- Shared base and layering rules: [`../inference/AGENTS.md`](../inference/AGENTS.md)
- This crate owns provider-independent Messages call orchestration: the entrypoint, call envelopes, provider selection, credential resolution, transport coordination, hooks, and stream lifecycle
  - shared API data contracts belong in `litellm-llms-types::formats::messages`
  - adapter contracts and execution inputs in `llms/src/base_llm/messages`
  - provider implementations in `llms/src/<provider>/messages`
- Select concrete provider adapters and invoke their contracts
  - delegate authentication policy, beta selection, payload rewriting, and response interpretation to those adapters
  - keep provider policy out of request preparation and transport handlers
  - calling a concrete provider helper for every provider is still a policy dependency
- Route types such as `MessagesCall`, provider calls, and response wrappers containing live streams describe execution. Reuse the shared Messages payload types inside them instead of defining another request or response schema here
- The route returns `litellm_host::call::CallOutput`: a completed response, or a stream head and chunks
- Per-call dependencies are grouped in `litellm_inference::context::CallContext`. `handler::execute` explicitly sequences the provider call build in Python's handler order, cache lookup, provider execution, result acceptance (`CallContext::result_ready`), and cache storage
- Preserve the order of validation, normalization, caller-requested parameter removal, and provider transformation when that order affects observable behavior
- Test provider dispatch, auth precedence, header handling, transformations, and responses through behavior, not source structure