File name max number of characters to upload to a Lakehouse folder

[ToddChitt]

I uploaded a file to a Lakehouse folder that was 63 characters long, including the ".xlsx" extension:

"Q1_2025 <...> Jan-Mar25.xlsx"

What Fabric named the file upon upoad was:

"Q1_202~2.XLS"

It ripped out about 50 characters from the middle of the file namen, AND changed the file extension.

It this expected behavior?

Is it a BUG?

Either way, it is EXTREMELY frustrating.

After upload I could RENAME the file back to its original just fine, but that would then mess up the Data Activator Trigger that is looking for new files in the folder of type ".xlsx".

[v-sshirivolu]

Hi @ToddChitt ,
Thanks for reaching out to Microsoft Fabric Community forum.

Thanks for sharing the details. From what you’ve described, it seems Fabric truncated your filename and changed the extension due to internal filename limits. Long filenames are often converted to the legacy 8.3 DOS format (8 characters for the name, 3 for the extension), which explains the changes you saw.

This is expected behavior rather than a bug, though I understand it’s frustrating especially if your Data Activator Trigger relies on the .xlsx extension. A few ways to handle it - use shorter filenames, adjust the trigger to accept altered names, or rename files after upload while updating dependent processes.

To avoid trigger issues, consider configuring your Data Activator to detect both original and truncated filenames, or include a consistent naming pattern that works within the filename limits.

[ToddChitt]

>>This is expected behavior rather than a bug<< ?!? WHAT? Someone at Microsoft needs to re-evaluate the design specification of this bit of code.

Comment by @justinjbird seems to indicate it does not happen to him, so why does it happen to me?

What is the limit on number of characters?

My particular workflow is that the Client is going to be uploading files to the lakehouse(s) via their OneLake data explorer. If that changes file names on them willy-nilly, they are going to freak out. Plus data loaded gets tagged with the file name associated with its import. I can't have rows in the warehouse with "Q1_202~2.XLS" as the file name source. And re-naming AFTER import then defeats the use of Data Activator Triggers that kick off a pipeline, possibly before the file can get renamed. 

Very disappointed. 

[v-sshirivolu]

Hi @ToddChitt ,

I understand your concerns, especially since being told this is “expected behavior” does not align with what others are experiencing. Community testing, including @justinjbird , shows that long filenames (60–70 characters) upload successfully via OneDrive, indicating that this is not a universal limit in Fabric Lakehouse. The issue appears to be specific to certain upload paths, such as OneLake Data Explorer.

Recommended workarounds:

Use OneDrive as a staging area: Upload files to the OneDrive folder linked to your Lakehouse to maintain full filenames and avoid 8.3 truncation.

Adopt a safer naming convention: When uploading directly through the Lakehouse UI or API, keep filenames under approximately 50 characters to prevent truncation.

Adjust trigger handling: Configure your Data Activator Trigger to handle both .xlsx and .XLS extensions, or include a renaming step in your pipeline or notebook to standardize filenames.

Escalate with detailed steps: If the issue persists, consider raising a support ticket with detailed reproduction steps, including upload method, file path, and filename length. This will help Microsoft determine if this is by design or a connector-specific issue.

Using OneDrive or shorter filenames can resolve the issue, and a support ticket may help clarify or address the underlying cause.

[justinjbird]

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@v-sshirivolu wrote:

I understand your concerns, especially since being told this is “expected behavior” does not align with what others are experiencing. Community testing, including @justinjbird , shows that long filenames (60–70 characters) upload successfully via OneDrive, indicating that this is not a universal limit in Fabric Lakehouse. The issue appears to be specific to certain upload paths, such as OneLake Data Explorer.

---

I didn't upload via OneDrive and there is no evidence yet that this is OneLake Data Explorer.

[justinjbird]

Because it's not expected behaviour. This is a Copilot response mess gone wrong. Copilot has even been citing the original response by @jaineshp as factual known issue. And what is further unhelpful is that it appears that the original response that was an inaccurate copilot response, and was tagged as the solution has now been removed leaving this whole thread a bit of a confusing mess.

I think it is likely that there are some specific conditions under which this is happening, but directly updating to onedrive files are worked fine for me, does it for you? When you say you uploaded, how? Was it the same way I did or some other way? What is the file system? If through onedrive explorer is it up to date etc?

[ToddChitt]

Ah, thanks, CoPilot for throwing a wrench in the gearbox!

Strange, I just did a quick test by creating a blank Excel file with a 50 character name (excluding extension) and uploading it to a Lakehouse (using the browser). Then renamed it to add 10 more characters to the name and uploaded again. Repeat until the filename is 100 characters long. 

So why did my other file get renamed? I don't think it was 100 characters, and certainly was not that large. 

Strange to say the least.

[ToddChitt]

Do you have a link to this issue?

I have marked your answer as a solution, but really, the solution is for Microsoft to fix the issue.

And FYI: Most of your suggestions are not procticle.

Renaming AFTER upload messes up the Trigger's FileName property.

My 'defect support system' will be to tell the client to keep their filenames shorter than about 50 characters.

So frustrating to sell something like Fabric to a client, then have to turn around and tell them that something as simple as retaining a file's name AND EXTENSION during an upload is BROKEN.

[AndyDDC]

That's a purely AI generated answer. I put your question into Copilot and it generated the exact response from that reply.

Use the advice at your own discretion.

[v-sshirivolu]

Hi @ToddChitt  ,

I wanted to check if you had the opportunity to review the information provided by @jaineshp  . Please feel free to contact us if you have any further questions.

Thank you and continue using Microsoft Fabric Community Forum.

[justinjbird]

For what it's worth I attempted to upload 3 xlsx files. Different filenmame lengths - 30, 60, 70. All uploaded fine and filename intact...

[justinjbird]

Where is this known issue?

[AndyDDC]

The answer has been AI generated. We have no way of knowing whether the answer has any validity.