diff --git a/nemo_rl/algorithms/grpo.py b/nemo_rl/algorithms/grpo.py
index 5435df757a..77a6d55eff 100644
--- a/nemo_rl/algorithms/grpo.py
+++ b/nemo_rl/algorithms/grpo.py
@@ -496,6 +496,7 @@ def _spinup_nemo_gym(base_urls, model_name):
             base_urls=base_urls,
             invalid_tool_call_patterns=invalid_tool_call_patterns,
             thinking_tags=thinking_tags,
+            require_routed_experts=router_replay_enabled(policy_config),
             initial_global_config_dict=nemo_gym_dict,
         )
         nemo_gym_opts = {}
diff --git a/nemo_rl/environments/nemo_gym.py b/nemo_rl/environments/nemo_gym.py
index 611751af36..b3f8dcbfbd 100644
--- a/nemo_rl/environments/nemo_gym.py
+++ b/nemo_rl/environments/nemo_gym.py
@@ -77,6 +77,9 @@ class NemoGymConfig(TypedDict):
     thinking_tags: NotRequired[
         List[str] | None
     ]  # Thinking tags to check for malformed usage
+    require_routed_experts: NotRequired[
+        bool
+    ]  # Require Gym output items to carry R3 routed_experts
 
 
 def _detect_invalid_tool_call_and_malformed_thinking(
@@ -343,15 +346,48 @@ def _postprocess_nemo_gym_to_nemo_rl_result(
             prompt_token_ids = output_item_dict.pop("prompt_token_ids")
             generation_token_ids = output_item_dict.pop("generation_token_ids")
             generation_log_probs = output_item_dict.pop("generation_log_probs")
+            routed_experts_raw = output_item_dict.pop("routed_experts", None)
             new_prompt_token_ids = prompt_token_ids[len(seen_token_ids) :]
 
-            nemo_rl_message_log.append(
-                {
-                    "role": "user",
-                    "content": "",
-                    "token_ids": torch.tensor(new_prompt_token_ids),
-                }
-            )
+            routed_experts = None
+            if routed_experts_raw is not None:
+                routed_experts = torch.as_tensor(routed_experts_raw, dtype=torch.int32)
+                if routed_experts.dim() != 3:
+                    raise ValueError(
+                        "NeMo Gym returned routed_experts with invalid shape. "
+                        "Expected [tokens, num_moe_layers, topk], got "
+                        f"{tuple(routed_experts.shape)}."
+                    )
+                expected_tokens = len(prompt_token_ids) + len(generation_token_ids)
+                if routed_experts.shape[0] < expected_tokens:
+                    raise ValueError(
+                        "NeMo Gym returned too few routed_experts rows for a "
+                        "trainable output item: "
+                        f"routes={routed_experts.shape[0]}, expected_at_least="
+                        f"{expected_tokens}."
+                    )
+            elif self.cfg.get("require_routed_experts", False):
+                raise ValueError(
+                    "policy.router_replay.enabled=true requires NeMo Gym output "
+                    "items to include routed_experts, but the field was missing. "
+                    "Make sure the Gym repo includes routed_experts propagation "
+                    "and the NeMo-RL vLLM OpenAI-compatible server is configured "
+                    "with enable_return_routed_experts."
+                )
+
+            prompt_start = len(seen_token_ids)
+            prompt_end = len(prompt_token_ids)
+            generation_start = prompt_end
+            generation_end = prompt_end + len(generation_token_ids)
+
+            user_message = {
+                "role": "user",
+                "content": "",
+                "token_ids": torch.tensor(new_prompt_token_ids),
+            }
+            if routed_experts is not None:
+                user_message["routed_experts"] = routed_experts[prompt_start:prompt_end]
+            nemo_rl_message_log.append(user_message)
             # Valid tool calls go through the structured API (tool_calls field) and get
             # executed by NeMo-Gym. If tool call patterns appear in the text content instead,
             # the call was invalid and never executed — flag it so training can penalize it.
@@ -365,16 +401,19 @@ def _postprocess_nemo_gym_to_nemo_rl_result(
                 )
             )
 
-            nemo_rl_message_log.append(
-                {
-                    "role": "assistant",
-                    "content": "",
-                    "token_ids": torch.tensor(generation_token_ids),
-                    "generation_logprobs": torch.tensor(generation_log_probs),
-                    "is_invalid_tool_call": is_invalid_tool_call,
-                    "has_malformed_thinking": has_malformed_thinking,
-                }
-            )
+            assistant_message = {
+                "role": "assistant",
+                "content": "",
+                "token_ids": torch.tensor(generation_token_ids),
+                "generation_logprobs": torch.tensor(generation_log_probs),
+                "is_invalid_tool_call": is_invalid_tool_call,
+                "has_malformed_thinking": has_malformed_thinking,
+            }
+            if routed_experts is not None:
+                assistant_message["routed_experts"] = routed_experts[
+                    generation_start:generation_end
+                ]
+            nemo_rl_message_log.append(assistant_message)
 
             seen_token_ids.extend(new_prompt_token_ids)
             seen_token_ids.extend(generation_token_ids)
diff --git a/nemo_rl/models/generation/vllm/utils.py b/nemo_rl/models/generation/vllm/utils.py
index 349d36fabf..9635fa389d 100644
--- a/nemo_rl/models/generation/vllm/utils.py
+++ b/nemo_rl/models/generation/vllm/utils.py
@@ -192,6 +192,95 @@ def pad_and_align_routed_expert_indices(
     return (full, stats) if return_stats else full
 
 
+def attach_routed_experts_to_chat_response_choices(
+    response: Any,
+    final_request_output: Any,
+    *,
+    device: torch.device,
+    logger: Any = None,
+) -> Any:
+    """Attach aligned routed experts to OpenAI chat response choices."""
+    outputs_by_index = {
+        output.index: output for output in getattr(final_request_output, "outputs", [])
+    }
+    prompt_token_count = len(
+        getattr(final_request_output, "prompt_token_ids", []) or []
+    )
+
+    choices = list(getattr(response, "choices", []))
+    attached_choice_indices = set()
+    for choice in choices:
+        generation_details = outputs_by_index.get(choice.index)
+        if generation_details is None:
+            continue
+        attached_choice_indices.add(choice.index)
+
+        generation_token_count = len(getattr(generation_details, "token_ids", []) or [])
+        routed_result = pad_and_align_routed_expert_indices(
+            final_request_output,
+            generation_details,
+            valid_length=prompt_token_count + generation_token_count,
+            padded_length=prompt_token_count + generation_token_count,
+            device=device,
+            require_complete_routed_experts=True,
+            return_stats=True,
+        )
+        if not isinstance(routed_result, tuple):
+            raise RuntimeError(
+                "Expected routed_experts alignment to return stats for the "
+                "OpenAI-compatible chat endpoint."
+            )
+        routed_experts, r3_stats = routed_result
+        if routed_experts is None:
+            raise RuntimeError(
+                "vLLM was asked to return routed experts for the "
+                "OpenAI-compatible chat endpoint but the generation "
+                "output did not include routed_experts."
+            )
+        if r3_stats["missing_routes"] > 0 and logger is not None:
+            logger.warning(
+                "R3 router replay fallback: vLLM returned incomplete "
+                "routed_experts for chat choice_idx=%d, "
+                "missing_token_routes=%d, actual_routes=%d, "
+                "expected_routes=%d. Megatron will use its own router "
+                "for those missing token routes.",
+                choice.index,
+                r3_stats["missing_routes"],
+                r3_stats["actual_routes"],
+                r3_stats["expected_routes"],
+            )
+        choice.message.routed_experts = routed_experts.to(dtype=torch.int32).tolist()
+
+    if len(attached_choice_indices) != len(choices):
+        missing_choice_indices = sorted(
+            choice.index
+            for choice in choices
+            if choice.index not in attached_choice_indices
+        )
+        raise RuntimeError(
+            "vLLM was asked to return routed experts for the "
+            "OpenAI-compatible chat endpoint but response choices could not be "
+            "matched to generation outputs: "
+            f"missing_choice_indices={missing_choice_indices}."
+        )
+
+    return response
+
+
+def model_dump_chat_response_with_routed_experts(response: Any) -> dict[str, Any]:
+    """Dump a vLLM OpenAI chat response while preserving dynamic R3 fields."""
+    response_dict = response.model_dump()
+    for choice, choice_dict in zip(
+        getattr(response, "choices", []), response_dict.get("choices", [])
+    ):
+        routed_experts = getattr(
+            getattr(choice, "message", None), "routed_experts", None
+        )
+        if routed_experts is not None:
+            choice_dict.setdefault("message", {})["routed_experts"] = routed_experts
+    return response_dict
+
+
 def aggregate_spec_decode_counters(
     worker_metrics: list[dict[str, float | list[float]]],
 ) -> dict[str | tuple[str, int], float]:
diff --git a/nemo_rl/models/generation/vllm/vllm_worker_async.py b/nemo_rl/models/generation/vllm/vllm_worker_async.py
index 591b929fbf..fab2e1330e 100644
--- a/nemo_rl/models/generation/vllm/vllm_worker_async.py
+++ b/nemo_rl/models/generation/vllm/vllm_worker_async.py
@@ -41,7 +41,9 @@
     verify_right_padding,
 )
 from nemo_rl.models.generation.vllm.utils import (
+    attach_routed_experts_to_chat_response_choices,
     format_prompt_for_vllm_generation,
+    model_dump_chat_response_with_routed_experts,
     pad_and_align_routed_expert_indices,
 )
 from nemo_rl.models.generation.vllm.vllm_worker import BaseVllmGenerationWorker
@@ -216,6 +218,14 @@ def __init__(
         self.llm = None
         self.vllm_device_ids = None
 
+    def _return_routed_experts_enabled(self) -> bool:
+        engine_args = getattr(self, "llm_async_engine_args", None)
+        if bool(getattr(engine_args, "enable_return_routed_experts", False)):
+            return True
+        return bool(
+            self.cfg.get("vllm_kwargs", {}).get("enable_return_routed_experts", False)
+        )
+
     def _reserve_port(self) -> None:
         """Bind and listen on a TCP socket to reserve a free port from the OS.
 
@@ -671,7 +681,45 @@ class NeMoRLChatCompletionRequest(
         # vLLM 0.20 routes both /v1/chat/completions and /tokenize through
         # OpenAIServingRender.preprocess_chat, so the prefix-token override
         # belongs on the render subclass.
-        class NeMoRLOpenAIServingChat(OpenAIServingChat):
+        worker_self = self
+
+        class NeMoRLOpenAIServingChatMixin:
+            async def chat_completion_full_generator(
+                self,
+                request,
+                result_generator,
+                *args,
+                **kwargs,
+            ):
+                final_res = None
+
+                async def capture_result_generator():
+                    nonlocal final_res
+                    async for res in result_generator:
+                        final_res = res
+                        yield res
+
+                response = await super().chat_completion_full_generator(
+                    request,
+                    capture_result_generator(),
+                    *args,
+                    **kwargs,
+                )
+                if (
+                    not worker_self._return_routed_experts_enabled()
+                    or not isinstance(response, ChatCompletionResponse)
+                    or final_res is None
+                ):
+                    return response
+
+                return attach_routed_experts_to_chat_response_choices(
+                    response,
+                    final_res,
+                    device=torch.device("cpu"),
+                    logger=LOGGER,
+                )
+
+        class NeMoRLOpenAIServingChat(NeMoRLOpenAIServingChatMixin, OpenAIServingChat):
             pass
 
         class NeMoRLOpenAIServingRender(NeMoRLOpenAIServingMixin, OpenAIServingRender):
@@ -753,7 +801,9 @@ async def create_chat_completion(
                 )
 
             elif isinstance(generator, ChatCompletionResponse):
-                return JSONResponse(content=generator.model_dump())
+                return JSONResponse(
+                    content=model_dump_chat_response_with_routed_experts(generator)
+                )
 
             return StreamingResponse(content=generator, media_type="text/event-stream")
 
@@ -1071,11 +1121,7 @@ async def process_single_sample(sample_idx):
             generation_details = final_request_output.outputs[0]
             generated_token_ids = list(generation_details.token_ids)
             num_generated_tokens = len(generated_token_ids)
-            return_routed_experts = bool(
-                self.cfg.get("vllm_kwargs", {}).get(
-                    "enable_return_routed_experts", False
-                )
-            )
+            return_routed_experts = self._return_routed_experts_enabled()
 
             original_input_ids_single_row = input_ids_batch[sample_idx]
             final_output_tensor_len = current_input_actual_length + num_generated_tokens
diff --git a/tests/unit/environments/test_nemo_gym_router_replay.py b/tests/unit/environments/test_nemo_gym_router_replay.py
new file mode 100644
index 0000000000..fdc7a021f7
--- /dev/null
+++ b/tests/unit/environments/test_nemo_gym_router_replay.py
@@ -0,0 +1,90 @@
+# Copyright (c) 2026, NVIDIA CORPORATION.  All rights reserved.
+#
+# 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
+#
+#     http://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.
+
+import pytest
+
+from nemo_rl.environments.nemo_gym import NemoGym
+
+
+class _Tokenizer:
+    def batch_decode(self, batch):
+        return [" ".join(map(str, token_ids)) for token_ids in batch]
+
+
+def _routes(num_tokens: int) -> list[list[list[int]]]:
+    return [[[token_idx, token_idx + 100]] for token_idx in range(num_tokens)]
+
+
+def test_nemo_gym_postprocess_slices_routed_experts():
+    nemo_gym_result = {
+        "response": {
+            "output": [
+                {
+                    "prompt_token_ids": [1, 2],
+                    "generation_token_ids": [3],
+                    "generation_log_probs": [-0.1],
+                    "routed_experts": _routes(3),
+                },
+                {
+                    "prompt_token_ids": [1, 2, 3, 4, 5],
+                    "generation_token_ids": [6, 7],
+                    "generation_log_probs": [-0.2, -0.3],
+                    "routed_experts": _routes(7),
+                },
+            ]
+        },
+        "responses_create_params": {"input": []},
+    }
+
+    class _MockSelf:
+        cfg = {"require_routed_experts": True}
+
+    result = (
+        NemoGym.__ray_metadata__.modified_class._postprocess_nemo_gym_to_nemo_rl_result(
+            _MockSelf(), nemo_gym_result, _Tokenizer()
+        )
+    )
+
+    message_log = result["message_log"]
+    assert message_log[0]["token_ids"].tolist() == [1, 2]
+    assert message_log[0]["routed_experts"].tolist() == _routes(2)
+    assert message_log[1]["token_ids"].tolist() == [3]
+    assert message_log[1]["routed_experts"].tolist() == _routes(3)[2:3]
+    assert message_log[2]["token_ids"].tolist() == [4, 5]
+    assert message_log[2]["routed_experts"].tolist() == _routes(7)[3:5]
+    assert message_log[3]["token_ids"].tolist() == [6, 7]
+    assert message_log[3]["routed_experts"].tolist() == _routes(7)[5:7]
+
+
+def test_nemo_gym_postprocess_requires_routed_experts_when_configured():
+    nemo_gym_result = {
+        "response": {
+            "output": [
+                {
+                    "prompt_token_ids": [1, 2],
+                    "generation_token_ids": [3],
+                    "generation_log_probs": [-0.1],
+                },
+            ]
+        },
+        "responses_create_params": {"input": []},
+    }
+
+    class _MockSelf:
+        cfg = {"require_routed_experts": True}
+
+    with pytest.raises(ValueError, match="requires NeMo Gym output items"):
+        NemoGym.__ray_metadata__.modified_class._postprocess_nemo_gym_to_nemo_rl_result(
+            _MockSelf(), nemo_gym_result, _Tokenizer()
+        )
diff --git a/tests/unit/models/generation/test_vllm_utils.py b/tests/unit/models/generation/test_vllm_utils.py
index 8e21abbf46..674a23b5fb 100644
--- a/tests/unit/models/generation/test_vllm_utils.py
+++ b/tests/unit/models/generation/test_vllm_utils.py
@@ -13,6 +13,7 @@
 # limitations under the License.
 
 import math
+from types import SimpleNamespace
 
 import pytest
 import torch
@@ -21,8 +22,10 @@
 from nemo_rl.models.generation.vllm.utils import (
     R3_MISSING_ROUTE_SENTINEL,
     aggregate_spec_decode_counters,
+    attach_routed_experts_to_chat_response_choices,
     compute_spec_decode_metrics,
     format_prompt_for_vllm_generation,
+    model_dump_chat_response_with_routed_experts,
     pad_and_align_routed_expert_indices,
 )
 
@@ -298,6 +301,95 @@ class Output:
         )
 
 
+def test_attach_routed_experts_to_chat_response_choices_reassociates_by_choice_index():
+    final_res = SimpleNamespace(
+        prompt_token_ids=[101, 102, 103],
+        prompt_routed_experts=torch.tensor([[[10]], [[11]]], dtype=torch.int32),
+        outputs=[
+            SimpleNamespace(
+                index=1,
+                token_ids=[201, 202],
+                routed_experts=torch.tensor([[[31]], [[32]]], dtype=torch.int32),
+            ),
+            SimpleNamespace(
+                index=0,
+                token_ids=[200],
+                routed_experts=torch.tensor([[[30]]], dtype=torch.int32),
+            ),
+        ],
+    )
+    response = SimpleNamespace(
+        choices=[
+            SimpleNamespace(index=0, message=SimpleNamespace()),
+            SimpleNamespace(index=1, message=SimpleNamespace()),
+        ]
+    )
+
+    attach_routed_experts_to_chat_response_choices(
+        response,
+        final_res,
+        device=torch.device("cpu"),
+    )
+
+    assert response.choices[0].message.routed_experts == [
+        [[10]],
+        [[11]],
+        [[30]],
+        [[0]],
+    ]
+    assert response.choices[1].message.routed_experts == [
+        [[10]],
+        [[11]],
+        [[31]],
+        [[32]],
+        [[0]],
+    ]
+
+
+def test_attach_routed_experts_to_chat_response_choices_requires_routed_experts():
+    final_res = SimpleNamespace(
+        prompt_token_ids=[101, 102],
+        outputs=[SimpleNamespace(index=0, token_ids=[200])],
+    )
+    response = SimpleNamespace(
+        choices=[SimpleNamespace(index=0, message=SimpleNamespace())]
+    )
+
+    with pytest.raises(RuntimeError, match="did not include routed_experts"):
+        attach_routed_experts_to_chat_response_choices(
+            response,
+            final_res,
+            device=torch.device("cpu"),
+        )
+
+
+def test_model_dump_chat_response_with_routed_experts_preserves_dynamic_field():
+    routed_experts = [[[1]], [[2]]]
+
+    class Response:
+        choices = [
+            SimpleNamespace(
+                message=SimpleNamespace(routed_experts=routed_experts),
+            )
+        ]
+
+        def model_dump(self):
+            return {
+                "choices": [
+                    {
+                        "message": {
+                            "role": "assistant",
+                            "content": "hello",
+                        }
+                    }
+                ]
+            }
+
+    response_dict = model_dump_chat_response_with_routed_experts(Response())
+
+    assert response_dict["choices"][0]["message"]["routed_experts"] == routed_experts
+
+
 @pytest.mark.vllm
 def test_vllm_speculative_decoding_patch_removed():
     # The speculative decoding patch was fixed upstream in vLLM >= 0.14.0: