apim-policies

name	apim-policies
description	Guide for creating Azure API Management (APIM) XML policies. Use when users want to create, modify, or understand APIM policies including inbound/outbound processing, authentication, rate limiting, caching, transformations, and policy expressions. This skill provides policy syntax, examples, and C# policy expressions for request/response manipulation.

APIM Policies

This skill provides guidance for creating Azure API Management XML policies.

Policy Document Structure

Every APIM policy document follows this structure:

<policies>
    <inbound>
        <base />
        <!-- Policies applied to incoming requests -->
    </inbound>
    <backend>
        <base />
        <!-- Policies applied before forwarding to backend -->
    </backend>
    <outbound>
        <base />
        <!-- Policies applied to outgoing responses -->
    </outbound>
    <on-error>
        <base />
        <!-- Policies applied when errors occur -->
    </on-error>
</policies>

The <base /> element inherits policies from parent scopes (Global → Product → API → Operation).

Policy Categories Quick Reference

Category	Common Policies	Section
Authentication	authentication-managed-identity, validate-azure-ad-token, validate-jwt	inbound
Rate Limiting	rate-limit-by-key, quota-by-key	inbound
Caching	cache-lookup, cache-store	inbound/outbound
Routing	set-backend-service, forward-request, retry	inbound/backend
Transformation	set-header, set-body, set-variable, rewrite-uri	any
Control Flow	choose, return-response, retry, wait	any

Essential Policies

Set Backend Service

Route requests to a specific backend:

<set-backend-service backend-id="my-backend" />

Authentication with Managed Identity

Authenticate to Azure services using APIM's managed identity:

<authentication-managed-identity resource="https://cognitiveservices.azure.com"
    output-token-variable-name="managed-id-access-token" ignore-error="false" />
<set-header name="Authorization" exists-action="override">
    <value>@("Bearer " + (string)context.Variables["managed-id-access-token"])</value>
</set-header>

Validate Azure AD Token

Validate JWT tokens from Microsoft Entra ID:

<validate-azure-ad-token tenant-id="{tenant-id}">
    <client-application-ids>
        <application-id>{client-app-id}</application-id>
    </client-application-ids>
</validate-azure-ad-token>

Conditional Logic (Choose)

Apply policies based on conditions:

<choose>
    <when condition="@(context.Request.Headers.GetValueOrDefault("X-Custom","") == "value")">
        <!-- policies when condition is true -->
    </when>
    <otherwise>
        <!-- fallback policies -->
    </otherwise>
</choose>

Return Custom Response

Return an immediate response without calling the backend:

<return-response>
    <set-status code="403" reason="Forbidden" />
    <set-header name="Content-Type" exists-action="override">
        <value>application/json</value>
    </set-header>
    <set-body>{"error": "Access denied"}</set-body>
</return-response>

Retry Logic

Retry failed requests with conditions:

<retry count="3" interval="1" first-fast-retry="true"
    condition="@(context.Response.StatusCode == 429 || context.Response.StatusCode >= 500)">
    <forward-request buffer-request-body="true" />
</retry>

Policy Expressions

Policy expressions use C# syntax within @() for single statements or @{} for multi-statement blocks.

Common Expressions

// Get header value
@(context.Request.Headers.GetValueOrDefault("header-name", "default"))

// Get query parameter
@(context.Request.Url.Query.GetValueOrDefault("param-name", "default"))

// Get URL path parameter
@(context.Request.MatchedParameters.GetValueOrDefault("param-name", "default"))

// Get subscription ID
@(context.Subscription.Id)

// Get client IP
@(context.Request.IpAddress)

// Read JSON body property
@(context.Request.Body.As<JObject>(preserveContent: true)["property"]?.ToString())

// Check header existence
@(context.Request.Headers.ContainsKey("header-name"))

// Get context variable
@(context.Variables.GetValueOrDefault<string>("var-name", "default"))

Multi-Statement Expression

<set-variable name="result" value="@{
    string[] value;
    if (context.Request.Headers.TryGetValue("Authorization", out value))
    {
        if(value != null && value.Length > 0)
        {
            return Encoding.UTF8.GetString(Convert.FromBase64String(value[0]));
        }
    }
    return null;
}" />

Allowed .NET Types and Members (CRITICAL)

Policy expressions may only reference the .NET Framework types and members on APIM's allow-list. Anything outside the list causes a deploy-time ValidationError: One or more fields contain incorrect values with no further detail, which is hard to diagnose.

Before using a type or member in a policy expression, verify it appears on the official allow-list. Common pitfalls:

• Whole namespaces are absent. System.Globalization (e.g. DateTimeStyles, CultureInfo), System.Threading, System.IO (except StringReader/StringWriter), System.Net.Http, System.Reflection, and System.Diagnostics are not allowed.
• System.DateTime is restricted. Allowed members include Parse, UtcNow, Now, AddSeconds, Subtract, ToString, Ticks. Not allowed: TryParse, TryParseExact, ParseExact, ToUniversalTime, ToLocalTime, SpecifyKind. For round-trip-safe time math, prefer System.DateTimeOffset (All members allowed) and ToUnixTimeSeconds() / ToUnixTimeMilliseconds().
• System.Enum is restricted to Parse, TryParse, ToString. No GetValues, GetNames, IsDefined.
• System.Text.RegularExpressions.Regex is restricted to the constructor plus IsMatch, Match, Matches, Replace, Unescape, Split. No CompileToAssembly, CacheSize.
• Numeric primitives are fully allowed, so int.TryParse, long.TryParse, double.TryParse are safe.
• JSON via Newtonsoft.Json is the only supported JSON library — do not use System.Text.Json.

When a member you need is not allowed, refactor to an equivalent that is. Examples:

Disallowed	Allowed replacement
DateTime.TryParse(s, ..., DateTimeStyles.RoundtripKind, out dt)	Store as Unix epoch via DateTimeOffset.UtcNow.ToUnixTimeSeconds(), parse with long.TryParse
DateTime.ParseExact(s, fmt, CultureInfo.InvariantCulture)	DateTime.Parse(s) (allowed) or epoch-based representation
Enum.GetValues(typeof(T))	Hard-code the comparison values or store as a string
System.Text.Json.JsonSerializer.Deserialize<T>(s)	JsonConvert.DeserializeObject<T>(s)

If you are unsure whether a member is allowed, fetch the allowed types table and confirm before writing the expression.

Reference Documentation

• Shared policies in this repo: shared/apim-policies/ (reusable policy XML fragments)

Official Documentation

• APIM Policy Reference
• Policy Expressions
• Policy Snippets Repository