---
title: Value *serialization* for memoization
description: >
  How CocoIndex serializes memoized function returns with msgspec — supported
  types (primitives, collections, dataclasses, Pydantic, NumPy), required type
  annotations, custom type registration, and troubleshooting tips.
---
## Overview

CocoIndex serializes and caches the return values of [memoized functions](./function#memoization) so that unchanged work can be skipped on subsequent runs. Most Python types work automatically — the key thing to get right is the **return type annotation**, which tells CocoIndex how to reconstruct your objects:

```python
@coco.fn(memo=True)
async def process_chunk(chunk: Chunk) -> Embedding:  # return type annotation
    return embed(chunk.text)
```

Without annotations, values may deserialize as basic Python types (`dict`, `list`, `str`, etc.) instead of their original types.

:::info[Advanced: other places where serialization and type annotations matter]
Serialization also applies to [memo states](../advanced_topics/memoization_keys#memo-state-validation) and [tracking records](../advanced_topics/custom_target_connector#targethandler-you-implement). If you're implementing these, add type annotations to:

- **`__coco_memo_state__` `prev_state` parameter** — annotate with the state type you return in `MemoStateOutcome(state=...)`. See [Memo state validation](../advanced_topics/memoization_keys#memo-state-validation).
- **`reconcile()` `prev_possible_records` parameter** — annotate with `Collection[YourTrackingRecord]`. See [Custom Target Connector](../advanced_topics/custom_target_connector#targethandler-you-implement).
:::

## Supported types

The following types all work out of the box — no registration needed:

| Category | Types |
|----------|-------|
| **Primitives** | `bool`, `int`, `float`, `str`, `bytes`, `None` |
| **Collections** | `list`, `tuple`, `dict`, `set`, `frozenset` |
| **Dataclasses** | Any `@dataclass` (including frozen) |
| **NamedTuples** | Any `NamedTuple` subclass |
| **Pydantic models** | Any `pydantic.BaseModel` subclass |
| **msgspec Structs** | Any `msgspec.Struct` subclass |
| **Date/time** | `datetime.datetime`, `datetime.date`, `datetime.time`, `datetime.timedelta`, `datetime.timezone` |
| **Other stdlib** | `uuid.UUID`, `complex`, `pathlib.Path`, `pathlib.PurePath` |
| **NumPy** | `numpy.ndarray`, `numpy.dtype` (when numpy is installed) |

More generally, all types [supported by msgspec](https://msgspec.dev/supported-types) work automatically. These types also work when nested inside collections or other structured types.

## Custom types

If your type isn't in the list above, register it with `@coco.serialize_by_pickle`:

```python
import cocoindex as coco

@coco.serialize_by_pickle
class MySpecialType:
    def __init__(self, data):
        self.data = data
```

For third-party types, call it as a regular function:

```python
import cocoindex as coco
from some_library import SomeType

coco.serialize_by_pickle(SomeType)
```

:::caution[Not for dataclasses, NamedTuples, or `msgspec.Struct`]
Don't apply `@coco.serialize_by_pickle` to dataclasses, NamedTuples, or `msgspec.Struct` — these are already supported natively. Applying it only works at the top level; when nested inside another supported type, the native encoding takes precedence and the decorator has no effect.

If serialization fails because of a problematic *field* inside a dataclass, register that field's type with `@coco.serialize_by_pickle` instead.
:::

## Union types

Unions of a custom type with `None` work fine (`MyDataclass | None`). However, unions involving multiple custom types or a custom type with other non-`None` types require tagged `msgspec.Struct` variants.

For example, this **won't work**:

```python
from dataclasses import dataclass

@dataclass
class Config:
    value: int

class Settings(NamedTuple):
    config: Config | str  # fails at deserialization
```

**Fix** — wrap each variant in a tagged [`msgspec.Struct`](https://msgspec.dev/structs#tagged-unions). The `tag=True` parameter embeds a type tag in the serialized data so that the correct variant can be identified during deserialization:

```python
import msgspec

class ConfigValue(msgspec.Struct, tag=True):
    value: int

class StringValue(msgspec.Struct, tag=True):
    value: str

class Settings(NamedTuple):
    config: ConfigValue | StringValue  # works — variants are distinguished by tag
```

## Troubleshooting

### `DeserializationError: Cannot build msgspec Decoder`

This typically means an unsupported union type. The error message includes a hint about the cause.

**Fix**: Restructure the union to use tagged `msgspec.Struct` variants. See [Union types](#union-types) above.

### `DeserializationError: Failed to deserialize msgspec payload`

The type annotation doesn't match the serialized data. Common causes:

- **Missing return type annotation** on a memoized function — add `-> YourType` to the function signature.
- **Changed type structure** between runs — if you renamed or restructured a dataclass, the cached data won't match. Rebuild the cache by running [`app.update(full_reprocess=True)`](./app#updating-an-app) or [`cocoindex update --full-reprocess`](../cli).
- **Forward reference not resolved** — if your type annotation uses a string forward reference, ensure the type is defined before the function is first called.

### `UnpicklingError: Forbidden global during unpickling`

```
_pickle.UnpicklingError: Forbidden global during unpickling: myapp.models.Summary
```

CocoIndex restricts which types can be deserialized for security. This error means your type isn't in the allow-list. Fix by either:

1. **Converting to a dataclass or NamedTuple** (recommended — supported natively, no registration needed)
2. **Using `@coco.serialize_by_pickle`** to register the type

:::note[Upgrading from older versions]
If you see this error after upgrading, previously cached data may reference types that aren't yet registered. You have two options:

- Add `@coco.serialize_by_pickle` to the type and re-run.
- If the type is already a dataclass or NamedTuple, add `@coco.unpickle_safe` to allow reading the old cached data. Once the cache is rebuilt, the decorator can be removed.
:::
