Evidence-Bound Gateway-Path Provenance for Third-Party LLM Inference

Third-party LLM gateways select providers, models, endpoints, and fallback paths while also authoring the records used to explain and bill those decisions. We present evidence-bound gateway-path provenance, which makes these decisions subject to client-verifiable request contracts. An attested mediation runtime intersects a signed service policy with the client's authorization, constructs provider-facing calls, and protects streamed responses. The contract distinguishes initial routes from fallback destinations and admits each transition only after an authorized failure. A signed close-out binds the execution trace, provider-reported usage, authorized pricing, and complete response. We formalize the protocol state machine and establish authorization preservation, execution and stream binding, charge integrity, and channel confidentiality relative to the public receipt metadata. The analysis also characterizes replay, runtime replacement, and the information disclosed by public payload commitments. Controlled software-path measurements show 0.442 ms paired p95 latency overhead at concurrency one, peak throughput of 10.65 thousand requests/s, and stream close-out below 0.4 ms p95. An AWS Nitro Enclaves prototype completed 30 live single-route streaming requests with attestation-bound session and signing keys. The design separates gateway administration from payload processing and supplies a verifiable account of each confirmed execution.