diff --git a/open-sse/services/compression/ultraHeuristic.ts b/open-sse/services/compression/ultraHeuristic.ts
index 8a82715918..546830a48b 100644
--- a/open-sse/services/compression/ultraHeuristic.ts
+++ b/open-sse/services/compression/ultraHeuristic.ts
@@ -3,6 +3,10 @@
  *
  * Scores tokens by information density and prunes low-value tokens
  * to achieve a target compression rate.
+ *
+ * #13454: Polarity/modality words (never, always, no, not, must, etc.) must
+ * NOT be prunable — dropping them flips the meaning of the sentence.
+ * "must never be deleted" → "must deleted" is worse than no compression.
  */
 
 export const STOPWORDS = new Set([
@@ -19,18 +23,17 @@ export const STOPWORDS = new Set([
   "have",
   "has",
   "had",
-  "do",
-  "does",
-  "did",
+  // #13454: "do/does/did" removed — carry polarity in imperatives
+  // ("do not push") and negations ("don't"). Dropping them flips
+  // instruction meaning.
   "will",
   "would",
   "could",
-  "should",
+  // #13454: "should" removed — modality word in instructions.
   "may",
   "might",
   "shall",
-  "can",
-  "need",
+  // #13454: "can/need" removed — modal auxiliaries in instructions.
   "dare",
   "ought",
   "used",
@@ -59,7 +62,7 @@ export const STOPWORDS = new Set([
   "and",
   "but",
   "or",
-  "nor",
+  // #13454: "nor" removed — negation word.
   "for",
   "yet",
   "so",
@@ -88,8 +91,8 @@ export const STOPWORDS = new Set([
   "even",
   "still",
   "already",
-  "always",
-  "never",
+  // #13454: "always/never" removed — polarity words, highest-value tokens
+  // in instructions. "never" → score 0.1 was the root cause of #13454.
   "often",
   "usually",
   "sometimes",
@@ -100,6 +103,20 @@ export const STOPWORDS = new Set([
 /** Regex for tokens that must never be pruned */
 export const FORCE_PRESERVE_RE = /\d|https?:\/\/|[._\/\\]|Error:|Exception:|```/i;
 
+// #13454: Polarity, modality, and negation words that must never be pruned.
+// Dropping these flips the meaning of the sentence they appear in.
+const POLARITY_WORDS = new Set([
+  "never", "always", "no", "not", "nor",
+  "must", "shall", "shall not",
+  "do", "does", "did",
+  "don't", "doesn't", "didn't",
+  "can", "cannot", "can't",
+  "should", "shouldn't",
+  "need", "needs", "mustn't",
+  "won't", "wouldn't",
+  "could", "couldn't",
+]);
+
 /**
  * Score a single token (word/symbol) for information value.
  * Returns 0.0 (prune candidate) to 1.0 (must keep).
@@ -107,6 +124,8 @@ export const FORCE_PRESERVE_RE = /\d|https?:\/\/|[._\/\\]|Error:|Exception:|```/
 export function scoreToken(token: string): number {
   if (FORCE_PRESERVE_RE.test(token)) return 1.0;
   const lower = token.toLowerCase();
+  // #13454: polarity words always score 1.0 — never prunable
+  if (POLARITY_WORDS.has(lower)) return 1.0;
   if (STOPWORDS.has(lower)) return 0.1;
   if (token.length <= 2) return 0.2;
   if (/^[A-Z]/.test(token)) return 0.8; // proper nouns / identifiers
@@ -152,6 +171,8 @@ export function pruneByScore(text: string, keepRate = 0.5, minScore = 0.3): stri
       return keep ? t : "";
     })
     .join("")
-    .replace(/\s{2,}/g, " ")
+    // #13454: Only collapse spaces/tabs, NOT newlines.
+    // Collapsing newlines destroys bullet lists, headings, and code fences.
+    .replace(/[ \t]{2,}/g, " ")
     .trim();
 }
diff --git a/scripts/check/compression-budget-baseline.json b/scripts/check/compression-budget-baseline.json
index f948293623..c9a6ed5f59 100644
--- a/scripts/check/compression-budget-baseline.json
+++ b/scripts/check/compression-budget-baseline.json
@@ -8,9 +8,9 @@
   },
   "caveman": {
     "tasks": {
-      "prose": 129,
-      "tool-output": 127,
-      "json": 160
+      "prose": 119,
+      "tool-output": 114,
+      "json": 136
     }
   },
   "aggressive": {
@@ -22,9 +22,9 @@
   },
   "ultra": {
     "tasks": {
-      "prose": 92,
-      "tool-output": 116,
-      "json": 117
+      "prose": 97,
+      "tool-output": 117,
+      "json": 126
     }
   },
   "rtk": {
diff --git a/tests/unit/compression/ultra-heuristic-polarity.test.ts b/tests/unit/compression/ultra-heuristic-polarity.test.ts
new file mode 100644
index 0000000000..4c46f507d6
--- /dev/null
+++ b/tests/unit/compression/ultra-heuristic-polarity.test.ts
@@ -0,0 +1,116 @@
+/**
+ * Tests for #13454: ultra heuristic must not prune polarity/modality words.
+ *
+ * The ultra heuristic engine scores tokens and prunes the lowest-scoring 50%.
+ * Before the fix, polarity words like "never", "always", "no", "not", "must"
+ * scored 0.1 (stopwords) or 0.2 (length ≤ 2), making them the first tokens
+ * pruned. This inverted instruction meaning:
+ *   "must never be deleted" → "must deleted"
+ *   "NEVER run rm -rf" → "run rm -rf"
+ *
+ * The fix adds polarity words to a force-preserve set (score 1.0) and stops
+ * collapsing newlines (which destroyed bullet lists and code fences).
+ */
+import { test } from "node:test";
+import assert from "node:assert/strict";
+import { scoreToken, pruneByScore } from "../../../open-sse/services/compression/ultraHeuristic.ts";
+
+test("scoreToken: polarity words score 1.0 (never prunable)", () => {
+  // These words MUST survive compression — they carry instruction polarity
+  const polarityWords = ["never", "always", "no", "not", "nor", "must", "do", "does", "did"];
+  for (const word of polarityWords) {
+    assert.equal(scoreToken(word), 1.0, `"${word}" should score 1.0 (force-preserved)`);
+  }
+});
+
+test("scoreToken: modal auxiliaries score 1.0 (never prunable)", () => {
+  // Modal auxiliaries in instructions must not be pruned
+  const modals = ["can", "should", "need", "shall"];
+  for (const word of modals) {
+    assert.equal(scoreToken(word), 1.0, `"${word}" should score 1.0 (modal auxiliary)`);
+  }
+});
+
+test("scoreToken: contractions score 1.0", () => {
+  const contractions = ["don't", "doesn't", "didn't", "can't", "cannot", "won't"];
+  for (const word of contractions) {
+    assert.equal(scoreToken(word), 1.0, `"${word}" should score 1.0 (contraction)`);
+  }
+});
+
+test("scoreToken: regular stopwords still score 0.1", () => {
+  // Words that are genuinely low-value should still be prunable
+  const stopwords = ["a", "the", "is", "are", "was", "were", "in", "of", "on"];
+  for (const word of stopwords) {
+    assert.equal(scoreToken(word), 0.1, `"${word}" should still score 0.1`);
+  }
+});
+
+test("pruneByScore: polarity words survive pruning", () => {
+  const block = `- NEVER run \`rm -rf\` on the target host. Always ask first.
+- Do not push to \`main\` directly; open a PR.
+- The backup files \`.app-prev-*\` must never be deleted.
+- Never store the SSH password on disk.
+- Always run \`npm test\` before \`npm run build\`.
+- Do NOT edit files under \`/etc\` by hand.`;
+
+  // Default engine settings: keepRate 0.5, minScore 0.3
+  const result = pruneByScore(block, 0.5, 0.3);
+
+  // All polarity words MUST survive
+  assert.ok(result.includes("never") || result.includes("NEVER"), "MUST preserve 'never'/'NEVER'");
+  assert.ok(
+    result.includes("always") || result.includes("Always"),
+    "MUST preserve 'always'/'Always'"
+  );
+  assert.ok(result.includes("not") || result.includes("NOT"), "MUST preserve 'not'/'NOT'");
+  assert.ok(result.includes("Do"), "MUST preserve 'Do'");
+});
+
+test("pruneByScore: newlines are preserved (not collapsed to spaces)", () => {
+  const block = `Line one
+Line two
+Line three`;
+
+  const result = pruneByScore(block, 1.0); // keepRate=1.0 means keep everything
+
+  // With keepRate=1.0 nothing is pruned, but we verify newlines survive
+  assert.ok(result.includes("\n"), "Newlines must be preserved when keepRate=1.0");
+  assert.equal(result, block, "Full keepRate should return identical text");
+});
+
+test("pruneByScore: newlines survive even with pruning", () => {
+  const block = `- NEVER do X
+- ALWAYS do Y
+- NEVER do Z`;
+
+  const result = pruneByScore(block, 0.7, 0.3);
+
+  // The line breaks between bullets should survive
+  const lines = result.split("\n");
+  assert.ok(lines.length >= 2, "Line breaks between bullets must be preserved");
+});
+
+test("pruneByScore: sample from #13454 issue preserves meaning", () => {
+  const block = `- NEVER run \`rm -rf\` on the target host. Always ask first.
+- Do not push to \`main\` directly; open a PR.
+- The backup files \`.app-prev-*\` must never be deleted.
+- Never store the SSH password on disk.
+- Always run \`npm test\` before \`npm run build\`.
+- Do NOT edit files under \`/etc\` by hand.`;
+
+  const result = pruneByScore(block, 0.5, 0.3);
+
+  // After the fix, NONE of these meaning-critical words should be pruned:
+  assert.ok(result.includes("NEVER") || result.includes("never"), "NEVER must survive");
+  assert.ok(result.includes("Always") || result.includes("always"), "Always must survive");
+  assert.ok(result.includes("not") || result.includes("NOT"), "not/NOT must survive");
+  assert.ok(result.includes("Do") || result.includes("do"), "Do/do must survive");
+  assert.ok(result.includes("must") || result.includes("MUST"), "must/MUST must survive");
+
+  // The critical test: "must never" must NOT become "must" alone
+  assert.ok(
+    !result.match(/\bmust\b(?![\s\S]*never)/) || result.includes("never"),
+    "must and never must both survive together"
+  );
+});