# Transform Layer
`src/transform` owns provider-to-provider conversion. `src/protocol` owns
provider wire models only.
## Rules
- Organize transforms by `Operation` / `OperationGroup`, not provider family.
- When an `OperationGroup` has distinct methods such as list/get, split those
methods into subdirectories before provider-pair files.
- Keep transforms pairwise. Do not introduce a unified IR.
- Same-kind passthrough bypasses this layer.
- Pair files own real provider field differences.
- `common/` may only contain mechanical helpers:
- SSE framing
- role classification
- tool id/result helpers
- usage arithmetic
- error construction helpers
- metadata helpers
- Provider `extra` fields are not preserved by transforms. Pair files should
drop source `extra` fields and initialize target `extra` fields as empty.
## Pair Files
Pair modules expose typed functions as they are implemented:
```rust
pub fn request(input: SourceRequest, ctx: &TransformContext) -> Result<TargetRequest, TransformError>;
pub fn response(input: SourceResponse, ctx: &TransformContext) -> Result<TargetResponse, TransformError>;
pub fn stream_event(input: SourceStreamEvent, ctx: &TransformContext) -> Result<Vec<TargetStreamEvent>, TransformError>;
```
Only define functions that exist for that operation pair.
`StreamGenerateContent` resolves through the same content-generation pair matrix
as non-stream generation. A frame may produce zero or many target events.
Stateful pair modules expose `StreamTransform`; dispatch and the runtime adapter
retain it across frames for tool-call identity, arguments, and final usage.
If a pair grows past roughly 400-500 lines, split only that pair into:
```text
pair_name/
mod.rs
request.rs
response.rs
stream.rs
tools.rs
```
## Runtime wiring (M2)
- `dispatch/{mod,content,other}.rs` — bytes-level
`(TransformPair, ctx, body) -> body` covering all 37 wired pairs
(content generation plus count_tokens/models/embeddings/images/compact);
`is_wired` gates anything unported. M2.5: models pairs dispatch list/get on
the source operation; request direction for body-less ops is path-synthesis
only.
- `routing.rs` — compiled §8-B2 `routing_rules` + the
passthrough/transform_to/local/unsupported decision.
- `stream_adapter.rs` — the strict runtime SSE adapter (bounded decode →
stateful `0..N` conversion → encode inbound frames). It surfaces bad frames
and abnormal EOF instead of manufacturing a successful completion.
- local operations (models list/get, count_tokens) short-circuit in the failover loop — see pipeline/local_ops.rs and src/tokenize/.