fix(sessions): keep inlineData mimeType when encoding or decoding content

core/src/main/java/com/google/adk/sessions/SessionUtils.java

@@ -45,7 +45,9 @@ public static Content encodeContent(Content content) {
             byte[] dataBytes = dataOptional.get();
             byte[] encodedData = Base64.getEncoder().encode(dataBytes);
             encodedParts.add(
-                part.toBuilder().inlineData(Blob.builder().data(encodedData).build()).build());
+                part.toBuilder()
+                    .inlineData(inlineDataBlob.toBuilder().data(encodedData).build())
+                    .build());
             isInlineDataPresent = true;
           }
         }
@@ -71,7 +73,9 @@ public static Content decodeContent(Content content) {
             byte[] dataBytes = dataOptional.get();
             byte[] decodedData = Base64.getDecoder().decode(dataBytes);
             decodedParts.add(
-                part.toBuilder().inlineData(Blob.builder().data(decodedData).build()).build());
+                part.toBuilder()
+                    .inlineData(inlineDataBlob.toBuilder().data(decodedData).build())
+                    .build());
             isInlineDataPresent = true;
           }
         }

core/src/test/java/com/google/adk/sessions/SessionUtilsTest.java

@@ -0,0 +1,93 @@
+/*
+ * Copyright 2026 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.google.adk.sessions;
+
+import static com.google.common.truth.Truth.assertThat;
+
+import com.fasterxml.jackson.databind.JsonNode;
+import com.fasterxml.jackson.databind.ObjectMapper;
+import com.google.adk.JsonBaseModel;
+import com.google.adk.events.Event;
+import com.google.genai.types.Blob;
+import com.google.genai.types.Content;
+import com.google.genai.types.Part;
+import java.nio.charset.StandardCharsets;
+import java.time.Instant;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.junit.runners.JUnit4;
+
+@RunWith(JUnit4.class)
+public class SessionUtilsTest {
+
+  private static final ObjectMapper objectMapper = JsonBaseModel.getMapper();
+  private static final byte[] PAYLOAD = "payload".getBytes(StandardCharsets.UTF_8);
+
+  private static Content contentWithInlineData() {
+    return Content.builder()
+        .role("user")
+        .parts(
+            Part.builder()
+                .inlineData(Blob.builder().mimeType("image/png").data(PAYLOAD).build())
+                .build())
+        .build();
+  }
+
+  @Test
+  public void encodeContent_inlineData_preservesMimeType() {
+    Content encoded = SessionUtils.encodeContent(contentWithInlineData());
+
+    Blob encodedBlob = encoded.parts().get().get(0).inlineData().get();
+    assertThat(encodedBlob.mimeType()).hasValue("image/png");
+  }
+
+  @Test
+  public void decodeContent_inlineData_preservesMimeType() {
+    Content decoded = SessionUtils.decodeContent(contentWithInlineData());
+
+    Blob decodedBlob = decoded.parts().get().get(0).inlineData().get();
+    assertThat(decodedBlob.mimeType()).hasValue("image/png");
+  }
+
+  @Test
+  public void decodeContent_encodeContent_roundTripsInlineData() {
+    Content roundTripped =
+        SessionUtils.decodeContent(SessionUtils.encodeContent(contentWithInlineData()));
+
+    Blob blob = roundTripped.parts().get().get(0).inlineData().get();
+    assertThat(blob.mimeType()).hasValue("image/png");
+    assertThat(blob.data().get()).isEqualTo(PAYLOAD);
+  }
+
+  /** The MIME type has to survive the JSON the Vertex AI Sessions API stores the event in. */
+  @Test
+  public void convertEventToJson_inlineData_persistsMimeType() throws Exception {
+    Event event =
+        Event.builder()
+            .author("user")
+            .invocationId("inv-123")
+            .timestamp(Instant.parse("2023-01-01T00:00:00Z").toEpochMilli())
+            .content(contentWithInlineData())
+            .build();
+
+    JsonNode jsonNode = objectMapper.readTree(SessionJsonConverter.convertEventToJson(event));
+
+    JsonNode inlineData = jsonNode.get("content").get("parts").get(0).get("inlineData");
+    assertThat(inlineData.has("mimeType")).isTrue();
+    assertThat(inlineData.get("mimeType").asText()).isEqualTo("image/png");
+  }
+}