Managing context

A model can only read so much text at once — its context window. Long coding sessions fill it up. Tau handles this with compaction (summarizing older history) and lets you tune how hard the model works with thinking modes.

Seeing context usage

Run /session in the TUI to see a rough estimate:

Estimated context tokens: <count>
Context token breakdown: system=<count>, messages=<count>, tools=<count>
Thinking mode: <mode>

The estimate is deterministic (roughly characters / 4 plus small per-message and per-tool overhead), not a provider tokenizer — treat it as approximate. It covers the system prompt, project context (AGENTS.md), skill metadata, the message history, and tool schemas.

Automatic compaction

By default, Tau compacts automatically when the estimate gets close to the model’s context window. It checks three moments:

• before a new prompt (to catch context added out-of-band),
• after a successful turn (to compact before your next turn), and
• after a context-overflow error (compact and retry once).

When it compacts, Tau asks the model to summarize older messages, keeps a recent suffix of the conversation, and continues. The original session file is never edited — only the active context sent to the provider changes.

The default threshold follows the model’s context window minus a reserve. You can override it for a run:

tau --auto-compact-threshold 100000

Automatic compaction is best-effort: if summarization fails, Tau logs it, keeps the original context, and carries on.

Manual compaction

Compact on demand any time:

/compact
/compact focus on the database migration work

Optional text after /compact is added as extra focus for the summary. Manual compaction summarizes the whole active context into one summary and fails visibly if the request fails.

Thinking modes

Some models can spend extra effort reasoning before answering. Tau exposes a thinking level you can cycle:

off → minimal → low → medium → high → xhigh

• Shift+Tab cycles the thinking level (default is medium).
• Ctrl+T toggles whether streamed reasoning tokens are shown (hidden by default).

Thinking is model-aware: Tau enables it only when the active provider declares supported levels for the active model. When it’s unavailable, /session shows the reason (e.g. the provider doesn’t declare thinking_levels, or the model isn’t listed). Custom providers can opt in via thinking_levels in their config — see Configuration.