Module codes 

Stable, machine-readable diagnostic codes.

Each constant is a permanent identifier for one class of authoring mistake. The code is rendered alongside every diagnostic and emitted as a structured field by webui build --format json, so editors, CI, and AI assistants can map an error to a deterministic fix or docs page regardless of the human-readable wording. Treat these as a stable API surface — rename only with a deliberate, documented migration.

Constants

AMBIGUOUS_MODULE_ENTRY

More than one build output shares the file name of an authored <script type="module" src>, so its shared chunks cannot be preloaded without guessing which output the URL serves. Severity: warning.

AUTHORED_WEBUI_HYDRATE

<webui-hydrate> was written by an application author. It is reserved for the compiler/handler-generated runtime hydration sentinel.

BOUNDARY_CROSSES_SCOPE

A <boundary> would cut through component host content or an inert/raw-text HTML element instead of independently wrapping it.

BOUNDARY_IN_FOSTER_CONTEXT

A <boundary> appears inside an HTML insertion mode that foster-parents unknown elements (<table>, <tbody>, <tr>, <select>, …), which would separate the generated <webui-hydrate> sentinel from its payload script.

BOUNDARY_IN_REPEAT

A <boundary> is reachable from a <for> repeat body, directly or transitively through <if>, routes/outlets, or a reusable component. A repeat iteration cannot suspend, so the declaration can never be paced independently.

BOUNDARY_OUTSIDE_BODY

A <boundary> appears before <body> opens or after it closes.

COMPONENT_ASSETS_WITH_ROUTES

Static component assets were requested for a graph containing <route>.

DUPLICATE_BOUNDARY_NAME

Two <boundary> directives in the same owning template share a name.

DUPLICATE_FOR_ID

More than one <for> body defines the same identifier in one file.

DYNAMIC_LIGHT_KEYFRAMEDeprecated

Legacy code for removed Light-local keyframe namespacing.

EXCESSIVE_MODULE_PRELOADS

A module entry’s static import closure exceeded the modulepreload hint cap. Preloads share one connection, so the remainder would delay the entry they are meant to accelerate. Severity: warning.

EXCESSIVE_NESTING

Template nesting exceeds the parser’s depth limit.

FAST_LIGHT_DOM_UNSUPPORTED

Compatibility alias for the FAST-owned Light DOM diagnostic code.

INCOMPATIBLE_FOR_ITEM

A named <for> reference uses a different item variable from its body.

INVALID_BOUNDARY_KEY

A <boundary key> is empty or has malformed binding delimiters.

INVALID_BOUNDARY_NAME

<boundary name> is not a static string literal (e.g. it contains a {{binding}}).

INVALID_COMPONENT_RENDER_POLICY

A component’s root rendering or hydration policy is malformed.

INVALID_CSS

A <style> block contains malformed CSS.

INVALID_EVENT_HANDLER

An @event handler value is not a valid {handler()} expression.

INVALID_FOR_EACH

<for each> value is not of the form item in collection.

INVALID_FOR_ID

<for> has an invalid, conflicting, or unsupported identifier attribute.

INVALID_FOR_IDENTIFIER

<for each> item or collection name uses disallowed characters.

INVALID_FOR_KEY

An explicit repeat key is malformed or used on an invalid element.

INVALID_IF_CONDITION

<if condition> value is not a parseable expression.

INVALID_RENDER_RESERVATION

A lazy-render intrinsic block-size estimate is malformed or misplaced.

INVALID_ROUTE_BOUNDARY_PLACEMENT

A route-local <boundary> is nested in ignored route markup instead of being a direct child of the <route>.

INVALID_SHADOW_ROOT_MODE

A declarative Shadow DOM wrapper has an unsupported mode or placement.

INVALID_W_REF

A w-ref binding is missing its required {braces}.

LIGHT_DOM_SLOT

An unwrapped Light DOM component contains a native <slot>.

LIGHT_SCOPE_COLLISIONDeprecated

Legacy code for the removed Light DOM scope-collision diagnostic.

MALFORMED_HTML_TAG

An HTML tag could not be parsed (missing >, stray <, …).

MISSING_BOUNDARY_KEY

A <boundary> declaration renders more than once — through repeated static callsites of its owning component — without a stable occurrence key.

MISSING_BOUNDARY_NAME

<boundary> is missing its required, non-empty name attribute.

MISSING_FOR_EACH

<for> element is missing its required each attribute.

MISSING_IF_CONDITION

<if> element is missing its required condition attribute.

MISSING_RENDER_RESERVATION

Lazy rendering omitted the intrinsic block-size estimate needed to reserve scroll geometry while the browser skips layout.

MISSING_THEME_TOKEN

A CSS token required by parser output is missing from the configured theme.

MULTIPLE_OUTLETS

More than one <outlet> appears in the same parsed route level. Severity: warning.

NESTED_BOUNDARY

A <boundary> is nested inside another <boundary>.

RECURSIVE_TEMPLATE

A template references itself (directly or transitively) at build time.

RESERVED_LIGHT_DOM_MARKERDeprecated

Legacy code for the removed compiler-owned Light DOM marker diagnostic.

SCRIPTLESS_EVENT_HANDLER

A scriptless HTML component contains event bindings and needs authored JS.

STREAMING_WITHOUT_PROJECTION

A build declares <boundary> streaming checkpoints but has no state-projection manifest, so every checkpoint serializes the whole state object instead of its own components’ keys. Severity: warning.

TOO_MANY_BOUNDARIES

The protocol-wide boundary declaration identity space was exhausted.

UNCLOSED_HTML_TAG

An HTML element is missing its closing tag.

UNEXPECTED_CLOSING_TAG

A closing tag has no matching opening tag.

UNKNOWN_COMPONENT

A component tag has no matching registered component.

UNKNOWN_FOR_ID

A named <for> reference has no definition in its owning file.

UNSAFE_MODULE_PRELOAD

A resolved modulepreload href contains characters that cannot be written verbatim into an HTML attribute, so the hint was dropped rather than emitted or escaped. Severity: warning.

UNSUPPORTED_LIGHT_CSS

Light component CSS contains a Shadow-only selector.

UNTERMINATED_HTML_COMMENT

An HTML comment was opened but never closed with -->.

UNTERMINATED_HTML_DECLARATION

An HTML declaration was opened but never closed with >.

UNTHEMED_TOKEN

A CSS token used only with a literal var() fallback and absent from every theme — non-fatal, but usually a typo. Severity: warning.