Streaming SSE Delta Assembler

Background

Streaming LLM responses arrive as Server-Sent Events: each data: {...} line carries a partial token (delta). The client must concatenate these deltas into the final text, skipping event-control lines and tolerating malformed entries without crashing.

Problem statement

Implement assemble_sse(lines). Walk the input lines; any line starting with data: is parsed as JSON; if it contains a delta field, append str(delta) to the output. Skip everything else (including malformed JSON) without raising.

Input

• lines — list[str] of SSE-format event lines, in stream order.

Output

Returns a str — the concatenated delta values.

Examples

Example 1 — standard streaming

Input:
  ['data: {"delta": "Hello"}',
   'data: {"delta": " world"}',
   '',
   'data: {"delta": "!"}']
Output: "Hello world!"

Example 2 — non-data lines skipped

Input:  ['event: open', 'data: {"delta": "x"}', 'id: 1']
Output: "x"

Example 3 — malformed JSON tolerated

Input:  ['data: not-json', 'data: {"delta": "ok"}']
Output: "ok"

Constraints

• Empty input returns "".
• Lines that don't start with data: are silently skipped.
• Malformed JSON lines are silently skipped, not raised.
• delta may be any JSON value; coerce to str before concatenating.

Notes

• Why be lenient. SSE streams routinely carry heartbeats, ID lines, and event-type markers. Raising on these would break every client.
• Production extras. Real assemblers also handle [DONE] sentinel, recovery from broken UTF-8, and reconnection with Last-Event-Id.