feat(lambda): read cloud.account.id from symlink in Lambda detector

CHANGELOG.md

@@ -2,6 +2,11 @@
 
 ## Unreleased
 
+### AWS Lambda resource detector
+
+- Read `cloud.account.id` from symlink created by the OTel Lambda Extension
+  ([#2619](https://github.com/open-telemetry/opentelemetry-java-contrib/pull/2619))
+
 ## Version 1.53.0 (2026-01-21)
 
 ### AWS X-Ray propagator

aws-resources/src/main/java/io/opentelemetry/contrib/aws/resource/LambdaResource.java

@@ -5,6 +5,7 @@
 
 package io.opentelemetry.contrib.aws.resource;
 
+import static io.opentelemetry.contrib.aws.resource.IncubatingAttributes.CLOUD_ACCOUNT_ID;
 import static io.opentelemetry.contrib.aws.resource.IncubatingAttributes.CLOUD_PLATFORM;
 import static io.opentelemetry.contrib.aws.resource.IncubatingAttributes.CLOUD_PROVIDER;
 import static io.opentelemetry.contrib.aws.resource.IncubatingAttributes.CLOUD_REGION;
@@ -17,12 +18,18 @@
 import io.opentelemetry.api.common.AttributesBuilder;
 import io.opentelemetry.sdk.resources.Resource;
 import io.opentelemetry.semconv.SchemaUrls;
+import java.io.IOException;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.nio.file.Paths;
 import java.util.Map;
 import java.util.stream.Stream;
 
 /** A factory for a {@link Resource} which provides information about the AWS Lambda function. */
 public final class LambdaResource {
 
+  private static final String ACCOUNT_ID_SYMLINK_PATH = "/tmp/.otel-account-id";
+
   private static final Resource INSTANCE = buildResource();
 
   /**
@@ -34,11 +41,16 @@ public static Resource get() {
   }
 
   private static Resource buildResource() {
-    return buildResource(System.getenv());
+    return buildResource(System.getenv(), Paths.get(ACCOUNT_ID_SYMLINK_PATH));
   }
 
   // Visible for testing
   static Resource buildResource(Map<String, String> environmentVariables) {
+    return buildResource(environmentVariables, Paths.get(ACCOUNT_ID_SYMLINK_PATH));
+  }
+
+  // Visible for testing
+  static Resource buildResource(Map<String, String> environmentVariables, Path accountIdSymlink) {
     String region = environmentVariables.getOrDefault("AWS_REGION", "");
     String functionName = environmentVariables.getOrDefault("AWS_LAMBDA_FUNCTION_NAME", "");
     String functionVersion = environmentVariables.getOrDefault("AWS_LAMBDA_FUNCTION_VERSION", "");
@@ -60,6 +72,15 @@ static Resource buildResource(Map<String, String> environmentVariables) {
       builder.put(FAAS_VERSION, functionVersion);
     }
 
+    try {
+      String accountId = Files.readSymbolicLink(accountIdSymlink).toString();
+      if (!accountId.isEmpty()) {
+        builder.put(CLOUD_ACCOUNT_ID, accountId);
+      }
+    } catch (IOException | UnsupportedOperationException e) {
+      // Symlink doesn't exist or readlink not supported — silently skip
+    }
+
     return Resource.create(builder.build(), SchemaUrls.V1_25_0);
   }
 

aws-resources/src/test/java/io/opentelemetry/contrib/aws/resource/LambdaResourceTest.java

@@ -6,6 +6,7 @@
 package io.opentelemetry.contrib.aws.resource;
 
 import static io.opentelemetry.sdk.testing.assertj.OpenTelemetryAssertions.assertThat;
+import static io.opentelemetry.semconv.incubating.CloudIncubatingAttributes.CLOUD_ACCOUNT_ID;
 import static io.opentelemetry.semconv.incubating.CloudIncubatingAttributes.CLOUD_PLATFORM;
 import static io.opentelemetry.semconv.incubating.CloudIncubatingAttributes.CLOUD_PROVIDER;
 import static io.opentelemetry.semconv.incubating.CloudIncubatingAttributes.CLOUD_REGION;
@@ -19,10 +20,14 @@
 import io.opentelemetry.sdk.autoconfigure.spi.ResourceProvider;
 import io.opentelemetry.sdk.resources.Resource;
 import io.opentelemetry.semconv.SchemaUrls;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.nio.file.Paths;
 import java.util.HashMap;
 import java.util.Map;
 import java.util.ServiceLoader;
 import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.io.TempDir;
 
 class LambdaResourceTest {
   @Test
@@ -65,6 +70,53 @@ void shouldAddAllAttributes() {
             entry(FAAS_VERSION, "1.2.3"));
   }
 
+  @Test
+  void shouldReadCloudAccountIdFromSymlink(@TempDir Path tempDir) throws Exception {
+    Path symlink = tempDir.resolve(".otel-account-id");
+    Files.createSymbolicLink(symlink, Paths.get("123456789012"));
+
+    Resource resource =
+        LambdaResource.buildResource(
+            singletonMap("AWS_LAMBDA_FUNCTION_NAME", "my-function"), symlink);
+    Attributes attributes = resource.getAttributes();
+
+    assertThat(attributes)
+        .containsOnly(
+            entry(CLOUD_PROVIDER, "aws"),
+            entry(CLOUD_PLATFORM, "aws_lambda"),
+            entry(FAAS_NAME, "my-function"),
+            entry(CLOUD_ACCOUNT_ID, "123456789012"));
+  }
+
+  @Test
+  void shouldSkipCloudAccountIdWhenSymlinkMissing(@TempDir Path tempDir) {
+    Path symlink = tempDir.resolve(".otel-account-id");
+
+    Resource resource =
+        LambdaResource.buildResource(
+            singletonMap("AWS_LAMBDA_FUNCTION_NAME", "my-function"), symlink);
+    Attributes attributes = resource.getAttributes();
+
+    assertThat(attributes)
+        .containsOnly(
+            entry(CLOUD_PROVIDER, "aws"),
+            entry(CLOUD_PLATFORM, "aws_lambda"),
+            entry(FAAS_NAME, "my-function"));
+  }
+
+  @Test
+  void shouldPreserveLeadingZerosInAccountId(@TempDir Path tempDir) throws Exception {
+    Path symlink = tempDir.resolve(".otel-account-id");
+    Files.createSymbolicLink(symlink, Paths.get("012345678901"));
+
+    Resource resource =
+        LambdaResource.buildResource(
+            singletonMap("AWS_LAMBDA_FUNCTION_NAME", "my-function"), symlink);
+    Attributes attributes = resource.getAttributes();
+
+    assertThat(attributes).containsEntry(CLOUD_ACCOUNT_ID, "012345678901");
+  }
+
   @Test
   void inServiceLoader() {
     // No practical way to test the attributes themselves so at least check the service loader picks