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# RFD D45: Elelem: a standalone LLM provider streaming crate

- **Status**: Draft
- **Category**: Design
- **Authors**: Jean Mertz <git@jeanmertz.com>
- **Date**: 2026-05-30
- **Extends**: [RFD 012]

## Summary

`elelem` is a standalone, feature-gated crate that owns the LLM streaming
pipeline: one SSE driver, one normalized event model, and a per-shape parser
seam, with each provider behind a Cargo feature.
It replaces the external provider SDKs JP currently wraps and in several cases
forks.

## Motivation

A recent bug, in which the `cerebras` and `llamacpp` SSE adapters silently
swallowed stream errors via `take_while(is_ok)`, exposed a structural problem:
JP has one streaming contract with several divergent implementations, and the
contract is written down nowhere that a compiler or test enforces it.
Each provider wraps a different external crate, and those crates do two things:
serde types and SSE stream processing.
Several of them JP forks and maintains.

This conflates two concerns.
Generic streaming plumbing (connect, drive the SSE stream, surface errors, emit
a normalized event stream with consistent flush and finish semantics) should be
DRY, owned, and tested once.
Provider-specific logic (request building, chunk parsing, quirks) is irreducible
and stays per provider.
Today the generic part is copied and re-derived per provider, which is how one
copy drifted into a silent-error bug.

Doing nothing means continued divergence, more latent bugs of this class, and
ongoing fork maintenance across several crates.

## Design

### What a consumer sees

One crate, one dependency, providers behind features:

```toml
elelem = { version = "0", features = ["anthropic"] }
```

The generic core is always available: the `Event` and `StreamError` types and
the `ChunkParser` trait.
Each provider feature adds that shape's typed wire request and response types
and its parser.
The SSE driver and the `reqwest` client builder sit behind the `transport`
feature, on by default, so a consumer that only wants the wire types and parsers
(to validate or transform chunks, or drive its own HTTP stack) can depend on
`elelem` with `default-features = false` and pull in no HTTP dependencies.
Provider features are the only public Cargo feature switches; the shared shape
module is enabled internally via `#[cfg(any(feature = "cerebras", …))]`, not
separately feature-gated:

```toml
[features]
default = ["transport"]
transport = ["dep:reqwest", "dep:reqwest-eventsource", "dep:tokio"]
cerebras   = []
llamacpp   = []
ollama     = []
openrouter = []
anthropic  = []
# ...
```

### What elelem owns, and what JP keeps

elelem owns the wire: the typed request and response types for each API shape,
the shape parsers, the SSE driver, and the `Event` and `StreamError` types.
It does not own request *building*.
JP populates elelem's exported request types from a `ChatQuery`, including every
provider quirk (cache control, thinking budgets, reasoning effort, schema
transforms, Ollama's forced-tool system message), and hands that to elelem as
data: the typed body, the base URL, and auth headers. elelem alone builds the
`reqwest` client and the `EventSource`; no public API accepts a prebuilt client
or stream, which is what makes the connect-timeout and `Never` guarantees
unforgeable.
Request construction has never been a source of streaming bugs, and a
provider-neutral input model expressive enough for every quirk would cost far
more than it saves, so it stays in JP.

This makes elelem a typed wire client plus a streaming engine, a lower tier than
a high-level `chat()` abstraction.
The reusable, bug-prone part (driving and normalizing the stream) is shared; the
request ergonomics are not.

### The normalized event model is the contract

`Event` and `StreamError` move out of `jp_llm` and become `elelem`'s public API;
`jp_llm` re-exports them so JP keeps one event type, not a parallel copy.
The model is small:

```rust
enum Event {
    // a typed delta: message, reasoning, structured, or tool-call chunk
    Part { index: usize, part: EventPart, metadata: Map },
    // commit the parts grouped under `index`
    Flush { index: usize, metadata: Map },
    // emitted exactly once
    Finished(FinishReason),
}
```

`index` is an opaque grouping key, not a fixed slot: parts sharing an index
accumulate until their `Flush`, and parsers emit flushes in stream order.
Some shapes number them `0/1/2+` (chat completions), others use provider-native
indices (Anthropic content blocks, OpenAI `output_index`, Google virtual
indices).
Callers must not attach meaning to the number.
This matches [RFD 012], which defined the index as a grouping key, not a
semantic slot.

### One SSE driver

A single driver owns the stream lifecycle, so it cannot diverge per provider:

- It is built only through the shared client builder, the one place that sets
  the connect timeout and `EventSource::set_retry_policy(Never)`.
  A new provider cannot forget either, which is the exact failure mode behind
  the original bug.
- It enforces a stream-idle timeout from a value the caller passes in (JP
  supplies `assistant.request.stream_idle_timeout_secs`, where `0` disables).
  elelem owns the timeout *mechanism* and emits a retryable `StreamError` when
  it fires; JP owns the *value*.
- It surfaces transport errors before completion as a retryable `StreamError`,
  converts a stream that ends without a terminal `Finished` into a retryable
  error, and drops the benign close that follows that `Finished`.
  The terminal signal is the shape's own (`[DONE]` for chat completions, a named
  event elsewhere); the driver keys off `Finished`, not any protocol literal.
  These rules are the regression guard the whole crate exists for.
- Retry budget, backoff, and user notification stay in JP. elelem never retries
  on its own, and the contract suite asserts no shape or driver does.

### The parser seam and the four shapes

A *shape* implements the parser, not a provider.
The driver feeds it lifecycle frames and the parser emits normalized events;
provider-specific parsers exist only when a wire shape is genuinely unique.

```rust
enum Frame<'a> {
    Open,
    Message { event_name: Option<&'a str>, data: &'a str },
    Eof,
}

trait ChunkParser {
    fn parse(&mut self, frame: Frame<'_>) -> Vec<Result<Event, StreamError>>;
}
```

The `Frame` makes the lifecycle explicit, which is the point: the parser flushes
any trailing state on `Eof`, and the driver, tracking whether a terminal
`Finished` was emitted, suppresses the benign post-`[DONE]` close and converts a
premature `Eof` into a retryable error.
`event_name` is carried because Anthropic sends named SSE events (`event:
content_block_delta`) while OpenAI-style streams are anonymous `data:` frames;
`reqwest_eventsource` already exposes the name.

A *provider* selects a shape and supplies its request construction and quirks.
The four chat-completions providers share one shape parser:

| Shape                           | Providers                              |
| ------------------------------- | -------------------------------------- |
| Chat Completions (OpenAI-style) | cerebras, llamacpp, ollama, openrouter |
| Responses (OpenAI)              | openai                                 |
| Messages (Anthropic)            | anthropic                              |
| Gemini                          | google                                 |

### Recovery: request rejection without a side channel

Some providers reject an otherwise-valid request because of stale metadata:
Anthropic and Google invalidate old thinking and thought signatures.
The fix is to strip the offending metadata from the conversation history and
retry.
That is recovery, not response content, so it rides the error channel, not the
event stream.
`Event` has no `Patch` variant and `FinishReason` has no `Retry`; both are
deleted.

```rust
enum StreamError {
    Timeout, Connect, RateLimit, Transient, /* ... */
    Recoverable { patches: Vec<Patch> },
}

struct Patch  { matcher: Match, action: Action }   // generic, content-addressed
enum   Match  { MetadataValue { key: String, value: String } }
enum   Action { RemoveMetadata(String) }
```

The shape builds the patch from the wire request and the rejection error (it
owns the metadata-key vocabulary it emitted on the success path) and surfaces it
as `StreamError::Recoverable`.
Because that needs the request and error, not stream frames, it is a separate
function on the shape from the frame-oriented `ChunkParser`; the shape owns both
seams.
JP applies it: find the persisted event whose `metadata[key] == value`, remove
the key, rebuild the request, and retry.
The match is by value, so JP needs no provider knowledge and stays a generic
applier.
`Event::Patch` was a transport hack that routed a history mutation through the
content channel; moving it to the error channel removes both the conflation and
any second `Event` type.

`Recoverable` is caller-action-required, not a transient transport failure, so
JP applies the patches and retries immediately, without backoff and without
spending the transient-error retry budget.
It carries the underlying provider error: a caller that will not patch, or where
no stored metadata matches, surfaces that error as an ordinary failure rather
than looping.
Each applied patch makes progress and an unmatched patch terminates, so recovery
is bounded.

### Transport is uniform SSE

Every provider streams SSE through `reqwest_eventsource`.
Two decisions make that hold:

- **Ollama** uses `/v1/chat/completions`, folding it into the chat shape with no
  new parser.
  That endpoint supports streaming, tools, vision, structured output, and
  reasoning via `reasoning_content` and `reasoning_effort`.
  Ollama's `/v1/responses` endpoint is non-stateful only and drops vision and
  structured output while adding nothing JP uses, so it is rejected.
  The chat endpoint has no `tool_choice`; JP keeps its existing forced-tool
  system-message workaround when it builds the request.
  See [Ollama OpenAI compatibility] and [Ollama Anthropic compatibility].
- **Gemini** uses `?alt=sse`.

### JP side

JP's `Provider` trait is unchanged from JP CLI's point of view: it still returns
a stream of `Event` (now elelem's, re-exported).
Below the trait, each provider implementation builds the wire request, calls
elelem to drive and parse it, and owns everything that is not single-request
wire handling: request construction and quirks, retry budget and backoff, the
idle-timeout value (passed to elelem per request, so JP no longer wraps the
stream with `with_idle_timeout`), recovery (applying `Recoverable` patches and
retrying), the multi-request orchestration some providers need (Anthropic
max-token chaining and forced-tool fallback, Google unexpected-tool-call retry),
and `EventBuilder`, which stays in `jp_llm` because it translates `EventPart`
into the persistence type `ConversationEvent`. elelem owns the OpenAI Responses
wire types for both streaming and non-streaming responses but only the streaming
transport, so the non-streaming fallback for `streaming_unsupported` models is
JP's HTTP call mapping elelem's response type into events.

## Drawbacks

- Re-inlining forked crates risks re-discovering edge cases they quietly encode.
  Chesterton's Fence applies per crate; the vendor-first step and the contract
  suite are the mitigations.
- `elelem`'s event model becomes a semver commitment once published.
  That is the price of "standalone, reusable."
- The effort is wide and asymmetric: chat-completions is nearly free (already
  hand-rolled), but Responses, Messages, and Gemini carry real request-schema
  work.
- A standalone multi-provider client enters a populated space (genai,
  async-openai, and others).
  Maintaining a public crate is its own ongoing cost.

## Alternatives

- **Extract a shared driver for cerebras/llamacpp only, keep the SDKs.** Fixes
  the immediate duplication but leaves the fork-maintenance burden, does nothing
  for the SDK-wrapped providers, and gives the plugin future no foundation.
- **A core crate plus N shape crates plus N provider crates.** More granular,
  but worse ergonomics for a reusable standalone library; consumers want one
  dependency and a feature, not a crate-assembly job.
- **One driver and one parser for all providers, dropping the SDKs entirely.** A
  Golden Hammer: the wire shapes genuinely differ, and forcing Anthropic and
  Gemini through a chat-completions parser would trade correctness for
  uniformity.
- **Route everything through the Responses API.** Responses earns its place for
  OpenAI proper (encrypted reasoning continuity, hosted tools, the o-series),
  but a chat-completions-compatible backend gains nothing from its Responses
  shim.

## Non-Goals

- Re-implementing `reqwest_eventsource`.
  The bug was JP's usage, not the crate; it stays.
- Adding new providers.
- Stateful Responses API support (JP does not use it for OpenAI either).

## Risks and Open Questions

- **Quirk parity.** The Anthropic and Google signature-strip-and-retry behaviors
  must survive the migration unchanged.
  The contract suite plus re-recorded cassettes are the guard.
- **Cassette re-recording needs live endpoints.** `llamacpp` and `ollama`
  require local servers; the stale `llamacpp` cassettes surfaced by the original
  fix are the first to re-record.
- **Publication timing.** Publishing elelem overrides the workspace `publish =
  false` and freezes its feature names, `Event`, `StreamError`, and per-shape
  request types as public API.
  Don't publish until at least two non-chat shapes are migrated, so the surface
  isn't frozen chat-biased.

## Implementation Plan

- **Phase 0: Vendor.** Pull the forked external crates into the workspace as
  plain members, keep every cassette green, and drop the upstream forks.
  Reviewable on its own, and independent of Phase 1, which extracts from the
  hand-rolled `cerebras` / `llamacpp` and needs no vendored SDK.
- **Phase 1: Core plus chat shape.** Create `crates/contrib/elelem`.
  Extract the generic core (types, `ChunkParser`, driver, client builder) from
  `cerebras` / `llamacpp`, implement the chat-completions shape, and seed the
  stream-contract test suite (the surface/swallow cases already written).
  Move `cerebras` and `llamacpp` onto it.
- **Phase 2: Fold the chat family.** Move `ollama` (switched to
  `/v1/chat/completions`) and `openrouter` onto the chat shape.
  Delete `ollama-rs` and `jp_openrouter`, including `openrouter`'s internal
  `backon` retry loop, since JP owns retry.
- **Phase 3: Anthropic Messages.** Add the shape, drop `async_anthropic`.
  First real exercise of the named-event seam and the `StreamError::Recoverable`
  path.
- **Phase 4: OpenAI Responses.** Add the shape, drop `openai_responses`.
- **Phase 5: Gemini.** Add the shape on `alt=sse`, drop `gemini_client_rs`.

Each phase is gated by the contract suite and re-recorded cassettes, and each
provider cuts over independently, so the migration stays reviewable and
reversible throughout.

## References

- [RFD 012], which defined the typed `Event` / `EventPart` streaming model that
  `elelem` relocates out of `jp_llm` and owns.
- The Ollama compatibility docs behind the transport decision.
- [RFD 043] (Discussion) stays JP-side: elelem emits raw tool-call chunks;
  `EventBuilder` owns their accumulation and any UI progress.
- [RFD 064] (Discussion) may later move patch application from in-place mutation
  to stored events applied at projection time.

[Ollama Anthropic compatibility]: https://docs.ollama.com/api/anthropic-compatibility
[Ollama OpenAI compatibility]: https://docs.ollama.com/api/openai-compatibility
[RFD 012]: ../012-typed-llm-streaming-events.md
[RFD 043]: ../043-incremental-tool-call-argument-streaming.md
[RFD 064]: ../064-non-destructive-conversation-compaction.md
