A production .NET API is running, but Application Insights shows missing or incomplete telemetry. How would you debug it?

Debug missing Application Insights telemetry from a .NET API

Investigate a .NET API that appears to run but sends little or no telemetry to Application Insights.

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TL;DR

I would first decide what is missing: all telemetry, only logs, only dependencies, only traces, or a sampled subset. Then I would verify the Application Insights connection string, environment-specific configuration, SDK or OpenTelemetry setup, and whether the app is sending telemetry to the expected resource. I would check sampling and ingestion limits before assuming data is lost. If the issue started after deployment, I would compare telemetry configuration and package changes with the previous release.

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Connection strings, SDK or OpenTelemetry configuration, sampling, ingestion failures, network restrictions, and telemetry pipeline checks.

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Core answer
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Why this question matters
Missing telemetry can originate in instrumentation, configuration, sampling, network delivery, ingestion, workspace routing, or the query used to find it. First identifying which telemetry types and instances are absent narrows the broken stage.
I would first decide what is missing: all telemetry, only logs, only dependencies, only traces, or a sampled subset. Then I would verify the Application Insights connection string, environment-specific configuration, SDK or OpenTelemetry setup, and whether the app is sending telemetry to the expected resource. I would check sampling and ingestion limits before assuming data is lost. If the issue started after deployment, I would compare telemetry configuration and package changes with the previous release.
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Stronger answer
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I would trace the telemetry path from app to Azure Monitor ingestion. The app may generate telemetry but fail to send it because of network restrictions, Entra authentication settings, wrong endpoint, missing role, TLS issues, or SDK misconfiguration. It may also send correctly while the portal query, sampling, or workspace configuration makes data look incomplete. I would use SDK diagnostics, a controlled test event, live metrics where appropriate, and Azure Monitor health signals to narrow the missing step.

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Details

Assuming no portal data means the app produced no telemetry.
Ignoring sampling and ingestion limits.
Sending telemetry to the wrong Application Insights resource.
Changing observability libraries without validating existing alerts.

A production .NET API is running, but Application Insights shows missing or incomplete telemetry. How would you debug it?

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