Diagnostics

The compiler reports problems as numbered diagnostics. An error (E) stops the compile and writes nothing; a warning (W) is printed and the modules are still generated. The same codes appear in the Studio plugin’s editor as you type.

Codes are banded by the stage that raises them – 1xxx reading characters, 2xxx parsing, 3xxx checking what was parsed – and never renumbered, so the gaps are codes that were retired. A code is shown as E or W by severity; 3020 can be either.

Reading a diagnostic

Game.blink

event Snapshot {
    From: Server,
    Type: Unreliable,
    Call: SingleSync,
    Data: struct { Names: string[], Tick: u32 }
}

[W3019] Warning: Unreliable event "Snapshot" may exceed the packet limit
    ╭─[Game.blink:1:7]
    │
001 │ event Snapshot {
    ┆       ────┬────
    ┆           │
    ┆           ╰── Unbounded, and the limit is 900 bytes
005 │     Data: struct { Names: string[], Tick: u32 }
    ┆                           ───┬───
    ┆                              │
    ┆                              ╰── This has no upper bound
    │ = note: Bound the variable-length fields -- `string(0..64)`, `u8[..16]` -- or send it reliably.
    │
────╯

The first line is the code, the severity and the message. Below it, each labelled line of the schema with its line number: the primary label under what is wrong, secondary labels under what explains it, and note lines with the fix. With --compact the CLI prints one line instead:

[W3019] [L001:L001] [Game.blink] Warning: Unreliable event "Snapshot" may exceed the packet limit

A program reading the diagnostics – an editor task, a CI step, an assistant – should use --json instead, which gives each one’s code, file, line and column, labels and notes as JSON. See JSON output.

Errors and warnings

Reading and parsing

Code	Message	What to do
E1001	Unexpected token	A character that begins no token of the language. Remove it, or check for a stray symbol or an unclosed string.
E2001	Unexpected end of file	A declaration or list was left open. Close the brace, bracket or parenthesis.
E2002	Unexpected token	The grammar expected something else here; the label says what. Often a missing comma, : or =.
E2003	Unknown option “X”	An option name that does not exist, or an event or function field that does not. The label lists the valid ones.
E2003	Invalid value for option “X”	A numeric option that is not a positive whole number, a Casing other than Pascal, Camel or Snake, or an InboundBurst below MaxPacketSize.
E2003	Unknown option “X” (on a field’s value)	A From, Type, Call or Yield value that is not one of the allowed words.
E2003	Invalid value for “Rate” / “Burst” / “Concurrency”	Rate must be above zero; Burst must be at least 1, since each event spends a whole token; Concurrency must be a whole number of at least 1.
E2005	Field “X” is missing	An event needs From, Type and Call; a function needs Yield. Add the field.

Declarations and types

Code	Message	What to do
E3001	Cannot cast “X” to “Y”	A reference to a declaration of the wrong kind, such as a map name where a struct is expected.
E3001	Expected a struct to merge	..Name inside a struct must name a struct.
E3001	A map cannot be keyed by …	A struct, tagged enum, map, set, array or type pack decodes to a fresh table that nothing can look up. Key by a string, number, boolean or enum.
E3002	Invalid optional type	A map’s key or value may not be optional.
E3002	Type cannot be optional	unknown and quat have no way to encode absence.
E3004	Field “X” was already specified	An event or function field given twice.
E3004	Duplicate field / flag / value / variant / element	A name repeated inside a struct, set, enum, tagged enum or type pack.
E3004	Merged struct contains a duplicate field	Two merged structs, or a merge and a field, share a field name.
E3005	Reserved identifier	StepReplication, SetRateLimitHandler and SetDecodeErrorHandler name module members. Rename the declaration.
E3005	“X” cannot name a type pack element	A named element becomes a parameter: it must be a Luau identifier, not a keyword, and not a name the generated function already uses, such as Player.
E3005	Reserved identifier (a type named number, Player, …)	A declared type is exported to Luau under its own name. It cannot take a built-in Luau type’s name (any, boolean, buffer, never, number, string, thread, unknown), and a top-level type cannot take the name of a Roblox type the module uses: Player, Instance, RemoteEvent, UnreliableRemoteEvent, CFrame, Vector3, Color3, DateTime, BrickColor, or a class the schema names in Instance(...). Rename it.
E3005	Enum tag used as field in variant	A tagged enum’s tag field may not also be a field of a variant.
E3007	Unknown reference	The name is not declared, or not declared yet: a declaration is in scope only after it closes.
E3008	Type doesn’t accept a range	Only numbers, string, buffer, vector and arrays take a range.
E3009	Malformed range / Expected an integer / Range outside bounds	Fix the range: integer types and lengths need whole numbers, and the range must fit the type (an array holds at most 65535).
E3010	Duplicate declaration	Two declarations, scopes or imports share a name in one scope, including across profiles.
E3012	Unit enums don’t support generics	Only structs, maps and tagged enums are generic.
E3013	… is not exportable / Generic types can’t be exported / Types containing an Instance or unknown can’t be exported	export applies to non-generic types whose values travel entirely in the buffer.
E3022	Recursive type	A type cannot contain itself: references are inlined where they appear.
E3023	Type doesn’t accept components / Too many components / Unknown or invalid primitive used as component	Components go on vector (one) and CFrame (two), and must be number types or quat.
E3023	quat is not a type on its own / Only a CFrame’s rotation can be a quaternion / A CFrame has one rotation	quat is valid only as one component of a CFrame.
E3029	Too many values / variants, or Empty enum	An enum travels as one byte: it holds 1 to 256 values or variants.
E3030	Too many reliable events and functions / Too many unreliable events	A channel numbers its declarations with one byte, so it holds 256. Imports count; declarations a profile leaves out do not. Split the schema’s traffic, or move events to the other channel.

Files, options and profiles

Code	Message	What to do
E3014	Unknown require	The imported file was not found. Check the path, relative to the importing file.
E3015	Cyclic import	Two files import each other, directly or through others. Move what they share into a third file.
E3016	Option set after start of file	Move every option above the first declaration.
W3017	Option “UseColon” is deprecated	It has no effect. Delete the line.
W3017	Field “Poll” is deprecated	Write Call: Polling and delete Poll. The Call value beside Poll: true was being ignored.
E3024	Duplicate option	An option set twice where one build would apply both. Each profile may set it once; set without a profile, it may not also be set with one.
E3025	Unknown attribute	The only attribute is @profile.
E3026	Unknown profile	Profiles are dev, debug, test and release.
E3027	Reference to an excluded declaration	A compiled declaration uses one this build’s profile leaves out. Give both the same profile, or neither.
E3028	Duplicate attribute / Attribute without a statement	A statement takes one @profile, and it must be followed by a declaration, import or option.

Network safety

Code	Message	What to do
E3018	Unreliable event “X” cannot fit in one packet	Its smallest payload is over MaxUnreliableSize. Send it Reliable, or shrink it.
W3019	Unreliable event “X” may exceed the packet limit	Its largest payload is over the limit. Bound the field the secondary label names, or send it reliably.
W3020	Rate limiting a Server event / function has no effect	Rate and concurrency limits apply to inbound traffic only. Remove Rate, Burst and Concurrency.
E3020	“Burst” was set without a “Rate”	Add a Rate (or option DefaultRate), or remove the Burst.
E3020	Inbound “X” has no rate limit	option RequireRates is set. Give it a Rate, or set option DefaultRate.
E3021	Array / Map of a type that costs nothing to decode	An unbounded repetition of elements that read no bytes gives a hostile count nothing to run out of. Bound it ([..16]) or give the element a sent field.

Errors without a code

A few problems are reported as plain errors rather than diagnostics:

Message	Cause
A client output path must be defined. / A server output path must be defined.	The CLI needs ClientOutput and ServerOutput.
Cannot use yield type: "Future", without providing a path to the future library.	A Yield: Future function needs option FutureLibrary; Promise likewise needs PromiseLibrary.

Runtime messages

The generated modules report problems through warn and error, prefixed [BlinkBlox]. Warnings the server prints about a client’s traffic are limited to one a second per player and kind.

Message	Side	Meaning
Dropped a malformed packet from X.	server	The remote was called with arguments that are not a buffer and a table.
Dropped an oversized packet from X.	server	Over MaxPacketSize.
Dropped a packet with too many instances from X.	server	Over MaxInstancesPerPacket.
Dropped the rest of a packet from X, too many events.	server	MaxEventsPerPacket events were decoded; the rest was dropped.
Dropped packets from X over the inbound budget of N bytes a second, N refused.	server	The player’s InboundBytesPerSecond budget ran out.
Rate limited event "E" from X, N refused.	server	The event’s Rate refused it.
Dropped a packet that could not be decoded, at event "E": ...	client	A packet from the server failed to decode and no decode-error handler is installed.
Event queue of "E" exceeded 256, did you forget to implement a listener?	client	Reliable events are queuing for a listener that never connected.
"E" already has a listener. ... keeps only the newest one, and the replaced listener's disconnect stops working.	both	A second On on a Single event.
Event "E" yielded in a Sync call, so the rest of the packet it arrived in was discarded. ...	both	A Sync listener yielded; see SyncValidation.
Dropped unreliable event "E", N bytes exceeds the N byte limit.	both	An unreliable send grew past MaxUnreliableSize and was not sent.
"F" was never answered and has been failed after N seconds.	both	An Invoke timed out; on the server the message names the player.
"F" encountered an error, ...	both	A function’s listener errored or returned a value that failed to serialise; the caller is failed.
There was an exception while processing "F".	both	Raised by a Coroutine Invoke whose call failed.
32 calls are already awaiting a response, this call has been dropped.	both	Raised by the 33rd outstanding Invoke.
Cannot invoke X, they are no longer in the game.	server	Raised by Invoke on a player who has left.
This client was built from a different schema than the server (client ..., server ...).	client	Raised on require; see Wire compatibility.
The server did not publish a schema signature. ...	client	Raised on require: the server is a build from before signatures, or not BlinkBlox.
The reliable remote is not a RemoteEvent. / The unreliable remote is not an UnreliableRemoteEvent.	both	Something else holds the remote’s name in ReplicatedStorage.
An instance of BlinkBlox is already running with the remote scope "S". ...	both	Two modules with the same RemoteScope were required in one Luau environment.

A decode failure passed to SetDecodeErrorHandler carries the error that stopped the decode: a range or length check (Expected "Value" to be smaller than or equal to 100, got 250 instead.), a length the packet cannot back, Unknown event index. for an index no event has, or This channel receives no events. for traffic on a channel this side never listens to.