fix(cron): preserve model fallbacks when agent overrides primary

When an agent config specifies `model: { primary: "..." }` without
an explicit `fallbacks` array, the existing code replaced the entire
model object from `agents.defaults`—discarding the default fallbacks.

This caused cron jobs (and agent sessions) to have only one model
candidate (the pinned model) plus the global primary as a final
fallback, skipping all intermediate fallback models.

The fix merges the agent model override into the existing defaults
model object using spread, so that keys like `fallbacks` survive
when the agent only overrides `primary`. Agents can still explicitly
override or clear fallbacks by providing their own `fallbacks` array.

Reproduction scenario:
- `agents.defaults.model = { primary: "codex", fallbacks: ["opus", "flash", "deepseek"] }`
- Agent config: `model: { primary: "codex" }`
- Cron job pins: `model: "flash"`
- Before fix: fallback candidates = [flash, codex] (3 models lost)
- After fix: fallback candidates = [flash, opus, deepseek, ..., codex]

src/cron/isolated-agent/run.model-fallback-preservation.test.ts

@@ -0,0 +1,106 @@
+import { describe, expect, it } from "vitest";
+import type { AgentDefaultsConfig } from "../../config/types.js";
+
+/**
+ * Tests for the model merge fix in runCronIsolatedAgentTurn.
+ *
+ * Bug: When an agent config defines `model: { primary: "..." }` without
+ * `fallbacks`, the merge into `agentCfg` replaced the entire model object
+ * from defaults—losing `fallbacks`.  This caused cron jobs to have only
+ * one model candidate (the pinned model) plus the global primary, skipping
+ * all intermediate fallbacks.
+ *
+ * Fix: Spread the existing `agentCfg.model` before applying the override,
+ * so keys like `fallbacks` from `agents.defaults.model` survive when the
+ * agent only overrides `primary`.
+ */
+describe("agent model override preserves default fallbacks", () => {
+  // Simulates the merge logic extracted from run.ts lines 148–159
+  function mergeAgentModel(
+    defaults: AgentDefaultsConfig,
+    overrideModel: { primary?: string; fallbacks?: string[] } | string | undefined,
+  ): AgentDefaultsConfig {
+    const agentCfg: AgentDefaultsConfig = { ...defaults };
+
+    // --- FIX: merge instead of replace ---
+    const existingModel =
+      agentCfg.model && typeof agentCfg.model === "object" ? agentCfg.model : {};
+    if (typeof overrideModel === "string") {
+      agentCfg.model = { ...existingModel, primary: overrideModel };
+    } else if (overrideModel) {
+      agentCfg.model = { ...existingModel, ...overrideModel };
+    }
+    return agentCfg;
+  }
+
+  const defaultFallbacks = [
+    "anthropic/claude-opus-4-6",
+    "google-gemini-cli/gemini-3-pro-preview",
+    "nvidia/deepseek-ai/deepseek-v3.2",
+  ];
+
+  const defaults: AgentDefaultsConfig = {
+    model: {
+      primary: "openai-codex/gpt-5.3-codex",
+      fallbacks: defaultFallbacks,
+    },
+  };
+
+  it("preserves fallbacks when agent overrides primary as string", () => {
+    const result = mergeAgentModel(defaults, "anthropic/claude-sonnet-4-5");
+    const model = result.model as { primary?: string; fallbacks?: string[] };
+
+    expect(model.primary).toBe("anthropic/claude-sonnet-4-5");
+    expect(model.fallbacks).toEqual(defaultFallbacks);
+  });
+
+  it("preserves fallbacks when agent overrides primary as object", () => {
+    const result = mergeAgentModel(defaults, {
+      primary: "anthropic/claude-sonnet-4-5",
+    });
+    const model = result.model as { primary?: string; fallbacks?: string[] };
+
+    expect(model.primary).toBe("anthropic/claude-sonnet-4-5");
+    expect(model.fallbacks).toEqual(defaultFallbacks);
+  });
+
+  it("allows agent to explicitly override fallbacks", () => {
+    const customFallbacks = ["nvidia/deepseek-ai/deepseek-v3.2"];
+    const result = mergeAgentModel(defaults, {
+      primary: "anthropic/claude-sonnet-4-5",
+      fallbacks: customFallbacks,
+    });
+    const model = result.model as { primary?: string; fallbacks?: string[] };
+
+    expect(model.primary).toBe("anthropic/claude-sonnet-4-5");
+    expect(model.fallbacks).toEqual(customFallbacks);
+  });
+
+  it("allows agent to explicitly clear fallbacks with empty array", () => {
+    const result = mergeAgentModel(defaults, {
+      primary: "anthropic/claude-sonnet-4-5",
+      fallbacks: [],
+    });
+    const model = result.model as { primary?: string; fallbacks?: string[] };
+
+    expect(model.primary).toBe("anthropic/claude-sonnet-4-5");
+    expect(model.fallbacks).toEqual([]);
+  });
+
+  it("leaves model untouched when override is undefined", () => {
+    const result = mergeAgentModel(defaults, undefined);
+    const model = result.model as { primary?: string; fallbacks?: string[] };
+
+    expect(model.primary).toBe("openai-codex/gpt-5.3-codex");
+    expect(model.fallbacks).toEqual(defaultFallbacks);
+  });
+
+  it("handles missing defaults model gracefully", () => {
+    const emptyDefaults: AgentDefaultsConfig = {};
+    const result = mergeAgentModel(emptyDefaults, "anthropic/claude-sonnet-4-5");
+    const model = result.model as { primary?: string; fallbacks?: string[] };
+
+    expect(model.primary).toBe("anthropic/claude-sonnet-4-5");
+    expect(model.fallbacks).toBeUndefined();
+  });
+});

src/cron/isolated-agent/run.ts

@@ -149,10 +149,14 @@ export async function runCronIsolatedAgentTurn(params: {
     params.cfg.agents?.defaults,
     agentOverrideRest as Partial<AgentDefaultsConfig>,
   );
+  // Merge agent model override with defaults instead of replacing, so that
+  // `fallbacks` from `agents.defaults.model` are preserved when the agent
+  // (or its per-cron model pin) only specifies `primary`.
+  const existingModel = agentCfg.model && typeof agentCfg.model === "object" ? agentCfg.model : {};
   if (typeof overrideModel === "string") {
-    agentCfg.model = { primary: overrideModel };
+    agentCfg.model = { ...existingModel, primary: overrideModel };
   } else if (overrideModel) {
-    agentCfg.model = overrideModel;
+    agentCfg.model = { ...existingModel, ...overrideModel };
   }
   const cfgWithAgentDefaults: OpenClawConfig = {
     ...params.cfg,