diff --git a/src/polymarket_insider_tracker/detector/__init__.py b/src/polymarket_insider_tracker/detector/__init__.py
index e82d348..cbb71f8 100644
--- a/src/polymarket_insider_tracker/detector/__init__.py
+++ b/src/polymarket_insider_tracker/detector/__init__.py
@@ -1,6 +1,12 @@
 """Anomaly detection layer - Suspicious activity identification."""
 
 from polymarket_insider_tracker.detector.fresh_wallet import FreshWalletDetector
-from polymarket_insider_tracker.detector.models import FreshWalletSignal
+from polymarket_insider_tracker.detector.models import FreshWalletSignal, SizeAnomalySignal
+from polymarket_insider_tracker.detector.size_anomaly import SizeAnomalyDetector
 
-__all__ = ["FreshWalletDetector", "FreshWalletSignal"]
+__all__ = [
+    "FreshWalletDetector",
+    "FreshWalletSignal",
+    "SizeAnomalyDetector",
+    "SizeAnomalySignal",
+]
diff --git a/src/polymarket_insider_tracker/detector/models.py b/src/polymarket_insider_tracker/detector/models.py
index d958c2e..f1aa7e4 100644
--- a/src/polymarket_insider_tracker/detector/models.py
+++ b/src/polymarket_insider_tracker/detector/models.py
@@ -4,7 +4,7 @@ from dataclasses import dataclass, field
 from datetime import UTC, datetime
 from decimal import Decimal
 
-from polymarket_insider_tracker.ingestor.models import TradeEvent
+from polymarket_insider_tracker.ingestor.models import MarketMetadata, TradeEvent
 from polymarket_insider_tracker.profiler.models import WalletProfile
 
 
@@ -71,3 +71,75 @@ class FreshWalletSignal:
             "factors": self.factors,
             "timestamp": self.timestamp.isoformat(),
         }
+
+
+@dataclass(frozen=True)
+class SizeAnomalySignal:
+    """Signal emitted when a trade has unusually large position size.
+
+    This signal is generated when a trade's size significantly impacts
+    the market volume or order book depth, indicating potential informed
+    trading activity.
+
+    Attributes:
+        trade_event: The original trade event that triggered this signal.
+        market_metadata: Metadata about the market being traded.
+        volume_impact: Trade size as fraction of 24h volume (0.0 if unknown).
+        book_impact: Trade size as fraction of order book depth (0.0 if unknown).
+        is_niche_market: Whether the market is considered niche/low-volume.
+        confidence: Overall confidence score (0.0 to 1.0).
+        factors: Individual factor scores contributing to confidence.
+        timestamp: When this signal was generated.
+    """
+
+    trade_event: TradeEvent
+    market_metadata: MarketMetadata
+    volume_impact: float
+    book_impact: float
+    is_niche_market: bool
+    confidence: float
+    factors: dict[str, float]
+    timestamp: datetime = field(default_factory=lambda: datetime.now(UTC))
+
+    @property
+    def wallet_address(self) -> str:
+        """Return the wallet address from the trade event."""
+        return self.trade_event.wallet_address
+
+    @property
+    def market_id(self) -> str:
+        """Return the market ID from the trade event."""
+        return self.trade_event.market_id
+
+    @property
+    def trade_size_usdc(self) -> Decimal:
+        """Return the trade size in USDC (notional value)."""
+        return self.trade_event.notional_value
+
+    @property
+    def is_high_confidence(self) -> bool:
+        """Return True if confidence exceeds 0.7."""
+        return self.confidence >= 0.7
+
+    @property
+    def is_very_high_confidence(self) -> bool:
+        """Return True if confidence exceeds 0.85."""
+        return self.confidence >= 0.85
+
+    def to_dict(self) -> dict[str, object]:
+        """Serialize to dictionary for Redis stream publishing."""
+        return {
+            "wallet_address": self.wallet_address,
+            "market_id": self.market_id,
+            "trade_id": self.trade_event.trade_id,
+            "trade_size": str(self.trade_size_usdc),
+            "trade_side": self.trade_event.side,
+            "trade_price": str(self.trade_event.price),
+            "market_category": self.market_metadata.category,
+            "volume_impact": self.volume_impact,
+            "book_impact": self.book_impact,
+            "is_niche_market": self.is_niche_market,
+            "confidence": self.confidence,
+            "factors": self.factors,
+            "timestamp": self.timestamp.isoformat(),
+        }
diff --git a/src/polymarket_insider_tracker/detector/size_anomaly.py b/src/polymarket_insider_tracker/detector/size_anomaly.py
new file mode 100644
index 0000000..b348dee
--- /dev/null
+++ b/src/polymarket_insider_tracker/detector/size_anomaly.py
@@ -0,0 +1,354 @@
+"""Position size anomaly detection algorithm.
+
+This module provides the SizeAnomalyDetector class that identifies trades
+with unusually large position sizes relative to market liquidity.
+"""
+
+import logging
+from decimal import Decimal
+
+from polymarket_insider_tracker.detector.models import SizeAnomalySignal
+from polymarket_insider_tracker.ingestor.metadata_sync import MarketMetadataSync
+from polymarket_insider_tracker.ingestor.models import MarketMetadata, TradeEvent
+
+logger = logging.getLogger(__name__)
+
+# Default configuration
+DEFAULT_VOLUME_THRESHOLD = 0.02  # 2% of daily volume
+DEFAULT_BOOK_THRESHOLD = 0.05  # 5% of order book depth
+DEFAULT_NICHE_VOLUME_THRESHOLD = Decimal("50000")  # $50k daily volume
+
+# Niche market categories - markets in these categories with low specificity
+# are more likely to have insider information value
+NICHE_PRONE_CATEGORIES = frozenset({"science", "tech", "finance", "other"})
+
+
+class SizeAnomalyDetector:
+    """Detector for unusually large trade sizes.
+
+    This detector analyzes trade events for size anomalies by comparing
+    the trade size against market liquidity metrics:
+    - Volume impact: trade size / 24h volume
+    - Book impact: trade size / order book depth
+
+    When volume data is unavailable, the detector uses category-based
+    heuristics to identify niche markets where large trades are more
+    significant.
+
+    Confidence scoring:
+    - Volume impact > threshold: base score from impact ratio
+    - Book impact > threshold: additional score from impact ratio
+    - Niche market multiplier: 1.5x for low-volume markets
+
+    Example:
+        ```python
+        sync = MarketMetadataSync(redis, clob_client)
+        detector = SizeAnomalyDetector(sync)
+
+        # Analyze a trade
+        signal = await detector.analyze(trade_event)
+        if signal is not None:
+            print(f"Size anomaly detected! Confidence: {signal.confidence}")
+        ```
+    """
+
+    def __init__(
+        self,
+        metadata_sync: MarketMetadataSync,
+        *,
+        volume_threshold: float = DEFAULT_VOLUME_THRESHOLD,
+        book_threshold: float = DEFAULT_BOOK_THRESHOLD,
+        niche_volume_threshold: Decimal = DEFAULT_NICHE_VOLUME_THRESHOLD,
+    ) -> None:
+        """Initialize the size anomaly detector.
+
+        Args:
+            metadata_sync: MarketMetadataSync for fetching market metadata.
+            volume_threshold: Threshold for volume impact (default 0.02 = 2%).
+            book_threshold: Threshold for book impact (default 0.05 = 5%).
+            niche_volume_threshold: Volume below which market is niche ($50k).
+        """
+        self._metadata_sync = metadata_sync
+        self._volume_threshold = volume_threshold
+        self._book_threshold = book_threshold
+        self._niche_volume_threshold = niche_volume_threshold
+
+    async def analyze(
+        self,
+        trade: TradeEvent,
+        *,
+        daily_volume: Decimal | None = None,
+        book_depth: Decimal | None = None,
+    ) -> SizeAnomalySignal | None:
+        """Analyze a trade event for size anomalies.
+
+        This method:
+        1. Fetches market metadata
+        2. Calculates volume and book impact (if data available)
+        3. Determines if market is niche
+        4. Calculates confidence score
+
+        Args:
+            trade: TradeEvent to analyze.
+            daily_volume: Optional 24h volume in USDC. If provided, enables
+                volume impact calculation.
+            book_depth: Optional order book depth in USDC. If provided,
+                enables book impact calculation.
+
+        Returns:
+            SizeAnomalySignal if the trade triggers anomaly detection,
+            None otherwise.
+        """
+        # Get market metadata
+        try:
+            metadata = await self._metadata_sync.get_market(trade.market_id)
+            if metadata is None:
+                logger.warning(
+                    "No metadata found for market %s, creating minimal metadata",
+                    trade.market_id,
+                )
+                metadata = self._create_minimal_metadata(trade)
+        except Exception as e:
+            logger.warning(
+                "Failed to get metadata for market %s: %s",
+                trade.market_id,
+                e,
+            )
+            metadata = self._create_minimal_metadata(trade)
+
+        trade_size = trade.notional_value
+
+        # Calculate impacts
+        volume_impact = self._calculate_volume_impact(trade_size, daily_volume)
+        book_impact = self._calculate_book_impact(trade_size, book_depth)
+
+        # Determine if niche market
+        is_niche = self._is_niche_market(metadata, daily_volume)
+
+        # Check if any threshold exceeded
+        exceeds_volume = volume_impact > self._volume_threshold
+        exceeds_book = book_impact > self._book_threshold
+
+        if not exceeds_volume and not exceeds_book and not is_niche:
+            logger.debug(
+                "Trade %s does not exceed thresholds: volume=%.4f, book=%.4f",
+                trade.trade_id,
+                volume_impact,
+                book_impact,
+            )
+            return None
+
+        # Calculate confidence score
+        confidence, factors = self.calculate_confidence(
+            volume_impact=volume_impact,
+            book_impact=book_impact,
+            is_niche=is_niche,
+        )
+
+        # Only emit signal if confidence is meaningful
+        if confidence < 0.1:
+            return None
+
+        logger.info(
+            "Size anomaly signal: market=%s, size=%s, volume_impact=%.4f, "
+            "book_impact=%.4f, niche=%s, confidence=%.2f",
+            trade.market_id[:10] + "...",
+            trade_size,
+            volume_impact,
+            book_impact,
+            is_niche,
+            confidence,
+        )
+
+        return SizeAnomalySignal(
+            trade_event=trade,
+            market_metadata=metadata,
+            volume_impact=volume_impact,
+            book_impact=book_impact,
+            is_niche_market=is_niche,
+            confidence=confidence,
+            factors=factors,
+        )
+
+    def _create_minimal_metadata(self, trade: TradeEvent) -> MarketMetadata:
+        """Create minimal metadata from trade event."""
+        from polymarket_insider_tracker.ingestor.models import Token
+
+        return MarketMetadata(
+            condition_id=trade.market_id,
+            question=trade.event_title or "Unknown Market",
+            description="",
+            tokens=(
+                Token(
+                    token_id=trade.asset_id,
+                    outcome=trade.outcome,
+                    price=trade.price,
+                ),
+            ),
+            category="other",
+        )
+
+    def _calculate_volume_impact(
+        self,
+        trade_size: Decimal,
+        daily_volume: Decimal | None,
+    ) -> float:
+        """Calculate trade size as fraction of daily volume.
+
+        Args:
+            trade_size: Trade notional value in USDC.
+            daily_volume: 24h trading volume in USDC.
+
+        Returns:
+            Volume impact ratio, or 0.0 if volume unknown.
+        """
+        if daily_volume is None or daily_volume <= 0:
+            return 0.0
+        return float(trade_size / daily_volume)
+
+    def _calculate_book_impact(
+        self,
+        trade_size: Decimal,
+        book_depth: Decimal | None,
+    ) -> float:
+        """Calculate trade size as fraction of order book depth.
+
+        Args:
+            trade_size: Trade notional value in USDC.
+            book_depth: Visible order book depth in USDC.
+
+        Returns:
+            Book impact ratio, or 0.0 if depth unknown.
+        """
+        if book_depth is None or book_depth <= 0:
+            return 0.0
+        return float(trade_size / book_depth)
+
+    def _is_niche_market(
+        self,
+        metadata: MarketMetadata,
+        daily_volume: Decimal | None,
+    ) -> bool:
+        """Determine if market is considered niche.
+
+        A market is niche if:
+        - Volume is below threshold ($50k), OR
+        - Category is prone to insider info AND volume is unknown
+
+        Args:
+            metadata: Market metadata with category.
+            daily_volume: Optional 24h volume.
+
+        Returns:
+            True if market is considered niche.
+        """
+        # If volume known and below threshold, it's niche
+        if daily_volume is not None and daily_volume < self._niche_volume_threshold:
+            return True
+
+        # If volume unknown, use category heuristics
+        return daily_volume is None and metadata.category in NICHE_PRONE_CATEGORIES
+
+    def calculate_confidence(
+        self,
+        *,
+        volume_impact: float,
+        book_impact: float,
+        is_niche: bool,
+    ) -> tuple[float, dict[str, float]]:
+        """Calculate confidence score based on impact metrics.
+
+        Confidence scoring:
+        - Volume impact: min(impact/threshold, 3) / 3 * 0.5
+        - Book impact: min(impact/threshold, 3) / 3 * 0.3
+        - Niche multiplier: 1.5x final score
+
+        Final confidence clamped to [0.0, 1.0].
+
+        Args:
+            volume_impact: Trade size / daily volume ratio.
+            book_impact: Trade size / book depth ratio.
+            is_niche: Whether market is niche.
+
+        Returns:
+            Tuple of (confidence_score, factors_dict).
+        """
+        factors: dict[str, float] = {}
+        confidence = 0.0
+
+        # Volume impact component
+        if volume_impact > self._volume_threshold:
+            ratio = min(volume_impact / self._volume_threshold, 3.0)
+            volume_score = ratio / 3.0 * 0.5
+            factors["volume_impact"] = volume_score
+            confidence += volume_score
+
+        # Book impact component
+        if book_impact > self._book_threshold:
+            ratio = min(book_impact / self._book_threshold, 3.0)
+            book_score = ratio / 3.0 * 0.3
+            factors["book_impact"] = book_score
+            confidence += book_score
+
+        # Niche market multiplier
+        if is_niche and confidence > 0:
+            factors["niche_multiplier"] = 1.5
+            confidence *= 1.5
+
+        # If niche but no other signals, give small base confidence
+        if is_niche and confidence == 0:
+            factors["niche_base"] = 0.2
+            confidence = 0.2
+
+        # Clamp to valid range
+        confidence = max(0.0, min(1.0, confidence))
+
+        return confidence, factors
+
+    async def analyze_batch(
+        self,
+        trades: list[TradeEvent],
+        *,
+        volume_data: dict[str, Decimal] | None = None,
+        book_data: dict[str, Decimal] | None = None,
+    ) -> list[SizeAnomalySignal]:
+        """Analyze multiple trades for size anomalies.
+
+        Processes trades in parallel for efficiency.
+
+        Args:
+            trades: List of trades to analyze.
+            volume_data: Optional dict mapping market_id to 24h volume.
+            book_data: Optional dict mapping market_id to book depth.
+
+        Returns:
+            List of SizeAnomalySignal for trades with anomalies.
+        """
+        import asyncio
+
+        volume_data = volume_data or {}
+        book_data = book_data or {}
+
+        tasks = [
+            self.analyze(
+                trade,
+                daily_volume=volume_data.get(trade.market_id),
+                book_depth=book_data.get(trade.market_id),
+            )
+            for trade in trades
+        ]
+        results = await asyncio.gather(*tasks, return_exceptions=True)
+
+        signals: list[SizeAnomalySignal] = []
+        for trade, result in zip(trades, results, strict=True):
+            if isinstance(result, BaseException):
+                logger.warning(
+                    "Failed to analyze trade %s: %s",
+                    trade.trade_id,
+                    result,
+                )
+                continue
+            if result is not None:
+                signals.append(result)
+
+        return signals
diff --git a/tests/detector/test_size_anomaly.py b/tests/detector/test_size_anomaly.py
new file mode 100644
index 0000000..e617838
--- /dev/null
+++ b/tests/detector/test_size_anomaly.py
@@ -0,0 +1,829 @@
+"""Tests for position size anomaly detection."""
+
+from datetime import UTC, datetime
+from decimal import Decimal
+from unittest.mock import AsyncMock
+
+import pytest
+
+from polymarket_insider_tracker.detector.models import SizeAnomalySignal
+from polymarket_insider_tracker.detector.size_anomaly import (
+    DEFAULT_BOOK_THRESHOLD,
+    DEFAULT_NICHE_VOLUME_THRESHOLD,
+    DEFAULT_VOLUME_THRESHOLD,
+    NICHE_PRONE_CATEGORIES,
+    SizeAnomalyDetector,
+)
+from polymarket_insider_tracker.ingestor.metadata_sync import MarketMetadataSync
+from polymarket_insider_tracker.ingestor.models import MarketMetadata, Token, TradeEvent
+
+# ============================================================================
+# Fixtures
+# ============================================================================
+
+
+@pytest.fixture
+def mock_metadata_sync() -> AsyncMock:
+    """Create a mock MarketMetadataSync."""
+    return AsyncMock(spec=MarketMetadataSync)
+
+
+@pytest.fixture
+def sample_token() -> Token:
+    """Create a sample token."""
+    return Token(
+        token_id="token_123",
+        outcome="Yes",
+        price=Decimal("0.65"),
+    )
+
+
+@pytest.fixture
+def sample_metadata(sample_token: Token) -> MarketMetadata:
+    """Create sample market metadata."""
+    return MarketMetadata(
+        condition_id="market_abc123",
+        question="Will it rain tomorrow?",
+        description="Weather prediction market",
+        tokens=(sample_token,),
+        category="science",
+    )
+
+
+@pytest.fixture
+def sample_trade() -> TradeEvent:
+    """Create a sample trade event."""
+    return TradeEvent(
+        market_id="market_abc123",
+        trade_id="tx_001",
+        wallet_address="0x1234567890abcdef",
+        side="BUY",
+        outcome="Yes",
+        outcome_index=0,
+        price=Decimal("0.65"),
+        size=Decimal("10000"),  # $6,500 notional
+        timestamp=datetime.now(UTC),
+        asset_id="token_123",
+        event_title="Weather Market",
+    )
+
+
+@pytest.fixture
+def large_trade() -> TradeEvent:
+    """Create a large trade event."""
+    return TradeEvent(
+        market_id="market_abc123",
+        trade_id="tx_002",
+        wallet_address="0xlargewallet",
+        side="BUY",
+        outcome="Yes",
+        outcome_index=0,
+        price=Decimal("0.50"),
+        size=Decimal("100000"),  # $50,000 notional
+        timestamp=datetime.now(UTC),
+        asset_id="token_123",
+        event_title="Big Market",
+    )
+
+
+# ============================================================================
+# SizeAnomalySignal Tests
+# ============================================================================
+
+
+class TestSizeAnomalySignal:
+    """Tests for the SizeAnomalySignal dataclass."""
+
+    def test_signal_creation(
+        self, sample_trade: TradeEvent, sample_metadata: MarketMetadata
+    ) -> None:
+        """Test basic signal creation."""
+        signal = SizeAnomalySignal(
+            trade_event=sample_trade,
+            market_metadata=sample_metadata,
+            volume_impact=0.05,
+            book_impact=0.10,
+            is_niche_market=True,
+            confidence=0.75,
+            factors={"volume_impact": 0.4, "niche_multiplier": 1.5},
+        )
+
+        assert signal.trade_event == sample_trade
+        assert signal.market_metadata == sample_metadata
+        assert signal.volume_impact == 0.05
+        assert signal.book_impact == 0.10
+        assert signal.is_niche_market is True
+        assert signal.confidence == 0.75
+        assert "volume_impact" in signal.factors
+
+    def test_wallet_address_property(
+        self, sample_trade: TradeEvent, sample_metadata: MarketMetadata
+    ) -> None:
+        """Test wallet_address property."""
+        signal = SizeAnomalySignal(
+            trade_event=sample_trade,
+            market_metadata=sample_metadata,
+            volume_impact=0.05,
+            book_impact=0.10,
+            is_niche_market=False,
+            confidence=0.5,
+            factors={},
+        )
+
+        assert signal.wallet_address == sample_trade.wallet_address
+
+    def test_market_id_property(
+        self, sample_trade: TradeEvent, sample_metadata: MarketMetadata
+    ) -> None:
+        """Test market_id property."""
+        signal = SizeAnomalySignal(
+            trade_event=sample_trade,
+            market_metadata=sample_metadata,
+            volume_impact=0.05,
+            book_impact=0.10,
+            is_niche_market=False,
+            confidence=0.5,
+            factors={},
+        )
+
+        assert signal.market_id == sample_trade.market_id
+
+    def test_trade_size_usdc_property(
+        self, sample_trade: TradeEvent, sample_metadata: MarketMetadata
+    ) -> None:
+        """Test trade_size_usdc property returns notional value."""
+        signal = SizeAnomalySignal(
+            trade_event=sample_trade,
+            market_metadata=sample_metadata,
+            volume_impact=0.05,
+            book_impact=0.10,
+            is_niche_market=False,
+            confidence=0.5,
+            factors={},
+        )
+
+        # notional = price * size = 0.65 * 10000 = 6500
+        assert signal.trade_size_usdc == Decimal("6500.00")
+
+    def test_is_high_confidence(
+        self, sample_trade: TradeEvent, sample_metadata: MarketMetadata
+    ) -> None:
+        """Test is_high_confidence threshold."""
+        high_signal = SizeAnomalySignal(
+            trade_event=sample_trade,
+            market_metadata=sample_metadata,
+            volume_impact=0.05,
+            book_impact=0.10,
+            is_niche_market=False,
+            confidence=0.70,
+            factors={},
+        )
+        low_signal = SizeAnomalySignal(
+            trade_event=sample_trade,
+            market_metadata=sample_metadata,
+            volume_impact=0.05,
+            book_impact=0.10,
+            is_niche_market=False,
+            confidence=0.69,
+            factors={},
+        )
+
+        assert high_signal.is_high_confidence is True
+        assert low_signal.is_high_confidence is False
+
+    def test_is_very_high_confidence(
+        self, sample_trade: TradeEvent, sample_metadata: MarketMetadata
+    ) -> None:
+        """Test is_very_high_confidence threshold."""
+        very_high = SizeAnomalySignal(
+            trade_event=sample_trade,
+            market_metadata=sample_metadata,
+            volume_impact=0.05,
+            book_impact=0.10,
+            is_niche_market=False,
+            confidence=0.85,
+            factors={},
+        )
+        high = SizeAnomalySignal(
+            trade_event=sample_trade,
+            market_metadata=sample_metadata,
+            volume_impact=0.05,
+            book_impact=0.10,
+            is_niche_market=False,
+            confidence=0.84,
+            factors={},
+        )
+
+        assert very_high.is_very_high_confidence is True
+        assert high.is_very_high_confidence is False
+
+    def test_to_dict_serialization(
+        self, sample_trade: TradeEvent, sample_metadata: MarketMetadata
+    ) -> None:
+        """Test to_dict produces valid serialization."""
+        signal = SizeAnomalySignal(
+            trade_event=sample_trade,
+            market_metadata=sample_metadata,
+            volume_impact=0.05,
+            book_impact=0.10,
+            is_niche_market=True,
+            confidence=0.75,
+            factors={"volume_impact": 0.5},
+        )
+
+        result = signal.to_dict()
+
+        assert result["wallet_address"] == sample_trade.wallet_address
+        assert result["market_id"] == sample_trade.market_id
+        assert result["trade_id"] == sample_trade.trade_id
+        assert result["trade_size"] == "6500.00"
+        assert result["trade_side"] == "BUY"
+        assert result["market_category"] == "science"
+        assert result["volume_impact"] == 0.05
+        assert result["book_impact"] == 0.10
+        assert result["is_niche_market"] is True
+        assert result["confidence"] == 0.75
+        assert result["factors"] == {"volume_impact": 0.5}
+        assert "timestamp" in result
+
+
+# ============================================================================
+# SizeAnomalyDetector Initialization Tests
+# ============================================================================
+
+
+class TestSizeAnomalyDetectorInit:
+    """Tests for SizeAnomalyDetector initialization."""
+
+    def test_default_initialization(self, mock_metadata_sync: AsyncMock) -> None:
+        """Test detector initializes with default values."""
+        detector = SizeAnomalyDetector(mock_metadata_sync)
+
+        assert detector._volume_threshold == DEFAULT_VOLUME_THRESHOLD
+        assert detector._book_threshold == DEFAULT_BOOK_THRESHOLD
+        assert detector._niche_volume_threshold == DEFAULT_NICHE_VOLUME_THRESHOLD
+
+    def test_custom_thresholds(self, mock_metadata_sync: AsyncMock) -> None:
+        """Test detector with custom thresholds."""
+        detector = SizeAnomalyDetector(
+            mock_metadata_sync,
+            volume_threshold=0.05,
+            book_threshold=0.10,
+            niche_volume_threshold=Decimal("100000"),
+        )
+
+        assert detector._volume_threshold == 0.05
+        assert detector._book_threshold == 0.10
+        assert detector._niche_volume_threshold == Decimal("100000")
+
+
+# ============================================================================
+# Volume Impact Tests
+# ============================================================================
+
+
+class TestVolumeImpactCalculation:
+    """Tests for volume impact calculation."""
+
+    def test_volume_impact_calculation(self, mock_metadata_sync: AsyncMock) -> None:
+        """Test correct volume impact calculation."""
+        detector = SizeAnomalyDetector(mock_metadata_sync)
+
+        # Trade size $1000, daily volume $50000 = 2% impact
+        impact = detector._calculate_volume_impact(
+            Decimal("1000"), Decimal("50000")
+        )
+        assert impact == pytest.approx(0.02)
+
+    def test_volume_impact_none_volume(self, mock_metadata_sync: AsyncMock) -> None:
+        """Test volume impact returns 0 when volume is None."""
+        detector = SizeAnomalyDetector(mock_metadata_sync)
+
+        impact = detector._calculate_volume_impact(Decimal("1000"), None)
+        assert impact == 0.0
+
+    def test_volume_impact_zero_volume(self, mock_metadata_sync: AsyncMock) -> None:
+        """Test volume impact returns 0 when volume is zero."""
+        detector = SizeAnomalyDetector(mock_metadata_sync)
+
+        impact = detector._calculate_volume_impact(Decimal("1000"), Decimal("0"))
+        assert impact == 0.0
+
+    def test_volume_impact_negative_volume(
+        self, mock_metadata_sync: AsyncMock
+    ) -> None:
+        """Test volume impact returns 0 when volume is negative."""
+        detector = SizeAnomalyDetector(mock_metadata_sync)
+
+        impact = detector._calculate_volume_impact(Decimal("1000"), Decimal("-1000"))
+        assert impact == 0.0
+
+
+# ============================================================================
+# Book Impact Tests
+# ============================================================================
+
+
+class TestBookImpactCalculation:
+    """Tests for order book impact calculation."""
+
+    def test_book_impact_calculation(self, mock_metadata_sync: AsyncMock) -> None:
+        """Test correct book impact calculation."""
+        detector = SizeAnomalyDetector(mock_metadata_sync)
+
+        # Trade size $5000, book depth $50000 = 10% impact
+        impact = detector._calculate_book_impact(Decimal("5000"), Decimal("50000"))
+        assert impact == pytest.approx(0.10)
+
+    def test_book_impact_none_depth(self, mock_metadata_sync: AsyncMock) -> None:
+        """Test book impact returns 0 when depth is None."""
+        detector = SizeAnomalyDetector(mock_metadata_sync)
+
+        impact = detector._calculate_book_impact(Decimal("5000"), None)
+        assert impact == 0.0
+
+    def test_book_impact_zero_depth(self, mock_metadata_sync: AsyncMock) -> None:
+        """Test book impact returns 0 when depth is zero."""
+        detector = SizeAnomalyDetector(mock_metadata_sync)
+
+        impact = detector._calculate_book_impact(Decimal("5000"), Decimal("0"))
+        assert impact == 0.0
+
+
+# ============================================================================
+# Niche Market Detection Tests
+# ============================================================================
+
+
+class TestNicheMarketDetection:
+    """Tests for niche market detection."""
+
+    def test_niche_market_low_volume(
+        self, mock_metadata_sync: AsyncMock, sample_metadata: MarketMetadata
+    ) -> None:
+        """Test market is niche when volume below threshold."""
+        detector = SizeAnomalyDetector(mock_metadata_sync)
+
+        # Volume $40k < $50k threshold
+        is_niche = detector._is_niche_market(sample_metadata, Decimal("40000"))
+        assert is_niche is True
+
+    def test_not_niche_high_volume(
+        self, mock_metadata_sync: AsyncMock, sample_metadata: MarketMetadata
+    ) -> None:
+        """Test market is not niche when volume above threshold."""
+        detector = SizeAnomalyDetector(mock_metadata_sync)
+
+        # Volume $100k > $50k threshold
+        is_niche = detector._is_niche_market(sample_metadata, Decimal("100000"))
+        assert is_niche is False
+
+    def test_niche_market_unknown_volume_niche_category(
+        self, mock_metadata_sync: AsyncMock, sample_token: Token
+    ) -> None:
+        """Test market is niche when volume unknown and category is niche-prone."""
+        detector = SizeAnomalyDetector(mock_metadata_sync)
+
+        for category in NICHE_PRONE_CATEGORIES:
+            metadata = MarketMetadata(
+                condition_id="test",
+                question="Test",
+                description="",
+                tokens=(sample_token,),
+                category=category,
+            )
+            is_niche = detector._is_niche_market(metadata, None)
+            assert is_niche is True, f"Category {category} should be niche"
+
+    def test_not_niche_unknown_volume_mainstream_category(
+        self, mock_metadata_sync: AsyncMock, sample_token: Token
+    ) -> None:
+        """Test market is not niche when volume unknown but category is mainstream."""
+        detector = SizeAnomalyDetector(mock_metadata_sync)
+
+        mainstream_categories = ["politics", "sports", "crypto", "entertainment"]
+        for category in mainstream_categories:
+            metadata = MarketMetadata(
+                condition_id="test",
+                question="Test",
+                description="",
+                tokens=(sample_token,),
+                category=category,
+            )
+            is_niche = detector._is_niche_market(metadata, None)
+            assert is_niche is False, f"Category {category} should not be niche"
+
+
+# ============================================================================
+# Confidence Scoring Tests
+# ============================================================================
+
+
+class TestConfidenceScoring:
+    """Tests for confidence score calculation."""
+
+    def test_confidence_volume_impact_only(
+        self, mock_metadata_sync: AsyncMock
+    ) -> None:
+        """Test confidence with only volume impact."""
+        detector = SizeAnomalyDetector(mock_metadata_sync)
+
+        # Volume impact 3x threshold = max score 0.5
+        confidence, factors = detector.calculate_confidence(
+            volume_impact=0.06,  # 3x the 0.02 threshold
+            book_impact=0.0,
+            is_niche=False,
+        )
+
+        assert confidence == pytest.approx(0.5)
+        assert "volume_impact" in factors
+        assert factors["volume_impact"] == pytest.approx(0.5)
+
+    def test_confidence_book_impact_only(self, mock_metadata_sync: AsyncMock) -> None:
+        """Test confidence with only book impact."""
+        detector = SizeAnomalyDetector(mock_metadata_sync)
+
+        # Book impact 3x threshold = max score 0.3
+        confidence, factors = detector.calculate_confidence(
+            volume_impact=0.0,
+            book_impact=0.15,  # 3x the 0.05 threshold
+            is_niche=False,
+        )
+
+        assert confidence == pytest.approx(0.3)
+        assert "book_impact" in factors
+        assert factors["book_impact"] == pytest.approx(0.3)
+
+    def test_confidence_combined_impacts(self, mock_metadata_sync: AsyncMock) -> None:
+        """Test confidence with both volume and book impact."""
+        detector = SizeAnomalyDetector(mock_metadata_sync)
+
+        # Both at 3x threshold = 0.5 + 0.3 = 0.8
+        confidence, factors = detector.calculate_confidence(
+            volume_impact=0.06,
+            book_impact=0.15,
+            is_niche=False,
+        )
+
+        assert confidence == pytest.approx(0.8)
+        assert "volume_impact" in factors
+        assert "book_impact" in factors
+
+    def test_confidence_niche_multiplier(self, mock_metadata_sync: AsyncMock) -> None:
+        """Test niche multiplier increases confidence."""
+        detector = SizeAnomalyDetector(mock_metadata_sync)
+
+        # Volume impact 2x threshold = 0.33, with 1.5x niche = 0.5
+        confidence, factors = detector.calculate_confidence(
+            volume_impact=0.04,  # 2x threshold
+            book_impact=0.0,
+            is_niche=True,
+        )
+
+        assert confidence == pytest.approx(0.5, rel=0.01)
+        assert "niche_multiplier" in factors
+        assert factors["niche_multiplier"] == 1.5
+
+    def test_confidence_niche_only_base(self, mock_metadata_sync: AsyncMock) -> None:
+        """Test niche market with no other signals gives base confidence."""
+        detector = SizeAnomalyDetector(mock_metadata_sync)
+
+        # No threshold exceeded, but is niche
+        confidence, factors = detector.calculate_confidence(
+            volume_impact=0.01,  # Below 0.02 threshold
+            book_impact=0.01,  # Below 0.05 threshold
+            is_niche=True,
+        )
+
+        assert confidence == 0.2
+        assert "niche_base" in factors
+        assert factors["niche_base"] == 0.2
+
+    def test_confidence_clamped_to_max(self, mock_metadata_sync: AsyncMock) -> None:
+        """Test confidence is clamped to 1.0."""
+        detector = SizeAnomalyDetector(mock_metadata_sync)
+
+        # High impacts with niche multiplier would exceed 1.0
+        confidence, factors = detector.calculate_confidence(
+            volume_impact=0.10,  # 5x threshold (capped at 3x)
+            book_impact=0.20,  # 4x threshold (capped at 3x)
+            is_niche=True,  # 1.5x multiplier
+        )
+
+        assert confidence == 1.0
+
+    def test_confidence_zero_no_signals(self, mock_metadata_sync: AsyncMock) -> None:
+        """Test confidence is zero with no signals."""
+        detector = SizeAnomalyDetector(mock_metadata_sync)
+
+        confidence, factors = detector.calculate_confidence(
+            volume_impact=0.01,  # Below threshold
+            book_impact=0.01,  # Below threshold
+            is_niche=False,
+        )
+
+        assert confidence == 0.0
+        assert len(factors) == 0
+
+
+# ============================================================================
+# Analyze Method Tests
+# ============================================================================
+
+
+class TestAnalyzeMethod:
+    """Tests for the analyze method."""
+
+    @pytest.mark.asyncio
+    async def test_analyze_high_volume_impact(
+        self,
+        mock_metadata_sync: AsyncMock,
+        sample_trade: TradeEvent,
+        sample_metadata: MarketMetadata,
+    ) -> None:
+        """Test analyze detects high volume impact trade."""
+        mock_metadata_sync.get_market.return_value = sample_metadata
+        detector = SizeAnomalyDetector(mock_metadata_sync)
+
+        # Trade notional = 6500, volume = 65000, impact = 10% > 2% threshold
+        signal = await detector.analyze(
+            sample_trade,
+            daily_volume=Decimal("65000"),
+        )
+
+        assert signal is not None
+        assert signal.volume_impact == pytest.approx(0.10)
+        assert signal.confidence > 0.1
+
+    @pytest.mark.asyncio
+    async def test_analyze_high_book_impact(
+        self,
+        mock_metadata_sync: AsyncMock,
+        sample_trade: TradeEvent,
+        sample_metadata: MarketMetadata,
+    ) -> None:
+        """Test analyze detects high book impact trade."""
+        mock_metadata_sync.get_market.return_value = sample_metadata
+        detector = SizeAnomalyDetector(mock_metadata_sync)
+
+        # Trade notional = 6500, book depth = 32500, impact = 20% > 5% threshold
+        signal = await detector.analyze(
+            sample_trade,
+            book_depth=Decimal("32500"),
+        )
+
+        assert signal is not None
+        assert signal.book_impact == pytest.approx(0.20)
+        assert signal.confidence > 0.1
+
+    @pytest.mark.asyncio
+    async def test_analyze_niche_market(
+        self,
+        mock_metadata_sync: AsyncMock,
+        sample_trade: TradeEvent,
+        sample_metadata: MarketMetadata,
+    ) -> None:
+        """Test analyze detects niche market trade."""
+        mock_metadata_sync.get_market.return_value = sample_metadata
+        detector = SizeAnomalyDetector(mock_metadata_sync)
+
+        # Low volume market (science category with volume unknown)
+        signal = await detector.analyze(sample_trade)
+
+        assert signal is not None
+        assert signal.is_niche_market is True
+        assert signal.confidence == 0.2  # niche_base
+
+    @pytest.mark.asyncio
+    async def test_analyze_no_anomaly(
+        self,
+        mock_metadata_sync: AsyncMock,
+        sample_token: Token,
+    ) -> None:
+        """Test analyze returns None for normal trade."""
+        # Politics category is not niche
+        metadata = MarketMetadata(
+            condition_id="market_politics",
+            question="Will Biden win?",
+            description="",
+            tokens=(sample_token,),
+            category="politics",
+        )
+        mock_metadata_sync.get_market.return_value = metadata
+
+        trade = TradeEvent(
+            market_id="market_politics",
+            trade_id="tx_normal",
+            wallet_address="0xnormal",
+            side="BUY",
+            outcome="Yes",
+            outcome_index=0,
+            price=Decimal("0.50"),
+            size=Decimal("100"),  # Small trade = $50 notional
+            timestamp=datetime.now(UTC),
+            asset_id="token_pol",
+        )
+
+        detector = SizeAnomalyDetector(mock_metadata_sync)
+
+        # High volume, large book depth = low impact
+        signal = await detector.analyze(
+            trade,
+            daily_volume=Decimal("1000000"),
+            book_depth=Decimal("500000"),
+        )
+
+        assert signal is None
+
+    @pytest.mark.asyncio
+    async def test_analyze_creates_minimal_metadata_on_missing(
+        self,
+        mock_metadata_sync: AsyncMock,
+        sample_trade: TradeEvent,
+    ) -> None:
+        """Test analyze creates minimal metadata when market not found."""
+        mock_metadata_sync.get_market.return_value = None
+        detector = SizeAnomalyDetector(mock_metadata_sync)
+
+        # Should still work with minimal metadata (category="other" which is niche)
+        signal = await detector.analyze(sample_trade)
+
+        assert signal is not None
+        assert signal.market_metadata.condition_id == sample_trade.market_id
+        assert signal.market_metadata.category == "other"
+
+    @pytest.mark.asyncio
+    async def test_analyze_handles_metadata_exception(
+        self,
+        mock_metadata_sync: AsyncMock,
+        sample_trade: TradeEvent,
+    ) -> None:
+        """Test analyze handles exception when fetching metadata."""
+        mock_metadata_sync.get_market.side_effect = Exception("Redis error")
+        detector = SizeAnomalyDetector(mock_metadata_sync)
+
+        # Should still work with minimal metadata
+        signal = await detector.analyze(sample_trade)
+
+        assert signal is not None
+        assert signal.market_metadata.category == "other"
+
+    @pytest.mark.asyncio
+    async def test_analyze_low_confidence_filtered(
+        self,
+        mock_metadata_sync: AsyncMock,
+        sample_token: Token,
+    ) -> None:
+        """Test analyze returns None when confidence is below 0.1."""
+        # Use a mainstream category with below-threshold impacts
+        metadata = MarketMetadata(
+            condition_id="market_sports",
+            question="Super Bowl winner?",
+            description="",
+            tokens=(sample_token,),
+            category="sports",
+        )
+        mock_metadata_sync.get_market.return_value = metadata
+
+        trade = TradeEvent(
+            market_id="market_sports",
+            trade_id="tx_small",
+            wallet_address="0xsmall",
+            side="BUY",
+            outcome="Yes",
+            outcome_index=0,
+            price=Decimal("0.50"),
+            size=Decimal("10"),  # Tiny trade
+            timestamp=datetime.now(UTC),
+            asset_id="token_sports",
+        )
+
+        detector = SizeAnomalyDetector(mock_metadata_sync)
+
+        # High volume but below threshold impacts
+        signal = await detector.analyze(
+            trade,
+            daily_volume=Decimal("10000000"),  # $10M volume
+            book_depth=Decimal("1000000"),  # $1M depth
+        )
+
+        assert signal is None
+
+
+# ============================================================================
+# Batch Analysis Tests
+# ============================================================================
+
+
+class TestBatchAnalysis:
+    """Tests for batch analysis."""
+
+    @pytest.mark.asyncio
+    async def test_analyze_batch_returns_signals(
+        self,
+        mock_metadata_sync: AsyncMock,
+        sample_metadata: MarketMetadata,
+    ) -> None:
+        """Test batch analysis returns signals for anomalous trades."""
+        mock_metadata_sync.get_market.return_value = sample_metadata
+
+        trades = [
+            TradeEvent(
+                market_id="market_abc123",
+                trade_id=f"tx_{i}",
+                wallet_address=f"0xwallet{i}",
+                side="BUY",
+                outcome="Yes",
+                outcome_index=0,
+                price=Decimal("0.50"),
+                size=Decimal("10000"),  # Large trade
+                timestamp=datetime.now(UTC),
+                asset_id="token_123",
+            )
+            for i in range(3)
+        ]
+
+        detector = SizeAnomalyDetector(mock_metadata_sync)
+        signals = await detector.analyze_batch(trades)
+
+        # All trades are in niche category with unknown volume
+        assert len(signals) == 3
+
+    @pytest.mark.asyncio
+    async def test_analyze_batch_with_volume_data(
+        self,
+        mock_metadata_sync: AsyncMock,
+        sample_metadata: MarketMetadata,
+    ) -> None:
+        """Test batch analysis uses provided volume data."""
+        mock_metadata_sync.get_market.return_value = sample_metadata
+
+        trades = [
+            TradeEvent(
+                market_id="market_abc123",
+                trade_id="tx_1",
+                wallet_address="0xwallet1",
+                side="BUY",
+                outcome="Yes",
+                outcome_index=0,
+                price=Decimal("0.50"),
+                size=Decimal("10000"),  # $5000 notional
+                timestamp=datetime.now(UTC),
+                asset_id="token_123",
+            )
+        ]
+
+        detector = SizeAnomalyDetector(mock_metadata_sync)
+
+        # $5000 trade / $50000 volume = 10% impact
+        signals = await detector.analyze_batch(
+            trades,
+            volume_data={"market_abc123": Decimal("50000")},
+        )
+
+        assert len(signals) == 1
+        assert signals[0].volume_impact == pytest.approx(0.10)
+
+    @pytest.mark.asyncio
+    async def test_analyze_batch_handles_errors(
+        self,
+        mock_metadata_sync: AsyncMock,
+    ) -> None:
+        """Test batch analysis handles individual trade errors."""
+        # First call succeeds, second fails
+        mock_metadata_sync.get_market.side_effect = [
+            Exception("Error"),
+            None,
+        ]
+
+        trades = [
+            TradeEvent(
+                market_id=f"market_{i}",
+                trade_id=f"tx_{i}",
+                wallet_address=f"0xwallet{i}",
+                side="BUY",
+                outcome="Yes",
+                outcome_index=0,
+                price=Decimal("0.50"),
+                size=Decimal("10000"),
+                timestamp=datetime.now(UTC),
+                asset_id="token_123",
+            )
+            for i in range(2)
+        ]
+
+        detector = SizeAnomalyDetector(mock_metadata_sync)
+        signals = await detector.analyze_batch(trades)
+
+        # Both should still produce signals (with minimal metadata fallback)
+        assert len(signals) == 2
+
+    @pytest.mark.asyncio
+    async def test_analyze_batch_empty_list(
+        self, mock_metadata_sync: AsyncMock
+    ) -> None:
+        """Test batch analysis with empty list."""
+        detector = SizeAnomalyDetector(mock_metadata_sync)
+        signals = await detector.analyze_batch([])
+
+        assert signals == []