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# Application Insights errors and missing telemetry

Telemetry can be lost at five places: the SDK or agent doesn't send it; the network blocks the ingestion endpoint; ingestion rejects or throttles it; the pipeline delays or drops it (service health); or the portal fails to query it. Microsoft's support article includes PowerShell and curl scripts that post a sample record straight to the ingestion endpoint: if that succeeds from the app's host, the problem is the SDK configuration; if it fails, it's the network.

## Ingestion responses (any language)

| Response | Cause → fix |
|---|---|
| `HTTP/1.1 400 Authentication not supported` | The resource is **Entra-only** (`DisableLocalAuth`) but the SDK sends unauthenticated telemetry to `v2/track`. Configure a credential so it uses `v2.1/track` |
| `HTTP/1.1 401 Authorization required` | The SDK is set up for Entra but can't get a token: look for Azure Identity errors in the SDK logs (wrong client ID, no identity on the host) |
| `HTTP/1.1 403 Unauthorized` | The token's identity lacks **Monitoring Metrics Publisher** on the Application Insights resource |
| Throttled | Above **32,000 events per second** (measured over a minute) |
| Item dropped or truncated | Item over **64 KB**; more than 64,000 items per batch; property values over 8,192 characters; names over 150; messages over 32,768 |

## Causes that send nothing at all

| Cause | Check → fix |
|---|---|
| DNS doesn't resolve the ingestion endpoint from the connection string, or resolves it wrongly | `nslookup` the `IngestionEndpoint` host from the app |
| Firewall or proxy blocks outbound HTTPS to it | Test with the support article's curl script |
| Client still offers TLS 1.0/1.1 | Ingestion requires TLS 1.2 |
| **Several Azure Monitor Private Link scopes** touching the same network overwrite each other's DNS records | The hub's reading: keep one AMPLS per shared DNS (Microsoft's private link design guidance covers this) |
| The distro starts after the libraries it should instrument | Load it first (`--import`, or the first `require`) |
| A **classic** Application Insights resource | It no longer ingests: convert to workspace-based |
| Instrumentation-key-only configuration | Use the full connection string (regional endpoints) |

## Telemetry that's there but incomplete

- **Sampling by design:** the Microsoft OpenTelemetry Distro keeps **5 traces per second** by default; Failures and Performance views look incomplete for busy apps. Raise the rate or use `samplingRatio`.
- **Daily cap:** Application Insights defaults to **100 GB per day**; at the cap, collection stops until the workspace's reset hour (`reference/monitor/log-analytics-workspaces.md`). Some data beyond the cap still arrives.
- **Duplicates:** two distros or an SDK plus codeless monitoring in the same app send everything twice: keep one.
- **Node.js exporter known issues:** dependency telemetry lacks the operation name; request and dependency telemetry lack the device model; database dependency names omit the server, so same-named tables on different servers aggregate together.

## Turning on the exporter's logs

`APPLICATIONINSIGHTS_INSTRUMENTATION_LOGGING_LEVEL=VERBOSE`, `APPLICATIONINSIGHTS_LOG_DESTINATION=file` and `APPLICATIONINSIGHTS_LOGDIR=<folder>`, or in code `diag.setLogger(new DiagConsoleLogger(), DiagLogLevel.ALL)` from `@opentelemetry/api`.


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