diff --git a/run_agent.py b/run_agent.py
index 3f10f36a0..f7a929118 100644
--- a/run_agent.py
+++ b/run_agent.py
@@ -7706,13 +7706,26 @@ class AIAgent:
 
         return msg
 
+    def _needs_kimi_tool_reasoning(self) -> bool:
+        """Return True when the current provider is Kimi / Moonshot thinking mode.
+
+        Kimi ``/coding`` and Moonshot thinking mode both require
+        ``reasoning_content`` on every assistant tool-call message; omitting
+        it causes the next replay to fail with HTTP 400.
+        """
+        return (
+            self.provider in {"kimi-coding", "kimi-coding-cn"}
+            or base_url_host_matches(self.base_url, "api.kimi.com")
+            or base_url_host_matches(self.base_url, "moonshot.ai")
+            or base_url_host_matches(self.base_url, "moonshot.cn")
+        )
+
     def _needs_deepseek_tool_reasoning(self) -> bool:
         """Return True when the current provider is DeepSeek thinking mode.
 
-        Used to decide whether to store reasoning_content on tool-call
-        assistant messages. DeepSeek V4 thinking mode requires this field
-        on every assistant tool-call turn; omitting it causes HTTP 400
-        when the message is replayed in a subsequent API request (#15250).
+        DeepSeek V4 thinking mode requires ``reasoning_content`` on every
+        assistant tool-call turn; omitting it causes HTTP 400 when the
+        message is replayed in a subsequent API request (#15250).
         """
         provider = (self.provider or "").lower()
         model = (self.model or "").lower()
@@ -7737,17 +7750,14 @@ class AIAgent:
             api_msg["reasoning_content"] = normalized_reasoning
             return
 
-        provider = (self.provider or "").lower()
-        model = (self.model or "").lower()
-        needs_tool_reasoning_echo = (
-            provider in {"kimi-coding", "kimi-coding-cn", "deepseek"}
-            or "deepseek" in model
-            or base_url_host_matches(self.base_url, "api.kimi.com")
-            or base_url_host_matches(self.base_url, "moonshot.ai")
-            or base_url_host_matches(self.base_url, "moonshot.cn")
-            or base_url_host_matches(self.base_url, "api.deepseek.com")
-        )
-        if needs_tool_reasoning_echo and source_msg.get("tool_calls"):
+        # Providers that require an echoed reasoning_content on every
+        # assistant tool-call turn. Detection logic lives in the per-provider
+        # helpers so both the creation path (_build_assistant_message) and
+        # this replay path stay in sync.
+        if source_msg.get("tool_calls") and (
+            self._needs_kimi_tool_reasoning()
+            or self._needs_deepseek_tool_reasoning()
+        ):
             api_msg["reasoning_content"] = ""
 
     @staticmethod
diff --git a/tests/run_agent/test_deepseek_reasoning_content_echo.py b/tests/run_agent/test_deepseek_reasoning_content_echo.py
new file mode 100644
index 000000000..98feea859
--- /dev/null
+++ b/tests/run_agent/test_deepseek_reasoning_content_echo.py
@@ -0,0 +1,213 @@
+"""Regression test: DeepSeek V4 thinking mode reasoning_content echo.
+
+DeepSeek V4-flash / V4-pro thinking mode requires ``reasoning_content`` on
+every assistant message that carries ``tool_calls``. When a persisted
+session replays an assistant tool-call turn that was recorded without the
+field, DeepSeek rejects the next request with HTTP 400::
+
+    The reasoning_content in the thinking mode must be passed back to the API.
+
+Fix covers three paths:
+
+1. ``_build_assistant_message`` — new tool-call messages without raw
+   reasoning_content get ``""`` pinned at creation time so nothing gets
+   persisted poisoned.
+2. ``_copy_reasoning_content_for_api`` — already-poisoned history replays
+   with ``reasoning_content=""`` injected defensively.
+3. Detection covers three signals: ``provider == "deepseek"``,
+   ``"deepseek" in model``, and ``api.deepseek.com`` host match. The third
+   catches custom-provider setups pointing at DeepSeek.
+
+Refs #15250 / #15353.
+"""
+
+from __future__ import annotations
+
+import pytest
+
+from run_agent import AIAgent
+
+
+def _make_agent(provider: str = "", model: str = "", base_url: str = "") -> AIAgent:
+    agent = object.__new__(AIAgent)
+    agent.provider = provider
+    agent.model = model
+    agent.base_url = base_url
+    return agent
+
+
+class TestNeedsDeepSeekToolReasoning:
+    """_needs_deepseek_tool_reasoning() recognises all three detection signals."""
+
+    def test_provider_deepseek(self) -> None:
+        agent = _make_agent(provider="deepseek", model="deepseek-v4-flash")
+        assert agent._needs_deepseek_tool_reasoning() is True
+
+    def test_model_substring(self) -> None:
+        # Custom provider pointing at DeepSeek with provider='custom'
+        agent = _make_agent(provider="custom", model="deepseek-v4-pro")
+        assert agent._needs_deepseek_tool_reasoning() is True
+
+    def test_base_url_host(self) -> None:
+        agent = _make_agent(
+            provider="custom",
+            model="some-aliased-name",
+            base_url="https://api.deepseek.com/v1",
+        )
+        assert agent._needs_deepseek_tool_reasoning() is True
+
+    def test_provider_case_insensitive(self) -> None:
+        agent = _make_agent(provider="DeepSeek", model="")
+        assert agent._needs_deepseek_tool_reasoning() is True
+
+    def test_non_deepseek_provider(self) -> None:
+        agent = _make_agent(
+            provider="openrouter",
+            model="anthropic/claude-sonnet-4.6",
+            base_url="https://openrouter.ai/api/v1",
+        )
+        assert agent._needs_deepseek_tool_reasoning() is False
+
+    def test_empty_everything(self) -> None:
+        agent = _make_agent()
+        assert agent._needs_deepseek_tool_reasoning() is False
+
+
+class TestCopyReasoningContentForApi:
+    """_copy_reasoning_content_for_api pads reasoning_content for DeepSeek tool-calls."""
+
+    def test_deepseek_tool_call_poisoned_history_gets_empty_string(self) -> None:
+        """Already-poisoned history (no reasoning_content, no reasoning) gets ''."""
+        agent = _make_agent(provider="deepseek", model="deepseek-v4-flash")
+        source = {
+            "role": "assistant",
+            "content": "",
+            "tool_calls": [{"id": "c1", "function": {"name": "terminal"}}],
+        }
+        api_msg: dict = {}
+        agent._copy_reasoning_content_for_api(source, api_msg)
+        assert api_msg.get("reasoning_content") == ""
+
+    def test_deepseek_assistant_no_tool_call_left_alone(self) -> None:
+        """Plain assistant turns without tool_calls don't get padded."""
+        agent = _make_agent(provider="deepseek", model="deepseek-v4-flash")
+        source = {"role": "assistant", "content": "hello"}
+        api_msg: dict = {}
+        agent._copy_reasoning_content_for_api(source, api_msg)
+        assert "reasoning_content" not in api_msg
+
+    def test_deepseek_explicit_reasoning_content_preserved(self) -> None:
+        """When reasoning_content is already set, it's copied verbatim."""
+        agent = _make_agent(provider="deepseek", model="deepseek-v4-flash")
+        source = {
+            "role": "assistant",
+            "reasoning_content": "<think>real chain of thought</think>",
+            "tool_calls": [{"id": "c1", "function": {"name": "terminal"}}],
+        }
+        api_msg: dict = {}
+        agent._copy_reasoning_content_for_api(source, api_msg)
+        assert api_msg["reasoning_content"] == "<think>real chain of thought</think>"
+
+    def test_deepseek_reasoning_field_promoted(self) -> None:
+        """When only 'reasoning' is set, it gets promoted to reasoning_content."""
+        agent = _make_agent(provider="deepseek", model="deepseek-v4-flash")
+        source = {
+            "role": "assistant",
+            "reasoning": "thought trace",
+            "tool_calls": [{"id": "c1", "function": {"name": "terminal"}}],
+        }
+        api_msg: dict = {}
+        agent._copy_reasoning_content_for_api(source, api_msg)
+        assert api_msg["reasoning_content"] == "thought trace"
+
+    def test_kimi_path_still_works(self) -> None:
+        """Existing Kimi detection still pads reasoning_content."""
+        agent = _make_agent(provider="kimi-coding", model="kimi-k2.5")
+        source = {
+            "role": "assistant",
+            "content": "",
+            "tool_calls": [{"id": "c1", "function": {"name": "terminal"}}],
+        }
+        api_msg: dict = {}
+        agent._copy_reasoning_content_for_api(source, api_msg)
+        assert api_msg.get("reasoning_content") == ""
+
+    def test_kimi_moonshot_base_url(self) -> None:
+        agent = _make_agent(
+            provider="custom", model="kimi-k2", base_url="https://api.moonshot.ai/v1"
+        )
+        source = {
+            "role": "assistant",
+            "content": "",
+            "tool_calls": [{"id": "c1", "function": {"name": "terminal"}}],
+        }
+        api_msg: dict = {}
+        agent._copy_reasoning_content_for_api(source, api_msg)
+        assert api_msg.get("reasoning_content") == ""
+
+    def test_non_thinking_provider_not_padded(self) -> None:
+        """Providers that don't require the echo are untouched."""
+        agent = _make_agent(
+            provider="openrouter",
+            model="anthropic/claude-sonnet-4.6",
+            base_url="https://openrouter.ai/api/v1",
+        )
+        source = {
+            "role": "assistant",
+            "content": "",
+            "tool_calls": [{"id": "c1", "function": {"name": "terminal"}}],
+        }
+        api_msg: dict = {}
+        agent._copy_reasoning_content_for_api(source, api_msg)
+        assert "reasoning_content" not in api_msg
+
+    def test_deepseek_custom_base_url(self) -> None:
+        """Custom provider pointing at api.deepseek.com is detected via host."""
+        agent = _make_agent(
+            provider="custom",
+            model="whatever",
+            base_url="https://api.deepseek.com/v1",
+        )
+        source = {
+            "role": "assistant",
+            "content": "",
+            "tool_calls": [{"id": "c1", "function": {"name": "terminal"}}],
+        }
+        api_msg: dict = {}
+        agent._copy_reasoning_content_for_api(source, api_msg)
+        assert api_msg.get("reasoning_content") == ""
+
+    def test_non_assistant_role_ignored(self) -> None:
+        """User/tool messages are left alone."""
+        agent = _make_agent(provider="deepseek", model="deepseek-v4-flash")
+        source = {"role": "user", "content": "hi"}
+        api_msg: dict = {}
+        agent._copy_reasoning_content_for_api(source, api_msg)
+        assert "reasoning_content" not in api_msg
+
+
+class TestNeedsKimiToolReasoning:
+    """The extracted _needs_kimi_tool_reasoning() helper keeps Kimi behavior intact."""
+
+    @pytest.mark.parametrize(
+        "provider,base_url",
+        [
+            ("kimi-coding", ""),
+            ("kimi-coding-cn", ""),
+            ("custom", "https://api.kimi.com/v1"),
+            ("custom", "https://api.moonshot.ai/v1"),
+            ("custom", "https://api.moonshot.cn/v1"),
+        ],
+    )
+    def test_kimi_signals(self, provider: str, base_url: str) -> None:
+        agent = _make_agent(provider=provider, model="kimi-k2", base_url=base_url)
+        assert agent._needs_kimi_tool_reasoning() is True
+
+    def test_non_kimi_provider(self) -> None:
+        agent = _make_agent(
+            provider="openrouter",
+            model="moonshotai/kimi-k2",
+            base_url="https://openrouter.ai/api/v1",
+        )
+        # model name contains 'moonshot' but host is openrouter — should be False
+        assert agent._needs_kimi_tool_reasoning() is False