diff --git a/tests/v1/structured_output/test_backend_guidance.py b/tests/v1/structured_output/test_backend_guidance.py
index 771076186a3b4..4c01560fc88c3 100644
--- a/tests/v1/structured_output/test_backend_guidance.py
+++ b/tests/v1/structured_output/test_backend_guidance.py
@@ -1,9 +1,14 @@
 # SPDX-License-Identifier: Apache-2.0
 # SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+import time
+from concurrent.futures import Future
+
+import pytest
 from transformers import AutoTokenizer
 
 from vllm.config import StructuredOutputsConfig, VllmConfig
 from vllm.config.model import ModelConfig
+from vllm.config.parallel import ParallelConfig
 from vllm.config.speculative import SpeculativeConfig
 from vllm.sampling_params import SamplingParams, StructuredOutputsParams
 from vllm.v1.request import Request
@@ -116,3 +121,72 @@ def test_grammar_bitmask_with_specdec():
         )  # EOS not the final token
         grammar_bitmask(request, prompt[i:])  # EOS not present
         grammar_bitmask(request, prompt[i:] + [tokenizer.eos_token_id])
+
+
+@pytest.mark.parametrize("async_grammar", [True, False])
+def test_grammar_init_async_and_sync(async_grammar):
+    """Test grammar initialization works correctly in both async and sync modes.
+
+    This test validates that the distributed_executor_backend config option
+    correctly controls whether grammar compilation happens asynchronously
+    (via executor.submit) or synchronously. When set to "external_launcher",
+    grammar compilation is synchronous to avoid deadlocks.
+    """
+    tokenizer = AutoTokenizer.from_pretrained(TOKENIZER)
+    prompt = tokenizer.encode('{"a": "b"}')
+
+    # Use "external_launcher" for sync mode, None for async mode
+    executor_backend = None if async_grammar else "external_launcher"
+    vllm_config = VllmConfig(
+        model_config=ModelConfig(tokenizer=TOKENIZER),
+        structured_outputs_config=StructuredOutputsConfig(backend="guidance"),
+        parallel_config=ParallelConfig(distributed_executor_backend=executor_backend),
+    )
+    structured_output_manager = StructuredOutputManager(vllm_config)
+
+    sampling_params = SamplingParams(
+        structured_outputs=StructuredOutputsParams(
+            json='{"type": "object"}',
+        ),
+    )
+    sampling_params.structured_outputs._backend = "guidance"
+
+    request = Request(
+        "test_request",
+        prompt_token_ids=prompt,
+        sampling_params=sampling_params,
+        pooling_params=None,
+        eos_token_id=tokenizer.eos_token_id,
+    )
+
+    structured_output_manager.grammar_init(request)
+
+    # Check the internal _grammar type immediately after init
+    # Before _check_grammar_completion is called, async mode should have a Future
+    raw_grammar = request.structured_output_request._grammar
+    if async_grammar:
+        assert isinstance(raw_grammar, Future), (
+            "Async mode should store a Future before completion"
+        )
+    else:
+        assert not isinstance(raw_grammar, Future), (
+            "Sync mode should store the grammar directly, not a Future"
+        )
+
+    # Wait for grammar to be ready (handles both async and sync cases)
+    start_time = time.time()
+    while not request.structured_output_request._check_grammar_completion():
+        if time.time() - start_time > 5:  # 5-second timeout
+            pytest.fail("Grammar compilation timed out")
+        time.sleep(0.01)
+
+    # After completion, _grammar should no longer be a Future
+    assert not isinstance(request.structured_output_request._grammar, Future)
+
+    # Verify grammar is properly initialized and functional
+    grammar = request.structured_output_request.grammar
+    assert grammar is not None
+    assert not grammar.is_terminated()
+
+    # Verify the grammar can accept valid tokens
+    assert grammar.accept_tokens(request.request_id, prompt)
diff --git a/vllm/v1/structured_output/__init__.py b/vllm/v1/structured_output/__init__.py
index d087d28b1dae3..5ee88178cdf60 100644
--- a/vllm/v1/structured_output/__init__.py
+++ b/vllm/v1/structured_output/__init__.py
@@ -40,6 +40,16 @@ class StructuredOutputManager:
         self.reasoner: ReasoningParser | None = None
         self.vllm_config = vllm_config
 
+        # When in external_launcher mode, async grammar compilation causes deadlocks
+        # due to external_launcher mode having a scheduler for each TP rank.
+        # Async grammar compilation causes the WAITING_FOR_FSM → WAITING transition to
+        # happen at different times on different TP ranks,
+        # breaking the determinism assumption that external_launcher relies on.
+        self._use_async_grammar_compilation = (
+            vllm_config.parallel_config.distributed_executor_backend
+            != "external_launcher"
+        )
+
         self._grammar_bitmask: torch.Tensor | None = None
         self._full_mask = torch.tensor(-1, dtype=torch.int32)
 
@@ -138,10 +148,13 @@ class StructuredOutputManager:
             else:
                 raise ValueError(f"Unsupported structured output backend: {backend}")
 
-        grammar = self.executor.submit(self._async_create_grammar, request)
+        if self._use_async_grammar_compilation:
+            grammar = self.executor.submit(self._create_grammar, request)
+        else:
+            grammar = self._create_grammar(request)  # type: ignore[assignment]
         request.structured_output_request.grammar = grammar  # type: ignore[assignment]
 
-    def _async_create_grammar(
+    def _create_grammar(
         self,
         request: Request,
     ) -> StructuredOutputGrammar: