fix: deep custom config should be optional

tests/plugin.test.ts

@@ -439,6 +439,26 @@ test("orchestration inherits deep-lo and falls back to good-lo with available wo
     ).toBe(false);
 });
 
+test("custom deep assignments are optional and a partial deep tier falls back to good-lo", async () => {
+    for (const omitted of [["deep-lo", "deep-hi"], ["deep-lo"], ["deep-hi"]] as const) {
+        const agents: Partial<ReturnType<typeof customAssignments>> = customAssignments();
+        for (const name of omitted) delete agents[name];
+        const result = await host(allModels, { mode: "custom", agents });
+        expect(result.agents.get("coordinate")?.model).toMatchObject({
+            id: "gpt-6-sol",
+            variant: "low",
+        });
+        expect(result.agents.get("coordinate")?.system).toContain("Deep agents are unavailable");
+        for (const name of ["fast-lo", "fast-hi", "good-lo", "good-hi"])
+            expect(result.agents.has(name)).toBe(true);
+        expect(result.agents.has("deep-lo")).toBe(false);
+        expect(result.agents.has("deep-hi")).toBe(false);
+        expect((await result.tools("coordinate")).subagent?.input.properties).toMatchObject({
+            agent: { type: "string", enum: ["fast-lo", "fast-hi", "good-lo", "good-hi"] },
+        });
+    }
+});
+
 test("registers the embedded anti-overengineering skill and makes it available to coordinate", async () => {
     const result = await host();
     const skill = result.skills.get("anti-overengineering")!;
@@ -499,6 +519,23 @@ test("custom assignments resolve duplicate model IDs by provider and refresh pro
 
 test("custom configuration requires qualified models and rejects obsolete fields", () => {
     expect(() => settings.parse({ mode: "custom", agents: customAssignments() })).not.toThrow();
+    for (const omitted of [["deep-lo", "deep-hi"], ["deep-lo"], ["deep-hi"]] as const) {
+        const agents: Partial<ReturnType<typeof customAssignments>> = customAssignments();
+        for (const name of omitted) delete agents[name];
+        expect(() => settings.parse({ mode: "custom", agents })).not.toThrow();
+    }
+    for (const name of ["fast-lo", "fast-hi", "good-lo", "good-hi"] as const) {
+        const agents: Partial<ReturnType<typeof customAssignments>> = customAssignments();
+        delete agents[name];
+        expect(() => settings.parse({ mode: "custom", agents })).toThrow();
+    }
+    for (const name of ["deep-lo", "deep-hi"] as const)
+        expect(() =>
+            settings.parse({
+                mode: "custom",
+                agents: { ...customAssignments(), [name]: { model: "test/model" } },
+            }),
+        ).toThrow();
     for (const model of ["bare", "/model", "provider/"])
         expect(() =>
             settings.parse({