diff --git a/packages/rstack/src/fmt/discovery.ts b/packages/rstack/src/fmt/discovery.ts
index aedb6f58..d8abd251 100644
--- a/packages/rstack/src/fmt/discovery.ts
+++ b/packages/rstack/src/fmt/discovery.ts
@@ -1,32 +1,26 @@
 import { resolveFmtOptions } from './config.ts';
 import { discoverFmtPaths } from './discoverPaths.ts';
 import { createFmtIgnoreMatcher } from './ignore.ts';
-import { resolveFmtParser } from './parser.ts';
 import { createFmtPluginResolver, type FmtPluginResolver } from './plugins.ts';
 import type { DiscoverFmtFilesOptions, FmtFileRequest, ResolvedFmtConfig } from './types.ts';
 
-const resolveFileRequest = async (
+const createFileRequest = (
   filePath: string,
   config: ResolvedFmtConfig,
   resolvePlugins: FmtPluginResolver,
-): Promise<FmtFileRequest | undefined> => {
+): FmtFileRequest => {
   const options = resolvePlugins(resolveFmtOptions(filePath, config));
-  const parser = await resolveFmtParser(filePath, options);
-  if (!parser) {
-    return;
-  }
 
   return {
     path: filePath,
     options: {
       ...options,
       filepath: filePath,
-      parser,
     },
   };
 };
 
-/** Discovers format-ready files without reading Prettier config files or `.prettierignore`. */
+/** Discovers worker-ready files without reading Prettier config files or `.prettierignore`. */
 const discoverFmtFiles = async ({
   cwd,
   patterns,
@@ -42,11 +36,8 @@ const discoverFmtFiles = async ({
     ? candidates.filter((filePath) => !isFmtIgnored(filePath))
     : candidates;
   const resolvePlugins = createFmtPluginResolver(config.rootPath);
-  const files = await Promise.all(
-    filePaths.map((filePath) => resolveFileRequest(filePath, config, resolvePlugins)),
-  );
 
-  return files.filter((file): file is FmtFileRequest => file !== undefined);
+  return filePaths.map((filePath) => createFileRequest(filePath, config, resolvePlugins));
 };
 
 export { discoverFmtFiles };
diff --git a/packages/rstack/src/fmt/format.ts b/packages/rstack/src/fmt/format.ts
index b6e52c4f..b2898316 100644
--- a/packages/rstack/src/fmt/format.ts
+++ b/packages/rstack/src/fmt/format.ts
@@ -11,7 +11,12 @@ const formatText = async (
   { filePath, cursorOffset, config }: FormatTextOptions,
 ): Promise<FormatTextResult> => {
   const options = createFmtPluginResolver(config.rootPath)(resolveFmtOptions(filePath, config));
-  const parser = await resolveFmtParser(filePath, options);
+  const formatOptions = {
+    ...options,
+    filepath: filePath,
+  };
+  const plugins = await getPrettierPlugins(formatOptions);
+  const parser = await resolveFmtParser(filePath, formatOptions, plugins);
 
   if (!parser) {
     return {
@@ -20,24 +25,18 @@ const formatText = async (
     };
   }
 
-  const formatOptions = {
-    ...options,
-    filepath: filePath,
-    parser,
-  };
-  const plugins = await getPrettierPlugins(formatOptions);
+  const resolvedOptions = { ...formatOptions, parser, plugins };
 
   if (cursorOffset === undefined) {
     return {
       status: 'formatted',
-      formatted: await format(source, { ...formatOptions, plugins }),
+      formatted: await format(source, resolvedOptions),
     };
   }
 
   const result = await formatWithCursor(source, {
-    ...formatOptions,
+    ...resolvedOptions,
     cursorOffset,
-    plugins,
   });
 
   return {
diff --git a/packages/rstack/src/fmt/parser.ts b/packages/rstack/src/fmt/parser.ts
index 614038db..f841220f 100644
--- a/packages/rstack/src/fmt/parser.ts
+++ b/packages/rstack/src/fmt/parser.ts
@@ -1,5 +1,6 @@
 import { getFileInfo, type FileInfoOptions, type Options as PrettierOptions } from 'prettier';
-import { getPrettierPlugins } from './prettierPlugins.ts';
+
+type PrettierPlugins = NonNullable<PrettierOptions['plugins']>;
 
 const fileInfoOptions = {
   ignorePath: [],
@@ -11,12 +12,13 @@ const fileInfoOptions = {
 const resolveFmtParser = async (
   filePath: string,
   options: PrettierOptions,
+  plugins: PrettierPlugins,
 ): Promise<PrettierOptions['parser'] | null> =>
   options.parser ??
   (
     await getFileInfo(filePath, {
       ...fileInfoOptions,
-      plugins: await getPrettierPlugins(options),
+      plugins,
     })
   ).inferredParser;
 
diff --git a/packages/rstack/src/fmt/runner.ts b/packages/rstack/src/fmt/runner.ts
index 4b251e07..a0fa109b 100644
--- a/packages/rstack/src/fmt/runner.ts
+++ b/packages/rstack/src/fmt/runner.ts
@@ -3,26 +3,30 @@ import type {
   FmtFileRequest,
   FmtFileResult,
   FmtRunResult,
+  FmtWorkerFileResult,
   RunFmtFilesOptions,
 } from './types.ts';
 
 /** Formats one file and reports whether its contents differ. */
-type FormatFile = (file: FmtFileRequest, shouldWrite: boolean) => Promise<boolean>;
+type FormatFile = (file: FmtFileRequest, shouldWrite: boolean) => Promise<FmtWorkerFileResult>;
 
 /** Converts a formatter outcome into the shared per-file result. */
 const runFmtFile = async (
   file: FmtFileRequest,
   shouldWrite: boolean,
   formatFile: FormatFile,
-): Promise<FmtFileResult> => {
+): Promise<FmtFileResult | undefined> => {
   const startTime = performance.now();
 
   try {
-    const changed = await formatFile(file, shouldWrite);
+    const result = await formatFile(file, shouldWrite);
+    if (result === 'unsupported') {
+      return;
+    }
 
     return {
       path: file.path,
-      status: changed ? (shouldWrite ? 'written' : 'different') : 'unchanged',
+      status: result === 'changed' ? (shouldWrite ? 'written' : 'different') : 'unchanged',
       durationMs: performance.now() - startTime,
     };
   } catch (error) {
@@ -45,7 +49,10 @@ const runFmtFilesWithWorkers = async (
   const worker = await createFmtWorker(files.length, maxWorkers);
 
   try {
-    return await Promise.all(files.map((file) => runFmtFile(file, shouldWrite, worker.formatFile)));
+    const results = await Promise.all(
+      files.map((file) => runFmtFile(file, shouldWrite, worker.formatFile)),
+    );
+    return results.filter((result): result is FmtFileResult => result !== undefined);
   } finally {
     worker.terminate();
   }
diff --git a/packages/rstack/src/fmt/types.ts b/packages/rstack/src/fmt/types.ts
index b02fbc3e..7be9afa9 100644
--- a/packages/rstack/src/fmt/types.ts
+++ b/packages/rstack/src/fmt/types.ts
@@ -66,10 +66,11 @@ interface DiscoverFmtFilesOptions {
 interface FmtFileRequest {
   /** Absolute path to the file. */
   path: string;
-  /** Final Prettier options with the parser and file path resolved. */
-  options: ResolvedFmtOptions & Required<Pick<PrettierOptions, 'filepath' | 'parser'>>;
+  /** Final per-file options with project plugins and the file path resolved. */
+  options: ResolvedFmtOptions & Required<Pick<PrettierOptions, 'filepath'>>;
 }
 
+type FmtWorkerFileResult = 'changed' | 'unchanged' | 'unsupported';
 type FmtMode = 'write' | 'check' | 'list-different';
 type FmtExitCode = 0 | 1 | 2;
 
@@ -129,6 +130,7 @@ export type {
   FmtMode,
   FmtPluginSpecifier,
   FmtRunResult,
+  FmtWorkerFileResult,
   FormatTextOptions,
   FormatTextResult,
   ResolvedFmtConfig,
diff --git a/packages/rstack/src/fmt/worker.ts b/packages/rstack/src/fmt/worker.ts
index 0b115eb3..ed81301e 100644
--- a/packages/rstack/src/fmt/worker.ts
+++ b/packages/rstack/src/fmt/worker.ts
@@ -2,8 +2,9 @@
 
 import { readFileSync, writeFileSync } from 'node:fs';
 import { format } from 'prettier';
+import { resolveFmtParser } from './parser.ts';
 import { getPrettierPlugins } from './prettierPlugins.ts';
-import type { FmtFileRequest } from './types.ts';
+import type { FmtFileRequest, FmtWorkerFileResult } from './types.ts';
 
 /**
  * Use synchronous direct I/O inside the dedicated worker to avoid libuv
@@ -12,22 +13,29 @@ import type { FmtFileRequest } from './types.ts';
 const formatFile = async (
   { path, options }: FmtFileRequest,
   shouldWrite: boolean,
-): Promise<boolean> => {
+): Promise<FmtWorkerFileResult> => {
+  const plugins = await getPrettierPlugins(options);
+  const parser = await resolveFmtParser(path, options, plugins);
+  if (!parser) {
+    return 'unsupported';
+  }
+
   const source = readFileSync(path, 'utf8');
   const formatted = await format(source, {
     ...options,
-    plugins: await getPrettierPlugins(options),
+    parser,
+    plugins,
   });
 
   if (source === formatted) {
-    return false;
+    return 'unchanged';
   }
 
   if (shouldWrite) {
     writeFileSync(path, formatted, 'utf8');
   }
 
-  return true;
+  return 'changed';
 };
 
 /** Confirms that the worker module and its runtime dependencies are ready. */
diff --git a/packages/rstack/tests/fmt/discovery.test.ts b/packages/rstack/tests/fmt/discovery.test.ts
index 56755011..ec4ed87c 100644
--- a/packages/rstack/tests/fmt/discovery.test.ts
+++ b/packages/rstack/tests/fmt/discovery.test.ts
@@ -47,7 +47,7 @@ test('applies config ignore patterns outside the config root', async () => {
   });
 });
 
-test('uses Yuku parsers by default and accepts an explicit parser', async () => {
+test('defers parser inference to workers and preserves an explicit parser', async () => {
   await withTempProject(async (rootPath) => {
     writeProjectFile(rootPath, 'index.js');
     writeProjectFile(rootPath, 'index.ts');
@@ -57,8 +57,13 @@ test('uses Yuku parsers by default and accepts an explicit parser', async () =>
     const inferredFiles = await discover(rootPath);
     const configuredFiles = await discover(rootPath, ['source.custom'], { parser: 'babel' });
 
-    expect(relativePaths(rootPath, inferredFiles)).toEqual(['index.js', 'index.ts']);
-    expect(inferredFiles.map((file) => file.options.parser)).toEqual(['yuku', 'yuku-ts']);
+    expect(relativePaths(rootPath, inferredFiles)).toEqual([
+      'index.js',
+      'index.ts',
+      'source.custom',
+      'unknown.extension',
+    ]);
+    expect(inferredFiles.every((file) => file.options.parser === undefined)).toBe(true);
     expect(configuredFiles[0].options).toMatchObject({
       filepath: path.join(rootPath, 'source.custom'),
       parser: 'babel',
@@ -108,13 +113,11 @@ test('resolves plugins after applying matching overrides', async () => {
     expect(files).toHaveLength(2);
     expect(files[0]).toMatchObject({
       options: {
-        parser: 'json',
         plugins: [pathToFileURL(pluginEntry).href],
       },
     });
     expect(files[1]).toMatchObject({
       options: {
-        parser: 'babel',
         plugins: [pathToFileURL(pluginEntry).href],
       },
     });
diff --git a/packages/rstack/tests/fmt/runner.test.ts b/packages/rstack/tests/fmt/runner.test.ts
index edd84ebf..ba8f018d 100644
--- a/packages/rstack/tests/fmt/runner.test.ts
+++ b/packages/rstack/tests/fmt/runner.test.ts
@@ -104,3 +104,20 @@ test('continues after a file fails and gives errors exit-code precedence', async
     expect(readFileSync(validPath, 'utf8')).toBe('const value=1');
   });
 });
+
+test('omits unsupported files from the result', async () => {
+  await withTempProject(async (rootPath) => {
+    const filePath = path.join(rootPath, 'example.unknown');
+    writeFileSync(filePath, 'plain text');
+
+    const result = await run([
+      {
+        path: filePath,
+        options: { filepath: filePath },
+      },
+    ]);
+
+    expect(result).toMatchObject({ exitCode: 0, files: [] });
+    expect(readFileSync(filePath, 'utf8')).toBe('plain text');
+  });
+});
diff --git a/packages/rstack/tests/fmt/worker.test.ts b/packages/rstack/tests/fmt/worker.test.ts
index d8a699e5..1d6a99bb 100644
--- a/packages/rstack/tests/fmt/worker.test.ts
+++ b/packages/rstack/tests/fmt/worker.test.ts
@@ -1,4 +1,5 @@
 import { readFileSync } from 'node:fs';
+import path from 'node:path';
 import { expect, test } from 'rstack/test';
 import { formatFile } from '../../src/fmt/worker.ts';
 import { withTempProject, writeProjectFile } from './helpers.ts';
@@ -18,8 +19,43 @@ test('writes formatted files', async () => {
         },
         true,
       ),
-    ).resolves.toBe(true);
+    ).resolves.toBe('changed');
 
     expect(readFileSync(filePath, 'utf8')).toBe('const value = 1;\n');
   });
 });
+
+test('infers the parser before formatting', async () => {
+  await withTempProject(async (rootPath) => {
+    const source = 'const value=1';
+    const filePath = writeProjectFile(rootPath, 'example.ts', source);
+
+    await expect(
+      formatFile(
+        {
+          path: filePath,
+          options: { filepath: filePath },
+        },
+        false,
+      ),
+    ).resolves.toBe('changed');
+
+    expect(readFileSync(filePath, 'utf8')).toBe(source);
+  });
+});
+
+test('skips unsupported files before reading them', async () => {
+  await withTempProject(async (rootPath) => {
+    const filePath = path.join(rootPath, 'missing.unknown');
+
+    await expect(
+      formatFile(
+        {
+          path: filePath,
+          options: { filepath: filePath },
+        },
+        true,
+      ),
+    ).resolves.toBe('unsupported');
+  });
+});