diff --git a/README.md b/README.md
index 4eed135..cf047cf 100644
--- a/README.md
+++ b/README.md
@@ -63,6 +63,10 @@ yours. `owned` also takes a single key of a shared file, such as `.github/workfl
 `package.json#devDependencies.lefthook`; repokeeper then leaves that key, and everything under it,
 alone. Every write command accepts `--dry-run`.
 
+Keep your formatter off the files repokeeper writes: a reformatted copy no longer matches, and
+`check` reports it as edited. With Biome (2.6 also formats Markdown and YAML) or Prettier in a Node
+repository, `init` and `update` name the new files to exclude in `biome.json` or `.prettierignore`.
+
 `init` applies the whole standard unless you say otherwise:
 
 - `repokeeper init --preset essential` writes only the editor settings, `.gitignore` and the CI
diff --git a/src/commands/report.ts b/src/commands/report.ts
index 36673c9..f12392b 100644
--- a/src/commands/report.ts
+++ b/src/commands/report.ts
@@ -90,6 +90,29 @@ export function printResult(io: Io, result: SyncResult): void {
 
 const WROTE: string[] = ["create", "write", "adopt"];
 
+/** Files Biome (2.6 and later) or Prettier would format: a formatted copy no longer matches what check expects. */
+const FORMATTED = /\.(m?js|cjs|json|jsonc|md|ya?ml)$/;
+
+/** Said when such a file is new: the formatter has to leave repokeeper's files, and its lock, alone. */
+function formatterStep(ctx: ModuleContext, result: SyncResult): string | null {
+  const jobs = ctx.stacks.filter((stack) => stack.id === "node" && !stack.directory).flatMap((stack) => stack.staged);
+  const tool = jobs.some((job) => job.name === "node:biome")
+    ? "Biome"
+    : jobs.some((job) => job.name === "node:prettier")
+      ? "Prettier"
+      : null;
+  if (!tool) return null;
+  const paths = result.decisions
+    .filter((d) => (d.output.kind === "file" || d.output.kind === "seed") && d.action === "create")
+    .map((d) => d.output.path)
+    .filter((path) => FORMATTED.test(path));
+  if (paths.length === 0) return null;
+  const why = `${tool} would reformat files repokeeper owns, and \`repokeeper check\` would then report them as edited`;
+  return tool === "Biome"
+    ? `${why}; add ${[...paths, ".repokeeper"].map((p) => `"!${p}"`).join(", ")} to files.includes in biome.json`
+    : `${why}; add ${[...paths, ".repokeeper/"].join(", ")} to .prettierignore`;
+}
+
 /** What the user still has to do after a write: install the hooks or the dependencies, renormalize line endings. */
 export async function nextSteps(root: string, ctx: ModuleContext, result: SyncResult): Promise<string[]> {
   const steps: string[] = [];
@@ -127,6 +150,8 @@ export async function nextSteps(root: string, ctx: ModuleContext, result: SyncRe
     const plan = publishPlan(ctx);
     if (!("problem" in plan)) steps.push(trustedPublisherStep(ctx, plan.registry));
   }
+  const formatter = formatterStep(ctx, result);
+  if (formatter) steps.push(formatter);
   if (wrote(".gitattributes")) {
     const crlf = await crlfTrackedFiles(root);
     if (crlf.length > 0) {
diff --git a/test/report.test.ts b/test/report.test.ts
index 152bc35..016c07a 100644
--- a/test/report.test.ts
+++ b/test/report.test.ts
@@ -3,7 +3,7 @@ import { nextSteps } from "../src/commands/report.js";
 import type { Output } from "../src/model.js";
 import type { SyncResult } from "../src/sync/sync.js";
 import { TOOL_VERSIONS } from "../src/version.js";
-import { makeContext } from "./helpers.js";
+import { makeContext, nodeResolved } from "./helpers.js";
 
 const hooksFile: Output = { kind: "file", module: "hooks", path: "lefthook.yml", content: "" };
 const lefthookRange = (path = "package.json"): Output => ({
@@ -40,7 +40,7 @@ describe("what to do after a write", () => {
   it("says it once when the hooks are new: installing them is the same step", async () => {
     const steps = await nextSteps(
       ".",
-      makeContext(),
+      makeContext({ stacks: [nodeResolved({ staged: [] })] }),
       result({ output: hooksFile, action: "create" }, { output: lefthookRange(), action: "create" }),
     );
     expect(steps).toEqual(["install dependencies; this installs the git hooks"]);
@@ -55,3 +55,37 @@ describe("what to do after a write", () => {
     expect(steps).toEqual([]);
   });
 });
+
+describe("telling a formatter to leave repokeeper's files alone", () => {
+  const file = (path: string): Output => ({ kind: "file", module: "health", path, content: "" });
+  const biome = nodeResolved({
+    staged: [{ name: "node:biome", glob: "*.{js,json}", run: "npx biome check --write {staged_files}" }],
+  });
+  const created = result(
+    { output: file("CODE_OF_CONDUCT.md"), action: "create" },
+    { output: file("commitlint.config.mjs"), action: "create" },
+    { output: file("LICENSE"), action: "create" },
+    { output: file("SECURITY.md"), action: "unchanged" },
+  );
+
+  // Biome 2.6 formats Markdown and YAML too; a formatted copy no longer matches, and check reports it as edited.
+  it("names the new files to exclude in biome.json", async () => {
+    const steps = await nextSteps(".", makeContext({ stacks: [biome] }), created);
+    expect(steps).toEqual([
+      'Biome would reformat files repokeeper owns, and `repokeeper check` would then report them as edited; add "!CODE_OF_CONDUCT.md", "!commitlint.config.mjs", "!.repokeeper" to files.includes in biome.json',
+    ]);
+  });
+
+  it("names them for .prettierignore with Prettier", async () => {
+    const steps = await nextSteps(".", makeContext(), created);
+    expect(steps).toEqual([
+      "Prettier would reformat files repokeeper owns, and `repokeeper check` would then report them as edited; add CODE_OF_CONDUCT.md, commitlint.config.mjs, .repokeeper/ to .prettierignore",
+    ]);
+  });
+
+  it("says nothing without a formatter, or when no such file is new", async () => {
+    expect(await nextSteps(".", makeContext({ stacks: [nodeResolved({ staged: [] })] }), created)).toEqual([]);
+    const unchanged = result({ output: file("CODE_OF_CONDUCT.md"), action: "unchanged" });
+    expect(await nextSteps(".", makeContext({ stacks: [biome] }), unchanged)).toEqual([]);
+  });
+});