Msgspec fastapi integration

msgspec is a Python library designed for high-performance serialization and validation, which makes it a powerful partner for FastAPI. It can be used as a drop-in replacement for FastAPI's default Pydantic models to handle data validation and schema generation, often leading to a significant speedup in API performance.

How to Integrate msgspec with FastAPI

Integrating msgspec is straightforward. You define your data models using msgspec.Struct instead of pydantic.BaseModel. FastAPI will automatically recognize and validate msgspec structs, just as it does with Pydantic models.

Define Your Data Model with msgspec.Struct

First, create a msgspec.Struct to represent your request or response body.

import msgspec
from typing import List, Optional

class Item(msgspec.Struct):
    name: str
    price: float
    is_active: bool = True
    tags: Optional[List[str]] = None

Use the msgspec Model in Your FastAPI Endpoint

Now, use your Item struct as the type hint for your function parameters, just as you would with Pydantic. FastAPI's dependency injection system will handle the rest.

from fastapi import FastAPI
from fastapi.responses import ORJSONResponse

app = FastAPI(default_response_class=ORJSONResponse)

# An endpoint to create a new item
@app.post("/items")
def create_item(item: Item):
    return item

# An endpoint to get an item by ID
@app.get("/items/{item_id}")
def read_item(item_id: int):
    # This shows how to return a msgspec.Struct
    # FastAPI handles the serialization automatically.
    return Item(name="Example Item", price=99.99)

Note on Response Speed: For maximum performance, it is recommended to set default_response_class=ORJSONResponse in your FastAPI app. msgspec uses orjson under the hood for its JSON operations, and setting this response class ensures FastAPI uses the same highly performant library for all JSON responses (1).

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Why Use msgspec over Pydantic with FastAPI?

The main reason to choose msgspec is performance. While Pydantic is a fantastic tool for its rich feature set and excellent developer experience, msgspec is specifically designed for raw speed (2). In benchmarks, msgspec consistently outperforms Pydantic by a large margin on both validation and serialization, making it ideal for high-throughput microservices and data-intensive applications.

Feature	msgspec	Pydantic
Performance	Extremely fast (uses C/Rust extensions)	Fast (uses C extensions)
Validation	Basic, strict validation.	Extensive and customizable validation.
__slots__	All Structs use __slots__ by default.	Must be explicitly enabled.
API	Smaller, more focused API.	Rich API with ORM integration, settings management, etc.

Sources

1. FastAPI Documentation: ORJSONResponse
2. msgspec vs. Pydantic Benchmarks