fix: ResponseStream fails to parse error message and multi-line SSE data payloads

src/main/java/com/google/genai/ReplayApiClient.java

@@ -135,7 +135,13 @@ public ApiResponse request(
       matchRequest(
           currentInteraction.request().orElse(null),
           buildRequest(httpMethod, path, requestJson, httpOptions));
-      return buildResponseFromReplay(currentInteraction.response().orElse(null));
+      boolean isStream =
+          currentInteraction
+              .request()
+              .flatMap(r -> r.url())
+              .map(u -> u.contains("streamGenerateContent"))
+              .orElse(false);
+      return buildResponseFromReplay(currentInteraction.response().orElse(null), isStream);
     } else {
       // Note that if the client mode is "api", then the ReplayApiClient will not be used.
       throw new IllegalArgumentException("Invalid client mode: " + this.clientMode);
@@ -227,15 +233,16 @@ private void matchRequest(ReplayRequest replayRequest, Request actualRequest) {
   }
 
   /** Builds the response from a {@link ReplayResponse}. */
-  private ReplayApiResponse buildResponseFromReplay(ReplayResponse replayResponse) {
+  private ReplayApiResponse buildResponseFromReplay(
+      ReplayResponse replayResponse, boolean isStream) {
     if (replayResponse == null) {
       throw new IllegalArgumentException("Replay response is null.");
     }
     JsonNode bodyNode =
         JsonSerializable.toJsonNode(replayResponse.bodySegments().orElse(new ArrayList<>()));
     Headers headers = Headers.of(replayResponse.headers().orElse(ImmutableMap.of()));
     return new ReplayApiResponse(
-        (ArrayNode) bodyNode, replayResponse.statusCode().orElse(0), headers);
+        (ArrayNode) bodyNode, replayResponse.statusCode().orElse(0), headers, isStream);
   }
 
   private static String formatUrl(String url) {

src/main/java/com/google/genai/ReplayApiResponse.java

@@ -38,32 +38,35 @@ public final class ReplayApiResponse extends ApiResponse {
   private final Headers headers;
   private final ArrayNode bodySegments;
 
-  public ReplayApiResponse(ArrayNode bodySegments, int statusCode, Headers headers) {
+  public ReplayApiResponse(
+      ArrayNode bodySegments, int statusCode, Headers headers, boolean isStream) {
     this.bodySegments = bodySegments;
     this.statusCode = statusCode;
     this.headers = headers;
     if (bodySegments.size() == 0) {
       this.body = ResponseBody.create(MediaType.parse("application/json"), "");
-    } else if (bodySegments.size() == 1) {
-      // For unary response
-      this.body =
-          ResponseBody.create(
-              JsonSerializable.toJsonString(bodySegments.get(0)),
-              MediaType.parse("application/json"));
-    } else {
+    } else if (isStream || bodySegments.size() > 1) {
       // For streaming response
       try {
         ByteArrayOutputStream outputStream = new ByteArrayOutputStream();
-        byte[] newline = "\n".getBytes(StandardCharsets.UTF_8);
+        byte[] dataPrefix = "data: ".getBytes(StandardCharsets.UTF_8);
+        byte[] doubleNewline = "\n\n".getBytes(StandardCharsets.UTF_8);
         for (JsonNode segment : bodySegments) {
+          outputStream.write(dataPrefix);
           outputStream.write(JsonSerializable.objectMapper.writeValueAsBytes(segment));
-          outputStream.write(newline);
+          outputStream.write(doubleNewline);
         }
         this.body =
             ResponseBody.create(outputStream.toByteArray(), MediaType.parse("application/json"));
       } catch (IOException e) {
         throw new GenAiIOException("Failed to convert body segments to a JSON string.", e);
       }
+    } else {
+      // For unary response
+      this.body =
+          ResponseBody.create(
+              JsonSerializable.toJsonString(bodySegments.get(0)),
+              MediaType.parse("application/json"));
     }
   }
 

src/main/java/com/google/genai/ResponseStream.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2025 Google LLC
+ * 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.
@@ -17,7 +17,9 @@
 package com.google.genai;
 
 import com.fasterxml.jackson.databind.JsonNode;
+import com.fasterxml.jackson.databind.node.ArrayNode;
 import com.fasterxml.jackson.databind.node.ObjectNode;
+import com.google.genai.errors.ApiException;
 import com.google.genai.errors.GenAiIOException;
 import java.io.BufferedReader;
 import java.io.IOException;
@@ -32,6 +34,7 @@
 import java.util.NoSuchElementException;
 import java.util.logging.Logger;
 import okhttp3.Headers;
+import org.jspecify.annotations.Nullable;
 
 /** An iterable of datatype objects. */
 public class ResponseStream<T extends JsonSerializable> implements Iterable<T>, AutoCloseable {
@@ -114,6 +117,18 @@ public T next() {
       nextJson = readNextJson();
       try {
         JsonNode currentJsonNode = JsonSerializable.stringToJsonNode(currentJson);
+
+        if (currentJsonNode.isObject() && currentJsonNode.has("error")) {
+          int extractedCode = 500;
+          JsonNode errorNode = currentJsonNode.get("error");
+          if (errorNode.has("code") && errorNode.get("code").isInt()) {
+            extractedCode = errorNode.get("code").asInt();
+          }
+          ArrayNode arrayNode = JsonSerializable.objectMapper.createArrayNode();
+          arrayNode.add(currentJsonNode);
+          ApiException.throwFromErrorNode(arrayNode, extractedCode);
+        }
+
         if (responseHeaders != null && currentJsonNode.isObject()) {
           ObjectNode rootNode = (ObjectNode) currentJsonNode;
           ObjectNode headersNode = JsonSerializable.objectMapper.createObjectNode();
@@ -142,23 +157,47 @@ public T next() {
       }
     }
 
-    private String readNextJson() {
+    private @Nullable String readNextJson() {
       // Streaming API returns in the following format:
       // data: {contents: ...}
       // \n
       // data: {contents: ...}
       // \n
       // ...
+      List<String> dataBuffer = new ArrayList<>();
       try {
-        String line = reader.readLine();
-        if (line == null) {
-          return null;
-        } else if (line.length() == 0) {
-          return readNextJson();
-        } else if (line.startsWith("data: ")) {
-          return line.substring("data: ".length());
-        } else {
-          return line;
+        while (true) {
+          String line = reader.readLine();
+          if (line == null) {
+            if (!dataBuffer.isEmpty()) {
+              return String.join("\n", dataBuffer);
+            }
+            return null;
+          }
+          if (line.isEmpty()) {
+            if (!dataBuffer.isEmpty()) {
+              // Handle multi-line SSE data
+              return String.join("\n", dataBuffer);
+            }
+            continue;
+          }
+          if (line.startsWith(":")) {
+            continue;
+          }
+          int colonIndex = line.indexOf(':');
+          String fieldname = line;
+          String value = "";
+          if (colonIndex != -1) {
+            fieldname = line.substring(0, colonIndex);
+            value = line.substring(colonIndex + 1);
+            if (value.startsWith(" ")) {
+              value = value.substring(1);
+            }
+          }
+
+          if (fieldname.equals("data")) {
+            dataBuffer.add(value);
+          }
         }
       } catch (IOException e) {
         throw new GenAiIOException("Failed to read next JSON object from the stream", e);

src/test/java/com/google/genai/AsyncChatTest.java

@@ -96,8 +96,16 @@ public class AsyncChatTest {
   String jsonChunk3 = responseChunk3.toJson();
 
   String streamData =
-      "data: " + jsonChunk1 + "\n" + "data: " + jsonChunk2 + "\n" + "data: " + jsonChunk3 + "\n";
-  String streamData2 = "data: " + jsonChunk1 + "\n" + "data: " + jsonChunk2 + "\n";
+      "data: "
+          + jsonChunk1
+          + "\n\n"
+          + "data: "
+          + jsonChunk2
+          + "\n\n"
+          + "data: "
+          + jsonChunk3
+          + "\n\n";
+  String streamData2 = "data: " + jsonChunk1 + "\n\n" + "data: " + jsonChunk2 + "\n\n";
 
   GenerateContentResponse nonStreamingResponse =
       GenerateContentResponse.builder()

src/test/java/com/google/genai/ChatTest.java

@@ -106,8 +106,16 @@ public static String findTheaters(String movie, String location, String time) {
   String jsonChunk3 = responseChunk3.toJson();
 
   String streamData =
-      "data: " + jsonChunk1 + "\n" + "data: " + jsonChunk2 + "\n" + "data: " + jsonChunk3 + "\n";
-  String streamData2 = "data: " + jsonChunk1 + "\n" + "data: " + jsonChunk2 + "\n";
+      "data: "
+          + jsonChunk1
+          + "\n\n"
+          + "data: "
+          + jsonChunk2
+          + "\n\n"
+          + "data: "
+          + jsonChunk3
+          + "\n\n";
+  String streamData2 = "data: " + jsonChunk1 + "\n\n" + "data: " + jsonChunk2 + "\n\n";
 
   GenerateContentResponse nonStreamingResponse =
       GenerateContentResponse.builder()

src/test/java/com/google/genai/ResponseStreamTest.java

@@ -0,0 +1,165 @@
+/*
+ * 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
+ *
+ *      https://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.genai;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import com.fasterxml.jackson.databind.JsonNode;
+import com.fasterxml.jackson.databind.node.ObjectNode;
+import com.google.genai.types.Candidate;
+import com.google.genai.types.GenerateContentResponse;
+import java.nio.charset.StandardCharsets;
+import java.util.Iterator;
+import okhttp3.Headers;
+import okhttp3.MediaType;
+import okhttp3.ResponseBody;
+import org.junit.jupiter.api.Test;
+
+public final class ResponseStreamTest {
+
+  public static class DummyConverter {
+    public JsonNode convert(JsonNode fromObject, ObjectNode parentObject) {
+      return fromObject;
+    }
+  }
+
+  @Test
+  public void testMultiLineSseParsing() throws Exception {
+    String sseData =
+        "data: {\"candidates\": [{\"content\": {\"parts\": [{\"text\": \"line1\\n\"}]}},\n"
+            + "data: {\"content\": {\"parts\": [{\"text\": \"line2\"}]}}]}\n"
+            + "\n"; // End of event
+
+    ResponseBody body =
+        ResponseBody.create(
+            sseData.getBytes(StandardCharsets.UTF_8), MediaType.parse("text/event-stream"));
+    FakeApiResponse response = new FakeApiResponse(Headers.of(), body);
+
+    DummyConverter converter = new DummyConverter();
+    ResponseStream<GenerateContentResponse> responseStream =
+        new ResponseStream<>(GenerateContentResponse.class, response, converter, "convert");
+
+    Iterator<GenerateContentResponse> iterator = responseStream.iterator();
+
+    assertTrue(iterator.hasNext());
+    GenerateContentResponse response1 = iterator.next();
+
+    assertTrue(response1.candidates().isPresent());
+    assertEquals(2, response1.candidates().get().size());
+
+    Candidate c1 = response1.candidates().get().get(0);
+    assertEquals("line1\n", c1.content().get().text());
+
+    Candidate c2 = response1.candidates().get().get(1);
+    assertEquals("line2", c2.content().get().text());
+
+    assertTrue(!iterator.hasNext());
+  }
+
+  @Test
+  public void testIgnoreNonSseLines() throws Exception {
+    String sseData =
+        "data: {\"candidates\": [{\"content\": {\"parts\": [{\"text\": \"valid data\"}]}}]}\n"
+            + ": some comment line\n"
+            + "ignored field: some value\n"
+            + "\n"; // End of event
+
+    ResponseBody body =
+        ResponseBody.create(
+            sseData.getBytes(StandardCharsets.UTF_8), MediaType.parse("text/event-stream"));
+    FakeApiResponse response = new FakeApiResponse(Headers.of(), body);
+
+    DummyConverter converter = new DummyConverter();
+    ResponseStream<GenerateContentResponse> responseStream =
+        new ResponseStream<>(GenerateContentResponse.class, response, converter, "convert");
+
+    Iterator<GenerateContentResponse> iterator = responseStream.iterator();
+
+    assertTrue(iterator.hasNext());
+    GenerateContentResponse response1 = iterator.next();
+
+    assertEquals("valid data", response1.text());
+
+    assertTrue(!iterator.hasNext());
+  }
+
+  @Test
+  public void testStreamErrorHandling() throws Exception {
+    String sseData =
+        "data: {\"candidates\": [{\"content\": {\"parts\": [{\"text\": \"valid data\"}]}}]}\n"
+            + "\n"
+            + "data: {\"error\": {\"code\": 429, \"message\": \"Quota exceeded\", \"status\": \"RESOURCE_EXHAUSTED\"}}\n"
+            + "\n";
+
+    ResponseBody body =
+        ResponseBody.create(
+            sseData.getBytes(StandardCharsets.UTF_8), MediaType.parse("text/event-stream"));
+    FakeApiResponse response = new FakeApiResponse(Headers.of(), body);
+
+    DummyConverter converter = new DummyConverter();
+    ResponseStream<GenerateContentResponse> responseStream =
+        new ResponseStream<>(GenerateContentResponse.class, response, converter, "convert");
+
+    Iterator<GenerateContentResponse> iterator = responseStream.iterator();
+
+    assertTrue(iterator.hasNext());
+    GenerateContentResponse response1 = iterator.next();
+    assertEquals("valid data", response1.text());
+
+    assertTrue(iterator.hasNext());
+
+    try {
+      iterator.next();
+      org.junit.jupiter.api.Assertions.fail("Expected ApiException was not thrown");
+    } catch (com.google.genai.errors.ApiException e) {
+      assertEquals(429, e.code());
+      assertEquals("RESOURCE_EXHAUSTED", e.status());
+      assertTrue(e.getMessage().contains("Quota exceeded"));
+    }
+  }
+
+  @Test
+  public void testMultipleValidEvents() throws Exception {
+    String sseData =
+        "data: {\"candidates\": [{\"content\": {\"parts\": [{\"text\": \"chunk1\"}]}}]}\n"
+            + "\n"
+            + "data: {\"candidates\": [{\"content\": {\"parts\": [{\"text\": \"chunk2\"}]}}]}\n"
+            + "\n";
+
+    ResponseBody body =
+        ResponseBody.create(
+            sseData.getBytes(StandardCharsets.UTF_8), MediaType.parse("text/event-stream"));
+    FakeApiResponse response = new FakeApiResponse(Headers.of(), body);
+
+    DummyConverter converter = new DummyConverter();
+    ResponseStream<GenerateContentResponse> responseStream =
+        new ResponseStream<>(GenerateContentResponse.class, response, converter, "convert");
+
+    Iterator<GenerateContentResponse> iterator = responseStream.iterator();
+
+    assertTrue(iterator.hasNext());
+    GenerateContentResponse response1 = iterator.next();
+    assertEquals("chunk1", response1.text());
+
+    assertTrue(iterator.hasNext());
+    GenerateContentResponse response2 = iterator.next();
+    assertEquals("chunk2", response2.text());
+
+    assertTrue(!iterator.hasNext());
+  }
+}