The Azure Functions host.json Guide

5 min·Jul 18, 2026

AI Summary

host.json configures the Functions runtime host-wide. The highest-impact settings are Application Insights sampling (controls telemetry cost), functionTimeout (capped at 10 min on Consumption), the app-wide retry policy, and HTTP concurrency limits. It never holds secrets — those go in application settings.

Azure Functions host.json

The global host.json for an Azure Functions app — Application Insights sampling, HTTP route prefix and concurrency, function timeout and retry policy — the runtime-wide behavior knobs.

{
  "version": "2.0",
  "logging": {
    "applicationInsights": {
      "samplingSettings": {
        "isEnabled": true,
        "maxTelemetryItemsPerSecond": 20,
        "excludedTypes": "Request"
      }
    },
    "logLevel": {
      "default": "Information",
      "Function.Host": "Warning"
    }
  },
  "extensions": {
    "http": {
      "routePrefix": "api",
      "maxConcurrentRequests": 100,
      "maxOutstandingRequests": 200
    }
  },
  "functionTimeout": "00:05:00",
  "retry": {
    "strategy": "fixedDelay",
    "maxRetryCount": 3,
    "delayInterval": "00:00:05"
  },
  "concurrency": {
    "dynamicConcurrencyEnabled": true,
    "snapshotPersistenceEnabled": true
  }
}

host.json configures the Azure Functions runtime host — not one function, but every function in the app. A handful of settings decide your telemetry bill and your reliability under load.

Control telemetry cost with sampling

Application Insights bills per ingested item. Sampling caps the rate so a traffic spike does not blow up your bill:

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

excludedTypes keeps every request but samples the noisier traces.

Cap execution time

"functionTimeout": "00:05:00" stops runaway executions. On the Consumption plan the hard ceiling is 10 minutes — set it explicitly rather than relying on the default.

Retry transient failures

An app-wide retry policy beats hand-rolled loops in every function:

"retry": {
  "strategy": "fixedDelay",
  "maxRetryCount": 3,
  "delayInterval": "00:00:05"
}

Shape HTTP behavior

extensions.http sets the routePrefix (default api) and concurrency limits — maxConcurrentRequests and maxOutstandingRequests protect a downstream database from being overwhelmed during a burst.

Security notes

host.json is not for secrets. Connection strings and keys belong in application settings or Key Vault references.
Lower logLevel in production so payloads do not leak into logs.

Open the host.json template and ask the workspace to "set the function timeout to 10 minutes" or "add a retry policy with fixed delay", then diff the result.

FAQ

Where does host.json live?

At the root of the Functions app, next to your function folders. It applies to every function in the app.

What is the max functionTimeout?

On the Consumption plan the ceiling is 10 minutes ("00:10:00"). Premium and Dedicated plans allow longer or unbounded execution.

Related templates

Azure Functions host.json