diff --git a/scripts/ch12_diagnostics.py b/scripts/ch12_diagnostics.py
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+++ b/scripts/ch12_diagnostics.py
@@ -0,0 +1,131 @@
+# SPDX-License-Identifier: MIT
+"""
+Chapter 12 — Regression Diagnostics & Influence (R → Python)
+
+Examples:
+  python scripts/ch12_diagnostics.py --save-plots
+  python scripts/ch12_diagnostics.py --formula "mpg ~ weight + model_year + C(cylinders) + horsepower" --save-plots --top-n 10
+"""
+import argparse, os, math
+import numpy as np
+import pandas as pd
+import statsmodels.formula.api as smf
+import statsmodels.api as sm
+from statsmodels.stats.outliers_influence import OLSInfluence, variance_inflation_factor
+import matplotlib
+matplotlib.use("Agg")
+import matplotlib.pyplot as plt
+
+def load_autompg(path: str) -> pd.DataFrame:
+    df = pd.read_csv(path, sep=None, engine="python")
+    # canonicalize names
+    ren = {}
+    for want, opts in {
+        "mpg": ["mpg"],
+        "cylinders": ["cyl","cylinders"],
+        "displacement": ["disp","displacement"],
+        "horsepower": ["hp","horsepower"],
+        "weight": ["wt","weight"],
+        "acceleration": ["acc","acceleration"],
+        "model_year": ["year","model_year"],
+        "origin": ["origin"],
+    }.items():
+        for o in opts:
+            if o in df.columns:
+                ren[o] = want; break
+    df = df.rename(columns=ren)
+    need = ["mpg","cylinders","displacement","horsepower","weight","acceleration","model_year"]
+    if not set(need).issubset(df.columns):
+        raise SystemExit(f"ERROR: missing columns; have {list(df.columns)}")
+    df = df.dropna()
+    df["cylinders"] = df["cylinders"].astype("category")
+    if "origin" in df.columns: df["origin"] = df["origin"].astype("category")
+    return df
+
+def default_formula(df: pd.DataFrame) -> str:
+    base = "mpg ~ weight + model_year + C(cylinders) + horsepower"
+    if "origin" in df.columns:
+        base += " + C(origin)"
+    return base
+
+def plot_resid_vs_fitted(fitted, resid, out):
+    plt.figure(figsize=(6,5))
+    plt.scatter(fitted, resid, s=12, alpha=0.7)
+    plt.axhline(0, color="k", lw=1)
+    plt.xlabel("Fitted")
+    plt.ylabel("Residuals")
+    plt.title("Residuals vs Fitted")
+    plt.tight_layout(); plt.savefig(out, dpi=150)
+
+def plot_qq(resid, out):
+    plt.figure(figsize=(6,5))
+    sm.ProbPlot(resid).qqplot(line="45", alpha=0.7)
+    plt.title("Normal Q-Q"); plt.tight_layout(); plt.savefig(out, dpi=150)
+
+def plot_scale_location(fitted, resid, out):
+    sr = np.sqrt(np.abs(resid))
+    plt.figure(figsize=(6,5))
+    plt.scatter(fitted, sr, s=12, alpha=0.7)
+    plt.xlabel("Fitted"); plt.ylabel("√|Residuals|")
+    plt.title("Scale-Location"); plt.tight_layout(); plt.savefig(out, dpi=150)
+
+def plot_leverage_resid2(hat, studres, cooks, out):
+    plt.figure(figsize=(6,5))
+    sz = 60 * (cooks / (cooks.max() + 1e-9)) + 10
+    plt.scatter(hat, studres, s=sz, alpha=0.6)
+    plt.xlabel("Leverage (hat)")
+    plt.ylabel("Studentized Residuals")
+    plt.title("Residuals vs Leverage (size ~ Cook's D)")
+    plt.tight_layout(); plt.savefig(out, dpi=150)
+
+def compute_vif(model) -> pd.DataFrame:
+    X = model.model.exog
+    cols = model.model.exog_names
+    # Intercept inflates VIF; skip it if present
+    start = 1 if cols[0] == "Intercept" else 0
+    data = []
+    for i in range(start, X.shape[1]):
+        data.append({"term": cols[i], "VIF": variance_inflation_factor(X, i)})
+    return pd.DataFrame(data).sort_values("VIF", ascending=False).reset_index(drop=True)
+
+def main():
+    ap = argparse.ArgumentParser()
+    ap.add_argument("--data", default="data/autompg.csv")
+    ap.add_argument("--formula", default="")
+    ap.add_argument("--save-plots", action="store_true")
+    ap.add_argument("--top-n", type=int, default=5, help="Top N influential rows to print")
+    args = ap.parse_args()
+
+    os.makedirs("outputs", exist_ok=True)
+    df = load_autompg(args.data)
+    formula = args.formula or default_formula(df)
+    print("=== Formula ==="); print(formula)
+
+    model = smf.ols(formula, data=df).fit()
+    print("\n=== Fit Stats ===")
+    print(f"R^2={model.rsquared:.4f}  AdjR^2={model.rsquared_adj:.4f}  AIC={model.aic:.3f}  BIC={model.bic:.3f}")
+
+    infl = OLSInfluence(model)
+    resid = model.resid
+    fitted = model.fittedvalues
+    studres = infl.resid_studentized_external
+    hat = infl.hat_matrix_diag
+    cooks = infl.cooks_distance[0]
+
+    print("\n=== Top Influential (by Cook's D) ===")
+    top = np.argsort(cooks)[::-1][:args.top_n]
+    for idx in top:
+        print(f"row={idx}  CookD={cooks[idx]:.4g}  hat={hat[idx]:.4g}  studres={studres[idx]:.3g}")
+
+    print("\n=== VIF ===")
+    print(compute_vif(model).to_string(index=False))
+
+    if args.save_plots:
+        plot_resid_vs_fitted(fitted, resid, "outputs/ch12_resid_vs_fitted.png")
+        plot_qq(resid, "outputs/ch12_qq.png")
+        plot_scale_location(fitted, resid, "outputs/ch12_scale_location.png")
+        plot_leverage_resid2(hat, studres, cooks, "outputs/ch12_leverage_resid2.png")
+        print("Saved plots -> outputs/ch12_resid_vs_fitted.png, ch12_qq.png, ch12_scale_location.png, ch12_leverage_resid2.png")
+
+if __name__ == "__main__":
+    main()
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