docs: add Browser REPL MCP reference

[dprevoznik]

Summary

• add a dedicated browser_repl MCP tool reference
• link it from the MCP server landing page and Tools navigation
• document persistent state, parameters, output behavior, runtime capabilities, and safety boundary

Validation

• parsed docs.json successfully
• git diff --check passed
• Mintlify CLI is not installed in the environment, so no local preview was run

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Note

Low Risk
Documentation-only changes with no runtime, auth, or API behavior impact.

Overview
Adds first-class documentation for the browser_repl MCP tool and wires it into the docs site.

A new reference page covers parameters (session_id, code, reset, timeout_sec, project), a JSON example and response shape, persistence vs execute_playwright_code, output channels, repl_terminated, and a security warning about unrestricted VM execution. The MCP server overview gets a browser_repl card, and docs.json lists the page under Tools. The Browser REPL guide now points MCP users at that reference instead of the code execution platform link.

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[masnwilliams]

Looks good overall. The page follows the sibling tool pages and the parameter table matches the browser_repl schema. I checked the page's claims against a live browser through the MCP tool, and all of these held:

• session_id accepts a session name.
• Empty code is rejected unless reset is true.
• timeout_sec above 150 is rejected.
• Bindings persist across calls.
• A timeout sets repl_terminated: true, and the next call gets a new repl_id with no earlier bindings.
• reset starts a fresh REPL.

Approving, with two suggestions I'd take before merging:

1. The example returns heading: null. example.com no longer has an <h1>. Neither the DOM nor the accessibility tree has a heading node, even after waitForLoad(). I ran the suggested replacement, and it returns the output shown.
2. Show the MCP response. The guide documents the SDK response, which differs from what MCP callers get. Images arrive as separate MCP image content, not data_b64. After a timeout, the response still carries the old repl_id, so repl_terminated is the only signal that state was lost.

The other inline comments are optional. One more optional follow-up: browsers/repl.mdx could link back to this page, the way browsers/playwright-execution.mdx links to execute_playwright_code.