Excessive session usage: reading a single MEMORY.md file (~150 lines) consumes ~9% session limit on Sonnet

Description

Reading a single auto-memory file (~150 lines of markdown) consumes approximately 9% of the session usage limit when using Sonnet (claude-sonnet-4-6). This is not an Opus issue — the model is explicitly set to Sonnet via /model sonnet.

Steps to Reproduce

1. Set model to Sonnet (/model sonnet)
2. Start a new session
3. The system loads MEMORY.md (~150 lines, ~5KB) as part of the auto-memory context injection
4. Observe session usage indicator jump by ~9%

Expected Behavior

Reading a small markdown file (~150 lines) should consume a negligible amount of session usage, similar to normal file reads.

Actual Behavior

A single MEMORY.md read consumes ~9% session limit on Sonnet. This is disproportionate for such a small operation.

Environment

• Claude Code version: latest (2.x)
• Model: claude-sonnet-4-6 (Sonnet)
• Platform: Windows 11
• Feature: auto-memory (~/.claude/projects/*/memory/MEMORY.md)

Additional Context

This appears related to issue #16856 (Claude Code 2.1.1+ uses 4x+ more usage), but specifically affects Sonnet — previously Sonnet would run all day without hitting 10% usage. Now a single session init with MEMORY.md loading immediately consumes ~9%.

The memory file contains ~150 lines of plain markdown (project notes, no code). The disproportionate usage suggests token counting or context management overhead, not the actual content size.

Related: #16856