Polymarket Bid-Ask Data

• Top-of-book: bid and ask with sizes, updated at 250ms for every market.
• Full-depth: every level (sports 35–36 per side) with size, as-of each frame.
• Spread derives cleanly: ask minus bid at the top level, with the composition visible in the level list.
• Sequence numbers make the two-sided record monotonic and exact for as-of replay.

At least 90% of sampled 250ms frames carry size on one side only — the bid-ask series is more often a bid-only or ask-only tape than a two-sided quote.

Where the book is two-sided, 98.6% of frames price at 0.99/0.995 — the tight, round grid that makes the spread narrow in cents and wide in risk.

Repricing is a depth event: the August 23 window saw ETH bid depth −96% and ask depth +491% in the same collapse that moved four lower bounds in 26 seconds.

• Spread regime: rolling median spread per market type from the bid-ask series.
• Depth asymmetry as a signal: ratio of bid size to ask size per frame, previously useful before repricings.
• Honest mid: mid only exists where both sides exist; a one-sided frame has no executable mid, and the series shows exactly when that is.

• Pull one market's books for an hour at 250ms and separate top-of-book rows from full-level rows; verify sizes and prices line up on the levels.
• Compute the frame-level spread series (ask minus bid) and mark frames where one side is empty — expect the high-90s one-sided share on the measured ladder types.
• Build the depth-asymmetry ratio (bid size over ask size) per frame for a reprice window and watch it invert across the move.
• Render the bid and ask series as two step-lines for one volatile hour; the split between them is the honest picture of liquidity, not the mid.

Bid-ask data is the liquidity microscope: it shows not just where the spread is but which side is actually resting, and Polymarket's answers are mostly one-sided.

The pair 90%+ one-sided frames and 98.6% at 0.99/0.995 defines the whole trading surface: tight round-grid quotes that are rarely two-sided at the same instant.

A mid is only meaningful where both sides exist; the bid-ask series is the only source that tells you when that condition is met and when it is quietly not.

Feed the bid-ask series into your spread analytics and your liquidity dashboards; it is the same 250ms rows the books give, filtered to the top of book.

When a study says "the bid held," it means the bid series did not empty; the series is the evidence, and the whole ladder is the audit.

The bid-ask series is the honesty layer of the books: it shows not the mid but the two sides that actually exist, and Polymarket's answer to "are both sides resting?" is mostly no — one-sided frames are the norm at the 90%+ level.

Use the series for spread regimes and depth-asymmetry signals, and treat an executable mid as the exception rather than the default: the series is the only place where the condition "both sides exist" is visible frame by frame.

The round-grid behavior — 98.6% of frames at 0.99/0.995 — is the positive half of the story: when the book is two-sided, it is tight and genuine; the risk is not width, it is the missing leg.

Render bid and ask as two step-lines in every study, never just the mid; the split between the lines is the liquidity picture, and merging it is how charts flatter their subject.

The bid-ask page is where the liquidity story gets its nuance: a tight round grid (98.6% at 0.99/0.995) paired with one-sidedness summed up as "the spread is often literally one leg."

Treat the mid as the exception, the two-sided frame as the event, and the one-sided frame as the norm; the series is the only record that distinguishes them second by second.

Depth-asymmetry analysis (bid size over ask size) is the page's practical weapon: it flags the moments a reprice is about to redistribute liquidity, the same kind you can verify on the August 23 event window.

Render bid and ask as separate step-lines forever; the split line is the liquidity picture, and anything that flattens it is flattering the market.

• spread explained
• depth explained
• mid price explained