Problem

Native OpenAI Responses and Anthropic Messages stream-error handling collapses provider failures into opaque generic strings such as:

• OpenAI Responses stream error
• OpenAI Responses response failed
• Anthropic Messages stream error

This hides important failure modes like rate limits, context-length errors, provider overloads, and nested response.error payloads.

Expected

• OpenAI Responses should surface top-level code/message and nested response.error details.
• Anthropic Messages should surface error.type and error.message.
• Empty message fields should fall back to code/type rather than hiding useful detail.

PR

• fix(llm): surface code, type, and nested fields on provider stream errors #28757