diff --git a/packages/core/src/git.ts b/packages/core/src/git.ts
index 224cd801cc54..d673b7b8ddb0 100644
--- a/packages/core/src/git.ts
+++ b/packages/core/src/git.ts
@@ -298,6 +298,9 @@ const layer = Layer.effect(
       yield* operation("reset", repository.worktree, ["reset", "--hard", revision])
     })
 
+    // Never pass an unbounded path list as arguments: Windows caps a command line at
+    // 32,767 characters (ENAMETOOLONG). Feed paths through stdin instead, e.g.
+    // `--pathspec-from-file=- --pathspec-file-nul`, `--stdin`, or `update-index --index-info`.
     const repositoryArgs = (repository: Repository, args: string[]) => [
       "--git-dir",
       repository.gitDirectory,
@@ -513,6 +516,7 @@ const layer = Layer.effect(
     /**
      * Three batched invocations over the tree pair instead of three per file. An
      * explicit empty selection diffs nothing; an absent one diffs every changed path.
+     * A selected path also selects everything beneath it, like a literal pathspec.
      * Patch output is capped like VCS diffs: files past the cap get an empty patch.
      */
     const treeDiff = Effect.fn("Git.tree.diff")(function* (input: {
@@ -523,17 +527,75 @@ const layer = Layer.effect(
       paths?: readonly RelativePath[]
     }) {
       if (input.paths?.length === 0) return []
-      const args = ["--no-renames", input.from, input.to, "--", ...(input.paths ?? [])]
+      if (!input.paths) return yield* diffFiles(input.repository, ["--no-renames", input.from, input.to], input.context)
+      // Never put the selection on the command line: a large revert selects thousands of
+      // files and the spawn fails with ENAMETOOLONG on Windows. `git diff` has no
+      // `--pathspec-from-file`, so stage only the selected changes of `to` over `from`
+      // in a private index (paths go through stdin) and diff that index against `from`.
+      // Normalize like a pathspec: `./a` and `a//b` mean `a` and `a/b`; a trailing slash keeps
+      // its directory-only meaning.
+      const selected = new Set(input.paths.map((file) => path.posix.normalize(file)))
+      const changes = nuls(
+        (yield* repositoryOperation("diff", input.repository, [
+          "diff",
+          "--raw",
+          "-z",
+          "--no-abbrev",
+          "--no-renames",
+          input.from,
+          input.to,
+        ])).text,
+      ).flatMap((meta, index, records) => {
+        const file = records[index + 1]
+        if (index % 2 !== 0 || !file) return []
+        // `:<old mode> <new mode> <old oid> <new oid> <status>`; a deletion has mode 000000.
+        const [before, mode, , oid] = meta.slice(1).split(" ")
+        if (!isSelected(selected, file, before === "160000" || mode === "160000")) return []
+        return [{ file, entry: `${mode === "000000" ? "0" : mode} ${oid}\t${file}\0` }]
+      })
+      if (!changes.length) return []
+      const chosen = new Set(changes.map((change) => change.file))
+      const index = path.join(input.repository.gitDirectory, `opencode-diff-${crypto.randomUUID()}.index`)
+      const env = { GIT_INDEX_FILE: index }
+      return yield* Effect.gen(function* () {
+        yield* repositoryOperation("diff", input.repository, ["read-tree", input.from], { env })
+        yield* repositoryOperation("diff", input.repository, ["update-index", "-z", "--index-info"], {
+          env,
+          stdin: changes.map((change) => change.entry).join(""),
+        })
+        // Staging a path over a file/directory conflict drops the other side from the index,
+        // so the index diff can include unselected deletions; keep only the selection.
+        return (yield* diffFiles(
+          input.repository,
+          ["--cached", "--no-renames", input.from],
+          input.context,
+          env,
+        )).filter((entry) => chosen.has(entry.file))
+      }).pipe(
+        Effect.ensuring(
+          Effect.forEach([index, `${index}.lock`], (file) => fs.remove(file, { force: true }).pipe(Effect.ignore), {
+            discard: true,
+          }),
+        ),
+      )
+    })
+
+    const diffFiles = Effect.fnUntraced(function* (
+      repository: Repository,
+      args: string[],
+      context = 3,
+      env?: Record<string, string>,
+    ) {
       // Patch headers have no -z form: unquoted paths keep chunksByFile matching non-ASCII names.
       const [names, numbers, patch] = yield* Effect.all(
         [
-          repositoryOperation("diff", input.repository, ["diff", "--name-status", "-z", ...args]),
-          repositoryOperation("diff", input.repository, ["diff", "--numstat", "-z", ...args]),
+          repositoryOperation("diff", repository, ["diff", "--name-status", "-z", ...args], { env }),
+          repositoryOperation("diff", repository, ["diff", "--numstat", "-z", ...args], { env }),
           repositoryOperation(
             "diff",
-            input.repository,
-            ["-c", "core.quotepath=false", "diff", "--no-ext-diff", `--unified=${input.context ?? 3}`, ...args],
-            { maxOutputBytes: VcsPatch.MAX_TOTAL_PATCH_BYTES },
+            repository,
+            ["-c", "core.quotepath=false", "diff", "--no-ext-diff", `--unified=${context}`, ...args],
+            { env, maxOutputBytes: VcsPatch.MAX_TOTAL_PATCH_BYTES },
           ),
         ],
         { concurrency: 3 },
@@ -744,6 +806,20 @@ function execute(cwd: string, proc: AppProcess.Interface, args: string[]) {
     )
 }
 
+/**
+ * Whether `file` or one of its parent directories is in the normalized selection. Like a
+ * pathspec, a trailing slash selects only a directory or a submodule (gitlink) entry.
+ */
+function isSelected(selected: ReadonlySet<string>, file: string, gitlink: boolean) {
+  if (selected.has(".") || selected.has("./") || selected.has(file)) return true
+  if (gitlink && selected.has(`${file}/`)) return true
+  for (let end = file.lastIndexOf("/"); end > 0; end = file.lastIndexOf("/", end - 1)) {
+    const directory = file.slice(0, end)
+    if (selected.has(directory) || selected.has(`${directory}/`)) return true
+  }
+  return false
+}
+
 /** Split NUL-terminated git output into its records. */
 function nuls(text: string) {
   return text.split("\0").filter(Boolean)
diff --git a/packages/core/test/git.test.ts b/packages/core/test/git.test.ts
index 39702b62d33e..67f73fee8efb 100644
--- a/packages/core/test/git.test.ts
+++ b/packages/core/test/git.test.ts
@@ -235,6 +235,103 @@ describe("Git trees", () => {
     }),
   )
 
+  it.live("selects tree diff paths with pathspec semantics", () =>
+    Effect.gen(function* () {
+      const root = yield* Effect.acquireRelease(
+        Effect.promise(() => tmpdir()),
+        (dir) => Effect.promise(() => dir[Symbol.asyncDispose]()),
+      )
+      yield* Effect.promise(async () => {
+        await initRepo(root.path)
+        await Bun.write(path.join(root.path, "swap"), "file\n")
+        await Bun.write(path.join(root.path, "plain"), "before\n")
+      })
+      const git = yield* Git.Service
+      const repository = yield* git.repo.discover(AbsolutePath.make(root.path))
+      if (!repository) throw new Error("Repository not found")
+      const before = yield* git.tree.capture({ repository, scopes: [RelativePath.make(".")] })
+      yield* Effect.promise(async () => {
+        await fs.rm(path.join(root.path, "swap"))
+        await Bun.write(path.join(root.path, "swap", "child.txt"), "child\n")
+        await Bun.write(path.join(root.path, "plain"), "after\n")
+      })
+      const after = yield* git.tree.capture({ repository, scopes: [RelativePath.make(".")] })
+      const select = (...paths: string[]) =>
+        git.tree
+          .diff({ repository, from: before, to: after, paths: paths.map((file) => RelativePath.make(file)) })
+          .pipe(Effect.map((diffs) => diffs.map((item) => [item.file, item.status])))
+
+      // Staging the child over the replaced file must not report the unselected file deletion.
+      expect(yield* select("swap/child.txt")).toEqual([["swap/child.txt", "added"]])
+      expect(yield* select("./swap//child.txt", "plain")).toEqual([
+        ["plain", "modified"],
+        ["swap/child.txt", "added"],
+      ])
+      // A trailing slash selects directories only, not the file that was replaced.
+      expect(yield* select("swap/", "plain/")).toEqual([["swap/child.txt", "added"]])
+      expect(yield* select("swap")).toEqual([
+        ["swap", "deleted"],
+        ["swap/child.txt", "added"],
+      ])
+    }),
+  )
+
+  it.live("diffs a path selection longer than any platform command line", () =>
+    Effect.gen(function* () {
+      const root = yield* Effect.acquireRelease(
+        Effect.promise(() => tmpdir()),
+        (dir) => Effect.promise(() => dir[Symbol.asyncDispose]()),
+      )
+      yield* Effect.promise(async () => {
+        await initRepo(root.path)
+        await fs.mkdir(path.join(root.path, "dir", "nested"), { recursive: true })
+        await Bun.write(path.join(root.path, "changed.txt"), "before\n")
+        await Bun.write(path.join(root.path, "removed.txt"), "removed\n")
+        await Bun.write(path.join(root.path, "dir", "nested", "file.txt"), "before\n")
+      })
+      const git = yield* Git.Service
+      const repository = yield* git.repo.discover(AbsolutePath.make(root.path))
+      if (!repository) throw new Error("Repository not found")
+      const before = yield* git.tree.capture({ repository, scopes: [RelativePath.make(".")] })
+      yield* Effect.promise(async () => {
+        await Bun.write(path.join(root.path, "changed.txt"), "after\n")
+        await fs.rm(path.join(root.path, "removed.txt"))
+        await Bun.write(path.join(root.path, "dir", "nested", "file.txt"), "after\n")
+        await Bun.write(path.join(root.path, "first.txt"), "first\n")
+        await Bun.write(path.join(root.path, "last.txt"), "last\n")
+        await Bun.write(path.join(root.path, "unselected.txt"), "unselected\n")
+      })
+      const after = yield* git.tree.capture({ repository, scopes: [RelativePath.make(".")] })
+      // About 3 MB of paths: past Windows' 32K command line and Linux's 2 MB ARG_MAX.
+      const missing = Array.from({ length: 30_000 }, (_, index) =>
+        RelativePath.make(`missing/${String(index).padStart(6, "0")}/${"x".repeat(80)}.txt`),
+      )
+      const paths = ["first.txt", "changed.txt", "removed.txt", "dir", ...missing, "last.txt"].map((file) =>
+        RelativePath.make(file),
+      )
+
+      const diffs = yield* git.tree.diff({ repository, from: before, to: after, paths })
+      expect(diffs.map((item) => [item.file, item.status, item.additions, item.deletions])).toEqual([
+        ["changed.txt", "modified", 1, 1],
+        ["dir/nested/file.txt", "modified", 1, 1],
+        ["first.txt", "added", 1, 0],
+        ["last.txt", "added", 1, 0],
+        ["removed.txt", "deleted", 0, 1],
+      ])
+      expect(diffs[0]?.patch).toContain("-before\n+after\n")
+      expect(diffs[2]?.patch).toContain("+first\n")
+      expect(diffs[4]?.patch).toContain("-removed\n")
+      expect(yield* git.tree.diff({ repository, from: before, to: after, paths: missing })).toEqual([])
+      // The private index is removed and the repository's own index is untouched.
+      expect(
+        (yield* Effect.promise(() => fs.readdir(repository.gitDirectory))).filter((file) =>
+          file.startsWith("opencode-diff-"),
+        ),
+      ).toEqual([])
+      expect(yield* git.tree.write(repository)).toBe(after)
+    }),
+  )
+
   it.live("captures, compares, previews, and restores scoped trees", () =>
     Effect.gen(function* () {
       const root = yield* Effect.acquireRelease(