Perplexity moved the answer text out of `markdown_block` into
`workflow_block` (`intended_usage: "workflow_root"`), streaming it as
RFC-6902 patches whose `field` is `"workflow_block"` and whose paths
address `/steps/<n>/items/<m>/payload/text_payload/chunks/<k>`.
`extractContent` recognised neither shape. Two independent guards dropped
every answer frame:
- `isAnswerTextUsage("workflow_root")` is false, so the block loop
`continue`d before any accumulation.
- the diff guard skipped every patch whose `field !== "markdown_block"`.
The stream therefore ran to `COMPLETED` with an empty accumulator and the
executor surfaced `Provider returned empty content` (502) even though the
upstream SSE carried the full answer. Every model was affected — the
carrying block is model-independent — so the provider was unusable.
Adds `workflow_block` to `PplxBlock`, an `applyWorkflowDiff` patch
applier for the streaming path, and `applyWorkflowBlock` for a
materialized block on the terminal frame. Answer tracks are keyed per
step+item so concurrent items cannot overwrite each other's chunk
indices, and only `variant: "answer"` payloads are accumulated — search
queries, sources and "thinking" items stay out of the message.
Fixtures in the regression test are trimmed from a live capture
(pplx-auto, mode=copilot); replaying the full 96 KB capture through the
patched extractor yields the complete 247-char answer over 7 incremental
deltas, against an empty string before the fix.
Co-authored-by: Jeyhun F. Aslanov <jeyhun.f.aslanov@Jeyhuns-MacBook-Pro.local>
05b1311884