diff --git a/back/src/main/java/com/back/domain/node/dto/decision/DecNodeDto.java b/back/src/main/java/com/back/domain/node/dto/decision/DecNodeDto.java
index be5955a..f98162f 100644
--- a/back/src/main/java/com/back/domain/node/dto/decision/DecNodeDto.java
+++ b/back/src/main/java/com/back/domain/node/dto/decision/DecNodeDto.java
@@ -1,12 +1,13 @@
 /**
- * [DTO-RES] DecisionNode 응답(보강)
- * - followPolicy/pinnedCommitId/virtual 플래그를 추가해 해석 상태를 노출
- * - effective* 필드는 Resolver가 계산한 최종 표현값(렌더 우선 사용)
+ * [DTO-RES] DecisionNode 응답(보강 최종본)
+ * - 정책/핀/가상(followPolicy/pinnedCommitId/virtual)과 effective* 해석값 노출
+ * - 렌더 편의: childrenIds/root/pivot 링크 + 단일 패스 렌더용 힌트(renderPhase, incoming*)
  */
 package com.back.domain.node.dto.decision;
 
 import com.back.domain.node.entity.FollowPolicy;
 import com.back.domain.node.entity.NodeCategory;
+
 import java.util.List;
 
 public record DecNodeDto(
@@ -40,10 +41,16 @@ public record DecNodeDto(
         List<String> effectiveOptions,
         String effectiveDescription,
 
+        // 기본 렌더 편의
         List<Long> childrenIds,        // 이 DECISION의 자식들 id(시간순)
         Boolean root,                  // 라인 헤더면 true
         Long pivotLinkBaseNodeId,      // 베이스 분기 슬롯에서 올라온 첫 노드면 해당 BaseNode id
-        Integer pivotSlotIndex         // 0/1 (분기 슬롯 인덱스), 아니면 null
+        Integer pivotSlotIndex,        // 0/1 (분기 슬롯 인덱스), 아니면 null
+
+        // 단일 패스 렌더용 최소 힌트
+        Integer renderPhase,           // 1=from-base 라인, 2..N=fork 깊이(부모 라인 +1)
+        Long incomingFromId,           // 이 노드로 "들어오는" 에지의 from 노드 id(루트면 포크 원본, 아니면 parentId)
+        String incomingEdgeType        // "normal" | "fork"
 ) {
     // === 호환 오버로드(기존 서비스 호출 유지) ===
     public DecNodeDto(
@@ -62,7 +69,7 @@ public DecNodeDto(
                 parentOptionIndex, description, aiNextSituation, aiNextRecommendedOption,
                 followPolicy, pinnedCommitId, virtual, effectiveCategory, effectiveSituation,
                 effectiveDecision, effectiveOptions, effectiveDescription,
-                null, null, null, null); // ← 새 필드는 null 기본값
+                null, null, null, null,
+                null, null, null); // 새 필드는 null 기본값
     }
-
 }
diff --git a/back/src/main/java/com/back/domain/node/service/DecisionFlowService.java b/back/src/main/java/com/back/domain/node/service/DecisionFlowService.java
index 7b88a6f..1b4a1ca 100644
--- a/back/src/main/java/com/back/domain/node/service/DecisionFlowService.java
+++ b/back/src/main/java/com/back/domain/node/service/DecisionFlowService.java
@@ -311,7 +311,6 @@ private Long currentUserId() {
     public DecisionLineLifecycleDto completeDecisionLine(Long decisionLineId) {
         DecisionLine line = support.requireDecisionLine(decisionLineId);
         try { line.complete(); } catch (RuntimeException e) { throw support.mapDomainToApi(e); }
-        decisionLineRepository.save(line);
         return new DecisionLineLifecycleDto(line.getId(), line.getStatus());
     }
 
diff --git a/back/src/main/java/com/back/domain/node/service/NodeQueryService.java b/back/src/main/java/com/back/domain/node/service/NodeQueryService.java
index ce03ba1..6d7dc4b 100644
--- a/back/src/main/java/com/back/domain/node/service/NodeQueryService.java
+++ b/back/src/main/java/com/back/domain/node/service/NodeQueryService.java
@@ -25,6 +25,7 @@
 import org.springframework.transaction.annotation.Transactional;
 
 import java.util.*;
+import java.util.stream.Collectors;
 
 @Service
 @RequiredArgsConstructor
@@ -52,13 +53,22 @@ public TreeDto getTreeForBaseLine(Long baseLineId) {
         List<BaseNode> orderedBase = support.getOrderedBaseNodes(baseLineId);
         List<BaseNodeDto> baseDtos = orderedBase.stream().map(mappers.BASE_READ::map).toList();
 
-        // 전체 라인 공용 pivot 역인덱스
+        // 가장 많이 사용하는: 전체 라인 공용 pivot 역인덱스
         Map<Long, PivotMark> pivotIndex = buildPivotIndex(baseLineId);
 
+        // ===== (1) 노드 수집 =====
+        record Key(Long baseId, Integer age) {}
+        record View(DecisionNode dn, DecNodeDto dto, boolean isRoot, Long baseId, Integer age,
+                    List<Long> childrenIds, Long pivotBaseId, Integer pivotSlot) {}
+
         List<DecNodeDto> decDtos = new ArrayList<>();
         List<DecisionLine> lines = decisionLineRepository.findByBaseLine_Id(baseLineId);
 
+        List<View> pool = new ArrayList<>();
+        Map<Long, List<View>> byLine = new HashMap<>();
+
         for (DecisionLine line : lines) {
+            // 가장 많이 사용하는 함수 호출 위에 한줄로만 요약 주석: 라인의 노드를 타임라인 정렬로 로드
             List<DecisionNode> ordered = decisionNodeRepository
                     .findByDecisionLine_IdOrderByAgeYearAscIdAsc(line.getId());
 
@@ -67,29 +77,126 @@ public TreeDto getTreeForBaseLine(Long baseLineId) {
             for (DecisionNode dn : ordered) {
                 DecNodeDto base = mappers.DECISION_READ.map(dn);
 
-                List<Long> childrenIds = childrenIndex.getOrDefault(dn.getId(), List.of());
                 boolean isRoot = (dn.getParent() == null);
+                List<Long> childrenIds = childrenIndex.getOrDefault(dn.getId(), List.of());
 
                 PivotMark mark = pivotIndex.get(base.id());
-                Long pivotBaseId = (mark != null) ? mark.baseNodeId() : null;
+                Long pivotBaseId = (mark != null) ? mark.baseNodeId() :
+                        (dn.getBaseNode() != null ? dn.getBaseNode().getId() : null);
                 Integer pivotSlot = (mark != null) ? mark.slotIndex() : null;
 
-                // effective*는 DECISION_READ에서 이미 계산/주입되어 있다면 그대로 사용
-                decDtos.add(new DecNodeDto(
-                        base.id(), base.userId(), base.type(), base.category(),
-                        base.situation(), base.decision(), base.ageYear(),
-                        base.decisionLineId(), base.parentId(), base.baseNodeId(),
-                        base.background(), base.options(), base.selectedIndex(),
-                        base.parentOptionIndex(), base.description(),
-                        base.aiNextSituation(), base.aiNextRecommendedOption(),
-                        base.followPolicy(), base.pinnedCommitId(), base.virtual(),
-                        base.effectiveCategory(), base.effectiveSituation(), base.effectiveDecision(),
-                        base.effectiveOptions(), base.effectiveDescription(),
-                        // ▼ 렌더 편의 필드
-                        List.copyOf(childrenIds), isRoot, pivotBaseId, pivotSlot
-                ));
+                Long baseId = pivotBaseId;
+                Integer age = dn.getAgeYear();
+
+                View v = new View(dn, base, isRoot, baseId, age, List.copyOf(childrenIds), pivotBaseId, pivotSlot);
+                pool.add(v);
+                byLine.computeIfAbsent(line.getId(), k -> new ArrayList<>()).add(v);
             }
         }
+
+        Map<Key, List<View>> byKey = pool.stream()
+                .filter(v -> v.baseId != null && v.age != null)
+                .collect(Collectors.groupingBy(v -> new Key(v.baseId, v.age)));
+
+        // ===== (2) 라인 간 포크 그래프 추론 → renderPhase 계산 =====
+        Map<Long, View> rootCand = new HashMap<>(); // lineId -> 루트 후보
+        for (Map.Entry<Long, List<View>> e : byLine.entrySet()) {
+            List<View> vs = e.getValue().stream()
+                    .sorted(Comparator.comparing((View x) -> x.age)
+                            .thenComparing(x -> x.dn.getId()))
+                    .toList();
+
+            View first = vs.get(0);
+            // 가장 중요한 함수: 라인 루트가 헤더면 다음 피벗을 루트 후보로 대체
+            View cand = (first.isRoot && first.baseId == null && vs.size() > 1) ? vs.get(1) : first;
+            rootCand.put(e.getKey(), cand);
+        }
+
+        Map<Long, Set<Long>> g = new HashMap<>();   // originLineId -> {forkLineId}
+        Map<Long, Integer> indeg = new HashMap<>(); // lineId -> indegree
+
+        for (Map.Entry<Long, View> e : rootCand.entrySet()) {
+            Long lineId = e.getKey();
+            View me = e.getValue();
+            indeg.putIfAbsent(lineId, 0);
+
+            if (me.baseId != null && me.age != null) {
+                List<View> same = byKey.getOrDefault(new Key(me.baseId, me.age), List.of());
+                Optional<View> origin = same.stream()
+                        .filter(o -> !o.dn.getDecisionLine().getId().equals(lineId))
+                        .sorted(Comparator
+                                .comparing((View o) -> o.dn.getDecisionLine().getId())
+                                .thenComparing(o -> o.dn.getId()))
+                        .findFirst();
+
+                if (origin.isPresent()) {
+                    Long originLineId = origin.get().dn.getDecisionLine().getId();
+                    g.computeIfAbsent(originLineId, k -> new HashSet<>()).add(lineId);
+                    indeg.put(lineId, indeg.getOrDefault(lineId, 0) + 1);
+                    indeg.putIfAbsent(originLineId, 0);
+                }
+            }
+        }
+
+        Map<Long, Integer> linePhase = new HashMap<>(); // lineId -> phase
+        ArrayDeque<Long> q = new ArrayDeque<>();
+        for (Map.Entry<Long, Integer> e : indeg.entrySet()) {
+            if (e.getValue() == 0) { // indegree==0 → from-base
+                linePhase.put(e.getKey(), 1);
+                q.add(e.getKey());
+            }
+        }
+        while (!q.isEmpty()) {
+            Long u = q.poll();
+            int next = linePhase.get(u) + 1;
+            for (Long v : g.getOrDefault(u, Set.of())) {
+                indeg.put(v, indeg.get(v) - 1);
+                linePhase.put(v, Math.max(linePhase.getOrDefault(v, 1), next));
+                if (indeg.get(v) == 0) q.add(v);
+            }
+        }
+
+        // ===== (3) DTO 주입: renderPhase + incomingFromId/incomingEdgeType =====
+        for (View v : pool) {
+            DecNodeDto b = v.dto;
+
+            Long lineId = v.dn.getDecisionLine().getId();
+            Integer renderPhase = linePhase.getOrDefault(lineId, 1);
+
+            Long incomingFromId = (v.isRoot)
+                    ? byKey.getOrDefault(new Key(v.baseId, v.age), List.of()).stream()
+                    .filter(o -> !o.dn.getDecisionLine().getId().equals(lineId))
+                    .sorted(Comparator
+                            .comparing((View o) -> o.dn.getDecisionLine().getId())
+                            .thenComparing(o -> o.dn.getId()))
+                    .map(o -> o.dn.getId())
+                    .findFirst()
+                    .orElse(null)
+                    : (v.dn.getParent() != null ? v.dn.getParent().getId() : null);
+
+            String incomingEdgeType = (v.isRoot && incomingFromId != null) ? "fork" : "normal";
+
+            decDtos.add(new DecNodeDto(
+                    b.id(), b.userId(), b.type(), b.category(),
+                    b.situation(), b.decision(), b.ageYear(),
+                    b.decisionLineId(), b.parentId(), b.baseNodeId(),
+                    b.background(), b.options(), b.selectedIndex(),
+                    b.parentOptionIndex(), b.description(),
+                    b.aiNextSituation(), b.aiNextRecommendedOption(),
+                    b.followPolicy(), b.pinnedCommitId(), b.virtual(),
+                    b.effectiveCategory(), b.effectiveSituation(), b.effectiveDecision(),
+                    b.effectiveOptions(), b.effectiveDescription(),
+                    v.childrenIds, v.isRoot, v.pivotBaseId, v.pivotSlot,
+                    renderPhase, incomingFromId, incomingEdgeType
+            ));
+        }
+
+        // 출력 순서 보장: phase → ageYear → id
+        decDtos.sort(Comparator
+                .comparing(DecNodeDto::renderPhase, Comparator.nullsLast(Integer::compareTo))
+                .thenComparing(DecNodeDto::ageYear, Comparator.nullsFirst(Integer::compareTo))
+                .thenComparing(DecNodeDto::id));
+
         return new TreeDto(baseDtos, decDtos);
     }
 
@@ -142,7 +249,8 @@ public DecisionLineListDto getDecisionLines(Long userId) {
     // 가장 중요한: 특정 라인의 상세를 childrenIds/root/pivotLink*와 함께 반환
     public DecisionLineDetailDto getDecisionLineDetail(Long decisionLineId) {
         DecisionLine line = decisionLineRepository.findById(decisionLineId)
-                .orElseThrow(() -> new ApiException(ErrorCode.DECISION_LINE_NOT_FOUND, "DecisionLine not found: " + decisionLineId));
+                .orElseThrow(() -> new ApiException(ErrorCode.DECISION_LINE_NOT_FOUND,
+                        "DecisionLine not found: " + decisionLineId));
 
         Long baseLineId   = line.getBaseLine().getId();
         Long baseBranchId = (line.getBaseBranch() != null) ? line.getBaseBranch().getId() : null;
@@ -153,7 +261,7 @@ public DecisionLineDetailDto getDecisionLineDetail(Long decisionLineId) {
 
         // parent→children 인덱스 구성
         Map<Long, List<Long>> childrenIndex = buildChildrenIndex(ordered);
-        // 베이스 분기 슬롯 역인덱스 구성(altOpt1/2TargetDecisionId → (baseNodeId, slot))
+        // 베이스 분기 슬롯 역인덱스 구성
         Map<Long, PivotMark> pivotIndex = buildPivotIndex(baseLineId);
 
         List<DecNodeDto> nodes = ordered.stream().map(n -> {
@@ -178,12 +286,12 @@ public DecisionLineDetailDto getDecisionLineDetail(Long decisionLineId) {
             if (verId != null) {
                 NodeAtomVersion v = versionRepo.findById(verId).orElse(null);
                 if (v != null) {
-                    if (v.getCategory()   != null) effCategory  = v.getCategory();
-                    if (v.getSituation()  != null) effSituation = v.getSituation();
-                    if (v.getDecision()   != null) effDecision  = v.getDecision();
+                    if (v.getCategory()    != null) effCategory  = v.getCategory();
+                    if (v.getSituation()   != null) effSituation = v.getSituation();
+                    if (v.getDecision()    != null) effDecision  = v.getDecision();
                     List<String> parsed = parseOptionsJson(v.getOptionsJson());
-                    if (parsed != null)           effOpts      = parsed;
-                    if (v.getDescription() != null) effDesc     = v.getDescription();
+                    if (parsed != null)             effOpts      = parsed;
+                    if (v.getDescription() != null) effDesc      = v.getDescription();
                 }
             }
 
@@ -194,6 +302,12 @@ public DecisionLineDetailDto getDecisionLineDetail(Long decisionLineId) {
             Long pivotBaseId = (mark != null) ? mark.baseNodeId() : null;
             Integer pivotSlot = (mark != null) ? mark.slotIndex() : null;
 
+            // ===== 라인 상세 전용 렌더 힌트 =====
+            // 한줄 요약: 상세 화면은 한 라인만 보므로 phase=1 고정, incoming은 parent 기준(normal)
+            Integer renderPhase = 1;
+            Long incomingFromId = isRoot ? null : n.getParent().getId();
+            String incomingEdgeType = "normal";
+
             return new DecNodeDto(
                     base.id(), base.userId(), base.type(), base.category(),
                     base.situation(), base.decision(), base.ageYear(),
@@ -202,10 +316,11 @@ public DecisionLineDetailDto getDecisionLineDetail(Long decisionLineId) {
                     base.parentOptionIndex(), base.description(),
                     base.aiNextSituation(), base.aiNextRecommendedOption(),
                     base.followPolicy(), base.pinnedCommitId(), base.virtual(),
-                    // effective*
+                    // effective* (최종 해석 반영)
                     effCategory, effSituation, effDecision, effOpts, effDesc,
-                    // ▼ 렌더 편의 필드
-                    List.copyOf(childrenIds), isRoot, pivotBaseId, pivotSlot
+                    // ▼ 렌더 편의 + 단일 패스 힌트(라인 내부 한정)
+                    List.copyOf(childrenIds), isRoot, pivotBaseId, pivotSlot,
+                    renderPhase, incomingFromId, incomingEdgeType
             );
         }).toList();
 
diff --git a/back/src/main/java/com/back/domain/scenario/controller/ScenarioController.java b/back/src/main/java/com/back/domain/scenario/controller/ScenarioController.java
index 6c94e0c..13646bf 100644
--- a/back/src/main/java/com/back/domain/scenario/controller/ScenarioController.java
+++ b/back/src/main/java/com/back/domain/scenario/controller/ScenarioController.java
@@ -1,5 +1,6 @@
 package com.back.domain.scenario.controller;
 
+import com.back.domain.node.dto.decision.DecisionNodeNextRequest;
 import com.back.domain.scenario.dto.*;
 import com.back.domain.scenario.service.ScenarioService;
 import com.back.global.common.PageResponse;
@@ -42,12 +43,13 @@ private Long getUserId(CustomUserDetails userDetails) {
     @PostMapping
     @Operation(summary = "시나리오 생성", description = "DecisionLine을 기반으로 AI 시나리오를 생성합니다.")
     public ResponseEntity<ScenarioStatusResponse> createScenario(
-            @Valid @RequestBody ScenarioCreateRequest request,
+            @Valid @RequestPart("scenario") ScenarioCreateRequest request,
+            @RequestPart(value = "lastDecision", required = false) DecisionNodeNextRequest lastDecision,
             @AuthenticationPrincipal CustomUserDetails userDetails
             ) {
         Long userId = getUserId(userDetails);
 
-        ScenarioStatusResponse scenarioCreateResponse = scenarioService.createScenario(userId, request);
+        ScenarioStatusResponse scenarioCreateResponse = scenarioService.createScenario(userId,request, lastDecision);
 
         return ResponseEntity.status(HttpStatus.CREATED).body(scenarioCreateResponse);
     }
diff --git a/back/src/main/java/com/back/domain/scenario/service/ScenarioService.java b/back/src/main/java/com/back/domain/scenario/service/ScenarioService.java
index bce06de..e46ed03 100644
--- a/back/src/main/java/com/back/domain/scenario/service/ScenarioService.java
+++ b/back/src/main/java/com/back/domain/scenario/service/ScenarioService.java
@@ -1,10 +1,11 @@
 package com.back.domain.scenario.service;
 
-import com.back.domain.node.entity.BaseLine;
-import com.back.domain.node.entity.DecisionLine;
-import com.back.domain.node.entity.NodeCategory;
+import com.back.domain.node.dto.decision.DecisionNodeNextRequest;
+import com.back.domain.node.entity.*;
 import com.back.domain.node.repository.BaseLineRepository;
 import com.back.domain.node.repository.DecisionLineRepository;
+import com.back.domain.node.repository.DecisionNodeRepository;
+import com.back.domain.node.service.DecisionFlowService;
 import com.back.domain.scenario.dto.*;
 import com.back.domain.scenario.entity.Scenario;
 import com.back.domain.scenario.entity.ScenarioStatus;
@@ -20,6 +21,7 @@
 import com.back.global.exception.ErrorCode;
 import com.fasterxml.jackson.core.type.TypeReference;
 import com.fasterxml.jackson.databind.ObjectMapper;
+import jakarta.annotation.Nullable;
 import lombok.Getter;
 import lombok.RequiredArgsConstructor;
 import lombok.extern.slf4j.Slf4j;
@@ -52,6 +54,7 @@ public class ScenarioService {
     // Node Repository 주입
     private final DecisionLineRepository decisionLineRepository;
     private final BaseLineRepository baseLineRepository;
+    private final DecisionNodeRepository decisionNodeRepository;
 
     // Object Mapper 주입
     private final ObjectMapper objectMapper;
@@ -62,9 +65,14 @@ public class ScenarioService {
     // Scenario Transaction Service 주입
     private final ScenarioTransactionService scenarioTransactionService;
 
+    // 노드 서비스 추가(시나리오 생성과 동시에 마지막 노드 처리용)
+    private final DecisionFlowService decisionFlowService;
+
     // 시나리오 생성
     @Transactional
-    public ScenarioStatusResponse createScenario(Long userId, ScenarioCreateRequest request) {
+    public ScenarioStatusResponse createScenario(Long userId,
+                                                 ScenarioCreateRequest request,
+                                                 @Nullable DecisionNodeNextRequest lastDecision) {
         // DecisionLine 존재 여부 확인
         DecisionLine decisionLine = decisionLineRepository.findById(request.decisionLineId())
                 .orElseThrow(() -> new ApiException(ErrorCode.DECISION_LINE_NOT_FOUND));
@@ -103,6 +111,18 @@ public ScenarioStatusResponse createScenario(Long userId, ScenarioCreateRequest
             }
         }
 
+        ensureOwnerEditable(userId, decisionLine);
+
+        if (lastDecision != null) {
+            ensureSameLine(decisionLine, lastDecision);
+            decisionFlowService.createDecisionNodeNext(lastDecision);
+        }
+
+        // 라인 완료 처리(외부 완료 API 제거 시 내부에서만 호출)
+        try { decisionLine.complete(); } catch (RuntimeException e) {
+            throw new ApiException(ErrorCode.INVALID_INPUT_VALUE, e.getMessage());
+        }
+
         // 새 시나리오 생성 (DataIntegrityViolationException 처리)
         try {
             // DecisionLine에서 BaseLine 가져오기
@@ -136,6 +156,62 @@ public ScenarioStatusResponse createScenario(Long userId, ScenarioCreateRequest
         }
     }
 
+    // 가장 많이 사용하는 함수 호출 위 한줄 요약: 시나리오 요청에서 lineId 필수 추출
+    private Long requireLineId(ScenarioCreateRequest scenario) {
+        if (scenario == null || scenario.decisionLineId() == null) {
+            throw new ApiException(ErrorCode.INVALID_INPUT_VALUE, "scenario.decisionLineId is required");
+        }
+        return scenario.decisionLineId();
+    }
+
+    // 한줄 요약: 소유자/편집 가능 상태 검증
+    private void ensureOwnerEditable(Long userId, DecisionLine line) {
+        if (!line.getUser().getId().equals(userId))
+            throw new ApiException(ErrorCode.HANDLE_ACCESS_DENIED, "not line owner");
+        if (line.getStatus() == DecisionLineStatus.COMPLETED || line.getStatus() == DecisionLineStatus.CANCELLED)
+            throw new ApiException(ErrorCode.INVALID_INPUT_VALUE, "line is not editable");
+    }
+
+    // lastDecision의 부모 노드가 같은 라인/같은 사용자/유효한 시퀀스인지 검증
+    private void ensureSameLine(DecisionLine line, DecisionNodeNextRequest lastDecision) {
+        if (lastDecision.parentDecisionNodeId() == null) {
+            throw new ApiException(ErrorCode.INVALID_INPUT_VALUE, "lastDecision.parentDecisionNodeId is required");
+        }
+
+        // 부모 노드 id로 조회(없으면 404)
+        DecisionNode parent = decisionNodeRepository.findById(lastDecision.parentDecisionNodeId())
+                .orElseThrow(() -> new ApiException(
+                        ErrorCode.NODE_NOT_FOUND,
+                        "parent decision node not found: " + lastDecision.parentDecisionNodeId()
+                ));
+
+        // 같은 라인인지 강제
+        if (!parent.getDecisionLine().getId().equals(line.getId())) {
+            throw new ApiException(ErrorCode.INVALID_INPUT_VALUE, "parent node does not belong to the target line");
+        }
+
+        // 소유자 일치 강제(추가 방어)
+        Long ownerIdOfLine = line.getUser().getId();
+        if (!parent.getUser().getId().equals(ownerIdOfLine)) {
+            throw new ApiException(ErrorCode.HANDLE_ACCESS_DENIED, "parent node is not owned by line owner");
+        }
+        if (!parent.getDecisionLine().getUser().getId().equals(ownerIdOfLine)) {
+            throw new ApiException(ErrorCode.HANDLE_ACCESS_DENIED, "line owner mismatch on parent node");
+        }
+
+        // 시퀀스 유효성: ageYear가 지정됐다면 반드시 부모 이후여야 함
+        Integer nextAge = lastDecision.ageYear();
+        if (nextAge != null && nextAge <= parent.getAgeYear()) {
+            throw new ApiException(ErrorCode.INVALID_INPUT_VALUE, "ageYear must be greater than parent.ageYear");
+        }
+
+        // 편집 가능 상태 재확인(이미 상위에서 검증했더라도 이중 방어)
+        if (line.getStatus() == DecisionLineStatus.COMPLETED || line.getStatus() == DecisionLineStatus.CANCELLED) {
+            throw new ApiException(ErrorCode.INVALID_INPUT_VALUE, "line is not editable");
+        }
+    }
+
+
     // FAILED 시나리오 재시도 로직 분리
     private ScenarioStatusResponse handleFailedScenarioRetry(Scenario failedScenario) {
         failedScenario.setStatus(ScenarioStatus.PENDING);
diff --git a/back/src/test/java/com/back/domain/scenario/controller/ScenarioControllerTest.java b/back/src/test/java/com/back/domain/scenario/controller/ScenarioControllerTest.java
index 92a7104..0c2a8f7 100644
--- a/back/src/test/java/com/back/domain/scenario/controller/ScenarioControllerTest.java
+++ b/back/src/test/java/com/back/domain/scenario/controller/ScenarioControllerTest.java
@@ -1,9 +1,12 @@
 package com.back.domain.scenario.controller;
 
+import com.back.domain.node.dto.decision.DecisionNodeNextRequest;
+import com.back.domain.node.entity.NodeCategory;
 import com.back.domain.scenario.dto.*;
 import com.back.domain.scenario.entity.ScenarioStatus;
 import com.back.domain.scenario.entity.Type;
 import com.back.domain.scenario.service.ScenarioService;
+import com.back.global.common.PageResponse;
 import com.back.global.exception.ApiException;
 import com.back.global.exception.ErrorCode;
 import com.fasterxml.jackson.databind.ObjectMapper;
@@ -15,11 +18,10 @@
 import org.springframework.boot.test.autoconfigure.web.servlet.AutoConfigureMockMvc;
 import org.springframework.boot.test.context.SpringBootTest;
 import org.springframework.boot.test.mock.mockito.MockBean;
-import com.back.global.common.PageResponse;
 import org.springframework.data.domain.Page;
 import org.springframework.data.domain.PageImpl;
 import org.springframework.data.domain.PageRequest;
-import org.springframework.http.MediaType;
+import org.springframework.mock.web.MockMultipartFile;
 import org.springframework.test.context.ActiveProfiles;
 import org.springframework.test.web.servlet.MockMvc;
 import org.springframework.transaction.annotation.Transactional;
@@ -31,7 +33,7 @@
 import static org.mockito.ArgumentMatchers.eq;
 import static org.mockito.BDDMockito.given;
 import static org.springframework.test.web.servlet.request.MockMvcRequestBuilders.get;
-import static org.springframework.test.web.servlet.request.MockMvcRequestBuilders.post;
+import static org.springframework.test.web.servlet.request.MockMvcRequestBuilders.multipart;
 import static org.springframework.test.web.servlet.result.MockMvcResultHandlers.print;
 import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.jsonPath;
 import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.status;
@@ -75,13 +77,41 @@ class CreateScenario {
                     "시나리오 생성이 시작되었습니다."
             );
 
-            given(scenarioService.createScenario(eq(1L), any(ScenarioCreateRequest.class)))
+            DecisionNodeNextRequest lastNode= new DecisionNodeNextRequest(
+                    1L,
+                    100L,
+                    NodeCategory.CAREER,
+                    "커리어 방향 선택",
+                    28,
+                    List.of("이직", "석사과정", "창업"),
+                    2,
+                    1,
+                    "리스크 감수"
+            );
+
+            given(scenarioService.createScenario(eq(1L), any(ScenarioCreateRequest.class),
+                    any(DecisionNodeNextRequest.class)))
                     .willReturn(mockResponse);
 
+            // request를 JSON으로 직렬화 (컨트롤러 @RequestPart("scenario")가 이걸 받음)
+            byte[] scenarioJson = objectMapper.writeValueAsBytes(request);
+
+            MockMultipartFile scenarioPart = new MockMultipartFile(
+                    "scenario",
+                    "", "application/json",
+                    scenarioJson
+            );
+            MockMultipartFile lastDecisionPart = new MockMultipartFile(
+                    "lastDecision",
+                    "", "application/json",
+                    objectMapper.writeValueAsBytes(lastNode)
+            );
+
             // When & Then
-            mockMvc.perform(post("/api/v1/scenarios")
-                            .contentType(MediaType.APPLICATION_JSON)
-                            .content(objectMapper.writeValueAsString(request)))
+            mockMvc.perform(multipart("/api/v1/scenarios")
+                            .file(scenarioPart)
+                            .file(lastDecisionPart)
+                            .with(req -> { req.setMethod("POST"); return req; }))
                     .andDo(print())
                     .andExpect(status().isCreated())
                     .andExpect(jsonPath("$.scenarioId").value(1001))
@@ -90,32 +120,63 @@ class CreateScenario {
         }
 
         @Test
-        @DisplayName("실패 - 잘못된 요청 데이터 (null decisionLineId)")
-        void createScenario_실패_잘못된요청() throws Exception {
-            // Given
-            String invalidRequest = "{\"decisionLineId\":null}";
+        @DisplayName("실패 - 잘못된 요청 데이터 (null decisionLineId) - multipart")
+        void createScenario_실패_잘못된요청_multipart() throws Exception {
+            // Given: decisionLineId=null인 잘못된 JSON을 그대로 scenario 파트에 실어 보냄
+            byte[] invalidScenarioJson = "{\"decisionLineId\":null}".getBytes();
+
+            MockMultipartFile scenarioPart = new MockMultipartFile(
+                    "scenario", "", "application/json", invalidScenarioJson
+            );
 
             // When & Then
-            mockMvc.perform(post("/api/v1/scenarios")
-                            .contentType(MediaType.APPLICATION_JSON)
-                            .content(invalidRequest))
+            mockMvc.perform(multipart("/api/v1/scenarios")
+                            .file(scenarioPart)
+                            .with(req -> { req.setMethod("POST"); return req; }))
                     .andDo(print())
                     .andExpect(status().isBadRequest());
         }
 
         @Test
-        @DisplayName("실패 - Service 예외 발생")
-        void createScenario_실패_Service예외() throws Exception {
+        @DisplayName("실패 - Service 예외 발생 - multipart")
+        void createScenario_실패_Service예외_multipart() throws Exception {
             // Given
             ScenarioCreateRequest request = new ScenarioCreateRequest(999L);
 
-            given(scenarioService.createScenario(eq(1L), any(ScenarioCreateRequest.class)))
-                    .willThrow(new ApiException(ErrorCode.DECISION_LINE_NOT_FOUND));
+            DecisionNodeNextRequest lastNode = new DecisionNodeNextRequest(
+                    1L,
+                    456L,
+                    NodeCategory.CAREER,
+                    "커리어 방향 선택",
+                    28,
+                    List.of("이직", "석사과정", "창업"),
+                    2,
+                    1,
+                    "리스크 감수"
+            );
+
+            // 서비스 예외 스텁
+            given(scenarioService.createScenario(
+                    eq(1L),
+                    any(ScenarioCreateRequest.class),
+                    any(DecisionNodeNextRequest.class)
+            )).willThrow(new ApiException(ErrorCode.DECISION_LINE_NOT_FOUND));
+
+            // 멀티파트 파트 구성
+            MockMultipartFile scenarioPart = new MockMultipartFile(
+                    "scenario", "", "application/json",
+                    objectMapper.writeValueAsBytes(request)
+            );
+            MockMultipartFile lastDecisionPart = new MockMultipartFile(
+                    "lastDecision", "", "application/json",
+                    objectMapper.writeValueAsBytes(lastNode)
+            );
 
             // When & Then
-            mockMvc.perform(post("/api/v1/scenarios")
-                            .contentType(MediaType.APPLICATION_JSON)
-                            .content(objectMapper.writeValueAsString(request)))
+            mockMvc.perform(multipart("/api/v1/scenarios")
+                            .file(scenarioPart)
+                            .file(lastDecisionPart)
+                            .with(req -> { req.setMethod("POST"); return req; }))
                     .andDo(print())
                     .andExpect(status().isNotFound());
         }
diff --git a/back/src/test/java/com/back/domain/scenario/service/ScenarioServiceTest.java b/back/src/test/java/com/back/domain/scenario/service/ScenarioServiceTest.java
index 6f2bc7b..a605cb3 100644
--- a/back/src/test/java/com/back/domain/scenario/service/ScenarioServiceTest.java
+++ b/back/src/test/java/com/back/domain/scenario/service/ScenarioServiceTest.java
@@ -92,7 +92,7 @@ class CreateScenarioTests {
             doNothing().when(scenarioService).processScenarioGenerationAsync(anyLong());
 
             // When
-            ScenarioStatusResponse result = scenarioService.createScenario(userId, request);
+            ScenarioStatusResponse result = scenarioService.createScenario(userId, request, null);
 
             // Then - 시나리오 생성 요청이 접수되고 PENDING 상태로 반환되는지만 검증
             assertThat(result).isNotNull();
@@ -121,7 +121,7 @@ class CreateScenarioTests {
                     .willReturn(Optional.empty());
 
             // When & Then
-            assertThatThrownBy(() -> scenarioService.createScenario(userId, request))
+            assertThatThrownBy(() -> scenarioService.createScenario(userId, request, null))
                     .isInstanceOf(ApiException.class)
                     .hasFieldOrPropertyWithValue("errorCode", ErrorCode.DECISION_LINE_NOT_FOUND);
 
@@ -150,7 +150,7 @@ class CreateScenarioTests {
                     .willReturn(Optional.of(mockDecisionLine));
 
             // When & Then
-            assertThatThrownBy(() -> scenarioService.createScenario(userId, request))
+            assertThatThrownBy(() -> scenarioService.createScenario(userId, request, null))
                     .isInstanceOf(ApiException.class)
                     .hasFieldOrPropertyWithValue("errorCode", ErrorCode.HANDLE_ACCESS_DENIED);
 
@@ -188,7 +188,7 @@ class CreateScenarioTests {
                     .willReturn(Optional.of(existingScenario)); // 기존 PENDING 시나리오 존재
 
             // When & Then
-            assertThatThrownBy(() -> scenarioService.createScenario(userId, request))
+            assertThatThrownBy(() -> scenarioService.createScenario(userId, request, null))
                     .isInstanceOf(ApiException.class)
                     .hasFieldOrPropertyWithValue("errorCode", ErrorCode.SCENARIO_ALREADY_IN_PROGRESS);