diff --git a/tests/parser/__init__.py b/tests/parser/__init__.py
new file mode 100644
index 000000000000..208f01a7cb5e
--- /dev/null
+++ b/tests/parser/__init__.py
@@ -0,0 +1,2 @@
+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
diff --git a/tests/parser/test_reasoning_tool_transition.py b/tests/parser/test_reasoning_tool_transition.py
new file mode 100644
index 000000000000..c1231b312b00
--- /dev/null
+++ b/tests/parser/test_reasoning_tool_transition.py
@@ -0,0 +1,203 @@
+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+"""
+Regression tests for Gemma4 reasoning-to-tool-call transition.
+
+Issue #40911: partial tool-call markers (e.g. "<|" from "<|tool_call>")
+can leak into content when the reasoning end token and the tool-call
+start token arrive in the same streaming delta.  The fix lives in
+Gemma4ReasoningParser.extract_reasoning_streaming(), which reconstructs
+content from token IDs instead of relying on text-based splitting.
+"""
+
+import pytest
+
+from vllm.entrypoints.openai.chat_completion.protocol import (
+    ChatCompletionRequest,
+)
+from vllm.parser.abstract_parser import _WrappedParser
+from vllm.reasoning.gemma4_reasoning_parser import Gemma4ReasoningParser
+from vllm.tokenizers.registry import get_tokenizer
+from vllm.tool_parsers.gemma4_tool_parser import Gemma4ToolParser
+
+TOKENIZER_NAME = "google/gemma-4-E2B-it"
+
+
+@pytest.fixture(scope="module")
+def tokenizer():
+    return get_tokenizer(TOKENIZER_NAME)
+
+
+@pytest.fixture(scope="module")
+def vocab(tokenizer):
+    return tokenizer.get_vocab()
+
+
+def _make_parser(tokenizer):
+    """Create a DelegatingParser with Gemma4 parsers using a local subclass
+    to avoid polluting _WrappedParser class attributes across tests."""
+
+    class _Gemma4TestParser(_WrappedParser):
+        reasoning_parser_cls = Gemma4ReasoningParser
+        tool_parser_cls = Gemma4ToolParser
+
+    return _Gemma4TestParser(tokenizer)
+
+
+def _encode(tokenizer, text: str) -> list[int]:
+    enc = getattr(tokenizer, "tokenizer", tokenizer)
+    try:
+        return enc.encode(text, add_special_tokens=False)
+    except TypeError:
+        return enc.encode(text)
+
+
+def _request():
+    return ChatCompletionRequest(messages=[], model="test-model")
+
+
+class TestReasoningToToolTransition:
+    """Verify that partial tool-call markers do not leak as content."""
+
+    def test_partial_marker_does_not_leak(self, tokenizer, vocab):
+        """
+        Regression test for #40911.
+
+        Simulates the actual failure mode: the reasoning end token and
+        tool-call start token arrive in the same delta, but delta_text
+        contains only a partial prefix of the tool marker (e.g. "<|")
+        due to incremental detokenization.  The partial prefix must not
+        appear as content.
+        """
+        parser = _make_parser(tokenizer)
+        request = _request()
+
+        channel_end_id = vocab["<channel|>"]
+        channel_start_id = vocab["<|channel>"]
+        tool_call_start_id = vocab.get("<|tool_call>")
+        if tool_call_start_id is None:
+            pytest.skip("<|tool_call> not in vocab")
+
+        # Phase 1: feed reasoning tokens one by one
+        reasoning_tokens = _encode(tokenizer, "thinking about tools")
+        for tid in [channel_start_id] + reasoning_tokens:
+            delta_text = tokenizer.decode([tid], skip_special_tokens=False)
+            parser.parse_delta(
+                delta_text=delta_text,
+                delta_token_ids=[tid],
+                request=request,
+            )
+
+        # Phase 2: reasoning end + tool call start in same delta.
+        # Construct delta_text with a partial prefix of the tool token
+        # to simulate the actual failure mode (incremental detokenization
+        # can produce a prefix-diff that splits the special token text).
+        channel_end_text = tokenizer.decode([channel_end_id], skip_special_tokens=False)
+        tool_token_text = tokenizer.decode(
+            [tool_call_start_id], skip_special_tokens=False
+        )
+        partial_prefix = tool_token_text[:2]  # e.g. "<|"
+        combined_ids = [channel_end_id, tool_call_start_id]
+        combined_text = channel_end_text + partial_prefix
+
+        msg = parser.parse_delta(
+            delta_text=combined_text,
+            delta_token_ids=combined_ids,
+            request=request,
+        )
+
+        if msg is not None and msg.content is not None:
+            assert partial_prefix not in msg.content, (
+                f"Partial tool-call marker leaked as content: {msg.content!r}"
+            )
+
+    def test_separate_deltas_no_leak(self, tokenizer, vocab):
+        """
+        When reasoning end and tool-call start arrive in separate deltas,
+        the tool marker must not leak as content.
+        """
+        parser = _make_parser(tokenizer)
+        request = _request()
+
+        channel_start_id = vocab["<|channel>"]
+        channel_end_id = vocab["<channel|>"]
+        tool_call_start_id = vocab.get("<|tool_call>")
+        if tool_call_start_id is None:
+            pytest.skip("<|tool_call> not in vocab")
+
+        # Reasoning start + content
+        delta_text = tokenizer.decode([channel_start_id], skip_special_tokens=False)
+        parser.parse_delta(
+            delta_text=delta_text,
+            delta_token_ids=[channel_start_id],
+            request=request,
+        )
+        for tid in _encode(tokenizer, "reasoning"):
+            delta_text = tokenizer.decode([tid], skip_special_tokens=False)
+            parser.parse_delta(
+                delta_text=delta_text,
+                delta_token_ids=[tid],
+                request=request,
+            )
+
+        # Reasoning end (separate delta)
+        delta_text = tokenizer.decode([channel_end_id], skip_special_tokens=False)
+        parser.parse_delta(
+            delta_text=delta_text,
+            delta_token_ids=[channel_end_id],
+            request=request,
+        )
+
+        # Tool call start (separate delta) — should not leak as content
+        delta_text = tokenizer.decode([tool_call_start_id], skip_special_tokens=False)
+        msg = parser.parse_delta(
+            delta_text=delta_text,
+            delta_token_ids=[tool_call_start_id],
+            request=request,
+        )
+
+        if msg is not None and msg.content is not None:
+            assert "<|tool_call>" not in msg.content, (
+                f"Tool marker leaked as content: {msg.content!r}"
+            )
+
+    def test_reasoning_end_only_no_leak(self, tokenizer, vocab):
+        """
+        When reasoning ends and no tool tokens follow in the same delta,
+        no content should leak.
+        """
+        parser = _make_parser(tokenizer)
+        request = _request()
+
+        channel_start_id = vocab["<|channel>"]
+        channel_end_id = vocab["<channel|>"]
+
+        # Start reasoning
+        delta_text = tokenizer.decode([channel_start_id], skip_special_tokens=False)
+        parser.parse_delta(
+            delta_text=delta_text,
+            delta_token_ids=[channel_start_id],
+            request=request,
+        )
+
+        # Some reasoning text
+        for tid in _encode(tokenizer, "some thought"):
+            delta_text = tokenizer.decode([tid], skip_special_tokens=False)
+            parser.parse_delta(
+                delta_text=delta_text,
+                delta_token_ids=[tid],
+                request=request,
+            )
+
+        # End reasoning (no tool call follows in this delta)
+        delta_text = tokenizer.decode([channel_end_id], skip_special_tokens=False)
+        msg = parser.parse_delta(
+            delta_text=delta_text,
+            delta_token_ids=[channel_end_id],
+            request=request,
+        )
+
+        if msg is not None and msg.content is not None:
+            assert msg.content.strip() == "", (
+                f"Unexpected content at reasoning end: {msg.content!r}"
+            )
diff --git a/vllm/reasoning/gemma4_reasoning_parser.py b/vllm/reasoning/gemma4_reasoning_parser.py
index 6f2241603f9a..2403a131c099 100644
--- a/vllm/reasoning/gemma4_reasoning_parser.py
+++ b/vllm/reasoning/gemma4_reasoning_parser.py
@@ -158,6 +158,23 @@ def extract_reasoning_streaming(
         if result is None:
             return None
 
+        # When reasoning ends in this delta, the base class splits
+        # delta_text on the end-token string to separate reasoning from
+        # content.  That text split can produce partial special-token
+        # fragments (e.g. "<|" from "<|tool_call>") because the
+        # incremental detokenizer text may not align with token
+        # boundaries.  Reconstruct content from token IDs instead,
+        # which are always exact.  (#40911)
+        if self.end_token_id in delta_token_ids and result.content is not None:
+            content_ids = self.extract_content_ids(list(delta_token_ids))
+            if content_ids:
+                result.content = self.model_tokenizer.decode(
+                    content_ids,
+                    skip_special_tokens=False,
+                )
+            else:
+                result.content = None
+
         if result.reasoning is None:
             return result