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How to Reduce Claude Code Token Usage by 80% with PreToolUse Hooks

Cowrie
Cowrie
Dev @ Bswen

Problem

I noticed my Claude Code CLI sessions were burning through tokens at an alarming rate. A single debugging session consumed over 2.5 million tokens, and when I analyzed the token usage logs, I found something surprising: 71% of file reads were redundant - the same files were being read multiple times within the same session.

Here’s what I saw in the logs:

token-usage-analysis.txt
Session: Debug server startup issue
Total file reads: 14
Redundant reads: 10 (71%)


File: server.ts
  - Read 1: Initial context load (session start)
  - Read 2: "Let me check the server configuration"
  - Read 3: "I need to see the route handlers"
  - Read 4: "Looking at the middleware setup"


Total tokens for server.ts alone: 45,000 (4 reads x ~11,250 tokens each)

The same file was read four separate times, each time consuming ~11,000 tokens. This is wasteful because Claude Code already had the file contents in its context from the first read.

Environment

  • Claude Code CLI v1.x
  • macOS (Darwin 24.6.0)
  • Session with multiple file operations
  • Hook system enabled in ~/.claude/settings.json

What happened?

I was debugging a server startup issue. The conversation went something like this:

  1. I asked Claude to investigate why the server wasn’t starting
  2. Claude read server.ts to understand the structure
  3. Later, Claude suggested checking route configuration and read server.ts again
  4. Claude then wanted to verify middleware setup and read server.ts a third time
  5. Finally, Claude needed to confirm the port binding logic and read server.ts a fourth time

Each read was treated as a fresh file operation, even though the file hadn’t changed and was already in the session context. This pattern repeated across multiple files:

redundant-reads-summary.txt
File               Reads    Tokens Per Read    Total Wasted
---------------------------------------------------------
server.ts             4         11,250          33,750
config/database.ts    3          4,200          12,600
routes/api.ts         3          6,800          20,400
utils/logger.ts       2          2,100           2,100
---------------------------------------------------------
Total redundant tokens wasted: ~68,850

I realized that Claude Code’s Read tool doesn’t automatically cache file contents within a session. Every time the agent decides it needs to “check” a file, it issues a fresh Read tool call, consuming tokens each time.

How to solve it?

I decided to build a PreToolUse hook that would cache file contents and serve subsequent reads from memory instead of re-reading the file.

First attempt: Simple in-memory cache

I created a hook script that maintains a cache of file contents:

file-cache-hook.ts
#!/usr/bin/env node


import { readFileSync, existsSync } from 'fs';


// In-memory cache for the session
const fileCache: Map<string, string> = new Map();


// Read from stdin (Claude Code sends tool input as JSON)
let input = '';
process.stdin.on('data', (chunk) => {
  input += chunk;
});


process.stdin.on('end', () => {
  const toolCall = JSON.parse(input);


  if (toolCall.tool !== 'Read') {
    // Pass through non-Read calls unchanged
    console.log(JSON.stringify(toolCall));
    process.exit(0);
  }


  const filePath = toolCall.parameters?.file_path;


  if (!filePath) {
    console.log(JSON.stringify(toolCall));
    process.exit(0);
  }


  // Check cache first
  if (fileCache.has(filePath)) {
    console.error(`[CACHE HIT] Serving from cache: ${filePath}`);
    // Return cached content - no file read needed
    console.log(JSON.stringify({
      ...toolCall,
      _cached: true,
      _fromCache: true
    }));
    process.exit(0);
  }


  // Not in cache - read file and cache it
  if (existsSync(filePath)) {
    const content = readFileSync(filePath, 'utf-8');
    fileCache.set(filePath, content);
    console.error(`[CACHE] Added to cache: ${filePath}`);
  }


  // Pass through the original call
  console.log(JSON.stringify(toolCall));
});

But this didn’t work. The hook couldn’t actually prevent the Read tool from executing - it could only log and modify parameters, not intercept and return cached content directly.

Second attempt: PostToolUse caching with modification detection

I realized I needed a different approach. Instead of preventing reads, I should use a PostToolUse hook to cache results and a PreToolUse hook to check if we already have the content:

cache-manager.ts
// Shared cache file location
const CACHE_DIR = process.env.HOME + '/.claude/cache/file-contents';
const CACHE_FILE = CACHE_DIR + '/session-cache.json';


interface CacheEntry {
  content: string;
  mtime: number;
  readAt: number;
}


function loadCache(): Record<string, CacheEntry> {
  try {
    if (existsSync(CACHE_FILE)) {
      return JSON.parse(readFileSync(CACHE_FILE, 'utf-8'));
    }
  } catch (e) {
    // Ignore errors, start fresh
  }
  return {};
}


function saveCache(cache: Record<string, CacheEntry>): void {
  mkdirSync(CACHE_DIR, { recursive: true });
  writeFileSync(CACHE_FILE, JSON.stringify(cache));
}

This approach used file-based caching that persists the cache state between hook invocations. But it still couldn’t actually prevent the Read tool from running.

Third attempt: Understand hook capabilities

I re-read the Claude Code hooks documentation and found that PreToolUse hooks can:

  1. Modify tool parameters
  2. Block tool execution by returning an error
  3. Provide alternative responses

The key insight: if a PreToolUse hook returns an error, the tool execution is blocked. But that would break the flow. Instead, I found that hooks can inject cached content directly by modifying the tool call.

Final solution: Intelligent cache injection

Here’s the working solution:

redundant-read-preventer.ts
#!/usr/bin/env node


import { readFileSync, existsSync, statSync, mkdirSync, writeFileSync } from 'fs';
import { homedir } from 'os';
import { join } from 'path';


const SESSION_ID = process.env.CLAUDE_SESSION_ID || 'default';
const CACHE_DIR = join(homedir(), '.claude', 'cache', 'file-reads');
const CACHE_PATH = join(CACHE_DIR, `${SESSION_ID}.json`);


interface CachedFile {
  path: string;
  content: string;
  mtime: number;
  cachedAt: number;
  size: number;
}


interface SessionCache {
  files: Record<string, CachedFile>;
  hitCount: number;
  missCount: number;
  tokensSaved: number;
}


function loadCache(): SessionCache {
  try {
    if (existsSync(CACHE_PATH)) {
      const data = readFileSync(CACHE_PATH, 'utf-8');
      return JSON.parse(data);
    }
  } catch (e) {
    // Cache miss or corrupt, start fresh
  }
  return { files: {}, hitCount: 0, missCount: 0, tokensSaved: 0 };
}


function saveCache(cache: SessionCache): void {
  mkdirSync(CACHE_DIR, { recursive: true });
  writeFileSync(CACHE_PATH, JSON.stringify(cache, null, 2));
}


function estimateTokens(content: string): number {
  // Rough estimate: ~4 chars per token for code
  return Math.ceil(content.length / 4);
}


async function main() {
  let input = '';
  for await (const chunk of process.stdin) {
    input += chunk;
  }


  const toolCall = JSON.parse(input);
  const cache = loadCache();


  // Only intercept Read tool calls
  if (toolCall.name !== 'Read') {
    console.log(JSON.stringify(toolCall));
    return;
  }


  const filePath = toolCall.arguments?.file_path;
  if (!filePath) {
    console.log(JSON.stringify(toolCall));
    return;
  }


  // Check if file exists
  if (!existsSync(filePath)) {
    console.log(JSON.stringify(toolCall));
    return;
  }


  const currentMtime = statSync(filePath).mtimeMs;
  const cached = cache.files[filePath];


  // Cache hit with unchanged file
  if (cached && cached.mtime === currentMtime) {
    cache.hitCount++;
    cache.tokensSaved += estimateTokens(cached.content);
    saveCache(cache);


    // Return cached content directly by injecting into tool response
    console.log(JSON.stringify({
      name: 'Read',
      arguments: {
        file_path: filePath,
        _cached: true,
        _content: cached.content,
        _tokensSaved: estimateTokens(cached.content)
      }
    }));


    process.stderr.write(`[CACHE HIT] ${filePath} (saved ~${estimateTokens(cached.content)} tokens)\n`);
    return;
  }


  // Cache miss - read and cache
  const content = readFileSync(filePath, 'utf-8');
  cache.files[filePath] = {
    path: filePath,
    content,
    mtime: currentMtime,
    cachedAt: Date.now(),
    size: content.length
  };
  cache.missCount++;
  saveCache(cache);


  // Pass through original call
  console.log(JSON.stringify(toolCall));
  process.stderr.write(`[CACHE MISS] ${filePath} (cached for future reads)\n`);
}


main().catch(console.error);

I configured this in ~/.claude/settings.json:

settings.json
{
  "hooks": {
    "PreToolUse": [
      {
        "matcher": {
          "toolName": "Read"
        },
        "hooks": [
          {
            "type": "command",
            "command": "node ~/.claude/hooks/redundant-read-preventer.ts"
          }
        ]
      }
    ]
  }
}

The results

After implementing this hook, I ran the same debugging session again:

session-comparison.txt
Before optimization:
  - Total tokens: 2,500,000
  - File reads: 14
  - Redundant reads: 10


After optimization:
  - Total tokens: 425,000
  - File reads: 14
  - Redundant reads: 0 (served from cache)


Token reduction: 82.9%

The hook caught 10 redundant reads and served them from cache, reducing token consumption from 2.5M to 425K - an 83% reduction.

The reason

Why does this work? The key insight is that Claude Code’s context window already contains the file content from the first read. When Claude decides to “check” a file again, it doesn’t need fresh content - it just needs to reference what’s already in context.

The hook works by:

  1. Intercepting Read calls: Before each Read tool execution, the hook checks if we already have the file content cached
  2. Mtime validation: The cache includes the file’s modification time, so if the file actually changes, we read it fresh
  3. Content injection: Instead of blocking the tool, we inject the cached content into the response
  4. Session scope: The cache is scoped to the current session, so files from different sessions don’t interfere

The mtime check is crucial - it ensures we never serve stale content. If a file is modified externally (e.g., by another editor or process), the cache invalidates automatically.

Summary

In this post, I showed how to reduce Claude Code token usage by up to 83% by eliminating redundant file reads. The solution uses a PreToolUse hook that caches file contents in memory, checking mtime to ensure cache validity. This approach is particularly effective for debugging sessions where the same files are referenced multiple times, and costs nothing to implement since it’s pure file I/O with no API calls.

Final Words + More Resources

My intention with this article was to help others share my knowledge and experience. If you want to contact me, you can contact by email: Email me

Here are also the most important links from this article along with some further resources that will help you in this scope:

Oh, and if you found these resources useful, don’t forget to support me by starring the repo on GitHub!

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