How to Prevent Cost Surges from Malformed JSON Requests

I recently faced a major issue where a single request, which should have cost around $0.43, unexpectedly spiked to $7.81 due to a recursive JSON object. This bloated into a massive 3.2MB payload that was sent to the LLM as its 'context'. The problem is that our monitoring system didn't catch this - we saw HTTP 200s all around, token usage seemed reasonable, and our cost alerts were delayed by more than 6 hours. We didn't have any checks for payload sizes either. To tackle this issue, I implemented a few fixes including a hard limit of 100KB at the API boundary, per-request cost tracking with a $3 circuit breaker, schema validation in CI to avoid circular references, and a deduplication script. After these changes, we noticed a 91% drop in duplicate requests and managed to avoid two more costly mistakes before they went to billing. I'm curious if anyone else has implemented similar strategies to validate payloads before they hit expensive APIs?