fix: normalize fallback updates for string model defaults

src/commands/models.set.test.ts

@@ -82,6 +82,25 @@ describe("models set + fallbacks", () => {
     });
   });
 
+  it("preserves primary when adding fallbacks to string defaults.model", async () => {
+    mockConfigSnapshot({ agents: { defaults: { model: "openai/gpt-4.1-mini" } } });
+    const runtime = makeRuntime();
+
+    await modelsFallbacksAddCommand("anthropic/claude-opus-4-6", runtime);
+
+    expect(writeConfigFile).toHaveBeenCalledTimes(1);
+    const written = getWrittenConfig();
+    expect(written.agents).toEqual({
+      defaults: {
+        model: {
+          primary: "openai/gpt-4.1-mini",
+          fallbacks: ["anthropic/claude-opus-4-6"],
+        },
+        models: { "anthropic/claude-opus-4-6": {} },
+      },
+    });
+  });
+
   it("normalizes provider casing in models set", async () => {
     mockConfigSnapshot({});
     const runtime = makeRuntime();

src/commands/models/fallbacks-shared.ts

@@ -2,12 +2,13 @@ import { buildModelAliasIndex, resolveModelRefFromString } from "../../agents/mo
 import type { OpenClawConfig } from "../../config/config.js";
 import { loadConfig } from "../../config/config.js";
 import { logConfigUpdated } from "../../config/logging.js";
+import { resolveAgentModelFallbackValues, toAgentModelListLike } from "../../config/model-input.js";
+import type { AgentModelConfig } from "../../config/types.agents-shared.js";
 import type { RuntimeEnv } from "../../runtime.js";
 import {
   DEFAULT_PROVIDER,
   ensureFlagCompatibility,
   mergePrimaryFallbackConfig,
-  type PrimaryFallbackConfig,
   modelKey,
   resolveModelTarget,
   resolveModelKeysFromEntries,
@@ -17,17 +18,14 @@ import {
 type DefaultsFallbackKey = "model" | "imageModel";
 
 function getFallbacks(cfg: OpenClawConfig, key: DefaultsFallbackKey): string[] {
-  const entry = cfg.agents?.defaults?.[key] as unknown as PrimaryFallbackConfig | undefined;
-  return entry?.fallbacks ?? [];
+  return resolveAgentModelFallbackValues(cfg.agents?.defaults?.[key]);
 }
 
 function patchDefaultsFallbacks(
   cfg: OpenClawConfig,
   params: { key: DefaultsFallbackKey; fallbacks: string[]; models?: Record<string, unknown> },
 ): OpenClawConfig {
-  const existing = cfg.agents?.defaults?.[params.key] as unknown as
-    | PrimaryFallbackConfig
-    | undefined;
+  const existing = toAgentModelListLike(cfg.agents?.defaults?.[params.key]);
   return {
     ...cfg,
     agents: {

src/commands/models/shared.ts

@@ -166,7 +166,8 @@ export function mergePrimaryFallbackConfig(
   existing: PrimaryFallbackConfig | undefined,
   patch: { primary?: string; fallbacks?: string[] },
 ): PrimaryFallbackConfig {
-  const next: PrimaryFallbackConfig = { ...existing };
+  const base = existing && typeof existing === "object" ? existing : undefined;
+  const next: PrimaryFallbackConfig = { ...base };
   if (patch.primary !== undefined) {
     next.primary = patch.primary;
   }