Response schema break

Your inference service scores transactions and returns a JSON response that a downstream payments service consumes synchronously to decide approve/decline. At 16:20 the payments team pages you: their decline rate jumped and they're seeing parse errors from your service, though your own dashboards look green, you return 200s at normal latency with no errors logged. Investigation: a deploy 20 minutes ago changed your response 'score' field from a float (0.0–1.0) to a nested object {value, version} to add model-version metadata; the downstream parser expects a top-level float, so it fails to read the score and falls back to a conservative 'decline'. Your contract/schema with them wasn't versioned and they weren't notified. How do you triage and respond?