google-gemini/gemini-cli · error · Error

Maximum number of parallel browser instances (${BrowserManag

Error message

Maximum number of parallel browser instances (${BrowserManager.MAX_PARALLEL_INSTANCES}) reached. Please wait for an existing browser task to complete before starting a new one.

What it means

Thrown by BrowserManager.getInstance in 'isolated' mode after scanning all parallel instance slots (key:suffix) and finding MAX_PARALLEL_INSTANCES (5) already in use. This caps concurrent isolated browser processes.

Source

Thrown at packages/core/src/agents/browser/browserManager.ts:174

      // Chrome prevents multiple instances from using the same profile, so
      // concurrent usage would cause "profile locked" errors.
      if (sessionMode === 'persistent' || sessionMode === 'existing') {
        throw new Error(
          `Cannot launch a concurrent browser agent in "${sessionMode}" session mode. ` +
            `The browser instance is already in use by another task. ` +
            `Please run browser tasks sequentially, or switch to "isolated" session mode for concurrent browser usage.`,
        );
      }

      // Isolated mode: allow parallel instances up to the limit.
      let inUseCount = 1; // primary is already in-use
      let suffix = 1;
      let parallelKey = `${key}:${suffix}`;
      let parallel = BrowserManager.instances.get(parallelKey);
      while (parallel?.inUse) {
        inUseCount++;
        if (inUseCount >= BrowserManager.MAX_PARALLEL_INSTANCES) {
          throw new Error(
            `Maximum number of parallel browser instances (${BrowserManager.MAX_PARALLEL_INSTANCES}) reached. ` +
              `Please wait for an existing browser task to complete before starting a new one.`,
          );
        }
        suffix++;
        parallelKey = `${key}:${suffix}`;
        parallel = BrowserManager.instances.get(parallelKey);
      }
      if (!parallel) {
        parallel = new BrowserManager(config);
        BrowserManager.instances.set(parallelKey, parallel);
        debugLogger.log(
          `Created parallel BrowserManager (key: ${parallelKey})`,
        );
      } else {
        debugLogger.log(
          `Reusing released parallel BrowserManager (key: ${parallelKey})`,
        );

View on GitHub (pinned to 5024443c72)

Solutions

1. Wait for at least one in-flight browser task to complete before launching another.
2. Lower the fan-out / parallelism of browser tasks to stay under 5 concurrent.
3. Audit that every browser invocation releases its instance in a finally block; leaked inUse flags exhaust the pool.
4. Call BrowserManager.resetAll() if the count is stale due to a prior crash, or restart the process.

Example fix

// problematic: fanning out 8 parallel browser subagents
await Promise.all(tasks.map(t => browserAgent.run(t))); // >5 concurrent

// fix: limit concurrency to <= MAX_PARALLEL_INSTANCES
await pMap(tasks, t => browserAgent.run(t), { concurrency: 4 });

import { BrowserManager } from './browserManager.js';
function countInUse() {
  let n = 0;
  for (const inst of BrowserManager.instances.values()) if (inst.isAcquired()) n++;
  return n;
}
// Before launching: assert countInUse() < BrowserManager.MAX_PARALLEL_INSTANCES.

try {
  const bm = BrowserManager.getInstance(config);
} catch (e) {
  if (e instanceof Error && /Maximum number of parallel/.test(e.message)) {
    // wait / queue / reduce concurrency
  }
  throw e;
}

When it happens

Trigger: sessionMode === 'isolated' and the primary plus slots :1..:4 all have inUse===true, so inUseCount reaches MAX_PARALLEL_INSTANCES during the while loop.

Common situations: Five or more concurrent browser subagents/tasks fanned out at once; isolated instances not released because of missing finally release(); long-running browser tasks stacking up.

Related errors

• Cannot launch a concurrent browser agent in "${sessionMode}"
• LegacyAgentSession.send() cannot be called while a stream is
• Failed to initialize chrome-devtools MCP client
• Domain not allowed: The requested domain is not in the allow
• Browser agent requires user consent to proceed. Please re-ru

AI-assisted analysis of google-gemini/gemini-cli@5024443c72 (2026-08-12). Data as JSON: /api/errors/12a2f1eabe78f626. Report an issue: GitHub.