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Describe the enhancement you want to request

Describe the enhancement

URLs in chat are handed to the OS default handler (the browser). File paths in chat are not: there is no way to open a referenced file in an external application on any platform, and OS file associations are never consulted.

Two levels of this feature, either would be valuable:

1. Minimal — delegate to the OS default application. When a file path is clicked/opened from chat, hand it to the platform handler (start on Windows, open on macOS, xdg-open on Linux). The user's system file associations decide what opens. No config needed.

2. Full — a config mapping file extensions to external commands, for users who want more control than the OS default:

{
  "file_open": {
    "*.md": "cmd /c start chrome",
    "*.pdf": "open",
    "*.html": "xdg-open"
  }
}

If a full mapper is not feasible, level 1 alone would already close the biggest gap — right now there is no external-open path at all.

Reproduction (observed)

On Windows (TUI, opencode 1.18.29):

1. Ask the agent to reference an absolute path, e.g. C:\Users\...\report.md
2. Plain click the path → nothing happens
3. Ctrl+click the path → nothing happens
4. A markdown file:// link to the same file → nothing happens

The same asymmetry (built-in viewer only, no external dispatch) applies on macOS and Linux; the click handling itself may differ per platform, which is why the proposed mechanism is deliberately platform-agnostic.

Why

• Symmetry: links already dispatch to the OS default handler; this is the same mechanism with one more dispatch type, useful on every platform.
• Recurring demand: related requests already exist:
  • Desktop: make file paths in chat clickable (open in editor / reveal in Finder) #37891 — make file paths in chat clickable (open in editor / system default app / Finder, macOS)
  • [FEATURE]: markdown preview of project files in the TUI #43598 — markdown preview of project files in the TUI
  • Desktop: file:// markdown links are not clickable #44902 — file:// markdown links are not clickable (Desktop)
  • tui: Ctrl+click does not open Markdown links when mouse capture is active #44442 — Ctrl+click does not open markdown links when mouse capture is active
  • PRs feat(tui): markdown preview panel #43599 and feat(app): markdown preview toggle for file viewer and clickable file paths in chat #46793 explore preview panels and clickable paths — this complements them.

Expected behavior

• Minimal: any file reference → OS default application, honoring system file associations.
• Full: mapped extensions → the user's configured command/app; unmapped extensions fall back to the minimal behavior.
• No new UI surface needed — just a dispatch to the OS.