Katashiro (紙代 / 形代) - OpenAB Companion ⛩️

Katashiro (紙代 / 形代) is a sleek Chrome Extension Side Panel companion designed for OpenAB (Open Agent Broker). It connects via WebSockets using the Agent Client Protocol (ACP) to provide a premium, LINE-style single chatroom interface for real-time two-way communication with multiple AI agents, each connected over its own ACP endpoint.

🔮 The Metaphor of Katashiro (形代 / 紙代)

In Japanese Onmyodo (陰陽道) and Shinto rituals, a Katashiro (形代 / 紙代) is a physical paper doll or vessel used to temporarily host spirits or represent individuals.

Under this system:

• The Chrome Extension UI (Side Panel) = Katashiro (紙代)
  • The HTML, CSS, and JavaScript form the physical vessel—an empty shell waiting in the browser.
• The OpenAB AI Agents = Shikigami (式神)
  • The spiritual intelligence and agent logic that is "summoned" and "injected" into the vessel over the WebSocket connection.
• Writing the Code = Drawing Talismans
  • Writing configuration files (manifest.json) and styling components acts as the ritual of drawing magical talismans to establish a connection path between the summoner and the spirit.
• Browser Tools = The Shikigami's Hands (施術)
  • Through MCP-over-ACP, the summoned agent no longer merely speaks through the vessel — it acts. The extension serves DOM-semantic tools (katashiro.click, katashiro.read_dom, katashiro.navigate, katashiro.type, katashiro.screenshot) so the shikigami can reach through the katashiro and operate the living page. Perception and action, not just a voice.

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🌟 Key Features

• Browser Control (MCP-over-ACP): the extension is an MCP server over the same /acp socket; the agent discovers and calls 18 DOM-semantic browser tools (8 read + 10 write — snapshot, read_dom, get_text, screenshot, scroll, hover, tabs, wait_for, click, click_text, type, select_option, press_key, navigate, history, reload, new_tab, switch_tab) — most execute in the active tab via chrome.scripting; the tab-management tools (tabs/new_tab/switch_tab) act on the browser's tab set. Perception is an accessibility-tree snapshot with stable element refs. Full surface in the tool table; roadmap in ROADMAP.

• Rich Chat: agent and user messages render as markdown → DOMPurify-sanitized HTML — GFM tables, syntax-highlighted code with one-click copy, hardened links. stop/retry a turn (ACP session/cancel), chat history + ACP session resume persisted per window (reopen the panel and the conversation — and the session — continue), and stick-to-bottom auto-scroll with a "jump to latest" pill. A clear-screen button (🧹) wipes the on-screen transcript and this window's persisted scrollback while keeping each agent's ACP session — the local view resets, the agents don't forget.

• Roster status at a glance: each agent chip carries two self-labelled badges — a connection badge (● 已連線 / ◐ 連線中 / ○ 連線失敗 / ◌ 已停用) and, when the agent is allowed browser access, a browser badge (🌐 可操作 / 🌐 唯讀 / 🌐 未連 / ⚠️ 無回應). A liveness heartbeat keeps them honest — a half-open socket surfaces as ⚠️ 無回應 rather than a stale "connected", and clicking an unhealthy chip reconnects it.

• Multi-Agent Room: several agents share one chatroom, each on its own ACP connection. @mention mode (the default) routes a message only to the agents named in it and broadcasts when none are; ambient mode gives everyone everything. Agent replies are relayed to the other agents wrapped as <message from="...">, so they can answer each other — bounded by a loop guard that pauses a runaway agent-to-agent cascade and resets the moment a human speaks. Per-agent connect/disconnect and browser-access toggles, and one openab session per Chrome window.

• Unified Chat Space: Optimized specifically for a single multi-party chatroom, bypassing cluttered sidebar lists to fit perfectly in a narrow Side Panel.

• LINE-style Chat Bubbles: Self-sent messages align to the right (green), while received agent messages align to the left (dark slate blue) with custom avatars, sender names, and timestamp markers.

• Connection Persistence: Leverages the Chrome Side Panel API to host the WebSocket connection, allowing it to persist even as the user navigates across browser tabs.

• Auto-Reconnection: Automatically attempts to reconnect to the OpenAB broker every 5 seconds if the connection drops.

• Premium Glassmorphic UI: Features a modern dark-mode interface with glassmorphism styling, clean glow backdrops, and custom scrollbars for an enhanced visual experience.

🛠️ Project Structure

• manifest.json: Configuration manifest using Manifest V3 and the Chrome Side Panel API.
• background.js: Background Service Worker that registers the extension trigger to open the Side Panel on click.
• sidepanel.html: The HTML layout for the chatroom interface.
• sidepanel.css: High-fidelity styling utilizing modern CSS design tokens.
• sidepanel.js: Main client-side script managing WebSockets, Chrome local storage, auto-reconnection, and message rendering.
• browser-mcp.js: The MCP server we serve back to the agent over the ACP tunnel — tool registry, schemas, and DOM tool bodies. See Serving an MCP server over reverse MCP-over-ACP.
• room-core.js: Multi-agent room logic — @mention routing, agent-to-agent relay, and the loop guard.
• markdown.js: The single sanitized renderMarkdown sink (markdown-it → DOMPurify) + copy-code and link/media hardening. See docs/adr/chat-markdown-rendering.md.
• page/a11y-walker.js: Content-script injected into the page — builds the accessibility-tree snapshot and resolves element refs (__katashiroResolve).
• vendor/: Prebuilt, eval-free IIFE bundles (MV3 script-src 'self'): dom-accessibility-api, markdown-it, dompurify, highlight.js. Rebuild steps in vendor/BUILD.md.
• test/: node --test suites (4 files, 103 tests). No Chrome required; chrome.*, crypto, and the socket are mocked.
• icon*.png: The extension icon set — icon16/32/48/128.png (manifest icons + toolbar) plus icon.png (side-panel brand logo). Cyberpunk digital paper-doll with neon circuitry.

🔌 Serving an MCP server over reverse MCP-over-ACP

Normally an MCP client connects out to MCP servers. Katashiro does the reverse: the agent reaches into the browser. OpenAB opens a tunnel over the existing /acp WebSocket and speaks MCP to us — we are the server, the agent is the client. That is the only way to reach a browser tab, which no outside process can dial into.

browser-mcp.js is meant to be read as the reference implementation. If you want to serve your own tools this way, you need exactly three things:

1. Declare the server in your session/new params, alongside any normal MCP servers:

   { "mcpServers": [ { "type": "acp", "id": "<uuid>", "name": "katashiro" } ] }
   

   id is minted fresh per connection; name is stable and is what the operator allowlists. The gateway answers with a server-initiated mcp/connect, which you reply to with a connectionId.
2. Answer tools/list with your tool set. OpenAB fetches once per declared server and caches — discovery is pull-based, so there is no list_changed notification to send.
3. Answer tools/call with an MCP CallToolResult. Return tool failures as isError: true results rather than protocol errors, so the agent can read what went wrong and adapt.

Inner MCP messages arrive flattened into an mcp/message frame (method / params inline); the outer ACP id is what correlates the reply. The wire format is specified in docs/mcp-over-acp-tunnel-contract.md in the OpenAB repo.

You can declare more than one. createServer({ id, name, tools }) builds an instance with its own name and registry, and the module routes by the connectionId handed back at mcp/connect — so a second client-side MCP server sits alongside katashiro on the same socket, each answering only its own tools/list and tools/call. That is the client end of OpenAB's multi-server fan-out. A connectionId you never minted is refused rather than served by whichever server seems likeliest — guessing there would hand one server's tools to a caller that was never granted a tunnel to it.

Two conventions worth copying:

• One registry, not two lists. TOOLS in browser-mcp.js holds each tool's schema and its implementation in the same entry, and the tools/list payload is derived from it. A tool cannot be advertised without an implementation, or implemented without being discoverable.
• Namespace your tool names. Ours are katashiro.*. A bare browser.* collided with a co-installed Playwright MCP's browser_* tools and the model could not tell the two surfaces apart. The operator allowlist is keyed on the declared name, and OpenAB admits tools as fetched ∩ allowed, so the prefix is load-bearing, not cosmetic.

The tools we serve

Most tools act on the active tab (tabs.query({ active: true, lastFocusedWindow: true })); DOM work runs injected in the page via chrome.scripting.executeScript. The exception is the tab-management tools (tabs, new_tab, switch_tab) — they operate on the browser's tab set, not a single page, so they don't require (and aren't blocked by) a scriptable active tab; opening or switching a tab is how the agent changes which tab is active. A tool that fails — selector matched nothing, no active tab — comes back as an MCP result with isError: true, not a protocol error, so the agent can read the reason and adapt.

Most tools take a ref (+ its snapshotId) from the most recent snapshot — the primary, cheapest way to perceive the page — with a CSS selector as a fallback. Action tools return the post-action snapshot so the agent rarely needs a follow-up read.

| Tool | | Params | Returns | | --- | --- | --- | --- | | katashiro.snapshot | read | — | Accessibility-tree snapshot as compact text, a stable ref on each interactive element, and a snapshotId. PRIMARY way to see the page. | | katashiro.read_dom | read | selector? (CSS) | outerHTML of the match, capped at 100k chars. No selector ⇒ document.body. | | katashiro.get_text | read | selector? (CSS) | innerText of the match (capped 100k). No selector ⇒ document.body. | | katashiro.screenshot | read | — | image/jpeg at quality 70. JPEG, not PNG: a full-page PNG base64 runs several MB and blows past the tunnel's per-frame cap. | | katashiro.scroll | read | to|direction+amount?|ref|selector | Scrolls to reveal content (perception aid — works in read-only). Returns the updated snapshot. | | katashiro.hover | read | ref|selector | Dispatches pointer events to reveal menus/tooltips. Returns the updated snapshot. | | katashiro.tabs | read | — | Lists all open tabs across every window (index, title, URL, active marker) — wider exposure than the active-tab-only tools, by design. The [index] is a live enumeration order, not a stable id. | | katashiro.new_tab | write | url?, active? | Opens a new tab and (default) switches to it so later tools act on it; active: false opens it in the background. Returns the new tab's index, plus the snapshot when it switched to a scriptable page. | | katashiro.switch_tab | write | index|url | Activates an existing tab — by index (from a fresh tabs) or by url substring (more stable). Focuses the tab and its window; returns the now-active tab, plus its snapshot for a scriptable page. | | katashiro.wait_for | read | selector|text, timeout? |