Simulated · no server required
Live Interactive Demo
Edit the envelope, pick a preset, press Run — watch a realistic HAPPI event stream play out token by token in your browser.
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HAPPI envelope (editable JSON)
ready
HAPPI event stream (NDJSON)
// event stream will appear here
One envelope in
Every HAPPI call is a single JSON object with v, id, cmd, and optional args. Provider-agnostic by design.
NDJSON stream out
The runtime emits one JSON object per line. Each line is a complete, self-describing event — parse with any JSON library, no framing protocol needed.
Seven event types
started → delta (×N) → completed. Plus error, tool_call, tool_result, and meta for richer flows.
Timestamps count up
The ts field is milliseconds since the request started. Use it to measure first-token latency, total stream duration, and per-token throughput.