diff --git a/pydantic_ai_slim/pydantic_ai/_chat_completions.py b/pydantic_ai_slim/pydantic_ai/_chat_completions.py
new file mode 100644
index 000000000..820348564
--- /dev/null
+++ b/pydantic_ai_slim/pydantic_ai/_chat_completions.py
@@ -0,0 +1,242 @@
+from __future__ import annotations as _annotations
+
+import base64
+from typing import TYPE_CHECKING, Annotated, Any, NotRequired
+
+from fastapi import FastAPI, HTTPException
+from openai.types.chat import (
+    ChatCompletion,
+    ChatCompletionAudioParam,
+    ChatCompletionMessage,
+    ChatCompletionMessageParam,
+    ChatCompletionReasoningEffort,
+    ChatCompletionToolParam,
+)
+from openai.types.chat.chat_completion import Choice
+from openai.types.completion_usage import CompletionUsage
+from pydantic import Field, TypeAdapter
+from typing_extensions import TypedDict
+
+from pydantic_ai import UserError, _utils
+from pydantic_ai.messages import (
+    BinaryContent,
+    ImageUrl,
+    ModelMessage,
+    ModelRequest,
+    ModelResponse,
+    ModelResponsePart,
+    SystemPromptPart,
+    TextPart,
+    ToolCallPart,
+    ToolReturnPart,
+    UserContent,
+    UserPromptPart,
+)
+from pydantic_ai.settings import ModelSettings
+
+if TYPE_CHECKING:
+    from pydantic_ai.agent import Agent, AgentRunResult
+
+Number = Annotated[float, Field(ge=-2.0, le=2.0)]
+PositiveNumber = Annotated[float, Field(ge=0.0, le=2.0)]
+
+
+class ChatCompletionInput(TypedDict):
+    messages: list[ChatCompletionMessageParam]
+    model: str
+    audio: NotRequired[ChatCompletionAudioParam]
+    frequency_penalty: NotRequired[float | None]
+    logit_bias: NotRequired[dict[str, int] | None]
+    logprobs: NotRequired[bool | None]
+    max_completion_tokens: NotRequired[int | None]
+    parallel_tool_calls: NotRequired[bool | None]
+    presence_penalty: NotRequired[Number | None]
+    reasoning_effort: NotRequired[ChatCompletionReasoningEffort | None]
+    seed: NotRequired[int | None]
+    stop: NotRequired[str | None]
+    stream: NotRequired[bool | None]
+    temperature: NotRequired[PositiveNumber | None]
+    # TODO(Marcelo): We need to support tools.
+    tools: NotRequired[list[ChatCompletionToolParam]]
+    top_logprobs: NotRequired[Annotated[int, Field(ge=0, le=20)] | None]
+    top_p: NotRequired[PositiveNumber | None]
+    user: NotRequired[str]
+    extra_headers: NotRequired[dict[str, str]]
+    extra_body: NotRequired[object]
+
+
+model_settings_ta = TypeAdapter(ModelSettings)
+
+
+def to_chat_completions(agent: Agent[None], path: str = '/chat/completions') -> FastAPI:
+    # TODO(Marcelo): PydanticAI deps should be created in the lifespan.
+    app = FastAPI()
+
+    async def chat_completions(body: ChatCompletionInput) -> ChatCompletion:
+        message_history = openai2pai(body['messages'])
+
+        if body.get('tools'):
+            raise HTTPException(status_code=400, detail='Tools are not supported yet')
+
+        try:
+            result = await agent.run(
+                model=body['model'],
+                message_history=message_history,
+                model_settings=model_settings_ta.validate_python(body),
+            )
+        except UserError as e:
+            raise HTTPException(status_code=400, detail=e.message)
+        return pai_result_2_openai(result=result, model=body['model'])
+
+    app.add_api_route(path, chat_completions, methods=['POST'])
+
+    return app
+
+
+def openai2pai(messages: list[ChatCompletionMessageParam]) -> list[ModelMessage]:
+    """Convert OpenAI ChatCompletionMessageParam list to pydantic-ai ModelMessage format."""
+    result: list[ModelMessage] = []
+    current_model_request = ModelRequest(parts=[])
+    current_model_response = ModelResponse(parts=[])
+
+    for message in messages:
+        if message['role'] == 'system' or message['role'] == 'developer':
+            content = message['content']
+            if not isinstance(content, str):
+                content = '\n'.join(part['text'] for part in content)
+            current_model_request.parts.append(SystemPromptPart(content=content))
+
+        elif message['role'] == 'user':
+            content = message['content']
+            user_content: str | list[UserContent]
+            if isinstance(content, str):
+                user_content = content
+            else:
+                user_content = []
+                for part in content:
+                    if part['type'] == 'text':
+                        user_content.append(part['text'])
+                    elif part['type'] == 'image_url':
+                        user_content.append(ImageUrl(url=part['image_url']['url']))
+                    elif part['type'] == 'input_audio':
+                        user_content.append(
+                            BinaryContent(
+                                data=base64.b64decode(part['input_audio']['data']),
+                                media_type=part['input_audio']['format'],
+                            )
+                        )
+                    elif part['type'] == 'file':
+                        assert 'file' in part['file']
+                        user_content.append(
+                            BinaryContent(
+                                data=base64.b64decode(part['file']['file_data']),
+                                media_type=part['file']['file']['type'],
+                            )
+                        )
+                    else:
+                        raise ValueError(f'Unknown content type: {part["type"]}')
+            current_model_request.parts.append(UserPromptPart(content=user_content))
+
+        elif message['role'] == 'assistant':
+            if current_model_request.parts:
+                result.append(current_model_request)
+                current_model_request = ModelRequest(parts=[])
+
+            response_parts: list[ModelResponsePart] = []
+            content = message.get('content')
+            tool_calls = message.get('tool_calls')
+
+            if content:
+                if isinstance(content, str):
+                    response_parts.append(TextPart(content=content))
+                else:
+                    content_text = '\n'.join(part['text'] for part in content if part['type'] == 'text')
+                    if content_text:
+                        response_parts.append(TextPart(content=content_text))
+
+            if tool_calls:
+                for tool_call in tool_calls:
+                    if tool_call['type'] == 'function':
+                        response_parts.append(
+                            ToolCallPart(
+                                tool_name=tool_call['function']['name'],
+                                args=tool_call['function']['arguments'],
+                                tool_call_id=tool_call['id'],
+                            )
+                        )
+
+            if response_parts:
+                current_model_response = ModelResponse(parts=response_parts)
+                result.append(current_model_response)
+                current_model_response = ModelResponse(parts=[])
+
+        elif message['role'] == 'tool':
+            tool_call_id = message['tool_call_id']
+            content = message['content']
+            tool_name = message.get('name', 'unknown')
+
+            current_model_request.parts.append(
+                ToolReturnPart(
+                    tool_name=tool_name,
+                    content=content,
+                    tool_call_id=tool_call_id,
+                )
+            )
+
+        elif message['role'] == 'function':
+            name = message['name']
+            content = message['content']
+
+            current_model_request.parts.append(
+                ToolReturnPart(
+                    tool_name=name,
+                    content=content,
+                    tool_call_id=f'call_{name}',
+                )
+            )
+
+        else:
+            raise ValueError(f'Unknown role: {message["role"]}')
+
+    if current_model_request.parts:
+        result.append(current_model_request)
+    if current_model_response.parts:
+        result.append(current_model_response)
+
+    return result
+
+
+def pai_result_2_openai(result: AgentRunResult[Any], model: str) -> ChatCompletion:
+    """Convert a PydanticAI AgentRunResult to OpenAI ChatCompletion format."""
+    content = ''
+    if result.output:
+        content = str(result.output)
+    elif result.all_messages():
+        # Get the last message content
+        last_message = result.all_messages()[-1]
+        if isinstance(last_message, ModelResponse):
+            for part in last_message.parts:
+                if isinstance(part, TextPart):
+                    content += part.content
+
+    return ChatCompletion(
+        id=f'chatcmpl-{_utils.now_utc().isoformat()}',
+        object='chat.completion',
+        created=int(_utils.now_utc().timestamp()),
+        model=model,
+        choices=[
+            Choice(
+                index=0,
+                message=ChatCompletionMessage(
+                    role='assistant',
+                    content=content,
+                ),
+                finish_reason='stop',
+            )
+        ],
+        usage=CompletionUsage(
+            prompt_tokens=0,
+            completion_tokens=0,
+            total_tokens=0,
+        ),
+    )
diff --git a/pydantic_ai_slim/pydantic_ai/agent.py b/pydantic_ai_slim/pydantic_ai/agent.py
index 51facd769..068d30e29 100644
--- a/pydantic_ai_slim/pydantic_ai/agent.py
+++ b/pydantic_ai_slim/pydantic_ai/agent.py
@@ -54,6 +54,7 @@
 UserPromptNode = _agent_graph.UserPromptNode
 
 if TYPE_CHECKING:
+    from fastapi import FastAPI
     from starlette.middleware import Middleware
     from starlette.routing import Route
     from starlette.types import ExceptionHandler, Lifespan
@@ -1769,6 +1770,11 @@ def to_a2a(
             lifespan=lifespan,
         )
 
+    def to_chat_completions(self) -> FastAPI:
+        from ._chat_completions import to_chat_completions
+
+        return to_chat_completions(self)  # type: ignore[reportArgumentType]
+
     async def to_cli(self: Self, deps: AgentDepsT = None, prog_name: str = 'pydantic-ai') -> None:
         """Run the agent in a CLI chat interface.
 
diff --git a/pydantic_ai_slim/pydantic_ai/settings.py b/pydantic_ai_slim/pydantic_ai/settings.py
index af22a2699..6aafaec5d 100644
--- a/pydantic_ai_slim/pydantic_ai/settings.py
+++ b/pydantic_ai_slim/pydantic_ai/settings.py
@@ -1,9 +1,11 @@
 from __future__ import annotations
 
 from httpx import Timeout
+from pydantic import ConfigDict, with_config
 from typing_extensions import TypedDict
 
 
+@with_config(ConfigDict(arbitrary_types_allowed=True))
 class ModelSettings(TypedDict, total=False):
     """Settings to configure an LLM.