[FEATURE]: support configurable external diff, pager, and Markdown renderer tools

[cycle2zhou]

Summary

Please consider adding explicit configuration hooks for external rendering tools in opencode, especially for reviewing diffs, paging long output, and rendering Markdown/document previews.

Examples of tools users may want to opt into:

• external diff tools: delta, difftastic
• external pagers: less, moar, most
• external Markdown renderers: glow, mdcat

Problem

opencode already has a strong built-in TUI renderer and theme system, including settings such as diff_style and theme color keys for diff/Markdown/syntax rendering. However, users who already have a preferred terminal toolchain cannot currently tell opencode to use those tools for specific output surfaces.

For example:

• Installing glow does not make opencode use it for Markdown rendering.
• Installing delta or difftastic does not make opencode use them for reviewing changes.
• Existing git pager settings such as core.pager or PAGER do not appear to be an explicit integration point for opencode's TUI diff/preview rendering.

This means users need to leave the opencode UI or run separate commands when they want their existing renderer/pager/diff workflow.

Proposed solution

Add optional configuration fields for external tools, for example in tui.json or another appropriate config surface:

{
  "external_tools": {
    "diff": {
      "command": "delta",
      "args": ["--side-by-side"]
    },
    "pager": {
      "command": "less",
      "args": ["-R"]
    },
    "markdown": {
      "command": "glow",
      "args": ["-"]
    }
  }
}

The exact schema is only illustrative. It may be better to split this into narrower settings, such as:

• external_diff_command
• external_pager_command
• external_markdown_renderer

Possible scope

This could be implemented incrementally:

1. Support an external diff command for reviewing code changes.
2. Respect PAGER / GIT_PAGER / git core.pager for CLI-oriented output where appropriate.
3. Support an external Markdown renderer for file/message preview surfaces.

Why this would be useful

• Lets users reuse their existing terminal rendering workflow.
• Makes tools like delta/difftastic/glow useful inside opencode instead of only in separate shell commands.
• Keeps the built-in renderer as the default while allowing advanced users to opt in.
• Helps users who prefer structural diffs, custom pager behavior, or specialized Markdown styling.

Alternatives

• Continue improving the built-in renderer only.
• Add more built-in diff/Markdown display modes.
• Expose only more theme keys for the current renderer.

Those are useful, but they do not cover users who specifically want their existing external renderer/pager/diff tools.

Notes

I searched for existing issues around external diff tools, external pagers, Markdown renderers, delta, difftastic, glow, git pager, and PAGER. I found related issues around built-in diff display modes and IDE diff preview, but did not find a direct request for configurable external renderer/pager/diff tools.

Verification

• I have searched existing issues to avoid duplicates.

[github-actions]

This issue doesn't fully meet our contributing guidelines.

What needs to be fixed:

• The title should start with [FEATURE]: to match the Feature Request template (e.g. [FEATURE]: support configurable external diff, pager, and Markdown renderer tools)
• The Feature Request template requires a verification checkbox confirming you've searched for existing issues

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

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[github-actions]

Thanks for updating the issue! It now meets the contributing guidelines and looks great. We appreciate the clear problem description, proposed solution, and verification. Your request has been noted.

[simonklee]

thanks for the suggestion this isn't currently on our roadmap to have these external tools that you hook in like this so i'm going to close this as not planned for now.