claude-code - 💡(How to fix) Fix EBUSY file lock error on Windows when multiple sessions write to .claude.json [1 comments, 2 participants]

claude-code2026-04-11 01:12:43

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anthropics/claude-code#46482•Fetched 2026-04-11 06:19:08

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jeffyaw

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Running multiple Claude Code sessions simultaneously on Windows causes intermittent EBUSY: resource busy or locked errors when writing to ~/.claude.json. This is Windows-specific — macOS and Linux don't exclusively lock files during writes the same way.

Error Message

ERROR EBUSY: resource busy or locked, open 'C:\Users\<user>\.claude.json'

  at writeFileSync (unknown)
  VZ8 (B:/~BUN/root/src/entrypoints/cli.js:135:1271)
  NcK (B:/~BUN/root/src/entrypoints/cli.js:442:23227)
  p6 (B:/~BUN/root/src/entrypoints/cli.js:442:21289)
  <anonymous> (B:/~BUN/root/src/entrypoints/cli.js:8305:475)
  Au (B:/~BUN/root/src/entrypoints/cli.js:492:63164)
  zm8 (B:/~BUN/root/src/entrypoints/cli.js:492:76228)
  dR (B:/~BUN/root/src/entrypoints/cli.js:492:76109)
  zm8 (B:/~BUN/root/src/entrypoints/cli.js:492:77018)
  dR (B:/~BUN/root/src/entrypoints/cli.js:492:76109)

Root Cause

writeFileSync on Windows acquires an exclusive file lock. When two Claude Code processes try to write to the same .claude.json simultaneously, the second process gets EBUSY.

This doesn't happen on macOS/Linux because POSIX file writes don't acquire exclusive locks by default.

Code Example

ERROR EBUSY: resource busy or locked, open 'C:\Users\<user>\.claude.json'

  at writeFileSync (unknown)
  VZ8 (B:/~BUN/root/src/entrypoints/cli.js:135:1271)
  NcK (B:/~BUN/root/src/entrypoints/cli.js:442:23227)
  p6 (B:/~BUN/root/src/entrypoints/cli.js:442:21289)
  <anonymous> (B:/~BUN/root/src/entrypoints/cli.js:8305:475)
  Au (B:/~BUN/root/src/entrypoints/cli.js:492:63164)
  zm8 (B:/~BUN/root/src/entrypoints/cli.js:492:76228)
  dR (B:/~BUN/root/src/entrypoints/cli.js:492:76109)
  zm8 (B:/~BUN/root/src/entrypoints/cli.js:492:77018)
  dR (B:/~BUN/root/src/entrypoints/cli.js:492:76109)

---

function writeFileSyncRetry(path, data, options, retries = 3, delay = 50) {
  for (let i = 0; i < retries; i++) {
    try {
      writeFileSync(path, data, options);
      return;
    } catch (err) {
      if ((err.code === 'EBUSY' || err.code === 'EPERM') && i < retries - 1) {
        const waitUntil = Date.now() + delay * (i + 1);
        while (Date.now() < waitUntil) {} // sync backoff
        continue;
      }
      throw err;
    }
  }
}

RAW_BUFFERClick to expand / collapse

Description

Error

ERROR EBUSY: resource busy or locked, open 'C:\Users\<user>\.claude.json'

  at writeFileSync (unknown)
  VZ8 (B:/~BUN/root/src/entrypoints/cli.js:135:1271)
  NcK (B:/~BUN/root/src/entrypoints/cli.js:442:23227)
  p6 (B:/~BUN/root/src/entrypoints/cli.js:442:21289)
  <anonymous> (B:/~BUN/root/src/entrypoints/cli.js:8305:475)
  Au (B:/~BUN/root/src/entrypoints/cli.js:492:63164)
  zm8 (B:/~BUN/root/src/entrypoints/cli.js:492:76228)
  dR (B:/~BUN/root/src/entrypoints/cli.js:492:76109)
  zm8 (B:/~BUN/root/src/entrypoints/cli.js:492:77018)
  dR (B:/~BUN/root/src/entrypoints/cli.js:492:76109)

Steps to Reproduce

Open two Claude Code sessions on Windows (e.g., two terminals, or CLI + desktop app)
Use both sessions actively
The error occurs when both try to write to .claude.json at the same time

Root Cause

writeFileSync on Windows acquires an exclusive file lock. When two Claude Code processes try to write to the same .claude.json simultaneously, the second process gets EBUSY.

This doesn't happen on macOS/Linux because POSIX file writes don't acquire exclusive locks by default.

Suggested Fix

Wrap the writeFileSync call to .claude.json with a retry-on-EBUSY pattern:

function writeFileSyncRetry(path, data, options, retries = 3, delay = 50) {
  for (let i = 0; i < retries; i++) {
    try {
      writeFileSync(path, data, options);
      return;
    } catch (err) {
      if ((err.code === 'EBUSY' || err.code === 'EPERM') && i < retries - 1) {
        const waitUntil = Date.now() + delay * (i + 1);
        while (Date.now() < waitUntil) {} // sync backoff
        continue;
      }
      throw err;
    }
  }
}

Alternatively, use atomic writes (write to a temp file, then rename) which avoids the lock contention entirely.

Environment

OS: Windows 11 Pro
Claude Code version: 0.9.560
Runtime: Bun

extent analysis

TL;DR

Implement a retry mechanism for writeFileSync calls to .claude.json to handle EBUSY errors on Windows.

Guidance

Identify all instances of writeFileSync calls to .claude.json and replace them with the suggested writeFileSyncRetry function to implement a retry-on-EBUSY pattern.
Consider using atomic writes as an alternative solution to avoid lock contention entirely.
Verify the fix by running multiple Claude Code sessions simultaneously and checking for the absence of EBUSY errors.
Test the retry mechanism with different retry counts and delay values to find the optimal configuration for your use case.

Example

// Replace writeFileSync with writeFileSyncRetry
writeFileSyncRetry('.claude.json', data, options);

Notes

The suggested fix is specific to Windows and may not be necessary on other platforms. The retry mechanism may introduce additional latency, so it's essential to test and optimize the retry count and delay values.

Recommendation

Apply the workaround by implementing the writeFileSyncRetry function, as it provides a straightforward solution to handle EBUSY errors on Windows without requiring significant changes to the existing codebase.

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claude-code - 💡(How to fix) Fix EBUSY file lock error on Windows when multiple sessions write to .claude.json [1 comments, 2 participants]

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