docs: add document for AI agents (AGENTS.md)

Author: mafredri

AGENTS.md

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+# Agents
+
+This document helps AI agents work effectively in this codebase. It explains the philosophy, patterns, and pitfalls behind the code, so you can make good decisions on any task, not just scenarios explicitly covered. Apply these principles while reading the code for specifics.
+
+## Philosophy
+
+This is a minimal, idiomatic WebSocket library. Simplicity is a feature. Before adding code, consider whether it's necessary. Before adding a dependency, don't; tests requiring external packages are isolated in `internal/thirdparty`.
+
+## Research
+
+**Understand the protocol first.** Code comments reference RFC 6455 (WebSocket), RFC 7692 (compression), and RFC 8441 (HTTP/2). When behavior seems odd, check the RFC section cited nearby.
+
+**Trace from public API inward.** Start with `Accept` (server) or `Dial` (client), then follow to `Conn`, then to `Reader`/`Writer`. The internal structure follows these paths.
+
+**Read tests for intent.** Tests often reveal why code exists. The autobahn-testsuite (`autobahn_test.go`) validates protocol compliance; if you're unsure whether behavior is correct, that's the authority.
+
+**Check both platforms.** Native Go files have `//go:build !js`. WASM lives in `ws_js.go` (imports `syscall/js`). They have the same API but different implementations. WASM wraps browser APIs and cannot control framing, compression, or masking, so don't try to implement those features there.
+
+## Making Changes
+
+**Every change needs a reason.** Don't reword comments, rename variables, or restructure code without justification. If you can't articulate why a change improves things, don't make it.
+
+**Understand before changing.** Research the code you're modifying. Trace the call paths, read the tests, check both platforms. A change with good intentions but incomplete understanding can break things in ways you won't notice.
+
+**Ask for clarification rather than assuming.** If requirements are ambiguous or you're unsure how something should work, stop and ask. A wrong assumption can waste more time than a quick question.
+
+**Don't delete existing comments.** Comments that explain non-obvious behavior preserve important context about why code works a certain way. If a comment seems wrong, verify before removing.
+
+**Ask: does this need to exist?** The library stays small by saying no. A feature that solves one user's problem but complicates the API for everyone is not worth it.
+
+**Ask: is this the user's job or the library's job?** The library handles protocol correctness. Application-level concerns (reconnection, auth, message routing) belong in user code.
+
+**Ask: what breaks if I'm wrong?** Context cancellation closes connections. Pooled objects must be returned. Locks must respect context. These invariants exist because violating them causes subtle bugs.
+
+## Code Style
+
+Follow Go conventions: [Effective Go](https://go.dev/doc/effective_go), [Go Code Review Comments](https://go.dev/wiki/CodeReviewComments), and [Go Proverbs](https://go-proverbs.github.io/). Be concise, declarative, and factual.
+
+Never use emdash. Use commas, semicolons, or separate sentences.
+
+**Doc comments** start with the name and state what it does. Put useful information where users make decisions (usually the constructor, not methods).
+
+**Inline comments** are terse. Prefer end-of-line when short enough.
+
+**Explain why, not what.** The code shows what it does; comments should explain reasoning, non-obvious decisions, or edge cases.
+
+**Wrap comments** at ~80 characters, continuing naturally at word boundaries. Don't put each sentence on its own line.
+
+**Avoid:**
+
+- Tautologies and redundant explanations the code already conveys
+- Filler phrases: "Note that", "This is because", "It should be noted"
+- Hedging: "basically", "actually", "really"
+
+## Key Invariants
+
+**Always read from connections.** Control frames (ping, pong, close) arrive on the read path. A connection that never reads will miss them and misbehave. `CloseRead` exists for write-only patterns.
+
+**Pooled objects must be returned.** `flate.Writer` is ~1.2MB. Leaking them causes memory growth. Follow `get*()` / `put*()` patterns; return on all paths including errors.
+
+**Locks must respect context.** The `mu` type in `conn.go` unblocks when context cancels or connection closes. Using `sync.Mutex` for user-facing operations would block forever on stuck connections.
+
+**Reads and writes are independent.** They have separate locks (`readMu`, `writeFrameMu`) and can happen concurrently. Don't create unnecessary coupling between them.
+
+**Masking is asymmetric.** Clients mask payloads; servers don't. The mask is applied into the bufio.Writer buffer, not in-place on source bytes.
+
+## Testing
+
+**Protocol compliance:** run autobahn tests with `AUTOBAHN=1 go test`. Some tests are intentionally skipped (UTF-8 validation adds overhead without value; `requestMaxWindowBits` is unimplemented due to `compress/flate` limitations).
+
+**Frame-level correctness:** compare wire bytes. When two implementations disagree, capture the bytes and check against the RFC.
+
+**WASM compatibility:** API changes need both implementations. Test that method signatures match.
+
+**Test style:** prefer table-driven tests and simple want/got comparisons.
+
+## RFC References
+
+- RFC 6455: The WebSocket Protocol
+- RFC 7692: Compression Extensions for WebSocket (permessage-deflate)
+- RFC 8441: Bootstrapping WebSockets with HTTP/2
+
+Section numbers in code comments refer to these RFCs.