diff --git a/.changeset/legal-nights-work.md b/.changeset/legal-nights-work.md
new file mode 100644
index 0000000..a845151
--- /dev/null
+++ b/.changeset/legal-nights-work.md
@@ -0,0 +1,2 @@
+---
+---
diff --git a/AGENTS.md b/AGENTS.md
index ca964fc..d21159c 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -7,7 +7,7 @@ _use_ a running sideshow lives in `guide/AGENT_SETUP.md`, served at `/setup`.)
 ## What this is and why
 
 A live visual surface for terminal coding agents: agents publish surfaces
-(multi-part cards — html, markdown, diff, terminal, image) over
+(multi-part cards — html, markdown, diff, terminal, image, mermaid) over
 CLI/MCP/HTTP; the user watches them render in a browser and comments back. The
 two-way loop — publish → live render → comment → revise/reply — is the product.
 When in doubt, optimize for the loop.
@@ -33,8 +33,8 @@ consciously, not as a side effect):
   (`/api/assets`, `/a/:id`), and the shared flow functions both REST and MCP call.
 - `server/types.ts` — data model + `Store` interface; no runtime imports. A
   surface is an ordered list of parts (`html` | `markdown` | `diff` | `terminal`
-  | `image`); a snippet is sugar for a single html part.
-  `firstHtml`/`htmlPart` bridge the legacy snippet shape. Assets (uploaded blobs)
+  | `image` | `mermaid`); a snippet is sugar for a single html part.
+  `htmlPart` bridges the legacy snippet shape. Assets (uploaded blobs)
   are a separate entity, referenced by `image` parts; `selectEvictions`
   is the reference-aware LRU policy.
 - `server/public.ts` — the `sideshow/server` package export (`createApp`,
@@ -44,7 +44,7 @@ consciously, not as a side effect):
   and both migrate legacy `snippets`/`snippetId` data to surfaces on load.
 - `server/surfacePage.ts` — sandboxed document for one html part: CSP allowlist
   and the postMessage bridge (resize, sendPrompt, openLink). Only html parts
-  reach here — markdown, diff, terminal, and image parts are data the
+  reach here — markdown, diff, terminal, image, and mermaid parts are data the
   viewer renders natively, never markup in the sandbox.
 - `server/mcpHttp.ts` — stateless MCP at `/mcp`. `mcp/server.ts` — stdio MCP,
   a thin client over the HTTP API (passes response fields through untouched).