# Context Mode: 94-100% Context Reduction in Claude Code

> Context Mode achieves 94-100% context reduction per tool call, extending typical Claude Code sessions from 30 minutes to about 3 hours.

Published: 2026-03-16
URL: https://daniliants.com/insights/github-mksglu-context-mode-mcp-context-virtualizat/
Tags: mcp, context-window, llm-agents, claude-code, session-continuity, context-optimization, ai-tools, self-hosting

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## Summary

Context Mode is an MCP server that intercepts tool calls (Bash, Read, WebFetch, Playwright, etc.) and executes them inside sandboxed subprocesses, returning only a compact summary to the context window instead of raw output. It achieves 94–100% context reduction per call (e.g. 56 KB Playwright snapshot → 299 B), extending a typical Claude Code session from ~30 minutes to ~3 hours. A SQLite + FTS5 session tracker survives context compaction by rebuilding full working state - active tasks, edited files, user decisions - before the conversation window is wiped.

## Key Insight

- **Context savings are dramatic and measured:** 315 KB of raw tool output per session becomes 5.4 KB on average. Playwright snapshots: 56 KB → 299 B (99%). GitHub Issues (20): 59 KB → 1.1 KB (98%). Access logs (500 lines): 45 KB → 155 B (100%).
- **Two distinct problems solved:** (1) context bloat from raw data dumped by MCP tools, (2) session amnesia after the model compacts the conversation. Both require hooks - instruction files alone (~60% compliance) are not enough.
- **Hooks are the difference between 60% and 98% savings.** Without hooks, one unrouted Playwright call undoes an entire session's savings. With hooks, every tool call is intercepted before execution.
- **Session continuity architecture:** PostToolUse hook captures every file edit, git op, error, task, and user decision into SQLite. PreCompact builds a priority-tiered XML snapshot (≤2 KB). SessionStart restores working state via a "Session Guide" injected into context - the model picks up from the last user prompt without being re-briefed.
- **Platform support varies sharply:** Full session continuity on Claude Code, Gemini CLI, VS Code Copilot, OpenClaw. Partial on Cursor and OpenCode. None on Codex CLI, Antigravity, Kiro.
- **Install for Claude Code is two commands:** `/plugin marketplace add mksglu/context-mode` then `/plugin install context-mode@context-mode`. Hooks, routing instructions (CLAUDE.md patch), and slash commands (`/ctx-stats`, `/ctx-doctor`, `/ctx-upgrade`) are configured automatically.
- **Privacy-first by design:** no telemetry, no cloud sync, no account. SQLite databases live locally and are deleted when session ends (unless `--continue` is used).
- **Security policy integration:** Deny/allow rules from `.claude/settings.json` are enforced inside the sandbox too - commands chained with `&&`, `;`, or `|` are split and each part checked.
- **FTS5 search with three-layer fallback:** Porter stemming → trigram substring → Levenshtein fuzzy correction. Enables the model to retrieve specific indexed content rather than re-loading full documents.