diff --git a/tests/entrypoints/openai/test_gptoss_structural_tags_integration.py b/tests/entrypoints/openai/test_gptoss_structural_tags_integration.py
new file mode 100644
index 0000000000..fbfae4f268
--- /dev/null
+++ b/tests/entrypoints/openai/test_gptoss_structural_tags_integration.py
@@ -0,0 +1,280 @@
+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+
+"""Integration tests for GPT-OSS structural tags functionality (PR #25515)."""
+
+import json
+from unittest.mock import Mock
+
+import pytest
+
+from vllm.entrypoints.openai.protocol import (
+    StructuredOutputsParams,
+)
+from vllm.entrypoints.tool_server import ToolServer
+from vllm.reasoning.gptoss_reasoning_parser import (
+    GptOssReasoningParser,
+)
+
+
+class TestGptOssStructuralTagsIntegration:
+    """Integration tests for structural tags in GPT-OSS tool calls."""
+
+    @pytest.fixture
+    def mock_tokenizer(self):
+        """Create a mock tokenizer."""
+        tokenizer = Mock()
+        tokenizer.encode = Mock(return_value=[1, 2, 3, 4, 5])
+        return tokenizer
+
+    @pytest.fixture
+    def gptoss_parser(self, mock_tokenizer):
+        """Create a real GptOssReasoningParser instance."""
+        return GptOssReasoningParser(mock_tokenizer)
+
+    @pytest.fixture
+    def tool_server_with_python(self):
+        """Create a tool server with Python tool enabled."""
+        tool_server = Mock(spec=ToolServer)
+        tool_server.has_tool = Mock(side_effect=lambda tool: tool == "python")
+        return tool_server
+
+    @pytest.fixture
+    def tool_server_empty(self):
+        """Create a tool server with no tools."""
+        tool_server = Mock(spec=ToolServer)
+        tool_server.has_tool = Mock(return_value=False)
+        return tool_server
+
+    def test_end_to_end_no_tools(self, gptoss_parser):
+        """Test end-to-end flow when no tools are available."""
+        # Test the parser directly
+        result = gptoss_parser.prepare_structured_tag(None, None)
+        parsed_result = json.loads(result)
+
+        # Verify basic structure
+        assert parsed_result["type"] == "structural_tag"
+        assert parsed_result["format"]["type"] == "triggered_tags"
+        assert len(parsed_result["format"]["tags"]) == 1
+
+        # Verify only analysis channel is allowed
+        analysis_tag = parsed_result["format"]["tags"][0]
+        assert analysis_tag["begin"] == "<|channel|>analysis<|message|>"
+        assert analysis_tag["content"]["type"] == "any_text"
+        assert analysis_tag["end"] == "<|end|>"
+
+        # Verify triggers
+        assert parsed_result["format"]["triggers"] == ["<|channel|>analysis"]
+        assert parsed_result["format"]["stop_after_first"] is False
+
+    def test_end_to_end_with_python_tool(self, gptoss_parser, tool_server_with_python):
+        """Test end-to-end flow with Python tool enabled."""
+        result = gptoss_parser.prepare_structured_tag(None, tool_server_with_python)
+        parsed_result = json.loads(result)
+
+        # Should have analysis tag + 2 python tags
+        assert len(parsed_result["format"]["tags"]) == 3
+
+        # Verify all expected tags are present
+        tag_begins = [tag["begin"] for tag in parsed_result["format"]["tags"]]
+        expected_begins = [
+            "<|channel|>analysis<|message|>",
+            "<|channel|>commentary to=python",
+            "<|channel|>analysis to=python",
+        ]
+
+        for expected in expected_begins:
+            assert expected in tag_begins
+
+        # Verify triggers include commentary
+        assert "<|channel|>analysis" in parsed_result["format"]["triggers"]
+        assert "<|channel|>commentary to=" in parsed_result["format"]["triggers"]
+
+    def test_structured_outputs_params_integration(
+        self, gptoss_parser, tool_server_with_python
+    ):
+        """Test integration with StructuredOutputsParams."""
+        # Generate structural tag
+        structural_tag = gptoss_parser.prepare_structured_tag(
+            None, tool_server_with_python
+        )
+
+        # Create StructuredOutputsParams
+        params = StructuredOutputsParams(structural_tag=structural_tag)
+
+        # Verify the tag is properly stored and accessible
+        assert params.structural_tag == structural_tag
+
+        # Verify the tag is valid JSON
+        parsed_tag = json.loads(params.structural_tag)
+        assert parsed_tag["type"] == "structural_tag"
+
+    @pytest.mark.parametrize(
+        "browser, python, container, expected_tags",
+        [
+            # No tools
+            (False, False, False, 1),
+            # Single tool
+            (True, False, False, 3),
+            # Multiple tools
+            (True, True, False, 5),
+            # All tools
+            (True, True, True, 7),
+        ],
+    )
+    def test_tool_server_interaction_flow(
+        self, gptoss_parser, browser, python, container, expected_tags
+    ):
+        """Test the complete tool server interaction flow."""
+
+        # Create a mock ToolServer
+        tool_server = Mock(spec=ToolServer)
+
+        # Simulate tool availability based on parameters
+        tool_server.has_tool = Mock(
+            side_effect=lambda tool: {
+                "browser": browser,
+                "python": python,
+                "container": container,
+            }.get(tool, False)
+        )
+
+        # Run the parser and verify results
+        result = gptoss_parser.prepare_structured_tag(None, tool_server)
+        parsed_result = json.loads(result)
+
+        # Validate number of tags
+        assert len(parsed_result["format"]["tags"]) == expected_tags
+
+        # Verify tool-specific tags exist for enabled tools
+        tag_begins = [tag["begin"] for tag in parsed_result["format"]["tags"]]
+        for tool, enabled in {
+            "browser": browser,
+            "python": python,
+            "container": container,
+        }.items():
+            if enabled:
+                assert f"<|channel|>commentary to={tool}" in tag_begins
+                assert f"<|channel|>analysis to={tool}" in tag_begins
+
+    def test_original_tag_preservation(self, gptoss_parser, tool_server_with_python):
+        """Test that original tags are preserved when provided."""
+        original_tag = '{"type": "custom_tag", "data": "preserved"}'
+
+        result = gptoss_parser.prepare_structured_tag(
+            original_tag, tool_server_with_python
+        )
+
+        # Should return original tag unchanged
+        assert result == original_tag
+
+    @pytest.mark.parametrize(
+        "tools",
+        [
+            [],
+            ["browser"],
+            ["python"],
+            ["container"],
+            ["browser", "python"],
+            ["browser", "container"],
+            ["python", "container"],
+            ["browser", "python", "container"],
+        ],
+    )
+    def test_json_validity_comprehensive(self, gptoss_parser, tools):
+        """Test JSON validity across all possible tool combinations."""
+
+        tool_server = Mock(spec=ToolServer)
+        tool_server.has_tool = Mock(side_effect=lambda tool: tool in tools)
+
+        result = gptoss_parser.prepare_structured_tag(None, tool_server)
+
+        # Should be valid JSON
+        parsed_result = json.loads(result)
+
+        # Should have correct structure
+        assert parsed_result["type"] == "structural_tag"
+        assert "format" in parsed_result
+        assert "tags" in parsed_result["format"]
+        assert "triggers" in parsed_result["format"]
+
+        # Tag count should be: 1 (analysis) + 2 * len(tools)
+        expected_tag_count = 1 + (2 * len(tools))
+        assert len(parsed_result["format"]["tags"]) == expected_tag_count
+
+    def test_error_handling_invalid_tool_server(self, gptoss_parser):
+        """Test error handling with invalid tool server."""
+        # Tool server that raises exceptions
+        tool_server = Mock(spec=ToolServer)
+        tool_server.has_tool = Mock(side_effect=Exception("Tool server error"))
+
+        # Should handle gracefully and still return a valid tag
+        with pytest.raises(Exception, match="Tool server error"):
+            gptoss_parser.prepare_structured_tag(None, tool_server)
+
+    def test_concurrent_requests_isolation(self, gptoss_parser):
+        """Test that concurrent requests don't interfere with each other."""
+        # Simulate concurrent requests with different tool servers
+        tool_server_1 = Mock(spec=ToolServer)
+        tool_server_1.has_tool = Mock(side_effect=lambda tool: tool == "python")
+
+        tool_server_2 = Mock(spec=ToolServer)
+        tool_server_2.has_tool = Mock(side_effect=lambda tool: tool == "browser")
+
+        # Generate tags concurrently
+        result_1 = gptoss_parser.prepare_structured_tag(None, tool_server_1)
+        result_2 = gptoss_parser.prepare_structured_tag(None, tool_server_2)
+
+        # Parse results
+        parsed_1 = json.loads(result_1)
+        parsed_2 = json.loads(result_2)
+
+        # Verify they have different tool configurations
+        tags_1 = [tag["begin"] for tag in parsed_1["format"]["tags"]]
+        tags_2 = [tag["begin"] for tag in parsed_2["format"]["tags"]]
+
+        # Result 1 should have python tags
+        assert "<|channel|>commentary to=python" in tags_1
+        assert "<|channel|>commentary to=browser" not in tags_1
+
+        # Result 2 should have browser tags
+        assert "<|channel|>commentary to=browser" in tags_2
+        assert "<|channel|>commentary to=python" not in tags_2
+
+    def test_tag_format_consistency(self, gptoss_parser):
+        """Test that all generated tags follow consistent format."""
+        tool_server = Mock(spec=ToolServer)
+        tool_server.has_tool = Mock(
+            side_effect=lambda tool: tool in ["python", "browser"]
+        )
+
+        result = gptoss_parser.prepare_structured_tag(None, tool_server)
+        parsed_result = json.loads(result)
+
+        # Verify all tags have required fields
+        for tag in parsed_result["format"]["tags"]:
+            assert "begin" in tag
+            assert "content" in tag
+            assert "end" in tag
+            assert tag["content"]["type"] == "any_text"
+            assert tag["end"] == "<|end|>"
+
+            # Verify begin format
+            assert tag["begin"].startswith("<|channel|>")
+
+    def test_trigger_configuration(self, gptoss_parser):
+        """Test trigger configuration for different tool setups."""
+        # Test with no tools
+        result_no_tools = gptoss_parser.prepare_structured_tag(None, None)
+        parsed_no_tools = json.loads(result_no_tools)
+        assert parsed_no_tools["format"]["triggers"] == ["<|channel|>analysis"]
+
+        # Test with tools
+        tool_server = Mock(spec=ToolServer)
+        tool_server.has_tool = Mock(side_effect=lambda tool: tool == "python")
+
+        result_with_tools = gptoss_parser.prepare_structured_tag(None, tool_server)
+        parsed_with_tools = json.loads(result_with_tools)
+
+        expected_triggers = ["<|channel|>analysis", "<|channel|>commentary to="]
+        assert set(parsed_with_tools["format"]["triggers"]) == set(expected_triggers)
diff --git a/tests/v1/entrypoints/llm/test_struct_output_generate.py b/tests/v1/entrypoints/llm/test_struct_output_generate.py
index cca9729b9d..014e6eca2e 100644
--- a/tests/v1/entrypoints/llm/test_struct_output_generate.py
+++ b/tests/v1/entrypoints/llm/test_struct_output_generate.py
@@ -864,3 +864,49 @@ def test_structured_output_batched_with_non_structured_outputs_requests(
             # non-structured outputs requests should not return a valid JSON here
             with pytest.raises(ValueError):
                 output_json = json.loads(generated_text)
+
+
+@pytest.mark.parametrize("guided_decoding_backend", ["xgrammar"])
+def test_structured_output_with_structural_tag(
+    monkeypatch: pytest.MonkeyPatch,
+    guided_decoding_backend: str,
+):
+    monkeypatch.setenv("VLLM_USE_V1", "1")
+
+    llm = LLM(
+        model="Qwen/Qwen2.5-1.5B-Instruct",
+        guided_decoding_backend=guided_decoding_backend,
+    )
+
+    structural_tag_config = {
+        "type": "structural_tag",
+        "format": {
+            "type": "triggered_tags",
+            "tags": [
+                {"begin": "hello_flag", "content": {"type": "any_text"}, "end": "hello"}
+            ],
+            "triggers": ["hello"],
+            "stop_after_first": False,
+        },
+    }
+
+    sampling_params = SamplingParams(
+        temperature=0.0,
+        max_tokens=500,
+        guided_decoding=StructuredOutputsParams(
+            structural_tag=json.dumps(structural_tag_config)
+        ),
+    )
+
+    prompt = "Hello and repete hello 10 times, do not say anything else. Only say hello hello hello, now start"
+    outputs = llm.generate(prompt, sampling_params=sampling_params, use_tqdm=True)
+    assert outputs is not None
+    for output in outputs:
+        assert output is not None
+        assert isinstance(output, RequestOutput)
+        prompt = output.prompt
+        generated_text = output.outputs[0].text
+        assert generated_text is not None
+        assert "hello_flag" in generated_text, (
+            f"Expected 'hello_flag' to be in generated text, but got: {generated_text}"
+        )
diff --git a/tests/v1/structured_output/test_gptoss_structural_tags.py b/tests/v1/structured_output/test_gptoss_structural_tags.py
new file mode 100644
index 0000000000..f0feabfb99
--- /dev/null
+++ b/tests/v1/structured_output/test_gptoss_structural_tags.py
@@ -0,0 +1,172 @@
+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+
+"""Unit tests for GPT-OSS structural tag support in reasoning (PR #25515)."""
+
+import json
+from unittest.mock import Mock
+
+import pytest
+
+from vllm.entrypoints.tool_server import ToolServer
+from vllm.reasoning.gptoss_reasoning_parser import (
+    GptOssReasoningParser,
+    from_builtin_tool_to_tag,
+    no_func_reaonsing_tag,
+    tag_with_builtin_funcs,
+)
+
+
+class TestGptOssReasoningParser:
+    """Test cases for GptOssReasoningParser structural tag functionality."""
+
+    @pytest.fixture
+    def mock_tokenizer(self):
+        """Create a mock tokenizer for testing."""
+        tokenizer = Mock()
+        tokenizer.encode = Mock(return_value=[1, 2, 3, 4, 5])
+        return tokenizer
+
+    @pytest.fixture
+    def reasoning_parser(self, mock_tokenizer):
+        """Create a GptOssReasoningParser instance."""
+        return GptOssReasoningParser(mock_tokenizer)
+
+    @pytest.fixture
+    def mock_tool_server_empty(self):
+        """Create a mock ToolServer with no tools."""
+        tool_server = Mock(spec=ToolServer)
+        tool_server.has_tool = Mock(return_value=False)
+        return tool_server
+
+    @pytest.fixture
+    def mock_tool_server_with_browser(self):
+        """Create a mock ToolServer with browser tool."""
+        tool_server = Mock(spec=ToolServer)
+        tool_server.has_tool = Mock(side_effect=lambda tool: tool == "browser")
+        return tool_server
+
+    @pytest.fixture
+    def mock_tool_server_with_all_tools(self):
+        """Create a mock ToolServer with all builtin tools."""
+        tool_server = Mock(spec=ToolServer)
+        tool_server.has_tool = Mock(
+            side_effect=lambda tool: tool in ["browser", "python", "container"]
+        )
+        return tool_server
+
+    def test_prepare_structured_tag_no_tool_server(self, reasoning_parser):
+        """Test prepare_structured_tag with no tool server."""
+        result = reasoning_parser.prepare_structured_tag(None, None)
+        expected = json.dumps(no_func_reaonsing_tag)
+
+        assert result == expected
+
+        # Verify the structure is correct
+        parsed = json.loads(result)
+        assert parsed["type"] == "structural_tag"
+        assert parsed["format"]["type"] == "triggered_tags"
+        assert len(parsed["format"]["tags"]) == 1
+        assert parsed["format"]["tags"][0]["begin"] == "<|channel|>analysis<|message|>"
+        assert parsed["format"]["triggers"] == ["<|channel|>analysis"]
+
+    def test_prepare_structured_tag_with_all_tools(
+        self, reasoning_parser, mock_tool_server_with_all_tools
+    ):
+        """Test prepare_structured_tag with all builtin tools."""
+        result = reasoning_parser.prepare_structured_tag(
+            None, mock_tool_server_with_all_tools
+        )
+        parsed = json.loads(result)
+
+        # Should have analysis tag + tags for all 3 tools (2 tags each)
+        assert len(parsed["format"]["tags"]) == 7  # 1 analysis + 6 tool tags
+
+        # Check all tool tags are present
+        tag_begins = [tag["begin"] for tag in parsed["format"]["tags"]]
+        for tool in ["browser", "python", "container"]:
+            assert f"<|channel|>commentary to={tool}" in tag_begins
+            assert f"<|channel|>analysis to={tool}" in tag_begins
+
+    def test_prepare_structured_tag_with_original_tag(self, reasoning_parser):
+        """Test prepare_structured_tag when original_tag is provided."""
+        original_tag = '{"custom": "tag"}'
+        result = reasoning_parser.prepare_structured_tag(original_tag, None)
+
+        # Should return the original tag unchanged
+        assert result == original_tag
+
+    def test_from_builtin_tool_to_tag(self):
+        """Test from_builtin_tool_to_tag function."""
+        tags = from_builtin_tool_to_tag("python")
+
+        assert len(tags) == 2
+        assert tags[0]["begin"] == "<|channel|>commentary to=python"
+        assert tags[0]["content"]["type"] == "any_text"
+        assert tags[0]["end"] == "<|end|>"
+
+        assert tags[1]["begin"] == "<|channel|>analysis to=python"
+        assert tags[1]["content"]["type"] == "any_text"
+        assert tags[1]["end"] == "<|end|>"
+
+    def test_tag_with_builtin_funcs(self):
+        """Test tag_with_builtin_funcs function."""
+        builtin_tools = ["browser", "python"]
+        result = tag_with_builtin_funcs(no_func_reaonsing_tag, builtin_tools)
+
+        assert result["type"] == "structural_tag"
+        # Should have original analysis tag + 2 tags per tool
+        assert len(result["format"]["tags"]) == 5  # 1 + 2*2
+
+        # Should have added commentary trigger
+        assert "<|channel|>commentary to=" in result["format"]["triggers"]
+        assert "<|channel|>analysis" in result["format"]["triggers"]
+
+    def test_tag_structure_invariants(self):
+        """Test that the basic tag structure follows expected format."""
+        # Test the base no_func_reaonsing_tag structure
+        assert no_func_reaonsing_tag["type"] == "structural_tag"
+        assert no_func_reaonsing_tag["format"]["type"] == "triggered_tags"
+        assert no_func_reaonsing_tag["format"]["stop_after_first"] is False
+
+        # Verify analysis tag structure
+        analysis_tag = no_func_reaonsing_tag["format"]["tags"][0]
+        assert analysis_tag["begin"] == "<|channel|>analysis<|message|>"
+        assert analysis_tag["content"]["type"] == "any_text"
+        assert analysis_tag["end"] == "<|end|>"
+
+    def test_json_serialization_valid(
+        self, reasoning_parser, mock_tool_server_with_all_tools
+    ):
+        """Test that all generated tags produce valid JSON."""
+        # Test with no tool server
+        result1 = reasoning_parser.prepare_structured_tag(None, None)
+        json.loads(result1)  # Should not raise
+
+        # Test with empty tool server
+        empty_server = Mock(spec=ToolServer)
+        empty_server.has_tool = Mock(return_value=False)
+        result2 = reasoning_parser.prepare_structured_tag(None, empty_server)
+        json.loads(result2)  # Should not raise
+
+        # Test with tools
+        result3 = reasoning_parser.prepare_structured_tag(
+            None, mock_tool_server_with_all_tools
+        )
+        json.loads(result3)  # Should not raise
+
+    @pytest.mark.parametrize("tool_name", ["browser", "python", "container"])
+    def test_single_tool_integration(self, reasoning_parser, tool_name):
+        """Test integration with individual tools."""
+        tool_server = Mock(spec=ToolServer)
+        tool_server.has_tool = Mock(side_effect=lambda tool: tool == tool_name)
+
+        result = reasoning_parser.prepare_structured_tag(None, tool_server)
+        parsed = json.loads(result)
+
+        # Should have 1 analysis + 2 tool-specific tags
+        assert len(parsed["format"]["tags"]) == 3
+
+        tag_begins = [tag["begin"] for tag in parsed["format"]["tags"]]
+        assert f"<|channel|>commentary to={tool_name}" in tag_begins
+        assert f"<|channel|>analysis to={tool_name}" in tag_begins
diff --git a/tests/v1/structured_output/test_reasoning_structured_output.py b/tests/v1/structured_output/test_reasoning_structured_output.py
new file mode 100644
index 0000000000..70047a993c
--- /dev/null
+++ b/tests/v1/structured_output/test_reasoning_structured_output.py
@@ -0,0 +1,207 @@
+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+
+"""Unit tests for reasoning-aware structured output functionality (PR #25515)."""
+
+from unittest.mock import Mock
+
+import pytest
+
+from vllm.config import ModelConfig, SchedulerConfig, VllmConfig
+from vllm.reasoning import ReasoningParser
+from vllm.v1.request import Request
+from vllm.v1.structured_output import StructuredOutputManager
+
+
+class TestReasoningStructuredOutput:
+    """Test reasoning-aware structured output functionality."""
+
+    @pytest.fixture
+    def mock_model_config(self):
+        """Create a mock ModelConfig."""
+        config = Mock(spec=ModelConfig)
+        config.skip_tokenizer_init = True  # Skip tokenizer init to avoid network calls
+        config.get_vocab_size = Mock(return_value=50000)
+        # Add missing runner_type attribute that tokenizer initialization expects
+        config.runner_type = "generate"
+        # Add other attributes that tokenizer initialization might need
+        config.tokenizer = "test-tokenizer"
+        config.tokenizer_mode = "auto"
+        config.trust_remote_code = False
+        config.tokenizer_revision = None
+        return config
+
+    @pytest.fixture
+    def mock_scheduler_config(self):
+        """Create a mock SchedulerConfig."""
+        config = Mock(spec=SchedulerConfig)
+        config.max_num_seqs = 128
+        return config
+
+    @pytest.fixture
+    def mock_vllm_config(self, mock_model_config, mock_scheduler_config):
+        """Create a mock VllmConfig."""
+        config = Mock(spec=VllmConfig)
+        config.model_config = mock_model_config
+        config.scheduler_config = mock_scheduler_config
+        config.structured_outputs_config = Mock()
+        config.structured_outputs_config.reasoning_parser = None
+        config.structured_outputs_config.enable_in_reasoning = False
+        config.speculative_config = None
+        return config
+
+    @pytest.fixture
+    def mock_reasoning_parser(self):
+        """Create a mock ReasoningParser."""
+        parser = Mock(spec=ReasoningParser)
+        parser.is_reasoning_end = Mock(return_value=False)
+        return parser
+
+    @pytest.fixture
+    def mock_request_with_structured_output(self):
+        """Create a mock request with structured output."""
+        request = Mock(spec=Request)
+        request.structured_output_request = Mock()
+        request.structured_output_request.reasoning_ended = None
+        request.structured_output_request.grammar = Mock()
+        request.structured_output_request.grammar.is_terminated = Mock(
+            return_value=False
+        )
+        request.use_structured_output = True
+        request.prompt_token_ids = [1, 2, 3, 4, 5]
+        request.all_token_ids = [1, 2, 3, 4, 5, 6, 7, 8]
+        return request
+
+    def test_should_fill_bitmask_with_enable_in_reasoning(
+        self, mock_vllm_config, mock_request_with_structured_output
+    ):
+        """Test should_fill_bitmask when enable_in_reasoning is True."""
+        # Enable enable_in_reasoning
+        mock_vllm_config.structured_outputs_config.enable_in_reasoning = True
+
+        manager = StructuredOutputManager(mock_vllm_config)
+
+        # Should always return True when enable_in_reasoning is enabled
+        result = manager.should_fill_bitmask(mock_request_with_structured_output)
+        assert result is True
+
+    def test_should_fill_bitmask_without_enable_in_reasoning(
+        self,
+        mock_vllm_config,
+        mock_request_with_structured_output,
+        mock_reasoning_parser,
+    ):
+        """Test should_fill_bitmask when enable_in_reasoning is False."""
+        # Keep enable_in_reasoning as False (default)
+        config = mock_vllm_config.structured_outputs_config
+        assert config.enable_in_reasoning is False
+
+        manager = StructuredOutputManager(mock_vllm_config)
+        manager.reasoner = mock_reasoning_parser
+
+        # Mock reasoning not ended
+        mock_reasoning_parser.is_reasoning_end.return_value = False
+
+        result = manager.should_fill_bitmask(mock_request_with_structured_output)
+
+        # Should set reasoning_ended and return its value
+        assert (
+            mock_request_with_structured_output.structured_output_request.reasoning_ended
+            is False
+        )
+        assert result is False
+
+    def test_should_fill_bitmask_no_reasoner(
+        self, mock_vllm_config, mock_request_with_structured_output
+    ):
+        """Test should_fill_bitmask when no reasoner is configured."""
+        manager = StructuredOutputManager(mock_vllm_config)
+        manager.reasoner = None
+
+        result = manager.should_fill_bitmask(mock_request_with_structured_output)
+
+        # Should default to True when no reasoner
+        assert result is True
+
+    def test_should_advance_with_enable_in_reasoning(
+        self,
+        mock_vllm_config,
+        mock_request_with_structured_output,
+        mock_reasoning_parser,
+    ):
+        """Test should_advance when enable_in_reasoning is True."""
+        # Enable enable_in_reasoning
+        mock_vllm_config.structured_outputs_config.enable_in_reasoning = True
+
+        manager = StructuredOutputManager(mock_vllm_config)
+        manager.reasoner = mock_reasoning_parser
+
+        # Should always return True when enable_in_reasoning is enabled
+        result = manager.should_advance(mock_request_with_structured_output)
+        assert result is True
+
+    def test_should_advance_reasoning_not_ended(
+        self,
+        mock_vllm_config,
+        mock_request_with_structured_output,
+        mock_reasoning_parser,
+    ):
+        """Test should_advance when reasoning has not ended."""
+        manager = StructuredOutputManager(mock_vllm_config)
+        manager.reasoner = mock_reasoning_parser
+
+        # Set reasoning as not ended
+        (
+            mock_request_with_structured_output.structured_output_request
+        ).reasoning_ended = False
+        mock_reasoning_parser.is_reasoning_end.return_value = False
+
+        result = manager.should_advance(mock_request_with_structured_output)
+
+        # Should return False since reasoning hasn't ended
+        assert result is False
+
+    def test_should_advance_reasoning_just_ended(
+        self,
+        mock_vllm_config,
+        mock_request_with_structured_output,
+        mock_reasoning_parser,
+    ):
+        """Test should_advance when reasoning ends in current step."""
+        manager = StructuredOutputManager(mock_vllm_config)
+        manager.reasoner = mock_reasoning_parser
+
+        # Set reasoning as not ended initially, but ends in this step
+        (
+            mock_request_with_structured_output.structured_output_request
+        ).reasoning_ended = False
+        mock_reasoning_parser.is_reasoning_end.return_value = True
+
+        result = manager.should_advance(mock_request_with_structured_output)
+
+        # Should set reasoning_ended to True but return False for this step
+        assert (
+            mock_request_with_structured_output.structured_output_request.reasoning_ended
+            is True
+        )
+        assert result is False
+
+    def test_should_advance_reasoning_already_ended(
+        self,
+        mock_vllm_config,
+        mock_request_with_structured_output,
+        mock_reasoning_parser,
+    ):
+        """Test should_advance when reasoning has already ended."""
+        manager = StructuredOutputManager(mock_vllm_config)
+        manager.reasoner = mock_reasoning_parser
+
+        # Set reasoning as already ended
+        (
+            mock_request_with_structured_output.structured_output_request
+        ).reasoning_ended = True
+
+        result = manager.should_advance(mock_request_with_structured_output)
+
+        # Should return True since reasoning has ended
+        assert result is True
diff --git a/vllm/config/structured_outputs.py b/vllm/config/structured_outputs.py
index 5111c9c77d..76b565006e 100644
--- a/vllm/config/structured_outputs.py
+++ b/vllm/config/structured_outputs.py
@@ -35,6 +35,8 @@ class StructuredOutputsConfig:
     reasoning_parser: str = ""
     """Select the reasoning parser depending on the model that you're using.
     This is used to parse the reasoning content into OpenAI API format."""
+    enable_in_reasoning: bool = False
+    """Whether to use structured input for reasoning."""
 
     def compute_hash(self) -> str:
         """
diff --git a/vllm/engine/arg_utils.py b/vllm/engine/arg_utils.py
index 432b1eca45..05958ca523 100644
--- a/vllm/engine/arg_utils.py
+++ b/vllm/engine/arg_utils.py
@@ -479,6 +479,7 @@ class EngineArgs:
         VllmConfig, "structured_outputs_config"
     )
     reasoning_parser: str = StructuredOutputsConfig.reasoning_parser
+
     # Deprecated guided decoding fields
     guided_decoding_backend: str | None = None
     guided_decoding_disable_fallback: bool | None = None
diff --git a/vllm/entrypoints/openai/protocol.py b/vllm/entrypoints/openai/protocol.py
index e73752b9d5..0d27e6707c 100644
--- a/vllm/entrypoints/openai/protocol.py
+++ b/vllm/entrypoints/openai/protocol.py
@@ -200,7 +200,7 @@ class JsonSchemaResponseFormat(OpenAIBaseModel):
     strict: bool | None = None
 
 
-class StructuralTag(OpenAIBaseModel):
+class LegacyStructuralTag(OpenAIBaseModel):
     begin: str
     # schema is the field, but that causes conflicts with pydantic so
     # instead use structural_tag_schema with an alias
@@ -208,10 +208,20 @@ class StructuralTag(OpenAIBaseModel):
     end: str
 
 
+class LegacyStructuralTagResponseFormat(OpenAIBaseModel):
+    type: Literal["structural_tag"]
+    structures: list[LegacyStructuralTag]
+    triggers: list[str]
+
+
 class StructuralTagResponseFormat(OpenAIBaseModel):
     type: Literal["structural_tag"]
-    structures: list[StructuralTag]
-    triggers: list[str]
+    format: Any
+
+
+AnyStructuralTagResponseFormat: TypeAlias = (
+    LegacyStructuralTagResponseFormat | StructuralTagResponseFormat
+)
 
 
 class ResponseFormat(OpenAIBaseModel):
@@ -220,7 +230,9 @@ class ResponseFormat(OpenAIBaseModel):
     json_schema: JsonSchemaResponseFormat | None = None
 
 
-AnyResponseFormat: TypeAlias = ResponseFormat | StructuralTagResponseFormat
+AnyResponseFormat: TypeAlias = (
+    ResponseFormat | StructuralTagResponseFormat | LegacyStructuralTagResponseFormat
+)
 
 
 class StreamOptions(OpenAIBaseModel):
@@ -823,7 +835,11 @@ class ChatCompletionRequest(OpenAIBaseModel):
                 elif response_format.type == "structural_tag":
                     structural_tag = response_format
                     assert structural_tag is not None and isinstance(
-                        structural_tag, StructuralTagResponseFormat
+                        structural_tag,
+                        (
+                            LegacyStructuralTagResponseFormat,
+                            StructuralTagResponseFormat,
+                        ),
                     )
                     s_tag_obj = structural_tag.model_dump(by_alias=True)
                     self.structured_outputs.structural_tag = json.dumps(s_tag_obj)
diff --git a/vllm/entrypoints/openai/serving_responses.py b/vllm/entrypoints/openai/serving_responses.py
index ffc692f099..1fdb6997bc 100644
--- a/vllm/entrypoints/openai/serving_responses.py
+++ b/vllm/entrypoints/openai/serving_responses.py
@@ -98,7 +98,7 @@ from vllm.logger import init_logger
 from vllm.logprobs import Logprob as SampleLogprob
 from vllm.logprobs import SampleLogprobs
 from vllm.outputs import CompletionOutput
-from vllm.sampling_params import SamplingParams
+from vllm.sampling_params import SamplingParams, StructuredOutputsParams
 from vllm.transformers_utils.tokenizer import AnyTokenizer
 from vllm.utils import random_uuid
 
@@ -365,6 +365,19 @@ class OpenAIServingResponses(OpenAIServing):
                         context = HarmonyContext(messages, available_tools)
                 else:
                     context = SimpleContext()
+
+                if self.reasoning_parser is not None:
+                    reasoning_parser = self.reasoning_parser(tokenizer)
+                    if sampling_params.structured_outputs is None:
+                        sampling_params.structured_outputs = StructuredOutputsParams()
+                    struct_out = sampling_params.structured_outputs
+                    if struct_out.all_non_structural_tag_constraints_none():
+                        sampling_params.structured_outputs.structural_tag = (
+                            reasoning_parser.prepare_structured_tag(
+                                sampling_params.structured_outputs.structural_tag,
+                                self.tool_server,
+                            )
+                        )
                 generator = self._generate_with_builtin_tools(
                     request_id=request.request_id,
                     request_prompt=request_prompts[i],
@@ -1919,6 +1932,7 @@ class OpenAIServingResponses(OpenAIServing):
                 processer = self._process_harmony_streaming_events
             else:
                 processer = self._process_simple_streaming_events
+            # TODO Hanchen make sampling params to include the structural tag
 
             initial_response = ResponsesResponse.from_request(
                 request,
diff --git a/vllm/reasoning/abs_reasoning_parsers.py b/vllm/reasoning/abs_reasoning_parsers.py
index 3a595a3076..ee890e662e 100644
--- a/vllm/reasoning/abs_reasoning_parsers.py
+++ b/vllm/reasoning/abs_reasoning_parsers.py
@@ -7,6 +7,7 @@ from collections.abc import Callable, Sequence
 from functools import cached_property
 from typing import TYPE_CHECKING, Any
 
+from vllm.entrypoints.tool_server import ToolServer
 from vllm.logger import init_logger
 from vllm.utils.collections import is_list_of
 from vllm.utils.import_utils import import_from_path
@@ -115,6 +116,17 @@ class ReasoningParser:
         previously been parsed and extracted (see constructor)
         """
 
+    def prepare_structured_tag(
+        self,
+        original_tag: str | None,
+        tool_server: ToolServer | None,
+    ) -> str:
+        """
+        Instance method that is implemented for preparing the structured tag
+        Otherwise, None is returned
+        """
+        return None
+
 
 class ReasoningParserManager:
     reasoning_parsers: dict[str, type] = {}
diff --git a/vllm/reasoning/gptoss_reasoning_parser.py b/vllm/reasoning/gptoss_reasoning_parser.py
index ccb2d9553c..e6766ddcbc 100644
--- a/vllm/reasoning/gptoss_reasoning_parser.py
+++ b/vllm/reasoning/gptoss_reasoning_parser.py
@@ -1,17 +1,61 @@
 # SPDX-License-Identifier: Apache-2.0
 # SPDX-FileCopyrightText: Copyright contributors to the vLLM project
-
+import json
 from collections.abc import Sequence
 
 from transformers import PreTrainedTokenizerBase
 
 from vllm.entrypoints.harmony_utils import parse_chat_output
 from vllm.entrypoints.openai.protocol import ChatCompletionRequest, DeltaMessage
+from vllm.entrypoints.tool_server import ToolServer
 from vllm.logger import init_logger
 from vllm.reasoning import ReasoningParser, ReasoningParserManager
 
 logger = init_logger(__name__)
 
+no_func_reaonsing_tag = {
+    "type": "structural_tag",
+    "format": {
+        "type": "triggered_tags",
+        "tags": [
+            {
+                "begin": "<|channel|>analysis<|message|>",
+                "content": {"type": "any_text"},
+                "end": "<|end|>",
+            }
+        ],
+        "triggers": ["<|channel|>analysis"],
+        "stop_after_first": False,
+    },
+}
+
+
+def from_builtin_tool_to_tag(tool: str) -> list[dict]:
+    tag = [
+        {
+            "begin": f"<|channel|>commentary to={tool}",
+            "content": {"type": "any_text"},
+            "end": "<|end|>",
+        },
+        {
+            "begin": f"<|channel|>analysis to={tool}",
+            "content": {"type": "any_text"},
+            "end": "<|end|>",
+        },
+    ]
+    return tag
+
+
+def tag_with_builtin_funcs(no_func_reaonsing_tag, builtin_tool_list: list[str]) -> dict:
+    import copy
+
+    new_tag = copy.deepcopy(no_func_reaonsing_tag)
+    new_tag["format"]["triggers"].append("<|channel|>commentary to=")
+
+    for tool in builtin_tool_list:
+        new_tag["format"]["tags"].extend(from_builtin_tool_to_tag(tool))
+    return new_tag
+
 
 @ReasoningParserManager.register_module("openai_gptoss")
 class GptOssReasoningParser(ReasoningParser):
@@ -81,3 +125,33 @@ class GptOssReasoningParser(ReasoningParser):
         raise NotImplementedError(
             "gpt-oss has a special branch for parsing reasoning in non-streaming mode. This method shouldn't be used."  # noqa: E501
         )
+
+    # This function prepares the structural tag to format reasoning output
+    def prepare_structured_tag(
+        self, original_tag: str | None, tool_server: ToolServer | None
+    ) -> str:
+        if original_tag is None:
+            if tool_server is None:
+                return json.dumps(no_func_reaonsing_tag)
+            else:
+                builtin_tool_list: list[str] = []
+                if tool_server.has_tool("browser"):
+                    builtin_tool_list.append("browser")
+                if tool_server.has_tool("python"):
+                    builtin_tool_list.append("python")
+                if tool_server.has_tool("container"):
+                    builtin_tool_list.append("container")
+
+                if len(builtin_tool_list) > 0:
+                    logger.info("Builtin_tool_list: %s", builtin_tool_list)
+                    func_tag = json.dumps(
+                        tag_with_builtin_funcs(no_func_reaonsing_tag, builtin_tool_list)
+                    )
+                else:
+                    logger.info("Builtin_tool_list is empty")
+                    func_tag = json.dumps(no_func_reaonsing_tag)
+
+                return func_tag
+        else:
+            # There is potential risk for appending the tag to the original tag
+            return original_tag
diff --git a/vllm/sampling_params.py b/vllm/sampling_params.py
index 3f583b393e..4b2a3bc4db 100644
--- a/vllm/sampling_params.py
+++ b/vllm/sampling_params.py
@@ -58,6 +58,7 @@ class StructuredOutputsParams:
                 self.choice is not None,
                 self.grammar is not None,
                 self.json_object is not None,
+                self.structural_tag is not None,
             ]
         )
         if count > 1:
@@ -66,6 +67,37 @@ class StructuredOutputsParams:
                 f"but multiple are specified: {self.__dict__}"
             )
 
+    def all_constraints_none(self) -> bool:
+        """
+        Returns True if all structured-output constraint fields are None.
+        """
+        return all(
+            getattr(self, field) is None
+            for field in (
+                "json",
+                "regex",
+                "choice",
+                "grammar",
+                "json_object",
+                "structural_tag",
+            )
+        )
+
+    def all_non_structural_tag_constraints_none(self) -> bool:
+        """
+        Returns True if all structured-output constraint fields are None.
+        """
+        return all(
+            getattr(self, field) is None
+            for field in (
+                "json",
+                "regex",
+                "choice",
+                "grammar",
+                "json_object",
+            )
+        )
+
 
 @dataclass
 class GuidedDecodingParams(StructuredOutputsParams):
diff --git a/vllm/v1/structured_output/__init__.py b/vllm/v1/structured_output/__init__.py
index 4fb26ab1ce..6f9dbeabd8 100644
--- a/vllm/v1/structured_output/__init__.py
+++ b/vllm/v1/structured_output/__init__.py
@@ -73,6 +73,10 @@ class StructuredOutputManager:
                 )
                 self.reasoner = reasoner_cls(tokenizer=self.tokenizer)
 
+        self.enable_in_reasoning = (
+            self.vllm_config.structured_outputs_config.enable_in_reasoning
+        )
+
     def grammar_init(self, request: Request) -> None:
         if request.structured_output_request is None:
             return
@@ -274,7 +278,13 @@ class StructuredOutputManager:
         return bitmask_tensor.numpy()
 
     def should_fill_bitmask(self, request: Request) -> bool:
+        # NOTE (Hanchen) if enable_in_reasoning is True, it means that
+        # the model needs to be constrained in reasoning. So we should always
+        # enable the bitmask filling.
+
         if self.reasoner is not None:
+            if self.enable_in_reasoning:
+                return True
             assert request.structured_output_request is not None
             if request.structured_output_request.reasoning_ended is None:
                 request.structured_output_request.reasoning_ended = (
@@ -297,6 +307,10 @@ class StructuredOutputManager:
         if self.reasoner is None:
             return True
 
+        # if the model needs structured in reasoning, we should advance
+        if self.enable_in_reasoning:
+            return True
+
         structured_req = request.structured_output_request
         if structured_req.reasoning_ended:
             return True
diff --git a/vllm/v1/structured_output/backend_xgrammar.py b/vllm/v1/structured_output/backend_xgrammar.py
index 074781fb66..4fe4f8848d 100644
--- a/vllm/v1/structured_output/backend_xgrammar.py
+++ b/vllm/v1/structured_output/backend_xgrammar.py
@@ -91,18 +91,20 @@ class XgrammarBackend(StructuredOutputBackend):
             ctx = self.compiler.compile_regex(grammar_spec)
         elif request_type == StructuredOutputOptions.STRUCTURAL_TAG:
             s_tag = json.loads(grammar_spec)
-            tags = [
-                xgr.StructuralTagItem(
-                    begin=s["begin"],
-                    schema=json.dumps(s["schema"]),
-                    end=s["end"],
-                )
-                for s in s_tag["structures"]
-            ]
-            structural_tag = xgr.StructuralTag.from_legacy_structural_tag(
-                tags, s_tag["triggers"]
-            )
-            ctx = self.compiler.compile_structural_tag(structural_tag)
+            if "structures" in s_tag:
+                # Falling back to deprecated method of compiling structural tag
+                tags = [
+                    xgr.StructuralTagItem(
+                        begin=s["begin"],
+                        schema=json.dumps(s["schema"]),
+                        end=s["end"],
+                    )
+                    for s in s_tag["structures"]
+                ]
+                ctx = self.compiler.compile_structural_tag(tags, s_tag["triggers"])
+            else:
+                logger.info("Compiling structural tag grammar_spec: %s", grammar_spec)
+                ctx = self.compiler.compile_structural_tag(grammar_spec)
         else:
             logger.error(
                 "Validation should have already occurred. Please file an issue."
@@ -320,17 +322,19 @@ def validate_xgrammar_grammar(sampling_params: SamplingParams) -> None:
     if so_params.structural_tag:
         try:
             s_tag = json.loads(so_params.structural_tag)
-            tags = [
-                xgr.StructuralTagItem(
-                    begin=s["begin"],
-                    schema=json.dumps(s["schema"]),
-                    end=s["end"],
-                )
-                for s in s_tag["structures"]
-            ]
-            structural_tag = xgr.StructuralTag.from_legacy_structural_tag(
-                tags, s_tag["triggers"]
-            )
-            xgr.Grammar.from_structural_tag(structural_tag)
+
+            # Using the deprecated method of compiling structural tag
+            if "structures" in s_tag:
+                tags = [
+                    xgr.StructuralTagItem(
+                        begin=s["begin"],
+                        schema=json.dumps(s["schema"]),
+                        end=s["end"],
+                    )
+                    for s in s_tag["structures"]
+                ]
+                xgr.Grammar.from_structural_tag(tags, s_tag["triggers"])
+            else:
+                xgr.Grammar.from_structural_tag(so_params.structural_tag)
         except Exception as e:
             raise ValueError("Invalid structural tag specification.") from e
diff --git a/vllm/v1/structured_output/request.py b/vllm/v1/structured_output/request.py
index afe0e4b3f3..94ae36a1ab 100644
--- a/vllm/v1/structured_output/request.py
+++ b/vllm/v1/structured_output/request.py
@@ -28,7 +28,12 @@ class StructuredOutputRequest:
         if sampling_params is None:
             return None
         params = sampling_params.structured_outputs
-        return StructuredOutputRequest(params=params) if params else None
+        if params:
+            if params.all_constraints_none():
+                return None
+            else:
+                return StructuredOutputRequest(params=params)
+        return None
 
     def _check_grammar_completion(self) -> bool:
         # NOTE: We have to lazy import to gate circular imports