diff --git a/src/auto-reply/reply/directive-handling.impl.model-persist.test.ts b/src/auto-reply/reply/directive-handling.impl.model-persist.test.ts
new file mode 100644
index 000000000..8ac0f12fb
--- /dev/null
+++ b/src/auto-reply/reply/directive-handling.impl.model-persist.test.ts
@@ -0,0 +1,174 @@
+import { describe, expect, it, vi } from "vitest";
+
+import type { ModelAliasIndex } from "../../agents/model-selection.js";
+import type { ClawdbotConfig } from "../../config/config.js";
+import type { SessionEntry } from "../../config/sessions.js";
+import { parseInlineDirectives } from "./directive-handling.js";
+import { handleDirectiveOnly } from "./directive-handling.impl.js";
+
+// Mock dependencies
+vi.mock("../../agents/agent-scope.js", () => ({
+  resolveAgentConfig: vi.fn(() => ({})),
+  resolveAgentDir: vi.fn(() => "/tmp/agent"),
+  resolveSessionAgentId: vi.fn(() => "main"),
+}));
+
+vi.mock("../../agents/sandbox.js", () => ({
+  resolveSandboxRuntimeStatus: vi.fn(() => ({ sandboxed: false })),
+}));
+
+vi.mock("../../config/sessions.js", () => ({
+  updateSessionStore: vi.fn(async () => {}),
+}));
+
+vi.mock("../../infra/system-events.js", () => ({
+  enqueueSystemEvent: vi.fn(),
+}));
+
+function baseAliasIndex(): ModelAliasIndex {
+  return { byAlias: new Map(), byKey: new Map() };
+}
+
+function baseConfig(): ClawdbotConfig {
+  return {
+    commands: { text: true },
+    agents: { defaults: {} },
+  } as unknown as ClawdbotConfig;
+}
+
+describe("handleDirectiveOnly model persist behavior (fixes #1435)", () => {
+  const allowedModelKeys = new Set(["anthropic/claude-opus-4-5", "openai/gpt-4o"]);
+  const allowedModelCatalog = [
+    { provider: "anthropic", id: "claude-opus-4-5" },
+    { provider: "openai", id: "gpt-4o" },
+  ];
+
+  it("shows success message when session state is available", async () => {
+    const directives = parseInlineDirectives("/model openai/gpt-4o");
+    const sessionEntry: SessionEntry = {
+      sessionId: "s1",
+      updatedAt: Date.now(),
+    };
+    const sessionStore = { "agent:main:dm:1": sessionEntry };
+
+    const result = await handleDirectiveOnly({
+      cfg: baseConfig(),
+      directives,
+      sessionEntry,
+      sessionStore,
+      sessionKey: "agent:main:dm:1",
+      storePath: "/tmp/sessions.json",
+      elevatedEnabled: false,
+      elevatedAllowed: false,
+      defaultProvider: "anthropic",
+      defaultModel: "claude-opus-4-5",
+      aliasIndex: baseAliasIndex(),
+      allowedModelKeys,
+      allowedModelCatalog,
+      resetModelOverride: false,
+      provider: "anthropic",
+      model: "claude-opus-4-5",
+      initialModelLabel: "anthropic/claude-opus-4-5",
+      formatModelSwitchEvent: (label) => `Switched to ${label}`,
+    });
+
+    expect(result?.text).toContain("Model set to");
+    expect(result?.text).toContain("openai/gpt-4o");
+    expect(result?.text).not.toContain("failed");
+  });
+
+  it("shows error message when sessionEntry is missing", async () => {
+    const directives = parseInlineDirectives("/model openai/gpt-4o");
+    const sessionStore = {};
+
+    const result = await handleDirectiveOnly({
+      cfg: baseConfig(),
+      directives,
+      sessionEntry: undefined, // Missing!
+      sessionStore,
+      sessionKey: "agent:main:dm:1",
+      storePath: "/tmp/sessions.json",
+      elevatedEnabled: false,
+      elevatedAllowed: false,
+      defaultProvider: "anthropic",
+      defaultModel: "claude-opus-4-5",
+      aliasIndex: baseAliasIndex(),
+      allowedModelKeys,
+      allowedModelCatalog,
+      resetModelOverride: false,
+      provider: "anthropic",
+      model: "claude-opus-4-5",
+      initialModelLabel: "anthropic/claude-opus-4-5",
+      formatModelSwitchEvent: (label) => `Switched to ${label}`,
+    });
+
+    expect(result?.text).toContain("failed");
+    expect(result?.text).toContain("session state unavailable");
+  });
+
+  it("shows error message when sessionStore is missing", async () => {
+    const directives = parseInlineDirectives("/model openai/gpt-4o");
+    const sessionEntry: SessionEntry = {
+      sessionId: "s1",
+      updatedAt: Date.now(),
+    };
+
+    const result = await handleDirectiveOnly({
+      cfg: baseConfig(),
+      directives,
+      sessionEntry,
+      sessionStore: undefined, // Missing!
+      sessionKey: "agent:main:dm:1",
+      storePath: "/tmp/sessions.json",
+      elevatedEnabled: false,
+      elevatedAllowed: false,
+      defaultProvider: "anthropic",
+      defaultModel: "claude-opus-4-5",
+      aliasIndex: baseAliasIndex(),
+      allowedModelKeys,
+      allowedModelCatalog,
+      resetModelOverride: false,
+      provider: "anthropic",
+      model: "claude-opus-4-5",
+      initialModelLabel: "anthropic/claude-opus-4-5",
+      formatModelSwitchEvent: (label) => `Switched to ${label}`,
+    });
+
+    expect(result?.text).toContain("failed");
+    expect(result?.text).toContain("session state unavailable");
+  });
+
+  it("shows no model message when no /model directive", async () => {
+    const directives = parseInlineDirectives("hello world");
+    const sessionEntry: SessionEntry = {
+      sessionId: "s1",
+      updatedAt: Date.now(),
+    };
+    const sessionStore = { "agent:main:dm:1": sessionEntry };
+
+    const result = await handleDirectiveOnly({
+      cfg: baseConfig(),
+      directives,
+      sessionEntry,
+      sessionStore,
+      sessionKey: "agent:main:dm:1",
+      storePath: "/tmp/sessions.json",
+      elevatedEnabled: false,
+      elevatedAllowed: false,
+      defaultProvider: "anthropic",
+      defaultModel: "claude-opus-4-5",
+      aliasIndex: baseAliasIndex(),
+      allowedModelKeys,
+      allowedModelCatalog,
+      resetModelOverride: false,
+      provider: "anthropic",
+      model: "claude-opus-4-5",
+      initialModelLabel: "anthropic/claude-opus-4-5",
+      formatModelSwitchEvent: (label) => `Switched to ${label}`,
+    });
+
+    // No model directive = no model message
+    expect(result?.text ?? "").not.toContain("Model set to");
+    expect(result?.text ?? "").not.toContain("failed");
+  });
+});
diff --git a/src/auto-reply/reply/directive-handling.impl.ts b/src/auto-reply/reply/directive-handling.impl.ts
index 63b1baa8c..a753085dc 100644
--- a/src/auto-reply/reply/directive-handling.impl.ts
+++ b/src/auto-reply/reply/directive-handling.impl.ts
@@ -461,7 +461,9 @@ export async function handleDirectiveOnly(params: {
       parts.push(`Auth profile set to ${profileOverride}.`);
     }
   } else if (modelSelection && !didPersistModel) {
-    parts.push(`Model switch to ${modelSelection.provider}/${modelSelection.model} failed (session state unavailable).`);
+    parts.push(
+      `Model switch to ${modelSelection.provider}/${modelSelection.model} failed (session state unavailable).`,
+    );
   }
   if (directives.hasQueueDirective && directives.queueMode) {
     parts.push(formatDirectiveAck(`Queue mode set to ${directives.queueMode}.`));