diff --git a/src/agents/pi-extensions/compaction-safeguard.test.ts b/src/agents/pi-extensions/compaction-safeguard.test.ts
index 3b6ba42dcad..36c6bc753ae 100644
--- a/src/agents/pi-extensions/compaction-safeguard.test.ts
+++ b/src/agents/pi-extensions/compaction-safeguard.test.ts
@@ -13,6 +13,7 @@ const {
   formatToolFailuresSection,
   splitPreservedRecentTurns,
   formatPreservedTurnsSection,
+  buildCompactionStructureInstructions,
   resolveRecentTurnsPreserve,
   computeAdaptiveChunkRatio,
   isOversizedForSummary,
@@ -400,6 +401,13 @@ describe("compaction-safeguard recent-turn preservation", () => {
     expect(resolveRecentTurnsPreserve(-1)).toBe(0);
     expect(resolveRecentTurnsPreserve(99)).toBe(12);
   });
+
+  it("builds structured instructions with required sections", () => {
+    const instructions = buildCompactionStructureInstructions("Keep security caveats.");
+    expect(instructions).toContain("## Decisions");
+    expect(instructions).toContain("## Exact identifiers");
+    expect(instructions).toContain("Keep security caveats.");
+  });
 });
 
 describe("compaction-safeguard extension model fallback", () => {
diff --git a/src/agents/pi-extensions/compaction-safeguard.ts b/src/agents/pi-extensions/compaction-safeguard.ts
index 8eb613da42e..a90df969068 100644
--- a/src/agents/pi-extensions/compaction-safeguard.ts
+++ b/src/agents/pi-extensions/compaction-safeguard.ts
@@ -31,6 +31,13 @@ const MAX_TOOL_FAILURE_CHARS = 240;
 const DEFAULT_RECENT_TURNS_PRESERVE = 3;
 const MAX_RECENT_TURNS_PRESERVE = 12;
 const MAX_RECENT_TURN_TEXT_CHARS = 600;
+const REQUIRED_SUMMARY_SECTIONS = [
+  "## Decisions",
+  "## Open TODOs",
+  "## Constraints/Rules",
+  "## Pending user asks",
+  "## Exact identifiers",
+] as const;
 
 type ToolFailure = {
   toolCallId: string;
@@ -254,6 +261,20 @@ function formatPreservedTurnsSection(messages: AgentMessage[]): string {
   return `\n\n## Recent turns preserved verbatim\n${lines.join("\n")}`;
 }
 
+function buildCompactionStructureInstructions(customInstructions?: string): string {
+  const sectionsTemplate = [
+    "Produce a compact, factual summary with these exact section headings:",
+    ...REQUIRED_SUMMARY_SECTIONS,
+    "For ## Exact identifiers, preserve literal values exactly as seen (IDs, URLs, file paths, ports, hashes, dates, times).",
+    "Do not omit unresolved asks from the user.",
+  ].join("\n");
+  const custom = customInstructions?.trim();
+  if (!custom) {
+    return sectionsTemplate;
+  }
+  return `${sectionsTemplate}\n\nAdditional focus:\n${custom}`;
+}
+
 /**
  * Read and format critical workspace context for compaction summary.
  * Extracts "Session Startup" and "Red Lines" from AGENTS.md.
@@ -415,6 +436,7 @@ export default function compactionSafeguardExtension(api: ExtensionAPI): void {
       });
       messagesToSummarize = summaryTargetMessages;
       const preservedTurnsSection = formatPreservedTurnsSection(preservedRecentMessages);
+      const structuredInstructions = buildCompactionStructureInstructions(customInstructions);
 
       // Use adaptive chunk ratio based on message sizes, reserving headroom for
       // the summarization prompt, system prompt, previous summary, and reasoning budget
@@ -439,7 +461,7 @@ export default function compactionSafeguardExtension(api: ExtensionAPI): void {
         reserveTokens,
         maxChunkTokens,
         contextWindow: contextWindowTokens,
-        customInstructions,
+        customInstructions: structuredInstructions,
         previousSummary: effectivePreviousSummary,
       });
 
@@ -453,7 +475,7 @@ export default function compactionSafeguardExtension(api: ExtensionAPI): void {
           reserveTokens,
           maxChunkTokens,
           contextWindow: contextWindowTokens,
-          customInstructions: TURN_PREFIX_INSTRUCTIONS,
+          customInstructions: `${TURN_PREFIX_INSTRUCTIONS}\n\n${structuredInstructions}`,
           previousSummary: undefined,
         });
         summary = `${historySummary}\n\n---\n\n**Turn Context (split turn):**\n\n${prefixSummary}`;
@@ -493,6 +515,7 @@ export const __testing = {
   formatToolFailuresSection,
   splitPreservedRecentTurns,
   formatPreservedTurnsSection,
+  buildCompactionStructureInstructions,
   resolveRecentTurnsPreserve,
   computeAdaptiveChunkRatio,
   isOversizedForSummary,