diff --git a/pydantic_ai_slim/pydantic_ai/models/openrouter.py b/pydantic_ai_slim/pydantic_ai/models/openrouter.py
index 3f910d76b6..07550b9a03 100644
--- a/pydantic_ai_slim/pydantic_ai/models/openrouter.py
+++ b/pydantic_ai_slim/pydantic_ai/models/openrouter.py
@@ -2,9 +2,9 @@
 
 from collections.abc import Iterable
 from dataclasses import dataclass, field
-from typing import Any, Literal, cast
+from typing import Annotated, Any, Literal, TypeAlias, cast
 
-from pydantic import BaseModel
+from pydantic import BaseModel, Discriminator
 from typing_extensions import TypedDict, assert_never, override
 
 from ..exceptions import ModelHTTPError
@@ -22,9 +22,13 @@
 try:
     from openai import APIError, AsyncOpenAI
     from openai.types import chat, completion_usage
-    from openai.types.chat import chat_completion, chat_completion_chunk
+    from openai.types.chat import chat_completion, chat_completion_chunk, chat_completion_message_function_tool_call
 
-    from .openai import OpenAIChatModel, OpenAIChatModelSettings, OpenAIStreamedResponse
+    from .openai import (
+        OpenAIChatModel,
+        OpenAIChatModelSettings,
+        OpenAIStreamedResponse,
+    )
 except ImportError as _import_error:
     raise ImportError(
         'Please install `openai` to use the OpenRouter model, '
@@ -341,6 +345,27 @@ def _into_reasoning_detail(thinking_part: ThinkingPart) -> _OpenRouterReasoningD
         assert_never(data.type)
 
 
+class _OpenRouterFunction(chat_completion_message_function_tool_call.Function):
+    arguments: str | None  # type: ignore[reportIncompatibleVariableOverride]
+    """
+    The arguments to call the function with, as generated by the model in JSON
+    format. Note that the model does not always generate valid JSON, and may
+    hallucinate parameters not defined by your function schema. Validate the
+    arguments in your code before calling your function.
+    """
+
+
+class _OpenRouterChatCompletionMessageFunctionToolCall(chat.ChatCompletionMessageFunctionToolCall):
+    function: _OpenRouterFunction  # type: ignore[reportIncompatibleVariableOverride]
+    """The function that the model called."""
+
+
+_OpenRouterChatCompletionMessageToolCallUnion: TypeAlias = Annotated[
+    _OpenRouterChatCompletionMessageFunctionToolCall | chat.ChatCompletionMessageCustomToolCall,
+    Discriminator(discriminator='type'),
+]
+
+
 class _OpenRouterCompletionMessage(chat.ChatCompletionMessage):
     """Wrapped chat completion message with OpenRouter specific attributes."""
 
@@ -350,6 +375,9 @@ class _OpenRouterCompletionMessage(chat.ChatCompletionMessage):
     reasoning_details: list[_OpenRouterReasoningDetail] | None = None
     """The reasoning details associated with the message, if any."""
 
+    tool_calls: list[_OpenRouterChatCompletionMessageToolCallUnion] | None = None  # type: ignore[reportIncompatibleVariableOverride]
+    """The tool calls generated by the model, such as function calls."""
+
 
 class _OpenRouterChoice(chat_completion.Choice):
     """Wraps OpenAI chat completion choice with OpenRouter specific attributes."""
diff --git a/tests/models/cassettes/test_openrouter/test_openrouter_tool_optional_parameters.yaml b/tests/models/cassettes/test_openrouter/test_openrouter_tool_optional_parameters.yaml
new file mode 100644
index 0000000000..a826576f09
--- /dev/null
+++ b/tests/models/cassettes/test_openrouter/test_openrouter_tool_optional_parameters.yaml
@@ -0,0 +1,86 @@
+interactions:
+- request:
+    headers:
+      accept:
+      - application/json
+      accept-encoding:
+      - gzip, deflate
+      connection:
+      - keep-alive
+      content-length:
+      - '362'
+      content-type:
+      - application/json
+      host:
+      - openrouter.ai
+    method: POST
+    parsed_body:
+      messages:
+      - content: Can you find me any education content?
+        role: user
+      model: anthropic/claude-sonnet-4.5
+      stream: false
+      tool_choice: auto
+      tools:
+      - function:
+          description: ''
+          name: find_education_content
+          parameters:
+            properties:
+              title:
+                anyOf:
+                - type: string
+                - type: 'null'
+                default: null
+            type: object
+        type: function
+    uri: https://openrouter.ai/api/v1/chat/completions
+  response:
+    headers:
+      access-control-allow-origin:
+      - '*'
+      connection:
+      - keep-alive
+      content-length:
+      - '611'
+      content-type:
+      - application/json
+      permissions-policy:
+      - payment=(self "https://checkout.stripe.com" "https://connect-js.stripe.com" "https://js.stripe.com" "https://*.js.stripe.com"
+        "https://hooks.stripe.com")
+      referrer-policy:
+      - no-referrer, strict-origin-when-cross-origin
+      transfer-encoding:
+      - chunked
+      vary:
+      - Accept-Encoding
+    parsed_body:
+      choices:
+      - finish_reason: tool_calls
+        index: 0
+        logprobs: null
+        message:
+          content: I'll search for education content for you.
+          reasoning: null
+          refusal: null
+          role: assistant
+          tool_calls:
+          - function:
+              name: find_education_content
+            id: toolu_vrtx_015QAXScZzRDPttiPoc34AdD
+            index: 0
+            type: function
+        native_finish_reason: tool_calls
+      created: 1764308342
+      id: gen-1764308342-FInFdBZR9TF8jmnOwZGZ
+      model: anthropic/claude-sonnet-4.5
+      object: chat.completion
+      provider: Google
+      usage:
+        completion_tokens: 48
+        prompt_tokens: 568
+        total_tokens: 616
+    status:
+      code: 200
+      message: OK
+version: 1
diff --git a/tests/models/test_openrouter.py b/tests/models/test_openrouter.py
index d6243bdfe5..c52ec6e546 100644
--- a/tests/models/test_openrouter.py
+++ b/tests/models/test_openrouter.py
@@ -358,3 +358,51 @@ async def test_openrouter_map_messages_reasoning(allow_model_requests: None, ope
             }
         ]
     )
+
+
+async def test_openrouter_tool_optional_parameters(allow_model_requests: None, openrouter_api_key: str) -> None:
+    provider = OpenRouterProvider(api_key=openrouter_api_key)
+
+    class FindEducationContentFilters(BaseModel):
+        title: str | None = None
+
+    model = OpenRouterModel('anthropic/claude-sonnet-4.5', provider=provider)
+    response = await model_request(
+        model,
+        [ModelRequest.user_text_prompt('Can you find me any education content?')],
+        model_request_parameters=ModelRequestParameters(
+            function_tools=[
+                ToolDefinition(
+                    name='find_education_content',
+                    description='',
+                    parameters_json_schema=FindEducationContentFilters.model_json_schema(),
+                )
+            ],
+            allow_text_output=True,  # Allow model to either use tools or respond directly
+        ),
+    )
+
+    assert len(response.parts) == 2
+
+    tool_call_part = response.parts[1]
+    assert isinstance(tool_call_part, ToolCallPart)
+    assert tool_call_part.tool_call_id == snapshot('toolu_vrtx_015QAXScZzRDPttiPoc34AdD')
+    assert tool_call_part.tool_name == 'find_education_content'
+    assert tool_call_part.args == snapshot(None)
+
+    mapped_messages = await model._map_messages([response], None)  # type: ignore[reportPrivateUsage]
+    tool_call_message = mapped_messages[0]
+    assert tool_call_message['role'] == 'assistant'
+    assert tool_call_message.get('content') == snapshot("I'll search for education content for you.")
+    assert tool_call_message.get('tool_calls') == snapshot(
+        [
+            {
+                'id': 'toolu_vrtx_015QAXScZzRDPttiPoc34AdD',
+                'type': 'function',
+                'function': {
+                    'name': 'find_education_content',
+                    'arguments': '{}',
+                },
+            }
+        ]
+    )