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A market is in one of three states, and each one changes what your requests do.

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Halted

A halt is temporary and specific to one market. New orders are rejected; existing orders stay on the book and you can still cancel them. That combination is deliberate: a halt should not trap you in a position you want out of the queue for. Trading elsewhere is unaffected. If your bot covers many markets, a halt on one is not a reason to stop.

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Resolved

Resolution is terminal. The market will never accept another order, its outcome is decided, and positions pay out — 100¢ per share on the winning outcome, 0¢ on the other. See Settlement. A resolved market stays readable: you can still fetch it, its trade history, and your positions in it. It just cannot be traded.

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Venue-wide states

Above individual markets there are two venue-wide stops. They return 409, not 422, and the difference from a market halt is the whole reason for the separate status code: venue_cancel_only is an orderly wind-down: you can always get out, never in. venue_halted is a total freeze — nothing is accepted, cancels included.
Branch on the status, not the message. 409 means the venue is not accepting this, so trying a different market will not help — back off and retry later. 422 market_halted means that one market, so trading elsewhere is fine. Collapsing them makes a bot hammer a closed venue.

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Listing and rolling

New markets appear over time as companies are listed and as ladders roll. Ladder events are quarterly: an event resolves at the end of its quarter, and a fresh event with a new tenor takes its place. The practical consequence for a long-running bot: market tickers are not permanent. ANTHROPIC-VALUATION-26Q3-1.05T stops trading when 26Q3 resolves, and its 26Q4 counterpart is a different market with a different ticker and its own book. Re-discover markets rather than hard-coding a list:
See Survival ladders for how the series, event, and market tickers nest.