Having just fixed one real bug (below), the same customer hit the same model failing again. The actual error this time: “Unsupported value: 'temperature' does not support 0.7 with this model. Only the default (1) value is supported.”
This is a second, unrelated restriction on the same model family — not a recurrence of the first bug. Every model we'd integrated before this one accepted a custom temperature value without complaint; this generation of reasoning models does not.
Fixed by omitting the parameter entirely for this model family, letting it fall back to its own default rather than sending a value it would reject.