diff --git a/src/multi_swarm/ga/__init__.py b/src/multi_swarm/ga/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/src/multi_swarm/ga/fitness.py b/src/multi_swarm/ga/fitness.py
new file mode 100644
index 0000000..234f085
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+++ b/src/multi_swarm/ga/fitness.py
@@ -0,0 +1,44 @@
+"""Fitness function v0 della Phase 1.
+
+Combina :class:`FalsificationReport` (metriche di robustezza) e
+:class:`AdversarialReport` (findings euristici) in uno scalare ``>= 0`` che il
+GA usa per selezione e ranking.
+
+Logica deliberatamente coarse: DSR penalizzato dal max drawdown, con due
+kill-switch hard (no-trade, finding HIGH adversarial) che azzerano la fitness.
+La penalita' lineare sul drawdown e' un compromesso volutamente semplice;
+versioni successive potranno usare Calmar o utility convessa.
+"""
+
+from __future__ import annotations
+
+from ..agents.adversarial import AdversarialReport, Severity
+from ..agents.falsification import FalsificationReport
+
+
+def compute_fitness(
+    falsification: FalsificationReport,
+    adversarial: AdversarialReport,
+    drawdown_penalty: float = 0.5,
+) -> float:
+    """Calcola la fitness scalare di una strategia.
+
+    Args:
+        falsification: report con DSR, max_drawdown, n_trades.
+        adversarial: report con eventuali findings euristici.
+        drawdown_penalty: peso lineare sul max drawdown (default 0.5).
+
+    Returns:
+        Fitness ``>= 0``. Zero indica strategia da scartare.
+
+    Logica:
+        1. ``n_trades == 0`` → 0 (nessuna evidenza, sega subito).
+        2. Almeno un finding ``HIGH`` adversarial → 0 (kill).
+        3. Altrimenti: ``dsr - drawdown_penalty * max_drawdown``, clamped a 0.
+    """
+    if falsification.n_trades == 0:
+        return 0.0
+    if any(f.severity == Severity.HIGH for f in adversarial.findings):
+        return 0.0
+    raw = falsification.dsr - drawdown_penalty * falsification.max_drawdown
+    return max(0.0, float(raw))
diff --git a/tests/unit/test_fitness.py b/tests/unit/test_fitness.py
new file mode 100644
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+++ b/tests/unit/test_fitness.py
@@ -0,0 +1,45 @@
+from multi_swarm.agents.adversarial import AdversarialReport, Finding, Severity
+from multi_swarm.agents.falsification import FalsificationReport
+from multi_swarm.ga.fitness import compute_fitness
+
+
+def make_falsification(
+    dsr: float = 0.7, max_dd: float = 0.2, n_trades: int = 30
+) -> FalsificationReport:
+    return FalsificationReport(
+        sharpe=1.5,
+        dsr=dsr,
+        dsr_pvalue=0.05,
+        max_drawdown=max_dd,
+        total_return=0.3,
+        n_trades=n_trades,
+        n_bars=500,
+    )
+
+
+def test_fitness_zero_trades_is_zero() -> None:
+    f = make_falsification(n_trades=0)
+    a = AdversarialReport()
+    assert compute_fitness(f, a) == 0.0
+
+
+def test_fitness_increases_with_dsr() -> None:
+    a = AdversarialReport()
+    f1 = make_falsification(dsr=0.5)
+    f2 = make_falsification(dsr=0.9)
+    assert compute_fitness(f2, a) > compute_fitness(f1, a)
+
+
+def test_fitness_decreases_with_drawdown() -> None:
+    a = AdversarialReport()
+    f1 = make_falsification(max_dd=0.1)
+    f2 = make_falsification(max_dd=0.4)
+    assert compute_fitness(f1, a) > compute_fitness(f2, a)
+
+
+def test_fitness_zeroed_by_high_severity_finding() -> None:
+    f = make_falsification()
+    a = AdversarialReport(
+        findings=[Finding(name="degenerate", severity=Severity.HIGH, detail="x")]
+    )
+    assert compute_fitness(f, a) == 0.0