diff --git a/packages/voice-assistant/src/server/acp/agent-manager.ts b/packages/voice-assistant/src/server/acp/agent-manager.ts
index 5d24d3a18..1358e1a0d 100644
--- a/packages/voice-assistant/src/server/acp/agent-manager.ts
+++ b/packages/voice-assistant/src/server/acp/agent-manager.ts
@@ -315,10 +315,10 @@ export class AgentManager {
       status: agent.status,
       createdAt: agent.createdAt,
       type: "claude" as const,
-      sessionId: agent.sessionId,
-      error: agent.error,
-      currentModeId: agent.currentModeId,
-      availableModes: agent.availableModes,
+      sessionId: agent.sessionId ?? null,
+      error: agent.error ?? null,
+      currentModeId: agent.currentModeId ?? null,
+      availableModes: agent.availableModes ?? null,
     }));
   }
 
@@ -356,23 +356,23 @@ export class AgentManager {
   /**
    * Get the current session mode for an agent
    */
-  getCurrentMode(agentId: string): string | undefined {
+  getCurrentMode(agentId: string): string | null {
     const agent = this.agents.get(agentId);
     if (!agent) {
       throw new Error(`Agent ${agentId} not found`);
     }
-    return agent.currentModeId;
+    return agent.currentModeId ?? null;
   }
 
   /**
    * Get available session modes for an agent
    */
-  getAvailableModes(agentId: string): SessionMode[] {
+  getAvailableModes(agentId: string): SessionMode[] | null {
     const agent = this.agents.get(agentId);
     if (!agent) {
       throw new Error(`Agent ${agentId} not found`);
     }
-    return agent.availableModes || [];
+    return agent.availableModes ?? null;
   }
 
   /**
diff --git a/packages/voice-assistant/src/server/acp/mcp-server.ts b/packages/voice-assistant/src/server/acp/mcp-server.ts
index 6f32ea78b..cca1aebf9 100644
--- a/packages/voice-assistant/src/server/acp/mcp-server.ts
+++ b/packages/voice-assistant/src/server/acp/mcp-server.ts
@@ -74,12 +74,12 @@ export async function createAgentMcpServer(
             "Current agent status: 'initializing', 'ready', 'processing', etc."
           ),
         cwd: z.string().describe("The resolved absolute working directory the agent is running in"),
-        currentModeId: z.string().optional().describe("The agent's current session mode"),
+        currentModeId: z.string().nullable().describe("The agent's current session mode"),
         availableModes: z.array(z.object({
           id: z.string(),
           name: z.string(),
           description: z.string().optional(),
-        })).optional().describe("Available session modes for this agent"),
+        })).nullable().describe("Available session modes for this agent"),
       },
     },
     async ({ cwd, initialPrompt, initialMode }) => {
@@ -100,8 +100,8 @@ export async function createAgentMcpServer(
         agentId,
         status,
         cwd: resolvedCwd,
-        currentModeId,
-        availableModes,
+        currentModeId: currentModeId ?? null,
+        availableModes: availableModes ?? null,
       };
 
       return {
@@ -162,14 +162,14 @@ export async function createAgentMcpServer(
             status: z.string(),
             createdAt: z.date(),
             type: z.literal("claude"),
-            sessionId: z.string().optional(),
-            error: z.string().optional(),
-            currentModeId: z.string().optional(),
+            sessionId: z.string().nullable(),
+            error: z.string().nullable(),
+            currentModeId: z.string().nullable(),
             availableModes: z.array(z.object({
               id: z.string(),
               name: z.string(),
               description: z.string().optional(),
-            })).optional(),
+            })).nullable(),
           })
           .describe("Detailed agent information"),
       },
@@ -210,14 +210,14 @@ export async function createAgentMcpServer(
             status: z.string(),
             createdAt: z.date(),
             type: z.literal("claude"),
-            sessionId: z.string().optional(),
-            error: z.string().optional(),
-            currentModeId: z.string().optional(),
+            sessionId: z.string().nullable(),
+            error: z.string().nullable(),
+            currentModeId: z.string().nullable(),
             availableModes: z.array(z.object({
               id: z.string(),
               name: z.string(),
               description: z.string().optional(),
-            })).optional(),
+            })).nullable(),
           })
         ),
       },
@@ -362,7 +362,7 @@ export async function createAgentMcpServer(
       },
       outputSchema: {
         success: z.boolean().describe("Whether the mode change succeeded"),
-        previousMode: z.string().optional().describe("The previous session mode"),
+        previousMode: z.string().nullable().describe("The previous session mode"),
         newMode: z.string().describe("The new session mode"),
       },
     },
@@ -372,7 +372,7 @@ export async function createAgentMcpServer(
 
       const result = {
         success: true,
-        previousMode,
+        previousMode: previousMode ?? null,
         newMode: modeId,
       };
 
diff --git a/packages/voice-assistant/src/server/acp/types.ts b/packages/voice-assistant/src/server/acp/types.ts
index afee4fcb6..e78d0ab9f 100644
--- a/packages/voice-assistant/src/server/acp/types.ts
+++ b/packages/voice-assistant/src/server/acp/types.ts
@@ -19,10 +19,10 @@ export interface AgentInfo {
   status: AgentStatus;
   createdAt: Date;
   type: "claude";
-  sessionId?: string;
-  error?: string;
-  currentModeId?: string;
-  availableModes?: SessionMode[];
+  sessionId: string | null;
+  error: string | null;
+  currentModeId: string | null;
+  availableModes: SessionMode[] | null;
 }
 
 /**