High startup context consumption from system prompt components (~28% before first user message)

Summary

A new session in a moderately-configured project consumes roughly 28% of the available context window before the user sends a single message. This significantly limits effective session length for any substantive work.

Environment

• Claude Code CLI (desktop app / zsh)
• macOS Darwin 25.3.0
• Model: claude-sonnet-4-6

Observed behavior

On session start, approximately 28% of context is already consumed. The contributing components (estimated, in rough size order):

1. Tool definition schemas — full JSON schemas for all available tools, including the deferred tool list (~200+ tool names with descriptions)
2. Project CLAUDE.md — project-specific instructions (in this case a long file covering ~15 protocol areas)
3. Global CLAUDE.md — user-wide engineering principles, system references, response style
4. MEMORY.md — auto-loaded project memory index (~80+ entries)
5. Session-start system reminders — MCP server instructions, skills list (~60+ skills), hook output injected via UserPromptSubmit

Expected behavior

Startup overhead should be low enough that a full session's worth of work can happen before context pressure forces a /clear or new session. For users with large CLAUDE.md configurations or many MCP servers/skills, 28% pre-consumption leaves limited headroom.

Notes

• The deferred tool list (tools that require ToolSearch before use) still appears to contribute significant token cost even though schemas are not loaded upfront — the names + descriptions alone add up
• The skills list in system reminders is verbose; a compact registry format could reduce cost substantially
• MEMORY.md auto-loading on every turn (via system-reminder) means the cost is paid repeatedly, not just at session start

Not a crash / correctness bug — reporting for awareness and potential optimization.