diff --git a/crates/df-ai/src/git_worktree.rs b/crates/df-ai/src/git_worktree.rs
index 657bc86..dcb9435 100644
--- a/crates/df-ai/src/git_worktree.rs
+++ b/crates/df-ai/src/git_worktree.rs
@@ -167,24 +167,35 @@ pub fn merge_branch(
     plan_wt_path: &Path,
     subtask_branch: &str,
 ) -> Result<MergeResult, WorktreeError> {
-    // 用旧版兼容的 merge-tree(base branch → 输出冲突树)
-    // --write-tree 需 Git 2.38+,改用三参数 merge-tree <base> <branch1> <branch2>
-    // 成功(exit 0)= 无冲突可自动合并;非0且有输出 = 冲突
-    let base = "HEAD";
-    let precheck = Command::new("git")
-        .args(["merge-tree", base, subtask_branch, base])
+    // 预检用旧版三参形态 merge-tree <共同祖先> <ours> <theirs>
+    // (--write-tree 需 Git 2.38+,为兼容旧 git 不采用)
+    // 第 1 参必须是 merge-base:此前误传 HEAD,branch2(HEAD) 相对 base 零差异,
+    // trivial merge 恒判无冲突(实测真冲突场景输出为空 exit 0),预检形同虚设
+    let mb = Command::new("git")
+        .args(["merge-base", "HEAD", subtask_branch])
         .current_dir(plan_wt_path)
         .output()
-        .map_err(|e| WorktreeError::GitFailed(format!("git merge-tree 失败: {}", e)))?;
+        .map_err(|e| WorktreeError::GitFailed(format!("git merge-base 失败: {}", e)))?;
 
-    // merge-tree 输出含 conflict marker(<<<<<<<)时表示有冲突
-    let precheck_stdout = String::from_utf8_lossy(&precheck.stdout);
-    if precheck_stdout.contains("<<<<<<<") || precheck_stdout.contains("=======") {
-        return Ok(MergeResult::Conflict {
-            branch: subtask_branch.to_string(),
-            details: precheck_stdout.to_string(),
-        });
+    // 旧式形态冲突时 exit 仍为 0,只能看 stdout 判定;只认 <<<<<<< 标记,
+    // 不查 =======:干净合并的 diff 上下文会带出文件内容,文档里的等号分隔行会误撞
+    if mb.status.success() {
+        let merge_base = String::from_utf8_lossy(&mb.stdout).trim().to_string();
+        let precheck = Command::new("git")
+            .args(["merge-tree", merge_base.as_str(), "HEAD", subtask_branch])
+            .current_dir(plan_wt_path)
+            .output()
+            .map_err(|e| WorktreeError::GitFailed(format!("git merge-tree 失败: {}", e)))?;
+
+        let precheck_stdout = String::from_utf8_lossy(&precheck.stdout);
+        if precheck_stdout.contains("<<<<<<<") {
+            return Ok(MergeResult::Conflict {
+                branch: subtask_branch.to_string(),
+                details: precheck_stdout.to_string(),
+            });
+        }
     }
+    // merge-base 失败 = 两分支无共同祖先,无法预检,交由下方实际 merge 裁决
 
     // 无冲突,执行实际 merge
     let merge = Command::new("git")
@@ -539,4 +550,91 @@ mod tests {
         // plan wt 目录也应被清理
         assert!(!plan_wt.0.exists(), "plan worktree 目录应已清理");
     }
+
+    #[test]
+    fn wt_08_precheck_hits_conflict_before_real_merge() {
+        let repo = setup_repo();
+        let root = repo.path();
+
+        std::fs::write(root.join("shared.rs"), "line1\nline2\n").unwrap();
+        Command::new("git").args(["add", "-A"]).current_dir(root).output().unwrap();
+        Command::new("git").args(["commit", "-m", "add shared"]).current_dir(root).output().unwrap();
+
+        let plan_wt = create_plan_worktree(root, "plan-prechk").unwrap();
+        let plan_wt_path = plan_wt.0.clone();
+        let plan_branch = plan_wt.1;
+
+        // A 和 B 改同一文件同一行
+        let a = create_worktree(root, "plan-prechk", "A", &plan_branch).unwrap();
+        std::fs::write(a.path.join("shared.rs"), "A_version\n").unwrap();
+        commit_worktree(&a.path, "A change shared").unwrap();
+
+        let b = create_worktree(root, "plan-prechk", "B", &plan_branch).unwrap();
+        std::fs::write(b.path.join("shared.rs"), "B_version\n").unwrap();
+        commit_worktree(&b.path, "B change shared").unwrap();
+
+        assert!(matches!(
+            merge_branch(&plan_wt_path, &a.branch).unwrap(),
+            MergeResult::Clean
+        ));
+
+        // 冲突必须由 merge-tree 预检命中(details 含标记),而非实际 merge 失败兜底
+        // (兜底 details 来自 merge stderr,只有 CONFLICT 文案无标记)
+        match merge_branch(&plan_wt_path, &b.branch).unwrap() {
+            MergeResult::Conflict { details, .. } => {
+                assert!(
+                    details.contains("<<<<<<<"),
+                    "冲突详情应含 merge-tree 预检输出的标记,而非 merge 兜底文案: {}",
+                    details
+                );
+            }
+            _ => panic!("同文件同行改应冲突"),
+        }
+
+        // 预检命中即返回,不应留下合并中状态
+        let merging = Command::new("git")
+            .args(["rev-parse", "-q", "--verify", "MERGE_HEAD"])
+            .current_dir(&plan_wt_path)
+            .output()
+            .unwrap();
+        assert!(!merging.status.success(), "预检命中不应留下 MERGE_HEAD");
+
+        remove_plan_worktrees(root, "plan-prechk").unwrap();
+    }
+
+    #[test]
+    fn wt_09_clean_merge_with_eq_marker_content() {
+        let repo = setup_repo();
+        let root = repo.path();
+
+        // 文件内容含等号分隔行(md 标题下划线常见),A/B 隔行修改,实际可干净合并
+        std::fs::write(root.join("doc.md"), "t0\n=======\nl3\nl4\nl5\n").unwrap();
+        Command::new("git").args(["add", "-A"]).current_dir(root).output().unwrap();
+        Command::new("git").args(["commit", "-m", "add doc"]).current_dir(root).output().unwrap();
+
+        let plan_wt = create_plan_worktree(root, "plan-eqmark").unwrap();
+        let plan_wt_path = plan_wt.0.clone();
+        let plan_branch = plan_wt.1;
+
+        let a = create_worktree(root, "plan-eqmark", "A", &plan_branch).unwrap();
+        std::fs::write(a.path.join("doc.md"), "A0\n=======\nl3\nl4\nl5\n").unwrap();
+        commit_worktree(&a.path, "A chg line1").unwrap();
+
+        let b = create_worktree(root, "plan-eqmark", "B", &plan_branch).unwrap();
+        std::fs::write(b.path.join("doc.md"), "t0\n=======\nl3\nl4\nB5\n").unwrap();
+        commit_worktree(&b.path, "B chg line5").unwrap();
+
+        assert!(matches!(
+            merge_branch(&plan_wt_path, &a.branch).unwrap(),
+            MergeResult::Clean
+        ));
+
+        // B 隔行修改应 Clean:预检不得因 diff 上下文中的等号行误判冲突
+        assert!(matches!(
+            merge_branch(&plan_wt_path, &b.branch).unwrap(),
+            MergeResult::Clean
+        ));
+
+        remove_plan_worktrees(root, "plan-eqmark").unwrap();
+    }
 }