diff --git a/src/llm/providers/astraflow.py b/src/llm/providers/astraflow.py
index 2bcc6d6b2..398242a2b 100644
--- a/src/llm/providers/astraflow.py
+++ b/src/llm/providers/astraflow.py
@@ -17,6 +17,7 @@ from llm.core.interface import (
 from llm.core.types import LLMInput, LLMOutput, ModelInfo, ProviderType, ToolCall
 from llm.providers.constants import EMPTY_FILTERED_RESPONSE_ERROR
 from llm.providers.openai_compat import to_openai_message_param
+from llm.providers.reasoning import strip_reasoning
 
 ASTRAFLOW_BASE_URL = "https://api.umodelverse.ai/v1"
 ASTRAFLOW_CN_BASE_URL = "https://api.modelverse.cn/v1"
@@ -105,7 +106,7 @@ class _AstraflowBaseProvider(LLMProvider):
                 }
 
             return LLMOutput(
-                content=choice.message.content or "",
+                content=strip_reasoning(choice.message.content or ""),
                 tool_calls=tool_calls,
                 model=response.model,
                 usage=usage,
diff --git a/src/llm/providers/atlas.py b/src/llm/providers/atlas.py
index d626509cc..8d1dc739f 100644
--- a/src/llm/providers/atlas.py
+++ b/src/llm/providers/atlas.py
@@ -17,6 +17,7 @@ from llm.core.interface import (
 from llm.core.types import LLMInput, LLMOutput, ModelInfo, ProviderType, ToolCall
 from llm.providers.constants import EMPTY_FILTERED_RESPONSE_ERROR
 from llm.providers.openai_compat import to_openai_message_param
+from llm.providers.reasoning import strip_reasoning
 
 ATLAS_BASE_URL = "https://api.atlascloud.ai/v1"
 DEFAULT_ATLAS_MODEL = "deepseek-ai/deepseek-v4-pro"
@@ -122,7 +123,7 @@ class AtlasProvider(LLMProvider):
                 }
 
             return LLMOutput(
-                content=choice.message.content or "",
+                content=strip_reasoning(choice.message.content or ""),
                 tool_calls=tool_calls,
                 model=response.model,
                 usage=usage,
diff --git a/src/llm/providers/ollama.py b/src/llm/providers/ollama.py
index 58505ac7b..6ee4aa8d4 100644
--- a/src/llm/providers/ollama.py
+++ b/src/llm/providers/ollama.py
@@ -19,6 +19,7 @@ from llm.core.types import (
     ProviderType,
     ToolCall,
 )
+from llm.providers.reasoning import prefilled_reasoning_from_env, strip_reasoning
 
 
 class OllamaProvider(LLMProvider):
@@ -28,7 +29,15 @@ class OllamaProvider(LLMProvider):
         self,
         base_url: str | None = None,
         default_model: str | None = None,
+        prefilled_reasoning: bool | None = None,
     ) -> None:
+        # Set for models whose chat template prefills <think>, so replies arrive
+        # as "...</think>answer" without the opening tag.
+        self.prefilled_reasoning = (
+            prefilled_reasoning
+            if prefilled_reasoning is not None
+            else prefilled_reasoning_from_env("OLLAMA_PREFILLED_REASONING")
+        )
         self.base_url = base_url or os.environ.get("OLLAMA_BASE_URL", "http://localhost:11434")
         self.default_model = default_model or os.environ.get("OLLAMA_MODEL", "llama3.2")
         self._models = [
@@ -85,7 +94,9 @@ class OllamaProvider(LLMProvider):
             with urllib.request.urlopen(req, timeout=60) as response:
                 result = json.loads(response.read().decode("utf-8"))
 
-            content = result.get("message", {}).get("content", "")
+            content = strip_reasoning(
+                result.get("message", {}).get("content", ""), prefilled=self.prefilled_reasoning
+            )
 
             tool_calls = None
             if result.get("message", {}).get("tool_calls"):
diff --git a/src/llm/providers/openai.py b/src/llm/providers/openai.py
index 7d00d333c..b81d4ab08 100644
--- a/src/llm/providers/openai.py
+++ b/src/llm/providers/openai.py
@@ -23,12 +23,25 @@ from llm.core.types import (
 )
 from llm.providers.constants import EMPTY_FILTERED_RESPONSE_ERROR
 from llm.providers.openai_compat import to_openai_message_param
+from llm.providers.reasoning import prefilled_reasoning_from_env, strip_reasoning
 
 
 class OpenAIProvider(LLMProvider):
     provider_type = ProviderType.OPENAI
 
-    def __init__(self, api_key: str | None = None, base_url: str | None = None) -> None:
+    def __init__(
+        self,
+        api_key: str | None = None,
+        base_url: str | None = None,
+        prefilled_reasoning: bool | None = None,
+    ) -> None:
+        # Set for self-hosted servers whose chat template prefills <think>, so
+        # replies arrive as "...</think>answer" without the opening tag.
+        self.prefilled_reasoning = (
+            prefilled_reasoning
+            if prefilled_reasoning is not None
+            else prefilled_reasoning_from_env("OPENAI_PREFILLED_REASONING")
+        )
         self.client = OpenAI(
             api_key=api_key or os.environ.get("OPENAI_API_KEY"),
             base_url=base_url,
@@ -106,7 +119,7 @@ class OpenAIProvider(LLMProvider):
                 }
 
             return LLMOutput(
-                content=choice.message.content or "",
+                content=strip_reasoning(choice.message.content or "", prefilled=self.prefilled_reasoning),
                 tool_calls=tool_calls,
                 model=response.model,
                 usage=usage,
diff --git a/src/llm/providers/reasoning.py b/src/llm/providers/reasoning.py
new file mode 100644
index 000000000..4a45c1fc1
--- /dev/null
+++ b/src/llm/providers/reasoning.py
@@ -0,0 +1,71 @@
+"""Helpers for reasoning text that self-hosted models leave in message content."""
+
+from __future__ import annotations
+
+import json
+import os
+
+THINK_OPEN = "<think>"
+THINK_CLOSE = "</think>"
+
+_TRUTHY = {"1", "true", "yes", "on"}
+
+
+def prefilled_reasoning_from_env(name: str) -> bool:
+    """Read a ``*_PREFILLED_REASONING`` flag from the environment."""
+    return os.environ.get(name, "").strip().lower() in _TRUTHY
+
+
+def strip_reasoning(content: str, *, prefilled: bool = False) -> str:
+    """Drop a leading reasoning block from model output.
+
+    Hosted APIs return reasoning in a separate field, but self-hosted reasoning
+    models (vLLM or llama.cpp without a reasoning parser, Ollama models without
+    a thinking template) put it in ``content``.
+
+    By default only a balanced block that opens the output is removed:
+    ``<think>...</think>answer``. Anything else is returned unchanged, so an
+    answer that merely mentions the tags (including a lone or reversed closing
+    tag) is never truncated, and neither is unterminated reasoning (for example
+    when ``max_tokens`` cut the model off before it reached an answer).
+
+    ``prefilled=True`` is for deployments whose chat template prefills the
+    opening tag in the generation prompt, so the model only ever emits
+    ``...</think>answer``. Only enable it when the model is known to behave this
+    way: everything up to the first closing tag is then treated as reasoning.
+    """
+    stripped = content.lstrip()
+    if stripped.startswith(THINK_OPEN):
+        close = stripped.find(THINK_CLOSE, len(THINK_OPEN))
+        if close == -1:
+            return content
+        return stripped[close + len(THINK_CLOSE) :].lstrip()
+
+    if not prefilled:
+        return content
+
+    close = content.find(THINK_CLOSE)
+    if close == -1 or THINK_OPEN in content[:close] or _is_json_document(content):
+        return content
+    return content[close + len(THINK_CLOSE) :].lstrip()
+
+
+def _is_json_document(content: str) -> bool:
+    """Whether ``content`` is one complete JSON object or array.
+
+    Reasoning followed by a closing tag is never valid JSON, so a structured
+    answer that carries the tag inside a string value is left whole. Input
+    nested too deeply for the parser to finish is kept whole as well: reasoning
+    does not open with that many brackets, so cutting it would only risk
+    truncating a real answer.
+    """
+    stripped = content.strip()
+    if not stripped.startswith(("{", "[")):
+        return False
+    try:
+        json.loads(stripped)
+    except RecursionError:
+        return True
+    except ValueError:
+        return False
+    return True
diff --git a/tests/test_reasoning.py b/tests/test_reasoning.py
new file mode 100644
index 000000000..c8d923c17
--- /dev/null
+++ b/tests/test_reasoning.py
@@ -0,0 +1,168 @@
+import json
+import urllib.request
+from io import BytesIO
+from types import SimpleNamespace
+from typing import Any, cast
+
+import pytest
+
+from llm.core.interface import LLMProvider
+from llm.core.types import LLMInput, LLMOutput, Message, Role
+from llm.providers.astraflow import AstraflowProvider
+from llm.providers.ollama import OllamaProvider
+from llm.providers.openai import OpenAIProvider
+from llm.providers.reasoning import strip_reasoning
+
+# Answers that mention the tags without carrying reasoning. None of them may be
+# changed, whatever the deployment's reasoning format.
+LITERAL_ANSWERS = [
+    "Wrap it in <think> and </think> tags.",
+    "Close it with </think> and nothing else.",
+    "</think> comes before <think> here.",
+    '{"closing":"</think>","opening":"<think>"}',
+    '{"note": "<think>x</think>"}',
+]
+
+
+@pytest.mark.parametrize(
+    ("content", "expected"),
+    [
+        ("<think>17 * 3 = 51.</think>51", "51"),
+        ("\n<think>\nplan\n</think>\n\nanswer", "answer"),
+        ('<think>build the object</think>{"answer": 51}', '{"answer": 51}'),
+        ('<think>plan</think>{"closing":"</think>"}', '{"closing":"</think>"}'),
+        ("plain answer", "plain answer"),
+        ("", ""),
+        ("<think>cut off by max_tokens", "<think>cut off by max_tokens"),
+        # Close-only output is ambiguous without a declared format, so it stays.
+        ("17 * 3 = 51.</think>51", "17 * 3 = 51.</think>51"),
+    ],
+)
+def test_strip_reasoning_default(content: str, expected: str) -> None:
+    assert strip_reasoning(content) == expected
+
+
+@pytest.mark.parametrize("content", LITERAL_ANSWERS)
+def test_strip_reasoning_default_preserves_literal_tags(content: str) -> None:
+    assert strip_reasoning(content) == content
+
+
+@pytest.mark.parametrize(
+    ("content", "expected"),
+    [
+        ("17 * 3 = 51.</think>51", "51"),
+        ('build the object\n</think>\n{"answer": 51}', '{"answer": 51}'),
+        ("<think>17 * 3 = 51.</think>51", "51"),
+        ("plain answer", "plain answer"),
+        ("Wrap it in <think> and </think> tags.", "Wrap it in <think> and </think> tags."),
+        ("17 * 3 = 51.</think>", ""),
+        # A complete JSON answer is never reasoning plus a closing tag.
+        ('{"closing":"</think>"}', '{"closing":"</think>"}'),
+        ('  {"closing":"</think>","opening":"<think>"}\n', '  {"closing":"</think>","opening":"<think>"}\n'),
+        ('["</think>", 1]', '["</think>", 1]'),
+    ],
+)
+def test_strip_reasoning_prefilled(content: str, expected: str) -> None:
+    assert strip_reasoning(content, prefilled=True) == expected
+
+
+def test_strip_reasoning_prefilled_survives_deeply_nested_json() -> None:
+    # Deep enough for json.loads to raise RecursionError on every supported Python.
+    depth = 100_000
+    content = "[" * depth + '"</think>"' + "]" * depth
+    assert strip_reasoning(content, prefilled=True) == content
+
+
+def _openai_provider(content: str, **kwargs: Any) -> OpenAIProvider:
+    provider = OpenAIProvider(api_key="test", **kwargs)
+    message = SimpleNamespace(content=content, tool_calls=None)
+    response = SimpleNamespace(
+        choices=[SimpleNamespace(message=message, finish_reason="stop")],
+        model="spark-x2.5",
+        usage=None,
+    )
+    client: Any = SimpleNamespace(
+        chat=SimpleNamespace(completions=SimpleNamespace(create=lambda **params: response))
+    )
+    provider.client = client
+    return provider
+
+
+def _ask(provider: LLMProvider, **kwargs: Any) -> LLMOutput:
+    return provider.generate(LLMInput(messages=[Message(role=Role.USER, content="17*3?")], **kwargs))
+
+
+def test_openai_provider_strips_balanced_reasoning() -> None:
+    assert _ask(_openai_provider("<think>17 * 3 = 51.</think>51")).content == "51"
+
+
+@pytest.mark.parametrize("content", LITERAL_ANSWERS)
+def test_openai_provider_keeps_literal_tags(content: str) -> None:
+    assert _ask(_openai_provider(content)).content == content
+
+
+def test_openai_provider_close_only_needs_opt_in(monkeypatch: pytest.MonkeyPatch) -> None:
+    monkeypatch.delenv("OPENAI_PREFILLED_REASONING", raising=False)
+    assert _ask(_openai_provider("17 * 3 = 51.</think>51")).content == "17 * 3 = 51.</think>51"
+    assert _ask(_openai_provider("17 * 3 = 51.</think>51", prefilled_reasoning=True)).content == "51"
+
+    monkeypatch.setenv("OPENAI_PREFILLED_REASONING", "true")
+    assert _ask(_openai_provider("17 * 3 = 51.</think>51")).content == "51"
+
+
+def test_hosted_provider_never_strips_close_only(monkeypatch: pytest.MonkeyPatch) -> None:
+    monkeypatch.setenv("OPENAI_PREFILLED_REASONING", "true")
+    provider = AstraflowProvider(api_key="test")
+    message = SimpleNamespace(content='{"closing":"</think>"}', tool_calls=None)
+    response = SimpleNamespace(
+        choices=[SimpleNamespace(message=message, finish_reason="stop")],
+        model="deepseek-v3.2",
+        usage=None,
+    )
+    client: Any = SimpleNamespace(
+        chat=SimpleNamespace(completions=SimpleNamespace(create=lambda **params: response))
+    )
+    provider.client = client
+
+    assert _ask(provider).content == '{"closing":"</think>"}'
+
+
+def _fake_ollama(monkeypatch: pytest.MonkeyPatch, content: str) -> list[dict[str, Any]]:
+    sent: list[dict[str, Any]] = []
+
+    def fake_urlopen(request: urllib.request.Request, timeout: float) -> BytesIO:
+        sent.append(json.loads(cast(bytes, request.data)))
+        return BytesIO(json.dumps({"message": {"content": content}, "done_reason": "stop"}).encode())
+
+    monkeypatch.setattr(urllib.request, "urlopen", fake_urlopen)
+    return sent
+
+
+@pytest.mark.parametrize("content", LITERAL_ANSWERS)
+def test_ollama_provider_keeps_literal_tags(monkeypatch: pytest.MonkeyPatch, content: str) -> None:
+    monkeypatch.delenv("OLLAMA_PREFILLED_REASONING", raising=False)
+    _fake_ollama(monkeypatch, content)
+
+    assert _ask(OllamaProvider()).content == content
+
+
+def test_ollama_provider_strips_prefilled_reasoning_when_declared(monkeypatch: pytest.MonkeyPatch) -> None:
+    monkeypatch.delenv("OLLAMA_PREFILLED_REASONING", raising=False)
+    _fake_ollama(monkeypatch, "17 * 3 = 51.</think>51")
+    assert _ask(OllamaProvider()).content == "17 * 3 = 51.</think>51"
+    assert _ask(OllamaProvider(prefilled_reasoning=True)).content == "51"
+
+    monkeypatch.setenv("OLLAMA_PREFILLED_REASONING", "1")
+    assert _ask(OllamaProvider()).content == "51"
+
+
+def test_ollama_stream_flag_still_strips_complete_message(monkeypatch: pytest.MonkeyPatch) -> None:
+    # The provider always requests a single complete message, even when the
+    # caller asks for streaming, so stripping sees the whole reply. If chunked
+    # streaming is added, this needs an incremental stripper instead.
+    sent = _fake_ollama(monkeypatch, '<think>plan</think>{"closing":"</think>"}')
+
+    output = _ask(OllamaProvider(), stream=True)
+
+    assert sent[0]["stream"] is False
+    assert output.content == '{"closing":"</think>"}'