diff --git a/src/llm_core.py b/src/llm_core.py
index 8da2c46e0..28e432e7b 100644
--- a/src/llm_core.py
+++ b/src/llm_core.py
@@ -832,7 +832,7 @@ def _sanitize_llm_messages(messages: List[Dict]) -> List[Dict]:
     (content=None, since Gemini/Ollama reject tool_calls alongside ""). Dropping
     it leaves the tool result dangling and breaks the next round.
     """
-    allowed = {"role", "content", "name", "tool_call_id", "tool_calls", "function_call"}
+    allowed = {"role", "content", "name", "tool_call_id", "tool_calls", "function_call", "reasoning_content"}
     cleaned = []
     for msg in messages or []:
         if not isinstance(msg, dict):
diff --git a/tests/test_sanitize_preserves_reasoning.py b/tests/test_sanitize_preserves_reasoning.py
new file mode 100644
index 000000000..d324992e5
--- /dev/null
+++ b/tests/test_sanitize_preserves_reasoning.py
@@ -0,0 +1,91 @@
+"""Regression: _sanitize_llm_messages must preserve reasoning_content.
+
+Providers like Moonshot (Kimi K2.5/K2.6) require reasoning_content on
+assistant tool-call messages. Stripping it causes HTTP 400 in multi-turn
+tool calling when thinking mode is enabled.
+
+See: https://github.com/pewdiepie-archdaemon/odysseus/issues/3118
+"""
+import sys
+from unittest.mock import MagicMock
+
+# Mock heavy dependencies before importing.
+for mod in [
+    'sqlalchemy', 'sqlalchemy.orm', 'sqlalchemy.ext', 'sqlalchemy.ext.declarative',
+    'sqlalchemy.ext.hybrid', 'sqlalchemy.sql', 'sqlalchemy.sql.expression',
+    'src.database', 'src.agent_tools', 'core.models', 'core.database',
+]:
+    if mod not in sys.modules:
+        sys.modules[mod] = MagicMock()
+
+from src.llm_core import _sanitize_llm_messages  # noqa: E402
+
+
+def test_sanitize_preserves_reasoning_content_on_assistant_tool_call():
+    """reasoning_content must survive sanitization.
+
+    Providers like Moonshot (Kimi K2.5/K2.6) require reasoning_content to be
+    present on assistant tool-call messages in multi-turn conversations.  Stripping
+    it causes HTTP 400: "thinking is enabled but reasoning_content is missing in
+    assistant tool call message at index N".
+    """
+    messages = [
+        {
+            "role": "assistant",
+            "content": None,
+            "reasoning_content": "Let me think about which tool to use...",
+            "tool_calls": [
+                {"id": "call_1", "type": "function",
+                 "function": {"name": "web_search", "arguments": '{"q":"test"}'}},
+            ],
+        },
+        {
+            "role": "tool",
+            "content": "search results here",
+            "tool_call_id": "call_1",
+        },
+    ]
+
+    out = _sanitize_llm_messages(messages)
+    assistant = next(m for m in out if m["role"] == "assistant")
+
+    assert assistant.get("reasoning_content") == "Let me think about which tool to use...", (
+        "reasoning_content was stripped during sanitization; Moonshot/Kimi API will "
+        "reject this as HTTP 400 in multi-turn tool calling"
+    )
+    assert assistant.get("tool_calls"), "tool_calls were lost"
+    assert assistant["content"] is None
+
+
+def test_sanitize_preserves_reasoning_content_on_plain_assistant():
+    """reasoning_content also survives on assistant messages without tool_calls."""
+    messages = [
+        {
+            "role": "assistant",
+            "content": "Here is my answer.",
+            "reasoning_content": "Internal reasoning that should be kept for the next turn.",
+        },
+    ]
+
+    out = _sanitize_llm_messages(messages)
+    assert len(out) == 1
+    assert out[0]["reasoning_content"] == "Internal reasoning that should be kept for the next turn."
+
+
+def test_sanitize_strips_unknown_fields_but_keeps_reasoning_content():
+    """Only allowed fields survive; reasoning_content is now in the allow-list."""
+    messages = [
+        {
+            "role": "assistant",
+            "content": "reply",
+            "reasoning_content": "thinking text",
+            "some_custom_field": "should be stripped",
+            "another_meta": 123,
+        },
+    ]
+
+    out = _sanitize_llm_messages(messages)
+    assert len(out) == 1
+    assert "reasoning_content" in out[0], "reasoning_content was stripped"
+    assert "some_custom_field" not in out[0], "custom field was not stripped"
+    assert "another_meta" not in out[0], "custom field was not stripped"