feat(tests): improve coverage to 56%, add benchmark regression tests

- Add EffectPlugin ABC with @abstractmethod decorators for interface enforcement
- Add runtime interface checking in discover_plugins() with issubclass()
- Add EffectContext factory with sensible defaults
- Standardize Display __init__ (remove redundant init in TerminalDisplay)
- Document effect behavior when ticker_height=0
- Evaluate legacy effects: document coexistence, no deprecation needed
- Research plugin patterns (VST, Python entry points)
- Fix pysixel dependency (removed broken dependency)

Test coverage improvements:
- Add DisplayRegistry tests
- Add MultiDisplay tests
- Add SixelDisplay tests
- Add controller._get_display tests
- Add effects controller command handling tests
- Add benchmark regression tests (@pytest.mark.benchmark)
- Add pytest marker for benchmark tests in pyproject.toml

Documentation updates:
- Update AGENTS.md with 56% coverage stats and effect plugin docs
- Update README.md with Sixel display mode and benchmark commands
- Add new modules to architecture section

AGENTS.md

@@ -159,6 +159,31 @@ mise run test-cov
 
 The project uses pytest with strict marker enforcement. Test configuration is in `pyproject.toml` under `[tool.pytest.ini_options]`.
 
+### Test Coverage Strategy
+
+Current coverage: 56% (336 tests)
+
+Key areas with lower coverage (acceptable for now):
+- **app.py** (8%): Main entry point - integration heavy, requires terminal
+- **scroll.py** (10%): Terminal-dependent rendering logic
+- **benchmark.py** (0%): Standalone benchmark tool, runs separately
+
+Key areas with good coverage:
+- **display/backends/null.py** (95%): Easy to test headlessly
+- **display/backends/terminal.py** (96%): Uses mocking
+- **display/backends/multi.py** (100%): Simple forwarding logic
+- **effects/performance.py** (99%): Pure Python logic
+- **eventbus.py** (96%): Simple event system
+- **effects/controller.py** (95%): Effects command handling
+
+Areas needing more tests:
+- **websocket.py** (48%): Network I/O, hard to test in CI
+- **ntfy.py** (50%): Network I/O, hard to test in CI
+- **mic.py** (61%): Audio I/O, hard to test in CI
+
+Note: Terminal-dependent modules (scroll, layers render) are harder to test in CI.
+Performance regression tests are in `tests/test_benchmark.py` with `@pytest.mark.benchmark`.
+
 ## Architecture Notes
 
 - **ntfy.py** and **mic.py** are standalone modules with zero internal dependencies
@@ -169,13 +194,15 @@ The project uses pytest with strict marker enforcement. Test configuration is in
 
 ### Display System
 
-- **Display abstraction** (`engine/display.py`): swap display backends via the Display protocol
+- **Display abstraction** (`engine/display/`): swap display backends via the Display protocol
-  - `TerminalDisplay` - ANSI terminal output
+  - `display/backends/terminal.py` - ANSI terminal output
-  - `WebSocketDisplay` - broadcasts to web clients via WebSocket
+  - `display/backends/websocket.py` - broadcasts to web clients via WebSocket
-  - `SixelDisplay` - renders to Sixel graphics (pure Python, no C dependency)
+  - `display/backends/sixel.py` - renders to Sixel graphics (pure Python, no C dependency)
-  - `MultiDisplay` - forwards to multiple displays simultaneously
+  - `display/backends/null.py` - headless display for testing
+  - `display/backends/multi.py` - forwards to multiple displays simultaneously
+  - `display/__init__.py` - DisplayRegistry for backend discovery
 
-- **WebSocket display** (`engine/websocket_display.py`): real-time frame broadcasting to web browsers
+- **WebSocket display** (`engine/display/backends/websocket.py`): real-time frame broadcasting to web browsers
   - WebSocket server on port 8765
   - HTTP server on port 8766 (serves HTML client)
   - Client at `client/index.html` with ANSI color parsing and fullscreen support
@@ -186,6 +213,15 @@ The project uses pytest with strict marker enforcement. Test configuration is in
   - `sixel` - Sixel graphics in supported terminals (iTerm2, mintty, etc.)
   - `both` - Terminal + WebSocket simultaneously
 
+### Effect Plugin System
+
+- **EffectPlugin ABC** (`engine/effects/types.py`): abstract base class for effects
+  - All effects must inherit from EffectPlugin and implement `process()` and `configure()`
+  - Runtime discovery via `effects_plugins/__init__.py` using `issubclass()` checks
+  
+- **EffectRegistry** (`engine/effects/registry.py`): manages registered effects
+- **EffectChain** (`engine/effects/chain.py`): chains effects in pipeline order
+
 ### Command & Control
 
 - C&C uses separate ntfy topics for commands and responses

README.md

@@ -75,6 +75,7 @@ Mainline supports multiple display backends:
 
 - **Terminal** (`--display terminal`): ANSI terminal output (default)
 - **WebSocket** (`--display websocket`): Stream to web browser clients
+- **Sixel** (`--display sixel`): Sixel graphics in supported terminals (iTerm2, mintty)
 - **Both** (`--display both`): Terminal + WebSocket simultaneously
 
 WebSocket mode serves a web client at http://localhost:8766 with ANSI color support and fullscreen mode.
@@ -131,10 +132,17 @@ engine/
   translate.py          Google Translate wrapper + region detection
   render.py             OTF → half-block pipeline (SSAA, gradient)
   effects/              plugin architecture for visual effects
-    controller.py       handles /effects commands
+    types.py            EffectPlugin ABC, EffectConfig, EffectContext
-    chain.py            effect pipeline chaining
     registry.py         effect registration and lookup
+    chain.py            effect pipeline chaining
+    controller.py       handles /effects commands
     performance.py      performance monitoring
+    legacy.py           legacy functional effects
+  effects_plugins/      effect plugin implementations
+    noise.py            noise effect
+    fade.py             fade effect
+    glitch.py           glitch effect
+    firehose.py         firehose effect
   fetch.py              RSS/Gutenberg fetching + cache
   ntfy.py               NtfyPoller — standalone, zero internal deps
   mic.py                MicMonitor — standalone, graceful fallback
@@ -147,8 +155,15 @@ engine/
   controller.py         coordinates ntfy/mic monitoring
   emitters.py           background emitters
   types.py              type definitions
-  display.py            Display protocol (Terminal, WebSocket, Multi)
+  display/              Display backend system
-  websocket_display.py  WebSocket server for browser clients
+    __init__.py         DisplayRegistry, get_monitor
+    backends/
+      terminal.py       ANSI terminal display
+      websocket.py      WebSocket server for browser clients
+      sixel.py         Sixel graphics (pure Python)
+      null.py          headless display for testing
+      multi.py         forwards to multiple displays
+  benchmark.py          performance benchmarking tool
 ```
 
 ---
@@ -172,17 +187,23 @@ With [mise](https://mise.jdx.dev/):
 ```bash
 mise run test            # run test suite
 mise run test-cov       # run with coverage report
+
 mise run lint           # ruff check
 mise run lint-fix       # ruff check --fix
 mise run format         # ruff format
 
 mise run run            # terminal display
 mise run run-websocket  # web display only
+mise run run-sixel     # sixel graphics
 mise run run-both       # terminal + web
 mise run run-client     # both + open browser
 
 mise run cmd            # C&C command interface
 mise run cmd-stats      # watch effects stats
+
+mise run benchmark      # run performance benchmarks
+mise run benchmark-json # save as JSON
+
 mise run topics-init    # initialize ntfy topics
 ```
 
@@ -191,8 +212,21 @@ mise run topics-init     # initialize ntfy topics
 ```bash
 uv run pytest
 uv run pytest --cov=engine --cov-report=term-missing
+
+# Run with mise
+mise run test
+mise run test-cov
+
+# Run performance benchmarks
+mise run benchmark
+mise run benchmark-json
+
+# Run benchmark hook mode (for CI)
+uv run python -m engine.benchmark --hook
 ```
 
+Performance regression tests are in `tests/test_benchmark.py` marked with `@pytest.mark.benchmark`.
+
 ### Linting
 
 ```bash

effects_plugins/__init__.py

@@ -5,6 +5,7 @@ PLUGIN_DIR = Path(__file__).parent
 
 def discover_plugins():
     from engine.effects.registry import get_registry
+    from engine.effects.types import EffectPlugin
 
     registry = get_registry()
     imported = {}
@@ -22,11 +23,13 @@ def discover_plugins():
                 attr = getattr(module, attr_name)
                 if (
                     isinstance(attr, type)
-                    and hasattr(attr, "name")
+                    and issubclass(attr, EffectPlugin)
-                    and hasattr(attr, "process")
+                    and attr is not EffectPlugin
                     and attr_name.endswith("Effect")
                 ):
                     plugin = attr()
+                    if not isinstance(plugin, EffectPlugin):
+                        continue
                     registry.register(plugin)
                     imported[plugin.name] = plugin
         except Exception:

effects_plugins/fade.py

@@ -3,7 +3,7 @@ import random
 from engine.effects.types import EffectConfig, EffectContext, EffectPlugin
 
 
-class FadeEffect:
+class FadeEffect(EffectPlugin):
     name = "fade"
     config = EffectConfig(enabled=True, intensity=1.0)
 
@@ -54,5 +54,5 @@ class FadeEffect:
                 i += 1
         return "".join(result)
 
-    def configure(self, cfg: EffectConfig) -> None:
+    def configure(self, config: EffectConfig) -> None:
-        self.config = cfg
+        self.config = config

effects_plugins/firehose.py

@@ -7,7 +7,7 @@ from engine.sources import FEEDS, POETRY_SOURCES
 from engine.terminal import C_DIM, G_DIM, G_LO, RST, W_GHOST
 
 
-class FirehoseEffect:
+class FirehoseEffect(EffectPlugin):
     name = "firehose"
     config = EffectConfig(enabled=True, intensity=1.0)
 
@@ -68,5 +68,5 @@ class FirehoseEffect:
             color = random.choice([G_LO, C_DIM, W_GHOST])
             return f"{color}{text}{RST}"
 
-    def configure(self, cfg: EffectConfig) -> None:
+    def configure(self, config: EffectConfig) -> None:
-        self.config = cfg
+        self.config = config

effects_plugins/glitch.py

@@ -5,7 +5,7 @@ from engine.effects.types import EffectConfig, EffectContext, EffectPlugin
 from engine.terminal import C_DIM, DIM, G_DIM, G_LO, RST
 
 
-class GlitchEffect:
+class GlitchEffect(EffectPlugin):
     name = "glitch"
     config = EffectConfig(enabled=True, intensity=1.0)
 
@@ -33,5 +33,5 @@ class GlitchEffect:
         o = random.randint(0, w - n)
         return " " * o + f"{G_LO}{DIM}" + c * n + RST
 
-    def configure(self, cfg: EffectConfig) -> None:
+    def configure(self, config: EffectConfig) -> None:
-        self.config = cfg
+        self.config = config

effects_plugins/noise.py

@@ -5,7 +5,7 @@ from engine.effects.types import EffectConfig, EffectContext, EffectPlugin
 from engine.terminal import C_DIM, G_DIM, G_LO, RST, W_GHOST
 
 
-class NoiseEffect:
+class NoiseEffect(EffectPlugin):
     name = "noise"
     config = EffectConfig(enabled=True, intensity=0.15)
 
@@ -32,5 +32,5 @@ class NoiseEffect:
             for _ in range(w)
         )
 
-    def configure(self, cfg: EffectConfig) -> None:
+    def configure(self, config: EffectConfig) -> None:
-        self.config = cfg
+        self.config = config

engine/display/backends/terminal.py

@@ -11,10 +11,6 @@ class TerminalDisplay:
     width: int = 80
     height: int = 24
 
-    def __init__(self):
-        self.width = 80
-        self.height = 24
-
     def init(self, width: int, height: int) -> None:
         from engine.terminal import CURSOR_OFF
 

engine/effects/__init__.py

@@ -10,7 +10,12 @@ from engine.effects.legacy import (
 )
 from engine.effects.performance import PerformanceMonitor, get_monitor, set_monitor
 from engine.effects.registry import EffectRegistry, get_registry, set_registry
-from engine.effects.types import EffectConfig, EffectContext, PipelineConfig
+from engine.effects.types import (
+    EffectConfig,
+    EffectContext,
+    PipelineConfig,
+    create_effect_context,
+)
 
 
 def get_effect_chain():
@@ -25,6 +30,7 @@ __all__ = [
     "EffectConfig",
     "EffectContext",
     "PipelineConfig",
+    "create_effect_context",
     "get_registry",
     "set_registry",
     "get_effect_chain",

engine/effects/legacy.py

@@ -1,6 +1,14 @@
 """
 Visual effects: noise, glitch, fade, ANSI-aware truncation, firehose, headline pool.
 Depends on: config, terminal, sources.
+
+These are low-level functional implementations of visual effects. They are used
+internally by the EffectPlugin system (effects_plugins/*.py) and also directly
+by layers.py and scroll.py for rendering.
+
+The plugin system provides a higher-level OOP interface with configuration
+support, while these legacy functions provide direct functional access.
+Both systems coexist - there are no current plans to deprecate the legacy functions.
 """
 
 import random

engine/effects/types.py

@@ -1,3 +1,24 @@
+"""
+Visual effects type definitions and base classes.
+
+EffectPlugin Architecture:
+- Uses ABC (Abstract Base Class) for interface enforcement
+- Runtime discovery via directory scanning (effects_plugins/)
+- Configuration via EffectConfig dataclass
+- Context passed through EffectContext dataclass
+
+Plugin System Research (see AGENTS.md for references):
+- VST: Standardized audio interfaces, chaining, presets (FXP/FXB)
+- Python Entry Points: Namespace packages, importlib.metadata discovery
+- Shadertoy: Shader-based with uniforms as context
+
+Current gaps vs industry patterns:
+- No preset save/load system
+- No external plugin distribution via entry points
+- No plugin metadata (version, author, description)
+"""
+
+from abc import ABC, abstractmethod
 from dataclasses import dataclass, field
 from typing import Any
 
@@ -22,15 +43,76 @@ class EffectConfig:
     params: dict[str, Any] = field(default_factory=dict)
 
 
-class EffectPlugin:
+class EffectPlugin(ABC):
+    """Abstract base class for effect plugins.
+
+    Subclasses must define:
+    - name: str - unique identifier for the effect
+    - config: EffectConfig - current configuration
+
+    And implement:
+    - process(buf, ctx) -> list[str]
+    - configure(config) -> None
+
+    Effect Behavior with ticker_height=0:
+    - NoiseEffect: Returns buffer unchanged (no ticker to apply noise to)
+    - FadeEffect: Returns buffer unchanged (no ticker to fade)
+    - GlitchEffect: Processes normally (doesn't depend on ticker_height)
+    - FirehoseEffect: Returns buffer unchanged if no items in context
+
+    Effects should handle missing or zero context values gracefully by
+    returning the input buffer unchanged rather than raising errors.
+    """
+
     name: str
     config: EffectConfig
 
+    @abstractmethod
     def process(self, buf: list[str], ctx: EffectContext) -> list[str]:
-        raise NotImplementedError
+        """Process the buffer with this effect applied.
 
+        Args:
+            buf: List of lines to process
+            ctx: Effect context with terminal state
+
+        Returns:
+            Processed buffer (may be same object or new list)
+        """
+        ...
+
+    @abstractmethod
     def configure(self, config: EffectConfig) -> None:
-        raise NotImplementedError
+        """Configure the effect with new settings.
+
+        Args:
+            config: New configuration to apply
+        """
+        ...
+
+
+def create_effect_context(
+    terminal_width: int = 80,
+    terminal_height: int = 24,
+    scroll_cam: int = 0,
+    ticker_height: int = 0,
+    mic_excess: float = 0.0,
+    grad_offset: float = 0.0,
+    frame_number: int = 0,
+    has_message: bool = False,
+    items: list | None = None,
+) -> EffectContext:
+    """Factory function to create EffectContext with sensible defaults."""
+    return EffectContext(
+        terminal_width=terminal_width,
+        terminal_height=terminal_height,
+        scroll_cam=scroll_cam,
+        ticker_height=ticker_height,
+        mic_excess=mic_excess,
+        grad_offset=grad_offset,
+        frame_number=frame_number,
+        has_message=has_message,
+        items=items or [],
+    )
 
 
 @dataclass

pyproject.toml

@@ -70,6 +70,9 @@ addopts = [
     "--tb=short",
     "-v",
 ]
+markers = [
+    "benchmark: marks tests as performance benchmarks (may be slow)",
+]
 filterwarnings = [
     "ignore::DeprecationWarning",
 ]

tests/test_benchmark.py

@@ -0,0 +1,100 @@
+"""
+Tests for engine.benchmark module - performance regression tests.
+"""
+
+from unittest.mock import patch
+
+import pytest
+
+from engine.display import NullDisplay
+
+
+class TestBenchmarkNullDisplay:
+    """Performance tests for NullDisplay - regression tests."""
+
+    @pytest.mark.benchmark
+    def test_null_display_minimum_fps(self):
+        """NullDisplay should meet minimum performance threshold."""
+        import time
+
+        display = NullDisplay()
+        display.init(80, 24)
+        buffer = ["x" * 80 for _ in range(24)]
+
+        iterations = 1000
+        start = time.perf_counter()
+        for _ in range(iterations):
+            display.show(buffer)
+        elapsed = time.perf_counter() - start
+
+        fps = iterations / elapsed
+        min_fps = 20000
+
+        assert fps >= min_fps, f"NullDisplay FPS {fps:.0f} below minimum {min_fps}"
+
+    @pytest.mark.benchmark
+    def test_effects_minimum_throughput(self):
+        """Effects should meet minimum processing throughput."""
+        import time
+
+        from effects_plugins import discover_plugins
+        from engine.effects import EffectContext, get_registry
+
+        discover_plugins()
+        registry = get_registry()
+        effect = registry.get("noise")
+        assert effect is not None, "Noise effect should be registered"
+
+        buffer = ["x" * 80 for _ in range(24)]
+        ctx = EffectContext(
+            terminal_width=80,
+            terminal_height=24,
+            scroll_cam=0,
+            ticker_height=20,
+            mic_excess=0.0,
+            grad_offset=0.0,
+            frame_number=0,
+            has_message=False,
+        )
+
+        iterations = 500
+        start = time.perf_counter()
+        for _ in range(iterations):
+            effect.process(buffer, ctx)
+        elapsed = time.perf_counter() - start
+
+        fps = iterations / elapsed
+        min_fps = 10000
+
+        assert fps >= min_fps, (
+            f"Effect processing FPS {fps:.0f} below minimum {min_fps}"
+        )
+
+
+class TestBenchmarkWebSocketDisplay:
+    """Performance tests for WebSocketDisplay."""
+
+    @pytest.mark.benchmark
+    def test_websocket_display_minimum_fps(self):
+        """WebSocketDisplay should meet minimum performance threshold."""
+        import time
+
+        with patch("engine.display.backends.websocket.websockets", None):
+            from engine.display import WebSocketDisplay
+
+            display = WebSocketDisplay()
+            display.init(80, 24)
+            buffer = ["x" * 80 for _ in range(24)]
+
+            iterations = 500
+            start = time.perf_counter()
+            for _ in range(iterations):
+                display.show(buffer)
+            elapsed = time.perf_counter() - start
+
+            fps = iterations / elapsed
+            min_fps = 10000
+
+            assert fps >= min_fps, (
+                f"WebSocketDisplay FPS {fps:.0f} below minimum {min_fps}"
+            )

tests/test_controller.py

@@ -5,7 +5,75 @@ Tests for engine.controller module.
 from unittest.mock import MagicMock, patch
 
 from engine import config
-from engine.controller import StreamController
+from engine.controller import StreamController, _get_display
+
+
+class TestGetDisplay:
+    """Tests for _get_display function."""
+
+    @patch("engine.controller.WebSocketDisplay")
+    @patch("engine.controller.TerminalDisplay")
+    def test_get_display_terminal(self, mock_terminal, mock_ws):
+        """returns TerminalDisplay for display=terminal."""
+        mock_terminal.return_value = MagicMock()
+        mock_ws.return_value = MagicMock()
+
+        cfg = config.Config(display="terminal")
+        display = _get_display(cfg)
+
+        mock_terminal.assert_called()
+        assert isinstance(display, MagicMock)
+
+    @patch("engine.controller.WebSocketDisplay")
+    @patch("engine.controller.TerminalDisplay")
+    def test_get_display_websocket(self, mock_terminal, mock_ws):
+        """returns WebSocketDisplay for display=websocket."""
+        mock_ws_instance = MagicMock()
+        mock_ws.return_value = mock_ws_instance
+        mock_terminal.return_value = MagicMock()
+
+        cfg = config.Config(display="websocket")
+        _get_display(cfg)
+
+        mock_ws.assert_called()
+        mock_ws_instance.start_server.assert_called()
+        mock_ws_instance.start_http_server.assert_called()
+
+    @patch("engine.controller.SixelDisplay")
+    def test_get_display_sixel(self, mock_sixel):
+        """returns SixelDisplay for display=sixel."""
+        mock_sixel.return_value = MagicMock()
+        cfg = config.Config(display="sixel")
+        _get_display(cfg)
+
+        mock_sixel.assert_called()
+
+    def test_get_display_unknown_returns_null(self):
+        """returns NullDisplay for unknown display mode."""
+        cfg = config.Config(display="unknown")
+        display = _get_display(cfg)
+
+        from engine.display import NullDisplay
+
+        assert isinstance(display, NullDisplay)
+
+    @patch("engine.controller.WebSocketDisplay")
+    @patch("engine.controller.TerminalDisplay")
+    @patch("engine.controller.MultiDisplay")
+    def test_get_display_both(self, mock_multi, mock_terminal, mock_ws):
+        """returns MultiDisplay for display=both."""
+        mock_terminal_instance = MagicMock()
+        mock_ws_instance = MagicMock()
+        mock_terminal.return_value = mock_terminal_instance
+        mock_ws.return_value = mock_ws_instance
+
+        cfg = config.Config(display="both")
+        _get_display(cfg)
+
+        mock_multi.assert_called()
+        call_args = mock_multi.call_args[0][0]
+        assert mock_terminal_instance in call_args
+        assert mock_ws_instance in call_args
 
 
 class TestStreamController:
@@ -68,6 +136,24 @@ class TestStreamController:
         assert mic_ok is False
         assert ntfy_ok is True
 
+    @patch("engine.controller.MicMonitor")
+    def test_initialize_sources_cc_subscribed(self, mock_mic):
+        """initialize_sources subscribes C&C handler."""
+        mock_mic_instance = MagicMock()
+        mock_mic_instance.available = False
+        mock_mic_instance.start.return_value = False
+        mock_mic.return_value = mock_mic_instance
+
+        with patch("engine.controller.NtfyPoller") as mock_ntfy:
+            mock_ntfy_instance = MagicMock()
+            mock_ntfy_instance.start.return_value = True
+            mock_ntfy.return_value = mock_ntfy_instance
+
+            controller = StreamController()
+            controller.initialize_sources()
+
+            mock_ntfy_instance.subscribe.assert_called()
+
 
 class TestStreamControllerCleanup:
     """Tests for StreamController cleanup."""

tests/test_display.py

@@ -2,7 +2,10 @@
 Tests for engine.display module.
 """
 
-from engine.display import NullDisplay, TerminalDisplay
+from unittest.mock import MagicMock
+
+from engine.display import DisplayRegistry, NullDisplay, TerminalDisplay
+from engine.display.backends.multi import MultiDisplay
 
 
 class TestDisplayProtocol:
@@ -25,6 +28,66 @@ class TestDisplayProtocol:
         assert hasattr(display, "cleanup")
 
 
+class TestDisplayRegistry:
+    """Tests for DisplayRegistry class."""
+
+    def setup_method(self):
+        """Reset registry before each test."""
+        DisplayRegistry._backends = {}
+        DisplayRegistry._initialized = False
+
+    def test_register_adds_backend(self):
+        """register adds a backend to the registry."""
+        DisplayRegistry.register("test", TerminalDisplay)
+        assert DisplayRegistry.get("test") == TerminalDisplay
+
+    def test_register_case_insensitive(self):
+        """register is case insensitive."""
+        DisplayRegistry.register("TEST", TerminalDisplay)
+        assert DisplayRegistry.get("test") == TerminalDisplay
+
+    def test_get_returns_none_for_unknown(self):
+        """get returns None for unknown backend."""
+        assert DisplayRegistry.get("unknown") is None
+
+    def test_list_backends_returns_all(self):
+        """list_backends returns all registered backends."""
+        DisplayRegistry.register("a", TerminalDisplay)
+        DisplayRegistry.register("b", NullDisplay)
+        backends = DisplayRegistry.list_backends()
+        assert "a" in backends
+        assert "b" in backends
+
+    def test_create_returns_instance(self):
+        """create returns a display instance."""
+        DisplayRegistry.register("test", NullDisplay)
+        display = DisplayRegistry.create("test")
+        assert isinstance(display, NullDisplay)
+
+    def test_create_returns_none_for_unknown(self):
+        """create returns None for unknown backend."""
+        display = DisplayRegistry.create("unknown")
+        assert display is None
+
+    def test_initialize_registers_defaults(self):
+        """initialize registers default backends."""
+        DisplayRegistry.initialize()
+        assert DisplayRegistry.get("terminal") == TerminalDisplay
+        assert DisplayRegistry.get("null") == NullDisplay
+        from engine.display.backends.sixel import SixelDisplay
+        from engine.display.backends.websocket import WebSocketDisplay
+
+        assert DisplayRegistry.get("websocket") == WebSocketDisplay
+        assert DisplayRegistry.get("sixel") == SixelDisplay
+
+    def test_initialize_idempotent(self):
+        """initialize can be called multiple times safely."""
+        DisplayRegistry.initialize()
+        DisplayRegistry._backends["custom"] = TerminalDisplay
+        DisplayRegistry.initialize()
+        assert "custom" in DisplayRegistry.list_backends()
+
+
 class TestTerminalDisplay:
     """Tests for TerminalDisplay class."""
 
@@ -77,3 +140,62 @@ class TestNullDisplay:
         """cleanup does nothing."""
         display = NullDisplay()
         display.cleanup()
+
+
+class TestMultiDisplay:
+    """Tests for MultiDisplay class."""
+
+    def test_init_stores_dimensions(self):
+        """init stores dimensions and forwards to displays."""
+        mock_display1 = MagicMock()
+        mock_display2 = MagicMock()
+        multi = MultiDisplay([mock_display1, mock_display2])
+
+        multi.init(120, 40)
+
+        assert multi.width == 120
+        assert multi.height == 40
+        mock_display1.init.assert_called_once_with(120, 40)
+        mock_display2.init.assert_called_once_with(120, 40)
+
+    def test_show_forwards_to_all_displays(self):
+        """show forwards buffer to all displays."""
+        mock_display1 = MagicMock()
+        mock_display2 = MagicMock()
+        multi = MultiDisplay([mock_display1, mock_display2])
+
+        buffer = ["line1", "line2"]
+        multi.show(buffer)
+
+        mock_display1.show.assert_called_once_with(buffer)
+        mock_display2.show.assert_called_once_with(buffer)
+
+    def test_clear_forwards_to_all_displays(self):
+        """clear forwards to all displays."""
+        mock_display1 = MagicMock()
+        mock_display2 = MagicMock()
+        multi = MultiDisplay([mock_display1, mock_display2])
+
+        multi.clear()
+
+        mock_display1.clear.assert_called_once()
+        mock_display2.clear.assert_called_once()
+
+    def test_cleanup_forwards_to_all_displays(self):
+        """cleanup forwards to all displays."""
+        mock_display1 = MagicMock()
+        mock_display2 = MagicMock()
+        multi = MultiDisplay([mock_display1, mock_display2])
+
+        multi.cleanup()
+
+        mock_display1.cleanup.assert_called_once()
+        mock_display2.cleanup.assert_called_once()
+
+    def test_empty_displays_list(self):
+        """handles empty displays list gracefully."""
+        multi = MultiDisplay([])
+        multi.init(80, 24)
+        multi.show(["test"])
+        multi.clear()
+        multi.cleanup()

tests/test_effects_controller.py

@@ -5,8 +5,10 @@ Tests for engine.effects.controller module.
 from unittest.mock import MagicMock, patch
 
 from engine.effects.controller import (
+    _format_stats,
     handle_effects_command,
     set_effect_chain_ref,
+    show_effects_menu,
 )
 
 
@@ -92,6 +94,29 @@ class TestHandleEffectsCommand:
                 assert "Reordered pipeline" in result
                 mock_chain_instance.reorder.assert_called_once_with(["noise", "fade"])
 
+    def test_reorder_failure(self):
+        """reorder returns error on failure."""
+        with patch("engine.effects.controller.get_registry") as mock_registry:
+            mock_registry.return_value.list_all.return_value = {}
+
+            with patch("engine.effects.controller._get_effect_chain") as mock_chain:
+                mock_chain_instance = MagicMock()
+                mock_chain_instance.reorder.return_value = False
+                mock_chain.return_value = mock_chain_instance
+
+                result = handle_effects_command("/effects reorder bad")
+
+                assert "Failed to reorder" in result
+
+    def test_unknown_effect(self):
+        """unknown effect returns error."""
+        with patch("engine.effects.controller.get_registry") as mock_registry:
+            mock_registry.return_value.list_all.return_value = {}
+
+            result = handle_effects_command("/effects unknown on")
+
+            assert "Unknown effect" in result
+
     def test_unknown_command(self):
         """unknown command returns error."""
         result = handle_effects_command("/unknown")
@@ -102,6 +127,105 @@ class TestHandleEffectsCommand:
         result = handle_effects_command("not a command")
         assert "Unknown command" in result
 
+    def test_invalid_intensity_value(self):
+        """invalid intensity value returns error."""
+        with patch("engine.effects.controller.get_registry") as mock_registry:
+            mock_plugin = MagicMock()
+            mock_registry.return_value.get.return_value = mock_plugin
+            mock_registry.return_value.list_all.return_value = {"noise": mock_plugin}
+
+            result = handle_effects_command("/effects noise intensity bad")
+
+            assert "Invalid intensity" in result
+
+    def test_missing_action(self):
+        """missing action returns usage."""
+        with patch("engine.effects.controller.get_registry") as mock_registry:
+            mock_plugin = MagicMock()
+            mock_registry.return_value.get.return_value = mock_plugin
+            mock_registry.return_value.list_all.return_value = {"noise": mock_plugin}
+
+            result = handle_effects_command("/effects noise")
+
+            assert "Usage" in result
+
+    def test_stats_command(self):
+        """stats command returns formatted stats."""
+        with patch("engine.effects.controller.get_monitor") as mock_monitor:
+            mock_monitor.return_value.get_stats.return_value = {
+                "frame_count": 100,
+                "pipeline": {"avg_ms": 1.5, "min_ms": 1.0, "max_ms": 2.0},
+                "effects": {},
+            }
+
+            result = handle_effects_command("/effects stats")
+
+            assert "Performance Stats" in result
+
+    def test_list_only_effects(self):
+        """list command works with just /effects."""
+        with patch("engine.effects.controller.get_registry") as mock_registry:
+            mock_plugin = MagicMock()
+            mock_plugin.config.enabled = False
+            mock_plugin.config.intensity = 0.5
+            mock_registry.return_value.list_all.return_value = {"noise": mock_plugin}
+
+            with patch("engine.effects.controller._get_effect_chain") as mock_chain:
+                mock_chain.return_value = None
+
+                result = handle_effects_command("/effects")
+
+                assert "noise: OFF" in result
+
+
+class TestShowEffectsMenu:
+    """Tests for show_effects_menu function."""
+
+    def test_returns_formatted_menu(self):
+        """returns formatted effects menu."""
+        with patch("engine.effects.controller.get_registry") as mock_registry:
+            mock_plugin = MagicMock()
+            mock_plugin.config.enabled = True
+            mock_plugin.config.intensity = 0.75
+            mock_registry.return_value.list_all.return_value = {"noise": mock_plugin}
+
+            with patch("engine.effects.controller._get_effect_chain") as mock_chain:
+                mock_chain_instance = MagicMock()
+                mock_chain_instance.get_order.return_value = ["noise"]
+                mock_chain.return_value = mock_chain_instance
+
+                result = show_effects_menu()
+
+                assert "EFFECTS MENU" in result
+                assert "noise" in result
+
+
+class TestFormatStats:
+    """Tests for _format_stats function."""
+
+    def test_returns_error_when_no_monitor(self):
+        """returns error when monitor unavailable."""
+        with patch("engine.effects.controller.get_monitor") as mock_monitor:
+            mock_monitor.return_value.get_stats.return_value = {"error": "No data"}
+
+            result = _format_stats()
+
+            assert "No data" in result
+
+    def test_formats_pipeline_stats(self):
+        """formats pipeline stats correctly."""
+        with patch("engine.effects.controller.get_monitor") as mock_monitor:
+            mock_monitor.return_value.get_stats.return_value = {
+                "frame_count": 50,
+                "pipeline": {"avg_ms": 2.5, "min_ms": 2.0, "max_ms": 3.0},
+                "effects": {"noise": {"avg_ms": 0.5, "min_ms": 0.4, "max_ms": 0.6}},
+            }
+
+            result = _format_stats()
+
+            assert "Pipeline" in result
+            assert "noise" in result
+
 
 class TestSetEffectChainRef:
     """Tests for set_effect_chain_ref function."""

tests/test_sixel.py

@@ -0,0 +1,123 @@
+"""
+Tests for engine.display.backends.sixel module.
+"""
+
+from unittest.mock import MagicMock, patch
+
+
+class TestSixelDisplay:
+    """Tests for SixelDisplay class."""
+
+    def test_init_stores_dimensions(self):
+        """init stores dimensions."""
+        from engine.display.backends.sixel import SixelDisplay
+
+        display = SixelDisplay()
+        display.init(80, 24)
+        assert display.width == 80
+        assert display.height == 24
+
+    def test_init_custom_cell_size(self):
+        """init accepts custom cell size."""
+        from engine.display.backends.sixel import SixelDisplay
+
+        display = SixelDisplay(cell_width=12, cell_height=18)
+        assert display.cell_width == 12
+        assert display.cell_height == 18
+
+    def test_show_handles_empty_buffer(self):
+        """show handles empty buffer gracefully."""
+        from engine.display.backends.sixel import SixelDisplay
+
+        display = SixelDisplay()
+        display.init(80, 24)
+
+        with patch("engine.display.backends.sixel._encode_sixel") as mock_encode:
+            mock_encode.return_value = ""
+            display.show([])
+
+    def test_show_handles_pil_import_error(self):
+        """show gracefully handles missing PIL."""
+        from engine.display.backends.sixel import SixelDisplay
+
+        display = SixelDisplay()
+        display.init(80, 24)
+
+        with patch.dict("sys.modules", {"PIL": None}):
+            display.show(["test line"])
+
+    def test_clear_sends_escape_sequence(self):
+        """clear sends clear screen escape sequence."""
+        from engine.display.backends.sixel import SixelDisplay
+
+        display = SixelDisplay()
+
+        with patch("sys.stdout") as mock_stdout:
+            display.clear()
+            mock_stdout.buffer.write.assert_called()
+
+    def test_cleanup_does_nothing(self):
+        """cleanup does nothing."""
+        from engine.display.backends.sixel import SixelDisplay
+
+        display = SixelDisplay()
+        display.cleanup()
+
+
+class TestSixelAnsiParsing:
+    """Tests for ANSI parsing in SixelDisplay."""
+
+    def test_parse_empty_string(self):
+        """handles empty string."""
+        from engine.display.backends.sixel import _parse_ansi
+
+        result = _parse_ansi("")
+        assert len(result) > 0
+
+    def test_parse_plain_text(self):
+        """parses plain text without ANSI codes."""
+        from engine.display.backends.sixel import _parse_ansi
+
+        result = _parse_ansi("hello world")
+        assert len(result) == 1
+        text, fg, bg, bold = result[0]
+        assert text == "hello world"
+
+    def test_parse_with_color_codes(self):
+        """parses ANSI color codes."""
+        from engine.display.backends.sixel import _parse_ansi
+
+        result = _parse_ansi("\033[31mred\033[0m")
+        assert len(result) == 2
+
+    def test_parse_with_bold(self):
+        """parses bold codes."""
+        from engine.display.backends.sixel import _parse_ansi
+
+        result = _parse_ansi("\033[1mbold\033[0m")
+        assert len(result) == 2
+
+    def test_parse_256_color(self):
+        """parses 256 color codes."""
+        from engine.display.backends.sixel import _parse_ansi
+
+        result = _parse_ansi("\033[38;5;196mred\033[0m")
+        assert len(result) == 2
+
+
+class TestSixelEncoding:
+    """Tests for Sixel encoding."""
+
+    def test_encode_empty_image(self):
+        """handles empty image."""
+        from engine.display.backends.sixel import _encode_sixel
+
+        with patch("PIL.Image.Image") as mock_image:
+            mock_img_instance = MagicMock()
+            mock_img_instance.convert.return_value = mock_img_instance
+            mock_img_instance.size = (0, 0)
+            mock_img_instance.load.return_value = {}
+            mock_image.return_value = mock_img_instance
+
+            result = _encode_sixel(mock_img_instance)
+            assert result == ""