diff --git a/src/app/api/diag-supplier/route.ts b/src/app/api/diag-supplier/route.ts
new file mode 100644
index 0000000..d3e7a6c
--- /dev/null
+++ b/src/app/api/diag-supplier/route.ts
@@ -0,0 +1,130 @@
+import { NextRequest, NextResponse } from "next/server";
+import { db } from "@/db";
+import { sql } from "drizzle-orm";
+
+/**
+ * Diagnostic endpoint for supplier attribution investigation.
+ * Usage:
+ *   /api/diag-supplier                      → top suppliers by CA (current month) both methods
+ *   /api/diag-supplier?code=BAZAR5000       → inspect artfou1 entries for that supplier's articles
+ */
+export async function GET(req: NextRequest) {
+    const { searchParams } = new URL(req.url);
+    const supplierCode = searchParams.get("code");
+
+    const mois = "2026-03";
+    const moisN1 = "2025-03";
+
+    try {
+        if (supplierCode) {
+            // Mode inspection : montrer les artfou1 entrées pour ce fournisseur
+            const artfou1Rows = await db.execute(sql`
+                SELECT af.art_no_id, af.no_id, af.code AS codefou, a.codein,
+                       f.raisonsociale AS nom_fournisseur
+                FROM artfou1 af
+                JOIN articles a ON a.no_id = af.art_no_id
+                LEFT JOIN fouadr1 f ON f.code = af.code AND f.sit_code = '000'
+                WHERE af.code = ${supplierCode}
+                ORDER BY af.art_no_id, af.no_id DESC
+                LIMIT 20
+            `);
+
+            // Pour ces articles, voir si d'autres fournisseurs existent dans artfou1
+            const multiSupplierArticles = await db.execute(sql`
+                SELECT a.codein, af.code AS codefou, af.no_id,
+                       f.raisonsociale AS nom,
+                       ROW_NUMBER() OVER (PARTITION BY af.art_no_id ORDER BY af.no_id DESC) AS rn
+                FROM artfou1 af
+                JOIN articles a ON a.no_id = af.art_no_id
+                LEFT JOIN fouadr1 f ON f.code = af.code AND f.sit_code = '000'
+                WHERE af.art_no_id IN (
+                    SELECT art_no_id FROM artfou1 WHERE code = ${supplierCode}
+                )
+                ORDER BY a.codein, af.no_id DESC
+                LIMIT 50
+            `);
+
+            return NextResponse.json({
+                mode: "inspect",
+                supplierCode,
+                artfou1Sample: artfou1Rows.rows,
+                multiSupplierArticles: multiSupplierArticles.rows,
+            });
+        }
+
+        // Mode comparaison : ancien (ASC) vs nouveau (DESC) pour le mois courant
+        const withAsc = await db.execute(sql`
+            WITH art_fou AS (
+                SELECT DISTINCT ON (art_no_id) art_no_id, code AS codefou
+                FROM artfou1
+                ORDER BY art_no_id, no_id ASC
+            )
+            SELECT af.codefou AS code, MAX(f.raisonsociale)::text AS nom,
+                   SUM(-m.mntmvtttc)::float AS ca_ttc
+            FROM mvtart m
+            JOIN articles a ON a.no_id = m.artnoid
+            JOIN art_fou af ON af.art_no_id = a.no_id
+            JOIN fouadr1 f ON f.code = af.codefou AND f.sit_code = '000'
+            WHERE TO_CHAR(m.datmvt, 'YYYY-MM') = ${mois}
+              AND m.site IN ('292', '579')
+              AND m.genremvt = 3
+            GROUP BY af.codefou
+            ORDER BY ca_ttc DESC
+            LIMIT 15
+        `);
+
+        const withDesc = await db.execute(sql`
+            WITH art_fou AS (
+                SELECT DISTINCT ON (art_no_id) art_no_id, code AS codefou
+                FROM artfou1
+                ORDER BY art_no_id, no_id DESC
+            )
+            SELECT af.codefou AS code, MAX(f.raisonsociale)::text AS nom,
+                   SUM(-m.mntmvtttc)::float AS ca_ttc
+            FROM mvtart m
+            JOIN articles a ON a.no_id = m.artnoid
+            JOIN art_fou af ON af.art_no_id = a.no_id
+            JOIN fouadr1 f ON f.code = af.codefou AND f.sit_code = '000'
+            WHERE TO_CHAR(m.datmvt, 'YYYY-MM') = ${mois}
+              AND m.site IN ('292', '579')
+              AND m.genremvt = 3
+            GROUP BY af.codefou
+            ORDER BY ca_ttc DESC
+            LIMIT 15
+        `);
+
+        // Combien d'articles ont plusieurs fournisseurs dans artfou1 ?
+        const multiStats = await db.execute(sql`
+            SELECT COUNT(*) AS articles_multi_fournisseur,
+                   MAX(cnt) AS max_fournisseurs_par_article
+            FROM (
+                SELECT art_no_id, COUNT(*) AS cnt
+                FROM artfou1
+                GROUP BY art_no_id
+                HAVING COUNT(*) > 1
+            ) t
+        `);
+
+        // Chercher BAZAR 5000 spécifiquement
+        const bazarCode = await db.execute(sql`
+            SELECT DISTINCT af.code, f.raisonsociale
+            FROM artfou1 af
+            JOIN fouadr1 f ON f.code = af.code AND f.sit_code = '000'
+            WHERE f.raisonsociale ILIKE '%bazar%'
+            LIMIT 5
+        `);
+
+        return NextResponse.json({
+            mode: "compare",
+            mois,
+            top15_method_ASC_oldest: withAsc.rows,
+            top15_method_DESC_newest: withDesc.rows,
+            multi_supplier_stats: multiStats.rows,
+            bazar_supplier_codes: bazarCode.rows,
+            hint: "Add ?code=BAZAR5000_CODE to inspect specific supplier's articles",
+        });
+
+    } catch (e) {
+        return NextResponse.json({ error: String(e) }, { status: 500 });
+    }
+}