What does Windows error code 565 (ERROR_TOO_MANY_THREADS) mean?

ERROR_TOO_MANY_THREADS

What this result means

ERROR_TOO_MANY_THREADS is a Windows system result. The process or subsystem has reached a thread-related limit for the requested operation. The practical problem is often unbounded concurrency or failure to retire workers.

Why it can appear

• a thread-per-request design creates workers faster than they exit
• threads are leaked because shutdown or error paths never join them
• desktop heap, address space, commit, job, or subsystem limits are exhausted before a nominal thread-count limit
• recursive or retry logic repeatedly creates helper threads

Troubleshooting steps

1. measure thread count over time and group stacks by start routine
2. Record process commit, handle count, desktop/session, job limits, and creation rate
3. look for blocked groups waiting on the same lock, network call, or event
4. Verify that thread pools are bounded and completion paths always release work items

Correct recovery and handling

Stop accepting new work, drain or restart the affected process, and replace unbounded creation with a bounded pool or asynchronous model. Raising limits without fixing growth usually delays the next outage.

Related and easily confused conditions

This can occur while plenty of CPU remains. Thread creation consumes kernel objects, stacks, commit, and scheduler capacity, so memory and handle telemetry are as important as the raw count.

References

• Microsoft: System error codes
• Microsoft: System error codes 500–999