litellm/litellm-rust/crates/host-python/AGENTS.md
yujonglee 8302f006dd
refactor(rust-bridge): return an unstarted Execution from native entrypoints and drive it in Python (#46299)
* refactor(rust-bridge): return an unstarted Execution from native entrypoints and drive it in Python

Rust's run_call built Python streams itself and imported
litellm.rust_bridge.streams by name, so sync calls were driven in Rust
while async calls came back as a Rust-made coroutine. Every native
inference entrypoint now returns one unstarted Execution and
public_execution.py drives it with lifecycle.run or lifecycle.drive,
wrapping streams and _hidden_params on the Python side. Rust only
imports the lifecycle step classes through StepTypes, and
python-bridge/src/lifecycle.rs, PythonLifecycle and streams.py are gone.

The adapter trusts the _native.pyi contract with a single cast instead
of a per-request runtime_checkable isinstance. Binding tests assert the
adapter starts the execution and returns its completed value rather
than checking callable identity.

* refactor(rust-bridge): fold lifecycle and public_execution into one execution module

The public stream wrappers only forwarded to the lifecycle streams and added the
header dict, so the streams now carry _hidden_params themselves. lifecycle.py no
longer held any call lifecycle, just the Execution driver, so it is renamed to
execution.py and absorbs the synchronous and asynchronous adapters

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

* fix(rust-bridge): keep execution.py free of third-party imports

The host-python tests load execution.py into a bare interpreter, so the
pydantic header validation broke them. The stream head comes from the
binding's encode_stream_head, which always builds a str-keyed dict

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

* test(host-python): preserve pre-async hook coverage after execution handoff

* style(rust-bridge): sort litellm_host_python imports

Co-Authored-By: Yujong Lee <yujong@berri.ai>

* test(rust-bridge): drive unstarted native calls through execution in the split lifecycle tests

Co-Authored-By: Yujong Lee <yujong@berri.ai>

* fix(rust-bridge): mark execution TypeVar as Final

Co-Authored-By: Yujong Lee <yujong@berri.ai>

* test(rust-bridge): cover stream close, read failure, and sync suspension in execution

Co-Authored-By: Yujong Lee <yujong@berri.ai>

---------

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>
Co-authored-by: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>
2026-10-14 17:50:44 -07:00

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- Target invariants. Implementation and runtime validation may lag these rules
## Boundary with Python consumers
This crate owns CPython execution mechanics for generic `litellm-host` machines and hooks. Consumers supply domain bindings, host operations, public result construction and callback policy. Neither Rust dependencies nor Python imports may require LiteLLM route modules or legacy `Logging`
`src/native.rs` belongs here: it runs a generic machine through the Python runtime and owns its pending execution and abort handle. Keep provider selection, request projection and public exception policy out of it. A rename to `machine_runner.rs` is optional and must not change behavior
`run_call` returns the unstarted `Execution` and never names a Python stream class or driver. The consumer drives it from Python and supplies the step classes through `StepTypes` rather than have this crate import a fixed `litellm.rust_bridge` module. Generic suspension and execution state validation belong here. Public stream wrappers and `_hidden_params` conventions belong to the consumer
Creating a resolved asyncio Future from an already constructed Python value belongs here, alongside runtime waiting, interpreter detachment and panic containment. Choosing which callable exceptions become a public `RuntimeError` belongs to the consumer. `python-bridge::callable::wrap_failure` owns that policy
The driver owns ordering: start, argument preparation, prepared-argument hooks, binding decode and machine start. Fallible per-call resource setup supplied by the consumer runs after all argument hooks, using the prepared argument view, and before provider work. Setup failure follows the existing terminal failure path. Creating or discarding an unstarted coroutine must not initialize clients, acquire credentials or capture execution context
Boundary tests exercise behavior with a supplied lifecycle binding without importing the LiteLLM Python package. Pin inline awaiting, awaitable final values, exception identity, cancellation, re-entry and release of retained objects, rather than module names or source layout
`HookChain` composes Python runtime hooks in order. Each argument, wire-request and response transformation feeds its result to the next hook. After all argument transformations, the driver calls `arguments_prepared` on every hook in order. Retained callback views must adopt that dictionary before later policy hooks can mutate or reject it. SDK policy is supplied by bridge composition as a hook, never a separate driver phase or parameter. Hooks implement only the stages they need. Default stages preserve the supplied values
Terminal notifications share the selected response or exception. An ordinary notification error is reported as unraisable and does not skip the next hook or replace the selected outcome. Preparation, interception and transformation errors stop the chain. Cancellation stops all further hook dispatch. Suspensions stay inline in the existing driver, and the chain traverses retained event values for GC
## Existing runtime invariants
- Keep this crate the CPython runtime adapter and nothing more: Serde marshalling, interpreter detachment, tokio/asyncio glue, the `Execution` handle, the call driver and the `PythonBinding`, `PythonHostCalls` and `PythonOwned` traits, and the `PythonRuntime` specialization of `host::hooks::CallHooks`
- No LiteLLM domain dependencies beyond `litellm-host`: no route types, no `Logging` policy, no public API registration, no cdylib build features
- `PythonCallHooks` only constrains the shared call-stage interface to `PythonRuntime` and Python ownership. It must not redeclare the stages
- `PythonCallEvent` is a specialization of the shared `CallEvent`, never a separately defined lifecycle. The driver emits `Succeeded` or `Failed` exactly once and never dispatches callbacks after a cancellation. Which Python objects consume those events is the legacy adapter's business
- `CallOptions` can publish snapshots independently of callback delivery. Terminal snapshots follow completed hook dispatch, and cancellation never calls a Python callback. For Python-driven calls, leave the machine's observation publisher unset so the driver is the sole publisher of intercepted provider-response snapshots
- `PythonBinding::decode_request` receives the keyword view returned by `prepare_arguments` and updated by `arguments_prepared`, not the caller's dict. A binding that decodes from it inherits all composed argument rewrites
- A native failure, including one a host op returns as `InvokeError::Native`, is classified exactly once through the binding's `map_error`. A Python exception raised inside the call, and a failure in `prepare_arguments` or `transform_response`, is raised as is
- A failing `map_error` is raised with the native error's text as its `__context__`, never swallowed
- Consumers import this crate's error as `use litellm_host_python::Error as HostError`, so it never shadows their own `Error`
- Use standard PyO3 ownership and conversion APIs
- Prefer `Bound<'py, T>` for attached operations/results, `Py<T>` for retention. Binding/unbinding does not copy payloads
- Use `pythonize` for selected Serde data, never a JSON-text round trip. Share conversion with `Pythonized<T>`
- Preserve `PythonizeError`'s standard conversion into `PyErr`. Do not stringify original Python exceptions into new `ValueError`s
- Keep serializer-panic containment in `Pythonized<T>`: async output conversion can run in an unjoined blocking task and otherwise strand delivery
- Use `Python::detach` for Rust-only work. Python operations require attachment
- Keep diagnostic counters in the consumer. Wrapper invocations do not measure every interpreter release
- Release exclusive class borrows/locks before Python calls or decrements that can invoke finalizers. Expose retained Python edges to GC without calling Python during traversal
- Keep coroutine driving in the shared Python driver and the native handle
- Shared driver implementation: `litellm/rust_bridge/execution.py`, handle: `src/handle.rs`, call driver: `src/driver.rs`, native-backed behavior tests: `tests/lifecycle.py`. The consumer supplies the lifecycle binding
- Every lifecycle suspension is awaited inline in the caller's task. `into_future` creates a separate task and cannot satisfy this contract
- References: [ownership](https://pyo3.rs/v0.29.2/types.html), [conversions](https://pyo3.rs/v0.29.2/conversions/traits.html), [pythonize errors](https://docs.rs/pythonize/0.29.0/src/pythonize/error.rs.html)
- [GC](https://pyo3.rs/v0.29.2/class/protocols.html#garbage-collector-integration), [re-entry](https://pyo3.rs/v0.29.2/class/call.html), [parallelism](https://pyo3.rs/v0.29.2/parallelism.html), [async delivery source](https://docs.rs/pyo3-async-runtimes/0.29.0/src/pyo3_async_runtimes/generic.rs.html)