Elastic Agent enrollment/check-in failures on 9.5.3 (and 9.4.6) with Fleet remote Elasticsearch output

We have patched Fleet Server versions 9.5.3 and 9.4.6 with corrected releases deployed as of September 10, 2026 at 21:20 UTC.

- Upgrading to the current 9.4.6 or 9.5.3 releases will not be affected by this bug as the updated release contains the fix.
- If you upgraded to 9.4.6 or 9.5.3 before 21:20 UTC on 10 September 2026, perform the following mitigation:
1. Force restart the Integration Server component of you affected deployment
2. Run cleanup procedures on affected Elastic Agents (see https://support.elastic.co/knowledge/bee1c75c) — required if agents failed to check in or remained offline after the Integration Server restart

IMPORTANT: The patched Fleet Server is not available outside Elastic Cloud Enterprise (ECE) Stack Packs and Elastic Cloud Hosted (ECH).

We have patched Fleet Server versions 9.5.3 and 9.4.6 with corrected releases deployed as of September 10, 2026 at 21:20 UTC.

- Upgrading to the current 9.4.6 or 9.5.3 releases will not be affected by this bug as the updated release contains the fix.
- If you upgraded to 9.4.6 or 9.5.3 before 21:20 UTC on 10 September 2026, perform the following mitigation:
1. Force restart the Integration Server component of you affected deployment
2. Run cleanup procedures on affected Elastic Agents (see https://support.elastic.co/knowledge/bee1c75c) — required if agents failed to check in or remained offline after the Integration Server restart

We've identified a bug in Fleet Server 9.5.3 (also present in 9.4.6) that can cause Elastic Agents to crash-loop and go offline when Fleet pushes a configuration update, including enrollment, a policy change, or a routine revision bump. This only affects policies that use Fleet's remote Elasticsearch output feature.

Recommendation: If you use Fleet's remote Elasticsearch output, do not upgrade to 9.5.3 or 9.4.6 until a fixed version is available. If you're already on an affected version and experiencing agent check-in failures, contact Support for remediation steps.

A fix has been merged and will ship in the next 9.5.x and 9.4.x releases. Known Issue documentation: fleet-server#7791, elastic-agent#16542.