Context Engineering

Four operations control everything about how context flows through an AI coding session. Master them and you control the quality of every response.

The Four Operations

1. Write — Persist Info Outside Context

Move information out of the context window into durable storage so it survives compaction and session boundaries.

Where to write:

Target	When	Example
CLAUDE.md	Permanent project rules	"Always use pnpm, never npm"
NOTES.md / scratchpad	Working state for current task	Architecture decisions, open questions
.claude/memory/	Learnings and patterns	[LEARN] rules from corrections
External files	Data too large for context	Test plans, migration checklists

Pattern — Scratchpad workflow:

1. Start complex task → create NOTES.md with goals and constraints
2. After research → write findings to NOTES.md
3. After compaction → NOTES.md survives, context does not
4. Resume → read NOTES.md to recover full state

2. Select — Retrieve Relevant Info

Pull the right information into context at the right time. Precision matters more than volume.

Methods ranked by precision:

1. @file references — exact file injection
2. grep / Glob — targeted pattern search
3. Subagent exploration — delegated deep search
4. RAG / embeddings — semantic retrieval for large codebases

Key principle: Focused 300 tokens > unfocused 113K tokens.

A surgical grep result that returns the exact function signature beats dumping an entire module into context. Every irrelevant token dilutes attention.

Pattern — Progressive retrieval:

1. Start with file names (Glob)
2. Narrow to specific functions (Grep)
3. Read only the relevant lines (Read with offset+limit)
4. Never read entire large files when you need one function

3. Compress — Reduce Tokens, Preserve Signal

Shrink context without losing the information that matters.

Compaction strategies:

Strategy	How	When
/compact with focus	/compact focus: auth module changes	Task boundaries
Microcompact	Ask Claude to summarize tool output inline	After large reads/searches
Head+tail	Read first 20 + last 20 lines of large output	Log analysis, test results
Tool result clearing	Subagent results auto-clear after reporting	Heavy exploration
Semantic selection	Summarize findings, discard raw data	Research phases

Compaction triggers:

• After planning, before implementation
• After completing a feature or milestone
• When context exceeds 50% (set CLAUDE_AUTOCOMPACT_PCT_OVERRIDE=50)
• Before switching task domains
• After heavy search/read operations

PostCompact hook — Re-inject critical context:

{
  "type": "PostCompact",
  "command": "cat .claude/critical-context.md"
}

Use this to ensure project rules, current task state, or architecture constraints survive every compaction.

4. Isolate — Partition Across Execution Spaces

Don't load everything into one context. Split work across independent execution spaces.

Method	Isolation Level	Use When
Subagents	Forked context	Heavy exploration, test runs, doc generation
Worktrees (claude -w)	Full repo copy	Parallel features, competing approaches
/btw (built-in Claude Code)	Temporary overlay	Quick questions without entering conversation history
Agent teams	Independent sessions	Cross-layer changes, parallel reviews
Fresh session (/resume)	Clean slate	Unrelated work, degraded context

Pattern — Subagent delegation:

Main session: planning, coordination, commits
Subagent 1: explore auth module, report findings
Subagent 2: run test suite, report failures
Subagent 3: generate migration script

Main context stays clean. Subagents handle the volume.

Context Budget Planning

Example baseline (calibrate with /context): a 200K window (Haiku 4.5; Fable 5.1, Opus 5.5, and Sonnet 5 carry 1M) with ~20K overhead (CLAUDE.md, tool definitions, MCP schemas) leaves ~180K usable. Quality still drops as context fills, so keep the same discipline on 1M tiers. Actual budgets vary by model and configuration.

Allocation	Budget	What Goes Here
Static context	20-30K	CLAUDE.md, tool schemas, MCP definitions
Dynamic context	150-180K	Code, conversation, tool results

Put static context first. CLAUDE.md and tool definitions load before conversation. Keeping them stable maximizes prompt cache hits — saves cost and latency.

Phase	Target Usage	Action If Over
Planning	< 20%	Keep plans concise, write to scratchpad
Implementation	< 50%	Compact between files, delegate reads
Testing	< 70%	Delegate test runs to subagents
Review	< 85%	Start fresh session if degraded

When to /clear vs /compact vs Subagent

Situation	Action
Task boundary, want to keep learnings	/compact with focus
Context degraded, Claude repeating itself	/compact, then /resume if still bad
Starting unrelated work	/clear or new session
Heavy read/search operation	Delegate to subagent
Quick side question	/btw (doesn't pollute main context)
Exploring multiple approaches	Worktrees or agent teams

Anti-Patterns

• Loading entire files when you need one function
• Keeping MCP tool results in context after extracting what you need
• Running 15+ MCPs (each adds tool schema overhead to every request)
• Vague prompts that force Claude to search broadly ("fix the code")
• Never compacting until auto-compact triggers at 95%

Add to CLAUDE.md

## Context Engineering

Write to NOTES.md for working state that must survive compaction.
Select with precision — grep first, read specific lines, never dump whole files.
Compact at 50% or task boundaries. Set CLAUDE_AUTOCOMPACT_PCT_OVERRIDE=50.
Isolate heavy work to subagents. Main session stays for coordination and commits.