DiffVisualizer — visual diffs in Agent Zero chat

Renders unified-diff fenced code blocks as side-by-side visual diffs directly in the Agent Zero chat UI — with a fullscreen maximize view, a copy-raw-source button, and theme-aware styling. It also ships an agent skill that teaches the agent to emit valid unified diffs, and a system-prompt nudge so the agent reaches for a diff whenever it proposes edits, shows uncommitted changes, or compares before/after.

Reviewing a proposed change in prose is slow; reviewing it as a rendered diff is instant. That's the whole plugin.

📸 What it looks like

Inline render in chat (side-by-side, per-file):

Maximized fullscreen review (Esc / backdrop / ✕ to close):

✨ Features

Feature	Detail
Auto-render	Any ```diff fence in chat becomes a visual diff (MutationObserver, debounced)
Side-by-side view	diff2html side-by-side output, line-matched
Multi-file fences	One fence containing several diff --git sections renders per-file
Maximize	Fullscreen overlay re-render at larger type; single-overlay invariant
Copy raw source	Copies the original diff text (never the rendered HTML)
Graceful fallback	CDN unreachable / malformed diff / empty block → the raw code block stays readable, nothing is swallowed
Theme-aware	Consumes A0's --color-* custom properties with sane fallbacks
Agent skill	skills/diff/SKILL.md — unified-diff anatomy, capture recipes, pitfalls
Behaviour nudge	System-prompt extension steering the agent toward diff output

⚙️ Architecture

💬 Usage

Every behaviour below is covered by a BDD scenario in tests/e2e/features/, and docs/BEHAVIOUR.md shows each one as a screenshot captured from a passing run — so what you read here is what CI proves on every push.

Nothing to drive

There is no button to press and no mode to switch on. Whenever a message contains a fenced ```diff block, it is replaced in place with a side-by-side visual diff. Ask the agent for a diff, paste one in yourself, or have a tool emit one — it renders either way.

Multi-file diffs render as one visual diff containing both files, so a patch that touches several files reads as a single reviewable unit rather than fragments.

Reviewing a large diff

Hover a rendered diff for its toolbar:

• Maximize opens a single fullscreen overlay for reading a long patch without the chat column squeezing it. Close it with the ✕, a backdrop click, or Escape — all three are asserted, and so is the fact that the inline diff survives the overlay closing.
• Copy puts the raw diff text on the clipboard, not the rendered HTML. You can paste it straight into git apply.

When the input is not a clean diff

Two failure modes are handled deliberately, because silently mangling text is worse than not rendering it:

• Malformed diff text falls back to the readable plain code block. You still see the content.
• A non-diff code fence is left completely untouched. A ```python block stays a Python block. The plugin only claims fences it can actually render.

Both are asserted scenarios rather than intentions.

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📦 Install

Plugin Hub (recommended): open Settings → Plugins in Agent Zero, find DiffVisualizer, click Install.

Manual zip:

git clone https://github.com/agent-zero-plugins/agent-zero-plugin-diff-visualizer
cd agent-zero-plugin-diff-visualizer
make package                       # → dist/diff_visualizer.zip
# Then: A0 Settings → Plugins → Install from file → pick the zip

Enable the plugin after installing (it is store-gated, not always-enabled).

🔧 Configuration

None. The plugin has no configurable options, no secrets, and no environment variables (default_config.yaml is intentionally empty; meta.yaml declares env: []).

Air-gapped note: the renderer lazy-loads diff2html@3.4.51 from jsDelivr. If the CDN is unreachable (offline / strict CSP), diff blocks simply stay as readable plain code — no crash, no data loss.

🛠️ Development

Repo layout follows the org-canonical devkit standard: the repo root IS the plugin (root layout — manifests, extensions/, skills/, webui/ at top level), devkit vendored at tests/_testkit/ (public submodule).

git submodule update --init --recursive   # devkit + nested .agent-zero
make verify                               # Tier-1 static BDD gates (bdd-lint)
make package                              # build dist/diff_visualizer.zip
python -m pytest tests/ -v  # L1 component suite (10 tests)
make e2e                                  # full BDD e2e in the nested-A0 harness

The e2e harness boots a nested Agent Zero via rootless podman, installs the built zip, runs a seam-off red-proof (the suite must fail without the plugin), then runs the BDD features in tests/e2e/features/ (playwright-bdd). Specs assert end state (rendered containers, overlay presence/absence), never transient markers.

Behaviour truth lives in docs/spec/behaviour-spec.md (BEH-1…9) — feature scenarios trace to those IDs.

🧪 Tests

Layer	Where	What
L1 component	tests/test_diff_visualizer.py, tests/test_smoke.py, tests/test_publish_nudge.py	surface validity, stray folders, dead hooks, manifest sanity, nudge publisher
L3 BDD e2e	tests/e2e/features/*.feature + tests/e2e/steps/	render, toolbar, maximize, 3 close paths, copy, multi-file, malformed fallback, non-diff negative

⚖️ License

Apache-2.0 — see LICENSE.