How to Fix “SerializationException: Type is not marked as serializable” in Unity Save Code

Save systems fail here when the save graph accidentally includes things that are not data — components, events, engine structs. The durable fix is a strict DTO boundary. Here is the shape.

How to fix it

1. Define pure DTOs for saves

Create plain classes holding primitives, strings, lists, and other DTOs only, each marked [Serializable] — never serialize MonoBehaviours or ScriptableObjects directly.

2. Mirror engine types

Store Vector3 as three floats (or a small serializable struct) and convert at the boundary; most serializers cannot roundtrip engine structs portably.

3. Strip live references and delegates

Events, callbacks, and object references drag entire object graphs into the serializer — replace them with IDs you resolve on load.

4. Prefer JSON for saves you must evolve

A text format plus explicit versioning survives refactors and lets you migrate old saves; binary graph serializers break on any rename.

Catching the ones you can't reproduce

The hardest version of this to fix is the one you can't reproduce — it only happens on a player's hardware, OS, driver, or save state, under conditions that simply aren't present on your machine. A report that says “it crashed” or “it froze” gives you nothing to act on, so the bug survives release after release while quietly costing you players.

Automatic error capture closes that gap. Each failure arrives with its full stack trace, the device and OS, the build number, and a breadcrumb trail of what the player did right before it broke, so even a failure you have never seen becomes a specific, reproducible issue. Fold identical failures into one signature ranked by how many players each hits, and your worklist sorts itself worst-first instead of arriving as a stream of vague complaints.

This is where a tool like Bugnet earns its place. Its SDK captures every Unity error automatically with the full stack trace plus device, OS, memory, build, and game-state context, folds duplicates into one grouped issue with an occurrence count, and ties each to the build it first appeared on — so you fix the problem that hurts the most players first and confirm it is gone when its signature disappears from the next release.