/deposit/sign`, `/clear/sign`, `/relay/*`, `/bridge/*` | | `solver` | 120 / 60 s | `/intents/solver` | `/v1/health`, `/v1/time` and `/openapi.json` are not limited. Over the limit the answer is `429` `{ "detail": "Too many requests" }` with `Retry-After: 60`. ## Load protection The MPC nodes are a fixed quorum: every signature is a round on each of them. So on top of the per-IP limits, every call that reaches the nodes (the `Signatures` routes, and the relay's own signing) is admitted globally, whoever asks: - **Rate per kind of signing**, by default 300 a minute each for withdrawals, deposits and clears: over it, `429` with `Retry-After`. - **Signings at once**, 60 in all, each kind its own share: over it, `503` `MPC nodes are busy, try again shortly` with `Retry-After`. Also `503` with `Retry-After` when the nodes answer `429` or too few of them are up. - **One signing per message.** A signature is kept for a day and answered again (a retry, another client, the relay), and while it is being made a second request waits for it (or gets `503` `This signature is being made, try again shortly` after 45 s). A refusal of the bridge (`400`) is answered again for 5 seconds. - **Refusals before the nodes**: a deposit omni already credited (`400` `Deposit … is already credited`), a withdrawal no payout can land (`400` `Withdrawal … : it can only be refunded`: it pays the token contract itself, the zero address, or a Stellar nonce past its 60 days). Retry a `429` or `503` after its `Retry-After`. servers: - url: https://mpc.hex-api.app description: Production security: [] tags: - name: Relay description: >- The recommended integration: the service sends the bridge's transactions and pays for them. Deposits are credited to NEAR Intents; withdrawals are paid out on their chain and cleared on omni. - name: Signatures description: >- The bridge's MPC signatures, for callers that submit the transactions themselves (HOR-compatible: same bodies, answers and errors). Each signature needs omni's word, and a deposit or clear also its AuthCall on chain, so a caller cannot forge or redirect one. - name: Registry description: >- The bridge registry: what users do on the bridge, as clients report it and as the relay sees it on chain. A post only records a claim: nothing is signed or sent for it, and what the relay reads on chain overrides it. Lookups answer 503 while the registry's database is not configured. - name: Monitor description: Health of every bridge HEX moves funds through, over 24 hours. - name: Intents description: >- NEAR Intents solver relay passthrough, used by HEX/HOT kit to publish signed intents. - name: Service description: Health, time and this document. paths: /withdraw/sign: post: tags: - Signatures summary: Sign a withdrawal's payout description: >- The withdraw wallet signs omni's transfer hash of the withdrawal `nonce`, for the target chain's bridge `withdraw`. The signature is base58 of 65 bytes, laid out as the chain's bridge takes it: `r ‖ s ‖ v` with `v` 27/28 on EVM; `v ‖ r ‖ s` (`v` 0/1) on TON (1117); `r ‖ s ‖ v` (`v` 0/1) on Stellar, Gonka and Juno. On EVM it signs only when the chain's bridge `verifyAddress()` is the MPC address (else `404 Cant sign by HOT Verify`). Withdrawals to Solana (1001) and TON v1 (1111) are not signed here. A signature is kept for a day, across the service: a retry (or anyone asking for the same one) answers the same one, and the nodes sign it once (see Load protection). A withdrawal no payout can land (to the token contract itself, to the zero address, or a Stellar nonce past the bridge's 60 days) is refused with a `400`: it can only be refunded. operationId: signWithdraw requestBody: required: true content: application/json: schema: $ref: '#/components/schemas/NonceRequest' examples: withdraw: summary: A withdrawal's nonce value: nonce: '1792107257000001836156' responses: '200': description: The signature. content: application/json: schema: $ref: '#/components/schemas/MetaSignOut' examples: evm: summary: EVM value: signature: >- DMStgVeGnFjpsYP8wPtWogP2feeHpsFYDjgP2jU1CpscSUQgfQWgffzdjMKppVqrtG6fGiUr6GjvX6NkrFxCu8KCf autopilot_hash: null '400': description: The withdrawal cannot be signed here. content: application/json: schema: $ref: '#/components/schemas/Error' examples: old: value: detail: Nonce is too old stellarOld: value: detail: Stellar Nonce is too old solana: value: detail: >- Chain 1001 withdrawals are signed by HOR's local key, not MPC sign: value: detail: Cant sign message refund: summary: No payout can land value: detail: >- Withdrawal 1792107257000001836156 pays the token contract itself: it can only be refunded '403': $ref: '#/components/responses/NodesUnavailable' '404': description: >- omni has no such withdrawal (never made, or already cleared), or the chain's bridge does not verify the MPC address. content: application/json: schema: $ref: '#/components/schemas/Error' examples: nonce: value: detail: Nonce not found or vas already cleared verify: value: detail: Cant sign by HOT Verify '413': $ref: '#/components/responses/PayloadTooLarge' '422': $ref: '#/components/responses/InvalidNonce' '429': $ref: '#/components/responses/TooManyRequests' '500': $ref: '#/components/responses/InternalError' '502': $ref: '#/components/responses/BadGateway' '503': $ref: '#/components/responses/SigningUnavailable' components: schemas: NonceRequest: type: object required: - nonce properties: nonce: $ref: '#/components/schemas/NonceInput' MetaSignOut: type: object description: HOR's signature answer. required: - signature - autopilot_hash properties: signature: $ref: '#/components/schemas/Base58' description: >- The signature, base58. Withdrawals: 65 bytes in the target bridge's layout. Deposits and clears: 64 bytes, `r ‖ s`. autopilot_hash: type: 'null' description: Always null; kept from HOR's answer model. Error: type: object description: 'Every error: FastAPI''s `{ detail }` with the HTTP status.' required: - detail properties: detail: type: string description: What went wrong, readable. examples: - detail: Nonce was not used on target chain NonceInput: description: >- omni's nonce: a decimal string (preferred) or a JSON integer, read exactly even above 2^53. Hex, floats and negatives are refused. oneOf: - $ref: '#/components/schemas/NonceString' - type: integer minimum: 0 Base58: type: string pattern: ^[1-9A-HJ-NP-Za-km-z]+$ description: Base58 (Bitcoin alphabet). NonceString: type: string pattern: ^\d{1,39}$ description: omni's nonce (`unix seconds * 10^12 + n`), decimal. examples: - '1792944123000000004211' responses: NodesUnavailable: description: The MPC nodes refused or did not answer. Retry later. content: application/json: schema: $ref: '#/components/schemas/Error' examples: nodes: value: detail: RPC is not responding PayloadTooLarge: description: 'The body exceeds 32 KiB (`/intents/solver`: 128 KiB).' content: application/json: schema: $ref: '#/components/schemas/Error' examples: tooLarge: value: detail: Body exceeds 32768 bytes InvalidNonce: description: The nonce is not a decimal integer, or the body is not a JSON object. content: application/json: schema: $ref: '#/components/schemas/Error' examples: nonce: value: detail: nonce must be a decimal integer json: value: detail: Body must be JSON TooManyRequests: description: >- Over the route's per-IP rate limit (60 s), or, on a route that signs, over the MPC nodes' signing rate (see Load protection). Wait `Retry-After` seconds. headers: Retry-After: $ref: '#/components/headers/RetryAfter' content: application/json: schema: $ref: '#/components/schemas/Error' examples: limited: value: detail: Too many requests signing: value: detail: Too many bridge_deposit signings, try again in 1 s InternalError: description: An unexpected error. Its message is not exposed. content: application/json: schema: $ref: '#/components/schemas/Error' examples: internal: value: detail: Internal Server Error BadGateway: description: >- A chain or NEAR RPC is unavailable, or an answer could not be read. Retry with backoff. content: application/json: schema: $ref: '#/components/schemas/Error' examples: near: value: detail: NEAR RPC is unavailable, try again chain: value: detail: Stellar RPC of chain 1100 is unavailable, try again SigningUnavailable: description: >- The MPC nodes cannot take the signing now (busy, rate limited by the nodes, too few of them up, or the same signature is being made), with `Retry-After`; or something the route needs is not configured. headers: Retry-After: $ref: '#/components/headers/RetryAfter' content: application/json: schema: $ref: '#/components/schemas/Error' examples: busy: value: detail: MPC nodes are busy, try again shortly down: value: detail: MPC nodes are unavailable, try again shortly inFlight: value: detail: This signature is being made, try again shortly nodes: value: detail: MPC nodes are not configured relay: value: detail: The relay's NEAR keys are not configured headers: RetryAfter: description: Seconds to wait. schema: type: integer example: 60 ```` This documentation is built and hosted on [Mintlify](https://mintlify.com), a developer documentation platform.