diff --git a/libs/partners/mistralai/langchain_mistralai/chat_models.py b/libs/partners/mistralai/langchain_mistralai/chat_models.py
index 809c1a6ae6f..e920fc6784c 100644
--- a/libs/partners/mistralai/langchain_mistralai/chat_models.py
+++ b/libs/partners/mistralai/langchain_mistralai/chat_models.py
@@ -373,6 +373,25 @@ def _clean_block(block: dict) -> dict:
     return new_block
 
 
+def _sanitize_chat_completions_content(content: Any) -> Any:
+    """Strip non-wire keys from text content blocks.
+
+    Mistral's chat completions endpoint rejects unknown fields on tool
+    message content blocks (e.g. the `id` that LangChain auto-generates on
+    `TextContentBlock`). For list content, keep only `type` and `text` on
+    text blocks; pass other blocks and non-list content through unchanged.
+    """
+    if not isinstance(content, list):
+        return content
+    sanitized: list[Any] = []
+    for block in content:
+        if isinstance(block, dict) and block.get("type") == "text" and "text" in block:
+            sanitized.append({"type": "text", "text": block["text"]})
+        else:
+            sanitized.append(block)
+    return sanitized
+
+
 def _format_message_content(content: Any) -> Any:
     """Format message content for the Mistral chat completions wire format.
 
@@ -484,7 +503,7 @@ def _convert_message_to_mistral_chat_message(
     if isinstance(message, ToolMessage):
         return {
             "role": "tool",
-            "content": message.content,
+            "content": _sanitize_chat_completions_content(message.content),
             "name": message.name,
             "tool_call_id": _convert_tool_call_id_to_mistral_compatible(
                 message.tool_call_id
diff --git a/libs/partners/mistralai/tests/unit_tests/test_chat_models.py b/libs/partners/mistralai/tests/unit_tests/test_chat_models.py
index b4cc3fd1531..90be8938812 100644
--- a/libs/partners/mistralai/tests/unit_tests/test_chat_models.py
+++ b/libs/partners/mistralai/tests/unit_tests/test_chat_models.py
@@ -16,6 +16,7 @@ from langchain_core.messages import (
     InvalidToolCall,
     SystemMessage,
     ToolCall,
+    ToolMessage,
 )
 from pydantic import SecretStr
 
@@ -26,11 +27,30 @@ from langchain_mistralai.chat_models import (  # type: ignore[import]
     _convert_tool_call_id_to_mistral_compatible,
     _format_message_content,
     _is_valid_mistral_tool_call_id,
+    _sanitize_chat_completions_content,
 )
 
 os.environ["MISTRAL_API_KEY"] = "foo"
 
 
+def test_sanitize_chat_completions_text_blocks_strips_id() -> None:
+    """LangChain auto-generated `id` on text blocks must not reach the wire.
+
+    Mistral's chat completions endpoint returns 422 with `extra_forbidden`
+    on `messages[*].tool.content.list[...].text.id` if not stripped.
+    """
+    message = ToolMessage(
+        content=[{"type": "text", "text": "foo", "id": "lc_abc123"}],
+        tool_call_id="abc12345",
+    )
+    result = _convert_message_to_mistral_chat_message(message)
+    assert result["content"] == [{"type": "text", "text": "foo"}]
+
+
+def test_sanitize_chat_completions_content_passthrough_string() -> None:
+    assert _sanitize_chat_completions_content("hello") == "hello"
+
+
 def test_mistralai_model_param() -> None:
     llm = ChatMistralAI(model="foo")  # type: ignore[call-arg]
     assert llm.model == "foo"