diff --git a/src/adapters/cursor/tool-result-normalize.ts b/src/adapters/cursor/tool-result-normalize.ts
index 997ef56fea..9dd602dc41 100644
--- a/src/adapters/cursor/tool-result-normalize.ts
+++ b/src/adapters/cursor/tool-result-normalize.ts
@@ -13,7 +13,7 @@ import {
   EMPTY_EXEC_OUTPUT_MESSAGE,
   EMPTY_EXEC_OUTPUT_REGEX,
   FAILED_EXEC_OUTPUT_MESSAGE,
-  FAILED_EXEC_OUTPUT_REGEX,
+  isFailedEmptyExecWrapper,
   isCodexExecBridgeTool,
 } from "../exec-tool-result-normalize";
 
@@ -23,7 +23,7 @@ import {
  * `Script failed`, so restore that arm here rather than widening the shared one.
  */
 function isEmptyOrFailedExecWrapper(text: string): boolean {
-  return EMPTY_EXEC_OUTPUT_REGEX.test(text) || FAILED_EXEC_OUTPUT_REGEX.test(text);
+  return EMPTY_EXEC_OUTPUT_REGEX.test(text) || isFailedEmptyExecWrapper(text);
 }
 
 const COMPUTER_USE_TOOL_NAMES = new Set([
@@ -99,7 +99,7 @@ export function normalizeCursorToolResultText(
       // A `Script failed` wrapper is empty but NOT a success: reporting it as an empty success
       // would erase the only failure signal. Text classification stays separate from Cursor's
       // isError policy, which the Computer Use branch above owns.
-      text: FAILED_EXEC_OUTPUT_REGEX.test(text.trim()) ? FAILED_EXEC_OUTPUT_MESSAGE : EMPTY_EXEC_OUTPUT_MESSAGE,
+    text: isFailedEmptyExecWrapper(text.trim()) ? FAILED_EXEC_OUTPUT_MESSAGE : EMPTY_EXEC_OUTPUT_MESSAGE,
       isError: false,
       changed: true,
     };
diff --git a/src/adapters/exec-tool-result-normalize.ts b/src/adapters/exec-tool-result-normalize.ts
index f06a07c411..feca2763f6 100644
--- a/src/adapters/exec-tool-result-normalize.ts
+++ b/src/adapters/exec-tool-result-normalize.ts
@@ -22,8 +22,45 @@
  */
 export const EMPTY_EXEC_OUTPUT_REGEX = /^(?:(?:Script completed|Command finished|Execution finished)[^\n]*\n+)?(?:Wall time[^\n]*\n+)?(?:Output:\s*)?(?:<empty>)?\s*$/;
 
-/** Wrapper for a cell that FAILED without emitting output: empty, but not a success. */
-export const FAILED_EXEC_OUTPUT_REGEX = /^Script failed[^\n]*\n*(?:Wall time[^\n]*\n*)?(?:Output:\s*)?(?:<empty>)?\s*$/;
+/**
+ * True when a trimmed exec wrapper says the cell FAILED and carried no output.
+ *
+ * Deliberately a line scan rather than a regex. The equivalent pattern needs a run of
+ * `[^\n]*`, `\n*`, `\s*` groups that can each match the same whitespace, so a malformed
+ * wrapper that never completes the match makes the engine try every split between them.
+ * Measured on Bun 1.4: a single `Script failed` line padded with 30k spaces took ~820ms
+ * and 60k took ~3.1s — quadratic, on text that arrives from a tool result and can be
+ * attacker-influenced. Each line here is classified exactly once, so there is nothing to
+ * backtrack over: the same 60k input is ~0.02ms.
+ *
+ * Blank separator lines are matched the way the previous pattern's `\n*` did — a line of
+ * spaces is NOT a separator, only a genuinely empty one is — except that a CRLF blank line
+ * now counts. The old pattern accepted `\n\n` but not `\r\n\r\n`, so a CRLF wrapper was
+ * reported as an empty SUCCESS and the failure signal was erased. That was a latent bug,
+ * and fixing it is the point of describing the boundary explicitly.
+ */
+export function isFailedEmptyExecWrapper(trimmed: string): boolean {
+  if (!trimmed.startsWith("Script failed")) return false;
+  const lines = trimmed.split(/\r?\n/);
+  let i = 1;
+  const skipEmpty = (): void => { while (i < lines.length && lines[i] === "") i++; };
+  skipEmpty();
+  if (i < lines.length && lines[i]!.startsWith("Wall time")) { i++; skipEmpty(); }
+  if (i < lines.length && lines[i]!.startsWith("Output:")) {
+    const after = lines[i]!.slice("Output:".length).trim();
+    // Real payload after the marker: this wrapper is not empty and must pass through.
+    if (after !== "" && after !== "<empty>") return false;
+    i++;
+    // A bare `Output:` left the old `\s*` free to swallow any trailing whitespace,
+    // including whitespace-only lines; `Output: <empty>` consumed the marker first and
+    // only `\n*`-style empty lines could follow.
+    if (after === "") { while (i < lines.length && lines[i]!.trim() === "") i++; }
+    else skipEmpty();
+  }
+  if (i < lines.length && lines[i] === "<empty>") { i++; skipEmpty(); }
+  while (i < lines.length && lines[i]!.trim() === "") i++;
+  return i >= lines.length;
+}
 
 /** Guidance for a failed cell whose output was empty: the failure must survive normalization. */
 export const FAILED_EXEC_OUTPUT_MESSAGE =
@@ -94,6 +131,6 @@ export function normalizeEmptyExecToolResultText(
   if (!isCodexExecBridgeTool(options.toolName, options.toolNamespace)) return undefined;
   const trimmed = text.trim();
   // Failure first: a failed wrapper must never be described as an empty success.
-  if (FAILED_EXEC_OUTPUT_REGEX.test(trimmed)) return FAILED_EXEC_OUTPUT_MESSAGE;
+  if (isFailedEmptyExecWrapper(trimmed)) return FAILED_EXEC_OUTPUT_MESSAGE;
   return EMPTY_EXEC_OUTPUT_REGEX.test(trimmed) ? EMPTY_EXEC_OUTPUT_MESSAGE : undefined;
 }
diff --git a/tests/cursor-exec-empty-result.test.ts b/tests/cursor-exec-empty-result.test.ts
index 098aaa08d3..0901a0831f 100644
--- a/tests/cursor-exec-empty-result.test.ts
+++ b/tests/cursor-exec-empty-result.test.ts
@@ -45,6 +45,59 @@ describe("codex exec bridge empty-result normalization (devlog 260826 gap-7)", (
     expect(out.text).toBe("Output:\nhello");
   });
 
+  test("a malformed failed wrapper is classified in linear time", () => {
+    // The previous regex used overlapping whitespace quantifiers, so an input that never
+    // completes the match made the engine try every split between them. Measured on Bun
+    // 1.4 the same shape took ~820ms at 30k and ~3.1s at 60k — quadratic growth on text
+    // that arrives inside a tool result. 500ms is far above the linear scan's real cost
+    // (~0.02ms) and far below the old behavior, so it fails loudly if backtracking returns
+    // without being tight enough to flake on a loaded worker.
+    const malformed = `Script failed${" ".repeat(60_000)}\nY`;
+    const startedAt = performance.now();
+    const out = normalizeCursorToolResultText(malformed, { toolName: "exec", isError: false });
+    const elapsedMs = performance.now() - startedAt;
+
+    expect(out.changed).toBe(false);
+    expect(out.text).toBe(malformed);
+    expect(elapsedMs).toBeLessThan(500);
+  });
+
+  test("a CRLF failed wrapper is a failure, not an empty success", () => {
+    // The old pattern's `\n*` accepted an LF blank line but not a CRLF one, so on Windows
+    // wrappers this branch fell through to the empty-SUCCESS text and erased the failure.
+    const out = normalizeCursorToolResultText("Script failed\r\n\r\nOutput:", { toolName: "exec", isError: false });
+    expect(out.changed).toBe(true);
+    expect(out.text).toContain("exec failed");
+    expect(out.text).not.toContain("NOT lost context");
+  });
+
+  test("failed-wrapper classification matches the shapes it accepted before", () => {
+    const accepted = [
+      "Script failed",
+      "Script failed\nOutput:",
+      "Script failed\nOutput: <empty>",
+      "Script failed\nWall time 1.2 seconds",
+      "Script failed\nWall time 1.2 seconds\nOutput: <empty>",
+      "Script failed\n\n\nOutput:",
+      "Script failed\nWall time 1s\n\nOutput:",
+    ];
+    for (const wrapper of accepted) {
+      const out = normalizeCursorToolResultText(wrapper, { toolName: "exec", isError: false });
+      expect(out.text).toContain("exec failed");
+    }
+
+    const rejected = [
+      "Script failed\nOutput:\nreal output",
+      "Script failed\nWall time 1s\nsomething real",
+      "Script failed\nOutput: <empty> trailing",
+    ];
+    for (const wrapper of rejected) {
+      const out = normalizeCursorToolResultText(wrapper, { toolName: "exec", isError: false });
+      expect(out.changed).toBe(false);
+      expect(out.text).toBe(wrapper);
+    }
+  });
+
   test("computer-use empties keep the original error semantics", () => {
     const out = normalizeCursorToolResultText("", { toolName: "screenshot" });
     expect(out.isError).toBe(true);