diff --git a/src/agents/agent-builder.test.ts b/src/agents/agent-builder.test.ts
new file mode 100644
index 000000000..56abb8ac7
--- /dev/null
+++ b/src/agents/agent-builder.test.ts
@@ -0,0 +1,67 @@
+import { describe, test, expect } from "bun:test"
+import { buildAgent } from "./agent-builder"
+import type { AgentFactory } from "./types"
+
+describe("#given an agent factory with mode", () => {
+  const mockFactory = ((model: string) => ({
+    name: "test-agent",
+    description: "Test",
+    instructions: "test",
+    model,
+    temperature: 0.1,
+  })) as AgentFactory
+  mockFactory.mode = "subagent"
+
+  test("#when building agent from factory", () => {
+    const agent = buildAgent(mockFactory, "test-model")
+    expect(agent.mode).toBe("subagent")
+  })
+})
+
+describe("#given an agent factory with mode=primary", () => {
+  const mockFactory = ((model: string) => ({
+    name: "primary-agent",
+    description: "Primary Test",
+    instructions: "test",
+    model,
+    temperature: 0.1,
+  })) as AgentFactory
+  mockFactory.mode = "primary"
+
+  test("#when building agent from factory", () => {
+    const agent = buildAgent(mockFactory, "test-model")
+    expect(agent.mode).toBe("primary")
+  })
+})
+
+describe("#given an agent config object without mode", () => {
+  const mockConfig = {
+    name: "config-agent",
+    description: "Config Test",
+    instructions: "test",
+    model: "test-model",
+    temperature: 0.1,
+  }
+
+  test("#when building agent from config object", () => {
+    const agent = buildAgent(mockConfig, "test-model")
+    expect(agent.mode).toBeUndefined()
+  })
+})
+
+describe("#given an agent factory with mode but config already has mode", () => {
+  const mockFactory = ((model: string) => ({
+    name: "override-agent",
+    description: "Override Test",
+    instructions: "test",
+    model,
+    temperature: 0.1,
+    mode: "all",
+  })) as AgentFactory
+  mockFactory.mode = "subagent"
+
+  test("#when building agent from factory", () => {
+    const agent = buildAgent(mockFactory, "test-model")
+    expect(agent.mode).toBe("all")
+  })
+})
diff --git a/src/agents/agent-builder.ts b/src/agents/agent-builder.ts
index 5747bb841..1a98a954f 100644
--- a/src/agents/agent-builder.ts
+++ b/src/agents/agent-builder.ts
@@ -33,5 +33,9 @@ export function buildAgent(
     }
   }
 
+  if (isFactory(source) && (base as AgentConfig & { mode?: string }).mode === undefined) {
+    ;(base as AgentConfig & { mode?: string }).mode = source.mode
+  }
+
   return base
 }