diff --git a/packages/subagent/tool-subagent/src/index.ts b/packages/subagent/tool-subagent/src/index.ts
index f677076d3f..31d6dce113 100644
--- a/packages/subagent/tool-subagent/src/index.ts
+++ b/packages/subagent/tool-subagent/src/index.ts
@@ -93,9 +93,13 @@ export interface Config {
 export const Config: z<Config> = z.object({
   provider: z.string().required(),
   toolName: z.string().default('subagent'),
+  // Omitted-object discipline (see the toolFilter note below): without the
+  // forced default an omitted `agentOptions` materializes `{}`, which reads as
+  // present — the request would carry `agentOptions: {}` and the presence
+  // check in execute() could never be false through config.
   agentOptions: z.object({
     model: z.string(),
-  }),
+  }).default(undefined as unknown as { model: string }),
   persona: z.string(),
   // A schemastery object materializes {} (with [] for nested arrays) when the
   // key is omitted — for toolFilter that would mean an EMPTY ALLOW-LIST, i.e.
@@ -229,7 +233,7 @@ export function apply(ctx: Context, config: Config): void {
           prompt: [{ type: 'text', text: args.prompt }],
           parent,
           ...exec.signal ? { signal: exec.signal } : {},
-          ...config.agentOptions ? { agentOptions: config.agentOptions } : {},
+          ...config.agentOptions !== undefined ? { agentOptions: config.agentOptions } : {},
           ...config.persona !== undefined ? { persona: config.persona } : {},
           ...config.toolFilter !== undefined ? { toolFilter: config.toolFilter } : {},
           ...config.maxDepth !== undefined ? { maxDepth: config.maxDepth } : {},
diff --git a/packages/subagent/tool-subagent/tests/tool-subagent.spec.ts b/packages/subagent/tool-subagent/tests/tool-subagent.spec.ts
index da560350e4..9d9941fdff 100644
--- a/packages/subagent/tool-subagent/tests/tool-subagent.spec.ts
+++ b/packages/subagent/tool-subagent/tests/tool-subagent.spec.ts
@@ -511,6 +511,36 @@ describe('dsh-tool-subagent', () => {
     expect(seen?.toolFilter).not.toHaveProperty('allow')
   })
 
+  it('an omitted agentOptions does not materialize an empty object onto the request', async () => {
+    // Same schemastery trap as toolFilter, adjacent field: an omitted
+    // `agentOptions` config key materializes `{}` without the forced default,
+    // which reads as present and puts a dishonest `agentOptions: {}` on every
+    // start request.
+    let seen: { agentOptions?: unknown } | undefined
+    const ctx = new Context()
+    await ctx.plugin(SystemPrompt)
+    await ctx.plugin(ToolRegistry)
+    await ctx.plugin(SubagentService)
+    ctx.subagents.registerProvider({
+      name: 'capture4',
+      capabilities: { outputSchema: false, depthLimit: false, toolFilter: false, persona: false },
+      inheritsParentContext: false,
+      start: (request) => {
+        seen = request
+        return {
+          id: AgentId('capture4-child'),
+          result: Promise.resolve({ output: [{ type: 'text', text: 'ok' }], stopReason: 'completed' as const }),
+          cancel() {},
+          dispose: async () => {},
+        }
+      },
+    })
+    await ctx.plugin(tool, { provider: 'capture4' })
+    await callSubagent(ctx, { description: 'd', prompt: 'p' })
+    expect(seen).toBeDefined()
+    expect(seen).not.toHaveProperty('agentOptions')
+  })
+
   it('an explicit empty toolFilter fails at plugin load, not at first delegation', async () => {
     const ctx = new Context()
     await ctx.plugin(SystemPrompt)