[BUG] VS Code extension context pie pins at "100% context used" ~12k tokens before auto-compact fires; popup mislabels it as "until auto-compact"

Summary

In the VS Code extension, the context-usage pie in the input footer reads 100% context used well before auto-compact will fire, and its hover popup labels the number as "% of context remaining until auto-compact." — which it is not. The pie's denominator is contextWindow - maxOutputTokens - 13000, while the worker triggers auto-compact off its own resolved auto-compact window minus a summary buffer and precompute fraction. The two disagree by roughly 12k tokens on a 200k window, so the meter saturates at 100% while auto-compact still has real headroom.

The practical effect: the UI tells you that you are out of context and that auto-compact is imminent, auto-compact does not fire, and you have to run /compact manually. It reads as "auto-compact is broken" when it is actually working as designed.

Where

webview/index.js (extension 2.1.234). The meter is fed:

b(rot, {
  usedTokens: e.usageData.value.totalTokens,
  contextWindow: e.usageData.value.contextWindow
             - e.usageData.value.maxOutputTokens
             - 13000,
  onCompact: l,
  ...
})

and rendered as:

let a = t > 0 ? Math.min(e / t * 100, 100) : 0;   // clamped at 100
let c = 100 - a;
if (t === 0) return null;
if (c >= 50) return null;                          // only shows past 50% used
// title:  `${Math.round(a)}% context used — click to compact`
// popup:  `${Math.round(c)}% of context remaining until auto-compact.`

usageData.contextWindow / maxOutputTokens come from the SDK result message's modelUsage[currentMainLoopModel]; usageData.totalTokens is input + cache_creation + cache_read + output of the latest turn.

Numbers

Model claude-opus-5, 200k window (1M mode off via CLAUDE_CODE_DISABLE_1M_CONTEXT=1), default max_output_tokens 32000:

| | tokens |
|---|---|
| Pie denominator (200000 - 32000 - 13000) | 155,000 |
| Pie reads "100% context used" from | 155,000 |
| Worker auto-compact actually fires at | ~167,000 |

The worker figure is not a guess — across 104 compact_boundary records with trigger: "auto" in local transcripts, preTokens falls in a tight band of 167,003 – 184,721, clustered hard around 167.2k–167.9k.

So there is a ~12,000-token dead zone (larger if max_output_tokens resolves to 64000, which puts the denominator at 123,000 and widens the gap to ~44,000) in which the pie is pinned at 100%, the popup claims 0% remaining until auto-compact, and nothing happens automatically.

Repro

1. VS Code extension, model Opus 5, 200k context window.
2. Grow a session past ~155k tokens without crossing ~167k.
3. The footer pie is full and its tooltip reads 100% context used; hovering shows 0% of context remaining until auto-compact.
4. Keep going — no auto-compact occurs. /compact has to be invoked by hand.

A concrete session on my machine peaked at 157,170 tokens with zero compact_boundary entries, i.e. sat at a pinned 100% pie for the rest of its life.

Expected

Either:

1. Feed the pie the worker's real numbers. The worker already computes and publishes exactly this state — the internal autocompact_state event carries effective_window, threshold, enforced and source (documented in-binary as "The token count where threshold-triggered compaction fires on the worker"). Driving the meter from threshold would make "100%" mean "auto-compact fires now", and enforced: false would let the UI stop promising a compaction that will not happen.
2. Or fix the label. If the pie is deliberately a "usable context" gauge rather than a countdown, the popup should not say "until auto-compact" — and it should not clamp silently at 100%, since that hides how much headroom is actually left.

Option 1 also fixes the parallel case the enforced flag exists for: when threshold compaction is not enforced, the popup's promise is wrong regardless of the denominator.

Environment

• Claude Code VS Code extension 2.1.234 (anthropic.claude-code-2.1.234-darwin-arm64), native binary install
• macOS (Darwin 25.5.0), arm64
• Model: claude-opus-5
• CLAUDE_CODE_DISABLE_1M_CONTEXT=1 in ~/.claude/settings.json
• autoCompactEnabled unset (default true); no DISABLE_AUTO_COMPACT / DISABLE_COMPACT
• Custom statusLine configured — note the extension panel does not render statusLine at all, so the pie is the only context indicator available in that surface

Investigated with Claude Code.