extension_dev

Run the extension while you edit it: dev build, hot module replacement, and a browser with the extension loaded.

Quick start

Run my extension in Chrome and let me act on it

Asked to run the extension, the agent starts the session and the browser opens with it loaded.

Asked to run the extension, the agent starts the session and the browser opens with it loaded.

How the agent uses it

Reach for this first when the ask is "run my extension": by default it waits for the session to be ready and answers with the readiness and the first log lines, so one call covers the ask; pass wait:false to return as soon as the server is spawned. A dev session carries the control channel that extension_storage, extension_reload, extension_open and extension_dom_snapshot need (local, same machine, same user) unless you pass allowControl:false; the eval channel that extension_eval needs is the one gate that stays off until you pass allowEval:true (which implies control, so you never need to pass both). Use extension_start instead to run the production build in a browser. When the answer's ready.status is "ready" the session is up and nothing else needs calling before extension_logs, extension_inspect or the control verbs; extension_wait is only for wait:false, an answer whose ready.status was not ready, or a build-only session.

Reference

Group	local, on by default
Tier	Run
Needs	none
Read only	no
Destructive	no
Idempotent	no
Reaches the open web	no

Inputs

Input	Type	Required	Default	What it does
projectPath	string	yes		Extension project root
browser	"chrome" | "chromium" | "edge" | "brave" | "opera" | "vivaldi" | "yandex" | "firefox" | "waterfox" | "librewolf" | "zen" | "floorp" | "safari" | "chromium-based" | "gecko-based" | "firefox-based" | "webkit-based"	no	"chrome"
port	number	no		Dev server port (0 for auto-assign)
noBrowser	boolean	no	false	Start the dev server without launching a browser
polyfill	boolean	no	true	Apply cross-browser polyfill
profile	string	no		Profile path, or "false" to reuse the real user profile. Omit for a throwaway one.
startingUrl	string	no		URL the browser opens on launch
chromiumBinary	string	no		Custom Chromium-based binary to launch; browser still names the target family the engine builds for (chromium-based when none is given).
geckoBinary	string	no		Custom Gecko/Firefox binary to launch; browser still names the target family the engine builds for (gecko-based when none is given).
host	string	no		Bind host, default 127.0.0.1. Use 0.0.0.0 in Docker or devcontainers.
publicHost	string	no		Host the browser dials for HMR and reload when it differs from the bind host
extensions	array of string	no		Extra extension paths or store URLs to load alongside the project
replace	boolean	no	false	Stop the live session for this projectPath first, reported as replacedSession. Without it a second call is refused rather than forking: two sessions fight over one profile and the newer browser dies on the lock.
allowControl	boolean	no	true	The agent-bridge control channel that extension_storage/reload/open/dom_snapshot ride (local, same machine). On unless you pass false; eval is the gate that stays off (allowEval).
wait	boolean	no	true	Wait for the session to be ready before answering (the same contract extension_wait reads, up to 45s) and include the first log lines. false returns as soon as the server is spawned; call extension_wait and extension_logs yourself then.
allowEval	boolean	no	false	Enable extension_eval (runs code in a context; writes a 0600 session token). Implies allowControl, so you never need to pass both.

Next

• MCP server: install, the two groups and the session gates
• Install in your client
• extension_start: Run the PRODUCTION build in a browser: build the project, serve it, and launch.
• extension_stop: Stop a session that extension_dev or extension_start is running: terminate the server and the browser it launched, and remove the live-preview carrier if extension_dev placed one.
• extension_wait: Wait for a running dev or start session to be ready.