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Every model has a finite context window. In a long coding session the conversation history — messages, tool inputs, tool outputs — grows continuously until it hits that limit. Claude Code handles this automatically with a layered compression system in src/services/compact/.

​
Context window layout

At any point in a session, the context window is structured as follows: 
┌─────────────────────────────────────────────────────┐
│  System Prompt (tools, permissions, CLAUDE.md)      │
│  ══════════════════════════════════════════════      │
│                                                     │
│  Conversation History                               │
│  ┌─────────────────────────────────────────────┐    │
│  │ [compacted summary of older messages]        │    │
│  │ ═══════════════════════════════════════════  │    │
│  │ [compact_boundary marker]                    │    │
│  │ ─────────────────────────────────────────── │    │
│  │ [recent messages — full fidelity]            │    │
│  │ user → assistant → tool_use → tool_result   │    │
│  └─────────────────────────────────────────────┘    │
│                                                     │
│  Current Turn (user + assistant response)            │
└─────────────────────────────────────────────────────┘
The compact_boundary marker is a special system message with subtype: "compact_boundary" written into the JSONL transcript. It divides the conversation into two regions:
  • Before the boundary — a summary produced by a previous compaction call. Full fidelity is lost but the key facts are preserved.
  • After the boundary — recent messages at full fidelity. These have not been compressed.
When Claude Code resumes a session, getMessagesAfterCompactBoundary() reads the transcript and reconstructs this two-region layout.

​
Three compression strategies

StrategyTriggerWhat it doesFeature flag
autoCompactToken count exceeds threshold during normalizeMessagesForAPI()Sends all messages before the boundary to the Claude API with a summarization prompt. Replaces them with a compact summary and writes a new compact_boundary to the transcript.Always available
snipCompactCalled alongside autoCompact when enabledRemoves “zombie” messages — stale tool results, orphaned progress markers, and superseded file reads that are no longer referenced. Reduces noise before summarization.HISTORY_SNIP feature flag
contextCollapseCalled on demand or by thresholdRestructures the entire context for maximum efficiency — reordering, deduplicating, and collapsing repetitive tool exchange patterns. More aggressive than autoCompact.CONTEXT_COLLAPSE feature flag
snipCompact and contextCollapse are feature-gated and not present in the published npm package. Only autoCompact is available in the standard build.

​
Compaction flow

messages[] ──> getMessagesAfterCompactBoundary()
                    │
                    ├── older messages (before boundary)
                    │       │
                    │       ▼
                    │   Claude API (summarize)
                    │       │
                    │       ▼
                    │   compact summary text
                    │
                    └── recent messages (after boundary, unchanged)
                            │
                            ▼
              [summary] + [compact_boundary] + [recent messages]
                            │
                            ▼
              recordTranscript() ──> appended to JSONL
The compaction call is a full Claude API request. It uses a dedicated summarization prompt that instructs the model to preserve task context, key decisions, file paths modified, and any open threads — enough for the session to continue coherently.

​
The /compact slash command

You can trigger compaction manually at any time using the /compact slash command: 
/compact
This runs autoCompact immediately, regardless of the current token count. Use it proactively before starting a large task if you want to preserve context headroom. You can also pass a custom focus instruction to guide the summary: 
/compact Focus on the authentication refactor we just completed
The instruction is appended to the summarization prompt so the model knows what to preserve in detail.

​
CLAUDE.md memory files

CLAUDE.md files are a separate mechanism for injecting persistent knowledge into the system prompt. They are not part of the conversation history and are not affected by compaction. Claude Code discovers CLAUDE.md files by walking up the directory tree from the current working directory. Files are loaded lazily — only when the agent navigates to a directory that contains one. This keeps the system prompt small at startup. 
project/
├── CLAUDE.md          # loaded when working in project/
├── src/
│   └── CLAUDE.md      # loaded when working in project/src/
└── tests/
    └── CLAUDE.md      # loaded when working in project/tests/
The nestedMemoryAttachmentTriggers and loadedNestedMemoryPaths sets on ToolUseContext track which CLAUDE.md paths have already been injected this session. This prevents the same file from being re-injected dozens of times in busy sessions where the LRU file state cache evicts entries.

​
Session persistence and resume

Every message — including compact_boundary markers — is appended to the session JSONL file as it is produced: 
~/.claude/projects/<project-hash>/sessions/<session-id>.jsonl
When you resume a session with --continue or --resume <id>, Claude Code:
  1. Reads the JSONL file
  2. Locates the most recent compact_boundary marker
  3. Reconstructs messages[] as [summary_message, compact_boundary, ...recent_messages]
  4. Resumes the agent loop from that state
This means compacted sessions resume just as efficiently as fresh sessions — only the recent full-fidelity messages are sent to the API on the first turn. 
# Resume the most recent session in the current directory
node cli.js --continue

# Resume a specific session by its ID
node cli.js --resume <session-id>

​
Transcript persistence strategy

Different message types use different write strategies to balance crash safety against performance:
Message typeWrite strategyReason
User messagesawait (blocking)Crash recovery — must survive a process kill
Assistant messagesFire-and-forget (ordered queue)High frequency; ordering is preserved by the queue
Progress messagesInline write (deduped on next query)Transient; only latest state matters
Compact boundaryawait (blocking)Structural — loss would corrupt resume

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