Module contract 

The host API contract (catalog/host.json) as constants, plus the client messages every platform host sends (tests/host_fixtures.rs gates the constants against the JSON).

Constants

A2UI_MESSAGE_KINDS

The four A2UI v0.9 server messages.

DEFAULT_FUNCTION_DECISION

The default function policy’s default.

DEFAULT_MEDIA_SCHEMES

The media loader’s schemes when the host lists none (no file).

DEFAULT_URL_SCHEMES

EXTENSION_MESSAGE_KINDS

The two Exponential UI extensions.

FUNCTION_DECISIONS

FUNCTION_DENIED

FUNCTION_NOT_FOUND

HOST_CONTRACT_VERSION

HOST_ERROR_CODES

INVALID_MESSAGE

MCP_ACTION_TOOL

MCP_MIME_TYPES

MEDIA_MAX_BYTES

media.limits: what every image loader enforces.

MEDIA_MAX_PIXELS

MEDIA_OPTION_KEYS

MEDIA_RULE_KEYS

MEDIA_TIMEOUT_MS

MESSAGE_KINDS

Every message kind the router takes, A2UI first.

OP_KINDS

What the router turns a message into.

PACKAGE_REQUIRED

PAINT_ERROR_HOOK

The renderer → host hook a failed painter calls (paint.hook).

PRIVATE_HOSTS

VAPP-107: the hosts decide_url denies by default (urls.privateHosts, the rule itself is super::is_private_host).

RECORD_KIND_PATTERN

VAPP-122: a record kind’s name (package.records.kindPattern).

RENDER_FAILED

A component’s painter failed (paint.errorCode).

SOURCE_DENIED

VAPP-107: a bindDataModel outside a package surface’s sources.

SOURCE_PARAMS_ENCODING

VAPP-122: how the bind action appends its params (sources.paramsEncoding).

SOURCE_PATTERN

A binding source URI (<scheme>:<name>[?k=v&…]).

SSE_EVENTS

SURFACE_COMMANDS

SURFACE_FORMATTER

SURFACE_NOT_FOUND

SURFACE_SETTINGS

Round 2: what a host hands each surface besides messages — the SurfaceSettings keys, the host formatter’s methods, the commands.

TEMPLATE_NOT_FOUND

UNSUPPORTED_CATALOG

URL_REASONS

VALIDATION_FAILED

Functions

action_message

A client action message (A2UI v0.9 action). context null = {}; payload only when it has keys (JS Object.keys(payload).length).

error_message

A client error message (A2UI v0.9 error); path only when given.

host_contract

catalog/host.json without its $comments, rebuilt from the constants.