Root cause: the context-window-monitor hook computes actualUsagePercentage = (input + cache.read) / actualLimit and renders both 'X% used' and '(1 - X) * 100% remaining' inside a [SYSTEM DIRECTIVE: OH-MY-OPENCODE - CONTEXT WINDOW MONITOR] block that is appended to bash tool output. When resolveActualContextLimit() underestimates the model's real context window (for example a 1M-context Anthropic model that falls back to the 200K default per #3450), totalInputTokens > actualLimit and the rendered numbers go nonsensical (issue #3655 reproduces 144.7% used / -44.7% remaining at 289,370 / 200,000 tokens). Safety-tuned models recognize the >100% / negative-remaining pattern as a tell-tale prompt injection and refuse to follow the directive.
Fix: clamp actualUsagePercentage to [0, 1] before formatting. The 70% threshold check still uses the raw value so the block continues to fire above threshold, and resolveActualContextLimit() is left untouched (the deeper resolver concern is tracked separately as #3450). When totalInputTokens exceeds actualLimit the displayed numbers now read '100.0% used / 0.0% remaining' instead of the impossible >100% / negative pair, and safety-tuned models stop flagging the block as an injection attempt.
Verification: added a regression test (input 289,370, limit 200,000) that asserts usedPct in [0,100] and remainingPct in [0,100]. Test fails before the fix (Received: 144.7) and passes after. Full context-window-monitor.test.ts and context-window-monitor.model-context-limits.test.ts: 15 pass / 0 fail. Typecheck clean.
YeonGyu-Kim