Compaction: preserve tail sections when reserved suffix exceeds cap

When suffix.length >= maxChars, slice(0, maxChars) kept the head (preserved
turns) and dropped the tail (workspace rules, diagnostics). Use slice(-maxChars)
to preserve the critical tail instead.

Made-with: Cursor

src/agents/pi-extensions/compaction-safeguard.test.ts

@@ -329,6 +329,24 @@ describe("compaction-safeguard summary budgets", () => {
     expect(capped).toContain("None.\n\n## Tool Failures");
     expect(capped).not.toMatch(/None\.## Tool Failures/);
   });
+
+  it("preserves tail sections when suffix exceeds cap (workspace rules, diagnostics over preserved turns)", () => {
+    const criticalTail =
+      "\n\n## Tool Failures\n- exec: failed\n\n<read-files>\nfoo.ts\n</read-files>\n\n" +
+      "<workspace-critical-rules>\n## Session Startup\nRead AGENTS.md\n</workspace-critical-rules>";
+    const preservedTurns =
+      "## Recent turns preserved verbatim\n- User: x\n- Assistant: y\n" +
+      "x".repeat(MAX_COMPACTION_SUMMARY_CHARS);
+    const oversizedSuffix = preservedTurns + criticalTail;
+
+    const capped = capCompactionSummaryPreservingSuffix("short body", oversizedSuffix);
+
+    expect(capped.length).toBeLessThanOrEqual(MAX_COMPACTION_SUMMARY_CHARS);
+    expect(capped).toContain("<workspace-critical-rules>");
+    expect(capped).toContain("## Tool Failures");
+    expect(capped).toContain("<read-files>");
+    expect(capped).toContain("## Session Startup");
+  });
 });
 
 describe("computeAdaptiveChunkRatio", () => {

src/agents/pi-extensions/compaction-safeguard.ts

@@ -270,7 +270,8 @@ function capCompactionSummaryPreservingSuffix(
     return capCompactionSummary(`${summaryBody}${suffix}`, maxChars);
   }
   if (suffix.length >= maxChars) {
-    return suffix.slice(0, maxChars);
+    // Preserve tail (workspace rules, diagnostics) over head (preserved turns).
+    return suffix.slice(-maxChars);
   }
   const bodyBudget = Math.max(0, maxChars - suffix.length);
   const cappedBody = capCompactionSummary(summaryBody, bodyBudget);