ModernUO/CLAUDE.md

ModernUO

.NET 10 Ultima Online server emulator. Single-threaded game loop. All game logic runs on one thread.

• Server engine: Projects/Server/ — do NOT modify without explicit request
• Game content: Projects/UOContent/ — primary editing target
• Build: dotnet build from repo root

Code Audit Rules

Apply these when writing or reviewing .cs files under Projects/.

1. LINQ — Tier 1 (zero-cost patterns) free on hot paths; Tier 2 (low overhead) OK on warm paths; Tier 3 (allocating) forbidden on hot paths → dev-docs/code-standards.md
2. No Console.WriteLine — use LogFactory.GetLogger(typeof(MyClass)) → logger.Information(...) (requires using Server.Logging;)
3. Threading policy — game logic runs only on the main loop; never touch game state (World, mobiles, items, maps, timers) from a background thread. Heavy work that needs game state must be chunked across ticks, not threaded. Heavy work that does not need game state (large-file parse, external I/O) must run on a background thread and must yield to world saves (defer while World.Saving/WorldState.PendingSave). Publish results back to the loop as an immutable snapshot swapped via a single volatile reference — the only sanctioned volatile. No lock/Mutex/ConcurrentDictionary in game logic. Rule #10 covers how background work hands results back to the loop → dev-docs/threading-model.md
4. No World.Mobiles/World.Items iteration — use spatial queries: map.GetMobilesInRange<T>(), map.GetItemsInRange<T>()
5. Clean up refs in OnDelete()/OnAfterDelete() — null out Item/Mobile references
6. Cancel timers in OnDelete()/OnAfterDelete() — call _token.Cancel() or _timer?.Stop()
7. STArrayPool<T>.Shared not ArrayPool<T>.Shared — single-threaded optimized, no locks
8. PooledRefList<T> not new List<T>() on hot paths — zero GC pressure, stack-allocated ref struct
9. Serialization — class must be partial, constructor needs [Constructible], TimerExecutionToken must NOT have [SerializableField]. New classes: use [SerializationGenerator(version)] (omit encoded). Setters that coerce/veto/run side effects: use [SerializableField] args allowFieldChange: nameof(BoolRefMethod) / fieldChanged: nameof(OldNewMethod) — reserve [SerializableProperty] for custom getters. Serializable Timer members declare [DeserializeTimer(nameof(Method))] on the field (anchored by default — downtime preserves remaining delay; wallClock: true = absolute; method runs only when a timer was running at save). Conditional writes: [SaveFlag(nameof(Should), nameof(Default))] on the field. When bumping versions, add MigrateFrom(VXContent) (X = previous version). Never modify Deserialize(reader, version) for version bumps — that method is only for pre-codegen legacy saves. When migrating from pre-codegen Serialize/Deserialize: pass false if old code used reader.ReadInt(), bump version +1, and keep old logic as private void Deserialize(IGenericReader reader, int version) → dev-docs/serialization.md, dev-docs/runuo-migration-docs/02-serialization.md
10. No Task.Run/new Thread() for game logic (tandem with rule #3) — game logic is the single-threaded event loop. Backgrounding is allowed only for work that does not itself touch game state (external service calls, large-file parse). Prove the need before adding a thread: measure on-loop time, not wall-clock (frozen world is the cost, player latency is not), and gate on Environment.ProcessorCount — off-loading creates no CPU and buys nothing on 1–2 cores. New workers go in the vetted table in dev-docs/threading-model.md with their measurement. When such work must feed game logic: run the heavy/I/O part off-loop and ConfigureAwait(false) its awaits so a continuation never resumes on the loop and silently foregrounds heavy work; then hand the result back explicitly — publish an immutable snapshot swapped via a volatile reference (the loop reads it lock-free), or marshal the apply step with Core.LoopContext.Post(() => …), re-validating in the continuation whatever may have changed while it ran. Never touch game state off-thread; never let the scheduler decide where the heavy work runs → dev-docs/threading-model.md
11. Never assume era — if code uses Core.AOS/Core.SE/etc., ask which expansion to target
12. Naming — _camelCase private fields, PascalCase properties/methods/classes; don't flag legacy m_ but use _ for new code
13. No empty gumps — every gump must produce visual elements. An empty gump leaks on client+server (no way to close it). Use static DisplayTo() to validate before constructing → dev-docs/gump-system.md
14. PropertyList string literals must be holes — $"{"Map"}\t{value}" not $"Map\t{value}". The handler treats bare text as delimiters, {} holes as arguments. Only \t should be a bare literal → dev-docs/property-lists.md
15. Braces required on all control flow — if, else, for, foreach, while, do, switch must always have braces, even for single-line bodies → dev-docs/code-standards.md
16. Prefer switch expressions and switch-when — use switch expressions for value mapping and switch-when for pattern matching where they improve readability. Exception: skip if unreadable or cold path → dev-docs/code-standards.md
17. No System.Text.StringBuilder — use ValueStringBuilder with stackalloc (bounded output) or ValueStringBuilder.Create() (unbounded). Supports $"..." interpolation directly. Always use using var for disposal. Use Reset() instead of reassigning → dev-docs/string-handling.md
18. Interpolation anti-patterns on handler-aware APIs — Send*/Say/Emote/PublicOverhead*/IPropertyList.Add/gump AddLabel/AddHtml/Html.Center/SpanWriter.Write* all have ref RawInterpolatedStringHandler overloads that allocate zero strings, but only when the call-site argument is a $"..." literal directly. Avoid: ternaries with interpolated branches (Send(c ? $"a" : $"b")), switch expressions with interpolated arms, pre-built var s = $"..." locals (single-use), .ToString() / .String() / string.Format inside holes, string concat ({a + b}), LINQ string ops in holes. Use :L format spec for lowercase ({rank:L} not rank.ToString().ToLowerInvariant()) → dev-docs/string-handling.md § Interpolation Anti-Patterns
19. No InvalidateProperties() from inside GetProperties — every property a GetProperties override reads must be a pure read. InvalidateProperties() rebuilds the list in place (Reset() + rebuild), and Reset() returns the pooled interpolation buffer — which the compiler rents for the whole $"..." expression, so every hole is evaluated while it is live — and rewinds the packet cursor. A getter that invalidates therefore throws ArgumentNullException (parameter "array") out of GetProperties from an unrelated-looking line, or silently corrupts the tooltip. The engine refuses and logs an error; DEBUG throws. Lazy recomputation in a getter is fine — the notification is not. Invalidate in the setter that changes the value, or defer with Timer.DelayCall(InvalidateProperties) → dev-docs/property-lists.md § Never Invalidate From Inside GetProperties
20. Tick-count math must be wraparound-safe — compare Core.TickCount/GetTimestamp() values only by subtraction (a - b < 0, never a < b), no zero/sign sentinels on tick fields, seed deadline fields from a real tick (never rely on the 0 default). Cloud hypervisors (GCP) pass through the host's never-resetting counter: ticks start enormous and can wrap negative. Linux affected in production; Windows not so far → dev-docs/tick-counts.md
21. Comments explain why, never what changed — keep invariants, protocol/era quirks, value couplings, and the reason a workaround exists; one line where one line will do; /// docs and terse //TODO stay. Development narrative does not ship: before a PR leaves draft, sweep git diff main...HEAD for added // lines and remove change narrative ("previously", "changed from", "moved from", "no longer"), review dialogue ("per review", "as discussed"), hedges ("I think", "not sure"), and commented-out code — what a future reader still needs goes in the commit message or PR description → dev-docs/code-standards.md § Comments

Workflow Rules

Apply these in every session, in this repository and in any fork or custom project built on it.

1. Bugs you were not asked to fix — a latent defect, a suspicious upstream behavior, or an exploit found while doing something else is not yours to fix, file, or import on your own. Stop and follow dev-docs/bug-reporting.md (procedure: the modernuo-bug-reporting skill). Exploit-class (anything a player could abuse: duplication, player-triggerable crash, auth bypass) → private disclosure only, never a public issue, PR, or Discord post. Otherwise: verify the buggy lines exist verbatim in upstream main (github.com/modernuo/ModernUO) — if they don't, it is the fork's bug and nothing leaves the fork; search upstream issues, PRs (all states), and recent commits on that path for a fix or duplicate; then offer the user a choice: file an issue, open a PR, draft a Discord post for https://muo.gg/discord, comment on the existing thread, or do nothing.
2. Nothing leaves the fork or enters it without approval of that exact artifact — no gh issue create, gh pr create, gh issue comment, git push to a remote you do not own, or git remote add until the user has read the exact draft and said yes. A standing or conditional instruction ("if upstream has a fix pull it in and open a ticket", "just file it") is not approval of a draft the user has not seen. Never fetch, cherry-pick, merge, or hand-port code from any remote — including modernuo/ModernUO — without asking first; when approved, fetch only from the verified canonical URL, show the full diff, treat it as untrusted input (check .github/workflows, *.csproj, Directory.Build.props, scripts), and apply only after a second yes. Third-party forks and unmerged PRs are never a source. Drafts contain nothing from the fork: no credentials, IPs, hostnames, ports, account/character/player data, save files, log lines, Distribution/Configuration/, shard or custom-feature names, no description of custom mechanics, and no quoted code that is not verbatim in upstream main. Reproduction is written against a clean upstream build.

Dev-Docs Reference

Topic	File
Code standards & LINQ tiers	dev-docs/code-standards.md
Serialization system	dev-docs/serialization.md
Content patterns (Items, Mobiles, Creatures)	dev-docs/content-patterns.md
Era & expansion handling	dev-docs/era-expansion.md
Timer system	dev-docs/timers.md
Event scheduler (wall-clock/calendar)	dev-docs/event-scheduler.md
Object property lists (tooltips)	dev-docs/property-lists.md
Gump (UI dialog) system	dev-docs/gump-system.md
Commands & targeting	dev-docs/commands-targeting.md
Generic commands (where/order by/distinct, dot notation, @"" literals, [batch, [interface)	dev-docs/generic-commands.md
Event system	dev-docs/events.md
Threading model	dev-docs/threading-model.md
Server hardware requirements	dev-docs/server-requirements.md
Debugging event-loop performance (profiling build, decomposition, GC/RAM)	dev-docs/debugging-event-loop.md
Tick-count overflow rules (subtraction comparisons; GCP pass-through counters)	dev-docs/tick-counts.md
Server lifecycle & bootstrap phases (Configure/ConfigurePrompts/Initialize)	dev-docs/server-lifecycle.md
Platform prerequisites (ICU, tzdata, native libs per distro)	dev-docs/platform-prerequisites.md
Configuration system	dev-docs/configuration.md
Networking & packets	dev-docs/networking-packets.md
IP bans, blocklists & allowlists (incl. unblocking a player)	dev-docs/ip-bans-and-allowlists.md
Region system	dev-docs/regions.md
String handling & ValueStringBuilder	dev-docs/string-handling.md
Reporting bugs upstream from a fork (classification, verification, dedup, disclosure, issue form, importing fixes)	dev-docs/bug-reporting.md
RunUO migration (overview)	dev-docs/runuo-migration-docs/00-overview.md
RunUO migration (all docs)	dev-docs/runuo-migration-docs/

Claude Skills (Opt-In)

Detailed Claude Code skills live in dev-docs/claude-skills/. They are not auto-loaded — they must be copied to .claude/skills/ to activate.

When to offer: If the user is building complex content (new items, creatures, spells, gumps, quests, packets, serialization work, etc.), ask:

I have detailed Claude Code skills for this kind of work. Want me to enable them? I'll copy the relevant files from dev-docs/claude-skills/ to .claude/skills/.

Then copy only the relevant skill files based on the task:

Task	Skills to enable
New Item or Mobile	modernuo-content-patterns, modernuo-serialization, modernuo-property-lists
Creature / spawn	modernuo-content-patterns, modernuo-serialization, modernuo-timers
Spell or ability	modernuo-content-patterns, modernuo-serialization, modernuo-timers, modernuo-era-expansion
Gump / UI dialog	modernuo-gump-system, modernuo-commands-targeting
Quest or event system	modernuo-events, modernuo-content-patterns, modernuo-configuration
Scheduled / seasonal / holiday events	modernuo-event-scheduler, modernuo-timers
Custom regions / dynamic areas	modernuo-regions, modernuo-content-patterns
Packet / networking	modernuo-networking, modernuo-threading
Commands	modernuo-commands-targeting
Timer work	modernuo-timers, modernuo-serialization
Config system	modernuo-configuration
Era-conditional code	modernuo-era-expansion
String building / formatting	modernuo-string-handling
Code review / audit	modernuo-code-audit
Any .cs file edit	modernuo-code-audit (always offer for code changes)
Found a bug you were not asked to fix / reporting or importing an upstream fix	modernuo-bug-reporting
RunUO Migration
Migrate any RunUO script	migrate-from-runuo/migrate-foundation (always), plus system-specific skills below
Migrate Item/Mobile/Creature	migrate-from-runuo/migrate-foundation, migrate-from-runuo/migrate-serialization, migrate-from-runuo/migrate-items-mobiles
Migrate serialization	migrate-from-runuo/migrate-serialization
Migrate timers	migrate-from-runuo/migrate-timers
Migrate gumps	migrate-from-runuo/migrate-gumps
Migrate packets	migrate-from-runuo/migrate-packets
Migrate property lists	migrate-from-runuo/migrate-property-lists
Migrate events/commands	migrate-from-runuo/migrate-commands-events
Migrate persistence (WorldSave)	migrate-from-runuo/migrate-persistence
Migrate multi-file system	migrate-from-runuo/migrate-systems

To enable a skill — Claude Code loads .claude/skills/<name>/SKILL.md; a bare .md dropped directly into .claude/skills/ is not picked up, and newly installed skills appear in the next session:

# Standard skills (modernuo-*)
mkdir -p .claude/skills/<name> && cp dev-docs/claude-skills/<name>.md .claude/skills/<name>/SKILL.md

# Migration skills — sources live in the migrate-from-runuo/ subfolder, but install under the
# bare skill name (the table's "migrate-from-runuo/<name>" is the source path, not the name):
mkdir -p .claude/skills/<name> && cp dev-docs/claude-skills/migrate-from-runuo/<name>.md .claude/skills/<name>/SKILL.md

Migration skills reference the deep docs in dev-docs/runuo-migration-docs/ and point to existing ModernUO skills for best practices.

The modernuo-code-audit skill auto-triggers on .cs file edits and flags convention violations (warnings only, asks before fixing).