# API Requests failing with "Failed to write chunked buffer" error

**URL:** <https://forum.ecmwf.int/t/api-requests-failing-with-failed-to-write-chunked-buffer-error/14562>\
**Category:** C3S - Datasets and Usage\
**Tags:** api, era5\
**Created:** [29 December 2025 13:59 UTC](https://forum.ecmwf.int/t/api-requests-failing-with-failed-to-write-chunked-buffer-error/14562 "2025-12-29T13:59:10Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![ktrs](https://forum.ecmwf.int/letter_avatar_proxy/v4/letter/k/a3d4f5/32.png) [@ktrs](https://forum.ecmwf.int/u/ktrs)\
**Post date:** [29 December 2025 13:59 UTC](https://forum.ecmwf.int/t/api-requests-failing-with-failed-to-write-chunked-buffer-error/14562/1 "2025-12-29T13:59:10Z")

</div>

Hello,

today my jobs have been failing after long running times (12 min. and 50 min.). The error messages say:

Full error message:  
mars - ERROR - 20251229.094235 - Failed to write chunked buffer 393132/2076588 (Illegal seek)  
mars - ERROR - 20251229.094237 - Mars server task finished in error  
mars - ERROR - 20251229.094237 - Double buffer error: Assertion failed: length == buffers\_[i].length\_ in run, line 292 of /home/deploy/git/mars-server/eckit/src/eckit/io/DblBuffer.cc(RemoteException from Connector[mvr1006,cadsmarsmvr-1006:9701]) [mars]  
mars - ERROR - 20251229.094237 - Error code is -2  
mars - ERROR - 20251229.094237 - Failed to write chunked header -1/20 (Broken pipe)  
mars - ERROR - 20251229.094237 - Request failed  
mars - ERROR - 20251229.094237 - Some errors reported (last error -2)

Any idea what is going on? The same query had returned results last week.