feat: support base path prefix for reverse proxy deployments

[carsongee]

Problem

When running opencode web behind a reverse proxy that adds a path prefix (e.g. https://host/notebook-sessions/<id>/ports/4096/), the web UI breaks. The HTML is served at the prefixed URL but all assets use absolute root paths (/assets/index-xxx.js), so the browser fetches them from the host root and gets 404s.

Proposed solution

Add a --base-path CLI flag, OPENCODE_SERVER_BASE_PATH env var, and server.basePath config option. When set, the server injects a <base href="/the/prefix/"> tag into every HTML response. Browsers resolve all asset and navigation URLs relative to that base, so no rebuild is needed. The SolidJS Router is also given the base so client-side routing stays within the prefix.

Usage

# CLI flag
opencode web --base-path /notebook-sessions/abc123/ports/4096

# Env var
OPENCODE_SERVER_BASE_PATH=/notebook-sessions/abc123/ports/4096 opencode web

# opencode.json
{ "server": { "basePath": "/notebook-sessions/abc123/ports/4096" } }

[github-actions]

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What needs to be fixed:

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Please edit this issue to address the above within 2 hours, or it will be automatically closed.

---

This issue might be a duplicate of existing issues. Please check:

• [FEATURE]: Base path / prefix routing support #7624: [FEATURE]: Base path / prefix routing support — same request for configurable base path/prefix so OpenCode can be served under a URL prefix behind a reverse proxy.

If you believe this was flagged incorrectly, please let a maintainer know.

[carsongee]

Closing as duplicate of #7624. The PR at #25777 has been updated to reference the existing issue.