diff --git a/libs/partners/openrouter/langchain_openrouter/chat_models.py b/libs/partners/openrouter/langchain_openrouter/chat_models.py
index 7a510912766..6ff873f5ce8 100644
--- a/libs/partners/openrouter/langchain_openrouter/chat_models.py
+++ b/libs/partners/openrouter/langchain_openrouter/chat_models.py
@@ -2,6 +2,7 @@
 
 from __future__ import annotations
 
+import contextlib
 import json
 import warnings
 from collections.abc import AsyncIterator, Callable, Iterator, Mapping, Sequence
@@ -48,6 +49,7 @@ from langchain_core.messages.ai import (
 from langchain_core.messages.block_translators.openai import (
     convert_to_openai_data_block,
 )
+from langchain_core.messages.tool import tool_call_chunk
 from langchain_core.output_parsers import JsonOutputParser, PydanticOutputParser
 from langchain_core.output_parsers.base import OutputParserLike
 from langchain_core.output_parsers.openai_tools import (
@@ -135,7 +137,10 @@ class ChatOpenRouter(BaseChatModel):
     """
 
     client: Any = Field(default=None, exclude=True)
-    """OpenRouter client instance (`openrouter.OpenRouter`)."""
+    """Underlying SDK client (`openrouter.OpenRouter`).
+
+    Created automatically during validation.
+    """
 
     model_name: str = Field(alias="model")
     """The name of the model, e.g. `'anthropic/claude-sonnet-4-5'`."""
@@ -539,6 +544,14 @@ class ChatOpenRouter(BaseChatModel):
         if not isinstance(response, dict):
             response = response.model_dump(by_alias=True)
 
+        if error := response.get("error"):
+            msg = (
+                f"OpenRouter API returned an error: "
+                f"{error.get('message', str(error))} "
+                f"(code: {error.get('code', 'unknown')})"
+            )
+            raise ValueError(msg)
+
         generations = []
         token_usage = response.get("usage") or {}
 
@@ -573,20 +586,21 @@ class ChatOpenRouter(BaseChatModel):
             if token_usage is not None:
                 for k, v in token_usage.items():
                     if v is None:
-                        overall_token_usage.setdefault(k, v)
-                    elif k not in overall_token_usage:
-                        overall_token_usage[k] = v
-                    elif isinstance(v, dict):
-                        for nested_k, nested_v in v.items():
-                            if (
-                                nested_k in overall_token_usage[k]
-                                and nested_v is not None
-                            ):
-                                overall_token_usage[k][nested_k] += nested_v
-                            else:
-                                overall_token_usage[k][nested_k] = nested_v
+                        continue
+                    if k in overall_token_usage:
+                        if isinstance(v, dict):
+                            for nested_k, nested_v in v.items():
+                                if (
+                                    nested_k in overall_token_usage[k]
+                                    and nested_v is not None
+                                ):
+                                    overall_token_usage[k][nested_k] += nested_v
+                                else:
+                                    overall_token_usage[k][nested_k] = nested_v
+                        else:
+                            overall_token_usage[k] += v
                     else:
-                        overall_token_usage[k] += v
+                        overall_token_usage[k] = v
         return {"token_usage": overall_token_usage, "model_name": self.model_name}
 
     def bind_tools(
@@ -889,7 +903,13 @@ def _convert_dict_to_message(_dict: Mapping[str, Any]) -> BaseMessage:  # noqa:
             tool_call_id=_dict.get("tool_call_id"),
             additional_kwargs=additional_kwargs,
         )
-    return ChatMessage(content=_dict.get("content", ""), role=role)  # type: ignore[arg-type]
+    if role is None:
+        msg = (
+            f"OpenRouter response message is missing the 'role' field. "
+            f"Message keys: {list(_dict.keys())}"
+        )
+        raise ValueError(msg)
+    return ChatMessage(content=_dict.get("content", ""), role=role)
 
 
 def _convert_chunk_to_message_chunk(
@@ -909,9 +929,19 @@ def _convert_chunk_to_message_chunk(
     role = cast("str", _dict.get("role"))
     content = cast("str", _dict.get("content") or "")
     additional_kwargs: dict = {}
+    tool_call_chunks: list = []
 
-    if _dict.get("tool_calls"):
-        additional_kwargs["tool_calls"] = _dict["tool_calls"]
+    if raw_tool_calls := _dict.get("tool_calls"):
+        with contextlib.suppress(KeyError):
+            tool_call_chunks = [
+                tool_call_chunk(
+                    name=rtc["function"].get("name"),
+                    args=rtc["function"].get("arguments"),
+                    id=rtc.get("id"),
+                    index=rtc["index"],
+                )
+                for rtc in raw_tool_calls
+            ]
 
     if role == "user" or default_class == HumanMessageChunk:
         return HumanMessageChunk(content=content)
@@ -920,13 +950,13 @@ def _convert_chunk_to_message_chunk(
             additional_kwargs["reasoning_content"] = reasoning
         if reasoning_details := _dict.get("reasoning_details"):
             additional_kwargs["reasoning_details"] = reasoning_details
-        # Extract usage from chunk if present
         usage_metadata = None
         if usage := chunk.get("usage"):
             usage_metadata = _create_usage_metadata(usage)
         return AIMessageChunk(
             content=content,
             additional_kwargs=additional_kwargs,
+            tool_call_chunks=tool_call_chunks,  # type: ignore[arg-type]
             usage_metadata=usage_metadata,  # type: ignore[arg-type]
             response_metadata={"model_provider": "openrouter"},
         )
diff --git a/libs/partners/openrouter/tests/unit_tests/test_chat_models.py b/libs/partners/openrouter/tests/unit_tests/test_chat_models.py
index 4849e45ffbf..37022ca0769 100644
--- a/libs/partners/openrouter/tests/unit_tests/test_chat_models.py
+++ b/libs/partners/openrouter/tests/unit_tests/test_chat_models.py
@@ -1042,7 +1042,11 @@ class TestStreamingChunks:
         }
         message_chunk = _convert_chunk_to_message_chunk(chunk, AIMessageChunk)
         assert isinstance(message_chunk, AIMessageChunk)
-        assert "tool_calls" in message_chunk.additional_kwargs
+        assert len(message_chunk.tool_call_chunks) == 1
+        assert message_chunk.tool_call_chunks[0]["name"] == "get_weather"
+        assert message_chunk.tool_call_chunks[0]["args"] == '{"loc'
+        assert message_chunk.tool_call_chunks[0]["id"] == "call_1"
+        assert message_chunk.tool_call_chunks[0]["index"] == 0
 
     def test_chunk_with_user_role(self) -> None:
         """Test that a chunk with role=user produces HumanMessageChunk."""