Some Ollama models (like Olmo, Nemotron-3-nano) don't properly respect
system messages, causing them to ignore tool-format instructions.
This fix modifies _format_messages_for_provider() to merge system message
content into the first user message for Ollama models, ensuring the
instructions are visible to the model.
Changes:
- Accumulate all system messages and merge into first user message
- Handle edge cases: system-only messages, system then assistant
- Preserve conversation history after system message merge
- Add comprehensive tests for all scenarios
Fixes#4117
Co-Authored-By: João <joao@crewai.com>
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