> For the complete documentation index, see [llms.txt](https://docs.onelitefeather.net/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.onelitefeather.net/falco/reference/replaceable-policies.md). # Replaceable policies The three services `FalcoAnvilLoader` consults, their interfaces, their shipped defaults, and how each is supplied. > **This page is a generation target, currently written by hand.** Source of truth: the policy interfaces and their `META-INF/services` registrations in [`falco-anvil`](https://github.com/OneLiteFeatherNET/Falco/tree/main/falco-anvil/README.md). ## `ChunkVersionPolicy` | | | | --------------- | --------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | Method | `void check(CompoundBinaryTag data, int minimumDataVersion) throws ChunkDataException` | | Shipped default | [`DefaultChunkVersionPolicy`](https://github.com/OneLiteFeatherNET/Falco/blob/main/falco-anvil/src/main/java/net/onelitefeather/falco/anvil/DefaultChunkVersionPolicy.java) | | Registered by | `falco-anvil`, in its own `META-INF/services` | | Builder slots | `versionPolicy(ChunkVersionPolicy)`, `discoverVersionPolicy()` | | Can be disabled | yes — `versionPolicy(null)` | | Consulted | once per load, after the raw compound is parsed, before anything is decoded | | Source | [`ChunkVersionPolicy`](https://github.com/OneLiteFeatherNET/Falco/blob/main/falco-anvil/src/main/java/net/onelitefeather/falco/anvil/ChunkVersionPolicy.java) | Decides whether a chunk is readable at all. A policy only decides and throws; it does not count or log — `FalcoAnvilLoader#checkVersion` does both regardless of which policy ran. ## `UnknownEntryPolicy` | | | | --------------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ | | Methods | `String onUnknownBlock(String name, @Nullable CompoundBinaryTag properties)`, `String onUnknownBiome(String name)` | | Returns | a replacement *name*, not an id | | Shipped default | [`DefaultUnknownEntryPolicy`](https://github.com/OneLiteFeatherNET/Falco/blob/main/falco-anvil/src/main/java/net/onelitefeather/falco/anvil/DefaultUnknownEntryPolicy.java) — `"minecraft:air"` and `"minecraft:plains"` | | Registered by | `falco-anvil`, in its own `META-INF/services` | | Builder slots | `unknownEntryPolicy(UnknownEntryPolicy)`, `discoverUnknownEntryPolicy()` | | Can be disabled | no — `unknownEntryPolicy(null)` falls back to the default | | Consulted | by `BlockPaletteResolver#toId` and `BiomePaletteResolver#toId` | | Source | [`UnknownEntryPolicy`](https://github.com/OneLiteFeatherNET/Falco/blob/main/falco-anvil/src/main/java/net/onelitefeather/falco/anvil/UnknownEntryPolicy.java) | Decides what an unknown palette entry becomes. Throwing `AnvilChunkException` from either method fails the chunk instead of carrying a substitute. The policy is not asked twice: if the name it returns is itself unknown, the resolver throws `IllegalStateException`. The resolvers report the name to `AnvilDiagnostics` and the log regardless of what the policy returns. ## `ChunkMigrator` | | | | ---------------- | --------------------------------------------------------------------------------------------------------------------------------------------------- | | Shipped default | none in `falco-anvil` — the engine lives in `falco-migration` | | Builder slot | selecting a `ChunkMigrationMode` | | Discovery | turned on by selecting a mode; no second call needed | | Consulted | only when a mode is selected | | Missing provider | the loader fails to build rather than migrating nothing | | Source | [`ChunkMigrator`](https://github.com/OneLiteFeatherNET/Falco/blob/main/falco-anvil/src/main/java/net/onelitefeather/falco/anvil/ChunkMigrator.java) | Translates a chunk an older version wrote, rather than deciding what to do about it. Runs *before* `ChunkVersionPolicy`. See [Reference Migration modes](/falco/reference/migration-modes.md). ## Resolution rules Implemented in [`ServiceResolution`](https://github.com/OneLiteFeatherNET/Falco/blob/main/falco-anvil/src/main/java/net/onelitefeather/falco/anvil/ServiceResolution.java) and shared by all three. | Rule | Behaviour | | ------------------------------------------------------------ | ----------------------------------------------------------------- | | Shipped default vs one foreign provider | the shipped default steps aside | | Two *foreign* providers | throws, naming both classes | | Builder slot and discovery both set | throws `IllegalStateException` | | Any call to `versionPolicy(...)` / `unknownEntryPolicy(...)` | turns discovery off for that policy, including when passed `null` | | Classloader used for discovery | the service's own, never the thread context classloader | | When resolution runs | once, when the loader is built — not per chunk | The opening log line of the loader reports which version policy was resolved, or the word `none`. Related: [How-to Replace the version and unknown-entry policies](/falco/how-to-guides/replace-the-version-and-unknown-entry-policies.md) · [Explanation The chunk version guard](/falco/explanation/the-chunk-version-guard.md) --- # Agent Instructions This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com. ## Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation by asking a question. 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