diff --git a/src/sdk/parser.ts b/src/sdk/parser.ts
index d9028dab..bfe09dd4 100644
--- a/src/sdk/parser.ts
+++ b/src/sdk/parser.ts
@@ -120,6 +120,11 @@ export function parseSummary(text: string, sessionId?: number): ParsedSummary |
   const summaryMatch = summaryRegex.exec(text);
 
   if (!summaryMatch) {
+    // Log when the response contains <observation> instead of <summary>
+    // to help diagnose prompt conditioning issues (see #1312)
+    if (/<observation>/.test(text)) {
+      logger.warn('PARSER', 'Summary response contained <observation> tags instead of <summary> — prompt conditioning may need strengthening', { sessionId });
+    }
     return null;
   }
 
diff --git a/src/sdk/prompts.ts b/src/sdk/prompts.ts
index 1cdb4127..774b86c7 100644
--- a/src/sdk/prompts.ts
+++ b/src/sdk/prompts.ts
@@ -130,7 +130,11 @@ export function buildSummaryPrompt(session: SDKSession, mode: ModeConfig): strin
     return '';
   })();
 
-  return `${mode.prompts.header_summary_checkpoint}
+  return `--- MODE SWITCH: PROGRESS SUMMARY ---
+Do NOT output <observation> tags. This is a summary request, not an observation request.
+Your response MUST use <summary> tags ONLY. Any <observation> output will be discarded.
+
+${mode.prompts.header_summary_checkpoint}
 ${mode.prompts.summary_instruction}
 
 ${mode.prompts.summary_context_label}