fix(ollama): ensure stable message ID for streaming responses

agentscope-core/src/main/java/io/agentscope/core/model/ChatResponse.java

@@ -101,6 +101,17 @@ public String getFinishReason() {
         return finishReason;
     }
 
+    /**
+     * Creates a new instance of ChatResponse with the specified ID,
+     * copying all other fields from this instance.
+     *
+     * @param newId the new identifier
+     * @return a new ChatResponse instance
+     */
+    public ChatResponse withId(String newId) {
+        return new ChatResponse(newId, this.content, this.usage, this.metadata, this.finishReason);
+    }
+
     /**
      * Creates a new builder for ChatResponse.
      *

agentscope-core/src/main/java/io/agentscope/core/model/OllamaChatModel.java

@@ -187,8 +187,25 @@ private Flux<ChatResponse> streamWithHttpClient(
         Flux<ChatResponse> responseFlux;
         if (stream) {
             responseFlux =
-                    httpClient.stream(request)
-                            .map(response -> formatter.parseResponse(response, Instant.now()));
+                    Flux.defer(
+                            () -> {
+                                final String[] finalStableId = new String[1];
+
+                                return httpClient.stream(request)
+                                        .map(
+                                                response -> {
+                                                    ChatResponse parsedResponse =
+                                                            formatter.parseResponse(
+                                                                    response, Instant.now());
+
+                                                    if (finalStableId[0] == null) {
+                                                        finalStableId[0] = parsedResponse.getId();
+                                                        return parsedResponse;
+                                                    }
+
+                                                    return parsedResponse.withId(finalStableId[0]);
+                                                });
+                            });
         } else {
             responseFlux =
                     Flux.defer(

agentscope-core/src/test/java/io/agentscope/core/model/OllamaChatModelTest.java

@@ -969,4 +969,59 @@ void testRealQwen3Thinking() {
         assertEquals("POST", request.getMethod());
         assertTrue(request.getUrl().endsWith("/api/chat"));
     }
+
+    @Test
+    @DisplayName("Should populate and maintain stable ID for streaming responses")
+    void testStreamStableMessageId() {
+        // no ID field is included, simulating real Ollama behavior
+        String part1 =
+                "{\"model\":\""
+                        + TEST_MODEL_NAME
+                        + "\",\"message\":{\"role\":\"assistant\",\"content\":\"Thinking...\"},\"done\":false}";
+        String part2 =
+                "{\"model\":\""
+                        + TEST_MODEL_NAME
+                        + "\",\"message\":{\"role\":\"assistant\",\"content\":\""
+                        + " Hello\"},\"done\":false}";
+        String part3 =
+                "{\"model\":\""
+                        + TEST_MODEL_NAME
+                        + "\",\"message\":{\"role\":\"assistant\",\"content\":\""
+                        + " World\"},\"done\":false}";
+        String part4 =
+                "{\"model\":\"" + TEST_MODEL_NAME + "\",\"done\":true,\"total_duration\":100}";
+
+        when(httpTransport.stream(any(HttpRequest.class)))
+                .thenReturn(Flux.just(part1, part2, part3, part4));
+
+        GenerateOptions genOptions = OllamaOptions.builder().build().toGenerateOptions();
+
+        List<ChatResponse> responses =
+                model.stream(
+                                List.of(Msg.builder().role(MsgRole.USER).textContent("Hi").build()),
+                                null,
+                                genOptions)
+                        .collectList()
+                        .block();
+
+        assertNotNull(responses);
+        assertFalse(responses.isEmpty(), "Stream responses should not be empty");
+
+        // collect all the IDs for assertion
+        List<String> collectedIds = new ArrayList<>();
+        for (int i = 0; i < responses.size(); i++) {
+            String id = responses.get(i).getId();
+
+            assertNotNull(id);
+            collectedIds.add(id);
+        }
+
+        // verify the stability of the ID (Stable ID)
+        long uniqueIdCount = collectedIds.stream().distinct().count();
+        assertEquals(
+                1,
+                uniqueIdCount,
+                "All stream chunks for a single response must share the same stable ID! Found IDs: "
+                        + collectedIds);
+    }
 }