Modify the output stream

The model loop should not know whether the browser wants Server-Sent Events, your worker wants JSONL, or your test wants raw TurnEvents.

This recipe keeps transport formatting at the edge. Build the same conversation stream you would use anywhere else, then map the output into the wire format you need.

Scenario. Your agent loop emits Stream<TurnEvent>. Serve it as text/event-stream for a web UI and as newline-delimited JSON for another consumer. No special runner, no alternate conversation type.

The Design Move

Transport is a projection over the output stream:

• toSSE — one TurnEvent → one SSE.Event, or drop it.
• toJSONL — one TurnEvent → one JSON line, or drop it.

Both are plain Result.Result filters, so they compose with Stream.filterMap directly. They live in the recipe, not core, because they encode product policy: this version forwards text and completion, and drops everything else.

The whole thing

import { Stream } from "effect"
import * as SSE from "@effect-uai/core/SSE"
import { conversation, toJSONL, toSSE } from "./index.js"

// Server-Sent Events on the wire.
const sseBytes = conversation.pipe(Stream.filterMap(toSSE), SSE.toBytes)
//   ^? Stream<Uint8Array, AiError, LanguageModel>

// Newline-delimited JSON lines.
const jsonl = conversation.pipe(Stream.filterMap(toJSONL))
//   ^? Stream<string, AiError, LanguageModel>

The recipe also exports asSSE and asJSONL, the curried form of the same mapping, which is handy when you want the formatter to read like a named stream transform:

const sse = conversation.pipe(asSSE)
const jsonl = conversation.pipe(asJSONL)

The recipe’s conversation is deliberately small: one streamed turn, no tools, no extra state. In a real app, the same mapping sits after a tool-using loop, approval flow, retry policy, or long-lived queue-driven chat.

What gets emitted

TurnEvent	SSE	JSONL
text_delta	event: text\ndata: {"text":"..."}	{"type":"text","text":"..."}
turn_complete	event: done\ndata: {"stop_reason",...}	{"type":"done","stop_reason":...,...}
reasoning, tool-call deltas	dropped	dropped

These local toSSE and toJSONL helpers are intentionally conservative. They keep text deltas and turn completion events, and drop internals such as reasoning or tool-call argument deltas.

If your product wants those events on the wire, write your own filterMap. That is the point of the design: the loop emits typed events; the edge decides which ones become protocol frames.

Run it

OPENAI_API_KEY=sk-... pnpm tsx recipes/modify-output-stream/run.ts

The runner prints both wire formats back-to-back so you can copy a frame straight from the terminal.

The full source lives next to this README at index.ts.