ux(wasm): Thinking... indicator during prompt prefill#26
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First-token latency can be several seconds on a 0.8B model in WASM (processing the full prompt through 28 layers in single-threaded WASM). Without feedback, users see a blank assistant bubble and think the demo is broken. Add a spinner + "Thinking..." message inside the assistant bubble that appears immediately after sending. Replaced by the first streamed token. Also show "Processing prompt..." in the stats bar. Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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Summary
Users reported the WASM demo feels "hung" after sending the first message — blank assistant bubble for several seconds before tokens start appearing.
Cause: Prompt prefill (tokenization + processing all prompt tokens through 28 layers) takes 3-10s in single-threaded WASM. No visual feedback during this phase.
Fix: Show a spinner + "Thinking..." in the assistant bubble immediately after sending. Replaced by the first streamed token. Stats bar shows "Processing prompt..." during prefill.
Test plan
🤖 Generated with Claude Code