diff --git a/packages/core/src/prompt-builder.ts b/packages/core/src/prompt-builder.ts
index 1455033a5..7b92a122e 100644
--- a/packages/core/src/prompt-builder.ts
+++ b/packages/core/src/prompt-builder.ts
@@ -167,7 +167,8 @@ export function buildPrompt(config: PromptBuildConfig): string | null {
   const hasUserPrompt = Boolean(config.userPrompt);
 
   // Nothing to compose — return null for backward compatibility
-  if (!hasIssue && !hasRules && !hasUserPrompt) {
+  // (but exploratory sessions always need the exploratory prompt)
+  if (!hasIssue && !hasRules && !hasUserPrompt && !config.exploratory) {
     return null;
   }