diff --git a/src/providers/model-rename-migration.ts b/src/providers/model-rename-migration.ts
index 3895386b5..7f0d17952 100644
--- a/src/providers/model-rename-migration.ts
+++ b/src/providers/model-rename-migration.ts
@@ -42,6 +42,11 @@ export interface ModelRename {
    * but lose the reasoning picker entirely.
    */
   dropReasoningEffortMap?: boolean;
+  /**
+   * Keep a saved default whose legacy id carries request semantics that the
+   * replacement id cannot encode. Other catalog fields are still renamed.
+   */
+  preserveDefaultModel?: boolean;
 }
 
 /**
@@ -77,6 +82,9 @@ export const MODEL_RENAMES: readonly ModelRename[] = [
     // The retired Flash tiers were wire ids, so any saved per-model record keyed by one
     // may also hold one as a value. 3.7 expresses tiers as thinkingLevel names instead.
     dropReasoningEffortMap: true,
+    // Tier-specific retired ids remain valid routing aliases. Replacing one when it is
+    // the saved default would discard its encoded tier and make 3.7 default to medium.
+    preserveDefaultModel: from !== "gemini-3.6-flash",
   })),
 ];
 
@@ -237,7 +245,7 @@ export function projectModelRenames(
       row[field] = next;
       touched = true;
     }
-    if (prov.defaultModel === rename.from) {
+    if (prov.defaultModel === rename.from && !rename.preserveDefaultModel) {
       prov.defaultModel = rename.to;
       touched = true;
     }
diff --git a/tests/gemini-37-flash-migration.test.ts b/tests/gemini-37-flash-migration.test.ts
index cb65846a1..cc97ae83c 100644
--- a/tests/gemini-37-flash-migration.test.ts
+++ b/tests/gemini-37-flash-migration.test.ts
@@ -156,7 +156,13 @@ describe("a saved config survives the retirement", () => {
 
   test("migrating a tier id does not duplicate an already-present 3.7 entry", () => {
     const { config } = projectModelRenames(configWith(["gemini-3.6-flash-high", "gemini-3.7-flash"]));
-    expect(config.providers["google-antigravity"]!.selectedModels).toEqual(["gemini-3.7-flash"]);
+    const prov = config.providers["google-antigravity"]!;
+    expect(prov.selectedModels).toEqual(["gemini-3.7-flash"]);
+    expect(prov.defaultModel).toBe("gemini-3.6-flash-high");
+    expect(resolveAntigravityEffortWireModel(prov.defaultModel!)).toEqual({
+      wireModelId: "gemini-3.7-flash",
+      thinkingLevel: "high",
+    });
   });
 
   test("an unrelated selection is left alone", () => {