The model you asked for, and the one that answered.
An operator routes a step to a model from another provider because it would rather not pay for the harness's default model on steps it has judged do not need it. The justification is a saving, and a saving is shown by a record of which model did the work and what it cost. Which model answered is the first thing a component reading plaintext in the request path cannot establish from its own position, and what the call was billed is the second. These notes set out what such a record can attest, and what it takes on trust.
The step is ordinary: classify this, pull those fields out, retry a call that failed on a transient error. An operator running a harness at volume would rather not pay for its default model on the steps it has decided do not need it, and routes those to a model from another provider. Whether to route them belongs to whoever builds the system, and these notes take none of those decisions.
A saving is a claim about two quantities: what the step cost where it ran, and what it would have cost where it did not. The second is counterfactual, and no record of the step that ran recovers it. The first arrives from the party that ran the step, counted with a counter nobody on this side can open.
Routing costs the conversation something too: a thinking block "is readable only by the model that produced it or a newer one, and the API ignores or drops the blocks the target model can't read".[1]
The reason to route is a saving. The evidence for a saving is a pair of facts — which model answered, and what it counted — that a component in the request path takes delivery of rather than establishes. A record presenting that pair as findings is stronger than its own position; a record presenting it as testimony, with the testifier named, is exactly as strong as its position, and can be relied on for what it is.
The question these notes work on came out of building a routing switch inside Gateward, a Digital One product. It supplies no evidence here: every quotation above is from a published page a reader can fetch, cited with the date it was read, and the Gateward repository is not public, so nothing in it is a claim a reader can check today. Deliberately absent: prices, plans and tiers, availability, and deployment internals.
References
- Anthropic, Extended thinking, Claude Developer Platform documentation, on thinking-block signatures and on passing thinking blocks between models, platform.claude.com/docs/en/build-with-claude/thinking, read 12 September 2026.