diff --git a/A00_Common/design/design_profile.ts b/A00_Common/design/design_profile.ts
index 45cf43496..8cbb093ff 100644
--- a/A00_Common/design/design_profile.ts
+++ b/A00_Common/design/design_profile.ts
@@ -71,6 +71,7 @@ import {
 } from "../../B05_Profile/B05_Profile_UI_Profile_Drag";
 import { planCurveWarning, sampleAt } from "../../B05_Profile/logic/B05_Profile_UI_Profile_Guard";
 import { listedBase } from "./design_profile_base";
+import { tidyEdits } from "../../B05_Profile/logic/B05_Profile_UI_Profile_Straighten";
 import * as pagePipeline from "./design_pipeline_page";
 import type { ElevationWindowResult } from "@util/common_util_chart_ywindow";
 import { frameOf } from "@util/common_util_view_window";
@@ -303,6 +304,7 @@ export function createDesignProfile(ctx: DesignProfileContext) {
   /** 편집을 적용한다. 법정 위반 정책이 block이면 새 위반이 생기는 편집을 막는다. false = 막힘. */
   function applyEdits(next: AlignmentEdits): boolean {
     if (!base || !alignment) return false;
+    next = tidyEdits(base, next); // 직선 위 R · 0 오프셋 지움 · 모양 그대로(426)
     const candidate = buildAlignment(base, next);
     if (
       base.policy.grade_violation_policy === "block" &&
diff --git a/A00_Common/design/logic/design_profile_free.ts b/A00_Common/design/logic/design_profile_free.ts
index c658fca63..77d0ae4a9 100644
--- a/A00_Common/design/logic/design_profile_free.ts
+++ b/A00_Common/design/logic/design_profile_free.ts
@@ -27,7 +27,9 @@ import {
 import {
   baseElevationAt,
   defaultCurveLength,
+  pinEnds,
 } from "../../../B05_Profile/logic/B05_Profile_UI_Profile_Straighten";
+import { isStraightNode } from "../../../B05_Profile/logic/B05_Profile_Profile_Straight";
 
 /** 두 chainage 를 같은 측점으로 볼 허용 오차(m). */
 const SAME_M = 1e-6;
@@ -131,7 +133,8 @@ export function buildFreeEdits(
   const toM = path[path.length - 1].chainage_m;
   if (toM - fromM <= SAME_M) return edits;
   const inner = new Map<string, number>();
-  buildAlignment(base, edits).pvi.forEach((node) => {
+  const current = buildAlignment(base, edits);
+  current.pvi.forEach((node) => {
     if (node.chainage_m > fromM + SAME_M && node.chainage_m < toM - SAME_M) {
       inner.set(chainageKey(node.chainage_m), node.chainage_m);
     }
@@ -141,6 +144,10 @@ export function buildFreeEdits(
     .forEach((vertex) => inner.set(chainageKey(vertex.chainage_m), vertex.chainage_m));
   const offsets = { ...edits.station_offsets };
   const radii = { ...edits.curve_radii };
+  // 끝이 꼭짓점 아닌 측점이면 지금 높이로 꼭짓점(426) — 직선화와 같음
+  pinEnds(base, current, [fromM, toM]).forEach(([key, value]) => {
+    offsets[key] = Number(value.toFixed(6));
+  });
   inner.forEach((chainage, key) => {
     const target = elevationOnPath(path, chainage);
     offsets[key] = Number((target - baseElevationAt(base, chainage)).toFixed(6));
@@ -150,7 +157,7 @@ export function buildFreeEdits(
     const gradeIn = gradePct(path[index - 1], path[index]) / 100;
     const gradeOut = gradePct(path[index], path[index + 1]) / 100;
     const deltaGrade = Math.abs(gradeOut - gradeIn);
-    if (deltaGrade < 1e-9) continue;
+    if (isStraightNode(path[index - 1], path[index], path[index + 1])) continue;
     radii[chainageKey(path[index].chainage_m)] = Number(
       (defaultCurveLength(base, deltaGrade) / deltaGrade).toFixed(6),
     );
diff --git a/B05_Profile/B05_Profile_Engine_Grade_Alignment.py b/B05_Profile/B05_Profile_Engine_Grade_Alignment.py
index d5e0f1223..9cba2cb6f 100644
--- a/B05_Profile/B05_Profile_Engine_Grade_Alignment.py
+++ b/B05_Profile/B05_Profile_Engine_Grade_Alignment.py
@@ -13,6 +13,9 @@
 수치 최적화(직선 분할 DP·표고 결정)는 [[B05_Profile_Engine_Grade_Solver]]가,
 기준 해석과 오케스트레이션은 [[B05_Profile_Engine_Grade]]가 담당한다. 이 모듈은
 scipy에 의존하지 않는 순수 기하 계산만 두어 프론트엔드 구현과 1:1로 대응시킨다.
+
+직선 판정 짝: `is_straight_bend` · `STRAIGHT_HEIGHT_M` ↔ TS `logic/B05_Profile_Profile_Straight.ts`
+(거울 시험 `test_426_profile_straight.py`).
 """
 
 from dataclasses import dataclass
@@ -27,6 +30,17 @@ ALIGNMENT_SCHEMA_VERSION = 1
 _KEY_DECIMALS = 3
 
 
+# 직선으로 볼 꼭짓점 높이 어긋남(m) — TS `STRAIGHT_HEIGHT_M` 과 같은 값(426).
+STRAIGHT_HEIGHT_M = 1e-5
+
+
+def is_straight_bend(delta: float, span_left: float, span_right: float) -> bool:
+    """꼭짓점이 이웃 둘 잇는 선에서 `STRAIGHT_HEIGHT_M` 안이면 직선 — 높이 = |Δ|·a·b/(a+b)."""
+    if not (span_left > 0 and span_right > 0):
+        return True
+    return abs(delta) * span_left * span_right / (span_left + span_right) < STRAIGHT_HEIGHT_M
+
+
 def chainage_key(value: float) -> str:
     """chainage를 편집 델타 dict의 키 문자열로 바꾼다(프론트와 동일 규칙)."""
     return f"{float(value):.{_KEY_DECIMALS}f}"
@@ -303,8 +317,8 @@ def build_curves(
         delta = grade_out - grade_in
         chainage = float(pvi_s[index])
         key = chainage_key(chainage)
-        if abs(delta) < 1e-9:
-            continue
+        if is_straight_bend(delta, float(spans[index - 1]), float(spans[index])):
+            continue  # 직선 판정 한 벌(426)
         if only_explicit and overrides.get(key) is None:
             continue
         # 법정 다-(3)-(다)는 "종단곡선을 두지 않을 수 있다"는 허용 조항이다.
diff --git a/B05_Profile/B05_Profile_UI_Profile_Balance.ts b/B05_Profile/B05_Profile_UI_Profile_Balance.ts
index 03013d8e5..5cbe53030 100644
--- a/B05_Profile/B05_Profile_UI_Profile_Balance.ts
+++ b/B05_Profile/B05_Profile_UI_Profile_Balance.ts
@@ -16,6 +16,7 @@
 
 import type { ProfileAlignment } from "./logic/B05_Profile_UI_Profile_Alignment";
 import type { MinCoverViolation } from "./B05_Profile_UI_Profile_MinCover";
+import { bentEditCount } from "./logic/B05_Profile_Profile_Straight";
 
 export interface BalanceBarParams {
   /** 표시줄 컨테이너. 그릴 때마다 통째로 갈아 끼운다. */
@@ -99,7 +100,7 @@ export function renderBalanceBar(params: BalanceBarParams): void {
   if (curvesNeeded) entries.push(["종단곡선 필요", `${curvesNeeded} 곳`, "over"]);
   // 횡단배수 최소고 표시는 2026-09-02 사용자 지시로 삭제했다. 편집 차단(강제)은
   // `B05_Profile_UI_Profile_Render.ts` 에 그대로 남아 있고 기본 해제다.
-  const editedCount = Object.keys(alignment.edits.station_offsets).length;
+  const editedCount = bentEditCount(alignment); // 꺾인 편집 꼭짓점만(426)
   if (editedCount) entries.push(["편집 측점", `${editedCount} 개`, "edited"]);
   entries.forEach(([label, value, tone]) => {
     const item = document.createElement("span");
diff --git a/B05_Profile/logic/B05_Profile_Profile_Straight.ts b/B05_Profile/logic/B05_Profile_Profile_Straight.ts
new file mode 100644
index 000000000..26ec4f71e
--- /dev/null
+++ b/B05_Profile/logic/B05_Profile_Profile_Straight.ts
@@ -0,0 +1,56 @@
+/* =============================================================================
+ * B05_Profile_Profile_Straight.ts
+ * 종단 「직선인가」 판정 한 벌(426 · 분석 424 6장 · 10장) — 꼭짓점이 이웃 둘을 잇는 선에서
+ * `STRAIGHT_HEIGHT_M` 안이면 직선(꺾임 아님).
+ *
+ * 짝: `B05_Profile_Engine_Grade_Alignment.py` `is_straight_bend` · `STRAIGHT_HEIGHT_M` —
+ * 거울 시험 `test_426_profile_straight.py`.
+ *
+ * 기울기 정확 비교(1e-9 · 1e-6)는 꼭짓점 높이를 소수 여섯째 자리로 끊은 값(±5e-7 m)이 측점
+ * 간격 1 m 안(구조물 측점)에서 기울기 차로 커져 직선을 꺾임으로 읽었다 — 높이로 잰다.
+ * 부르는 곳: 곡선 넣기(`buildCurves`) · ▲▼ 축(`pivotAxes`) · 직선 찾기(`detectStraightRun`) ·
+ * 편집 정리(`tidyEdits`) · 편집 측점 수. 표의 대수차 빈칸은 안전망으로 따로 둔다.
+ * ========================================================================== */
+
+import type { AlignmentNode, ProfileAlignment } from "./B05_Profile_UI_Profile_Alignment";
+
+/** 직선으로 볼 꼭짓점 높이 어긋남(m) — 파이썬 `STRAIGHT_HEIGHT_M` 과 같은 값. */
+export const STRAIGHT_HEIGHT_M = 1e-5;
+
+/** 대수차 · 좌우 마디 길이로 본 직선 — 꼭짓점이 이웃 잇는 선에서 떨어진 높이 = |Δ|·a·b/(a+b). */
+export function isStraightBend(delta: number, spanLeft: number, spanRight: number): boolean {
+  const span = spanLeft + spanRight;
+  if (!(spanLeft > 0 && spanRight > 0)) return true;
+  return (Math.abs(delta) * spanLeft * spanRight) / span < STRAIGHT_HEIGHT_M;
+}
+
+/** 세 꼭짓점의 가운데가 직선 위인가. */
+export function isStraightNode(
+  prev: AlignmentNode,
+  node: AlignmentNode,
+  next: AlignmentNode,
+): boolean {
+  const spanLeft = node.chainage_m - prev.chainage_m;
+  const spanRight = next.chainage_m - node.chainage_m;
+  if (!(spanLeft > 0 && spanRight > 0)) return true;
+  const delta =
+    (next.elevation_m - node.elevation_m) / spanRight -
+    (node.elevation_m - prev.elevation_m) / spanLeft;
+  return isStraightBend(delta, spanLeft, spanRight);
+}
+
+/** index 번째 꼭짓점이 직선 위인가 — 시종점은 꺾임으로 본다(false). */
+export function straightAt(pvi: ReadonlyArray<AlignmentNode>, index: number): boolean {
+  if (index <= 0 || index >= pvi.length - 1) return false;
+  return isStraightNode(pvi[index - 1], pvi[index], pvi[index + 1]);
+}
+
+/** 「편집 측점 N개」 — 높이를 고친 꼭짓점 중 실제로 꺾인 것(시종점은 오프셋이 있으면). */
+export function bentEditCount(alignment: ProfileAlignment): number {
+  const last = alignment.pvi.length - 1;
+  return alignment.pvi.filter((node, index) => {
+    if (node.source !== "user") return false;
+    if (index === 0 || index === last) return true;
+    return !straightAt(alignment.pvi, index);
+  }).length;
+}
diff --git a/B05_Profile/logic/B05_Profile_UI_Profile_Alignment.ts b/B05_Profile/logic/B05_Profile_UI_Profile_Alignment.ts
index 1612a8dca..9462f9b31 100644
--- a/B05_Profile/logic/B05_Profile_UI_Profile_Alignment.ts
+++ b/B05_Profile/logic/B05_Profile_UI_Profile_Alignment.ts
@@ -14,6 +14,7 @@
  * ========================================================================== */
 
 import { pivotAxes } from "./B05_Profile_UI_Profile_Pivot";
+import { isStraightBend } from "./B05_Profile_Profile_Straight";
 
 export interface AlignmentPolicy {
   station_interval_m: number;
@@ -305,7 +306,7 @@ export function buildCurves(
     const gradeIn = (pvi[index].elevation_m - pvi[index - 1].elevation_m) / spanLeft;
     const gradeOut = (pvi[index + 1].elevation_m - pvi[index].elevation_m) / spanRight;
     const delta = gradeOut - gradeIn;
-    if (Math.abs(delta) < 1e-9) continue;
+    if (isStraightBend(delta, spanLeft, spanRight)) continue; // 직선 판정 한 벌(426)
     const chainage = pvi[index].chainage_m;
     const key = chainageKey(chainage);
     const skipAllowed = !policy.paved && Math.abs(delta) <= skipDelta + 1e-12;
diff --git a/B05_Profile/logic/B05_Profile_UI_Profile_Pivot.ts b/B05_Profile/logic/B05_Profile_UI_Profile_Pivot.ts
index 6afe531e8..0977a27c3 100644
--- a/B05_Profile/logic/B05_Profile_UI_Profile_Pivot.ts
+++ b/B05_Profile/logic/B05_Profile_UI_Profile_Pivot.ts
@@ -10,9 +10,8 @@
  * ========================================================================== */
 
 import type { ProfileAlignment } from "./B05_Profile_UI_Profile_Alignment";
+import { straightAt } from "./B05_Profile_Profile_Straight";
 
-/** 같은 직선으로 볼 기울기 차이 — `B05_Profile_UI_Profile_Straighten` 과 같은 값. */
-const COLLINEAR_EPSILON = 1e-6;
 const SAME_STATION_M = 1e-6;
 
 export interface PivotAxes {
@@ -26,21 +25,17 @@ export interface PivotAxes {
 
 /** 누른 점의 두 축과 사이 변화점 — 시종점 · 계획선 밖이면 null. */
 export function pivotAxes(alignment: ProfileAlignment, chainageM: number): PivotAxes | null {
-  const { pvi, segments } = alignment;
+  const { pvi } = alignment;
   const index = pvi.findIndex((node) => Math.abs(node.chainage_m - chainageM) < SAME_STATION_M);
   // 꼭짓점 아닌 자리 = 그 점을 품은 마디 하나가 좌우 마디
   const right = index >= 0 ? index : pvi.findIndex((node) => node.chainage_m > chainageM);
   if (right <= 0 || (index >= 0 && index >= pvi.length - 1)) return null;
   const [leftSeg, rightSeg] = index >= 0 ? [index - 1, index] : [right - 1, right - 1];
-  const grade = (at: number): number => {
-    const segment = segments[at];
-    return segment && segment.length_m > 1e-12 ? segment.height_m / segment.length_m : 0;
-  };
-  const same = (a: number, b: number): boolean => Math.abs(grade(a) - grade(b)) < COLLINEAR_EPSILON;
+  // 이웃끼리 — 꼭짓점이 직선 위(높이 기준 · 426)면 건넘
   let from = leftSeg;
-  while (from > 0 && same(from - 1, leftSeg)) from -= 1;
+  while (from > 0 && straightAt(pvi, from)) from -= 1;
   let to = rightSeg + 1;
-  while (to < pvi.length - 1 && same(to, rightSeg)) to += 1;
+  while (to < pvi.length - 1 && straightAt(pvi, to)) to += 1;
   return {
     fromM: pvi[from].chainage_m,
     fromZ: pvi[from].elevation_m,
diff --git a/B05_Profile/logic/B05_Profile_UI_Profile_Straighten.ts b/B05_Profile/logic/B05_Profile_UI_Profile_Straighten.ts
index 74bc054e7..843024404 100644
--- a/B05_Profile/logic/B05_Profile_UI_Profile_Straighten.ts
+++ b/B05_Profile/logic/B05_Profile_UI_Profile_Straighten.ts
@@ -26,9 +26,8 @@ import {
   chainageKey,
   controlElevationAt,
 } from "./B05_Profile_UI_Profile_Alignment";
+import { isStraightNode, straightAt } from "./B05_Profile_Profile_Straight";
 
-/** 같은 직선 위에 있다고 볼 기울기 차이(무차원). 0.1m 편집 단위의 반올림 오차보다 크게 둔다. */
-const COLLINEAR_EPSILON = 1e-6;
 /** 두 chainage를 같은 측점으로 볼 허용 오차(m). */
 const SAME_STATION_M = 1e-6;
 
@@ -63,17 +62,10 @@ export function baseElevationAt(base: AlignmentBase, chainageM: number): number
   return last.elevation_m;
 }
 
-/** 변화점 사이 기울기 (index 번째 구간). */
-function gradeAt(alignment: ProfileAlignment, index: number): number {
-  const segment = alignment.segments[index];
-  if (!segment || segment.length_m <= 1e-12) return 0;
-  return segment.height_m / segment.length_m;
-}
-
 /**
  * 그 자리를 지나는 **최대 직선 구간**을 찾는다.
  *
- * 인접 구간의 기울기가 같으면(= 변화점에 꺾임이 없으면) 한 직선으로 본다. 전체 측점
+ * 사이 꼭짓점이 직선 위(높이 기준 · 426)면 한 직선으로 본다. 전체 측점
  * 폴리라인에서는 직선화하지 않은 자리마다 기울기가 달라지므로, 직선화한 구간만
  * 두 측점 이상으로 이어진 직선이 된다.
  */
@@ -89,18 +81,10 @@ export function detectStraightRun(
       chainageM >= segment.from_m - SAME_STATION_M && chainageM <= segment.to_m + SAME_STATION_M,
   );
   if (index < 0) return null;
-  const grade = gradeAt(alignment, index);
   let first = index;
   let last = index;
-  while (first > 0 && Math.abs(gradeAt(alignment, first - 1) - grade) < COLLINEAR_EPSILON) {
-    first -= 1;
-  }
-  while (
-    last < alignment.segments.length - 1 &&
-    Math.abs(gradeAt(alignment, last + 1) - grade) < COLLINEAR_EPSILON
-  ) {
-    last += 1;
-  }
+  while (first > 0 && straightAt(alignment.pvi, first)) first -= 1;
+  while (last < alignment.segments.length - 1 && straightAt(alignment.pvi, last + 1)) last += 1;
   // 한 구간(측점 두 개)뿐이면 직선화된 라인이 아니라 그냥 폴리라인 한 마디다.
   if (last === first) return null;
   const fromM = alignment.segments[first].from_m;
@@ -122,12 +106,7 @@ export function detectStraightRun(
  * (2026-09-03 사용자 보고). 안내를 나누려면 이 판정이 필요하다.
  */
 export function hasStraightRuns(alignment: ProfileAlignment): boolean {
-  for (let index = 1; index < alignment.segments.length; index += 1) {
-    if (Math.abs(gradeAt(alignment, index) - gradeAt(alignment, index - 1)) < COLLINEAR_EPSILON) {
-      return true;
-    }
-  }
-  return false;
+  return alignment.pvi.some((_node, index) => straightAt(alignment.pvi, index));
 }
 
 /** 두 측점 사이(끝 제외)의 변화점 chainage. */
@@ -177,7 +156,47 @@ export function straightenBetween(
     const target = startZ + ((endZ - startZ) * (chainage - fromM)) / span;
     return [chainageKey(chainage), target - baseElevationAt(base, chainage)];
   });
-  return withOffsets(edits, updates, inner.map(chainageKey));
+  return withOffsets(
+    edits,
+    [...updates, ...pinEnds(base, current, [fromM, toM])],
+    inner.map(chainageKey),
+  );
+}
+
+/**
+ * 범위 끝이 꼭짓점 아닌 측점(관 · 구조물 · 덧 · 곡선 측점)이면 지금 높이로 꼭짓점을 세운다(426) —
+ * 안 세우면 끝 높이가 옮긴 안쪽 점을 따라 바뀌어 직선이 다음 꼭짓점부터 시작했다.
+ */
+export function pinEnds(
+  base: AlignmentBase,
+  current: ProfileAlignment,
+  ends: number[],
+): Array<[string, number]> {
+  return ends
+    .filter((at) => !current.pvi.some((node) => Math.abs(node.chainage_m - at) < SAME_STATION_M))
+    .map((at) => [chainageKey(at), controlElevationAt(current, at) - baseElevationAt(base, at)]);
+}
+
+/**
+ * 편집 정리(426) — 직선 위 꼭짓점의 기억 R 을 지우고, 0 으로 돌아와 모양을 안 바꾸는 오프셋을
+ * 지운다(자동 꼭짓점 자리 · 또는 직선 위). 모양은 그대로 — 적용 한 곳(`applyEdits`)이 부른다.
+ */
+export function tidyEdits(base: AlignmentBase, edits: AlignmentEdits): AlignmentEdits {
+  const current = buildAlignment(base, edits);
+  const pvi = current.pvi;
+  const straight = new Set(
+    pvi
+      .filter((_node, index) => straightAt(pvi, index))
+      .map((node) => chainageKey(node.chainage_m)),
+  );
+  const auto = new Set(base.basePvi.map((node) => chainageKey(node.chainage_m)));
+  const offsets = { ...current.edits.station_offsets };
+  const radii = { ...current.edits.curve_radii };
+  Object.keys(radii).forEach((key) => straight.has(key) && delete radii[key]);
+  Object.entries(offsets).forEach(([key, value]) => {
+    if (Math.abs(value) < 1e-6 && (auto.has(key) || straight.has(key))) delete offsets[key];
+  });
+  return { station_offsets: offsets, curve_radii: radii };
 }
 
 /** 정책에서 라운드 기본 길이 L(m)을 읽는다 — 옛 저장분은 R 기준으로 되돌아간다. */
@@ -209,9 +228,8 @@ export function tiltStraightRun(
   );
   const startZ = controlElevationAt(current, run.fromM);
   const endZ = controlElevationAt(current, run.toM);
-  const span = run.toM - run.fromM;
-  const pivotZ =
-    startZ + ((endZ - startZ) * (pivot - run.fromM)) / (span > 1e-12 ? span : 1) + delta;
+  // 지금 경첩 높이 + 걸음(426) — 처음 직선 위 높이로 다시 잡으면 ▲ 두 번째가 안 오르고 ▼ 가 두 칸
+  const pivotZ = controlElevationAt(current, pivot) + delta;
 
   const updates: Array<[string, number]> = [
     [chainageKey(pivot), pivotZ - baseElevationAt(base, pivot)],
@@ -236,7 +254,16 @@ export function tiltStraightRun(
     updates,
     inner.filter((chainage) => Math.abs(chainage - pivot) >= SAME_STATION_M).map(chainageKey),
   );
-  if (deltaGrade < 1e-9) return next;
+  const straightPivot = isStraightNode(
+    { chainage_m: run.fromM, elevation_m: startZ },
+    { chainage_m: pivot, elevation_m: pivotZ },
+    { chainage_m: run.toM, elevation_m: endZ },
+  );
+  if (straightPivot) {
+    const curveRadii = { ...next.curve_radii };
+    delete curveRadii[chainageKey(pivot)];
+    return { ...next, curve_radii: curveRadii };
+  }
   return {
     ...next,
     curve_radii: {
diff --git a/resources/tester/helper_234_1_structure_station.cjs b/resources/tester/helper_234_1_structure_station.cjs
index 1ee3da8d5..c599bdb60 100644
--- a/resources/tester/helper_234_1_structure_station.cjs
+++ b/resources/tester/helper_234_1_structure_station.cjs
@@ -159,9 +159,13 @@ Promise.resolve(
     out.second = { count: snap.snapOldStations(again), toasts: [...toasts] };
 
     // ④ 계획고 편집 따라감 — 한꺼번에 여럿 옮김(사라진 둘 · 새 둘)
-    const pivot = loadTs("B05_Profile/logic/B05_Profile_UI_Profile_Pivot.ts");
+    const straightJudge = loadTs("B05_Profile/logic/B05_Profile_Profile_Straight.ts");
+    const pivot = loadTs("B05_Profile/logic/B05_Profile_UI_Profile_Pivot.ts", {
+      "./B05_Profile_Profile_Straight": straightJudge,
+    });
     const alignment = loadTs("B05_Profile/logic/B05_Profile_UI_Profile_Alignment.ts", {
       "./B05_Profile_UI_Profile_Pivot": pivot,
+      "./B05_Profile_Profile_Straight": straightJudge,
     });
     const gone = [
       { id: "pipe-80.17", chainage_m: 80.17 },
diff --git a/resources/tester/helper_344_2_route_pairs.cjs b/resources/tester/helper_344_2_route_pairs.cjs
index 222cbd91c..275b27073 100644
--- a/resources/tester/helper_344_2_route_pairs.cjs
+++ b/resources/tester/helper_344_2_route_pairs.cjs
@@ -20,9 +20,13 @@ function loadTs(file, stubs = {}) {
   return box.exports;
 }
 
-const pivot = loadTs("B05_Profile/logic/B05_Profile_UI_Profile_Pivot.ts");
+const straightJudge = loadTs("B05_Profile/logic/B05_Profile_Profile_Straight.ts");
+const pivot = loadTs("B05_Profile/logic/B05_Profile_UI_Profile_Pivot.ts", {
+  "./B05_Profile_Profile_Straight": straightJudge,
+});
 const alignment = loadTs("B05_Profile/logic/B05_Profile_UI_Profile_Alignment.ts", {
   "./B05_Profile_UI_Profile_Pivot": pivot,
+  "./B05_Profile_Profile_Straight": straightJudge,
 });
 const ground = loadTs("B05_Profile/logic/B05_Profile_Grade_Ground.ts", {
   "./B05_Profile_UI_Profile_Alignment": alignment,
diff --git a/resources/tester/helper_349_0v_span.cjs b/resources/tester/helper_349_0v_span.cjs
index 571141758..473788435 100644
--- a/resources/tester/helper_349_0v_span.cjs
+++ b/resources/tester/helper_349_0v_span.cjs
@@ -21,9 +21,13 @@ function loadTs(file, stubs = {}) {
 }
 
 const curve = loadTs("B05_Profile/logic/B05_Profile_UI_RouteEdit_Curve.ts");
-const pivot = loadTs("B05_Profile/logic/B05_Profile_UI_Profile_Pivot.ts");
+const straightJudge = loadTs("B05_Profile/logic/B05_Profile_Profile_Straight.ts");
+const pivot = loadTs("B05_Profile/logic/B05_Profile_UI_Profile_Pivot.ts", {
+  "./B05_Profile_Profile_Straight": straightJudge,
+});
 const alignment = loadTs("B05_Profile/logic/B05_Profile_UI_Profile_Alignment.ts", {
   "./B05_Profile_UI_Profile_Pivot": pivot,
+  "./B05_Profile_Profile_Straight": straightJudge,
 });
 const carry = loadTs("B05_Profile/logic/B05_Profile_Grade_Carry.ts", {
   "./B05_Profile_UI_Profile_Alignment": alignment,
diff --git a/resources/tester/helper_400_profile_carry.cjs b/resources/tester/helper_400_profile_carry.cjs
index 1416357d7..758fd809a 100644
--- a/resources/tester/helper_400_profile_carry.cjs
+++ b/resources/tester/helper_400_profile_carry.cjs
@@ -20,9 +20,13 @@ function loadTs(file, stubs = {}) {
   return box.exports;
 }
 
-const pivot = loadTs("B05_Profile/logic/B05_Profile_UI_Profile_Pivot.ts");
+const straightJudge = loadTs("B05_Profile/logic/B05_Profile_Profile_Straight.ts");
+const pivot = loadTs("B05_Profile/logic/B05_Profile_UI_Profile_Pivot.ts", {
+  "./B05_Profile_Profile_Straight": straightJudge,
+});
 const alignment = loadTs("B05_Profile/logic/B05_Profile_UI_Profile_Alignment.ts", {
   "./B05_Profile_UI_Profile_Pivot": pivot,
+  "./B05_Profile_Profile_Straight": straightJudge,
 });
 const carry = loadTs("B05_Profile/logic/B05_Profile_Grade_Carry.ts", {
   "./B05_Profile_UI_Profile_Alignment": alignment,
diff --git a/resources/tester/helper_426_profile_straight.cjs b/resources/tester/helper_426_profile_straight.cjs
new file mode 100644
index 000000000..e94097ecd
--- /dev/null
+++ b/resources/tester/helper_426_profile_straight.cjs
@@ -0,0 +1,189 @@
+/* 426 종단 직선 판정 한 벌 단위검증 헬퍼 — `B05_Profile_UI_Profile_Alignment.ts` · `_Straighten.ts` ·
+ * `_Pivot.ts` · `_Profile_Straight.ts` · `design_profile_free.ts` 를 그 자리에서 트랜스파일해 돌린다.
+ * 붙은 측점(100/100.4 · 120/120.25) 직선화 · 끝점 꼭짓점 아님 · 자유지정 R · 기울이기 누름 쌓임 ·
+ * 편집 측점 수 · 0 오프셋 정리 + 파이썬 거울 입력(base · edits · TS 곡선)을 JSON 으로 낸다.
+ */
+const fs = require("fs");
+const path = require("path");
+const ts = require(path.join(__dirname, "..", "..", "config", "node_modules", "typescript"));
+
+const ROOT = path.join(__dirname, "..", "..");
+const cache = new Map();
+function loadTs(file) {
+  const full = path.resolve(file);
+  if (cache.has(full)) return cache.get(full);
+  const js = ts.transpileModule(fs.readFileSync(full, "utf8"), {
+    compilerOptions: { module: ts.ModuleKind.CommonJS, target: ts.ScriptTarget.ES2020 },
+  }).outputText;
+  const box = { exports: {} };
+  cache.set(full, box.exports);
+  const localRequire = (request) => {
+    if (request.startsWith(".")) return loadTs(path.join(path.dirname(full), `${request}.ts`));
+    return require(request);
+  };
+  new Function("exports", "module", "require", js)(box.exports, box, localRequire);
+  return box.exports;
+}
+
+const LOGIC = path.join(ROOT, "B05_Profile", "logic");
+const align = loadTs(path.join(LOGIC, "B05_Profile_UI_Profile_Alignment.ts"));
+const straight = loadTs(path.join(LOGIC, "B05_Profile_UI_Profile_Straighten.ts"));
+const pivot = loadTs(path.join(LOGIC, "B05_Profile_UI_Profile_Pivot.ts"));
+const judge = loadTs(path.join(LOGIC, "B05_Profile_Profile_Straight.ts"));
+const free = loadTs(path.join(ROOT, "A00_Common", "design", "logic", "design_profile_free.ts"));
+
+const policy = {
+  station_interval_m: 20,
+  curve_radius_ratio: 1,
+  curve_tangent_max_ratio: 0.5,
+  curve_skip_legal_exception: false,
+  default_curve_length_m: 15,
+  default_curve_radius_m: 300,
+  balance_tolerance_percent: 10,
+  edit_step_m: 0.1,
+  grade_violation_policy: "warn",
+  max_grade_pct: 14,
+  curve_skip_delta_pct: 0,
+  paved: true,
+};
+const regular = Array.from({ length: 11 }, (_, i) => i * 20);
+const groundAt = (s) => 100 + 0.015 * s + 0.4 * Math.sin(s / 7) + 0.0371 * Math.cos(s * 3.1);
+const all = [...regular, 100.4, 120.25].sort((a, b) => a - b);
+/** 붙은 측점까지 꼭짓점(A) · 정규 측점만 꼭짓점(B). */
+function makeBase(pviS) {
+  return {
+    policy,
+    onlyExplicitCurves: false,
+    basePvi: pviS.map((s) => ({ chainage_m: s, elevation_m: Number(groundAt(s).toFixed(6)) })),
+    chainage: all,
+    ground: all.map((s) => Number(groundAt(s).toFixed(6))),
+    stations: all.map((s) => ({ station_id: `S${s}`, chainage_m: s })),
+  };
+}
+const baseA = makeBase(all);
+const baseB = makeBase(regular);
+const empty = align.emptyEdits();
+const build = (base, edits) => align.buildAlignment(base, edits);
+const curvesIn = (alignment, from, to) =>
+  alignment.curves
+    .filter((c) => c.chainage_m > from + 1e-6 && c.chainage_m < to - 1e-6)
+    .map((c) => c.chainage_m);
+const planAt = (alignment, s) => align.planElevationAt(alignment, s);
+const r6 = (v) => Number(v.toFixed(6));
+
+// ① 붙은 측점 직선화 40 → 140 (A)
+const sA = straight.straightenBetween(baseA, empty, 40, 140);
+const alA = build(baseA, sA);
+// 옛 판정(대수차 1e-9)이면 꺾임으로 읽혔을 안쪽 꼭짓점 수 — 시험이 반올림 흠을 실제로 거치는지
+let noiseBends = 0;
+alA.pvi.forEach((node, i) => {
+  if (i === 0 || i === alA.pvi.length - 1) return;
+  if (node.chainage_m <= 40 + 1e-6 || node.chainage_m >= 140 - 1e-6) return;
+  const g = (a, b) => (b.elevation_m - a.elevation_m) / (b.chainage_m - a.chainage_m);
+  if (Math.abs(g(node, alA.pvi[i + 1]) - g(alA.pvi[i - 1], node)) >= 1e-9) noiseBends += 1;
+});
+const runA = straight.detectStraightRun(alA, 70);
+const axesA = pivot.pivotAxes(alA, 80);
+const upA = align.adjustStation(baseA, sA, 80, 0.1);
+const alUp = build(baseA, upA);
+const gradesUp = alUp.segments
+  .filter((seg) => seg.from_m >= 40 - 1e-6 && seg.to_m <= 140 + 1e-6)
+  .map((seg) => r6(seg.grade_percent));
+const straightened = {
+  noiseBends,
+  curvesInside: curvesIn(alA, 40, 140),
+  radiiKeys: Object.keys(sA.curve_radii),
+  run: runA ? [runA.fromM, runA.toM] : null,
+  axes: axesA ? [axesA.fromM, axesA.toM] : null,
+  pressRise: r6(planAt(alUp, 80) - planAt(alA, 80)),
+  curvesAfterPress: curvesIn(alUp, 40, 140),
+  gradesUp,
+  editedCount: judge.bentEditCount(alA),
+  offsetCount: Object.keys(sA.station_offsets).length,
+};
+
+// ② 끝점 꼭짓점 아님(B) — 100.4 → 160 직선화 · 120.25 → 180 자유지정
+const alB = build(baseB, empty);
+const sB = straight.straightenBetween(baseB, empty, 100.4, 160);
+const alSB = build(baseB, sB);
+const line = (a, b, s) => a[1] + ((b[1] - a[1]) * (s - a[0])) / (b[0] - a[0]);
+const endA = [100.4, planAt(alB, 100.4)];
+const endB = [160, align.controlElevationAt(alB, 160)];
+const endpoint = {
+  // 끝 꼭짓점 = 직선화 전 그 자리 계획고(꼭짓점 높이 · 그 라운드는 직선의 접선)
+  endHeld: r6(align.controlElevationAt(alSB, 100.4) - planAt(alB, 100.4)),
+  endIsVertex: alSB.pvi.some((n) => Math.abs(n.chainage_m - 100.4) < 1e-6),
+  curvesInside: curvesIn(alSB, 100.4, 160),
+  innerResidual: [120, 120.25, 140].map((s) => r6(planAt(alSB, s) - line(endA, endB, s))),
+};
+const z140 = planAt(alB, 140) + 1;
+const fEdits = free.buildFreeEdits(baseB, empty, [
+  { chainage_m: 120.25, elevation_m: align.controlElevationAt(alB, 120.25) },
+  { chainage_m: 140, elevation_m: z140 },
+  { chainage_m: 180, elevation_m: align.controlElevationAt(alB, 180) },
+]);
+const alF = build(baseB, fEdits);
+// 자유지정 뒤 ▼ 로 140 을 다시 120.25 · 180 잇는 선 위로 → 직선 → 정리하면 R 없음
+const back =
+  line(
+    [120.25, align.controlElevationAt(alF, 120.25)],
+    [180, align.controlElevationAt(alF, 180)],
+    140,
+  ) - planAt(alF, 140);
+const downF = align.adjustStation(baseB, fEdits, 140, back);
+const tidyF = straight.tidyEdits(baseB, downF);
+const alTidyF = build(baseB, tidyF);
+const freeCase = {
+  endHeld: r6(align.controlElevationAt(alF, 120.25) - align.controlElevationAt(alB, 120.25)),
+  radiusAt140: fEdits.curve_radii["140.000"] ?? null,
+  curveAt140: alF.curves.some((c) => Math.abs(c.chainage_m - 140) < 1e-6),
+  curvesInsideF: curvesIn(alF, 120.25, 180),
+  radiusKeptRaw: downF.curve_radii["140.000"] !== undefined,
+  radiusAfterTidy: tidyF.curve_radii["140.000"] ?? null,
+  curvesAfterTidy: curvesIn(alTidyF, 120.25, 180),
+  sameShape: r6(planAt(alTidyF, 140) - planAt(build(baseB, downF), 140)),
+};
+
+// ③ 고른 직선 기울이기 — 누름 쌓임 · 되돌아오면 R 없음 (A)
+const run = straight.detectStraightRun(alA, 70);
+let e = sA;
+const rises = [];
+const pivotM = 80; // 40 · 140 가운데 = 90 → 가장 가까운 안쪽 꼭짓점(80 · 100 중 앞선 80)
+[0.1, 0.1, -0.1, -0.1].forEach((d) => {
+  e = straight.tiltStraightRun(baseA, e, run, d);
+  rises.push(r6(align.controlElevationAt(build(baseA, e), pivotM) - planAt(alA, pivotM)));
+});
+const tiltOnce = straight.tiltStraightRun(baseA, sA, run, 0.1);
+const tilt = {
+  rises,
+  finalRadii: Object.keys(e.curve_radii),
+  finalCurves: curvesIn(build(baseA, e), 40, 140),
+  editedAfterOne: judge.bentEditCount(build(baseA, tiltOnce)),
+};
+
+// ④ 0 오프셋 정리
+const tidyZero = straight.tidyEdits(baseB, {
+  station_offsets: { "60.000": 0, "80.000": 0.3 },
+  curve_radii: {},
+});
+
+console.log(
+  JSON.stringify({
+    straightened,
+    endpoint,
+    freeCase,
+    tilt,
+    tidyZero: Object.keys(tidyZero.station_offsets),
+    mirror: {
+      base: baseA,
+      cases: [
+        { edits: sA, curves: alA.curves.map((c) => [r6(c.chainage_m), r6(c.r_m)]) },
+        { edits: upA, curves: alUp.curves.map((c) => [r6(c.chainage_m), r6(c.r_m)]) },
+        {
+          edits: tiltOnce,
+          curves: build(baseA, tiltOnce).curves.map((c) => [r6(c.chainage_m), r6(c.r_m)]),
+        },
+      ],
+    },
+  }),
+);
diff --git a/resources/tester/test_426_profile_straight.py b/resources/tester/test_426_profile_straight.py
new file mode 100644
index 000000000..b743d8789
--- /dev/null
+++ b/resources/tester/test_426_profile_straight.py
@@ -0,0 +1,132 @@
+# -*- coding: utf-8 -*-
+"""426 B05 종단 직선 판정 한 벌 — 높이 기준(1e-5 m) · 끝점 꼭짓점 · 누름 쌓임 · 편집 측점 수.
+
+요구 259.
+
+계산(`logic/B05_Profile_Profile_Straight.ts` · `_Alignment.ts` buildCurves · `_Pivot.ts` ·
+`_Straighten.ts` · `design_profile_free.ts`)은 Node 헬퍼(`helper_426_profile_straight.cjs`)가
+트랜스파일해 돌리고, 파이썬 짝(`B05_Profile_Engine_Grade_Alignment.py` `is_straight_bend`)은
+같은 base · 편집으로 곡선 자리 · R 이 같은지 거울로 본다.
+./venv/Scripts/python.exe -m pytest resources/tester/test_426_profile_straight.py -q
+"""
+
+import json
+import os
+import subprocess
+import sys
+from pathlib import Path
+
+import numpy as np
+import pytest
+
+HERE = Path(__file__).resolve().parent
+ROOT = HERE.parent.parent
+sys.path.insert(0, str(ROOT))
+
+from B05_Profile.B05_Profile_Engine_Grade_Alignment import (  # noqa: E402
+    STRAIGHT_HEIGHT_M,
+    AlignmentPolicy,
+    build_alignment,
+    is_straight_bend,
+)
+
+
+def _read(relative: str) -> str:
+    return (ROOT / relative).read_text(encoding="utf-8")
+
+
+@pytest.fixture(scope="module")
+def out():
+    proc = subprocess.run(
+        ["node", os.path.join(HERE, "helper_426_profile_straight.cjs")],
+        capture_output=True,
+        text=True,
+        encoding="utf-8",
+        timeout=60,
+    )
+    assert proc.returncode == 0, proc.stderr
+    return json.loads(proc.stdout)
+
+
+def test_붙은_측점_직선화_뒤_R_없음_한_직선(out):
+    got = out["straightened"]
+    # 옛 판정(대수차 1e-9)이면 꺾임으로 읽혔을 반올림 흠이 실제로 있음 — 시험이 그 길을 거침
+    assert got["noiseBends"] > 0
+    assert got["curvesInside"] == [] and got["radiiKeys"] == []
+    assert got["run"] == [40, 140]
+    assert got["axes"] == [40, 140]
+
+
+def test_직선_안_누름_톱니_없음(out):
+    got = out["straightened"]
+    assert got["pressRise"] == pytest.approx(0.1, abs=1e-4)
+    assert got["curvesAfterPress"] == [80]  # 누른 점 하나만 꺾임
+    left, right = got["gradesUp"][:2], got["gradesUp"][2:]
+    assert max(left) - min(left) < 1e-3 and max(right) - min(right) < 1e-3  # 0.4 m 조각 톱니 없음
+
+
+def test_편집_측점_수는_꺾인_꼭짓점(out):
+    assert out["straightened"]["offsetCount"] > 0
+    assert out["straightened"]["editedCount"] == 0
+    assert out["tilt"]["editedAfterOne"] == 1
+
+
+def test_끝점_꼭짓점_아니면_지금_높이로_세움(out):
+    got = out["endpoint"]
+    assert got["endHeld"] == pytest.approx(0, abs=1e-6)
+    assert got["endIsVertex"]
+    assert got["curvesInside"] == []
+    assert all(abs(value) < 1e-5 for value in got["innerResidual"])
+
+
+def test_자유지정_R_뒤_직선되면_R_지움(out):
+    got = out["freeCase"]
+    assert got["endHeld"] == pytest.approx(0, abs=1e-6)
+    assert got["radiusAt140"] and got["curveAt140"] and got["curvesInsideF"] == [140]
+    assert got["radiusKeptRaw"]  # ▼ 만으로는 기억 R 이 남음 → 적용 한 곳의 정리가 지움
+    assert got["radiusAfterTidy"] is None and got["curvesAfterTidy"] == []
+    assert got["sameShape"] == pytest.approx(0, abs=1e-9)
+
+
+def test_고른_직선_기울이기_누름_쌓임(out):
+    got = out["tilt"]
+    assert got["rises"] == pytest.approx([0.1, 0.2, 0.1, 0.0], abs=1e-6)
+    assert got["finalRadii"] == [] and got["finalCurves"] == []
+
+
+def test_0_오프셋_정리(out):
+    assert out["tidyZero"] == ["80.000"]
+
+
+def test_파이썬_짝_같은_기준():
+    assert STRAIGHT_HEIGHT_M == 1e-5
+    assert is_straight_bend(2.5e-6, 0.4, 20.0)  # 반올림 흠(0.4 m 조각) = 직선
+    assert not is_straight_bend(1e-3, 20.0, 20.0)  # 0.1 % 꺾임 = 꺾임
+    assert "STRAIGHT_HEIGHT_M = 1e-5" in _read("B05_Profile/logic/B05_Profile_Profile_Straight.ts")
+
+
+def test_파이썬_거울_곡선_자리_R(out):
+    base = out["mirror"]["base"]
+    policy = AlignmentPolicy.from_dict(base["policy"])
+    for case in out["mirror"]["cases"]:
+        got = build_alignment(
+            base_s=np.array([n["chainage_m"] for n in base["basePvi"]]),
+            base_z=np.array([n["elevation_m"] for n in base["basePvi"]]),
+            chainage=np.array(base["chainage"]),
+            ground=np.array(base["ground"]),
+            stations=base["stations"],
+            policy=policy,
+            edits=case["edits"],
+        )
+        py = [[c["chainage_m"], c["r_m"]] for c in got["curves"]]
+        assert [c[0] for c in py] == pytest.approx([c[0] for c in case["curves"]], abs=1e-6)
+        assert [c[1] for c in py] == pytest.approx([c[1] for c in case["curves"]], rel=1e-6)
+
+
+def test_부르는_곳():
+    assert "tidyEdits(base, next)" in _read("A00_Common/design/design_profile.ts")
+    assert "bentEditCount(alignment)" in _read("B05_Profile/B05_Profile_UI_Profile_Balance.ts")
+    assert "isStraightBend(delta, spanLeft, spanRight)" in _read(
+        "B05_Profile/logic/B05_Profile_UI_Profile_Alignment.ts"
+    )
+    assert "is_straight_bend(delta," in _read("B05_Profile/B05_Profile_Engine_Grade_Alignment.py")