My Tool-Calling Loop Worked Fine, Until Compliance Wanted a Second Model to Check It

Small ask, on paper. A clinician types something like "any allergy conflicts for this patient's current meds?", and before the model answers it needs to actually go get the medication list and the allergy list rather than guess at what's plausible. Two functions, both of which already existed. The interesting part was never the lookups, it was getting a model to decide when to call them and hand back arguments I could trust. Then compliance sat in on the review and asked the question I should have seen coming: "what checks this model's answer?" Fair question, this is going in front of a clinician. Their answer was a second model, from a different provider, running the same lookups independently and flagging if it disagreed. Reasonable. Also, as it turned out, the thing that broke my code. The first version worked. That was the problem. I had the OpenAI SDK already wired into this project, so version one was straightforward: define tools , send the request, read message.tool_calls , run whichever function it asked for, push a role: "tool" message back with the result, loop until it stops asking. Twenty minutes, maybe. It worked on the first real test and I remember thinking this was going to be a short ticket. It was a short ticket, right up until "second model, different provider" landed in the same sprint. I went to point the exact same loop at Claude and it just doesn't speak that dialect - Anthropic sends tool requests back as tool_use blocks sitting inside the message content, not a separate tool_calls array, and the result has to go back as a tool_result block inside a user message. There's no tool role at all on their side. Same idea, completely different shape, and I was about two minutes from just writing a second version of the loop and calling it a day. I'd already installed the thing that fixes this I only stopped because I already had @aviasole/shapecraft in this project for the FHIR schema work, and figured it was worth thirty seconds to check whether g