synvael/AGENTS.md

AGENTS.md

Guidance for AI coding agents (and human contributors) working in this repository. This file is the single source of truth for architecture, conventions, and workflow. Tool-specific entry points (e.g. CLAUDE.md) import it rather than duplicating it.

Project goal

Voxel-based game with souls-like combat. Built in Rust; rendering targets Vulkan via ash (raw Vulkan bindings, not a higher-level wrapper like wgpu or vulkano).

World is procedurally generated. Voxel edge length is 0.5 m, so the player occupies 3 blocks tall × 2 blocks wide. This finer grid is load-bearing for design decisions: collision, mesh chunking, LOD thresholds, and network bandwidth all need to assume ~8× the voxel count of a 1 m-grid world per unit volume, so pick chunk sizes and data layouts accordingly. The rationale and trade-offs of this scale are recorded in ADR-0002. Supports both single-player and multiplayer via a dedicated server. That dual mode is why server exists as its own crate even for solo play (the single-player path is expected to run the server logic in-process or invoke the same crate, rather than having a separate offline code path).

Documentation map

Documentation is layered by altitude; keep content at the layer it belongs to so no single file accretes everything.

• This file (AGENTS.md): cross-cutting engineering conventions and architecture invariants, i.e. the rules that apply regardless of which feature is being touched. There is a finite set of these, so this file should grow slowly. Subsystem-specific specs do not belong here.
• docs/ + Rust module docs (//!, ///): per-subsystem technical implementation docs. How an individual system (meshing, networking, worldgen, ...) is built. Prefer module docs next to the code; promote to a docs/<subsystem>.md note when the design spans multiple files.
• docs/adr/: Architecture Decision Records capturing the why behind significant, hard-to-reverse choices, one append-only file per decision. See docs/README.md for the full scheme and docs/adr/0001-record-architecture-decisions.md for the practice.

The canonical game-design specification (intent, world rules, gameplay behaviour) is maintained separately and is not part of this repository; this repo documents how that design is implemented.

Workspace layout

Cargo workspace (resolver = "3", edition 2024) with four crates under crates/:

• client: binary. Windowed application using winit 0.30 (ApplicationHandler pattern, ControlFlow::Poll). Also pulls in image. Player-facing app titled "Synvael"; handles input, windowing, and drives the renderer.
• server: binary. Authoritative game simulation (voxel world, combat, players). Used both for dedicated multiplayer hosts and as the simulation backend for single-player.
• renderer: library. Voxel/scene rendering on Vulkan via ash, decoupled from windowing so it can be driven by client.
• shared: library. Types and protocol shared between client and server (world/voxel data, network messages, combat primitives). Stays lean and dep-light; no mlua, no rendering, no engine internals.
• scripting: library. Lua modding API and bindings (owns the mlua dependency, UserData wrappers around shared types, API table registration, mod loader). Both client and server depend on it.

When adding code, keep the boundary tight: protocol/data types and game-rule primitives go in shared; Lua API surface and mlua integration in scripting; GPU/draw code in renderer; only input, windowing, and presentation glue live in client. Avoid growing client with simulation logic since it must work identically against either a local or remote server.

Modding API (Lua): dogfooded

The game exposes a Lua modding API, and the base game itself is built on top of that same API rather than treating it as a separate add-on layer. Built-in content (blocks, items, entities, recipes, etc.) is defined through the modding API so that mod authors can read the shipped code as reference for what's possible and how to do it.

This has hard implications when adding features:

• Any new gameplay primitive (a new block type, item, entity, ability, ...) needs to be reachable through the Lua API, not just a Rust-only path. If you add a Rust-side concept without an API surface, you've broken the dogfooding contract.
• Prefer extending the API and then using it from the engine over adding a parallel Rust-only entry point.
• Keep the API stable and discoverable, since mod authors will be reading it. Avoid leaking engine internals through it.
• The API and its bindings live in the scripting crate. It owns the mlua dependency, the API table registration, and the mod loader. Both client and server depend on it; shared does not, since shared stays the lean protocol/data layer.
• Authoritative APIs (world mutation, combat resolution) are defined in scripting but gated so the client-side Lua VM can't invoke them. One API surface, two execution contexts: client VM = read-only/UI/effects, server VM = authoritative.
• Prefer wrapper newtypes inside scripting over impl UserData for SharedType in shared, to avoid coupling the protocol crate to mlua.

The decision to build the base game on top of the modding API, and the client/server VM gating that follows from it, are recorded in ADR-0006.

Assets

All game assets live under /assets at the repo root, organised into subfolders by kind: icons/, models/, shaders/, sounds/, textures/, scripts/. New assets must be placed in the matching subfolder; do not drop loose files into /assets itself, and do not scatter assets inside crate directories.

Assets are published openly under CC-BY-NC-SA 4.0 (see LICENSE.md). Binary assets (textures, models, sounds, compiled shaders) are tracked via Git LFS; keep .gitattributes up to date when adding a new binary file type. Lua scripts and JSON data live in plain Git (text files).

Script locations

Three distinct locations, do not mix them:

• /assets/scripts/: the base game's own Lua, shipped with the binary. This is the dogfooded "first-party mod" the engine loads through the same API mod authors use. Mirror the structure modders will use (e.g. scripts/blocks/, scripts/items/, scripts/entities/) so it serves as a working reference.
• /mods/ (top-level): in-repo example mods or test fixtures. Kept out of /assets/ because they're not engine-shipped content, and out of crates/ because they're not Rust source.
• <user-data-dir>/mods/: player-installed mods, loaded at runtime only. Resolved via the directories / dirs crate (Linux: ~/.local/share/synvael/mods/, with platform equivalents elsewhere). Never read from a hard-coded path.

Data packs & resource packs

Two distinct, orthogonal systems; keep them separate, do not collapse them into one "pack" concept. Resource packs are client-side asset overlays (textures, sounds, models, fonts, language files; no logic). Data packs are declarative content definitions (JSON/TOML/RON): blocks, items, recipes, loot tables, biomes, tags.

The load-bearing rule: do not build a parallel registration system. The data-pack loader reads the declarative files and calls the same Lua API the engine and Lua mods use, giving one source of truth (data/blocks/stone.json → loader → blocks.register{ ... }). Each schema is a stable contract; version it deliberately. The decision is recorded in ADR-0007.

Full subsystem detail (load order, repo and user-data layout, resolution semantics) lives in docs/packs.md.

Contributing workflow

Before committing a change, verify it against the actual repo state rather than assuming it is correct: read the files, inspect git diff, and run cargo check / cargo clippy / cargo test as appropriate. Then follow this loop for each change:

1. Verify the change is actually present and correct in the working tree.
2. Run the linter and formatter to ensure no regressions or style issues were introduced: cargo clippy --all-targets --all-features -- -D warnings, cargo fmt --all -- --check, selene ., and stylua ..
3. Ensure useful comments are present before committing: function doc comments (///) and inline comments above non-obvious logic, following the documentation style below.
4. Create a focused git commit following the commit conventions below.

Keep commits scoped to a single concept. Do not batch multiple unrelated changes into one commit, and do not leave a verified change uncommitted before moving on to the next.

Concurrency model

The game is multithreaded by design: single-threaded would not meet the perf budget for voxel meshing, worldgen, rendering, networking, and simulation running together. Code should assume multiple threads and design data ownership accordingly:

• Prefer message-passing (channels: crossbeam-channel, flume, or std::sync::mpsc) and per-thread ownership over shared mutable state.
• When sharing is unavoidable, use the right primitive for the access pattern: Arc<Mutex<_>> for low-contention shared state, Arc<RwLock<_>> for read-heavy, atomics (AtomicU32, AtomicBool, ...) for counters and flags, lock-free structures (crossbeam, dashmap) for hot paths. Avoid wrapping large hot data in a single Mutex "just in case", which is how you accidentally serialise the whole engine.
• Worldgen and chunk meshing are the obvious parallelism wins. A thread pool (e.g. rayon, or a hand-rolled one) feeding meshing/generation jobs is expected.
• Vulkan command-buffer recording can be parallelised too, but Vulkan queues are not free-threaded: only one thread submits to a given queue at a time. Plan ownership of vk::Queue accordingly.
• The Lua VMs (one per execution context: client, server) are not thread-safe in mlua's default config; treat each VM as owned by a single thread, and dispatch work to/from it via channels.

Logging & error handling

• Logging: tracing (with tracing-subscriber as the output backend). Use info! / warn! / error! / debug! / trace! macros at appropriate levels, and use spans (#[tracing::instrument], info_span!) to scope work, which is how you keep multithreaded log output legible. Don't reach for println!/eprintln! for diagnostics; if it's worth printing, it's worth a tracing event.
• Errors in libraries (shared, renderer, scripting): typed error enums via thiserror (#[derive(Error)]). Each variant is a distinct, matchable failure mode. Don't expose anyhow::Error from a library API.
• Errors in binaries (client, server): anyhow at the top level, with .context("...") for human-readable layering. Library errors compose into anyhow::Error cleanly via ?.
• Never .unwrap() or .expect() outside main / setup / tests, except where the invariant is genuinely impossible to violate. In the hot path, propagate with ? and let the caller decide.

Documentation style

• Objective Tone: All comments (both doc comments /// and inline //) must be written in a formal, objective, and neutral tone.
• No Personal Pronouns: Avoid first-person ("we", "our", "us") or second-person ("you", "your") pronouns.
• Voice: Use the passive voice or neutral descriptive language. Instead of "We initialize the buffer," use "The buffer is initialized." Instead of "Your vertex shader needs this," use "The vertex shader requires this."
• Focus: Describe the code's behavior, the system's state, or technical invariants.
• Struct Documentation: Every field in a public or internal struct must have a doc comment (///) explaining its purpose and any invariants.
• Stability: Treat the documentation as a technical specification for the engine.
• Line breaks: Do not insert line returns inside a comment unless necessary. A comment that fits on a single line stays on a single line; do not pre-wrap at ~80 chars for aesthetics. Only break across lines when the comment is genuinely long (multi-sentence prose, enumerated invariants) or when a hard break carries meaning (separating an intro line from a bullet list, for instance).

Target platforms

Linux and Windows only. No macOS, no mobile, no console, no web/WASM.

• Both platforms have native Vulkan via vendor ICDs (NVIDIA / AMD / Intel). No translation layer (no MoltenVK story), so modern Vulkan extensions can be adopted freely without consulting a portability matrix.
• File paths: always use std::path::Path / PathBuf and the directories (or dirs) crate for user-data lookup. Never hard-code /home/... or ~. Linux follows XDG ($XDG_DATA_HOME etc.); Windows uses %APPDATA%.
• Line endings: repo is LF-only. Set core.autocrlf = false and/or a .gitattributes with * text eol=lf to keep diffs clean across the two OSes.
• Filename casing: never have two files differing only in case. Linux is case-sensitive; Windows isn't; the mismatch produces confusing "works on my machine" bugs.

Determinism stance

• Worldgen is seed-deterministic. Given the same seed, worldgen must produce bit-for-bit the same world on any platform, any time. This constrains worldgen code: use a fixed RNG algorithm (e.g. wyrand, xoshiro), never rand::thread_rng() or anything seeded from the OS. Do not depend on HashMap iteration order (Rust's default hasher is randomised); use BTreeMap, IndexMap, or sort explicitly when iteration order feeds into RNG draws or content placement. See ADR-0003.
• Simulation is server-authoritative. The server runs the truth; clients send inputs and receive state snapshots, predicting locally for responsiveness and reconciling on disagreement. Combat, physics, mob AI, and item drops are computed once, on the server.
• Full simulation determinism (lockstep / rollback / replay-from-inputs) is a non-goal. This means floats, hash-map iteration, and platform-specific math are all fair game outside of worldgen. Don't pay the cost of cross-platform float reproducibility for a feature we're not building. See ADR-0004.

Content IDs & namespacing

All registered content (blocks, items, recipes, biomes, entities, ...) is identified by a namespaced string of the form "namespace:id". The full rationale is in ADR-0005.

• Engine's reserved namespace: core:. All first-party content registered by the base game uses it ("core:stone", "core:iron_sword"). Mods pick their own short namespace ("mymod:weird_dirt").
• Strict form required. A bare ID with no : is an error at registration / parse time, not silently coerced to core:. Same rule everywhere: engine scripts, data packs, Lua mods, recipe references, save files. No exceptions; the symmetry is the point.
• Charset: namespace and id are each [a-z0-9_-]+, exactly one : between them. Lowercase ASCII only. No uppercase, no Unicode, no spaces, no dots, no slashes. Keeps IDs greppable, filesystem-safe, and unambiguous in logs and save files.
• Runtime representation: intern each ID string into a small integer handle (e.g. BlockId(u32)) at registration time. Hot paths compare handles, not strings. Keep the original string for display, save/load, and the Lua API surface.

Project name note: The project is named Synvael ("Catalyst" was the working codename). The engine namespace is deliberately core: (not the project name), so it stays stable independent of branding.

Coordinate system & units

• Up axis: +Y.
• Handedness: right-handed (default math convention; +X right, +Y up, +Z toward the viewer / out of the screen).
• World unit: 1 unit = 1 block. Blocks are simply 0.5 m in physical scale, but inside the engine everything is counted in blocks, not metres. A player is therefore 3 units tall × 2 units wide in world coordinates.

Implementation gotchas that arise because neighbouring tools use different conventions (Vulkan clip space, Blender import, glTF) are collected in docs/rendering.md. They are not convention changes, only mismatches to handle in one agreed place.

Branching Strategy & Workflow

• main vs dev: The repository follows a strict workflow. The main branch is reserved purely for stable releases. All active development happens on the dev branch.
• Feature Branches: For any large feature, always create a new branch off of dev (e.g., feat/new-worldgen). Do not commit large, work-in-progress features directly to dev. Once the feature is complete and verified, merge it back into dev.
• Continuous Integration: The project enforces strict linting and formatting via GitHub Actions. This includes workspace-level clippy rules (banning unwrap and println!), cargo fmt, selene for Lua, and stylua. Always ensure your code passes these tools locally before pushing.

Commit conventions

Conventional Commits with mandatory crate-name scope.

Format:

<type>(<crate>): <imperative subject>

[optional body]

[optional footer(s)]

• Type (required, exactly one): feat (new feature), fix (bug fix), refactor (no behaviour change), perf, docs, test, chore (build/tooling/deps), build, ci. Breaking changes append ! before the colon: feat(scripting)!: ....
• Scope (required): the crate the change primarily affects, one of client, server, renderer, shared, scripting. For changes that genuinely span the whole workspace (e.g. workspace-level Cargo config, repo-wide .gitattributes), use workspace. For changes confined to non-Rust assets, use assets. Avoid omitting the scope, and avoid inventing per-commit scopes.
• Subject: imperative mood ("add", not "added" / "adds"), lowercase, no trailing period, ≤ ~72 chars.
• Body: keep commit messages short and simple, usually subject only, no body. The exception is fix(...) commits for non-trivial bugs, where a body explaining the root cause and why the fix works is valuable. Don't pad routine feat/refactor/chore/docs commits with bodies.

Examples:

feat(scripting): expose blocks.register to lua
fix(renderer): clamp swapchain extent to surface caps
refactor(shared): split network message types into submodule
chore(workspace): bump ash to 0.39
docs(assets): document texture-pack overlay layout
feat(server)!: change tick rate from 20 to 30 Hz

If a single commit truly touches multiple crates and can't be reasonably split, that's a signal to split it. Only fall back to workspace scope when the change is intrinsically workspace-wide.

Do not add any AI assistant as a co-author on commits. No Co-Authored-By: ... trailers for assistants, no "Generated with ..." footers. Commits are authored by the human running the work.

Common commands

cargo build                          # build all crates
cargo run -p client                  # run the windowed client
cargo run -p server                  # run the server
cargo test                           # run all tests
cargo test -p renderer it_works      # run a single test by name
cargo check -p <crate>               # fast type-check one crate
cargo clippy --all-targets --all-features -- -D warnings
cargo fmt
selene .
stylua .