feat(cast): add --sponsor-url for remote Tempo fee payer sponsorship#14790
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Adds --sponsor-url (alias --tempo.sponsor-url) to cast send. When set, the transaction is signed locally and forwarded to the remote sponsor service via eth_sendRawTransaction. The service adds its fee payer signature and broadcasts the sponsored transaction. Usage: cast send 0x... --sponsor-url https://sponsor.tempo.xyz/tp_abc123 This works with the existing fee-payer service in tempo-apps without any changes — no new RPC methods required. The sponsor URL can also be set via TEMPO_SPONSOR_URL env var. Amp-Thread-ID: https://ampcode.com/threads/T-019e2d8c-7750-71a5-924c-2394c0c4e070
…nsor_url - Use eth_signRawTransaction to get the sponsored tx from the service, then submit via the regular RPC's send_raw_transaction. This matches the viem withFeePayer sign-only protocol. - Clone sponsor_url instead of take() so it's still present when TempoOpts::apply() runs, ensuring nonce_key=0 is set and the tx builds as a Tempo AA (0x76) type instead of EIP-1559 (0x02). - Handle sponsor service error responses that have name/code but no message field. Amp-Thread-ID: https://ampcode.com/threads/T-019e2d8c-7750-71a5-924c-2394c0c4e070
- Sign sponsored Tempo payload by setting a placeholder fee_payer_signature before local signing, so the sender commits to the sponsored variant - Bail out when --sponsor-url is combined with --unlocked, --browser, or a Tempo access key instead of silently ignoring it - Validate sponsor URL scheme (require https://, allow http only for localhost/127.0.0.1) - Tighten JSON-RPC response parsing: accept arbitrary result types, drop non-standard 'name' error field Co-authored-by: Amp <amp@ampcode.com>
mattsse
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i think this pr has scope creep
The previous commit bundled unrelated anvil backend refactoring (amsterdam_eip8037_enabled plumbing) into a CI fix commit.
The tempo-alloy dependency tree pulls in criterion, which depends on a crates.io package also named cast (v0.3.0). Using -p cast is now ambiguous; --bin cast unambiguously targets the Foundry binary.
mablr
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0xrusowsky,
DaniPopes,
figtracer,
grandizzy and
stevencartavia
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mablr
requested review from
0xrusowsky,
figtracer,
grandizzy and
stevencartavia
as code owners
The old check accepted bypass hostnames like localhost.evil.com and 127.0.0.1.evil.com; the new check inspects scheme() and host_str() precisely.
mablr
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May 25, 2026
mattsse
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Approving based on follow-up: the noted tradeoffs are acceptable for this PR.
figtracer
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May 25, 2026
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Summary
Adds
--sponsor-url(alias--tempo.sponsor-url) tocast send. When set, Foundry signs locally and routes the raw transaction throughtempo-alloy's sponsor relay transport so the sponsor service can add its fee-payer signature.Usage
Or via env var:
How it works
FEE_PAYER_SIGNATURE_MARKERso the sender signs the sponsored payloadRelayConnectorwithSponsorshipMode::SignOnlyto call the sponsor serviceChanges
Cargo.toml/Cargo.lock: Bumped Tempo crates to thetempo-alloy@1.7.3release rev.crates/cast/src/cmd/send.rs: Replaced manual sponsor JSON-RPC/reqwestflow withtempo-alloyrelay transport types.crates/cast/Cargo.toml: Removed directreqwestusage and added directalloy-rpc-clientusage for built-in connection strings.Prompted by: @0xrusowsky