diff --git a/.task/feature-task-start-merged-guard.md b/.task/feature-task-start-merged-guard.md
index 00df6e21..fdde936f 100644
--- a/.task/feature-task-start-merged-guard.md
+++ b/.task/feature-task-start-merged-guard.md
@@ -6,11 +6,13 @@
 
 - 权威仓：ssh://git@git-ssh.u1680832.nyat.app:31411/weichen/Agent-System-Dev.git；目标分支：main；R0（initial_base_sha）= 43d33aa4921078bc202393b99cba017d7f731b98
 - 工作区：/home/weichen/projects/Agent-System-Dev-wt-feature-task-start-merged-guard（worktree）；分支：feature/task-start-merged-guard
-- 授权与交付终点：<本次允许的动作与终点；未授权事项写明>
+- 授权与交付终点：owner 2026-10-05 授权立项（决策清单 #7）；改 scripts/task.sh 加守护 + 测试 + PR 合入为本卡终点；不改 task.sh 其他行为。
 
 ## 目标与非目标
 
-<预期结果、受影响入口/模块/契约；明确不做什么。>
+- **目标**：`cmd_start` 在分支名校验后新增守护——本地或远端存在同名分支且已包含于 `origin/main` 时拒绝启动（exit 非零），提示「沿用原分支或换新 slug」。
+- **背景**：2026-10-05 EXT-1 复核轮实测，合入后再 `task-start` 同名分支会从新 main 长出第二段历史、空卡覆盖已合入任务卡（两轮证据差点丢失，靠 git show 恢复）；旧逻辑只校验本地分支能否创建，不查是否刚合入。
+- **非目标**：不改 precheck/task-sync/finish 等其他子命令；不做「恢复原卡」的自动修复（拒绝即可，恢复留给人工）；不引入新依赖。
 
 ## 同步记录
 
@@ -29,3 +31,58 @@
 - 结果：
 - 验证：<结论摘要 PASS/FAIL/SKIP/NOT_RUN 逐项；逐条证据由 make evidence 写入上方证据表>
 - 影响与剩余项：
+| 2026-10-05T12:28:53Z | `bash -n scripts/task.sh` | 94f8d4760b | 0 | SELF | e3b0c44298fc1c14 |
+
+<!-- evidence:run 2026-10-05T12:28:53Z -->
+```
+$ bash -n scripts/task.sh
+[exit 0 | HEAD 94f8d4760b | sha256:e3b0c44298fc1c14] 输出尾部 15 行：
+
+```
+| 2026-10-05T12:28:53Z | `bash scripts/task.sh start feature/ext-1-inproc-handler && exit 1 \|\| echo 'guard refused merged branch as expected'` | 94f8d4760b | 0 | SELF | 937f2c0a1987b4f7 |
+
+<!-- evidence:run 2026-10-05T12:28:53Z -->
+```
+$ bash scripts/task.sh start feature/ext-1-inproc-handler && exit 1 || echo 'guard refused merged branch as expected'
+[exit 0 | HEAD 94f8d4760b | sha256:937f2c0a1987b4f7] 输出尾部 15 行：
+ERROR: branch feature/ext-1-inproc-handler already merged into origin/main — reuse the original branch or pick a new slug (task-start must not recreate merged cards)
+guard refused merged branch as expected
+```
+| 2026-10-05T12:29:05Z | `make test` | 94f8d4760b | 0 | SELF | 84092b2ad053ee79 |
+
+<!-- evidence:run 2026-10-05T12:29:05Z -->
+```
+$ make test
+[exit 0 | HEAD 94f8d4760b | sha256:84092b2ad053ee79] 输出尾部 20 行：
+
+ ✓ test/ui.test.tsx (6 tests) 66ms
+ ✓ test/toolblock.test.tsx (4 tests) 76ms
+ ✓ test/overlay-host.test.tsx (6 tests) 68ms
+ ✓ test/file-preview.test.tsx (9 tests) 89ms
+ ✓ test/inbox.test.tsx (13 tests) 93ms
+ ✓ test/card-anchor.test.tsx (4 tests) 96ms
+ ✓ test/inline-confirm.test.tsx (7 tests) 107ms
+ ✓ test/capability.test.tsx (14 tests) 153ms
+
+ Test Files  14 passed (14)
+      Tests  103 passed (103)
+   Start at  12:29:37
+   Duration  619ms (transform 415ms, setup 0ms, import 1.36s, tests 955ms, environment 3.25s)
+
+knip unused exports 55 <= 55
+cd deps/agent-command && go vet ./... && go test -count=1 ./...
+ok  	agent-command	0.002s
+ok  	agent-command/conformance	0.002s
+make[1]: Leaving directory '/home/weichen/projects/Agent-System-Dev-wt-feature-task-start-merged-guard'
+```
+| 2026-10-05T12:30:24Z | `make task-sync` | 94f8d4760b | 2 | SELF | c59e97420676c4fa |
+
+<!-- evidence:run 2026-10-05T12:30:24Z -->
+```
+$ make task-sync
+[exit 2 | HEAD 94f8d4760b | sha256:c59e97420676c4fa] 输出尾部 15 行：
+make[1]: Entering directory '/home/weichen/projects/Agent-System-Dev-wt-feature-task-start-merged-guard'
+ERROR: dirty tree — 先显式提交；不隐式 stash（collaboration.md:11）
+make[1]: *** [Makefile:36: task-sync] Error 1
+make[1]: Leaving directory '/home/weichen/projects/Agent-System-Dev-wt-feature-task-start-merged-guard'
+```
diff --git a/scripts/task.sh b/scripts/task.sh
index 8a31e3ab..5505f5ba 100755
--- a/scripts/task.sh
+++ b/scripts/task.sh
@@ -102,9 +102,18 @@ cmd_precheck() {
 
 cmd_start() {
   need_repo
-  local name="${1:-}"; [ -n "$name" ] || die "usage: task.sh start <feature|fix|hotfix>/<slug>"
+  local name="${1:-}"; [ -n "$name" ] || die "usage: task.sh start <feature|fix|hotfix|release>/<slug>"
   echo "$name" | grep -qE '^(feature|fix|hotfix|release)/[A-Za-z0-9._-]+$' || die "branch name must match (feature|fix|hotfix|release)/<slug>"
   git fetch -q origin "${TARGET_BRANCH}" 2>/dev/null || true
+  # 已合入的同名分支不得重建（2026-10-05 实测：合入后再 task-start 同名会从新 main 长出
+  # 第二段历史，空卡覆盖已合入任务卡，两轮证据差点丢失）。复核/追加沿用原分支或换新 slug。
+  local guard_ref
+  for guard_ref in "refs/heads/${name}" "refs/remotes/origin/${name}"; do
+    if git rev-parse -q --verify "${guard_ref}" >/dev/null \
+      && git merge-base --is-ancestor "${guard_ref}" "origin/${TARGET_BRANCH}"; then
+      die "branch ${name} already merged into origin/${TARGET_BRANCH} — reuse the original branch or pick a new slug (task-start must not recreate merged cards)"
+    fi
+  done
   [ "$(git status --porcelain | { grep -v '^.. \.zcode/' || true; } | wc -l)" -eq 0 ] || die "dirty tree — resolve first (precheck verdict BUSY)"
   [ "$(git submodule status --recursive 2>/dev/null | { grep -cE '^\+|^U' || true; })" -eq 0 ] || die "submodule drift/conflict — resolve first (precheck verdict BUSY)"
   git merge-base --is-ancestor "origin/${TARGET_BRANCH}" HEAD || die "local ${TARGET_BRANCH} behind origin — pull --ff-only first"