msgspec

msgspec Skill

msgspec is a high-performance Python library for serialization, deserialization, and validation. Structs are ~5x more memory-efficient than regular classes and serialize faster than Pydantic or dataclasses.

Code Style Rules

• Use PEP 604 for unions: T | None (not Optional[T])
• from __future__ import annotations rule — Library/shared modules that define runtime-introspected msgspec.Struct subclasses should avoid postponed annotations unless the consuming tool resolves them. Consumer modules that only use Structs MAY use future annotations.
• Always annotate all fields; msgspec requires type annotations
• Use kw_only=True for Structs with more than 2 fields

Quick Reference

Struct Definition

import msgspec

# Basic struct
class User(msgspec.Struct):
    id: int
    name: str
    email: str | None = None

# Performance options
class Event(msgspec.Struct, frozen=True, gc=False):
    """frozen=True: immutable + hashable. gc=False: skip GC for short-lived objects."""
    event_type: str
    payload: dict[str, object]

# Keyword-only (recommended for >2 fields)
class Config(msgspec.Struct, kw_only=True):
    host: str
    port: int = 5432
    ssl: bool = False

# Array-like encoding (tuple encoding, more compact)
class Point(msgspec.Struct, array_like=True):
    x: float
    y: float

# Rename fields for serialization
class ApiResponse(msgspec.Struct, rename="camel"):
    user_id: int         # serialized as "userId"
    created_at: str      # serialized as "createdAt"

# Reject unknown fields at API boundaries
class StrictInput(msgspec.Struct, forbid_unknown_fields=True):
    name: str
    value: int

Validation Constraints

from typing import Annotated
import msgspec
from msgspec import Meta

class Product(msgspec.Struct):
    name: Annotated[str, Meta(min_length=1, max_length=100)]
    price: Annotated[float, Meta(gt=0)]
    quantity: Annotated[int, Meta(ge=0, le=10_000)]
    sku: Annotated[str, Meta(pattern=r"^[A-Z]{2}-\d{4}$")]
    weight_kg: Annotated[float, Meta(multiple_of=0.001)]

# Reusable constraint aliases
PositiveInt = Annotated[int, Meta(gt=0)]
NonEmptyStr = Annotated[str, Meta(min_length=1)]
Percentage = Annotated[float, Meta(ge=0.0, le=100.0)]

class Order(msgspec.Struct):
    id: PositiveInt
    label: NonEmptyStr
    discount: Percentage = 0.0

Serialization

import msgspec

# JSON -- singleton encoder/decoder (cache these!)
encoder = msgspec.json.Encoder()
decoder = msgspec.json.Decoder(User)

data = encoder.encode(user)          # bytes
user = decoder.decode(b'{"id":1,"name":"Alice"}')

# Functional API (convenience, slightly slower)
data = msgspec.json.encode(user)
user = msgspec.json.decode(b'...', type=User)

# MessagePack (binary, more compact)
data = msgspec.msgpack.encode(user)
user = msgspec.msgpack.decode(data, type=User)

# Custom hooks for non-native types (datetime, UUID, Decimal)
from datetime import datetime
import uuid

def enc_hook(obj: object) -> object:
    if isinstance(obj, datetime):
        return obj.isoformat()
    if isinstance(obj, uuid.UUID):
        return str(obj)
    raise TypeError(f"Unsupported type: {type(obj)}")

def dec_hook(type: type, obj: object) -> object:
    if type is datetime:
        return datetime.fromisoformat(obj)
    if type is uuid.UUID:
        return uuid.UUID(obj)
    raise TypeError(f"Unsupported type: {type}")

encoder = msgspec.json.Encoder(enc_hook=enc_hook)
decoder = msgspec.json.Decoder(MyStruct, dec_hook=dec_hook)

Canonical Litestar serializers (match-your-stack)

Litestar apps typically need to_json(value, as_bytes=True) that handles UUID / datetime / Enum / Decimal for Channels broadcasts, log contexts, and JSONB writes. Pick the branch that matches your project.

Branch A — sqlspec is in-stack. Re-export sqlspec's serializer; it already installs an enc_hook covering UUID, datetime, Enum, Decimal, Pydantic, dataclasses, attrs, and msgspec.Struct.

# myapp/utils/serialization.py
from sqlspec.utils.serializers import from_json, to_json

__all__ = ("from_json", "to_json")

Usage:

from myapp.utils.serialization import to_json

payload = to_json(order, as_bytes=True)
await backend.publish(payload, channels=[f"orders:{order.id}:events"])

Branch B — sqlspec is not in-stack. Hand-roll an Encoder with an enc_hook.

# myapp/utils/serialization.py
import datetime as _dt
import json
from typing import Any
from uuid import UUID

import msgspec


def _default(value: Any) -> str:
    if isinstance(value, UUID):
        return str(value)
    if isinstance(value, _dt.datetime):
        return value.astimezone(_dt.UTC).strftime("%Y-%m-%dT%H:%M:%SZ")
    if isinstance(value, _dt.date):
        return value.isoformat()
    return str(value)


_encoder = msgspec.json.Encoder(enc_hook=_default)


def to_json(value: Any) -> bytes:
    if isinstance(value, bytes):
        return value
    return _encoder.encode(value)

Type Coercion with convert()

import msgspec

raw = {"id": "42", "name": "Alice"}  # id is a string

# Strict mode (default): raises on type mismatch
user = msgspec.convert(raw, User)  # ValidationError: id must be int

# Lax mode: coerces compatible types
user = msgspec.convert(raw, User, strict=False)  # id coerced to 42

# str_keys: dict keys are strings (useful for JSON-loaded dicts)
data = {"1": "Alice", "2": "Bob"}
result = msgspec.convert(data, dict[int, str], str_keys=True)

# Convert with dec_hook for custom types
user = msgspec.convert(raw, UserWithUUID, dec_hook=dec_hook)

# Convert dataclass/dict/object to Struct
from dataclasses import dataclass

@dataclass
class LegacyUser:
    id: int
    name: str

legacy = LegacyUser(id=1, name="Alice")
user = msgspec.convert(msgspec.structs.asdict(legacy), User)
# Or directly:
user = msgspec.convert(legacy, User)

Dynamic Struct Creation

import msgspec

# Runtime struct from field definitions
fields = [
    ("id", int),
    ("name", str),
    ("score", Annotated[float, Meta(ge=0.0)]),
]
DynamicModel = msgspec.defstruct("DynamicModel", fields, kw_only=True)

# With defaults
fields_with_defaults = [
    ("id", int),
    ("active", bool, True),   # (name, type, default)
]
FlexModel = msgspec.defstruct("FlexModel", fields_with_defaults)

Tagged Unions (Discriminated Unions)

import msgspec
from typing import Literal

# Default tag field is "type", tag value is the class name
class Dog(msgspec.Struct, tag=True):
    name: str
    breed: str

class Cat(msgspec.Struct, tag=True):
    name: str
    indoor: bool

Animal = Dog | Cat

# Deserialize: inspects "type" field to pick correct class
animal = msgspec.json.decode(b'{"type":"Dog","name":"Rex","breed":"Lab"}', type=Animal)

# Custom tag values
class CreateEvent(msgspec.Struct, tag="create"):
    resource: str

class DeleteEvent(msgspec.Struct, tag="delete"):
    resource: str
    soft: bool = True

Event = CreateEvent | DeleteEvent

# Custom tag field name
class V1Request(msgspec.Struct, tag="v1", tag_field="version"):
    payload: str

class V2Request(msgspec.Struct, tag="v2", tag_field="version"):
    payload: str
    metadata: dict[str, str] = {}

Request = V1Request | V2Request

Workflow

Step 1: Define Structs

Create msgspec Structs for all data shapes. Use kw_only=True for Structs with more than 2 fields. Use frozen=True for immutable value objects. Use forbid_unknown_fields=True for API-boundary input validation.

Step 2: Add Constraints

Annotate fields with Annotated[Type, Meta(...)] for numeric ranges, string lengths, and regex patterns. Define reusable constraint aliases at module level to avoid repetition.

Step 3: Choose Serialization Strategy

Use msgspec.json for JSON APIs and msgspec.msgpack for binary protocols or internal messaging. Instantiate Encoder/Decoder once at module level as singletons. Add enc_hook/dec_hook for custom types (datetime, UUID, Decimal, Enum).

Step 4: Handle Polymorphism

Use tagged unions (tag=True or tag="value") for discriminated unions. Define a union type alias (Event = CreateEvent | DeleteEvent) and decode against it. Use tag_field to customize the discriminator field name.

Step 5: Validate

Test round-trip encode/decode. Confirm ValidationError is raised for constraint violations. Verify tag dispatch selects the correct Struct type for all union variants.

Guardrails

• Always annotate all fields -- msgspec requires type annotations; unannotated fields are ignored silently.
• Cache Encoder/Decoder as singletons -- instantiation is expensive; create once at module level and reuse.
• Use kw_only=True for Structs with >2 fields -- prevents positional argument confusion and makes instantiation self-documenting.
• Use forbid_unknown_fields=True at API boundaries -- rejects payloads with unexpected keys, preventing silent data loss.
• Prefer Meta constraints over manual validation -- zero runtime overhead; constraints are checked during decode, not after.
• Use gc=False for short-lived, non-circular objects -- eliminates GC overhead for hot-path objects like request/response shapes.
• Tagged unions for polymorphism -- faster than manual dispatch and eliminates isinstance chains.
• from __future__ import annotations rule — Library/shared modules that define runtime-introspected types (advanced-alchemy models, sqlspec configs, msgspec Structs, dishka providers) avoid postponed annotations unless their consumers resolve them. Consumer applications MAY use it. The restriction applies only to modules that define introspected types, not handler/service/test modules that use them.
• Use strict=False only at trust boundaries -- lax coercion is useful for converting legacy dicts but can mask type errors in internal code.
• Prefer sqlspec.utils.serializers.to_json when sqlspec is in-stack — its built-in enc_hook covers UUID, datetime, Enum, Decimal, Pydantic, msgspec.Struct, dataclasses, and attrs in one import. Hand-rolling is only needed when sqlspec is not a dependency.

Validation Checkpoint

Before delivering msgspec code, verify:

• All Struct fields have explicit type annotations
• If this library/shared module defines runtime-introspected types, avoid from __future__ import annotations unless all consumers resolve postponed annotations. Consumer modules may use it.
• Encoder/Decoder instances are module-level singletons (not created per-request)
• API-boundary Structs use forbid_unknown_fields=True
• Numeric/string constraints use Meta (not manual if checks)
• enc_hook/dec_hook handle all non-native types used in Structs
• Tagged union tag values are unique across all variants in a union
• kw_only=True on Structs with more than 2 fields
• If sqlspec is in-stack, to_json is imported from sqlspec.utils.serializers (not hand-rolled)

Example

Task: Define an event system with tagged unions, constraints, and JSON serialization.

# Library/shared modules that define runtime-introspected Structs usually avoid postponed annotations.

# events.py
from typing import Annotated, Literal
from datetime import datetime
import uuid
import msgspec
from msgspec import Meta

# --- Constraint aliases ---
NonEmptyStr = Annotated[str, Meta(min_length=1, max_length=255)]
PositiveInt = Annotated[int, Meta(gt=0)]

# --- Event variants (tagged union) ---
class UserCreatedEvent(msgspec.Struct, tag="user.created", tag_field="event_type", kw_only=True, gc=False):
    event_id: uuid.UUID
    user_id: PositiveInt
    email: NonEmptyStr
    occurred_at: datetime

class UserDeletedEvent(msgspec.Struct, tag="user.deleted", tag_field="event_type", kw_only=True, gc=False):
    event_id: uuid.UUID
    user_id: PositiveInt
    occurred_at: datetime
    reason: str | None = None

UserEvent = UserCreatedEvent | UserDeletedEvent

# --- Custom hooks for datetime and UUID ---
def enc_hook(obj: object) -> object:
    if isinstance(obj, datetime):
        return obj.isoformat()
    if isinstance(obj, uuid.UUID):
        return str(obj)
    raise TypeError(f"Unsupported type: {type(obj)}")

def dec_hook(type: type, obj: object) -> object:
    if type is datetime:
        return datetime.fromisoformat(obj)
    if type is uuid.UUID:
        return uuid.UUID(obj)
    raise TypeError(f"Unsupported type: {type}")

# --- Singleton codec ---
_encoder = msgspec.json.Encoder(enc_hook=enc_hook)
_decoder = msgspec.json.Decoder(UserEvent, dec_hook=dec_hook)

def encode_event(event: UserEvent) -> bytes:
    return _encoder.encode(event)

def decode_event(data: bytes) -> UserEvent:
    return _decoder.decode(data)

# --- Usage ---
event = UserCreatedEvent(
    event_id=uuid.uuid4(),
    user_id=42,
    email="[email protected]",
    occurred_at=datetime.utcnow(),
)
payload = encode_event(event)
# b'{"event_type":"user.created","event_id":"...","user_id":42,"email":"[email protected]","occurred_at":"..."}'

recovered = decode_event(payload)
assert isinstance(recovered, UserCreatedEvent)

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References Index

For detailed guides and reference tables, refer to the following documents in references/:

• Meta Constraints Reference -- Full table of all Meta constraint parameters with examples for numeric, string, bytes, and OpenAPI metadata.
• Tagged Union Patterns -- Discriminated union patterns: default tags, custom tag fields/values, nested unions, API versioning, and event systems.
• Litestar Patterns — CamelizedBaseStruct, sqlspec-vs-manual to_json branches, hybrid msgspec + Pydantic schema pattern, __post_init__ validation for Litestar apps.

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Official References

• https://jcristharif.com/msgspec/
• https://jcristharif.com/msgspec/structs.html
• https://jcristharif.com/msgspec/constraints.html
• https://jcristharif.com/msgspec/json.html
• https://jcristharif.com/msgspec/msgpack.html
• https://jcristharif.com/msgspec/converters.html
• https://jcristharif.com/msgspec/api.html
• https://github.com/jcrist/msgspec

Shared Styleguide Baseline

• Use shared styleguides for generic language/framework rules to reduce duplication in this skill.
• General Principles
• Python
• Keep this skill focused on tool-specific workflows, edge cases, and integration details.