diff --git a/tests/parser/engine/test_qwen3_reasoning.py b/tests/parser/engine/test_qwen3_reasoning.py
index cac3e3b2a6..a9bec494bc 100644
--- a/tests/parser/engine/test_qwen3_reasoning.py
+++ b/tests/parser/engine/test_qwen3_reasoning.py
@@ -27,6 +27,8 @@ _THINK_START_ID = 50
 _THINK_END_ID = 51
 _TOOL_CALL_ID = 60
 _TOOL_CALL_END_ID = 61
+_IM_START_ID = 70
+_IM_END_ID = 71
 _TEXT_ID = 100
 
 _QWEN3_VOCAB = {
@@ -34,6 +36,8 @@ _QWEN3_VOCAB = {
     "</think>": _THINK_END_ID,
     "<tool_call>": _TOOL_CALL_ID,
     "</tool_call>": _TOOL_CALL_END_ID,
+    "<|im_start|>": _IM_START_ID,
+    "<|im_end|>": _IM_END_ID,
 }
 
 
@@ -192,6 +196,72 @@ class TestIsReasoningEnd:
         assert not parser.is_reasoning_end([])
 
 
+class TestIsReasoningEndTurnBoundaries:
+    """Reasoning markers from earlier turns must not mark the new turn's
+    reasoning as finished (think-marker injection / interleaved-thinking
+    replay).
+    """
+
+    def test_injected_think_end_in_history_not_end(self, parser):
+        assert not parser.is_reasoning_end(
+            [_IM_START_ID, _TEXT_ID, _THINK_END_ID, _IM_END_ID, _IM_START_ID]
+        )
+
+    def test_replayed_reasoning_block_in_history_not_end(self, parser):
+        assert not parser.is_reasoning_end(
+            [
+                _IM_START_ID,
+                _THINK_START_ID,
+                _TEXT_ID,
+                _THINK_END_ID,
+                _TEXT_ID,
+                _IM_END_ID,
+                _IM_START_ID,
+            ]
+        )
+
+    def test_think_end_in_current_turn_is_end(self, parser):
+        assert parser.is_reasoning_end(
+            [_IM_START_ID, _TEXT_ID, _IM_END_ID, _IM_START_ID, _THINK_END_ID]
+        )
+
+    def test_unpaired_tool_call_in_history_not_end(self, parser):
+        assert not parser.is_reasoning_end(
+            [_TOOL_CALL_ID, _TEXT_ID, _IM_END_ID, _IM_START_ID]
+        )
+
+    def test_continued_turn_with_closed_thinking_is_end(self, parser):
+        assert parser.is_reasoning_end(
+            [_IM_START_ID, _THINK_START_ID, _TEXT_ID, _THINK_END_ID, _TEXT_ID]
+        )
+
+    def test_continued_turn_with_open_thinking_not_end(self, parser):
+        assert not parser.is_reasoning_end([_IM_START_ID, _THINK_START_ID, _TEXT_ID])
+
+    def test_empty_boundary_config_keeps_global_walk(self):
+        """Configs without turn_boundary_tokens keep the pre-existing
+        behavior: any </think> in the sequence ends reasoning."""
+        cfg = qwen3_config(turn_boundary_tokens=frozenset())
+        parser = Qwen3Parser(
+            make_mock_tokenizer(_QWEN3_VOCAB), parser_engine_config=cfg
+        )
+        assert parser.is_reasoning_end(
+            [_IM_START_ID, _TEXT_ID, _THINK_END_ID, _IM_END_ID, _IM_START_ID]
+        )
+
+    def test_boundary_tokens_missing_from_vocab_keep_global_walk(self):
+        """Boundary tokens absent from the vocabulary resolve to nothing,
+        so subclasses reusing the default on a non-ChatML vocab keep the
+        pre-existing behavior."""
+        vocab = {
+            token: token_id
+            for token, token_id in _QWEN3_VOCAB.items()
+            if not token.startswith("<|im_")
+        }
+        parser = Qwen3Parser(make_mock_tokenizer(vocab))
+        assert parser.is_reasoning_end([_TEXT_ID, _THINK_END_ID, _TEXT_ID])
+
+
 class TestDelegatingPromptDetection:
     def test_prompt_tool_example_does_not_skip_streaming_reasoning(
         self, mock_tokenizer, mock_request
@@ -211,6 +281,56 @@ class TestDelegatingPromptDetection:
         assert delta.reasoning == "thinking"
         assert delta.content is None
 
+    def test_injected_think_end_in_history_keeps_streaming_reasoning(
+        self, mock_tokenizer, mock_request
+    ):
+        parser = _Qwen3DelegatingParser(mock_tokenizer)
+        prompt_ids = [
+            _IM_START_ID,
+            _TEXT_ID,
+            _THINK_END_ID,
+            _TEXT_ID,
+            _IM_END_ID,
+            _IM_START_ID,
+        ]
+
+        delta = parser.parse_delta(
+            "thinking",
+            [_TEXT_ID],
+            mock_request,
+            prompt_token_ids=prompt_ids,
+            finished=False,
+        )
+
+        assert delta is not None
+        assert delta.reasoning == "thinking"
+        assert delta.content is None
+
+    def test_think_end_in_generation_prompt_still_seeds_reasoning_ended(
+        self, mock_tokenizer, mock_request
+    ):
+        parser = _Qwen3DelegatingParser(mock_tokenizer)
+        prompt_ids = [
+            _IM_START_ID,
+            _TEXT_ID,
+            _IM_END_ID,
+            _IM_START_ID,
+            _THINK_START_ID,
+            _THINK_END_ID,
+        ]
+
+        delta = parser.parse_delta(
+            "answer",
+            [_TEXT_ID],
+            mock_request,
+            prompt_token_ids=prompt_ids,
+            finished=False,
+        )
+
+        assert delta is not None
+        assert delta.reasoning is None
+        assert delta.content == "answer"
+
 
 class TestStreaming:
     def test_basic_streaming(self, parser):
diff --git a/vllm/parser/engine/parser_engine.py b/vllm/parser/engine/parser_engine.py
index 5ed837d22d..545bec6500 100644
--- a/vllm/parser/engine/parser_engine.py
+++ b/vllm/parser/engine/parser_engine.py
@@ -147,6 +147,11 @@ class ParserEngine(Parser):
             self._reasoning_start_token_id = vocab.get(start_text)
         if end_text:
             self._reasoning_end_token_id = vocab.get(end_text)
+        self._turn_boundary_token_ids: frozenset[int] = frozenset(
+            token_id
+            for token in parser_engine_config.turn_boundary_tokens
+            if (token_id := vocab.get(token)) is not None
+        )
 
     @property
     def reasoning_start_str(self) -> str | None:
@@ -601,11 +606,17 @@ class ParserEngine(Parser):
         if end_id is not None:
             if not input_ids:
                 return self.parser_engine_config.initial_state != ParserState.REASONING
+            boundary_ids = self._turn_boundary_token_ids
             for i in range(len(input_ids) - 1, -1, -1):
-                if input_ids[i] == end_id:
+                token_id = input_ids[i]
+                if token_id == end_id:
                     return True
-                if start_id is not None and input_ids[i] == start_id:
+                if start_id is not None and token_id == start_id:
                     return False
+                if token_id in boundary_ids:
+                    return (
+                        self.parser_engine_config.initial_state != ParserState.REASONING
+                    )
             return False
         return self._reasoning_ended
 
diff --git a/vllm/parser/engine/parser_engine_config.py b/vllm/parser/engine/parser_engine_config.py
index ad83e33149..4799604dab 100644
--- a/vllm/parser/engine/parser_engine_config.py
+++ b/vllm/parser/engine/parser_engine_config.py
@@ -84,6 +84,10 @@ class ParserEngineConfig:
     # Special tokens exempt from auto-drop but not state-machine terminals.
     preserve_tokens: frozenset[str] = field(default_factory=frozenset)
 
+    # Special tokens delimiting conversation turns in the prompt only
+    # considers reasoning markers after the last boundary token.
+    turn_boundary_tokens: frozenset[str] = field(default_factory=frozenset)
+
     # Prevents trailing-whitespace accumulation across multi-turn conversations.
     strip_trailing_reasoning_whitespace: bool = True
 
diff --git a/vllm/parser/nemotron_v3.py b/vllm/parser/nemotron_v3.py
index 7884480fee..3cdd8b70d7 100644
--- a/vllm/parser/nemotron_v3.py
+++ b/vllm/parser/nemotron_v3.py
@@ -16,7 +16,7 @@ import dataclasses
 import functools
 from typing import TYPE_CHECKING
 
-from vllm.parser.qwen3 import Qwen3Parser, qwen3_config
+from vllm.parser.qwen3 import CHATML_TURN_BOUNDARIES, Qwen3Parser, qwen3_config
 
 if TYPE_CHECKING:
     from vllm.entrypoints.openai.chat_completion.protocol import (
@@ -33,7 +33,7 @@ if TYPE_CHECKING:
 @functools.cache
 def nemotron_v3_config(thinking: bool = True) -> ParserEngineConfig:
     return dataclasses.replace(
-        qwen3_config(thinking=thinking),
+        qwen3_config(thinking=thinking, turn_boundary_tokens=CHATML_TURN_BOUNDARIES),
         name="nemotron_v3",
         strip_trailing_reasoning_whitespace=True,
     )
diff --git a/vllm/parser/qwen3.py b/vllm/parser/qwen3.py
index 0041a46eaa..8edf42e263 100644
--- a/vllm/parser/qwen3.py
+++ b/vllm/parser/qwen3.py
@@ -42,6 +42,7 @@ THINK_START = "<think>"
 THINK_END = "</think>"
 TOOL_CALL_START = "<tool_call>"
 TOOL_CALL_END = "</tool_call>"
+CHATML_TURN_BOUNDARIES = frozenset(("<|im_start|>", "<|im_end|>"))
 FUNC_PREFIX = "<function="
 FUNC_END = "</function>"
 PARAM_START = "<parameter="
@@ -94,10 +95,12 @@ def qwen3_config(
     think_end: str = THINK_END,
     tool_start: str = TOOL_CALL_START,
     tool_end: str = TOOL_CALL_END,
+    turn_boundary_tokens: frozenset[str] = frozenset(),
 ) -> ParserEngineConfig:
     return ParserEngineConfig(
         name=name,
         initial_state=ParserState.REASONING if thinking else ParserState.CONTENT,
+        turn_boundary_tokens=turn_boundary_tokens,
         terminals={
             # Reasoning terminals
             "THINK_START": think_start,
@@ -215,6 +218,7 @@ class Qwen3Parser(ParserEngine):
     THINK_END = THINK_END
     TOOL_START = TOOL_CALL_START
     TOOL_END = TOOL_CALL_END
+    TURN_BOUNDARIES: frozenset[str] = CHATML_TURN_BOUNDARIES
 
     def __init__(
         self,
@@ -233,6 +237,7 @@ class Qwen3Parser(ParserEngine):
                 think_end=self.THINK_END,
                 tool_start=self.TOOL_START,
                 tool_end=self.TOOL_END,
+                turn_boundary_tokens=self.TURN_BOUNDARIES,
             ),
         )
         super().__init__(
@@ -259,6 +264,7 @@ class Qwen3Parser(ParserEngine):
         tool_call_id = self._tool_call_token_id
         tool_call_end_id = self._tool_call_end_token_id
         reasoning_start_id = self._reasoning_start_token_id
+        boundary_ids = self._turn_boundary_token_ids
         if tool_call_id is not None:
             for i in range(len(input_ids) - 1, -1, -1):
                 if (
@@ -266,6 +272,8 @@ class Qwen3Parser(ParserEngine):
                     and input_ids[i] == reasoning_start_id
                 ):
                     return False
+                if input_ids[i] in boundary_ids:
+                    return False
                 if input_ids[i] == tool_call_id:
                     if tool_call_end_id is not None and any(
                         input_ids[j] == tool_call_end_id