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The marketplace hosts one plugin, coherent-gameface: a generic toolkit for driving a running Coherent Gameface UI, whatever the game or application embedding it.

Coherent Gameface (Cohtml) is the HTML/CSS/JS UI engine many games embed. The plugin ships gameface-devtools-mcp, an MCP server that lets your agent drive any Gameface UI over a direct Chrome DevTools Protocol (CDP) connection: evaluate JavaScript, take screenshots, inspect and drive the DOM, capture the console, and even set JS breakpoints.

It also ships two skills.

• The gameface skill teaches the agent the engine itself: how Gameface is, and is not, a browser; version-gating features against the changelog; data binding; and how to probe the running game for what the docs cannot promise.
• The gameface-driving skill is the operating manual for the game_* tools: field-verified procedures for finding elements, acting then verifying, surviving reloads, and debugging without freezing the game; the traps the tool schemas cannot tell.

[!NOTE] Generic, but developed against Cities: Skylines II. The server makes no assumptions about a specific application; it works against any Gameface CDP endpoint. It is developed and verified against Cities: Skylines II's Gameface UI, which is the reference target.

[!TIP] Not using Claude Code or Codex CLI? The MCP server is also published on npm as @csmodding/gameface-devtools-mcp and works with any MCP client (Cursor, Gemini CLI, VS Code, …). See mcp/README.md for the tool reference and per-client install snippets.

Table of contents

• What it looks like in practice
• Requirements
• Install
  • Claude Code
  • Codex CLI
• Configuration
• Development
• Troubleshooting

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What it looks like in practice

Short, abridged transcripts. The skill supplies the engine knowledge; the server supplies the hands.

"My panel looks right in Chrome but wrong in-game"

You: My settings panel is a two-column grid in Chrome, but in the game everything stacks into one column. Why?

Agent: Gameface has no CSS Grid in any version, and unsupported CSS is silently ignored: your display: grid just falls back to the default, and in Gameface every element defaults to display: flex in column direction, hence the single column. I rewrote the layout with nested flex rows, and a game_screenshot of .settings-panel confirms the two columns now render in-game.

"The UI stops updating after the first refresh"

You: My mod pushes stats to the UI every second, but the panel freezes on the first value. No errors anywhere.

Agent: game_console shows no exceptions, and game_eval on the bound nodes shows they are stuck on the first payload. This is a known engine gotcha: Cohtml caches a model's shape by its type name, and two payloads sharing a type name must expose the same set of PRESENT properties. Your second update omits delta when nothing changed; an omitted property is a different shape than a null one, and the cached binding breaks silently. Send delta: null instead. Verified in-game across two refresh cycles.

"Test the new toolbar button end to end"

You: I just built the milestones toolbar button. Can you verify the whole flow in the running game?

Agent: game_click on [data-tooltip="Milestones"]: Gameface accepts CDP input commands but never delivers them to the UI, so the click is dispatched as real bubbling DOM events, which your React handlers receive. game_wait until .milestones-panel is visible, game_screenshot to check the render, game_console shows no new errors. The flow works; screenshot attached.

"Something throws when the trade panel opens, no idea where"

You: Opening the trade panel logs TypeError: cannot read 'income' of undefined and I cannot reproduce it outside the game.

Agent: game_console captures the stack, pointing at trade-panel.js:412. game_debug_set_breakpoint there with the condition district == null, then game_click on the trade button. It pauses; the UI thread is frozen while paused, so I inspect quickly: game_debug_pause_state shows district is undefined exactly when the row is an outside connection, then game_debug_step resume. Fix: guard the outside-connection case, which has no district.

Requirements

• A Gameface application running with its CDP debug endpoint reachable (default http://localhost:9444). Verify with:

  curl http://localhost:9444/json/list
  

  You should get back a JSON array containing a "type": "page" target. Set the host/port to match your application if it differs (see Configuration).
• Node 22.4+ to launch the server. No npm install is needed: the plugin launches the server from a committed, self-contained bundle, so it works offline and stays version-locked to the plugin.

Install

Both harnesses add this repository as a marketplace, install the plugin from it, and launch the same committed server bundle. The full tool reference (UI tools and JS debugger tools) lives in mcp/README.md.

Claude Code

In Claude Code, add this repository as a marketplace, then install the plugin from it:

/plugin marketplace add CitiesSkylinesModding/agents-plugins
/plugin install coherent-gameface@csmodding

Or from your terminal:

claude plugin marketplace add CitiesSkylinesModding/agents-plugins
claude plugin install coherent-gameface@csmodding

Once enabled, Claude Code autoloads the gameface MCP server from the plugin's .mcp.json. Run /mcp to confirm it connected, then Claude will use this MCP when it needs it. You can ask it to call game_status to check the MCP is working properly.

Codex CLI

Add this repository as a marketplace, then install the plugin from it:

codex plugin marketplace add CitiesSkylinesModding/agents-plugins
codex plugin add coherent-gameface@csmodding

Once enabled, Codex autoloads the gameface MCP server from .codex-plugin/mcp.json. Run /mcp to confirm it connected, then Codex will use this MCP when it needs it. You can ask it to call game_status to check the MCP is working properly.

Configuration

The server reads these environment variables (all optional):

Variable	Default	Purpose
GAMEFACE_HOST	localhost	Host of the Gameface CDP endpoint.
GAMEFACE_PORT	9444	Port of the Gameface CDP endpoint.
GAMEFACE_CONNECT_TIMEOUT_MS	5000	HTTP discovery / WebSocket open timeout.
GAMEFACE_CALL_TIMEOUT_MS	15000	Per-command reply timeout.

On Claude Code, the plugin's .mcp.json forwards them from your environment (${VAR:-default}), and an extra GAMEFACE_MCP_RUNTIME variable (default node) overrides the runtime used to launch the server.

On Codex CLI, the plugin config passes no environment block (Codex does not interpolate ${VAR} placeholders, and ~/.codex/config.toml cannot override a plugin-provided server), so the server always starts with the defaults above. If you need non-default settings, register the npm-published server manually with codex mcp add and the environment you want; it replaces the plugin's copy under the same name (see mcp/README.md).

Development

Uses mise + bun (never npx). The repository is a marketplace hosting the plugin(s) under plugins/, and a bun workspace: the root package.json carries the lint/format tooling (oxlint, oxfmt) and the lefthook git hooks, while the MCP server lives in the plugins/coherent-gameface/mcp/ workspace package (published on npm as @csmodding/gameface-devtools-mcp).

bun install   # install all workspace deps (also installs the git hooks)
mise check    # type-check, lint (with safe auto-fixes), and format
mise build    # rebuild the server bundle (commit the result)

Run mise tasks for the full list.

After changing anything under plugins/coherent-gameface/mcp/src/, run mise check, rebuild, and commit the updated dist/server.mjs.

Troubleshooting

• /mcp shows the server failed / tools error with "Cannot reach …": the Gameface application is not running or the debug port is not reachable. Check curl http://localhost:9444/json/list. Use game_status for a structured diagnosis.
• Runtime not found: ensure node (22.4+) is on your PATH.
• Read the MCP server logs: Claude Code records each server's connection attempts and captured stderr to per-project .jsonl files, the fastest way to see why a launch failed (e.g., a -32000 Connection closed from a bad command/path before any game_* tool runs). They live under the Claude CLI cache, in an mcp-logs-gameface/ folder keyed by the project path (separators replaced with -); newest .jsonl first, and each Server stderr: ... line is what the server printed:
  • Windows: %LocalAppData%\claude-cli-nodejs\Cache\<project-path>\mcp-logs-gameface\
  • macOS / Linux: ~/.cache/claude-cli-nodejs/Cache/<project-path>/mcp-logs-gameface/