Limitations

Fairyfly trades breadth for speed and auditability. This page lists every hard cap; each guide links back here from its own summary table.

HTTP server

Area	Limit	When hit
Concurrent connections	512	The 513th waits for a free slot
Request body	~4 KB (shares the 4 KB read buffer)	Beyond → `413 Payload Too Large`
Response body	10 MB hard cap (64 KB stage buffer spills to the heap)	Beyond 10 MB → `500`
Response headers	2048 bytes	Headers beyond the cap are dropped
Hung handler	30s	Connection fails with `504`
Bind address	Always `0.0.0.0`	No host option, no `FF_PORT`
Plain listener protocol	HTTP/1.1	No HTTP/2 framing without TLS
HTTP/2	TLS + ALPN only (nghttp2)	Plain sockets stay HTTP/1.1
`FF_ECHO`	Any value set (even empty — only unsetting disables)	Every request gets canned `200 {"message":"ok"}`

Fetch (client)

Area	Limit	When hit
Concurrent fetches	16	The 17th waits for a free slot
Body stage per slot	64KB	Larger bodies spill to chunks/heap
I/O timeout	30s	Slow servers fail
Redirects	Followed, max 5 hops	Throws `Too many redirects`
Abort / `signal`	Not supported	`signal` / `timeout` / `redirect` init keys are ignored
Protocol	HTTP/1.1	No HTTP/2 client
Proxy	HTTP proxies via env vars (`HTTP_PROXY` / `HTTPS_PROXY` / `ALL_PROXY`); `NO_PROXY` bypass	No SOCKS support; no `https://` proxy URLs (TLS to the proxy); env is read once at launch

WebSocket

Area	Limit	When hit
Client sockets	64 per process	Extra connects fail
Message size	16384 bytes	Larger frames truncate
Handshake URL path	512 bytes	Longer paths fail
Server socket API	`send` / `sendBinary` only	No `id`, `readyState`, or `close` on server sockets
Proxy	Same env vars as `fetch`	`ws://` needs a CONNECT-capable proxy

UDP

Area	Limit	When hit
Concurrent sockets	64 (shared pool)	`udp.socket: no socket slots available`
In-flight `sendto` calls	64	`send: SendQueueFull`
Maximum datagram	65507 bytes	`send: MessageTooLong`
Pending receives per socket	1	`recvfrom: RecvPending`
Addresses	IP literals only	Hostnames rejected (`...: invalid address`)
`setsockopt` / multicast	Missing	No socket options

Timers and workers

Area	Limit	When hit
Live timers	128	`TypeError: too many timers (max 128)`
Extra timer args	8 after `ms`	Extras ignored
Workers	8	`too many workers (max 8)`
Worker heap	32 MB + 1 MB JS stack (8 MB thread stack)	—
Worker message	4 MB per message	Larger `postMessage` payloads are rejected
`process.nextTick`	Missing	Use `queueMicrotask`

// instead of process.nextTick(fn):
queueMicrotask(fn);

Filesystem

Area	Limit	When hit
Path length	4096 bytes	Longer paths fail
Read per call	10MB, sync	Larger reads fail
API style	Sync + six `*Async` Promise variants (`readFileAsync`, `writeFileAsync`, `existsAsync`, `mkdirAsync`, `rmAsync`, `readdirAsync`)	No `fs.watch` — poll with timers

Console

Area	Type	Description
Arguments printed	All per call	Every argument prints, joined with spaces
Destination	stderr	`console.log > out.txt` captures nothing

Modules and language

Area	Limit	When hit
Module system	ESM only	No `require()`, no CommonJS
Relative extensions	Optional for `.js`	`./x` probes `./x.js`, then `./x/index.js`; other extensions must be written out
Bare specifiers	Resolved via `node_modules`	Needs `ff imprint` install first
`Buffer`	Missing	Use `Uint8Array`
DOM / `window`	Missing	Backend only
Subprocess API	Separate global	`child_process.spawn` / `exec` / `execSync` — see the child_process section below

SQLite

Area	Limit	When hit
`db.exec`	First statement only	Multi-statement SQL needs `db.execNoArgs`
Parameters	Positional `?` only	Named `:x` / `$x` are not supported
Integer precision	±2⁵³	Beyond → precision loss in JS

Postgres

Area	Limit	When hit
Pool	10 connections	Extra queries queue
TLS (`sslmode`)	Not supported	—
Connection/socket timeouts	None	Slow queries wait indefinitely
`rowCount`	Returned rows only	INSERT/UPDATE row counts are not reported
COPY	Not supported	—

Process

Area	Limit	When hit
`process.env`	Snapshot at boot	Assigning to it does not affect the OS environment
stdout/stdin streams	Missing	No `process.stdout.write` or stdin reads
`hrtime`, `kill`	Missing	—
Runaway JS watchdog	None	An infinite loop hangs the process

child_process

Area	Limit	When hit
Concurrent children	8	The 9th `spawn` throws / `exec` rejects
`maxBuffer`	1 MB per stream (default)	Overflow SIGTERMs the child; `exec` rejects, `spawn` sets `truncated`
Platform	POSIX only (macOS/Linux)	Windows throws
stdin, streams, events	Missing	Use `wait()` + captured `stdout` / `stderr`
ff exit	Children are not killed	Call `kill()` yourself

TLS and bytecode

Area	Limit	When hit
TLS versions	TLS 1.2 (BearSSL build)	No TLS 1.3 in the default build
`ff compile` input	20MB source	Larger inputs fail
`ff test` file	10MB	Larger test files are not loaded
REPL line	8192 bytes	Longer lines truncate
`ff init`	Always overwrites	Back up `ff.json` if you customized it

Packages

Area	Limit	When hit
Package type	Pure-JS ESM only	`require(`, `module.exports`, or `node:` imports are rejected
Version graph	One flat `node_modules`	A conflicting transitive version is a hard error
Direct sources	Public HTTPS only; single package at the archive root	Private repos, auth, monorepo subdirectories unsupported
Direct archive size	128 MB download cap	Larger archives fail
Raw archive URLs	Must end in `.tar.gz` or `.tgz`	Other URL forms are rejected
Build scripts	Not run	The archive must already contain runnable JS
Direct refs	Default to `HEAD`; branch/tag names are mutable	The lock's integrity check fails closed if the archive changes

What is not limited (common misconceptions)

res.blob() and res.formData() work on Request and Response.
HTTP redirects are followed (up to 5 hops) by fetch.
Port shorthand http.serve(3000, handler) works; the default port is 3000.
make install targets ~/.local/bin/ff, not /usr/local/bin.

#Limitations

#HTTP server

#Fetch (client)

#WebSocket

#UDP

#Timers and workers

#Filesystem

#Console

#Modules and language

#SQLite

#Postgres

#Process

#child_process

#TLS and bytecode

#Packages

#What is not limited (common misconceptions)

Limitations

HTTP server

Fetch (client)

WebSocket

UDP

Timers and workers

Filesystem

Console

Modules and language

SQLite

Postgres

Process

child_process

TLS and bytecode

Packages

What is not limited (common misconceptions)