@@ -0,0 +1,187 @@
+// Copyright (c) 2026 ObjectStack. Licensed under the Apache-2.0 license.
+//
+// Two DTS passes over one source give the same bytes, whatever order rollup
+// loads the modules in (#30061).
+//
+// This package's DTS pass (`BUILD_DTS=true tsup`, the docblock above
+// `mainConfig` in `tsup.config.ts`) runs every entry through ONE ts.Program.
+// Its checker creates each type once, on first use, and the declaration
+// printer writes union members, and the object members derived from them, in
+// type-creation order. tsup's bundled rollup-plugin-dts emitted each file when
+// rollup TRANSFORMED it, and rollup transforms in the order its async loads
+// finish, which differs between runs. So whichever file came first decided the
+// order of every union it shared with a later one. On `main` at 1f13eb31, two
+// clean builds of the same tree differed in 18 of 134 declaration files, chunk
+// names included, with the same public surface.
+//
+// `patches/tsup@8.5.1.patch` now emits every source file once, in the
+// program's own file order, before rollup is handed any of them. This file
+// pins that with the smallest tree that shows the defect: two entries that
+// infer the same three string literals in opposite orders. It runs the real
+// tsup CLI twice and forces the two runs to load the entries in opposite
+// orders, by delaying the read of one entry in each run.
+//
+// The two controls make the equality mean something:
+//  - the fixture really is order-sensitive under this TypeScript: emitting
+//    it in the two orders by hand gives two different texts. If a TypeScript
+//    release starts sorting union members, this control fails, so the pin
+//    cannot go quietly vacuous;
+//  - the delay really reached rollup's loader: each run's read log shows the
+//    entries loaded in the order this file asked for.
+//
+// Why a fixture and not two builds of this package: one DTS pass of this
+// package takes ~45 s and ~3.5 GB of RSS, and of four unforced builds of
+// `main` measured on #30061 three agreed byte for byte. Two full builds would
+// double that cost in CI and still pass a regression most of the time. The
+// forced order turns the same defect into a failure on every run.
+
+import { execFileSync } from 'node:child_process';
+import { mkdirSync, mkdtempSync, readdirSync, readFileSync, rmSync, writeFileSync } from 'node:fs';
+import { createRequire } from 'node:module';
+import path from 'node:path';
+import ts from 'typescript';
+import { afterAll, describe, expect, it } from 'vitest';
+
+/** This package's root. Every read and write below stays inside it. */
+const PKG_DIR = path.resolve(__dirname, '..');
+const CACHE_DIR = path.join(PKG_DIR, 'node_modules', '.cache');
+mkdirSync(CACHE_DIR, { recursive: true });
+const WORK_DIR = mkdtempSync(path.join(CACHE_DIR, 'dts-emit-order-'));
+afterAll(() => rmSync(WORK_DIR, { recursive: true, force: true }));
+
+/** The tsup CLI this package's `build` script runs. */
+const TSUP_CLI = path.join(
+  path.dirname(createRequire(path.join(PKG_DIR, 'package.json')).resolve('tsup/package.json')),
+  'dist',
+  'cli-default.js',
+);
+
+const ENTRIES = ['a.ts', 'b.ts'] as const;
+
+// Each entry infers the union of the same three literals, written in opposite
+// orders. Whichever file the checker reads first creates the three literal
+// types, and their creation order is the order both declarations print.
+const SOURCES: Record<string, string> = {
+  'lib.ts': 'export function oneOf<T extends string>(...values: T[]): T {\n  return values[0];\n}\n',
+  'a.ts': "import { oneOf } from './lib';\nexport const a = oneOf('zeta', 'alpha', 'mid');\n",
+  'b.ts': "import { oneOf } from './lib';\nexport const b = oneOf('mid', 'alpha', 'zeta');\n",
+};
+
+// tsup sets `baseUrl`, which TypeScript 6 reports as deprecated.
+const TSCONFIG = {
+  compilerOptions: {
+    target: 'ES2022',
+    module: 'ESNext',
+    moduleResolution: 'Bundler',
+    strict: true,
+    declaration: true,
+    skipLibCheck: true,
+    ignoreDeprecations: '6.0',
+  },
+  include: ['src'],
+};
+
+// Loaded into tsup's DTS worker through NODE_OPTIONS: delays rollup's read of
+// the files named in DTS_ORDER_DELAY, and logs every fixture read in the
+// order it finishes.
+const DELAY_PRELOAD = `
+const fsp = require('node:fs/promises');
+const path = require('node:path');
+const slow = new Set((process.env.DTS_ORDER_DELAY || '').split(',').filter(Boolean));
+const log = process.env.DTS_ORDER_LOG;
+const src = process.env.DTS_ORDER_SRC;
+const readFile = fsp.readFile;
+fsp.readFile = async function (file, ...rest) {
+  const name = typeof file === 'string' && path.dirname(file) === src ? path.basename(file) : undefined;
+  if (name && slow.has(name)) await new Promise((resolve) => setTimeout(resolve, 400));
+  const result = await readFile.call(this, file, ...rest);
+  if (name && log) require('node:fs').appendFileSync(log, name + '\\n');
+  return result;
+};
+`;
+
+const FIXTURE_DIR = path.join(WORK_DIR, 'fixture');
+const SRC_DIR = path.join(FIXTURE_DIR, 'src');
+mkdirSync(SRC_DIR, { recursive: true });
+for (const [name, text] of Object.entries(SOURCES)) writeFileSync(path.join(SRC_DIR, name), text);
+writeFileSync(path.join(FIXTURE_DIR, 'tsconfig.json'), JSON.stringify(TSCONFIG, null, 2));
+writeFileSync(path.join(FIXTURE_DIR, 'package.json'), '{ "name": "dts-emit-order-fixture", "private": true }\n');
+const PRELOAD = path.join(WORK_DIR, 'delay-preload.cjs');
+writeFileSync(PRELOAD, DELAY_PRELOAD);
+
+/** One DTS-only tsup pass with `delayed` read late; returns the output tree and the read order. */
+function dtsPass(label: string, delayed: string): { files: Map<string, string>; readOrder: string[] } {
+  const outDir = path.join(WORK_DIR, label);
+  const log = path.join(WORK_DIR, `${label}.reads`);
+  writeFileSync(log, '');
+  execFileSync(
+    process.execPath,
+    [
+      TSUP_CLI,
+      ...ENTRIES.map((entry) => `src/${entry}`),
+      '--dts-only',
+      '--format',
+      'esm,cjs',
+      '--tsconfig',
+      'tsconfig.json',
+      '--out-dir',
+      outDir,
+      '--silent',
+    ],
+    {
+      cwd: FIXTURE_DIR,
+      env: {
+        ...process.env,
+        NODE_OPTIONS: `--require ${PRELOAD}`,
+        DTS_ORDER_DELAY: delayed,
+        DTS_ORDER_LOG: log,
+        DTS_ORDER_SRC: SRC_DIR,
+      },
+      stdio: 'pipe',
+    },
+  );
+  const files = new Map<string, string>();
+  for (const name of readdirSync(outDir).sort()) files.set(name, readFileSync(path.join(outDir, name), 'utf8'));
+  const readOrder = readFileSync(log, 'utf8')
+    .split('\n')
+    .filter((name) => (ENTRIES as readonly string[]).includes(name));
+  return { files, readOrder };
+}
+
+/** The fixture's declarations, emitted by hand in `order` through one program. */
+function emitInOrder(order: readonly string[]): string {
+  const options: ts.CompilerOptions = { ...ts.convertCompilerOptionsFromJson(TSCONFIG.compilerOptions, FIXTURE_DIR).options, emitDeclarationOnly: true };
+  const roots = ENTRIES.map((entry) => path.join(SRC_DIR, entry));
+  const program = ts.createProgram(roots, options);
+  let text = '';
+  for (const entry of order) {
+    program.emit(program.getSourceFile(path.join(SRC_DIR, entry)), (_, out) => {
+      text += out;
+    }, undefined, true);
+  }
+  return text.split('\n').filter((line) => line.startsWith('export declare const')).sort().join('\n');
+}
+
+describe('the DTS pass emits the same bytes whatever order rollup loads modules in (#30061)', () => {
+  it('the fixture is order-sensitive: emitting a then b and b then a gives different declarations', () => {
+    const aFirst = emitInOrder(['a.ts', 'b.ts']);
+    const bFirst = emitInOrder(['b.ts', 'a.ts']);
+    expect(aFirst).toContain('"zeta" | "alpha" | "mid"');
+    expect(bFirst).toContain('"mid" | "alpha" | "zeta"');
+    expect(aFirst, 'this TypeScript no longer prints unions in creation order, so the pin below proves nothing').not.toBe(bFirst);
+  });
+
+  it('two tsup DTS passes that load the entries in opposite orders write the same files, byte for byte', () => {
+    const aLate = dtsPass('a-late', 'a.ts');
+    const bLate = dtsPass('b-late', 'b.ts');
+
+    // The control: the delay reached rollup's loader, so the two passes really
+    // loaded the entries in opposite orders.
+    expect(aLate.readOrder, 'the a.ts delay did not reach rollup').toEqual(['b.ts', 'a.ts']);
+    expect(bLate.readOrder, 'the b.ts delay did not reach rollup').toEqual(['a.ts', 'b.ts']);
+
+    expect([...aLate.files.keys()]).toEqual(['a.d.mts', 'a.d.ts', 'b.d.mts', 'b.d.ts']);
+    expect(aLate.files.get('a.d.ts')).toMatch(/declare const a: "(zeta|mid)" \| "alpha" \| "(mid|zeta)";/);
+    expect(Object.fromEntries(bLate.files)).toEqual(Object.fromEntries(aLate.files));
+  });
+});
@@ -441,17 +441,28 @@ async function modulesReachingTheSeam(
  * `build` script went 189s → 87s. For scale, `tsc --noEmit` over the whole
  * package peaked at 1103 MB live on 8cdbe0c6e5.
  *
- * NOT BYTE-STABLE: this pass does not emit the same bytes twice. TypeScript
- * prints union members, and the members of object types derived from them, in
- * type-creation order, and that order follows emit order. Three runs of one
- * commit gave three different trees, and rollup's content-hashed chunk names
- * moved with them. The one-program pass also reorders top-level statements
- * and `import`/`export` specifiers, and names one shared chunk after a
- * different module (`data-engine` → `analytics.zod`). Normalise all of those
- * (sort union and property-only type-literal members, statements and
- * specifiers; strip chunk hashes; pair chunks by content), and the unpatched
- * and patched trees are the same tree: 130 files, 0 differing. ⇒ Compare two
- * declaration trees in that order-insensitive form, never by a byte digest.
+ * BYTE-STABLE FOR ONE TREE, ORDER-SENSITIVE ACROSS TREES. TypeScript prints
+ * union members, and the members of object types derived from them, in
+ * type-creation order, and in one shared program that order follows emit
+ * order. The bundled rollup-plugin-dts emitted each file when rollup
+ * transformed it, in the order rollup's async loads happened to finish, so
+ * two builds of one commit wrote different bytes and chunk names (18 of 134
+ * files on 1f13eb31, #30061). The same `patches/tsup@8.5.1.patch` now emits
+ * every source file once, in the program's own file order, before rollup
+ * reads any of them. Two builds of one tree are byte-identical, chunk names
+ * included, and `scripts/dts-emit-order.pin.test.ts` pins that.
+ *
+ * The order is still the PROGRAM's. A change that moves the program's file
+ * order (an entry added or reordered, an import moved, a module split in two)
+ * can reorder members in files it never touched. The one-program pass also
+ * reordered top-level statements and `import`/`export` specifiers against the
+ * per-entry programs, and named one shared chunk after a different module
+ * (`data-engine` → `analytics.zod`). Normalise all of those (sort union and
+ * property-only type-literal members, statements and specifiers; strip chunk
+ * hashes; pair chunks by content), and the unpatched and patched trees are the
+ * same tree: 130 files, 0 differing. ⇒ Compare the declaration trees of two
+ * DIFFERENT sources in that order-insensitive form, never by a byte digest; a
+ * byte digest answers only "same source, same bytes?".
  *
  * If this pass starts failing with `ERR_WORKER_OUT_OF_MEMORY`, the live type
  * graph has outgrown 6144 — that is a loud, actionable failure and the point of
 
@@ -1,5 +1,5 @@
 diff --git a/dist/rollup.js b/dist/rollup.js
-index e128b61b9558318b9f86dc11d72c31f09a8eb7db..35f85e39c2af6c645fb2d3da115742df445d2343 100644
+index e128b61b9558318b9f86dc11d72c31f09a8eb7db..b69ec9c39d208a378cdc96b3886e4279c34b3b27 100644
 --- a/dist/rollup.js
 +++ b/dist/rollup.js
@@ -4915,6 +4915,11 @@ var require_rollup_plugin_dts = _chunkTWFEYLU4js.__commonJS.call(void 0, {
@@ -14,3 +14,67 @@ index e128b61b9558318b9f86dc11d72c31f09a8eb7db..35f85e39c2af6c645fb2d3da115742df
        const { fileNames, options, errors } = configByPath.get(cacheKey);
        dtsFiles = fileNames.filter((name) => DTS_EXTENSIONS.test(name));
        if (errors.length) {
+@@ -6351,6 +6356,38 @@ export { ${[...exportedNames].join(", ")} };
+         return null;
+       }
+     }
++    var emittedInProgramOrder = /* @__PURE__ */ new WeakMap();
++    function emitInProgramOrder(program) {
++      // objectstack patch: one ts.Program serves every entry, so its checker
++      // creates each type once, on first use, and the declaration printer
++      // writes union members (and the object members derived from them) in
++      // type-creation order. Emitting per file in rollup's transform order --
++      // set by async module loading, different on every run -- made the
++      // published .d.ts bytes and chunk hashes differ between two builds of
++      // one source. Emitting every source file once, in the program's own
++      // (deterministic) file order, before any is handed to rollup, fixes the
++      // type-creation order. Each file is still emitted exactly once.
++      let emitted = emittedInProgramOrder.get(program);
++      if (emitted) return emitted;
++      emitted = /* @__PURE__ */ new Map();
++      emittedInProgramOrder.set(program, emitted);
++      for (const sourceFile of program.getSourceFiles()) {
++        if (sourceFile.isDeclarationFile || program.isSourceFileFromExternalLibrary(sourceFile) || !TS_EXTENSIONS.test(sourceFile.fileName)) {
++          continue;
++        }
++        let declarationText;
++        const { emitSkipped, diagnostics } = program.emit(
++          sourceFile,
++          (_, text) => {
++            declarationText = text;
++          },
++          void 0,
++          true
++        );
++        emitted.set(sourceFile.fileName, { declarationText, emitSkipped, diagnostics });
++      }
++      return emitted;
++    }
+     var plugin = (options = {}) => {
+       const transformPlugin = transform();
+       const ctx = { programs: [], resolvedOptions: resolveDefaultOptions(options) };
+@@ -6413,15 +6450,21 @@ export { ${[...exportedNames].join(", ")} };
+             watchFiles(module2);
+             const declarationId = id.replace(TS_EXTENSIONS, dts);
+             let generated;
+-            const { emitSkipped, diagnostics } = module2.program.emit(
++            // objectstack patch: read this file's declaration off the
++            // program's one ordered emit (emitInProgramOrder) instead of
++            // emitting it here, in whatever order rollup loads modules.
++            const { declarationText, emitSkipped, diagnostics } = emitInProgramOrder(module2.program).get(module2.source.fileName) || module2.program.emit(
+               module2.source,
+-              (_, declarationText) => {
+-                generated = transformPlugin.transform.call(this, declarationText, declarationId);
++              (_, text) => {
++                generated = transformPlugin.transform.call(this, text, declarationId);
+               },
+               void 0,
+               // cancellationToken
+               true
+             );
++            if (declarationText !== void 0) {
++              generated = transformPlugin.transform.call(this, declarationText, declarationId);
++            }
+             if (emitSkipped) {
+               const errors = diagnostics.filter((diag) => diag.category === ts2.DiagnosticCategory.Error);
+               if (errors.length) {