# @gensparx/diffs

Read-only diff viewer plugin for **Gensparx** agents.

It gives agents one tool, `diffs`, that can:

- render a gateway-hosted diff viewer for canvas use
- render the same diff to a file (PNG or PDF)
- accept either arbitrary `before` and `after` text or a unified patch

## What Agents Get

The tool can return:

- `details.viewerUrl`: a gateway URL that can be opened in the canvas
- `details.filePath`: a local rendered artifact path when file rendering is requested
- `details.fileFormat`: the rendered file format (`png` or `pdf`)

When the plugin is enabled, it also ships a companion skill from `skills/` and prepends stable tool-usage guidance into system-prompt space via `before_prompt_build`. The hook uses `prependSystemContext`, so the guidance stays out of user-prompt space while still being available every turn.

This means an agent can:

- call `diffs` with `mode=view`, then pass `details.viewerUrl` to `canvas present`
- call `diffs` with `mode=file`, then send the file through the normal `message` tool using `path` or `filePath`
- call `diffs` with `mode=both` when it wants both outputs

## Tool Inputs

Before and after:

```json
{
  "before": "# Hello\n\nOne",
  "after": "# Hello\n\nTwo",
  "path": "docs/example.md",
  "mode": "view"
}
```

Patch:

```json
{
  "patch": "diff --git a/src/example.ts b/src/example.ts\n--- a/src/example.ts\n+++ b/src/example.ts\n@@ -1 +1 @@\n-const x = 1;\n+const x = 2;\n",
  "mode": "both"
}
```

Useful options:

- `mode`: `view`, `file`, or `both`
- `layout`: `unified` or `split`
- `theme`: `light` or `dark` (default: `dark`)
- `fileFormat`: `png` or `pdf` (default: `png`)
- `fileQuality`: `standard`, `hq`, or `print`
- `fileScale`: device scale override (`1`-`4`)
- `fileMaxWidth`: max width override in CSS pixels (`640`-`2400`)
- `expandUnchanged`: expand unchanged sections (per-call option only, not a plugin default key)
- `path`: display name for before and after input
- `title`: explicit viewer title
- `ttlSeconds`: artifact lifetime
- `baseUrl`: override the gateway base URL used in the returned viewer link (origin or origin+base path only; no query/hash)

Input safety limits:

- `before` and `after`: max 512 KiB each
- `patch`: max 2 MiB
- patch rendering cap: max 128 files / 120,000 lines

## Plugin Defaults

Set plugin-wide defaults in `~/.gensparx/gensparx.json`:

```json5
{
  plugins: {
    entries: {
      diffs: {
        enabled: true,
        config: {
          defaults: {
            fontFamily: "Fira Code",
            fontSize: 15,
            lineSpacing: 1.6,
            layout: "unified",
            showLineNumbers: true,
            diffIndicators: "bars",
            wordWrap: true,
            background: true,
            theme: "dark",
            fileFormat: "png",
            fileQuality: "standard",
            fileScale: 2,
            fileMaxWidth: 960,
            mode: "both",
          },
        },
      },
    },
  },
}
```

Explicit tool parameters still win over these defaults.

Security options:

- `security.allowRemoteViewer` (default `false`): allows non-loopback access to `/plugins/diffs/view/...` token URLs

## Example Agent Prompts

Open in canvas:

```text
Use the `diffs` tool in `view` mode for this before and after content, then open the returned viewer URL in the canvas.

Path: docs/example.md

Before:
# Hello

This is version one.

After:
# Hello

This is version two.
```

Render a file (PNG or PDF):

```text
Use the `diffs` tool in `file` mode for this before and after input. After it returns `details.filePath`, use the `message` tool with `path` or `filePath` to send me the rendered diff file.

Path: README.md

Before:
gensparx supports plugins.

After:
gensparx supports plugins and hosted diff views.
```

Do both:

```text
Use the `diffs` tool in `both` mode for this diff. Open the viewer in the canvas and then send the rendered file by passing `details.filePath` to the `message` tool.

Path: src/demo.ts

Before:
const status = "old";

After:
const status = "new";
```

Patch input:

```text
Use the `diffs` tool with this unified patch in `view` mode. After it returns the viewer URL, present it in the canvas.

diff --git a/src/example.ts b/src/example.ts
--- a/src/example.ts
+++ b/src/example.ts
@@ -1,3 +1,3 @@
 export function add(a: number, b: number) {
-  return a + b;
+  return a + b + 1;
 }
```

## Notes

- The viewer is hosted locally through the gateway under `/plugins/diffs/...`.
- Artifacts are ephemeral and stored in the plugin temp subfolder (`$TMPDIR/gensparx-diffs`).
- Default viewer URLs use loopback (`127.0.0.1`) unless you set `baseUrl` (or use `gateway.bind=custom` + `gateway.customBindHost`).
- Remote viewer misses are throttled to reduce token-guess abuse.
- PNG or PDF rendering requires a Chromium-compatible browser. Set `browser.executablePath` if auto-detection is not enough.
- If your delivery channel compresses images heavily (for example Telegram or WhatsApp), prefer `fileFormat: "pdf"` to preserve readability.
- `N unmodified lines` rows may not always include expand controls for patch input, because many patch hunks do not carry full expandable context data.
- Diff rendering is powered by [Diffs](https://diffs.com).
