Function app logs are not being output

Question

Function app logs are not being output

dla-d 0

For some function apps, call logs and Application Insights logs are not being output after June 19, 2025. Logs were output before June 19, 2025. In addition, logs are being output without any problems for other function apps in the same tenant.

The situation does not change even if the function app is restarted, so please let me know if there is a solution.

Loknathsatyasaivarma Mahali 2,740 Reputation points Microsoft External Staff Moderator

2025-06-25T03:43:00.3666667+00:00
Hello dla-d,

Thanks for reaching out, please try the following checks to identify why logs are not being output after June 19, 2025:

Verify Application Insights Connection: Go to Configuration > Application settings and ensure the APPLICATIONINSIGHTS_CONNECTION_STRING is present and correctly points to the intended Application Insights resource.

Check for Aggressive Sampling: Review the host.json file's samplingSettings. If sampling is enabled, it might be discarding logs too aggressively. Try disabling it temporarily to isolate the issue.

Review Network and Firewall Rules: Check networking configurations for both the Function App and Application Insights. A recent firewall change may be blocking telemetry data.

Confirm Logging Is Not Disabled: Ensure the setting AzureWebJobsDisableLogging is not set to true in the application settings.

Validate the Application Insights Resource: Confirm that the linked Application Insights resource still exists and hasn’t been deleted or moved.

Inspect for Runtime Errors: Use the Log stream feature in the Azure portal. Startup or runtime errors can prevent the logging pipeline from initializing.

For more detailed steps, refer to the official Microsoft troubleshooting guide: Troubleshoot Azure Functions monitoring and Application Insights logs
dla-d 0 Reputation points

2025-06-25T07:14:20.7333333+00:00

Hello Loknathsatyasaivarma Mahali,

This was caused by an incorrect Application Insights InstrumentationKey being set as the value of the environment variable APPLICATIONINSIGHTS_CONNECTION_STRING. I set the correct Application Insights InstrumentationKey and logs started being output. After that, the incorrect InstrumentationKey value was automatically set again and logs were no longer being output. Do you know why this is?
Krishna Chowdary Paricharla 90 Reputation points Microsoft External Staff Moderator

2025-06-25T11:23:33.5033333+00:00

Hello dla-d,

Could you please check the private message and provide the required details for troubleshooting the issue further.

Here is the reference link on how to access & data retention policy of private messages in Microsoft Q&A.
dla-d 0 Reputation points

2025-06-26T09:00:06.4433333+00:00

Hello Loknathsatyasaivarma Mahali,

Is it possible that the value of the environment variable APPLICATIONINSIGHTS_CONNECTION_STRING will be changed automatically?
Krishna Chowdary Paricharla 90 Reputation points Microsoft External Staff Moderator

2025-06-26T11:48:17.8766667+00:00
Hello dla-d,
It is possible that in certain Azure environments, the value of the APPLICATIONINSIGHTS_CONNECTION_STRING environment variable could be automatically reset.

If you use deployment slots and the slot has a different Application Insights configuration, swapping the slots could overwrite your connection string.
Krishna Chowdary Paricharla 90 Reputation points Microsoft External Staff Moderator

2025-06-27T13:07:52.8966667+00:00

Hello dla-d,

Just checking in to see if you've had a chance to review my previous response and let me know if you have any additional questions.

Your answer

Loknathsatyasaivarma Mahali 2,740 Reputation points Microsoft External Staff Moderator

2025-06-25T03:43:00.3666667+00:00

Hello dla-d,

Thanks for reaching out, please try the following checks to identify why logs are not being output after June 19, 2025:

Verify Application Insights Connection: Go to Configuration > Application settings and ensure the APPLICATIONINSIGHTS_CONNECTION_STRING is present and correctly points to the intended Application Insights resource.

Check for Aggressive Sampling: Review the host.json file's samplingSettings. If sampling is enabled, it might be discarding logs too aggressively. Try disabling it temporarily to isolate the issue.

Review Network and Firewall Rules: Check networking configurations for both the Function App and Application Insights. A recent firewall change may be blocking telemetry data.

Confirm Logging Is Not Disabled: Ensure the setting AzureWebJobsDisableLogging is not set to true in the application settings.

Validate the Application Insights Resource: Confirm that the linked Application Insights resource still exists and hasn’t been deleted or moved.

Inspect for Runtime Errors: Use the Log stream feature in the Azure portal. Startup or runtime errors can prevent the logging pipeline from initializing.

For more detailed steps, refer to the official Microsoft troubleshooting guide: Troubleshoot Azure Functions monitoring and Application Insights logs
dla-d 0 Reputation points

2025-06-25T07:14:20.7333333+00:00

Hello Loknathsatyasaivarma Mahali,

This was caused by an incorrect Application Insights InstrumentationKey being set as the value of the environment variable APPLICATIONINSIGHTS_CONNECTION_STRING. I set the correct Application Insights InstrumentationKey and logs started being output. After that, the incorrect InstrumentationKey value was automatically set again and logs were no longer being output. Do you know why this is?
Krishna Chowdary Paricharla 90 Reputation points Microsoft External Staff Moderator

2025-06-25T11:23:33.5033333+00:00

Hello dla-d,

Could you please check the private message and provide the required details for troubleshooting the issue further.

Here is the reference link on how to access & data retention policy of private messages in Microsoft Q&A.
dla-d 0 Reputation points

2025-06-26T09:00:06.4433333+00:00

Hello Loknathsatyasaivarma Mahali,

Is it possible that the value of the environment variable APPLICATIONINSIGHTS_CONNECTION_STRING will be changed automatically?
Krishna Chowdary Paricharla 90 Reputation points Microsoft External Staff Moderator

2025-06-26T11:48:17.8766667+00:00

Hello dla-d,
It is possible that in certain Azure environments, the value of the APPLICATIONINSIGHTS_CONNECTION_STRING environment variable could be automatically reset.

If you use deployment slots and the slot has a different Application Insights configuration, swapping the slots could overwrite your connection string.
Krishna Chowdary Paricharla 90 Reputation points Microsoft External Staff Moderator

2025-06-27T13:07:52.8966667+00:00

Hello dla-d,

Just checking in to see if you've had a chance to review my previous response and let me know if you have any additional questions.

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Function app logs are not being output

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