@actagent/diffs

Read-only diff viewer plugin for ACTAgent agents.

Install

actagent plugins install @actagent/diffs

Restart the Gateway after installing or updating the plugin.

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)
• details.artifactId and details.expiresAt: artifact identity and TTL metadata
• details.context: available routing metadata such as agentId, sessionId, messageChannel, and agentAccountId

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:

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

Patch:

{
  "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 Deprecated alias: image behaves like file and is still accepted for backward compatibility.
• 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
• lang: language hint for before/after input; unknown values fall back to plain text
• Default syntax highlighting covers common source, config, and documentation languages. Install diffs-language-pack for the extended language catalog.
• title: explicit viewer title
• ttlSeconds: artifact lifetime for viewer and standalone file outputs
• baseUrl: override the gateway base URL used in the returned viewer link (origin or origin+base path only; no query/hash)
• viewerBaseUrl plugin config: persistent fallback used when a tool call omits baseUrl

Legacy input aliases still accepted for backward compatibility:

• format -> fileFormat
• imageFormat -> fileFormat
• imageQuality -> fileQuality
• imageScale -> fileScale
• imageMaxWidth -> fileMaxWidth

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 ~/.actagent/actagent.json:

{
  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",
            ttlSeconds: 21600,
          },
        },
      },
    },
  },
}

Explicit tool parameters still win over these defaults.

Docs

• https://docs.actagent.ai/tools/diffs

Package

• Plugin id: diffs
• Package: @actagent/diffs
• Minimum ACTAgent host: 2026.4.30

Security options:

• security.allowRemoteViewer (default false): allows non-loopback access to /plugins/diffs/view/... token URLs
• viewerBaseUrl (optional): persistent viewer-link origin/path fallback for shareable URLs
• defaults.ttlSeconds (default 1800, max 21600): default artifact lifetime for viewer and standalone file outputs

Example:

{
  plugins: {
    entries: {
      diffs: {
        enabled: true,
        config: {
          viewerBaseUrl: "https://gateway.example.com/actagent",
        },
      },
    },
  },
}

Example Agent Prompts

Open in canvas:

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):

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:
ACTAgent supports plugins.

After:
ACTAgent supports plugins and hosted diff views.

Do both:

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:

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/actagent-diffs).
• Default viewer URLs use loopback (127.0.0.1) unless you set plugin viewerBaseUrl, pass baseUrl, or use gateway.bind=custom + gateway.customBindHost.
• If gateway.trustedProxies includes loopback for a same-host proxy (for example Tailscale Serve), raw 127.0.0.1 viewer requests without forwarded client-IP headers fail closed by design.
• In that topology, prefer mode=file / mode=both for attachments, or intentionally enable remote viewers and set plugin viewerBaseUrl (or pass a proxy/public baseUrl) when you need a shareable viewer URL.
• 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.