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Mod Hot Reload

and assets, requires sandbox approval, requires every changed item to advance its revision, and caps each reload batch at 64 changes.

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This document covers TODOS.phase-72.72.14.1.9: development hot reload for changed mod scripts and assets without restarting the game.

Runtime Surface#

  • module: V2Modding
  • policy: FV2ModHotReloadPolicy
  • change: FV2ModHotReloadChange
  • issue: FV2ModHotReloadIssue
  • plan: FV2ModHotReloadPlan
  • helper functions: BuildDefaultModHotReloadPolicy, BuildModScriptHotReloadChange, BuildModAssetHotReloadChange, EvaluateModHotReload, and ExecuteModHotReloadPlan
  • subsystem forwarding: UV2ModLoadingSubsystem::EvaluateDevelopmentHotReload
  • automation: V2/ue/Source/V2Tests/Private/Modding/ModLoadingSystem.spec.cpp
  • contract: V2/ue/Content/V2/Modding/ModHotReload_V2_Contract.json

Policy#

BuildDefaultModHotReloadPolicy enables development-only reloads for scripts and assets, requires sandbox approval, requires every changed item to advance its revision, and caps each reload batch at 64 changes.

The evaluator blocks reloads when development hot reload is disabled, when the session is not a development session, when sandbox approval is missing, when a change targets another mod, when a changed script or asset id is unknown, when an integrity hash is missing, or when a revision does not advance.

Script And Asset Changes#

BuildModScriptHotReloadChange creates a script change that reuses the existing script runtime binding and validates through EvaluateModScriptHotReload. Runtime, entrypoint, permission, sandbox, integrity, and stale-revision checks remain centralized in the scripting integration path.

BuildModAssetHotReloadChange creates an asset change that reuses the manifest asset metadata and validates through EvaluateModAssetLoadRequest. Asset kind, mount root, load phase, size, integrity, sandbox, and engine asset registry checks remain centralized in the asset loading path.

No-Restart Execution#

EvaluateModHotReload returns an accepted FV2ModHotReloadPlan only when all changed scripts and assets are accepted. Accepted plans set bRequiresGameRestart to false. ExecuteModHotReloadPlan marks accepted asset mounts as mounted and registered with the engine asset system while preserving bRequiresGameRestart == false, so development reloads can apply without restarting the game.

Verification#

bash
python3 V2/ue/Tools/check-v2-mod-hot-reload.py
python3 V2/ue/Tools/check-v2-mod-scripting-integration.py
python3 V2/ue/Tools/check-v2-mod-asset-loading.py
python3 V2/ue/Tools/check-v2-ci-workflow.py
python3 V2/tools/validate-v2-docs.py
python3 -m py_compile V2/ue/Tools/check-v2-mod-hot-reload.py V2/ue/Tools/check-v2-mod-load-order-management.py V2/ue/Tools/check-v2-ci-workflow.py
python3 -m json.tool V2/ue/Content/V2/Modding/ModHotReload_V2_Contract.json