diff --git a/src/commands/models/fallbacks.ts b/src/commands/models/fallbacks.ts
index fcb729b05b0..a3e57390c60 100644
--- a/src/commands/models/fallbacks.ts
+++ b/src/commands/models/fallbacks.ts
@@ -5,6 +5,8 @@ import { logConfigUpdated } from "../../config/logging.js";
 import {
   DEFAULT_PROVIDER,
   ensureFlagCompatibility,
+  mergePrimaryFallbackConfig,
+  type PrimaryFallbackConfig,
   modelKey,
   resolveModelTarget,
   resolveModelKeysFromEntries,
@@ -55,20 +57,16 @@ export async function modelsFallbacksAddCommand(modelRaw: string, runtime: Runti
       return cfg;
     }
 
-    const existingModel = cfg.agents?.defaults?.model as
-      | { primary?: string; fallbacks?: string[] }
-      | undefined;
-
     return {
       ...cfg,
       agents: {
         ...cfg.agents,
         defaults: {
           ...cfg.agents?.defaults,
-          model: {
-            ...(existingModel?.primary ? { primary: existingModel.primary } : undefined),
-            fallbacks: [...existing, targetKey],
-          },
+          model: mergePrimaryFallbackConfig(
+            cfg.agents?.defaults?.model as unknown as PrimaryFallbackConfig | undefined,
+            { fallbacks: [...existing, targetKey] },
+          ),
           models: nextModels,
         },
       },
@@ -104,20 +102,16 @@ export async function modelsFallbacksRemoveCommand(modelRaw: string, runtime: Ru
       throw new Error(`Fallback not found: ${targetKey}`);
     }
 
-    const existingModel = cfg.agents?.defaults?.model as
-      | { primary?: string; fallbacks?: string[] }
-      | undefined;
-
     return {
       ...cfg,
       agents: {
         ...cfg.agents,
         defaults: {
           ...cfg.agents?.defaults,
-          model: {
-            ...(existingModel?.primary ? { primary: existingModel.primary } : undefined),
-            fallbacks: filtered,
-          },
+          model: mergePrimaryFallbackConfig(
+            cfg.agents?.defaults?.model as unknown as PrimaryFallbackConfig | undefined,
+            { fallbacks: filtered },
+          ),
         },
       },
     };
@@ -129,19 +123,16 @@ export async function modelsFallbacksRemoveCommand(modelRaw: string, runtime: Ru
 
 export async function modelsFallbacksClearCommand(runtime: RuntimeEnv) {
   await updateConfig((cfg) => {
-    const existingModel = cfg.agents?.defaults?.model as
-      | { primary?: string; fallbacks?: string[] }
-      | undefined;
     return {
       ...cfg,
       agents: {
         ...cfg.agents,
         defaults: {
           ...cfg.agents?.defaults,
-          model: {
-            ...(existingModel?.primary ? { primary: existingModel.primary } : undefined),
-            fallbacks: [],
-          },
+          model: mergePrimaryFallbackConfig(
+            cfg.agents?.defaults?.model as unknown as PrimaryFallbackConfig | undefined,
+            { fallbacks: [] },
+          ),
         },
       },
     };
diff --git a/src/commands/models/image-fallbacks.ts b/src/commands/models/image-fallbacks.ts
index 53e1fd2306b..43d1f51c60a 100644
--- a/src/commands/models/image-fallbacks.ts
+++ b/src/commands/models/image-fallbacks.ts
@@ -5,6 +5,8 @@ import { logConfigUpdated } from "../../config/logging.js";
 import {
   DEFAULT_PROVIDER,
   ensureFlagCompatibility,
+  mergePrimaryFallbackConfig,
+  type PrimaryFallbackConfig,
   modelKey,
   resolveModelTarget,
   resolveModelKeysFromEntries,
@@ -55,20 +57,16 @@ export async function modelsImageFallbacksAddCommand(modelRaw: string, runtime:
       return cfg;
     }
 
-    const existingModel = cfg.agents?.defaults?.imageModel as
-      | { primary?: string; fallbacks?: string[] }
-      | undefined;
-
     return {
       ...cfg,
       agents: {
         ...cfg.agents,
         defaults: {
           ...cfg.agents?.defaults,
-          imageModel: {
-            ...(existingModel?.primary ? { primary: existingModel.primary } : undefined),
-            fallbacks: [...existing, targetKey],
-          },
+          imageModel: mergePrimaryFallbackConfig(
+            cfg.agents?.defaults?.imageModel as unknown as PrimaryFallbackConfig | undefined,
+            { fallbacks: [...existing, targetKey] },
+          ),
           models: nextModels,
         },
       },
@@ -106,20 +104,16 @@ export async function modelsImageFallbacksRemoveCommand(modelRaw: string, runtim
       throw new Error(`Image fallback not found: ${targetKey}`);
     }
 
-    const existingModel = cfg.agents?.defaults?.imageModel as
-      | { primary?: string; fallbacks?: string[] }
-      | undefined;
-
     return {
       ...cfg,
       agents: {
         ...cfg.agents,
         defaults: {
           ...cfg.agents?.defaults,
-          imageModel: {
-            ...(existingModel?.primary ? { primary: existingModel.primary } : undefined),
-            fallbacks: filtered,
-          },
+          imageModel: mergePrimaryFallbackConfig(
+            cfg.agents?.defaults?.imageModel as unknown as PrimaryFallbackConfig | undefined,
+            { fallbacks: filtered },
+          ),
         },
       },
     };
@@ -133,19 +127,16 @@ export async function modelsImageFallbacksRemoveCommand(modelRaw: string, runtim
 
 export async function modelsImageFallbacksClearCommand(runtime: RuntimeEnv) {
   await updateConfig((cfg) => {
-    const existingModel = cfg.agents?.defaults?.imageModel as
-      | { primary?: string; fallbacks?: string[] }
-      | undefined;
     return {
       ...cfg,
       agents: {
         ...cfg.agents,
         defaults: {
           ...cfg.agents?.defaults,
-          imageModel: {
-            ...(existingModel?.primary ? { primary: existingModel.primary } : undefined),
-            fallbacks: [],
-          },
+          imageModel: mergePrimaryFallbackConfig(
+            cfg.agents?.defaults?.imageModel as unknown as PrimaryFallbackConfig | undefined,
+            { fallbacks: [] },
+          ),
         },
       },
     };
diff --git a/src/commands/models/set-image.ts b/src/commands/models/set-image.ts
index 82531414ef2..0b15ff5e670 100644
--- a/src/commands/models/set-image.ts
+++ b/src/commands/models/set-image.ts
@@ -1,6 +1,11 @@
 import type { RuntimeEnv } from "../../runtime.js";
 import { logConfigUpdated } from "../../config/logging.js";
-import { resolveModelTarget, updateConfig } from "./shared.js";
+import {
+  mergePrimaryFallbackConfig,
+  type PrimaryFallbackConfig,
+  resolveModelTarget,
+  updateConfig,
+} from "./shared.js";
 
 export async function modelsSetImageCommand(modelRaw: string, runtime: RuntimeEnv) {
   const updated = await updateConfig((cfg) => {
@@ -10,19 +15,16 @@ export async function modelsSetImageCommand(modelRaw: string, runtime: RuntimeEn
     if (!nextModels[key]) {
       nextModels[key] = {};
     }
-    const existingModel = cfg.agents?.defaults?.imageModel as
-      | { primary?: string; fallbacks?: string[] }
-      | undefined;
     return {
       ...cfg,
       agents: {
         ...cfg.agents,
         defaults: {
           ...cfg.agents?.defaults,
-          imageModel: {
-            ...(existingModel?.fallbacks ? { fallbacks: existingModel.fallbacks } : undefined),
-            primary: key,
-          },
+          imageModel: mergePrimaryFallbackConfig(
+            cfg.agents?.defaults?.imageModel as unknown as PrimaryFallbackConfig | undefined,
+            { primary: key },
+          ),
           models: nextModels,
         },
       },
diff --git a/src/commands/models/set.ts b/src/commands/models/set.ts
index 83db6723fbd..a6291f72696 100644
--- a/src/commands/models/set.ts
+++ b/src/commands/models/set.ts
@@ -1,6 +1,11 @@
 import type { RuntimeEnv } from "../../runtime.js";
 import { logConfigUpdated } from "../../config/logging.js";
-import { resolveModelTarget, updateConfig } from "./shared.js";
+import {
+  mergePrimaryFallbackConfig,
+  type PrimaryFallbackConfig,
+  resolveModelTarget,
+  updateConfig,
+} from "./shared.js";
 
 export async function modelsSetCommand(modelRaw: string, runtime: RuntimeEnv) {
   const updated = await updateConfig((cfg) => {
@@ -10,19 +15,16 @@ export async function modelsSetCommand(modelRaw: string, runtime: RuntimeEnv) {
     if (!nextModels[key]) {
       nextModels[key] = {};
     }
-    const existingModel = cfg.agents?.defaults?.model as
-      | { primary?: string; fallbacks?: string[] }
-      | undefined;
     return {
       ...cfg,
       agents: {
         ...cfg.agents,
         defaults: {
           ...cfg.agents?.defaults,
-          model: {
-            ...(existingModel?.fallbacks ? { fallbacks: existingModel.fallbacks } : undefined),
-            primary: key,
-          },
+          model: mergePrimaryFallbackConfig(
+            cfg.agents?.defaults?.model as unknown as PrimaryFallbackConfig | undefined,
+            { primary: key },
+          ),
           models: nextModels,
         },
       },
diff --git a/src/commands/models/shared.ts b/src/commands/models/shared.ts
index 002e839e23e..9f65eebf8f1 100644
--- a/src/commands/models/shared.ts
+++ b/src/commands/models/shared.ts
@@ -153,6 +153,22 @@ export function resolveKnownAgentId(params: {
   return agentId;
 }
 
+export type PrimaryFallbackConfig = { primary?: string; fallbacks?: string[] };
+
+export function mergePrimaryFallbackConfig(
+  existing: PrimaryFallbackConfig | undefined,
+  patch: { primary?: string; fallbacks?: string[] },
+): PrimaryFallbackConfig {
+  const next: PrimaryFallbackConfig = { ...existing };
+  if (patch.primary !== undefined) {
+    next.primary = patch.primary;
+  }
+  if (patch.fallbacks !== undefined) {
+    next.fallbacks = patch.fallbacks;
+  }
+  return next;
+}
+
 export { modelKey };
 export { DEFAULT_MODEL, DEFAULT_PROVIDER };