diff --git a/agentscope-core/src/main/java/io/agentscope/core/agent/accumulator/ToolCallsAccumulator.java b/agentscope-core/src/main/java/io/agentscope/core/agent/accumulator/ToolCallsAccumulator.java
index 289e02382a..3d85ed9ad3 100644
--- a/agentscope-core/src/main/java/io/agentscope/core/agent/accumulator/ToolCallsAccumulator.java
+++ b/agentscope-core/src/main/java/io/agentscope/core/agent/accumulator/ToolCallsAccumulator.java
@@ -39,6 +39,8 @@
  */
 public class ToolCallsAccumulator implements ContentAccumulator<ToolUseBlock> {
 
+    private static final String UNKNOWN_TOOL_NAME = "unknown_tool";
+
     // Map to support multiple parallel tool calls
     // Key: tool identifier (ID, name, or index)
     private final Map<String, ToolCallBuilder> builders = new LinkedHashMap<>();
@@ -103,7 +105,7 @@ ToolUseBlock build() {
 
             return ToolUseBlock.builder()
                     .id(toolId != null ? toolId : generateId())
-                    .name(name)
+                    .name(name != null ? name : UNKNOWN_TOOL_NAME)
                     .input(finalArgs)
                     .content(rawContentStr.isEmpty() ? "{}" : rawContentStr)
                     .metadata(metadata.isEmpty() ? null : metadata)
diff --git a/agentscope-core/src/main/java/io/agentscope/core/formatter/openai/OpenAIResponseParser.java b/agentscope-core/src/main/java/io/agentscope/core/formatter/openai/OpenAIResponseParser.java
index 1c70958215..69ccb5c18d 100644
--- a/agentscope-core/src/main/java/io/agentscope/core/formatter/openai/OpenAIResponseParser.java
+++ b/agentscope-core/src/main/java/io/agentscope/core/formatter/openai/OpenAIResponseParser.java
@@ -461,8 +461,15 @@ protected ChatResponse parseChunkResponse(OpenAIResponse response, Instant start
                                         }
                                     }
 
-                                    if (toolCallId == null) {
-                                        toolCallId = "streaming_" + System.currentTimeMillis();
+                                    if (toolCallId == null || toolCallId.isEmpty()) {
+                                        // Use index as stable synthetic id so chunks from the same
+                                        // tool call can be consistently merged across stream
+                                        // updates.
+                                        if (toolIndex != null) {
+                                            toolCallId = "streaming_idx_" + toolIndex;
+                                        } else {
+                                            toolCallId = "streaming_" + System.currentTimeMillis();
+                                        }
                                     }
                                     if (toolName == null) {
                                         toolName = "";
@@ -538,7 +545,7 @@ protected ChatResponse parseChunkResponse(OpenAIResponse response, Instant start
 
                                         contentBlocks.add(
                                                 ToolUseBlock.builder()
-                                                        .id("")
+                                                        .id(toolCallId)
                                                         .name(FRAGMENT_PLACEHOLDER)
                                                         .input(new HashMap<>())
                                                         .content(arguments)
diff --git a/agentscope-core/src/test/java/io/agentscope/core/agent/accumulator/ToolCallsAccumulatorTest.java b/agentscope-core/src/test/java/io/agentscope/core/agent/accumulator/ToolCallsAccumulatorTest.java
index eb197d5748..594720b3a0 100644
--- a/agentscope-core/src/test/java/io/agentscope/core/agent/accumulator/ToolCallsAccumulatorTest.java
+++ b/agentscope-core/src/test/java/io/agentscope/core/agent/accumulator/ToolCallsAccumulatorTest.java
@@ -323,4 +323,33 @@ void testMultipleParallelToolCallsWithStreamingChunks() {
         List<ToolUseBlock> allCalls = accumulator.getAllAccumulatedToolCalls();
         assertEquals(2, allCalls.size());
     }
+
+    @Test
+    @DisplayName("Should fallback to non-null name when only fragments are received")
+    void testOnlyFragmentsFallbackName() {
+        ToolUseBlock fragment1 =
+                ToolUseBlock.builder()
+                        .id("streaming_idx_0")
+                        .name("__fragment__")
+                        .content("{\"city\":")
+                        .build();
+        ToolUseBlock fragment2 =
+                ToolUseBlock.builder()
+                        .id("streaming_idx_0")
+                        .name("__fragment__")
+                        .content("\"Beijing\"}")
+                        .build();
+
+        accumulator.add(fragment1);
+        accumulator.add(fragment2);
+
+        List<ToolUseBlock> result = accumulator.buildAllToolCalls();
+        assertEquals(1, result.size());
+        ToolUseBlock toolCall = result.get(0);
+
+        assertNotNull(toolCall.getName());
+        assertEquals("unknown_tool", toolCall.getName());
+        assertEquals("{\"city\":\"Beijing\"}", toolCall.getContent());
+        assertEquals("Beijing", toolCall.getInput().get("city"));
+    }
 }
diff --git a/agentscope-core/src/test/java/io/agentscope/core/formatter/openai/OpenAIResponseParserTest.java b/agentscope-core/src/test/java/io/agentscope/core/formatter/openai/OpenAIResponseParserTest.java
index 737875574a..53e9da3af9 100644
--- a/agentscope-core/src/test/java/io/agentscope/core/formatter/openai/OpenAIResponseParserTest.java
+++ b/agentscope-core/src/test/java/io/agentscope/core/formatter/openai/OpenAIResponseParserTest.java
@@ -709,6 +709,78 @@ void testStreamingToolCallFragment() {
             assertEquals(OpenAIResponseParser.FRAGMENT_PLACEHOLDER, toolBlock.getName());
         }
 
+        @Test
+        @DisplayName("Should use stable synthetic id for streaming tool calls by index")
+        void testStreamingToolCallUsesStableSyntheticIdByIndex() {
+            // First chunk: fragment only (no id, no name), has index
+            OpenAIResponse firstChunk = new OpenAIResponse();
+            firstChunk.setObject("chat.completion.chunk");
+
+            OpenAIFunction firstFunction = new OpenAIFunction();
+            firstFunction.setName(null);
+            firstFunction.setArguments("{\"city\":");
+
+            OpenAIToolCall firstToolCall = new OpenAIToolCall();
+            firstToolCall.setId(null);
+            firstToolCall.setIndex(0);
+            firstToolCall.setType("function");
+            firstToolCall.setFunction(firstFunction);
+
+            OpenAIMessage firstDelta = new OpenAIMessage();
+            firstDelta.setToolCalls(List.of(firstToolCall));
+            firstDelta.setRole("assistant");
+
+            OpenAIChoice firstChoice = new OpenAIChoice();
+            firstChoice.setDelta(firstDelta);
+            firstChoice.setIndex(0);
+            firstChunk.setChoices(List.of(firstChoice));
+
+            ChatResponse firstResult = parser.parseResponse(firstChunk, startTime);
+            ToolUseBlock firstBlock =
+                    firstResult.getContent().stream()
+                            .filter(block -> block instanceof ToolUseBlock)
+                            .map(block -> (ToolUseBlock) block)
+                            .findFirst()
+                            .orElse(null);
+            assertNotNull(firstBlock);
+            assertEquals("streaming_idx_0", firstBlock.getId());
+            assertEquals(OpenAIResponseParser.FRAGMENT_PLACEHOLDER, firstBlock.getName());
+
+            // Second chunk: same index, now with tool name, still no id
+            OpenAIResponse secondChunk = new OpenAIResponse();
+            secondChunk.setObject("chat.completion.chunk");
+
+            OpenAIFunction secondFunction = new OpenAIFunction();
+            secondFunction.setName("get_weather");
+            secondFunction.setArguments("\"Beijing\"}");
+
+            OpenAIToolCall secondToolCall = new OpenAIToolCall();
+            secondToolCall.setId(null);
+            secondToolCall.setIndex(0);
+            secondToolCall.setType("function");
+            secondToolCall.setFunction(secondFunction);
+
+            OpenAIMessage secondDelta = new OpenAIMessage();
+            secondDelta.setToolCalls(List.of(secondToolCall));
+            secondDelta.setRole("assistant");
+
+            OpenAIChoice secondChoice = new OpenAIChoice();
+            secondChoice.setDelta(secondDelta);
+            secondChoice.setIndex(0);
+            secondChunk.setChoices(List.of(secondChoice));
+
+            ChatResponse secondResult = parser.parseResponse(secondChunk, startTime);
+            ToolUseBlock secondBlock =
+                    secondResult.getContent().stream()
+                            .filter(block -> block instanceof ToolUseBlock)
+                            .map(block -> (ToolUseBlock) block)
+                            .findFirst()
+                            .orElse(null);
+            assertNotNull(secondBlock);
+            assertEquals("streaming_idx_0", secondBlock.getId());
+            assertEquals("get_weather", secondBlock.getName());
+        }
+
         @Test
         @DisplayName("Should parse chunk with reasoning content")
         void testChunkWithReasoningContent() {