diff --git a/crates/ml-alpha/examples/alpha_gate.rs b/crates/ml-alpha/examples/alpha_gate.rs
new file mode 100644
index 000000000..ed18ca033
--- /dev/null
+++ b/crates/ml-alpha/examples/alpha_gate.rs
@@ -0,0 +1,94 @@
+//! Validation gate runner — reads two `alpha_train_summary.json`
+//! artifacts (one CfC, one Mamba2 baseline), emits `phase_a_gate.json`,
+//! exits 0 on PASS, 1 on FAIL.
+//!
+//! Both summaries must be generated against the SAME data window
+//! (train + val quarters identical) for a meaningful comparison.
+//! Run via:
+//!   alpha_gate \
+//!     --cfc-summary    artifacts/cfc/alpha_train_summary.json \
+//!     --mamba2-summary artifacts/mamba2/alpha_train_summary.json \
+//!     --tolerance      0.01 \
+//!     --out            artifacts/phase_a_gate.json
+
+use anyhow::{Context, Result};
+use clap::Parser;
+use ml_alpha::gate::cfc_vs_mamba2::{
+    gate_verdict, load_summary, GateReport, GateVerdict, HORIZONS,
+};
+use serde::Serialize;
+use std::path::PathBuf;
+
+#[derive(Parser)]
+#[command(name = "alpha_gate")]
+struct Cli {
+    #[arg(long)]
+    cfc_summary: PathBuf,
+
+    #[arg(long)]
+    mamba2_summary: PathBuf,
+
+    #[arg(long, default_value_t = 0.01)]
+    tolerance: f32,
+
+    #[arg(long)]
+    out: PathBuf,
+}
+
+#[derive(Serialize)]
+struct GateOutput {
+    report: GateReport,
+    verdict: GateVerdict,
+}
+
+fn main() -> Result<()> {
+    tracing_subscriber::fmt::init();
+    let cli = Cli::parse();
+
+    let cfc = load_summary(&cli.cfc_summary).context("load CfC summary")?;
+    let mamba2 = load_summary(&cli.mamba2_summary).context("load Mamba2 summary")?;
+
+    anyhow::ensure!(
+        cfc.horizons == mamba2.horizons,
+        "horizon mismatch: CfC={:?}, Mamba2={:?}",
+        cfc.horizons,
+        mamba2.horizons
+    );
+
+    let report = GateReport {
+        horizons: HORIZONS,
+        cfc_auc: cfc.final_val_auc,
+        mamba2_auc: mamba2.final_val_auc,
+        tolerance: cli.tolerance,
+    };
+    let verdict = gate_verdict(&report);
+
+    if let Some(parent) = cli.out.parent() {
+        std::fs::create_dir_all(parent).with_context(|| format!("mkdir {}", parent.display()))?;
+    }
+    let output = GateOutput { report: report.clone(), verdict: verdict.clone() };
+    std::fs::write(
+        &cli.out,
+        serde_json::to_vec_pretty(&output).context("serialize gate output")?,
+    )
+    .with_context(|| format!("write {}", cli.out.display()))?;
+
+    if verdict.pass {
+        tracing::info!(
+            cfc_auc = ?report.cfc_auc,
+            mamba2_auc = ?report.mamba2_auc,
+            deltas = ?verdict.deltas,
+            "CfC-vs-Mamba2 gate: PASS"
+        );
+        Ok(())
+    } else {
+        tracing::error!(
+            failing_horizons = ?verdict.failing_horizons,
+            cfc_auc = ?report.cfc_auc,
+            mamba2_auc = ?report.mamba2_auc,
+            deltas = ?verdict.deltas,
+            "CfC-vs-Mamba2 gate: FAIL"
+        );
+        std::process::exit(1);
+    }
+}
diff --git a/crates/ml-alpha/src/gate/cfc_vs_mamba2.rs b/crates/ml-alpha/src/gate/cfc_vs_mamba2.rs
new file mode 100644
index 000000000..b6920da8e
--- /dev/null
+++ b/crates/ml-alpha/src/gate/cfc_vs_mamba2.rs
@@ -0,0 +1,143 @@
+//! CfC-vs-Mamba2 validation gate logic.
+//!
+//! Per spec Section 4 (with 2026-05-16 stacked amendment):
+//! "stacked AUC ≥ Mamba2-only baseline AUC at every horizon"
+//!
+//! This module owns the verdict semantics. The binary
+//! `examples/alpha_gate.rs` orchestrates running both trainers and
+//! consuming this module's verdict.
+
+use anyhow::{Context, Result};
+use serde::{Deserialize, Serialize};
+use std::path::Path;
+
+pub const HORIZONS: [usize; 5] = [30, 100, 300, 1000, 6000];
+
+/// Summary emitted by `alpha_train` (one per backbone).
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct AlphaTrainSummary {
+    pub epochs: usize,
+    pub n_hid: usize,
+    pub seq_len: usize,
+    pub horizons: [usize; 5],
+    pub final_train_loss: f32,
+    pub final_val_loss: f32,
+    pub final_val_auc: [f32; 5],
+    pub n_train_seqs_consumed: usize,
+    pub n_val_seqs_consumed: usize,
+}
+
+pub fn load_summary(path: &Path) -> Result<AlphaTrainSummary> {
+    let bytes = std::fs::read(path).with_context(|| format!("read {}", path.display()))?;
+    serde_json::from_slice(&bytes).with_context(|| format!("parse {}", path.display()))
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct GateReport {
+    pub horizons: [usize; 5],
+    pub cfc_auc: [f32; 5],
+    pub mamba2_auc: [f32; 5],
+    pub tolerance: f32,
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct GateVerdict {
+    pub pass: bool,
+    pub failing_horizons: Vec<usize>,
+    pub deltas: [f32; 5],
+}
+
+impl GateVerdict {
+    pub fn is_pass(&self) -> bool { self.pass }
+    pub fn failing_horizons(&self) -> Vec<usize> { self.failing_horizons.clone() }
+}
+
+/// Verdict: CfC AUC must be ≥ Mamba2 AUC − tolerance at every horizon.
+pub fn gate_verdict(r: &GateReport) -> GateVerdict {
+    let mut deltas = [0f32; 5];
+    let mut failing = Vec::new();
+    let mut pass = true;
+    for i in 0..5 {
+        let delta = r.cfc_auc[i] - r.mamba2_auc[i];
+        deltas[i] = delta;
+        if delta < -r.tolerance {
+            pass = false;
+            failing.push(r.horizons[i]);
+        }
+    }
+    GateVerdict { pass, failing_horizons: failing, deltas }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn gate_passes_when_cfc_meets_or_exceeds_mamba2() {
+        let report = GateReport {
+            horizons: HORIZONS,
+            cfc_auc:    [0.59, 0.62, 0.65, 0.68, 0.71],
+            mamba2_auc: [0.58, 0.61, 0.64, 0.67, 0.70],
+            tolerance: 0.01,
+        };
+        let v = gate_verdict(&report);
+        assert!(v.is_pass());
+        assert!(v.failing_horizons().is_empty());
+    }
+
+    #[test]
+    fn gate_passes_within_tolerance() {
+        // CfC 0.005 below Mamba2 at every horizon, tolerance 0.01 -> PASS.
+        let report = GateReport {
+            horizons: HORIZONS,
+            cfc_auc:    [0.575, 0.605, 0.635, 0.665, 0.695],
+            mamba2_auc: [0.58,  0.61,  0.64,  0.67,  0.70],
+            tolerance: 0.01,
+        };
+        let v = gate_verdict(&report);
+        assert!(v.is_pass(), "CfC within tolerance should PASS");
+    }
+
+    #[test]
+    fn gate_fails_when_cfc_lags_at_long_horizon() {
+        let report = GateReport {
+            horizons: HORIZONS,
+            cfc_auc:    [0.59, 0.62, 0.65, 0.62, 0.63],
+            mamba2_auc: [0.58, 0.61, 0.64, 0.67, 0.70],
+            tolerance: 0.01,
+        };
+        let v = gate_verdict(&report);
+        assert!(!v.is_pass());
+        assert_eq!(v.failing_horizons(), vec![1000, 6000]);
+    }
+
+    #[test]
+    fn gate_fails_when_cfc_lags_at_short_horizon() {
+        let report = GateReport {
+            horizons: HORIZONS,
+            cfc_auc:    [0.50, 0.62, 0.65, 0.68, 0.71],
+            mamba2_auc: [0.58, 0.61, 0.64, 0.67, 0.70],
+            tolerance: 0.01,
+        };
+        let v = gate_verdict(&report);
+        assert!(!v.is_pass());
+        assert_eq!(v.failing_horizons(), vec![30]);
+    }
+
+    #[test]
+    fn deltas_sign_tracks_relative_perf() {
+        let report = GateReport {
+            horizons: HORIZONS,
+            cfc_auc:    [0.70, 0.50, 0.65, 0.60, 0.80],
+            mamba2_auc: [0.60, 0.60, 0.65, 0.70, 0.70],
+            tolerance: 0.01,
+        };
+        let v = gate_verdict(&report);
+        // CfC ahead at h=30, 6000; behind at 100, 1000; tied at 300
+        assert!(v.deltas[0] > 0.0);
+        assert!(v.deltas[1] < 0.0);
+        assert!((v.deltas[2]).abs() < 1e-6);
+        assert!(v.deltas[3] < 0.0);
+        assert!(v.deltas[4] > 0.0);
+    }
+}
diff --git a/crates/ml-alpha/src/gate/mod.rs b/crates/ml-alpha/src/gate/mod.rs
index 4fd0a4399..5bd924ffd 100644
--- a/crates/ml-alpha/src/gate/mod.rs
+++ b/crates/ml-alpha/src/gate/mod.rs
@@ -5,4 +5,4 @@
 //! framing to "stacked AUC ≥ Mamba2-only baseline AUC").
 
 pub mod auc;
-// pub mod cfc_vs_mamba2; — Task 17 will land this
+pub mod cfc_vs_mamba2;