Extension registry

Extensions are named, optional capability sets a writer declares in the top-level extensions array (see SPEC, "Extensions"). Core v1 parsing is never affected: a reader that doesn't know an extension ignores it — the field is never a validation input and never a reason to reject.

{
	"curt": "curtis-conversation",
	"version": 1,
	"extensions": ["images-inline"],
	"conversation": { }
}

Three properties make the mechanism safe:

1. Declaration costs nothing. The array is itself an unknown field to old readers — never a validation input, whatever its shape.
2. Extensions are behaviorally inert at the core. Behavior lives entirely in the fields an extension reads and writes inside the conversation and its messages; an extension must not change how the core validates.
3. The registry is additive. New entries never change how existing v1 files parse, and entries cannot conflict with core field names.

Status rule

Entries start draft and move to stable when two independent implementations round-trip them. Same-author emissions do not count — the registry states this per entry where it applies. deprecated retires an entry.

Registry

Name	Status	Emitted by	Purpose
images-inline	stable	curtis plugin, porter, curtis-web, curtis-cloud, curtis-ai-tech	Image payloads ride inside content
usage-metadata	stable	porter, curtis-web, curtis-ai-tech	Per-message cost/token/provider fields
tool-linkage	draft	porter, curtis-ai-tech, curt-convert (same author — does not count toward stability)	Tool-call requests and their results
agent-context	draft	curtis plugin, porter, curtis-cloud, curtis-ai-tech	Agent/swarm provenance on conversations and messages
consensus-receipt	draft	tractum	Signed k-of-n consensus decision bound to the transcript evidence

The browsable per-extension pages above are hand-authored views; the registry's full normative text is rendered at Extension registry — full text.

Full per-consumer emit/accept data lives in capabilities.json.

Adding an entry

Open a PR against EXTENSIONS.md — see governance for the required sections and the stability bar.