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Enable Token-budget context windows in Settings → Experiments to replace usage-triggered automatic summaries with fresh context windows. The experiment is off by default.

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Threshold and precedence

Use the existing context-usage slider to choose the per-model threshold. The Rolls over by N% label includes the five-percentage-point force buffer: a 70% slider setting displays Rolls over by 75%. Automatic rollover is evaluated when sending and after a settled tool step. The displayed percentage is an upper bound; the hard request ceiling takes precedence if reached first. Rollover starts a fresh window without summarizing earlier messages. The transcript shows a Context window rollover divider; earlier messages remain on disk, in the UI, and in exports.
  • Manual /compact and idle compaction still summarize normally.
  • Continuous compaction and effective RLM take precedence over rollover.
  • Setting the usage threshold to 100% disables automatic rollover and its warning. Hard request-size checks still apply, including after settled tool steps: the turn can pause without queuing a rollover or discarding completed tool results.
  • session_history must be allowed by the agent’s inherited tool policy and any caller restrictions. Built-in Exec, Plan, and Explore already allow it. Narrow custom agents can add session_history or a matching wildcard to tools.add. If access is omitted or disabled, rollover pauses before sealing existing context instead of falling back to a lossy summary.
Rollover also pauses when applicable request middleware can change the toolset, before clearing context state or saving a boundary. Context-only integrations, including sandboxed plugin context hooks, remain supported. Xum pins the workspace’s applicable hook registrations when admitting a rollover and uses that snapshot throughout the turn and its fallback attempts; later registration changes apply to subsequent requests. Plugin revocation still takes effect. Hooks explicitly scoped to another workspace do not block rollover. Ordinary requests and manual /compact retain their existing middleware behavior.

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Keeping useful context

Once per window, a machine-authored warning asks the agent to write important context to the conventional workspace/context-notes.md file, up to 8 KiB, if the workspace is writable. This is an opportunity to preserve notes, not a guarantee that the agent writes them. While token-budget mode is active, Xum can preload the notes as an additional ninth memory, without replacing the normal eight or using their existing byte/token budgets. The extra excerpt is separately bounded to 8 KiB / 2,000 tokens, including formatting, and is not duplicated if already selected normally. This still requires Memory and Memory Hot Set; the experiment does not enable either. With token-budget mode inactive, notes follow the ordinary memory-selection rules. The next window receives a model-only lead-in, not a summary. While the experiment is enabled, the agent can use session_history to list windows, search, or read earlier messages in the same workspace. Results are capped at 16 KiB per call, with scans bounded to 2 MiB, 500 rows, and 1 MiB per line. Large histories may require further bounded calls. The newest manual /clear --soft is a privacy floor: the tool cannot retrieve messages before it. Manual reset behavior and edited-file carryover are unchanged. Turning the experiment off removes retrieval access without deleting old windows.

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Pauses and size limits

Rollover stops only after a tool step settles, preserving tool call/result pairs. Only one rollover may be pending; it is handled on the next send. Restart leaves the workspace paused rather than resurrecting a queued continuation, and the next message re-evaluates pressure from history. The boundary, lead-in, and triggering message or continuation are saved as one atomic, all-or-nothing batch. Recovery also tolerates incomplete batches in legacy or externally modified histories. Requests estimated to exceed a fresh window are blocked before contacting the provider; rollover cannot make oversized attachments or instructions fit. Text guards use real encodings, but provider-family, media, and framing estimates can still differ from the provider’s accounting.