ADR-041: Playground live run streaming 

Context 

The playground inspector (ADR-040) ran the whole bounded agent loop server-side and returned the complete trace as one JSON document. The browser therefore showed nothing until the entire run — every model round-trip and tool execution — had finished, then rendered all at once. For a multi-round run against a slow local model that is several seconds of a blank pane, and it hides when each step happened.

The goal is a live inspector: each step appears the moment it is recorded.

Decision 

Stream the run as newline-delimited JSON (NDJSON), one event per line, over the existing admin AJAX route:

• Opt-in. The client sends stream=1; the controller then streams. Without it the controller keeps the batch path — one respondJson() document — which remains the no-JavaScript fallback and the shape the functional tests assert.
• Per-step callback. RunTrace takes an optional onRecord closure fired the instant each RunStep is recorded. The streaming controller passes a closure that echoes one {"event":"step","step":…} line and flushes; a final {"event":"done",…} (or {"event":"error",…}) line carries the summary. When no closure is passed (every production and test caller) the loop is byte-for-byte unchanged.
• Beat the proxy buffer. A TYPO3 backend response is buffered by the reverse proxy until a chunk clears its flush threshold, so small lines all arrive at the end. The stream disables output buffering and zlib compression at runtime and pads every line past  4 KB with trailing whitespace (ignored by JSON.parse), which makes each event flush immediately.
• NullResponse. Having written output directly, the controller returns TYPO3CMSCoreHttpNullResponse, which AbstractApplication::sendResponse() skips — TYPO3 emits nothing further, avoiding a double body or a headers-already-sent warning.
• Same UTF-8 guard. Each line is encoded with JSON_INVALID_UTF8_SUBSTITUTE (as ADR-040 established for the batch response), so a malformed byte substitutes rather than aborting the stream.

The client reads the response body stream, splits on newlines, parses each line, appends the step to a live trace and re-renders. If the browser cannot read a streaming body it falls back to the batch request.

Consequences 

• Steps render as they happen; the summary strip fills in live from the per-round token counts and finalises on done.
• The 4 KB padding is transfer overhead (a few KB per event) that never reaches the user — an accepted cost for reliable incremental flushing across proxies.
• Direct echo/flush plus NullResponse is deliberately outside the PSR-7 body abstraction; it is confined to the one streaming method and the batch path stays a normal response.
• A run whose final model round stops on finishReason: length is now flagged with a truncation banner, and Max tokens/Temperature are exposed as per-run controls so the operator can lift the cap.