diff --git a/CHANGELOG.md b/CHANGELOG.md
index cb8e33ef..2d3124bc 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -8,6 +8,13 @@ project follows [Semantic Versioning](https://semver.org/).
 
 ### Added
 
+- `ActivityContext::activityExecutionId` exposes the received logical activity
+  execution ID for downstream idempotency keys. Missing or invalid legacy
+  metadata stays null; task and attempt IDs are never used as substitutes.
+  Prepared local callbacks receive their validated Server execution identity;
+  inline local and Server-free callbacks without one report null
+  (durable-workflow/.github#211).
+
 - `Client::iterateWorkflows()` and `iterateWorkflowHistory()` lazily read
   complete workflow listings and exact-run histories, retaining one page at a
   time. Existing one-page methods stay available. Iteration preserves opaque
diff --git a/docs/portal/build/workflows-activities.md b/docs/portal/build/workflows-activities.md
index d4498bc5..52fec96c 100644
--- a/docs/portal/build/workflows-activities.md
+++ b/docs/portal/build/workflows-activities.md
@@ -517,6 +517,30 @@ heartbeats. Prepared local activities use supervised callback processes.
 
 ## Inspect retry budget and the owning workflow
 
+The unreleased `ActivityContext::activityExecutionId` property contains the
+logical `activity_execution_id` received with the claim. Ordinary and
+cooperatively supervised remote callbacks expose that same execution identity
+across retry attempts, while `activityAttemptId` identifies each attempt.
+Use the logical ID in a downstream idempotency key when the destination honors
+that key, including your application scope where needed.
+
+```php
+function activityIdempotencyKey(
+    \DurableWorkflow\Worker\ActivityContext $context,
+    string $businessKey,
+): string {
+    return $context->activityExecutionId ?? $businessKey;
+}
+```
+
+Older claims can omit the field. Missing, null, empty or non-string values
+produce null, so supply a business key when it is unavailable. Prepared local
+callbacks expose the execution ID from their validated Server admission.
+Inline local and Server-free callbacks without that admission report null.
+Session activity callbacks expose the field when their actual claim includes
+it; omission also produces null. The property is read-only and performs no
+additional runtime request.
+
 PHP SDK 3.6.0 introduced retry metadata on `ActivityContext`. Use 3.6.1 or later
 for the original owning workflow and run IDs from public activity claims.
 `$context->retryInfo` retains the original attempt number and retry policy,
diff --git a/docs/sdk-reference.md b/docs/sdk-reference.md
index 988f6367..3391bcaa 100644
--- a/docs/sdk-reference.md
+++ b/docs/sdk-reference.md
@@ -295,6 +295,18 @@ the supported runtime URL forms, role-specific environment variables,
 package-owned source files, and expected result. Documentation builds verify
 that the visible examples still match those runnable files.
 
+## Activity execution identity
+
+The unreleased `ActivityContext::activityExecutionId` is the nullable logical
+execution identity supplied by an activity claim. `workflowId` and `runId`
+identify its owning workflow/run; `activityAttemptId` remains attempt-specific.
+Use a caller-owned business key when logical identity is unavailable. The SDK
+preserves received ID spelling and never manufactures an execution ID from a
+task, attempt or session ID. Prepared local callbacks expose the validated
+Server execution ID; inline local/Server-free callbacks without one report
+null. Session callbacks use the actual claim field when present. The same
+accessor is used in ordinary and cooperative remote execution.
+
 ## Workflow handles and control-plane APIs
 
 `WorkflowHandle` distinguishes the stable workflow instance from a selected
diff --git a/src/Worker.php b/src/Worker.php
index e40f067c..9be7f4e0 100644
--- a/src/Worker.php
+++ b/src/Worker.php
@@ -2337,7 +2337,8 @@ function (RequestBudget $budget): void { $this->heartbeatIfDue($budget); },
             }
             return $this->invokeActivity($handler, new ActivityContext($this->client, $attempt->taskId, $attempt->attemptId,
                 $attempt->leaseOwner, $activityType, $attempt->attemptNumber, localHeartbeat: $heartbeat,
-                taskMetadata: ['workflow_instance_id' => $task['workflow_id'] ?? null, 'workflow_run_id' => $task['run_id'] ?? null]), $arguments,
+                taskMetadata: ['workflow_instance_id' => $task['workflow_id'] ?? null, 'workflow_run_id' => $task['run_id'] ?? null,
+                    'activity_execution_id' => $attempt->executionId]), $arguments,
                 ['activity_id' => $attempt->executionId] + ExecutionTelemetry::taskFields($task, $this->client->namespace, $this->taskQueue), true);
         };
 
diff --git a/src/Worker/ActivityContext.php b/src/Worker/ActivityContext.php
index ca0471b8..b38c56e4 100644
--- a/src/Worker/ActivityContext.php
+++ b/src/Worker/ActivityContext.php
@@ -16,6 +16,8 @@ final class ActivityContext
     public readonly ?string $workflowId;
     /** Original run_id from the public claim; unavailable legacy identity stays null. */
     public readonly ?string $runId;
+    /** Logical execution from the claim, never inferred from task/attempt identity. */
+    public readonly ?string $activityExecutionId;
 
     /**
      * @param (Closure(array<array-key, mixed>): array<string, mixed>)|null $heartbeatRequest
@@ -39,6 +41,7 @@ public function __construct(
             ? $taskMetadata['workflow_id'] : ($taskMetadata['workflow_instance_id'] ?? null));
         $this->runId = self::receivedIdentity(array_key_exists('run_id', $taskMetadata)
             ? $taskMetadata['run_id'] : ($taskMetadata['workflow_run_id'] ?? null));
+        $this->activityExecutionId = self::receivedIdentity($taskMetadata['activity_execution_id'] ?? null);
     }
 
     private static function receivedIdentity(mixed $value): ?string
diff --git a/tests/ActivityExecutionIdentityTest.php b/tests/ActivityExecutionIdentityTest.php
new file mode 100644
index 00000000..ef3e091b
--- /dev/null
+++ b/tests/ActivityExecutionIdentityTest.php
@@ -0,0 +1,83 @@
+<?php
+
+declare(strict_types=1);
+
+namespace DurableWorkflow\Tests;
+
+use DurableWorkflow\Client;
+use DurableWorkflow\Tests\Support\FakeTransport;
+use DurableWorkflow\Worker;
+use DurableWorkflow\Worker\ActivityContext;
+use PHPUnit\Framework\Attributes\DataProvider;
+use PHPUnit\Framework\TestCase;
+use RuntimeException;
+
+final class ActivityExecutionIdentityTest extends TestCase
+{
+    /** @return iterable<string, array{array<string, mixed>, ?string}> */
+    public static function receivedIdentities(): iterable
+    {
+        yield 'canonical identity' => [['activity_execution_id' => 'logical-execution'], 'logical-execution'];
+        yield 'opaque spelling' => [['activity_execution_id' => '01AbC'], '01AbC'];
+        yield 'zero string' => [['activity_execution_id' => '0'], '0'];
+        yield 'legacy omission' => [[], null];
+        yield 'explicit null' => [['activity_execution_id' => null, 'execution_id' => 'unrelated'], null];
+        yield 'empty' => [['activity_execution_id' => ''], null];
+        yield 'numeric' => [['activity_execution_id' => 42], null];
+        yield 'boolean' => [['activity_execution_id' => true], null];
+        yield 'unrelated aliases' => [['execution_id' => 'other', 'task_id' => 'task', 'activity_attempt_id' => 'attempt'], null];
+    }
+
+    #[DataProvider('receivedIdentities')]
+    public function testOnlyTheReceivedLogicalIdentityIsExposed(array $metadata, ?string $expected): void
+    {
+        $context = new ActivityContext(new Client('https://server.example'), 'task', 'attempt', 'owner', 'activity', 1,
+            taskMetadata: $metadata);
+        self::assertSame($expected, $context->activityExecutionId);
+        self::assertSame('task', $context->taskId);
+        self::assertSame('attempt', $context->activityAttemptId);
+    }
+
+    public function testOldConstructorsRemainValidWithoutInventingIdentity(): void
+    {
+        $context = new ActivityContext(new Client('https://server.example'), 'task', 'attempt', 'owner', 'activity', 1);
+        self::assertNull($context->activityExecutionId);
+    }
+
+    public function testReceivedRetryClaimsKeepLogicalIdentityAndDifferentAttempts(): void
+    {
+        $polls = 0;
+        $failures = 0;
+        $completions = 0;
+        $seen = [];
+        $transport = new FakeTransport(handler: static function (string $method, string $uri) use (&$polls, &$failures, &$completions): array {
+            if (str_ends_with($uri, '/worker/register')) { return ['registered' => true]; }
+            if (str_ends_with($uri, '/activity-tasks/poll')) {
+                ++$polls;
+                return ['task' => ['task_id' => 'task-'.$polls, 'activity_attempt_id' => 'attempt-'.$polls,
+                    'activity_execution_id' => 'logical-execution', 'workflow_id' => 'original-workflow', 'run_id' => 'original-run',
+                    'lease_owner' => 'owner', 'activity_type' => 'identity', 'attempt_number' => $polls, 'payload_codec' => 'avro']];
+            }
+            if (str_ends_with($uri, '/fail')) { ++$failures; return ['failed' => true]; }
+            if (str_ends_with($uri, '/complete')) { ++$completions; return ['completed' => true]; }
+            if (str_ends_with($uri, '/poll')) { return ['task' => null]; }
+            self::fail('Unexpected request: '.$method.' '.$uri);
+        });
+        $worker = new Worker(new Client('https://server.example', transport: $transport), 'queue', workerId: 'owner');
+        $worker->registerActivity('identity', static function (ActivityContext $context) use (&$seen): string {
+            $seen[] = [$context->activityExecutionId, $context->activityAttemptId, $context->workflowId, $context->runId];
+            if ($context->attemptNumber === 1) {
+                throw new RuntimeException('Retry the received activity.');
+            }
+            return 'done';
+        });
+        self::assertTrue($worker->tick(0));
+        self::assertTrue($worker->tick(0));
+        self::assertSame([
+            ['logical-execution', 'attempt-1', 'original-workflow', 'original-run'],
+            ['logical-execution', 'attempt-2', 'original-workflow', 'original-run'],
+        ], $seen);
+        self::assertSame(1, $failures);
+        self::assertSame(1, $completions);
+    }
+}
diff --git a/tests/ActivityRetryInfoTest.php b/tests/ActivityRetryInfoTest.php
index 1fa67ac3..4d96e90b 100644
--- a/tests/ActivityRetryInfoTest.php
+++ b/tests/ActivityRetryInfoTest.php
@@ -67,6 +67,7 @@ public function testLegacyConstructorAndServerFreeActivitiesHaveUnknownMetadata(
         self::assertNull($context->retryInfo->isLastConfiguredAttempt());
         self::assertNull($context->workflowId);
         self::assertNull($context->runId);
+        self::assertNull($context->activityExecutionId);
     }
 
     public static function originalIdentities(): array
@@ -123,13 +124,15 @@ public function testInlineLocalActivityRetainsParentIdentityWithoutInventingRetr
         $worker->registerWorkflow('flow', static fn (WorkflowContext $context): mixed => $context->localActivity('metadata'));
         $worker->registerActivity('metadata', static fn (ActivityContext $context): array => [
             'workflow_id' => $context->workflowId, 'run_id' => $context->runId,
+            'activity_execution_id' => $context->activityExecutionId,
             'provided' => $context->retryInfo->policyProvided, 'last' => $context->retryInfo->isLastConfiguredAttempt(),
         ]);
         self::assertTrue($worker->tick(0));
         self::assertNotNull($completion);
         $finished = array_values(array_filter($completion['commands'], static fn (array $command): bool => $command['type'] === 'complete_workflow'));
         self::assertCount(1, $finished);
-        self::assertSame(['workflow_id' => 'original-workflow', 'run_id' => 'original-run', 'provided' => false, 'last' => null],
+        self::assertSame(['workflow_id' => 'original-workflow', 'run_id' => 'original-run', 'activity_execution_id' => null,
+            'provided' => false, 'last' => null],
             (new AvroPayloadCodec())->decodeEnvelope($finished[0]['result']));
     }
 
diff --git a/tests/PreparedLocalActivityWorkerTest.php b/tests/PreparedLocalActivityWorkerTest.php
index 5f09e1dd..65fec2b6 100644
--- a/tests/PreparedLocalActivityWorkerTest.php
+++ b/tests/PreparedLocalActivityWorkerTest.php
@@ -47,6 +47,7 @@ public function test_prepared_activity_retains_parent_ids_with_unknown_server_re
         $worker->registerWorkflow('prepared', static fn (WorkflowContext $context): mixed => $context->localActivity('effect'));
         $worker->registerActivity('effect', static fn (ActivityContext $context): array => [
             'workflow_id' => $context->workflowId, 'run_id' => $context->runId,
+            'activity_execution_id' => $context->activityExecutionId,
             'provided' => $context->retryInfo->policyProvided, 'last' => $context->retryInfo->isLastConfiguredAttempt(),
         ]);
         $worker->run(0);
@@ -54,7 +55,8 @@ public function test_prepared_activity_retains_parent_ids_with_unknown_server_re
         self::assertCount(1, $transport->operations('outcome'));
         $command = $transport->completions[0]['commands'][0];
         self::assertSame('complete_workflow', $command['type']);
-        self::assertSame(['workflow_id' => 'workflow', 'run_id' => 'run', 'provided' => false, 'last' => null],
+        self::assertSame(['workflow_id' => 'workflow', 'run_id' => 'run', 'activity_execution_id' => 'backend-execution-1',
+            'provided' => false, 'last' => null],
             (new AvroPayloadCodec())->decodeEnvelope($command['result']));
     }