temporalio · donald-pinckney · Apr 23, 2026 · Apr 21, 2026 · Apr 22, 2026 · Apr 22, 2026
@@ -114,6 +114,22 @@ public class MyTools {
 
 Auto-detected and executed as Nexus operations, similar to activity stubs.
 
+## Media in messages
+
+If you attach media (images, audio, etc.) to a `UserMessage` or an `AssistantMessage`, prefer passing it by URI rather than raw bytes:
+
+```java
+// Good — only the URL crosses the activity boundary.
+Media image = new Media(MimeTypeUtils.IMAGE_PNG, URI.create("https://cdn.example.com/pic.png"));
+
+// Works, but size-limited — see below.
+Media image = new Media(MimeTypeUtils.IMAGE_PNG, new ByteArrayResource(bytes));
+```
+
+Raw `byte[]` media gets serialized into every chat activity's input *and* result payload, which end up inside Temporal workflow history events. Server-side history events have a fixed 2 MiB size limit; to leave headroom for messages, tool definitions, and options, the plugin enforces a **1 MiB default cap** on inline media bytes and fails fast with an `IllegalArgumentException` pointing you at the URI alternative.
+
+Override the cap by setting the system property `io.temporal.springai.maxMediaBytes` before your worker starts (pass a positive integer; `0` disables the check). For anything larger than a small thumbnail, the URI route is the right answer — have an activity write the bytes to blob storage, then pass only the URL into the conversation.
+
 ## Optional Integrations
 
 Auto-configured when their dependencies are on the classpath:

@@ -239,6 +239,7 @@ private ChatModelTypes.MediaContent fromMedia(Media media) {
     if (media.getData() instanceof String uri) {
       return new ChatModelTypes.MediaContent(mimeType, uri);
     } else if (media.getData() instanceof byte[] data) {
+      ChatModelTypes.checkMediaSize(data);
       return new ChatModelTypes.MediaContent(mimeType, data);
     }
     throw new IllegalArgumentException(

@@ -372,6 +372,7 @@ private ChatModelTypes.MediaContent toMediaContent(Media media) {
     if (media.getData() instanceof String uri) {
       return new ChatModelTypes.MediaContent(mimeType, uri);
     } else if (media.getData() instanceof byte[] data) {
+      ChatModelTypes.checkMediaSize(data);
       return new ChatModelTypes.MediaContent(mimeType, data);
     }
     throw new IllegalArgumentException(

@@ -4,6 +4,7 @@
 import com.fasterxml.jackson.annotation.JsonIgnoreProperties;
 import com.fasterxml.jackson.annotation.JsonInclude;
 import com.fasterxml.jackson.annotation.JsonProperty;
+import io.temporal.failure.ApplicationFailure;
 import java.time.Duration;
 import java.util.List;
 import javax.annotation.Nullable;
@@ -16,6 +17,50 @@
  */
 public final class ChatModelTypes {
 
+  /**
+   * Maximum size, in bytes, of a single {@link MediaContent#data()} byte array carried across the
+   * chat activity boundary. Bytes above this threshold land inside workflow history events, which
+   * have a fixed 2 MiB per-event limit on the Temporal server. 1 MiB leaves headroom for the rest
+   * of a chat payload (messages, tool definitions, options).
+   *
+   * <p>Users who want to raise or lower the cap can set the system property {@code
+   * io.temporal.springai.maxMediaBytes} to a positive integer before the chat activity runs; values
+   * &lt;= 0 disable the guard entirely. For most workloads, pass media by URI instead — write the
+   * bytes to a binary store from an activity, and pass only the URL across the conversation.
+   */
+  public static final long MAX_MEDIA_BYTES_IN_HISTORY =
+      Long.getLong("io.temporal.springai.maxMediaBytes", 1L * 1024 * 1024);
+
+  /** Failure type on the {@link ApplicationFailure} thrown by {@link #checkMediaSize(byte[])}. */
+  public static final String MEDIA_SIZE_EXCEEDED_FAILURE_TYPE = "MediaSizeExceeded";
+
+  /**
+   * Throws a non-retryable {@link ApplicationFailure} if {@code data} exceeds {@link
+   * #MAX_MEDIA_BYTES_IN_HISTORY}. Non-retryable because this is a permanent, programmer-level error
+   * — retrying the same oversized payload will never succeed, and using a plain {@link
+   * RuntimeException} here would cause the workflow task to be retried forever (or the activity to
+   * churn through its {@code maxAttempts}) rather than surfacing the real problem. The failure
+   * message points the caller at the URI-based {@code Media} constructor. Pass-through otherwise.
+   */
+  public static void checkMediaSize(byte[] data) {
+    if (data == null) {
+      return;
+    }
+    long limit = MAX_MEDIA_BYTES_IN_HISTORY;
+    if (limit > 0 && data.length > limit) {
+      throw ApplicationFailure.newNonRetryableFailure(
+          "Media byte[] is "
+              + data.length
+              + " bytes, which exceeds the "
+              + limit
+              + "-byte limit for inline media in Temporal workflow history. Pass the media by "
+              + "URI instead: store the bytes outside the workflow (e.g. S3) and construct "
+              + "Media(mimeType, URI). Set the system property "
+              + "'io.temporal.springai.maxMediaBytes' to override this limit (or 0 to disable).",
+          MEDIA_SIZE_EXCEEDED_FAILURE_TYPE);
+    }
+  }
+
   private ChatModelTypes() {}
 
   /**

@@ -0,0 +1,249 @@
+package io.temporal.springai;
+
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import io.temporal.client.WorkflowClient;
+import io.temporal.client.WorkflowException;
+import io.temporal.client.WorkflowOptions;
+import io.temporal.failure.ApplicationFailure;
+import io.temporal.springai.activity.ChatModelActivityImpl;
+import io.temporal.springai.model.ActivityChatModel;
+import io.temporal.springai.model.ChatModelTypes;
+import io.temporal.testing.TestWorkflowEnvironment;
+import io.temporal.worker.Worker;
+import io.temporal.workflow.WorkflowInterface;
+import io.temporal.workflow.WorkflowMethod;
+import java.net.URI;
+import java.util.List;
+import org.junit.jupiter.api.AfterEach;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+import org.springframework.ai.chat.messages.AssistantMessage;
+import org.springframework.ai.chat.messages.UserMessage;
+import org.springframework.ai.chat.model.ChatModel;
+import org.springframework.ai.chat.model.ChatResponse;
+import org.springframework.ai.chat.model.Generation;
+import org.springframework.ai.chat.prompt.Prompt;
+import org.springframework.ai.content.Media;
+import org.springframework.core.io.ByteArrayResource;
+import org.springframework.util.MimeType;
+import org.springframework.util.MimeTypeUtils;
+
+/**
+ * Unit tests around {@link ChatModelTypes#checkMediaSize(byte[])} plus integration-style tests
+ * against a live TestWorkflowEnvironment to make sure the guard fires on both the inbound (workflow
+ * → activity) and outbound (activity → workflow) conversion paths.
+ */
+class MediaSizeGuardTest {
+
+  private static final String TASK_QUEUE = "test-spring-ai-media-size-guard";
+
+  private TestWorkflowEnvironment testEnv;
+  private WorkflowClient client;
+
+  @BeforeEach
+  void setUp() {
+    testEnv = TestWorkflowEnvironment.newInstance();
+    client = testEnv.getWorkflowClient();
+  }
+
+  @AfterEach
+  void tearDown() {
+    testEnv.close();
+  }
+
+  @Test
+  void checkMediaSize_smallPayload_passes() {
+    byte[] small = new byte[500 * 1024]; // 500 KiB, well under 1 MiB
+    assertDoesNotThrow(() -> ChatModelTypes.checkMediaSize(small));
+  }
+
+  @Test
+  void checkMediaSize_oversizedPayload_throwsNonRetryableApplicationFailure() {
+    byte[] big = new byte[(int) ChatModelTypes.MAX_MEDIA_BYTES_IN_HISTORY + 1];
+    ApplicationFailure ex =
+        assertThrows(ApplicationFailure.class, () -> ChatModelTypes.checkMediaSize(big));
+    assertTrue(ex.isNonRetryable(), "guard must throw a non-retryable ApplicationFailure");
+    assertEquals(ChatModelTypes.MEDIA_SIZE_EXCEEDED_FAILURE_TYPE, ex.getType());
+    String msg = ex.getOriginalMessage();
+    assertTrue(msg.contains("URI"), "message should point at the URI alternative: " + msg);
+    assertTrue(
+        msg.contains("io.temporal.springai.maxMediaBytes"),
+        "message should mention the override system property: " + msg);
+  }
+
+  @Test
+  void checkMediaSize_null_passes() {
+    assertDoesNotThrow(() -> ChatModelTypes.checkMediaSize(null));
+  }
+
+  @Test
+  void inboundPath_oversizedUserMessageMedia_failsTheWorkflow() {
+    // Workflow → activity direction: the workflow builds a Prompt with a huge byte[] media,
+    // ActivityChatModel.createActivityInput calls toMediaContent → checkMediaSize throws.
+    Worker worker = testEnv.newWorker(TASK_QUEUE);
+    worker.registerWorkflowImplementationTypes(BigInboundMediaWorkflowImpl.class);
+    worker.registerActivitiesImplementations(new ChatModelActivityImpl(new StubChatModel()));
+    testEnv.start();
+
+    ChatWorkflow workflow =
+        client.newWorkflowStub(
+            ChatWorkflow.class, WorkflowOptions.newBuilder().setTaskQueue(TASK_QUEUE).build());
+    WorkflowException ex = assertThrows(WorkflowException.class, () -> workflow.chat("hi"));
+    String message = rootMessage(ex);
+    assertTrue(
+        message.contains(ChatModelTypes.MEDIA_SIZE_EXCEEDED_FAILURE_TYPE)
+            || message.contains("-byte limit"),
+        "expected size-guard failure, got: " + message);
+  }
+
+  @Test
+  void inboundPath_smallMedia_passes() {
+    Worker worker = testEnv.newWorker(TASK_QUEUE);
+    worker.registerWorkflowImplementationTypes(SmallInboundMediaWorkflowImpl.class);
+    worker.registerActivitiesImplementations(new ChatModelActivityImpl(new StubChatModel()));
+    testEnv.start();
+
+    ChatWorkflow workflow =
+        client.newWorkflowStub(
+            ChatWorkflow.class, WorkflowOptions.newBuilder().setTaskQueue(TASK_QUEUE).build());
+    assertEquals("pong", workflow.chat("hi"));
+  }
+
+  @Test
+  void inboundPath_uriMedia_passes_regardlessOfSize() {
+    // URI-based media is not subject to the byte[] guard — bytes stay out of workflow history.
+    Worker worker = testEnv.newWorker(TASK_QUEUE);
+    worker.registerWorkflowImplementationTypes(UriMediaWorkflowImpl.class);
+    worker.registerActivitiesImplementations(new ChatModelActivityImpl(new StubChatModel()));
+    testEnv.start();
+
+    ChatWorkflow workflow =
+        client.newWorkflowStub(
+            ChatWorkflow.class, WorkflowOptions.newBuilder().setTaskQueue(TASK_QUEUE).build());
+    assertEquals("pong", workflow.chat("hi"));
+  }
+
+  @Test
+  void outboundPath_assistantEchoesOversizedMedia_failsTheActivity() {
+    // Activity → workflow direction: the stub ChatModel returns an assistant message with a
+    // huge byte[] media, ChatModelActivityImpl.fromMedia → checkMediaSize throws.
+    Worker worker = testEnv.newWorker(TASK_QUEUE);
+    worker.registerWorkflowImplementationTypes(EchoMediaWorkflowImpl.class);
+    worker.registerActivitiesImplementations(
+        new ChatModelActivityImpl(new BigOutboundMediaChatModel()));
+    testEnv.start();
+
+    ChatWorkflow workflow =
+        client.newWorkflowStub(
+            ChatWorkflow.class, WorkflowOptions.newBuilder().setTaskQueue(TASK_QUEUE).build());
+    WorkflowException ex = assertThrows(WorkflowException.class, () -> workflow.chat("hi"));
+    String message = rootMessage(ex);
+    assertTrue(
+        message.contains("exceeds the") && message.contains("-byte limit"),
+        "expected size-guard failure on return path, got: " + message);
+  }
+
+  private static String rootMessage(Throwable t) {
+    Throwable cur = t;
+    while (cur.getCause() != null) {
+      cur = cur.getCause();
+    }
+    return cur.getMessage() == null ? "" : cur.getMessage();
+  }
+
+  @WorkflowInterface
+  public interface ChatWorkflow {
+    @WorkflowMethod
+    String chat(String message);
+  }
+
+  public static class BigInboundMediaWorkflowImpl implements ChatWorkflow {
+    @Override
+    public String chat(String message) {
+      byte[] big = new byte[(int) ChatModelTypes.MAX_MEDIA_BYTES_IN_HISTORY + 1];
+      UserMessage userMessage =
+          UserMessage.builder()
+              .text(message)
+              .media(List.of(new Media(MimeTypeUtils.IMAGE_PNG, new ByteArrayResource(big))))
+              .build();
+      ActivityChatModel chatModel = ActivityChatModel.forDefault();
+      return chatModel.call(new Prompt(List.of(userMessage))).getResult().getOutput().getText();
+    }
+  }
+
+  public static class SmallInboundMediaWorkflowImpl implements ChatWorkflow {
+    @Override
+    public String chat(String message) {
+      byte[] small = new byte[16 * 1024]; // 16 KiB
+      UserMessage userMessage =
+          UserMessage.builder()
+              .text(message)
+              .media(List.of(new Media(MimeTypeUtils.IMAGE_PNG, new ByteArrayResource(small))))
+              .build();
+      ActivityChatModel chatModel = ActivityChatModel.forDefault();
+      return chatModel.call(new Prompt(List.of(userMessage))).getResult().getOutput().getText();
+    }
+  }
+
+  public static class UriMediaWorkflowImpl implements ChatWorkflow {
+    @Override
+    public String chat(String message) {
+      UserMessage userMessage =
+          UserMessage.builder()
+              .text(message)
+              .media(
+                  List.of(
+                      new Media(
+                          MimeTypeUtils.IMAGE_PNG, URI.create("https://cdn.example.com/huge.png"))))
+              .build();
+      ActivityChatModel chatModel = ActivityChatModel.forDefault();
+      return chatModel.call(new Prompt(List.of(userMessage))).getResult().getOutput().getText();
+    }
+  }
+
+  public static class EchoMediaWorkflowImpl implements ChatWorkflow {
+    @Override
+    public String chat(String message) {
+      ActivityChatModel chatModel = ActivityChatModel.forDefault();
+      return chatModel.call(new Prompt(message)).getResult().getOutput().getText();
+    }
+  }
+
+  /** Returns "pong" — used to verify non-failing paths. */
+  private static class StubChatModel implements ChatModel {
+    @Override
+    public ChatResponse call(Prompt prompt) {
+      return ChatResponse.builder()
+          .generations(List.of(new Generation(new AssistantMessage("pong"))))
+          .build();
+    }
+
+    @Override
+    public reactor.core.publisher.Flux<ChatResponse> stream(Prompt prompt) {
+      throw new UnsupportedOperationException();
+    }
+  }
+
+  /** Returns an assistant message carrying a huge byte[] media, to trip the outbound guard. */
+  private static class BigOutboundMediaChatModel implements ChatModel {
+    @Override
+    public ChatResponse call(Prompt prompt) {
+      byte[] big = new byte[(int) ChatModelTypes.MAX_MEDIA_BYTES_IN_HISTORY + 1];
+      AssistantMessage assistant =
+          AssistantMessage.builder()
+              .content("")
+              .media(List.of(new Media(MimeType.valueOf("image/png"), new ByteArrayResource(big))))
+              .build();
+      return ChatResponse.builder().generations(List.of(new Generation(assistant))).build();
+    }
+
+    @Override
+    public reactor.core.publisher.Flux<ChatResponse> stream(Prompt prompt) {
+      throw new UnsupportedOperationException();
+    }
+  }
+}