[FEATURE] Add context window usage to agent's environment info block

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

The agent receives an environment block (working
directory, platform, shell, etc.) but has zero
visibility into context window usage. This makes
CLAUDE.md rules like "save state at 75%"
unenforceable — the agent can't evaluate thresholds
it can't observe.

Existing tools don't help the agent:

• StatusLine shows used_percentage to the

user, not the agent

• PreCompact hook stdout isn't injected into

agent context

• SessionStart compact matcher only helps

after the damage is done

Proposed Solution

Add 2 lines to the existing env block:

Context usage: 73% (145,800 / 200,000 tokens)
Auto-compact threshold: 95%

Why this approach: Zero new APIs (StatusLine
already has this data), zero new tools, backward
compatible. Users can then write CLAUDE.md
threshold rules:

```markdown
## Context Management

• At 75%: save progress to .claude/plans/
• At 85%: suggest /compact while summary quality is

still high

Real-world impact

I maintain a large codebase with .claude/plans/ and
.claude/explorations/ to survive auto-compact.
Despite detailed CLAUDE.md save rules, agent
compliance is low — it can't know when it's urgent.
My workaround (PreCompact→file→SessionStart relay)
handles recovery but not prevention.

Related issues

#23457 (Closed/locked), #24320 (Closed), #13521
(Open/stale), #16510, #16255, #7627, #5942 — all
closed, duped, or too broad. This focuses on the
smallest change with the most value.

Alternative Solutions

_No response_

Priority

High - Significant impact on productivity

Feature Category

Performance and speed

Use Case Example

_No response_

Additional Context

_No response_