> For the complete documentation index, see [llms.txt](https://assay.gitbook.io/assay-docs/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://assay.gitbook.io/assay-docs/how-it-works/receipts.md). # Receipts Every field of a receipt body, how it is hashed and signed, and how it reaches you. **Where:** SDK `buildReceipt` and `receiptHash` in `sdk/src/receipt.ts`. The host returns each receipt in the `X-Assay-Receipt` header of `POST /v1/chat/completions`. A receipt is canonical JSON that the host signs for every response. Its id is `receiptHash = sha256(JCS(body))`, and every signature and Merkle proof refers to that hash. ## Fields | Field | Meaning | | ------------------------------- | -------------------------------------------------------------------------------------------------------------------- | | `v` | Always `"assay-receipt/0"`. | | `model` | The model the host claims it ran, for example `"z-ai/glm-5.3"`. | | `host.agentId` | The host's ERC-8004 identity as `"erc8004::"`, for example `"erc8004:10143:1962"`. | | `host.keyId` | The `kid` of the signing key in the host's JWKS. | | `host.alg` | `"ES256"` (P-256). `"ES256K"` is reserved for secp256k1 hosts. | | `req.commit` | `sha256(salt ‖ utf8(JCS({messages, params})))`. | | `req.params` | Every request field except `messages`, `model`, `stream` and `provider`, as the client sent it. | | `req.cosigner` | Optional. `keccak256(abi.encode(qx, qy))` of the passkey allowed to co-sign. Set from the `X-Assay-Cosigner` header. | | `res.commit` | `sha256(salt ‖ utf8(output))`, where output is `choices[0].message.content` exactly as returned. | | `res.tokensIn`, `res.tokensOut` | Token counts from the upstream `usage`, as non-negative integers. | | `res.finish` | The upstream `finish_reason`, or `"unknown"`. | | `price` | Optional `{asset, amount}`, with `amount` as a string. | | `t` | Issue time in Unix milliseconds, a positive integer. | | `nonce` | 16 random bytes as hex. | ## Example body ```json { "v": "assay-receipt/0", "model": "z-ai/glm-5.3", "host": { "agentId": "erc8004:10143:1962", "keyId": "host-key-2026-09", "alg": "ES256" }, "req": { "commit": "0x…", "params": { "temperature": 0, "max_tokens": 512 } }, "res": { "commit": "0x…", "tokensIn": 812, "tokensOut": 143, "finish": "stop" }, "t": 1790500000000, "nonce": "0x…" } ``` ## How a receipt is made 1. You send a request with `X-Assay-Salt`: 32 random bytes as 64 hex characters, with or without `0x`. 2. The host forwards the request and gets the model's answer. 3. The host computes `req.commit` from the bytes it received and `res.commit` from the text it returns. 4. The host signs the JCS bytes of the body as a compact ES256 JWS. The JWS payload is exactly `JCS(body)`. 5. The host returns `X-Assay-Receipt` (base64url of the JSON `{body, jws}`) and `X-Assay-Receipt-Hash`. 6. The receipt waits in the host's queue until the next batch is anchored. ## Errors when building a receipt | Error | Cause | | --------------------------------------------- | ------------------------------------------------------------------------------------------------ | | `req.commit must be 32 bytes of hex` | The commit is not a 32-byte hex string. The same check runs for `res.commit` and `req.cosigner`. | | `res.tokensIn must be a non-negative integer` | A token count is a float, negative or unsafe. | | `t must be a positive integer (ms)` | `t` is zero, negative or not an integer. | | `jcs: undefined at $.…` | A field holds `undefined`, which JCS cannot represent. | ## Where else this shows up | Place | What it does | | ---------------------------------------------------------- | --------------------------------------------------------- | | [SDK reference](/assay-docs/for-developers/sdk.md) | `buildReceipt`, `receiptHash`, `verifyReceiptJws`, `wrap` | | [Host API](/assay-docs/for-developers/host-api.md) | The headers and status codes around a receipt | | [Web app](/assay-docs/for-developers/web-app.md) | The Verify tab accepts the `X-Assay-Receipt` value as is | | [Anchoring batches](/assay-docs/how-it-works/anchoring.md) | What happens to the receipt hash next | {% hint style="warning" %} Assay's own fields stay integers or strings so every JCS library hashes them the same way. `req.params` may still hold floats such as `temperature: 0.7`. A verifier in another language must use an RFC 8785 conformant JCS library, or its hash will not match. {% endhint %} Next: [Anchoring batches](/assay-docs/how-it-works/anchoring.md) --- # Agent Instructions This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com. ## Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation by asking a question. Perform an HTTP GET request on the following URL with the `ask` and `goal` query parameters: ``` GET https://assay.gitbook.io/assay-docs/how-it-works/receipts.md?ask=&goal= ``` `ask` is the immediate question: it should be specific, self-contained, and written in natural language. `goal` is what the user is ultimately trying to achieve, the reason they need the answer. Sharing it helps GitBook give you a better, more relevant answer. A goal is most helpful when it describes the outcome the user wants rather than restating the question. For example, with `ask=how do I create an API token`, a goal like `automate deployments from our CI pipeline` lets GitBook tailor the answer to that use case. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.