fix(models): treat a part-less candidate as an error, not a success

core/src/main/java/com/google/adk/models/LlmResponse.java

@@ -191,8 +191,22 @@ public final Builder response(GenerateContentResponse response) {
       if (candidatesOpt.isPresent() && !candidatesOpt.get().isEmpty()) {
         Candidate candidate = candidatesOpt.get().get(0);
         this.finishReason(candidate.finishReason().orElse(null));
-        if (candidate.content().isPresent()) {
-          this.content(candidate.content().get());
+        // A blocked or truncated candidate still carries a content object, so its presence says
+        // nothing about whether the model produced anything. Decide on the parts instead, and let a
+        // candidate that stopped normally with nothing to say remain a successful, empty turn.
+        boolean hasParts =
+            candidate
+                .content()
+                .flatMap(Content::parts)
+                .map(parts -> !parts.isEmpty())
+                .orElse(false);
+        boolean stopped =
+            candidate
+                .finishReason()
+                .map(reason -> reason.knownEnum() == FinishReason.Known.STOP)
+                .orElse(false);
+        if (hasParts || stopped) {
+          this.content(candidate.content().orElse(null));
           this.groundingMetadata(candidate.groundingMetadata().orElse(null));
         } else {
           candidate.finishReason().ifPresent(this::errorCode);

core/src/test/java/com/google/adk/models/LlmResponseTest.java

@@ -23,9 +23,11 @@
 import com.fasterxml.jackson.databind.ObjectMapper;
 import com.google.adk.JsonBaseModel;
 import com.google.common.collect.ImmutableList;
+import com.google.genai.types.Candidate;
 import com.google.genai.types.Content;
 import com.google.genai.types.FinishReason;
 import com.google.genai.types.FunctionCall;
+import com.google.genai.types.GenerateContentResponse;
 import com.google.genai.types.GenerateContentResponseUsageMetadata;
 import com.google.genai.types.Part;
 import com.google.genai.types.Transcription;
@@ -250,4 +252,134 @@ public void testDeserialization_someOptionalFieldsMissingSomePresent()
     assertThat(deserializedResponse.interrupted()).isEmpty();
     assertThat(deserializedResponse.usageMetadata()).isEmpty();
   }
+
+  private LlmResponse createFromCandidate(Candidate candidate) {
+    return LlmResponse.create(
+        GenerateContentResponse.builder().candidates(ImmutableList.of(candidate)).build());
+  }
+
+  /** The shape a blocked or budget-exhausted candidate arrives in: content present, no parts. */
+  private Content emptyModelContent() {
+    return Content.builder().role("model").build();
+  }
+
+  @Test
+  public void testCreate_partlessCandidateOutOfTokens_isReportedAsError() {
+    LlmResponse response =
+        createFromCandidate(
+            Candidate.builder()
+                .content(emptyModelContent())
+                .finishReason(new FinishReason(FinishReason.Known.MAX_TOKENS))
+                .build());
+
+    assertThat(response.errorCode()).hasValue(new FinishReason(FinishReason.Known.MAX_TOKENS));
+    assertThat(response.finishReason()).hasValue(new FinishReason(FinishReason.Known.MAX_TOKENS));
+    assertThat(response.content()).isEmpty();
+  }
+
+  @Test
+  public void testCreate_partlessCandidateBlocked_reportsFinishMessageAsError() {
+    LlmResponse response =
+        createFromCandidate(
+            Candidate.builder()
+                .content(emptyModelContent())
+                .finishReason(new FinishReason(FinishReason.Known.SAFETY))
+                .finishMessage("Blocked by safety filters.")
+                .build());
+
+    assertThat(response.errorCode()).hasValue(new FinishReason(FinishReason.Known.SAFETY));
+    assertThat(response.errorMessage()).hasValue("Blocked by safety filters.");
+    assertThat(response.content()).isEmpty();
+  }
+
+  @Test
+  public void testCreate_candidateWithEmptyPartsList_isReportedAsError() {
+    LlmResponse response =
+        createFromCandidate(
+            Candidate.builder()
+                .content(Content.builder().role("model").parts(ImmutableList.of()).build())
+                .finishReason(new FinishReason(FinishReason.Known.MAX_TOKENS))
+                .build());
+
+    assertThat(response.errorCode()).hasValue(new FinishReason(FinishReason.Known.MAX_TOKENS));
+    assertThat(response.content()).isEmpty();
+  }
+
+  @Test
+  public void testCreate_candidateWithEmptyTextPart_isReportedAsSuccess() {
+    Content content =
+        Content.builder().role("model").parts(ImmutableList.of(Part.fromText(""))).build();
+
+    LlmResponse response =
+        createFromCandidate(
+            Candidate.builder()
+                .content(content)
+                .finishReason(new FinishReason(FinishReason.Known.MAX_TOKENS))
+                .build());
+
+    assertThat(response.errorCode()).isEmpty();
+    assertThat(response.errorMessage()).isEmpty();
+    assertThat(response.content()).hasValue(content);
+  }
+
+  @Test
+  public void testCreate_partlessCandidateStoppedNormally_isReportedAsSuccess() {
+    Content content = emptyModelContent();
+
+    LlmResponse response =
+        createFromCandidate(
+            Candidate.builder()
+                .content(content)
+                .finishReason(new FinishReason(FinishReason.Known.STOP))
+                .build());
+
+    assertThat(response.errorCode()).isEmpty();
+    assertThat(response.errorMessage()).isEmpty();
+    assertThat(response.content()).hasValue(content);
+  }
+
+  @Test
+  public void testCreate_truncatedCandidateWithText_isReportedAsSuccess() {
+    Content content = createSampleContent("The bicycle began as the");
+
+    LlmResponse response =
+        createFromCandidate(
+            Candidate.builder()
+                .content(content)
+                .finishReason(new FinishReason(FinishReason.Known.MAX_TOKENS))
+                .build());
+
+    assertThat(response.errorCode()).isEmpty();
+    assertThat(response.errorMessage()).isEmpty();
+    assertThat(response.content()).hasValue(content);
+  }
+
+  /**
+   * A candidate that produced nothing and reported no reason has no failure to raise, so no error
+   * code is set. The part-less content object is not carried over either — it holds nothing, and
+   * this pins the behavior so a later change to the predicate cannot alter it unnoticed.
+   */
+  @Test
+  public void testCreate_partlessCandidateWithoutFinishReason_reportsNeitherContentNorError() {
+    LlmResponse response =
+        createFromCandidate(Candidate.builder().content(emptyModelContent()).build());
+
+    assertThat(response.errorCode()).isEmpty();
+    assertThat(response.errorMessage()).isEmpty();
+    assertThat(response.finishReason()).isEmpty();
+    assertThat(response.content()).isEmpty();
+  }
+
+  /** A turn that ended normally is not an error, even when the candidate carries no content. */
+  @Test
+  public void testCreate_contentlessCandidateStoppedNormally_isReportedAsSuccess() {
+    LlmResponse response =
+        createFromCandidate(
+            Candidate.builder().finishReason(new FinishReason(FinishReason.Known.STOP)).build());
+
+    assertThat(response.errorCode()).isEmpty();
+    assertThat(response.errorMessage()).isEmpty();
+    assertThat(response.finishReason()).hasValue(new FinishReason(FinishReason.Known.STOP));
+    assertThat(response.content()).isEmpty();
+  }
 }