Testing 16 LLMs: Tool Calling Failures Come Down to Schema Constraints
mastra_ai · reddit · 2026-07-25
A dev team tested 30 schema constraints across 16 major LLMs and found that every provider breaks tool calls in its own unique way:
- OpenAI reasoning models: Error out completely if they encounter a schema property they dislike.
- Gemini: The worst offender, as it silently ignores constraints and continues executing without throwing errors, making it a nightmare to debug later.
- DeepSeek and Llama: Sometimes outright refuse to call the tool.
The team discovered that instead of fighting with prompts, the easiest fix is to move the constraint text directly into the property's description field.
Related event: Testing 16 LLMs Reveals Schema Issues Behind Tool Call Failures(2 posts)→
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