diff --git a/docs/source-2.0/aws/rules-engine/built-ins.rst b/docs/source-2.0/aws/rules-engine/built-ins.rst
index c91a632032f..cc8deefe344 100644
--- a/docs/source-2.0/aws/rules-engine/built-ins.rst
+++ b/docs/source-2.0/aws/rules-engine/built-ins.rst
@@ -78,6 +78,16 @@ Description
 Type
     ``string``
 
+.. _rules-engine-aws-built-ins-account-id-endpoint-mode:
+
+``AWS::Auth::AccountIdEndpointMode`` built-in
+=============================================
+
+Description
+    The AccountId Endpoint Mode.
+Type
+    ``string``
+
 .. _rules-engine-aws-built-ins-credential-scope:
 
 ``AWS::Auth::CredentialScope`` built-in
diff --git a/smithy-aws-endpoints/src/main/java/software/amazon/smithy/rulesengine/aws/language/functions/AwsBuiltIns.java b/smithy-aws-endpoints/src/main/java/software/amazon/smithy/rulesengine/aws/language/functions/AwsBuiltIns.java
index f5fb0c4596c..5a47c5bb1bd 100644
--- a/smithy-aws-endpoints/src/main/java/software/amazon/smithy/rulesengine/aws/language/functions/AwsBuiltIns.java
+++ b/smithy-aws-endpoints/src/main/java/software/amazon/smithy/rulesengine/aws/language/functions/AwsBuiltIns.java
@@ -64,6 +64,17 @@ public final class AwsBuiltIns {
                     .documentation("The AWS AccountId used for the request.")
                     .build();
 
+    /**
+     * Built-in parameter representing the AccountId Endpoint Mode.
+     */
+    public static final Parameter ACCOUNT_ID_ENDPOINT_MODE =
+            Parameter.builder()
+                    .name("AccountIdEndpointMode")
+                    .type(ParameterType.STRING)
+                    .builtIn("AWS::Auth::AccountIdEndpointMode")
+                    .documentation("The AccountId Endpoint Mode.")
+                    .build();
+
     /**
      * Built-in parameter representing the Credential Scope.
      */
diff --git a/smithy-aws-endpoints/src/main/java/software/amazon/smithy/rulesengine/aws/language/functions/AwsRuleSetExtension.java b/smithy-aws-endpoints/src/main/java/software/amazon/smithy/rulesengine/aws/language/functions/AwsRuleSetExtension.java
index fd151a5a7d6..7fceddb7e7a 100644
--- a/smithy-aws-endpoints/src/main/java/software/amazon/smithy/rulesengine/aws/language/functions/AwsRuleSetExtension.java
+++ b/smithy-aws-endpoints/src/main/java/software/amazon/smithy/rulesengine/aws/language/functions/AwsRuleSetExtension.java
@@ -23,6 +23,7 @@ public List<Parameter> getBuiltIns() {
                 AwsBuiltIns.FIPS,
                 AwsBuiltIns.REGION,
                 AwsBuiltIns.ACCOUNT_ID,
+                AwsBuiltIns.ACCOUNT_ID_ENDPOINT_MODE,
                 AwsBuiltIns.CREDENTIAL_SCOPE,
                 AwsBuiltIns.S3_ACCELERATE,
                 AwsBuiltIns.S3_DISABLE_MRAP,
diff --git a/smithy-aws-endpoints/src/main/java/software/amazon/smithy/rulesengine/aws/validators/RuleSetAwsBuiltInValidator.java b/smithy-aws-endpoints/src/main/java/software/amazon/smithy/rulesengine/aws/validators/RuleSetAwsBuiltInValidator.java
index df0375a015f..b4131e74956 100644
--- a/smithy-aws-endpoints/src/main/java/software/amazon/smithy/rulesengine/aws/validators/RuleSetAwsBuiltInValidator.java
+++ b/smithy-aws-endpoints/src/main/java/software/amazon/smithy/rulesengine/aws/validators/RuleSetAwsBuiltInValidator.java
@@ -27,6 +27,7 @@
 public class RuleSetAwsBuiltInValidator extends AbstractValidator {
     private static final Set<String> ADDITIONAL_CONSIDERATION_BUILT_INS = SetUtils.of(
             AwsBuiltIns.ACCOUNT_ID.getBuiltIn().get(),
+            AwsBuiltIns.ACCOUNT_ID_ENDPOINT_MODE.getBuiltIn().get(),
             AwsBuiltIns.CREDENTIAL_SCOPE.getBuiltIn().get());
     private static final String ADDITIONAL_CONSIDERATION_MESSAGE = "The `%s` built-in used requires additional "
            + "consideration of the rules that use it.";