diff --git a/src/agents/agent-builder.test.ts b/src/agents/agent-builder.test.ts
index 56abb8ac7..f9be614aa 100644
--- a/src/agents/agent-builder.test.ts
+++ b/src/agents/agent-builder.test.ts
@@ -1,16 +1,15 @@
-import { describe, test, expect } from "bun:test"
+import { describe, expect, test } from "bun:test"
 import { buildAgent } from "./agent-builder"
 import type { AgentFactory } from "./types"
 
 describe("#given an agent factory with mode", () => {
-  const mockFactory = ((model: string) => ({
+  const mockFactory: AgentFactory = Object.assign((model: string) => ({
     name: "test-agent",
     description: "Test",
     instructions: "test",
     model,
     temperature: 0.1,
-  })) as AgentFactory
-  mockFactory.mode = "subagent"
+  }), { mode: "subagent" as const })
 
   test("#when building agent from factory", () => {
     const agent = buildAgent(mockFactory, "test-model")
@@ -19,14 +18,13 @@ describe("#given an agent factory with mode", () => {
 })
 
 describe("#given an agent factory with mode=primary", () => {
-  const mockFactory = ((model: string) => ({
+  const mockFactory: AgentFactory = Object.assign((model: string) => ({
     name: "primary-agent",
     description: "Primary Test",
     instructions: "test",
     model,
     temperature: 0.1,
-  })) as AgentFactory
-  mockFactory.mode = "primary"
+  }), { mode: "primary" as const })
 
   test("#when building agent from factory", () => {
     const agent = buildAgent(mockFactory, "test-model")
@@ -50,15 +48,14 @@ describe("#given an agent config object without mode", () => {
 })
 
 describe("#given an agent factory with mode but config already has mode", () => {
-  const mockFactory = ((model: string) => ({
+  const mockFactory: AgentFactory = Object.assign((model: string) => ({
     name: "override-agent",
     description: "Override Test",
     instructions: "test",
     model,
     temperature: 0.1,
-    mode: "all",
-  })) as AgentFactory
-  mockFactory.mode = "subagent"
+    mode: "all" as const,
+  }), { mode: "subagent" as const })
 
   test("#when building agent from factory", () => {
     const agent = buildAgent(mockFactory, "test-model")
diff --git a/src/agents/builtin-agents/available-skills.test.ts b/src/agents/builtin-agents/available-skills.test.ts
index 2505af5bb..fe5ea8441 100644
--- a/src/agents/builtin-agents/available-skills.test.ts
+++ b/src/agents/builtin-agents/available-skills.test.ts
@@ -2,10 +2,12 @@ import { describe, expect, test } from "bun:test"
 
 import { buildAvailableSkills } from "./available-skills"
 
+type DiscoveredSkills = Parameters<typeof buildAvailableSkills>[0]
+
 describe("buildAvailableSkills", () => {
   test("includes team-mode when team mode is enabled", () => {
     // given
-    const discoveredSkills = []
+    const discoveredSkills: DiscoveredSkills = []
 
     // when
     const availableSkills = buildAvailableSkills(discoveredSkills, undefined, undefined, true)
@@ -16,7 +18,7 @@ describe("buildAvailableSkills", () => {
 
   test("excludes team-mode when team mode is disabled", () => {
     // given
-    const discoveredSkills = []
+    const discoveredSkills: DiscoveredSkills = []
 
     // when
     const availableSkills = buildAvailableSkills(discoveredSkills, undefined, undefined, false)