What makes providers hard to abstract

Providers disagree at the wire level about nearly everything:

• Tool-schema format: each wants its own shape, and some reject edge cases (an empty parameter object) that others accept.
• Message format: content blocks vs. parts vs. input items; some require you to replay the model's prior turn verbatim, sometimes including opaque signatures that must be echoed exactly.
• Streaming events: different event vocabularies for text, reasoning, and tool calls.
• Reasoning/thinking: different switches and different ways the reasoning is surfaced.
• History chaining: some resend full history; some use a server-side "previous response id."

If any of this leaks into your agent loop, every provider-specific quirk becomes load-bearing and a fourth provider means a rewrite.