Model arena

Tab

Moves through Leaderboard, the Blind switch, the prompt field, Run, the sample chips, each answer (a scrollable region), Retry where shown, and the vote buttons or Next round.

Enter

In the prompt field, runs the prompt. While the models are answering, Run becomes Stop.

Command EnterorCtrl Enter

Runs the prompt from the field, Run or a chip.

Escape

Stops a run in progress from the prompt row, and closes the leaderboard popover, returning focus to its button.

Arrow leftorArrow right, Home, End

On the narrow layout's answer tabs, moves between model A and model B.

Arrow up, Page up, Home

Inside a focused answer, scrolls back and stops it following the stream; scrolling to the end follows again.

Enter, Space

On a vote button, records the vote and moves focus to Next round, which starts the next blind round and returns focus to Stop.

• Each pane is a labelled region ("Model A", or "Model A, Claude Opus 5.5" once revealed). Its answer is a focusable scroll region marked aria-busy while it streams.
• In blind mode no model name, mark or title reaches the page or the accessibility tree until the vote.
• Every status pairs an icon with a word: Thinking, Writing, Done, Finished first, Stopped, Failed. Color only reinforces it.
• A polite status region narrates the round: that a run started (and that the preview is simulated), which model finished first, that voting is open, the vote result with both real models, and a leaderboard reset.
• On narrow containers the panes use tab semantics: a tablist with aria-selected tabs, each controlling a tabpanel; the hidden panel leaves the tab order.
• The vote buttons form a labelled group. Leaderboard rows read their rank and win rate as text, the bars are hidden from assistive technology, and the Leaderboard button's name says when a vote has updated it.
• Run carries aria-keyshortcuts for its shortcut. Pointer focus draws no ring; keyboard focus shows Arc's shared focus ring, inset inside scroll regions.

• Answers stream in chunks of whole words every 30 to 40ms, each fading in over 160ms with no blur or travel, at the same pace in both panes; settled text never re-renders, and each answer follows its stream until the reader scrolls up.
• The reveal is one minimal crossfade: "Model A" fades out in 100ms while the brand mark and name fade in over 160ms from 2px below, and the label's measured width follows on a spring with no bounce. Status words and tab labels swap the same way; no character morph, blur or per letter motion anywhere.
• Working statuses shimmer with Text shimmer, and each status word mounts with its own shimmer so a change is a crossfade, never a morph inside the label.
• First token time counts live while the model thinks; speed and cost are sampled twice a second while it writes and roll on Animated counter digits, so each roll settles before the next.
• The vote row crossfades between waiting, voting and result while its height follows on the smooth spring. Run and Stop share one cell, so the button never changes width.
• Opening the leaderboard after a vote replays it: rows start at their earlier ranks and glide to the new ones on a critically damped spring while win rates roll and bars scale in each model's series color; ranks stay put while rows move. After the first vote, the bars grow from empty.
• Narrow tabs slide their highlight on a 220ms tween with no bounce, and the chosen pane fades in.
• Reduced motion keeps every state, label and announcement but shows chunks without fading, swaps labels instantly, reorders rows in place and stops the spinner and shimmer.

• The card is up to 800px wide. From 560px the panes sit side by side; below that they become two tabs, each showing the model's name and status, so both statuses stay visible while you read one answer.
• Each answer scrolls inside a fixed height, so the card never grows while text streams. Code blocks scroll sideways inside the pane.
• Below 480px the Leaderboard button collapses to its icon with an accessible name, the shortcut hint hides, sample chips scroll sideways in one line, and the vote buttons form a two by two grid.

• Streamed chunks are memoized spans that never re-render once shown, so long answers stay cheap.
• The first token clock ticks on animation frames only while a model thinks, and speed and cost are sampled twice a second only while it writes.
• Every stream has its own AbortController: Stop, a new prompt, Next round and Retry abort the work they replace, and the arena aborts everything on unmount.
• The leaderboard rows mount only while the popover is open. The simulation runs on requestAnimationFrame and pauses with the tab.

• Pass run to stream real models; it receives the ArenaModel, the prompt text and an AbortSignal. Timing (first token, speed) is measured from your stream on the client, so yield as soon as text arrives rather than buffering.
• Report the provider's usage by yielding { inputTokens, outputTokens } (with or without text) for exact cost; otherwise tokens are estimated from characters and cost uses the model's price.
• Throw from run to fail one pane; the other keeps streaming. ArenaRunError(message, { retryable: false }) hides the retry button.
• Persist votes in onVote. To show standings from earlier sessions, pass initialStandings keyed by model id; a tie counts as half a win and Both bad counts as a loss for each model.
• Give each model a color from --series-1 to --series-4 so providers read as families on the leaderboard; the default pool colors by provider.
• In blind mode cost and speed can hint at a model's identity; leave them for honest measurement, or pass the same price for every contestant if identity must stay fully hidden.
• Model names, marks and non-Anthropic prices in the default pool are sample content, not endorsements or published benchmarks.

The full library index for assistants is at /llms.txt.