diff --git a/tests/v1/engine/test_engine_core_client.py b/tests/v1/engine/test_engine_core_client.py
index ab5946ad3ba..8327b5b0763 100644
--- a/tests/v1/engine/test_engine_core_client.py
+++ b/tests/v1/engine/test_engine_core_client.py
@@ -27,12 +27,13 @@ from vllm.platforms import current_platform
 from vllm.pooling_params import LateInteractionParams, PoolingParams
 from vllm.usage.usage_lib import UsageContext
 from vllm.utils.torch_utils import set_default_torch_num_threads
-from vllm.v1.engine import EngineCoreRequest
+from vllm.v1.engine import EngineCoreReadyResponse, EngineCoreRequest
 from vllm.v1.engine.core import EngineCore
 from vllm.v1.engine.core_client import (
     AsyncMPClient,
     DPLBAsyncMPClient,
     EngineCoreClient,
+    MPClient,
     SyncMPClient,
 )
 from vllm.v1.engine.utils import CoreEngineProcManager
@@ -236,6 +237,30 @@ def test_dplb_non_late_interaction_still_uses_lb():
     assert client.lb_engines[1][0] == 1
 
 
+def test_apply_ready_response_syncs_block_size():
+    import msgspec
+
+    client = object.__new__(MPClient)
+    client.vllm_config = SimpleNamespace(
+        cache_config=SimpleNamespace(block_size=16, num_gpu_blocks=0),
+        model_config=SimpleNamespace(max_model_len=8192),
+    )
+    client.stats_update_address = None
+
+    payload = msgspec.msgpack.encode(
+        EngineCoreReadyResponse(
+            max_model_len=8192,
+            num_gpu_blocks=100,
+            block_size=1056,
+            dp_stats_address=None,
+            dtype="bfloat16",
+            vllm_version="test",
+        )
+    )
+    client._apply_ready_response(payload)
+    assert client.vllm_config.cache_config.block_size == 1056
+
+
 def loop_until_done(client: EngineCoreClient, outputs: dict):
     while True:
         engine_core_outputs = client.get_output().outputs
diff --git a/vllm/v1/engine/__init__.py b/vllm/v1/engine/__init__.py
index aa1756bf682..848f530ce33 100644
--- a/vllm/v1/engine/__init__.py
+++ b/vllm/v1/engine/__init__.py
@@ -74,6 +74,7 @@ class EngineCoreReadyResponse:
 
     max_model_len: int
     num_gpu_blocks: int
+    block_size: int
     dp_stats_address: str | None
     dtype: str
     vllm_version: str
diff --git a/vllm/v1/engine/core.py b/vllm/v1/engine/core.py
index c21a4de5d30..d08bbc951fa 100644
--- a/vllm/v1/engine/core.py
+++ b/vllm/v1/engine/core.py
@@ -1462,6 +1462,7 @@ class EngineCoreProc(EngineCore):
             ready_response = EngineCoreReadyResponse(
                 max_model_len=self.vllm_config.model_config.max_model_len,
                 num_gpu_blocks=self.vllm_config.cache_config.num_gpu_blocks or 0,
+                block_size=self.vllm_config.cache_config.block_size,
                 dp_stats_address=self.frontend_stats_publish_address,
                 dtype=str(self.vllm_config.model_config.dtype).removeprefix("torch."),
                 vllm_version=VLLM_VERSION,
diff --git a/vllm/v1/engine/core_client.py b/vllm/v1/engine/core_client.py
index c26380e6e15..14257b020ee 100644
--- a/vllm/v1/engine/core_client.py
+++ b/vllm/v1/engine/core_client.py
@@ -713,6 +713,10 @@ class MPClient(EngineCoreClient):
         num_gpu_blocks += response.num_gpu_blocks
         vllm_config.cache_config.num_gpu_blocks = num_gpu_blocks
 
+        # Sync block_size: may be enlarged by _align_hybrid_block_size in the
+        # worker for hybrid Mamba models.
+        vllm_config.cache_config.block_size = response.block_size
+
         # In external DP LB mode, the coordinator address that the
         # front-end procs connect to is obtained by each engine via it's
         # initial handshake with the rank 0 front-end.