kalshi-cpp follows Kalshi's published Predictions contracts. The copies in spec/ are byte-identical to what Kalshi served on 2026-11-03.

File	Version	SHA-256
`spec/openapi.yaml` from https://docs.kalshi.com/openapi.yaml	3.35.0	`b00862e39b9c543065c5cee75f7d16d26e682ba8c7c4cd4c329ae34d0ecded74`
`spec/asyncapi.yaml` from https://docs.kalshi.com/asyncapi.yaml	2.0.0	`33fd603e0f7a95acd0da82a70cfc9126c31ddc27528a6e696efb2c3806b56b23`

Kalshi's Margin API (https://docs.kalshi.com/perps_openapi.yaml) has its own host, authentication, and risk model, so this client leaves it out.

Authentication

Each request signs timestamp_ms + METHOD + path with the account's key. The path starts at /trade-api/v2 (REST) or /trade-api/ws/v2 (WebSocket) and excludes the query string. The signature goes in KALSHI-ACCESS-SIGNATURE, next to KALSHI-ACCESS-KEY and KALSHI-ACCESS-TIMESTAMP. RSA keys sign with RSA-PSS and SHA-256; Ed25519 keys sign the message directly. See Kalshi's API keys guide.

Contract limits

The upstream files contain inconsistent descriptions and incomplete constraints. Keep the captured copies unchanged; do not treat their examples as server guarantees. Client validation covers order-group limits, exit-trigger legs, and subscription identifiers. Handle partial batch fills according to the operation documentation.

Timestamp uses Unix time, which excludes leap seconds. parse_timestamp returns ParseError for second 60 rather than moving it to a different instant.

Updating the contract

curl -fsS https://docs.kalshi.com/openapi.yaml -o spec/openapi.yaml
curl -fsS https://docs.kalshi.com/asyncapi.yaml -o spec/asyncapi.yaml
shasum -a 256 spec/*.yaml
make codegen test

If make codegen stops on a spec feature it can't map, extend tools/codegen/ to support it before committing the new spec. Review the diff of the generated files, update the table above, and note user-visible changes in CHANGELOG.md. Kalshi's changelog at https://docs.kalshi.com/changelog explains most changes.