2027635 - [firefox-devtools-mcp] UID-based tools partially broken in connect-existing mode (element serialization)

Status: RESOLVED FIXED

(Developer Infrastructure :: AI for Development, defect, P3)

Description

[Julian Descottes [:jdescottes]]

Bug

GeckodriverHttpDriver returns GeckodriverElement wrappers from findElement(), but these don't serialize to W3C element references ({"element-6066-...": "id"}). When passed as arguments to executeScript or actions().move(), geckodriver doesn't recognize them.

Impact by tool

Tool	Status	Reason
click_by_uid	Works	Uses GeckodriverElement.click() (direct HTTP)
fill_by_uid	Works	Uses el.clear() + el.sendKeys() (direct HTTP)
hover_by_uid	Broken	actions().move({ origin: el }) — el serializes wrong
drag_by_uid_to_uid	Broken	executeScript('...', fromEl, toEl) — elements become null in browser
upload_file_by_uid	Partial	executeScript part broken, sendKeys works
screenshot_by_uid	Partial	scrollIntoView broken, takeScreenshot() works via HTTP
evaluate_script with UID args	Broken	Elements passed as raw args

Root cause

GeckodriverHttpDriver.executeScript() sends args raw via JSON:

async executeScript<T>(script: string, ...args: unknown[]): Promise<T> {
  return (await this.cmd('POST', '/execute/sync', { script, args })) as T;
}

GeckodriverElement is a plain JS object — it serializes to {}, not a W3C element reference.

Possible fix

GeckodriverElement needs a toJSON() method (or similar) that returns the W3C element reference format, and executeScript/actions need to convert elements before sending. Alternatively, implement ELEMENT_KEY serialization like selenium-webdriver does.

README currently states for connect-existing: "all other features work normally" — this is inaccurate for the tools listed above.

Refs: src/firefox/core.ts:303, src/firefox/core.ts:320, src/firefox/core.ts:419, src/firefox/dom.ts:184, src/firefox/dom.ts:221, src/firefox/dom.ts:274, src/firefox/dom.ts:336

Comment 1

[Julian Descottes [:jdescottes]]

PR at https://github.com/mozilla/firefox-devtools-mcp/pull/58

Comment 2

[Julian Descottes [:jdescottes]]

PR merged

https://github.com/mozilla/firefox-devtools-mcp/commit/4a235290f0f6db83481efe46d29831cf474ffdbc