diff --git a/v2/sidecar/agent-runner-deno.ts b/v2/sidecar/agent-runner-deno.ts
index 0470e36..9617825 100644
--- a/v2/sidecar/agent-runner-deno.ts
+++ b/v2/sidecar/agent-runner-deno.ts
@@ -103,7 +103,9 @@ async function handleQuery(msg: QueryMessage) {
         permissionMode: (permissionMode ?? "bypassPermissions") as "bypassPermissions" | "default",
         allowDangerouslySkipPermissions: (permissionMode ?? "bypassPermissions") === "bypassPermissions",
         settingSources: settingSources ?? ["user", "project"],
-        systemPrompt: systemPrompt ?? undefined,
+        systemPrompt: systemPrompt
+          ? systemPrompt
+          : { type: "preset" as const, preset: "claude_code" as const },
         model: model ?? undefined,
         additionalDirectories: additionalDirectories ?? undefined,
       },
diff --git a/v2/sidecar/agent-runner.ts b/v2/sidecar/agent-runner.ts
index 667f517..144de58 100644
--- a/v2/sidecar/agent-runner.ts
+++ b/v2/sidecar/agent-runner.ts
@@ -116,7 +116,9 @@ async function handleQuery(msg: QueryMessage) {
         permissionMode: (permissionMode ?? 'bypassPermissions') as 'bypassPermissions' | 'default',
         allowDangerouslySkipPermissions: (permissionMode ?? 'bypassPermissions') === 'bypassPermissions',
         settingSources: settingSources ?? ['user', 'project'],
-        systemPrompt: systemPrompt ?? undefined,
+        systemPrompt: systemPrompt
+          ? systemPrompt
+          : { type: 'preset' as const, preset: 'claude_code' as const },
         model: model ?? undefined,
         additionalDirectories: additionalDirectories ?? undefined,
       },