Crash when used with HM API

[lunaynx]

Not sure who is at fault here, but here's a minimal reproduction.

Mod List

• Fabric API [0.128.1+1.21.5] by FabricMC (fabric-api-0.128.1+1.21.5.jar)
• HM Api [1.0.1+1.21.2] by AzureAaron (hm-api-1.0.1+1.21.2.jar)
• Polymer [0.12.6+1.21.5] by Patbox (polymer-bundled-0.12.6+1.21.5.jar)

Crash Log

---- Minecraft Crash Report ----
// Shall we play a game?

Time: 2025-07-27 11:54:00
Description: Initializing game

java.lang.RuntimeException: Could not execute entrypoint stage 'main' due to errors, provided by 'fabric-data-attachment-api-v1' at 'net.fabricmc.fabric.impl.attachment.sync.AttachmentSync'!
	at net.fabricmc.loader.impl.FabricLoaderImpl.lambda$invokeEntrypoints$2(FabricLoaderImpl.java:403)
	at net.fabricmc.loader.impl.util.ExceptionUtil.gatherExceptions(ExceptionUtil.java:33)
	at net.fabricmc.loader.impl.FabricLoaderImpl.invokeEntrypoints(FabricLoaderImpl.java:401)
	at net.fabricmc.loader.impl.game.minecraft.Hooks.startClient(Hooks.java:52)
	at knot//net.minecraft.class_310.<init>(class_310.java:473)
	at knot//net.minecraft.client.main.Main.main(Main.java:249)
	at net.fabricmc.loader.impl.game.minecraft.MinecraftGameProvider.launch(MinecraftGameProvider.java:480)
	at net.fabricmc.loader.impl.launch.knot.Knot.launch(Knot.java:74)
	at net.fabricmc.loader.impl.launch.knot.KnotClient.main(KnotClient.java:23)
	at org.prismlauncher.launcher.impl.StandardLauncher.launch(StandardLauncher.java:105)
	at org.prismlauncher.EntryPoint.listen(EntryPoint.java:129)
	at org.prismlauncher.EntryPoint.main(EntryPoint.java:70)
Caused by: java.lang.ExceptionInInitializerError
	at knot//net.fabricmc.fabric.mixin.attachment.CustomPayloadC2SPacketAccessor.getMaxPayloadSize(CustomPayloadC2SPacketAccessor.java)
	at knot//net.fabricmc.fabric.impl.attachment.sync.AttachmentChange.<clinit>(AttachmentChange.java:63)
	at knot//net.fabricmc.fabric.impl.attachment.sync.s2c.AttachmentSyncPayloadS2C.<clinit>(AttachmentSyncPayloadS2C.java:30)
	at knot//net.fabricmc.fabric.impl.attachment.sync.AttachmentSync.onInitialize(AttachmentSync.java:103)
	at net.fabricmc.loader.impl.FabricLoaderImpl.invokeEntrypoints(FabricLoaderImpl.java:399)
	... 9 more
Caused by: java.lang.ClassCastException: class net.azureaaron.hmapi.network.WrappedPacketCodec cannot be cast to class eu.pb4.polymer.networking.impl.ExtCustomPayloadCodec (net.azureaaron.hmapi.network.WrappedPacketCodec and eu.pb4.polymer.networking.impl.ExtCustomPayloadCodec are in unnamed module of loader 'knot' @61009542)
	at knot//net.minecraft.class_2817.modifyExpressionValue$ben000$polymer-networking$modifyCodec(class_2817.java:1517)
	at knot//net.minecraft.class_2817.<clinit>(class_2817.java:18)
	... 14 more


A detailed walkthrough of the error, its code path and all known details is as follows:
---------------------------------------------------------------------------------------

-- Head --
Thread: Render thread
Stacktrace:
	at net.fabricmc.loader.impl.FabricLoaderImpl.lambda$invokeEntrypoints$2(FabricLoaderImpl.java:403)
	at net.fabricmc.loader.impl.util.ExceptionUtil.gatherExceptions(ExceptionUtil.java:33)
	at net.fabricmc.loader.impl.FabricLoaderImpl.invokeEntrypoints(FabricLoaderImpl.java:401)
	at net.fabricmc.loader.impl.game.minecraft.Hooks.startClient(Hooks.java:52)
	at knot//net.minecraft.class_310.<init>(class_310.java:473)

-- Initialization --
Details:
	Modules: 
Stacktrace:
	at knot//net.minecraft.client.main.Main.main(Main.java:249)
	at net.fabricmc.loader.impl.game.minecraft.MinecraftGameProvider.launch(MinecraftGameProvider.java:480)
	at net.fabricmc.loader.impl.launch.knot.Knot.launch(Knot.java:74)
	at net.fabricmc.loader.impl.launch.knot.KnotClient.main(KnotClient.java:23)
	at org.prismlauncher.launcher.impl.StandardLauncher.launch(StandardLauncher.java:105)
	at org.prismlauncher.EntryPoint.listen(EntryPoint.java:129)
	at org.prismlauncher.EntryPoint.main(EntryPoint.java:70)

-- System Details --
Details:
	Minecraft Version: 1.21.5
	Minecraft Version ID: 1.21.5
	Operating System: Mac OS X (aarch64) version 15.5
	Java Version: 21.0.7, Microsoft
	Java VM Version: OpenJDK 64-Bit Server VM (mixed mode), Microsoft
	Memory: 200699648 bytes (191 MiB) / 826277888 bytes (788 MiB) up to 4294967296 bytes (4096 MiB)
	CPUs: 8
	Processor Vendor: Apple Inc.
	Processor Name: Apple M1
	Identifier: Apple Inc. Family 0x1b588bb3 Model 0 Stepping 0
	Microarchitecture: ARM64 SoC: Firestorm + Icestorm
	Frequency (GHz): 3.20
	Number of physical packages: 1
	Number of physical CPUs: 8
	Number of logical CPUs: 8
	Graphics card #0 name: Apple M1
	Graphics card #0 vendor: Apple (0x106b)
	Graphics card #0 VRAM (MiB): 0.00
	Graphics card #0 deviceId: unknown
	Graphics card #0 versionInfo: unknown
	Memory slot #0 capacity (MiB): 0.00
	Memory slot #0 clockSpeed (GHz): 0.00
	Memory slot #0 type: unknown
	Virtual memory max (MiB): 10240.00
	Virtual memory used (MiB): 7140.69
	Swap memory total (MiB): 2048.00
	Swap memory used (MiB): 635.38
	Space in storage for jna.tmpdir (MiB): <path not set>
	Space in storage for org.lwjgl.system.SharedLibraryExtractPath (MiB): <path not set>
	Space in storage for io.netty.native.workdir (MiB): <path not set>
	Space in storage for java.io.tmpdir (MiB): available: 94968.50, total: 233752.44
	Space in storage for workdir (MiB): available: 94968.50, total: 233752.44
	JVM Flags: 2 total; -Xms512m -Xmx4096m
	Fabric Mods: 
		fabric-api: Fabric API 0.128.1+1.21.5
			fabric-api-base: Fabric API Base 0.4.62+73a52b4b49
			fabric-api-lookup-api-v1: Fabric API Lookup API (v1) 1.6.96+86c3a9f149
			fabric-biome-api-v1: Fabric Biome API (v1) 16.0.7+2dd063df49
			fabric-block-api-v1: Fabric Block API (v1) 1.1.0+ed91556f49
			fabric-block-view-api-v2: Fabric BlockView API (v2) 1.0.26+aa6d566c49
			fabric-blockrenderlayer-v1: Fabric BlockRenderLayer Registration (v1) 2.0.16+86c3a9f149
			fabric-client-tags-api-v1: Fabric Client Tags 1.1.37+86c3a9f149
			fabric-command-api-v1: Fabric Command API (v1) 1.2.70+f71b366f49
			fabric-command-api-v2: Fabric Command API (v2) 2.2.49+73a52b4b49
			fabric-commands-v0: Fabric Commands (v0) 0.2.87+df3654b349
			fabric-content-registries-v0: Fabric Content Registries (v0) 10.0.14+3e6c1f7d49
			fabric-convention-tags-v1: Fabric Convention Tags 2.1.33+7f945d5b49
			fabric-convention-tags-v2: Fabric Convention Tags (v2) 2.15.2+6d9989f349
			fabric-crash-report-info-v1: Fabric Crash Report Info (v1) 0.3.12+86c3a9f149
			fabric-data-attachment-api-v1: Fabric Data Attachment API (v1) 1.8.5+6d9989f349
			fabric-data-generation-api-v1: Fabric Data Generation API (v1) 22.5.2+a0fab30c49
			fabric-dimensions-v1: Fabric Dimensions API (v1) 4.0.17+3e6c1f7d49
			fabric-entity-events-v1: Fabric Entity Events (v1) 2.1.0+3ce7866349
			fabric-events-interaction-v0: Fabric Events Interaction (v0) 4.0.15+64e3057949
			fabric-game-rule-api-v1: Fabric Game Rule API (v1) 1.0.70+c327076a49
			fabric-item-api-v1: Fabric Item API (v1) 11.4.1+e46fd76a49
			fabric-item-group-api-v1: Fabric Item Group API (v1) 4.2.9+3459fc6149
			fabric-key-binding-api-v1: Fabric Key Binding API (v1) 1.0.63+ecf51cdc49
			fabric-keybindings-v0: Fabric Key Bindings (v0) 0.2.61+df3654b349
			fabric-lifecycle-events-v1: Fabric Lifecycle Events (v1) 2.6.0+230071a049
			fabric-loot-api-v2: Fabric Loot API (v2) 3.0.48+3f89f5a549
			fabric-loot-api-v3: Fabric Loot API (v3) 1.0.36+86c3a9f149
			fabric-message-api-v1: Fabric Message API (v1) 6.1.0+fe971bba49
			fabric-model-loading-api-v1: Fabric Model Loading API (v1) 5.2.3+5281b90749
			fabric-networking-api-v1: Fabric Networking API (v1) 4.5.0+775be32c49
			fabric-object-builder-api-v1: Fabric Object Builder API (v1) 21.1.2+b8d6ba7049
			fabric-particles-v1: Fabric Particles (v1) 4.1.2+112e550e49
			fabric-recipe-api-v1: Fabric Recipe API (v1) 8.1.8+3235ab3249
			fabric-registry-sync-v0: Fabric Registry Sync (v0) 6.1.22+cd64004349
			fabric-renderer-api-v1: Fabric Renderer API (v1) 6.1.2+bdf4842249
			fabric-renderer-indigo: Fabric Renderer - Indigo 3.1.2+5281b90749
			fabric-rendering-data-attachment-v1: Fabric Rendering Data Attachment (v1) 0.3.64+73761d2e49
			fabric-rendering-fluids-v1: Fabric Rendering Fluids (v1) 3.1.27+86c3a9f149
			fabric-rendering-v1: Fabric Rendering (v1) 11.3.0+a272b33849
			fabric-resource-conditions-api-v1: Fabric Resource Conditions API (v1) 5.0.21+73a52b4b49
			fabric-resource-loader-v0: Fabric Resource Loader (v0) 3.1.7+847e5f5c49
			fabric-screen-api-v1: Fabric Screen API (v1) 2.0.46+86c3a9f149
			fabric-screen-handler-api-v1: Fabric Screen Handler API (v1) 1.3.129+c327076a49
			fabric-sound-api-v1: Fabric Sound API (v1) 1.0.38+86c3a9f149
			fabric-tag-api-v1: Fabric Tag API (v1) 1.0.17+ecf51cdc49
			fabric-transfer-api-v1: Fabric Transfer API (v1) 5.4.24+7b20cbb049
			fabric-transitive-access-wideners-v1: Fabric Transitive Access Wideners (v1) 6.3.17+f17a180c49
		fabricloader: Fabric Loader 0.16.14
			mixinextras: MixinExtras 0.4.1
		hm-api: HM Api 1.0.1+1.21.2
		java: OpenJDK 64-Bit Server VM 21
		minecraft: Minecraft 1.21.5
		polymer-bundled: Polymer 0.12.6+1.21.5
			polymer-autohost: Polymer (Auto Host) 0.12.6+1.21.5
				io_netty_netty-codec-http: netty-codec-http 4.1.82
			polymer-blocks: Polymer (Textured Blocks) 0.12.6+1.21.5
			polymer-common: Polymer (Common) 0.12.6+1.21.5
				packet_tweaker: Packet Tweaker 0.6.0-pre.1+1.21.2-pre3
			polymer-core: Polymer (Core) 0.12.6+1.21.5
				polymer-networking: Polymer (Networking) 0.12.6+1.21.5
				polymer-registry-sync-manipulator: Polymer (Registry Sync Manipulator) 0.12.6+1.21.5
			polymer-resource-pack: Polymer (Resource Pack) 0.12.6+1.21.5
			polymer-resource-pack-extras: Polymer (Resource Pack Extras) 0.12.6+1.21.5
			polymer-sound-patcher: Polymer (Sound Patcher) 0.12.6+1.21.5
			polymer-virtual-entity: Polymer (Virtual Entity) 0.12.6+1.21.5
	Launched Version: 1.21.5
	Launcher name: PrismLauncher
	Backend library: LWJGL version 3.3.3-snapshot
	Backend API: Unknown
	Window size: <not initialized>
	GFLW Platform: <error>
	Render Extensions: ERR
	GL debug messages: <no renderer available>
	Is Modded: Definitely; Client brand changed to 'fabric'
	Universe: 404
	Type: Client (map_client.txt)
	Locale: en_US
	System encoding: UTF-8
	File encoding: UTF-8
	CPU: 8x Apple M1

[lunaynx]

Seems like it is likely HM API's fault for modifying the return value to an incompatible subclass, though Polymer is also doing an unchecked cast here.

[Patbox]

They shouldn't be wrapping the payload codec here, instead implement things with mixins.
Polymer's implementation is similar (as in functionally the same) to one provided by fabric api.

Polymer implementation

https://github.com/Patbox/polymer/blob/dev/1.21.6/polymer-networking/src/main/java/eu/pb4/polymer/networking/mixin/CustomPayloadC2SPacketMixin.java

Fabric API's implementation

https://github.com/FabricMC/fabric/blob/1.21.8/fabric-networking-api-v1/src/main/java/net/fabricmc/fabric/mixin/networking/CustomPayloadC2SPacketMixin.java

HM's implementation

https://github.com/AzureAaron/hm-api/blob/75f86ae350484e8644aa4755eb46c4ed988d05cd/src/main/java/net/azureaaron/hmapi/mixins/CustomPayloadC2SPacketMixin.java

[Patbox]

Please keep it open until it's actually solved, to prevent duplicates / confusion