diff --git a/apps/cli/src/commands/split.ts b/apps/cli/src/commands/split.ts
new file mode 100644
index 00000000..9342dea5
--- /dev/null
+++ b/apps/cli/src/commands/split.ts
@@ -0,0 +1,75 @@
+import {
+  getSplittableFiles,
+  split as splitCmd,
+} from "@array/core/commands/split";
+import type { ArrContext } from "@array/core/engine";
+import { cyan, dim, formatSuccess, hint, message, red } from "../utils/output";
+import { textInput } from "../utils/prompt";
+import { unwrap } from "../utils/run";
+
+export async function split(
+  paths: string[],
+  options: { message?: string },
+  ctx: ArrContext,
+): Promise<void> {
+  // Get splittable files for validation/preview
+  const filesResult = unwrap(await getSplittableFiles());
+
+  if (filesResult.length === 0) {
+    message(`${red("error:")} No files in parent change to split`);
+    return;
+  }
+
+  if (paths.length === 0) {
+    message(`${red("error:")} No paths provided to split`);
+    hint(`Files in parent: ${filesResult.map((f) => f.path).join(", ")}`);
+    return;
+  }
+
+  // Show preview of what will be split
+  const matchingFiles = filesResult.filter((f) =>
+    paths.some((p) => f.path === p || f.path.startsWith(`${p}/`)),
+  );
+
+  if (matchingFiles.length === 0) {
+    message(
+      `${red("error:")} None of the specified paths match files in parent change`,
+    );
+    hint(`Files in parent: ${filesResult.map((f) => f.path).join(", ")}`);
+    return;
+  }
+
+  message(
+    `Splitting ${cyan(String(matchingFiles.length))} file${matchingFiles.length === 1 ? "" : "s"} into new change:`,
+  );
+  for (const file of matchingFiles) {
+    console.log(`  ${dim(file.status)} ${file.path}`);
+  }
+  console.log();
+
+  // Get description - from option or prompt
+  let description = options.message;
+  if (!description) {
+    const input = await textInput("Description for new change");
+    if (!input) {
+      message(`${red("error:")} Description is required`);
+      return;
+    }
+    description = input;
+  }
+
+  const result = unwrap(
+    await splitCmd({
+      paths,
+      description,
+      engine: ctx.engine,
+    }),
+  );
+
+  message(
+    formatSuccess(
+      `Split ${cyan(String(result.fileCount))} file${result.fileCount === 1 ? "" : "s"} into "${result.description}"`,
+    ),
+  );
+  hint(`Tracking: ${cyan(result.bookmarkName)}`);
+}
diff --git a/apps/cli/src/registry.ts b/apps/cli/src/registry.ts
index d04e078f..632fb3dd 100644
--- a/apps/cli/src/registry.ts
+++ b/apps/cli/src/registry.ts
@@ -8,6 +8,7 @@ import { mergeCommand } from "@array/core/commands/merge";
 import { modifyCommand } from "@array/core/commands/modify";
 import { resolveCommand } from "@array/core/commands/resolve";
 import { restackCommand } from "@array/core/commands/restack";
+import { splitCommand } from "@array/core/commands/split";
 import { squashCommand } from "@array/core/commands/squash";
 import { statusCommand } from "@array/core/commands/status";
 import { submitCommand } from "@array/core/commands/submit";
@@ -36,6 +37,7 @@ import { merge } from "./commands/merge";
 import { modify } from "./commands/modify";
 import { resolve } from "./commands/resolve";
 import { restack } from "./commands/restack";
+import { split } from "./commands/split";
 import { squash } from "./commands/squash";
 import { status } from "./commands/status";
 import { submit } from "./commands/submit";
@@ -102,6 +104,7 @@ export const COMMANDS = {
   delete: deleteCommand.meta,
   modify: modifyCommand.meta,
   resolve: resolveCommand.meta,
+  split: splitCommand.meta,
   squash: squashCommand.meta,
   merge: mergeCommand.meta,
   undo: undoCommand.meta,
@@ -134,6 +137,12 @@ export const HANDLERS: Record<string, CommandHandler> = {
     deleteChange(p.args[0], ctx!, { yes: !!p.flags.yes || !!p.flags.y }),
   modify: () => modify(),
   resolve: () => resolve(),
+  split: (p, ctx) =>
+    split(
+      p.args,
+      { message: (p.flags.message ?? p.flags.m) as string | undefined },
+      ctx!,
+    ),
   squash: (p, ctx) => squash(p.args[0], ctx!),
   merge: (p, ctx) => merge(p.flags, ctx!),
   undo: () => undo(),
diff --git a/apps/cli/src/utils/args.ts b/apps/cli/src/utils/args.ts
index 5026c4a1..a1742c0a 100644
--- a/apps/cli/src/utils/args.ts
+++ b/apps/cli/src/utils/args.ts
@@ -23,7 +23,14 @@ export function parseArgs(argv: string[]): ParsedCommand {
         flags[key] = true;
       }
     } else if (arg.startsWith("-") && arg.length === 2) {
-      flags[arg.slice(1)] = true;
+      const key = arg.slice(1);
+      const nextArg = allArgs[i + 1];
+      if (nextArg && !nextArg.startsWith("-")) {
+        flags[key] = nextArg;
+        i++;
+      } else {
+        flags[key] = true;
+      }
     } else if (command === "__guided") {
       command = arg;
     } else {
diff --git a/apps/cli/src/utils/prompt.ts b/apps/cli/src/utils/prompt.ts
index 1817cc93..675c208d 100644
--- a/apps/cli/src/utils/prompt.ts
+++ b/apps/cli/src/utils/prompt.ts
@@ -57,6 +57,31 @@ export async function confirm(
   return result === "yes";
 }
 
+export async function textInput(message: string): Promise<string | null> {
+  const { stdin, stdout } = process;
+  const readline = await import("node:readline");
+
+  if (!stdin.isTTY) {
+    return null;
+  }
+
+  const rl = readline.createInterface({
+    input: stdin,
+    output: stdout,
+  });
+
+  return new Promise((resolve) => {
+    rl.question(`${cyan("?")} ${bold(message)} ${dim("›")} `, (answer) => {
+      rl.close();
+      if (answer.trim() === "") {
+        resolve(null);
+      } else {
+        resolve(answer.trim());
+      }
+    });
+  });
+}
+
 export async function select<T extends string>(
   message: string,
   options: { label: string; value: T; hint?: string }[],
diff --git a/packages/core/src/commands/split.ts b/packages/core/src/commands/split.ts
new file mode 100644
index 00000000..c1c23ff0
--- /dev/null
+++ b/packages/core/src/commands/split.ts
@@ -0,0 +1,218 @@
+import { resolveBookmarkConflict } from "../bookmark-utils";
+import type { Engine } from "../engine";
+import { ensureBookmark, list, runJJ, status } from "../jj";
+import { createError, err, ok, type Result } from "../result";
+import { datePrefixedLabel } from "../slugify";
+import type { Command } from "./types";
+
+interface SplitResult {
+  /** Number of files that were split out */
+  fileCount: number;
+  /** The paths that were split out */
+  paths: string[];
+  /** Description of the new commit (the split-out changes) */
+  description: string;
+  /** Bookmark name for the split-out commit */
+  bookmarkName: string;
+  /** Change ID of the split-out commit */
+  changeId: string;
+}
+
+interface SplitOptions {
+  /** File paths to split out into a new commit */
+  paths: string[];
+  /** Description for the new commit containing the split-out changes */
+  description: string;
+  /** Engine for tracking */
+  engine: Engine;
+}
+
+/**
+ * Split the parent change by moving specified files into a new grandparent.
+ * Like `arr modify`, this targets the parent (the change you're "on").
+ *
+ * Before: trunk -> parent (with all changes) -> WC (empty)
+ * After:  trunk -> new (selected files) -> parent (remaining) -> WC (empty)
+ *
+ * Uses `jj split -r @- -m "<description>" <paths...>` under the hood.
+ */
+export async function split(
+  options: SplitOptions,
+): Promise<Result<SplitResult>> {
+  const { paths, description, engine } = options;
+
+  if (paths.length === 0) {
+    return err(createError("INVALID_STATE", "No paths provided to split"));
+  }
+
+  if (!description.trim()) {
+    return err(
+      createError("INVALID_STATE", "Description is required for split"),
+    );
+  }
+
+  // Get current status
+  const statusResult = await status();
+  if (!statusResult.ok) return statusResult;
+
+  const { parents, modifiedFiles } = statusResult.value;
+
+  // If WC has changes, tell user to create first
+  if (modifiedFiles.length > 0) {
+    return err(
+      createError(
+        "INVALID_STATE",
+        'You have uncommitted changes. Run `arr create "message"` first.',
+      ),
+    );
+  }
+
+  // Get the parent (the change we're splitting)
+  const parent = parents[0];
+  if (!parent) {
+    return err(createError("INVALID_STATE", "No parent change to split"));
+  }
+
+  if (parent.isEmpty) {
+    return err(createError("INVALID_STATE", "Cannot split an empty change"));
+  }
+
+  // Get the parent's modified files
+  const parentDiffResult = await runJJ(["diff", "-r", "@-", "--summary"]);
+  if (!parentDiffResult.ok) return parentDiffResult;
+
+  const parentFiles = parentDiffResult.value.stdout
+    .trim()
+    .split("\n")
+    .filter(Boolean)
+    .map((line) => {
+      const status = line[0];
+      const path = line.slice(2).trim();
+      return { status, path };
+    });
+
+  if (parentFiles.length === 0) {
+    return err(
+      createError("INVALID_STATE", "Parent change has no files to split"),
+    );
+  }
+
+  // Check if any of the specified paths match parent's files
+  const changedPaths = new Set(parentFiles.map((f) => f.path));
+  const matchingPaths: string[] = [];
+
+  for (const path of paths) {
+    // Check for exact match or prefix match (for directories)
+    const matches = parentFiles.filter(
+      (f) => f.path === path || f.path.startsWith(`${path}/`),
+    );
+    if (matches.length > 0) {
+      matchingPaths.push(...matches.map((m) => m.path));
+    } else if (!changedPaths.has(path)) {
+      return err(
+        createError(
+          "INVALID_STATE",
+          `Path "${path}" is not in the parent change's files`,
+        ),
+      );
+    } else {
+      matchingPaths.push(path);
+    }
+  }
+
+  const uniquePaths = [...new Set(matchingPaths)];
+
+  // Generate bookmark name for the split-out commit
+  const timestamp = new Date();
+  const initialBookmarkName = datePrefixedLabel(description, timestamp);
+
+  // Check GitHub for name conflicts
+  const conflictResult = await resolveBookmarkConflict(initialBookmarkName);
+  if (!conflictResult.ok) return conflictResult;
+
+  const bookmarkName = conflictResult.value.resolvedName;
+
+  // Run jj split on the parent (-r @-) with the description and paths
+  const splitResult = await runJJ([
+    "split",
+    "-r",
+    "@-",
+    "-m",
+    description.trim(),
+    ...uniquePaths,
+  ]);
+
+  if (!splitResult.ok) return splitResult;
+
+  // After split on @-, the new structure is:
+  // grandparent (split-out) -> parent (remaining, keeps bookmark) -> WC
+  // So the split-out commit is the grandparent (parent of @-)
+  const grandparentResult = await list({ revset: "@--" });
+  if (!grandparentResult.ok) return grandparentResult;
+
+  const splitChangeId = grandparentResult.value[0]?.changeId;
+  if (!splitChangeId) {
+    return err(createError("INVALID_STATE", "Could not find split change"));
+  }
+
+  // Create bookmark on the split-out commit
+  const bookmarkResult = await ensureBookmark(bookmarkName, splitChangeId);
+  if (!bookmarkResult.ok) return bookmarkResult;
+
+  // Export to git
+  const exportResult = await runJJ(["git", "export"]);
+  if (!exportResult.ok) return exportResult;
+
+  // Track the new bookmark in the engine
+  const refreshResult = await engine.refreshFromJJ(bookmarkName);
+  if (!refreshResult.ok) return refreshResult;
+
+  return ok({
+    fileCount: uniquePaths.length,
+    paths: uniquePaths,
+    description: description.trim(),
+    bookmarkName,
+    changeId: splitChangeId,
+  });
+}
+
+/**
+ * Get the list of files in the parent change that can be split.
+ * Returns the parent's files (since split targets @-).
+ */
+export async function getSplittableFiles(): Promise<
+  Result<{ path: string; status: string }[]>
+> {
+  const parentDiffResult = await runJJ(["diff", "-r", "@-", "--summary"]);
+  if (!parentDiffResult.ok) return parentDiffResult;
+
+  const files = parentDiffResult.value.stdout
+    .trim()
+    .split("\n")
+    .filter(Boolean)
+    .map((line) => {
+      const statusChar = line[0];
+      const path = line.slice(2).trim();
+      const statusMap: Record<string, string> = {
+        M: "modified",
+        A: "added",
+        D: "deleted",
+        R: "renamed",
+      };
+      return { path, status: statusMap[statusChar] ?? statusChar };
+    });
+
+  return ok(files);
+}
+
+export const splitCommand: Command<SplitResult, [SplitOptions]> = {
+  meta: {
+    name: "split",
+    args: "<paths...>",
+    description:
+      "Split files from the parent change into a new change below it",
+    aliases: ["sp"],
+    category: "management",
+  },
+  run: split,
+};