host.json Reference¶

The host.json file configures the Azure Functions runtime behaviour. It lives in the root of your function app (same directory as function_app.py) and applies to all functions in the app. This reference covers the most important settings for Python function apps.

flowchart TD
    H[host.json] --> V[version]
    H --> L[logging]
    H --> E[extensions]
    H --> T[functionTimeout]
    H --> C[concurrency]
    E --> HTTP[http]
    E --> Q[queues]

Complete Annotated Example¶

{
  "version": "2.0",

  "logging": {
    "logLevel": {
      "default": "Information",
      "Host.Results": "Error",
      "Function": "Information",
      "Host.Aggregator": "Trace"
    },
    "applicationInsights": {
      "samplingSettings": {
        "isEnabled": true,
        "maxTelemetryItemsPerSecond": 20,
        "excludedTypes": "Request;Exception"
      },
      "enableLiveMetrics": true,
      "httpAutoCollectionOptions": {
        "enableHttpTriggerExtendedInfoCollection": true,
        "enableW3CDistributedTracing": true,
        "enableResponseHeaderInjection": true
      }
    }
  },

  "extensions": {
    "http": {
      "routePrefix": "api",
      "maxOutstandingRequests": 200,
      "maxConcurrentRequests": 100,
      "dynamicThrottlesEnabled": false
    },
    "queues": {
      "maxPollingInterval": "00:00:02",
      "visibilityTimeout": "00:00:30",
      "batchSize": 16,
      "maxDequeueCount": 5,
      "newBatchThreshold": 8
    }
  },

  "extensionBundle": {
    "id": "Microsoft.Azure.Functions.ExtensionBundle",
    "version": "[4.*, 5.0.0)"
  },

  "functionTimeout": "00:05:00",

  "concurrency": {
    "dynamicConcurrencyEnabled": true,
    "snapshotPersistenceEnabled": true
  }
}

Key Settings¶

version (Required)¶

{
  "version": "2.0"
}

Always set to "2.0" for Azure Functions v4 runtime. This is the only supported version for the v2 Python programming model.

extensionBundle (Required)¶

{
  "extensionBundle": {
    "id": "Microsoft.Azure.Functions.ExtensionBundle",
    "version": "[4.*, 5.0.0)"
  }
}

The extension bundle provides pre-compiled binding extensions (Cosmos DB, Storage, Event Grid, etc.) without requiring you to install them individually. The version range [4.*, 5.0.0) means any 4.x version but not 5.x.

Important: Without the extension bundle, non-HTTP bindings will not work. HTTP triggers work without it since they are part of the core runtime.

functionTimeout¶

{
  "functionTimeout": "00:05:00"
}

Maximum execution time for a single function invocation. The format is hh:mm:ss.

For the Consumption plan, if a function exceeds the timeout, the host terminates the process. Flex Consumption defaults to 30 minutes and supports unbounded execution (-1); if a bounded timeout is set, the host terminates the process when it is exceeded.

{
  "functionTimeout": "00:10:00"
}

Tip: Use a reasonable upper bound based on your SLOs and downstream limits even on plans that support unbounded timeout. Prefer bounded execution plus retries/orchestration; use Durable Functions for long-running workflows.

logging¶

Control log verbosity and Application Insights behaviour:

{
  "logging": {
    "logLevel": {
      "default": "Information",
      "Host.Results": "Error",
      "Function": "Information",
      "Function.my_func": "Debug"
    }
  }
}

logging.applicationInsights.samplingSettings¶

Control how much telemetry is sent to Application Insights:

{
  "logging": {
    "applicationInsights": {
      "samplingSettings": {
        "isEnabled": true,
        "maxTelemetryItemsPerSecond": 20,
        "excludedTypes": "Request;Exception"
      }
    }
  }
}

Cost Tip: Set excludedTypes to "Request;Exception" to ensure you never lose error data, while allowing traces and dependencies to be sampled. See Cost Optimization.

extensions.http¶

Configure HTTP trigger behaviour:

{
  "extensions": {
    "http": {
      "routePrefix": "api",
      "maxOutstandingRequests": 200,
      "maxConcurrentRequests": 100,
      "dynamicThrottlesEnabled": false
    }
  }
}

To remove the /api prefix (so routes are https://your-func.azurewebsites.net/health instead of https://your-func.azurewebsites.net/api/health):

{
  "extensions": {
    "http": {
      "routePrefix": ""
    }
  }
}

extensions.queues¶

Configure Queue trigger behaviour (see Queue recipe):

{
  "extensions": {
    "queues": {
      "maxPollingInterval": "00:00:02",
      "visibilityTimeout": "00:00:30",
      "batchSize": 16,
      "maxDequeueCount": 5,
      "newBatchThreshold": 8
    }
  }
}

concurrency¶

Enable dynamic concurrency for automatic tuning:

{
  "concurrency": {
    "dynamicConcurrencyEnabled": true,
    "snapshotPersistenceEnabled": true
  }
}

When enabled, the runtime automatically adjusts concurrency limits per trigger type based on observed performance. snapshotPersistenceEnabled persists the learned concurrency settings across restarts.

Minimal host.json¶

The minimum viable host.json for an HTTP-only Python function app:

{
  "version": "2.0",
  "extensionBundle": {
    "id": "Microsoft.Azure.Functions.ExtensionBundle",
    "version": "[4.*, 5.0.0)"
  }
}

Local vs. Production Differences¶

The host.json file is deployed with your code and applies in both local and production environments. For settings that should differ between environments, use app settings (environment variables) and reference them in your code rather than putting environment-specific values in host.json.

See Also¶

• Environment Variables
• Observability
• Cost Optimization

Sources¶

• host.json Reference (Microsoft Learn)