fix(assistant): treat malformed tool arguments as failed tool calls

Co-authored-by: DeepSeek V4.1 Flash (via OpenRouter) <noreply@deepseek.com>

Author: sampottinger

engine/src/main/java/org/kigalisim/assistant/model/OpenAiModelProtocolStrategy.java

@@ -173,11 +173,10 @@ private AssistantToolInvocation parseToolCall(JsonObject toolCall) {
   }
 
   /**
-   * Parses tool arguments, requiring them to be a JSON object.
+   * Parses tool arguments, falling back to an empty object when they are malformed.
    *
    * @param argumentsRaw The arguments value as received.
-   * @return The parsed argument object.
-   * @throws IllegalStateException If the arguments are not a JSON object.
+   * @return The parsed argument object, or an empty object when malformed.
    */
   private JsonObject parseArguments(Object argumentsRaw) {
     if (argumentsRaw instanceof JsonObject arguments) {
@@ -187,18 +186,12 @@ private JsonObject parseArguments(Object argumentsRaw) {
         Object parsed = Jsoner.deserialize(argumentsString);
         if (parsed instanceof JsonObject arguments) {
           return arguments;
-        } else {
-          throw new IllegalStateException(
-              "Tool arguments were not a JSON object: " + abbreviate(argumentsString));
         }
       } catch (JsonException e) {
-        throw new IllegalStateException(
-            "Unable to parse tool arguments: " + abbreviate(argumentsString), e);
+        return new JsonObject();
       }
-    } else {
-      throw new IllegalStateException(
-          "Tool arguments were not a JSON object: " + abbreviate(String.valueOf(argumentsRaw)));
     }
+    return new JsonObject();
   }
 
   /**

engine/src/test/java/org/kigalisim/assistant/UntimedAssistantTurnStrategyTest.java

@@ -274,20 +274,24 @@ public void testUnknownToolProducesFailure() {
   }
 
   /**
-   * Test that malformed tool arguments fail the query rather than being silently ignored.
+   * Test that malformed tool arguments produce a failed tool call without failing the turn.
    */
   @Test
-  public void testMalformedArgumentsFailQuery() {
+  public void testMalformedArgumentsProduceFailure() {
     Harness harness = new Harness(25);
     JsonArray firstCalls = new JsonArray();
     firstCalls.add(toolCall("call_bad", "message_user", "{\"message\":"));
     harness.wire.enqueuePayload(buildResponse(assistantMessage(firstCalls, null), "tool_calls"));
 
+    JsonArray secondCalls = new JsonArray();
+    secondCalls.add(toolCall("call_end", "end_turn", "{}"));
+    harness.wire.enqueuePayload(buildResponse(assistantMessage(secondCalls, null), "tool_calls"));
+
     harness.strategy.runTurn(harness.toolbelt, "chat", "hi", harness.callback);
 
     List<AssistantResult> actions = harness.callback.actions;
-    assertEquals(1, actions.size(), "Malformed arguments should fail the model query");
-    assertFalse(actions.get(0).getWasSuccessful(), "Malformed arguments should fail");
+    assertFalse(actions.get(1).getWasSuccessful(), "Malformed arguments should fail");
+    assertTrue(actions.get(1).getToolInvocation().orElseThrow().getCall().isEmpty());
     assertEquals(1, harness.callback.completions.size());
   }
 

engine/src/test/java/org/kigalisim/assistant/model/OpenAiModelProtocolStrategyTest.java

@@ -164,15 +164,20 @@ public void testParseMultipleToolCallsInOrder() throws IOException {
   }
 
   /**
-   * Test that malformed arguments cause parsing to fail.
+   * Test that malformed arguments become an empty object without failing the parse.
    *
    * @throws IOException If the fixture cannot be read.
    */
   @Test
-  public void testParseMalformedArgumentsThrows() throws IOException {
+  public void testParseMalformedArguments() throws IOException {
     OpenAiModelProtocolStrategy strategy = new OpenAiModelProtocolStrategy("test-model");
-    String raw = readFixture("openai_response_malformed_arguments.json");
-    assertThrows(IllegalStateException.class, () -> strategy.parseResponse(raw));
+    AssistantModelResponse response =
+        strategy.parseResponse(readFixture("openai_response_malformed_arguments.json"));
+
+    assertEquals(2, response.getToolCalls().size(), "Both calls should still parse");
+    assertTrue(response.getToolCalls().get(0).getCall().isEmpty(),
+        "Malformed arguments should become an empty object");
+    assertEquals("Good", response.getToolCalls().get(1).getCall().get("message"));
   }
 
   /**