diff --git a/src/model-catalog/index.ts b/src/model-catalog/index.ts
index b8d6680af95..dac4ecbcc0d 100644
--- a/src/model-catalog/index.ts
+++ b/src/model-catalog/index.ts
@@ -10,6 +10,7 @@ export {
 } from "./normalize.js";
 export { planManifestModelCatalogRows } from "./manifest-planner.js";
 export type {
+  ManifestModelCatalogConflict,
   ManifestModelCatalogPlan,
   ManifestModelCatalogPlanEntry,
   ManifestModelCatalogPlugin,
diff --git a/src/model-catalog/manifest-planner.test.ts b/src/model-catalog/manifest-planner.test.ts
index a4490bd97b9..1535c2a484b 100644
--- a/src/model-catalog/manifest-planner.test.ts
+++ b/src/model-catalog/manifest-planner.test.ts
@@ -51,6 +51,7 @@ describe("manifest model catalog planner", () => {
       },
     ]);
     expect(plan.rows.map((row) => row.ref)).toEqual(["moonshot/kimi-k2.6"]);
+    expect(plan.conflicts).toEqual([]);
   });
 
   it("filters providers before row planning", () => {
@@ -84,9 +85,10 @@ describe("manifest model catalog planner", () => {
 
     expect(plan.entries.map((entry) => entry.pluginId)).toEqual(["openrouter"]);
     expect(plan.rows.map((row) => row.ref)).toEqual(["openrouter/anthropic/claude-sonnet-4.6"]);
+    expect(plan.conflicts).toEqual([]);
   });
 
-  it("keeps the first registry row for duplicate provider/model keys", () => {
+  it("reports duplicate provider/model keys and excludes conflicted rows", () => {
     const plan = planManifestModelCatalogRows({
       registry: {
         plugins: [
@@ -95,7 +97,10 @@ describe("manifest model catalog planner", () => {
             modelCatalog: {
               providers: {
                 openai: {
-                  models: [{ id: "gpt-5.4", name: "First GPT-5.4" }],
+                  models: [
+                    { id: "gpt-5.4", name: "First GPT-5.4" },
+                    { id: "gpt-5.5", name: "GPT-5.5" },
+                  ],
                 },
               },
             },
@@ -115,10 +120,20 @@ describe("manifest model catalog planner", () => {
     });
 
     expect(plan.entries).toHaveLength(2);
+    expect(plan.conflicts).toEqual([
+      {
+        mergeKey: "openai::gpt-5.4",
+        ref: "openai/gpt-5.4",
+        provider: "openai",
+        modelId: "gpt-5.4",
+        firstPluginId: "z-first",
+        secondPluginId: "a-second",
+      },
+    ]);
     expect(plan.rows).toHaveLength(1);
     expect(plan.rows[0]).toMatchObject({
-      mergeKey: "openai::gpt-5.4",
-      name: "First GPT-5.4",
+      mergeKey: "openai::gpt-5.5",
+      name: "GPT-5.5",
     });
   });
 });
diff --git a/src/model-catalog/manifest-planner.ts b/src/model-catalog/manifest-planner.ts
index eb7543bfa67..e0f0f42843e 100644
--- a/src/model-catalog/manifest-planner.ts
+++ b/src/model-catalog/manifest-planner.ts
@@ -17,9 +17,19 @@ export type ManifestModelCatalogPlanEntry = {
   rows: readonly NormalizedModelCatalogRow[];
 };
 
+export type ManifestModelCatalogConflict = {
+  mergeKey: string;
+  ref: string;
+  provider: string;
+  modelId: string;
+  firstPluginId: string;
+  secondPluginId: string;
+};
+
 export type ManifestModelCatalogPlan = {
   rows: readonly NormalizedModelCatalogRow[];
   entries: readonly ManifestModelCatalogPlanEntry[];
+  conflicts: readonly ManifestModelCatalogConflict[];
 };
 
 export function planManifestModelCatalogRows(params: {
@@ -37,20 +47,36 @@ export function planManifestModelCatalogRows(params: {
     }
   }
 
-  const rows: NormalizedModelCatalogRow[] = [];
-  const seenMergeKeys = new Set<string>();
+  const rowCandidates: NormalizedModelCatalogRow[] = [];
+  const seenRows = new Map<string, { pluginId: string; row: NormalizedModelCatalogRow }>();
+  const conflicts = new Map<string, ManifestModelCatalogConflict>();
   for (const entry of entries) {
     for (const row of entry.rows) {
-      if (seenMergeKeys.has(row.mergeKey)) {
+      const seen = seenRows.get(row.mergeKey);
+      if (seen) {
+        if (!conflicts.has(row.mergeKey)) {
+          conflicts.set(row.mergeKey, {
+            mergeKey: row.mergeKey,
+            ref: seen.row.ref,
+            provider: seen.row.provider,
+            modelId: seen.row.id,
+            firstPluginId: seen.pluginId,
+            secondPluginId: entry.pluginId,
+          });
+        }
         continue;
       }
-      seenMergeKeys.add(row.mergeKey);
-      rows.push(row);
+      seenRows.set(row.mergeKey, { pluginId: entry.pluginId, row });
+      rowCandidates.push(row);
     }
   }
 
+  const conflictedMergeKeys = new Set(conflicts.keys());
+  const rows = rowCandidates.filter((row) => !conflictedMergeKeys.has(row.mergeKey));
+
   return {
     entries,
+    conflicts: [...conflicts.values()],
     rows: rows.toSorted(
       (left, right) =>
         left.provider.localeCompare(right.provider) || left.id.localeCompare(right.id),