Save and Load

Define the persistence contract first

Read references/system-selector.md.

1. List state that must survive a checkpoint, map travel, application restart, device change, patch, account change, and multiplayer session separately.
2. Assign each datum an authority, stable ID, owner file/slot, lifetime, default, validation rule, and migration policy.
3. Separate immutable definitions from mutable saved state. Persist definition IDs, not asset objects.
4. Capture a consistent snapshot, write through a serialized per-user operation queue, and acknowledge success/failure visibly.
5. Load into data first; validate type/header/version, migrate, resolve definitions, travel/construct the world, then apply state in a deterministic phase.
6. Preserve a last-known-good path and never overwrite a readable older/newer save after a failed load.
7. Test real previous-version files, corruption, missing content, low storage, interrupted operations, multiple users, and packaged target builds.

Route neighboring work deliberately:

• definition assets, Primary Asset IDs, tables, and redirects -> unreal-data-assets-tables;
• subsystem/Actor interface architecture -> unreal-blueprint-architecture;
• save-related loading/memory hitches -> unreal-memory-streaming and unreal-insights-profiling;
• release artifact, patch, platform packaging, and crash collection -> unreal-packaging-deployment;
• replicated runtime authority -> unreal-replication.

Load only what applies:

• references/schema-stable-identifiers.md
• references/save-load-sequence-async.md
• references/world-state-actors.md
• references/checkpoints-autosaves.md
• references/versioning-migrations.md
• references/slots-users-platforms-cloud.md
• references/corruption-failure-recovery.md
• references/multiplayer-authority-security.md
• references/performance-testing-debugging.md
• references/use-case-recipes.md

Hard rules

• Do not serialize live Actor/UObject pointers as durable identity. Persist stable domain IDs, Primary Asset IDs, tags, names, GUIDs, and value records with explicit resolution behavior.
• Do not save immutable definition data into every slot. Save the definition ID plus minimal mutable state.
• AsyncSaveGameToSlot still serializes on the game thread; it moves the platform write to a worker. Keep snapshot/schema cost bounded and measure hitches.
• Serialize save/load operations per platform user/slot. UE warns that some platforms may not support simultaneous save and load; completion order must not decide data authority.
• Treat callbacks as stale-capable. Bind weak ownership/request generations and verify slot/user/request.
• Never mutate the live world incrementally from unvalidated bytes. Validate and migrate a data snapshot before applying it.
• Never overwrite an unknown newer-version, corrupt, or failed-to-migrate save automatically.
• Do not rely on application shutdown as the only persistence point.
• Local save data is untrusted for competitive authority, paid entitlement, or server economy.
• SaveGameToSlot documents that it writes all non-transient properties and does not check the SaveGame property flag. Do not repeat the common claim that the flag filters this built-in call.
• Verify path, quota, user identity, slot enumeration, cloud behavior, suspend/resume, and atomicity with the target platform's current save API; desktop .sav behavior is not a universal contract.

See references/sources.md for the UE 5.8 primary-source trail.