diff --git a/web/src/api/projectSettingsError.test.ts b/web/src/api/projectSettingsError.test.ts
new file mode 100644
index 00000000..0a5351fe
--- /dev/null
+++ b/web/src/api/projectSettingsError.test.ts
@@ -0,0 +1,54 @@
+import { describe, it } from 'node:test';
+import assert from 'node:assert/strict';
+import {
+  ProjectSettingsApiError,
+  parseProjectSettingsErrorBody,
+  projectSettingsErrorFromResponse,
+  recoverFromProjectSettingsError,
+  retainDraftOnFrozenConflict,
+} from './projectSettingsError';
+
+describe('projectSettingsError', () => {
+  it('keeps status, code, message and issues on PLAN_FROZEN', () => {
+    const err = projectSettingsErrorFromResponse(
+      409,
+      JSON.stringify({ error: 'PLAN_FROZEN', message: '工作流已激活，规划已冻结' }),
+      '保存工作流规划失败: 409',
+    );
+    assert.equal(err instanceof ProjectSettingsApiError, true);
+    assert.equal(err.status, 409);
+    assert.equal(err.code, 'PLAN_FROZEN');
+    assert.equal(err.message, '工作流已激活，规划已冻结');
+    assert.deepEqual(err.issues, []);
+    assert.equal(recoverFromProjectSettingsError(err), 'refresh_and_freeze');
+  });
+
+  it('maps revision and concurrent conflicts to distinct recovery', () => {
+    assert.equal(
+      recoverFromProjectSettingsError({ code: 'PLAN_REVISION_CONFLICT', status: 409 }),
+      'reload_revision',
+    );
+    assert.equal(
+      recoverFromProjectSettingsError({ code: 'ERR_CONCURRENT_SWITCH', status: 409 }),
+      'retry_after_switch',
+    );
+  });
+
+  it('joins validation issues but still keeps typed codes', () => {
+    const body = parseProjectSettingsErrorBody(
+      JSON.stringify({
+        error: 'VALIDATION_FAILED',
+        message: '工作流配置校验未通过',
+        issues: [{ code: 'DUTY_PARTIAL_ASSIGNMENT', field: 'intake.duties', message: '不能只指派部分职责' }],
+      }),
+    );
+    assert.equal(body.code, 'VALIDATION_FAILED');
+    assert.equal(body.message, '不能只指派部分职责');
+    assert.equal(body.issues[0]?.code, 'DUTY_PARTIAL_ASSIGNMENT');
+  });
+
+  it('retains the local draft when a late tab hits PLAN_FROZEN', () => {
+    const draft = { workflow_definition_key: 'main', workflow_version: 1, stages: [] };
+    assert.equal(retainDraftOnFrozenConflict(draft), draft);
+  });
+});
diff --git a/web/src/api/projectSettingsError.ts b/web/src/api/projectSettingsError.ts
new file mode 100644
index 00000000..4a3c151c
--- /dev/null
+++ b/web/src/api/projectSettingsError.ts
@@ -0,0 +1,83 @@
+export type ProjectSettingsIssue = {
+  code?: string;
+  field?: string;
+  message?: string;
+};
+
+export type ProjectSettingsErrorRecovery =
+  | 'refresh_and_freeze'
+  | 'reload_revision'
+  | 'retry_after_switch'
+  | 'show_message';
+
+export class ProjectSettingsApiError extends Error {
+  readonly status: number;
+  readonly code: string;
+  readonly issues: ProjectSettingsIssue[];
+
+  constructor(input: {
+    status: number;
+    code: string;
+    message: string;
+    issues?: ProjectSettingsIssue[];
+  }) {
+    super(input.message);
+    this.name = 'ProjectSettingsApiError';
+    this.status = input.status;
+    this.code = input.code;
+    this.issues = input.issues ?? [];
+  }
+}
+
+export function parseProjectSettingsErrorBody(text: string): {
+  message: string;
+  code: string;
+  issues: ProjectSettingsIssue[];
+} {
+  try {
+    const parsed = JSON.parse(text) as {
+      error?: string;
+      message?: string;
+      issues?: ProjectSettingsIssue[];
+    };
+    const issues = Array.isArray(parsed.issues) ? parsed.issues : [];
+    const issueText = issues
+      .map((issue) => issue.message)
+      .filter((msg): msg is string => Boolean(msg))
+      .join('；');
+    return {
+      code: String(parsed.error || ''),
+      message: issueText || parsed.message || parsed.error || text || '请求失败',
+      issues,
+    };
+  } catch {
+    return { code: '', message: text || '请求失败', issues: [] };
+  }
+}
+
+export function projectSettingsErrorFromResponse(
+  status: number,
+  text: string,
+  fallback: string,
+): ProjectSettingsApiError {
+  const parsed = parseProjectSettingsErrorBody(text);
+  return new ProjectSettingsApiError({
+    status,
+    code: parsed.code,
+    message: parsed.message || fallback,
+    issues: parsed.issues,
+  });
+}
+
+export function recoverFromProjectSettingsError(
+  err: Pick<ProjectSettingsApiError, 'code' | 'status'>,
+): ProjectSettingsErrorRecovery {
+  if (err.code === 'PLAN_FROZEN') return 'refresh_and_freeze';
+  if (err.code === 'PLAN_REVISION_CONFLICT') return 'reload_revision';
+  if (err.code === 'ERR_CONCURRENT_SWITCH') return 'retry_after_switch';
+  return 'show_message';
+}
+
+export function retainDraftOnFrozenConflict<T>(draft: T): T {
+  return draft;
+}
diff --git a/web/src/lib/projectSettingsMode.test.ts b/web/src/lib/projectSettingsMode.test.ts
new file mode 100644
index 00000000..64a551ad
--- /dev/null
+++ b/web/src/lib/projectSettingsMode.test.ts
@@ -0,0 +1,106 @@
+import { describe, it } from 'node:test';
+import assert from 'node:assert/strict';
+import {
+  afterFrozenConflictReload,
+  frozenDutySnapshot,
+  projectSettingsChrome,
+  resolveProjectSettingsMode,
+  shouldShowPlanEditor,
+  shouldShowSavePreview,
+} from './projectSettingsMode';
+import type { WorkflowPlan } from '../types/projectSettings';
+
+describe('projectSettingsMode', () => {
+  it('enters frozen when can_edit_plan is false or the project is active', () => {
+    assert.equal(
+      resolveProjectSettingsMode({
+        settings: { can_edit_plan: false, lifecycle_status: 'planning' },
+      }),
+      'frozen',
+    );
+    assert.equal(
+      resolveProjectSettingsMode({
+        settings: { can_edit_plan: true, lifecycle_status: 'active' },
+      }),
+      'frozen',
+    );
+    assert.equal(
+      resolveProjectSettingsMode({
+        settings: {
+          can_edit_plan: true,
+          lifecycle_status: 'planning',
+          live_instance: { id: 89, status: 'active', config_revision: 1 },
+        },
+      }),
+      'frozen',
+    );
+    assert.equal(shouldShowPlanEditor('frozen'), false);
+    assert.equal(shouldShowSavePreview('frozen'), false);
+    const chrome = projectSettingsChrome('frozen');
+    assert.equal(chrome.showWorkflowSelector, false);
+    assert.equal(chrome.showDutySelects, false);
+    assert.equal(chrome.showSavePreview, false);
+    assert.equal(chrome.showFrozenSnapshot, true);
+  });
+
+  it('stays planning only for an editable draft', () => {
+    assert.equal(
+      resolveProjectSettingsMode({
+        settings: { can_edit_plan: true, lifecycle_status: 'planning' },
+      }),
+      'planning',
+    );
+    assert.equal(shouldShowPlanEditor('planning'), true);
+    assert.equal(shouldShowSavePreview('planning'), true);
+  });
+
+  it('lists enabled duties for the frozen snapshot and skips disabled tasks', () => {
+    const plan: WorkflowPlan = {
+      workflow_definition_key: 'main',
+      workflow_version: 2,
+      stages: [
+        {
+          stage_key: 'V0.1_triage',
+          enabled: true,
+          doc_tasks: [
+            {
+              task_key: 'intake',
+              task_title: '用户初始需求收集表',
+              enabled: true,
+              duties: { author_user_id: 9, reviewer_user_id: 2, approver_user_id: 3 },
+            },
+            {
+              task_key: 'research',
+              enabled: false,
+              duties: { author_user_id: 9 },
+            },
+          ],
+        },
+      ],
+    };
+    assert.deepEqual(frozenDutySnapshot(plan), [
+      {
+        taskKey: 'intake',
+        title: '用户初始需求收集表',
+        author: 9,
+        reviewer: 2,
+        approver: 3,
+      },
+    ]);
+  });
+
+  it('keeps the local draft after PLAN_FROZEN refresh and switches to frozen', () => {
+    const draft: WorkflowPlan = {
+      workflow_definition_key: 'main',
+      workflow_version: 1,
+      stages: [],
+    };
+    const recovered = afterFrozenConflictReload(
+      { can_edit_plan: false, lifecycle_status: 'active' },
+      draft,
+    );
+    assert.equal(recovered.plan, draft);
+    assert.equal(recovered.mode, 'frozen');
+    assert.equal(projectSettingsChrome(recovered.mode).showSavePreview, false);
+  });
+});
diff --git a/web/src/lib/projectSettingsMode.ts b/web/src/lib/projectSettingsMode.ts
new file mode 100644
index 00000000..924a866d
--- /dev/null
+++ b/web/src/lib/projectSettingsMode.ts
@@ -0,0 +1,75 @@
+import type { ProjectSettings, WorkflowPlan } from '../types/projectSettings';
+
+export type ProjectSettingsMode = 'loading' | 'planning' | 'frozen' | 'error';
+
+export function resolveProjectSettingsMode(input: {
+  loading?: boolean;
+  settings?: Pick<ProjectSettings, 'can_edit_plan' | 'lifecycle_status' | 'live_instance'> | null;
+  loadError?: boolean;
+}): ProjectSettingsMode {
+  if (input.loadError && !input.settings) return 'error';
+  if (input.loading && !input.settings) return 'loading';
+  const settings = input.settings;
+  if (!settings) return 'loading';
+  if (!settings.can_edit_plan) return 'frozen';
+  if (settings.lifecycle_status === 'active') return 'frozen';
+  const live = settings.live_instance;
+  if (live && live.status !== 'completed' && live.status !== 'terminated') return 'frozen';
+  return 'planning';
+}
+
+export function shouldShowPlanEditor(mode: ProjectSettingsMode): boolean {
+  return mode === 'planning';
+}
+
+export function shouldShowSavePreview(mode: ProjectSettingsMode): boolean {
+  return mode === 'planning';
+}
+
+export function projectSettingsChrome(mode: ProjectSettingsMode): {
+  showWorkflowSelector: boolean;
+  showDutySelects: boolean;
+  showSavePreview: boolean;
+  showStepPills: boolean;
+  showFrozenSnapshot: boolean;
+} {
+  const planning = mode === 'planning';
+  return {
+    showWorkflowSelector: planning,
+    showDutySelects: planning,
+    showSavePreview: planning,
+    showStepPills: planning,
+    showFrozenSnapshot: mode === 'frozen',
+  };
+}
+
+export function afterFrozenConflictReload(
+  settings: Pick<ProjectSettings, 'can_edit_plan' | 'lifecycle_status' | 'live_instance'>,
+  draftPlan: WorkflowPlan,
+): { plan: WorkflowPlan; mode: ProjectSettingsMode } {
+  return {
+    plan: draftPlan,
+    mode: resolveProjectSettingsMode({ settings }),
+  };
+}
+
+export function frozenDutySnapshot(plan?: WorkflowPlan | null): Array<{
+  taskKey: string;
+  title: string;
+  author?: number;
+  reviewer?: number;
+  approver?: number;
+}> {
+  if (!plan) return [];
+  return plan.stages.flatMap((stage) =>
+    stage.doc_tasks
+      .filter((task) => task.enabled)
+      .map((task) => ({
+        taskKey: task.task_key,
+        title: task.task_title || task.task_key,
+        author: task.duties.author_user_id,
+        reviewer: task.duties.reviewer_user_id,
+        approver: task.duties.approver_user_id,
+      })),
+  );
+}