diff --git a/packages/@clc/dependencies/package.json b/packages/@clc/dependencies/package.json
index b6f4eda0..5d800a99 100644
--- a/packages/@clc/dependencies/package.json
+++ b/packages/@clc/dependencies/package.json
@@ -21,7 +21,7 @@
         "default": "./dist/esm/index.mjs"
       },
       "require": {
-        "types": "./dist/cjs-types/index.d.ts",
+        "types": "./dist/cjs-types/index.d.cts",
         "default": "./dist/cjs/index.cjs"
       }
     },
diff --git a/packages/@clc/nx/executors/package-json/executor.mjs b/packages/@clc/nx/executors/package-json/executor.mjs
index 015f8e9b..90dcb5c3 100644
--- a/packages/@clc/nx/executors/package-json/executor.mjs
+++ b/packages/@clc/nx/executors/package-json/executor.mjs
@@ -82,10 +82,9 @@ async function config(pkg, mjs, mts, type, context) {
         default: mjs ? './src/index.mjs' : './dist/esm/index.mjs',
       },
       require: {
-        types:
-          mjs || mts
-            ? './dist/cjs-types/index.d.ts'
-            : './dist/types/index.d.ts',
+        // A `.d.ts` under `"type": "module"` is read as ESM, so the CommonJS
+        // condition needs the `.d.cts` that `scripts/dts-to-dcts.mjs` emits.
+        types: './dist/cjs-types/index.d.cts',
         default: './dist/cjs/index.cjs',
       },
       /* eslint-enable sort-keys */
diff --git a/packages/@clc/nx/src/create-nodes.mjs b/packages/@clc/nx/src/create-nodes.mjs
index 90e90028..31699fd0 100644
--- a/packages/@clc/nx/src/create-nodes.mjs
+++ b/packages/@clc/nx/src/create-nodes.mjs
@@ -182,16 +182,13 @@ export const createNodes = [
           '{workspaceRoot}/tsconfig.base.json',
           '{workspaceRoot}/tsconfig.references.json',
           '{workspaceRoot}/tsconfig.json',
+          '{workspaceRoot}/scripts/dts-to-dcts.mjs',
           '{projectRoot}/tsconfig.json',
           '{projectRoot}/package.json',
-          '{projectRoot}/src/**/*.[jt]s?(x)',
+          '{projectRoot}/src/**/*.?(m|c)[jt]s?(x)',
         ],
         options: {
-          command: mjs
-            ? `tsc --project {projectRoot}/tsconfig.json && scripts/dmts-to-dts {projectRoot}`
-            : mts
-              ? `tsc --project {projectRoot}/tsconfig.json && scripts/dmts-to-dts {projectRoot}`
-              : `tsc --project {projectRoot}/tsconfig.json`,
+          command: `tsc --project {projectRoot}/tsconfig.json && node scripts/dts-to-dcts.mjs {projectRoot}`,
         },
         outputs: [
           '{projectRoot}/dist/.tsconfig.tsbuildinfo',
diff --git a/packages/@clc/test-helpers/package.json b/packages/@clc/test-helpers/package.json
index 9872a4eb..33c1e891 100644
--- a/packages/@clc/test-helpers/package.json
+++ b/packages/@clc/test-helpers/package.json
@@ -18,7 +18,7 @@
         "default": "./dist/esm/index.mjs"
       },
       "require": {
-        "types": "./dist/cjs-types/index.d.ts",
+        "types": "./dist/cjs-types/index.d.cts",
         "default": "./dist/cjs/index.cjs"
       }
     },
diff --git a/packages/@code-like-a-carpenter/assert/package.json b/packages/@code-like-a-carpenter/assert/package.json
index d54c6e2f..144d953c 100644
--- a/packages/@code-like-a-carpenter/assert/package.json
+++ b/packages/@code-like-a-carpenter/assert/package.json
@@ -13,7 +13,7 @@
         "default": "./src/index.mjs"
       },
       "require": {
-        "types": "./dist/cjs-types/index.d.ts",
+        "types": "./dist/cjs-types/index.d.cts",
         "default": "./dist/cjs/index.cjs"
       }
     },
diff --git a/packages/@code-like-a-carpenter/aws-env-loader/package.json b/packages/@code-like-a-carpenter/aws-env-loader/package.json
index 4b035dc6..a9a670c6 100644
--- a/packages/@code-like-a-carpenter/aws-env-loader/package.json
+++ b/packages/@code-like-a-carpenter/aws-env-loader/package.json
@@ -8,7 +8,7 @@
         "default": "./dist/esm/index.mjs"
       },
       "require": {
-        "types": "./dist/types/index.d.ts",
+        "types": "./dist/cjs-types/index.d.cts",
         "default": "./dist/cjs/index.cjs"
       }
     },
diff --git a/packages/@code-like-a-carpenter/cli-core/package.json b/packages/@code-like-a-carpenter/cli-core/package.json
index cf8d51b3..b294a4d5 100644
--- a/packages/@code-like-a-carpenter/cli-core/package.json
+++ b/packages/@code-like-a-carpenter/cli-core/package.json
@@ -18,7 +18,7 @@
         "default": "./src/index.mjs"
       },
       "require": {
-        "types": "./dist/cjs-types/index.d.ts",
+        "types": "./dist/cjs-types/index.d.cts",
         "default": "./dist/cjs/index.cjs"
       }
     },
diff --git a/packages/@code-like-a-carpenter/cli-plugin-example/package.json b/packages/@code-like-a-carpenter/cli-plugin-example/package.json
index 7bc3fc7a..f7a4e90a 100644
--- a/packages/@code-like-a-carpenter/cli-plugin-example/package.json
+++ b/packages/@code-like-a-carpenter/cli-plugin-example/package.json
@@ -17,7 +17,7 @@
         "default": "./dist/esm/index.mjs"
       },
       "require": {
-        "types": "./dist/types/index.d.ts",
+        "types": "./dist/cjs-types/index.d.cts",
         "default": "./dist/cjs/index.cjs"
       }
     },
diff --git a/packages/@code-like-a-carpenter/cli/package.json b/packages/@code-like-a-carpenter/cli/package.json
index fb4ee133..a8843dac 100644
--- a/packages/@code-like-a-carpenter/cli/package.json
+++ b/packages/@code-like-a-carpenter/cli/package.json
@@ -42,7 +42,7 @@
         "default": "./dist/esm/index.mjs"
       },
       "require": {
-        "types": "./dist/types/index.d.ts",
+        "types": "./dist/cjs-types/index.d.cts",
         "default": "./dist/cjs/index.cjs"
       }
     },
diff --git a/packages/@code-like-a-carpenter/contract-tests/package.json b/packages/@code-like-a-carpenter/contract-tests/package.json
index b75d142e..ed0c15ec 100644
--- a/packages/@code-like-a-carpenter/contract-tests/package.json
+++ b/packages/@code-like-a-carpenter/contract-tests/package.json
@@ -19,7 +19,7 @@
         "default": "./dist/esm/index.mjs"
       },
       "require": {
-        "types": "./dist/types/index.d.ts",
+        "types": "./dist/cjs-types/index.d.cts",
         "default": "./dist/cjs/index.cjs"
       }
     },
diff --git a/packages/@code-like-a-carpenter/dotenv/package.json b/packages/@code-like-a-carpenter/dotenv/package.json
index f82ea354..f9e810aa 100644
--- a/packages/@code-like-a-carpenter/dotenv/package.json
+++ b/packages/@code-like-a-carpenter/dotenv/package.json
@@ -13,7 +13,7 @@
         "default": "./dist/esm/index.mjs"
       },
       "require": {
-        "types": "./dist/types/index.d.ts",
+        "types": "./dist/cjs-types/index.d.cts",
         "default": "./dist/cjs/index.cjs"
       }
     },
diff --git a/packages/@code-like-a-carpenter/env/package.json b/packages/@code-like-a-carpenter/env/package.json
index 6418b7ae..d17c80e8 100644
--- a/packages/@code-like-a-carpenter/env/package.json
+++ b/packages/@code-like-a-carpenter/env/package.json
@@ -8,7 +8,7 @@
         "default": "./src/index.mjs"
       },
       "require": {
-        "types": "./dist/cjs-types/index.d.ts",
+        "types": "./dist/cjs-types/index.d.cts",
         "default": "./dist/cjs/index.cjs"
       }
     },
diff --git a/packages/@code-like-a-carpenter/errors/package.json b/packages/@code-like-a-carpenter/errors/package.json
index 685bd290..7fc0c418 100644
--- a/packages/@code-like-a-carpenter/errors/package.json
+++ b/packages/@code-like-a-carpenter/errors/package.json
@@ -13,7 +13,7 @@
         "default": "./dist/esm/index.mjs"
       },
       "require": {
-        "types": "./dist/types/index.d.ts",
+        "types": "./dist/cjs-types/index.d.cts",
         "default": "./dist/cjs/index.cjs"
       }
     },
diff --git a/packages/@code-like-a-carpenter/exception/package.json b/packages/@code-like-a-carpenter/exception/package.json
index 2cff852c..c3bdcdc4 100644
--- a/packages/@code-like-a-carpenter/exception/package.json
+++ b/packages/@code-like-a-carpenter/exception/package.json
@@ -8,7 +8,7 @@
         "default": "./dist/esm/index.mjs"
       },
       "require": {
-        "types": "./dist/types/index.d.ts",
+        "types": "./dist/cjs-types/index.d.cts",
         "default": "./dist/cjs/index.cjs"
       }
     },
diff --git a/packages/@code-like-a-carpenter/foundation-intermediate-representation/package.json b/packages/@code-like-a-carpenter/foundation-intermediate-representation/package.json
index 73d7c334..1714a5b5 100644
--- a/packages/@code-like-a-carpenter/foundation-intermediate-representation/package.json
+++ b/packages/@code-like-a-carpenter/foundation-intermediate-representation/package.json
@@ -13,7 +13,7 @@
         "default": "./dist/esm/index.mjs"
       },
       "require": {
-        "types": "./dist/types/index.d.ts",
+        "types": "./dist/cjs-types/index.d.cts",
         "default": "./dist/cjs/index.cjs"
       }
     },
diff --git a/packages/@code-like-a-carpenter/foundation-parser/package.json b/packages/@code-like-a-carpenter/foundation-parser/package.json
index 6b22c2d6..6b52c3c2 100644
--- a/packages/@code-like-a-carpenter/foundation-parser/package.json
+++ b/packages/@code-like-a-carpenter/foundation-parser/package.json
@@ -13,7 +13,7 @@
         "default": "./dist/esm/index.mjs"
       },
       "require": {
-        "types": "./dist/types/index.d.ts",
+        "types": "./dist/cjs-types/index.d.cts",
         "default": "./dist/cjs/index.cjs"
       }
     },
diff --git a/packages/@code-like-a-carpenter/foundation-plugin-cloudformation/package.json b/packages/@code-like-a-carpenter/foundation-plugin-cloudformation/package.json
index 7e7ace0d..d1c71af1 100644
--- a/packages/@code-like-a-carpenter/foundation-plugin-cloudformation/package.json
+++ b/packages/@code-like-a-carpenter/foundation-plugin-cloudformation/package.json
@@ -13,7 +13,7 @@
         "default": "./dist/esm/index.mjs"
       },
       "require": {
-        "types": "./dist/types/index.d.ts",
+        "types": "./dist/cjs-types/index.d.cts",
         "default": "./dist/cjs/index.cjs"
       }
     },
diff --git a/packages/@code-like-a-carpenter/foundation-plugin-typescript/package.json b/packages/@code-like-a-carpenter/foundation-plugin-typescript/package.json
index 305d3022..92b59a92 100644
--- a/packages/@code-like-a-carpenter/foundation-plugin-typescript/package.json
+++ b/packages/@code-like-a-carpenter/foundation-plugin-typescript/package.json
@@ -13,7 +13,7 @@
         "default": "./dist/esm/index.mjs"
       },
       "require": {
-        "types": "./dist/types/index.d.ts",
+        "types": "./dist/cjs-types/index.d.cts",
         "default": "./dist/cjs/index.cjs"
       }
     },
diff --git a/packages/@code-like-a-carpenter/foundation-runtime/package.json b/packages/@code-like-a-carpenter/foundation-runtime/package.json
index c06f75da..81e34c64 100644
--- a/packages/@code-like-a-carpenter/foundation-runtime/package.json
+++ b/packages/@code-like-a-carpenter/foundation-runtime/package.json
@@ -13,7 +13,7 @@
         "default": "./dist/esm/index.mjs"
       },
       "require": {
-        "types": "./dist/types/index.d.ts",
+        "types": "./dist/cjs-types/index.d.cts",
         "default": "./dist/cjs/index.cjs"
       }
     },
diff --git a/packages/@code-like-a-carpenter/foundation-transform-environment/package.json b/packages/@code-like-a-carpenter/foundation-transform-environment/package.json
index d31fb2cc..44e51687 100644
--- a/packages/@code-like-a-carpenter/foundation-transform-environment/package.json
+++ b/packages/@code-like-a-carpenter/foundation-transform-environment/package.json
@@ -18,7 +18,7 @@
         "default": "./dist/esm/index.mjs"
       },
       "require": {
-        "types": "./dist/types/index.d.ts",
+        "types": "./dist/cjs-types/index.d.cts",
         "default": "./dist/cjs/index.cjs"
       }
     },
diff --git a/packages/@code-like-a-carpenter/graphql-codegen-helpers/package.json b/packages/@code-like-a-carpenter/graphql-codegen-helpers/package.json
index 996535ab..a10651d9 100644
--- a/packages/@code-like-a-carpenter/graphql-codegen-helpers/package.json
+++ b/packages/@code-like-a-carpenter/graphql-codegen-helpers/package.json
@@ -17,7 +17,7 @@
         "default": "./dist/esm/index.mjs"
       },
       "require": {
-        "types": "./dist/types/index.d.ts",
+        "types": "./dist/cjs-types/index.d.cts",
         "default": "./dist/cjs/index.cjs"
       }
     },
diff --git a/packages/@code-like-a-carpenter/interact/package.json b/packages/@code-like-a-carpenter/interact/package.json
index a49eda02..c51fb51b 100644
--- a/packages/@code-like-a-carpenter/interact/package.json
+++ b/packages/@code-like-a-carpenter/interact/package.json
@@ -13,7 +13,7 @@
         "default": "./dist/esm/index.mjs"
       },
       "require": {
-        "types": "./dist/types/index.d.ts",
+        "types": "./dist/cjs-types/index.d.cts",
         "default": "./dist/cjs/index.cjs"
       }
     },
diff --git a/packages/@code-like-a-carpenter/lambda-handlers/package.json b/packages/@code-like-a-carpenter/lambda-handlers/package.json
index 0fd21ca5..559a37b4 100644
--- a/packages/@code-like-a-carpenter/lambda-handlers/package.json
+++ b/packages/@code-like-a-carpenter/lambda-handlers/package.json
@@ -23,7 +23,7 @@
         "default": "./dist/esm/index.mjs"
       },
       "require": {
-        "types": "./dist/types/index.d.ts",
+        "types": "./dist/cjs-types/index.d.cts",
         "default": "./dist/cjs/index.cjs"
       }
     },
diff --git a/packages/@code-like-a-carpenter/logger/package.json b/packages/@code-like-a-carpenter/logger/package.json
index c27cf07a..bb7b4f84 100644
--- a/packages/@code-like-a-carpenter/logger/package.json
+++ b/packages/@code-like-a-carpenter/logger/package.json
@@ -13,7 +13,7 @@
         "default": "./dist/esm/index.mjs"
       },
       "require": {
-        "types": "./dist/types/index.d.ts",
+        "types": "./dist/cjs-types/index.d.cts",
         "default": "./dist/cjs/index.cjs"
       }
     },
diff --git a/packages/@code-like-a-carpenter/parallel/package.json b/packages/@code-like-a-carpenter/parallel/package.json
index f485a185..b56534cb 100644
--- a/packages/@code-like-a-carpenter/parallel/package.json
+++ b/packages/@code-like-a-carpenter/parallel/package.json
@@ -13,7 +13,7 @@
         "default": "./dist/esm/index.mjs"
       },
       "require": {
-        "types": "./dist/types/index.d.ts",
+        "types": "./dist/cjs-types/index.d.cts",
         "default": "./dist/cjs/index.cjs"
       }
     },
diff --git a/packages/@code-like-a-carpenter/sentry/package.json b/packages/@code-like-a-carpenter/sentry/package.json
index 6ffea8ab..ef216672 100644
--- a/packages/@code-like-a-carpenter/sentry/package.json
+++ b/packages/@code-like-a-carpenter/sentry/package.json
@@ -13,7 +13,7 @@
         "default": "./dist/esm/index.mjs"
       },
       "require": {
-        "types": "./dist/types/index.d.ts",
+        "types": "./dist/cjs-types/index.d.cts",
         "default": "./dist/cjs/index.cjs"
       }
     },
diff --git a/packages/@code-like-a-carpenter/telemetry/package.json b/packages/@code-like-a-carpenter/telemetry/package.json
index d27baa68..0a31371a 100644
--- a/packages/@code-like-a-carpenter/telemetry/package.json
+++ b/packages/@code-like-a-carpenter/telemetry/package.json
@@ -13,7 +13,7 @@
         "default": "./dist/esm/index.mjs"
       },
       "require": {
-        "types": "./dist/types/index.d.ts",
+        "types": "./dist/cjs-types/index.d.cts",
         "default": "./dist/cjs/index.cjs"
       }
     },
diff --git a/packages/@code-like-a-carpenter/tool-deps/package.json b/packages/@code-like-a-carpenter/tool-deps/package.json
index dca6f612..2978e8c1 100644
--- a/packages/@code-like-a-carpenter/tool-deps/package.json
+++ b/packages/@code-like-a-carpenter/tool-deps/package.json
@@ -21,7 +21,7 @@
         "default": "./src/index.mjs"
       },
       "require": {
-        "types": "./dist/cjs-types/index.d.ts",
+        "types": "./dist/cjs-types/index.d.cts",
         "default": "./dist/cjs/index.cjs"
       }
     },
diff --git a/packages/@code-like-a-carpenter/tool-foundation/package.json b/packages/@code-like-a-carpenter/tool-foundation/package.json
index 799b4ec6..74db66be 100644
--- a/packages/@code-like-a-carpenter/tool-foundation/package.json
+++ b/packages/@code-like-a-carpenter/tool-foundation/package.json
@@ -14,7 +14,7 @@
         "default": "./dist/esm/index.mjs"
       },
       "require": {
-        "types": "./dist/types/index.d.ts",
+        "types": "./dist/cjs-types/index.d.cts",
         "default": "./dist/cjs/index.cjs"
       }
     },
diff --git a/packages/@code-like-a-carpenter/tool-inliner/package.json b/packages/@code-like-a-carpenter/tool-inliner/package.json
index 4b6c1884..fb1465b9 100644
--- a/packages/@code-like-a-carpenter/tool-inliner/package.json
+++ b/packages/@code-like-a-carpenter/tool-inliner/package.json
@@ -13,7 +13,7 @@
         "default": "./src/index.mjs"
       },
       "require": {
-        "types": "./dist/cjs-types/index.d.ts",
+        "types": "./dist/cjs-types/index.d.cts",
         "default": "./dist/cjs/index.cjs"
       }
     },
diff --git a/packages/@code-like-a-carpenter/tool-json-schema/package.json b/packages/@code-like-a-carpenter/tool-json-schema/package.json
index 74afad13..f95febd0 100644
--- a/packages/@code-like-a-carpenter/tool-json-schema/package.json
+++ b/packages/@code-like-a-carpenter/tool-json-schema/package.json
@@ -13,7 +13,7 @@
         "default": "./src/index.mjs"
       },
       "require": {
-        "types": "./dist/cjs-types/index.d.ts",
+        "types": "./dist/cjs-types/index.d.cts",
         "default": "./dist/cjs/index.cjs"
       }
     },
diff --git a/packages/@code-like-a-carpenter/tool-stack/package.json b/packages/@code-like-a-carpenter/tool-stack/package.json
index f41737f9..2a7a7817 100644
--- a/packages/@code-like-a-carpenter/tool-stack/package.json
+++ b/packages/@code-like-a-carpenter/tool-stack/package.json
@@ -13,7 +13,7 @@
         "default": "./src/index.mjs"
       },
       "require": {
-        "types": "./dist/cjs-types/index.d.ts",
+        "types": "./dist/cjs-types/index.d.cts",
         "default": "./dist/cjs/index.cjs"
       }
     },
diff --git a/packages/@code-like-a-carpenter/tool-tool/package.json b/packages/@code-like-a-carpenter/tool-tool/package.json
index 0822f0a7..1f52be44 100644
--- a/packages/@code-like-a-carpenter/tool-tool/package.json
+++ b/packages/@code-like-a-carpenter/tool-tool/package.json
@@ -25,7 +25,7 @@
         "default": "./src/index.mjs"
       },
       "require": {
-        "types": "./dist/cjs-types/index.d.ts",
+        "types": "./dist/cjs-types/index.d.cts",
         "default": "./dist/cjs/index.cjs"
       }
     },
diff --git a/packages/@code-like-a-carpenter/tooling-common/package.json b/packages/@code-like-a-carpenter/tooling-common/package.json
index eda60293..61187908 100644
--- a/packages/@code-like-a-carpenter/tooling-common/package.json
+++ b/packages/@code-like-a-carpenter/tooling-common/package.json
@@ -13,7 +13,7 @@
         "default": "./src/index.mjs"
       },
       "require": {
-        "types": "./dist/cjs-types/index.d.ts",
+        "types": "./dist/cjs-types/index.d.cts",
         "default": "./dist/cjs/index.cjs"
       }
     },
diff --git a/packages/@code-like-a-carpenter/wait-for/package.json b/packages/@code-like-a-carpenter/wait-for/package.json
index 089fbac9..f8d24fe8 100644
--- a/packages/@code-like-a-carpenter/wait-for/package.json
+++ b/packages/@code-like-a-carpenter/wait-for/package.json
@@ -13,7 +13,7 @@
         "default": "./dist/esm/index.mjs"
       },
       "require": {
-        "types": "./dist/types/index.d.ts",
+        "types": "./dist/cjs-types/index.d.cts",
         "default": "./dist/cjs/index.cjs"
       }
     },
diff --git a/packages/@code-like-a-carpenter/workbench-config/package.json b/packages/@code-like-a-carpenter/workbench-config/package.json
index 74af2488..51f88905 100644
--- a/packages/@code-like-a-carpenter/workbench-config/package.json
+++ b/packages/@code-like-a-carpenter/workbench-config/package.json
@@ -13,7 +13,7 @@
         "default": "./src/index.mjs"
       },
       "require": {
-        "types": "./dist/cjs-types/index.d.ts",
+        "types": "./dist/cjs-types/index.d.cts",
         "default": "./dist/cjs/index.cjs"
       }
     },
diff --git a/scripts/dmts-to-dts b/scripts/dmts-to-dts
deleted file mode 100755
index f1984026..00000000
--- a/scripts/dmts-to-dts
+++ /dev/null
@@ -1,12 +0,0 @@
-#!/usr/bin/env bash
-
-set -euo pipefail
-
-PROJECT_ROOT=$1
-EXTENSION=${2:-}
-
-for f in $(find $PROJECT_ROOT/dist/types -type f); do
-  outfile=$(echo $f | sed -e 's#/types/#/cjs-types/#' | sed -e 's#.d.mts#.d.ts#')
-  mkdir -p "$(dirname "$outfile")"
-  sed -e "s#.mjs#${EXTENSION}#g" "$f" | sed -e "s#.mts#${EXTENSION}#g" > "$outfile"
-done
diff --git a/scripts/dts-to-dcts.mjs b/scripts/dts-to-dcts.mjs
new file mode 100755
index 00000000..f039c6e8
--- /dev/null
+++ b/scripts/dts-to-dcts.mjs
@@ -0,0 +1,258 @@
+#!/usr/bin/env node
+
+/**
+ * Produces `dist/cjs-types` from `dist/types`.
+ *
+ * Every package is `"type": "module"`, so a `.d.ts` under it is read as ESM no
+ * matter what `exports` says. The CommonJS half of `exports` therefore needs
+ * declarations whose extension forces CJS: `.d.cts`, whose relative specifiers
+ * resolve to sibling `.d.cts` files (a `.cjs` specifier resolves to `.d.cts`
+ * under node16).
+ */
+
+import {cp, mkdir, readdir, readFile, rm, writeFile} from 'node:fs/promises';
+import path from 'node:path';
+import process from 'node:process';
+
+const SPECIFIER_EXTENSIONS = [
+  '.mts',
+  '.mjs',
+  '.cts',
+  '.cjs',
+  '.tsx',
+  '.jsx',
+  '.ts',
+  '.js',
+];
+
+const DECLARATION_SUFFIXES = [
+  '.d.mts.map',
+  '.d.cts.map',
+  '.d.ts.map',
+  '.d.mts',
+  '.d.cts',
+  '.d.ts',
+];
+
+/**
+ * Extensions TypeScript resolves without help from this script: JSON keeps
+ * resolving as JSON, and the file itself is copied through unchanged.
+ */
+const PASSTHROUGH_EXTENSIONS = ['.json'];
+
+/**
+ * Treats every quoted string starting with `./` or `../` as a module
+ * specifier, which covers `from`, bare `import`, `import()` types, `require()`,
+ * and `declare module` alike. Any other quoted relative path gets the same
+ * treatment — a string literal type, a template literal type, or a doc comment
+ * — and an extensionless one throws. No declaration in this repo has one, and
+ * the alternative is parsing TypeScript here.
+ */
+const RELATIVE_SPECIFIER = /(['"])(\.\.?\/[^'"\n]*)\1/g;
+
+/**
+ * Reference paths need their own pass because, unlike a module specifier, one
+ * may name a sibling without a leading `./`.
+ */
+const REFERENCE_PATH = /(<reference\s+path\s*=\s*(['"]))([^'"\n]*)(\2)/g;
+
+const SOURCE_MAPPING_URL = /(\/\/#\s*sourceMappingURL=)(\S+)/g;
+
+/**
+ * @param {string} name
+ * @returns {string | undefined}
+ */
+function declarationSuffix(name) {
+  return DECLARATION_SUFFIXES.find((suffix) => name.endsWith(suffix));
+}
+
+/**
+ * A specifier that already names a declaration file points at the sibling
+ * `.d.cts` rather than at a `.cjs` implementation.
+ *
+ * An extensionless or directory specifier has no rewrite that works: node16
+ * CommonJS resolution finds neither `./x.d.cts` through `./x` nor
+ * `./dir/index.d.cts` through `./dir`, so the declaration would resolve to
+ * nothing. Fail rather than emit output that type-checks as an unresolved
+ * module.
+ *
+ * @param {string} specifier
+ * @param {string} source the declaration being converted, for the error
+ * @returns {string}
+ */
+function toCjsSpecifier(specifier, source) {
+  if (declarationSuffix(specifier)) {
+    return toCjsFilename(specifier);
+  }
+  for (const extension of SPECIFIER_EXTENSIONS) {
+    if (specifier.endsWith(extension)) {
+      return `${specifier.slice(0, -extension.length)}.cjs`;
+    }
+  }
+  if (
+    PASSTHROUGH_EXTENSIONS.some((extension) => specifier.endsWith(extension))
+  ) {
+    return specifier;
+  }
+  throw new Error(
+    `${source}: relative specifier "${specifier}" has no extension to rewrite; ` +
+      `CommonJS declarations cannot resolve it. Give the import an explicit extension.`
+  );
+}
+
+/**
+ * @param {string} filename
+ * @returns {string}
+ */
+function toCjsFilename(filename) {
+  const suffix = declarationSuffix(filename);
+  if (suffix) {
+    const replacement = suffix.endsWith('.map') ? '.d.cts.map' : '.d.cts';
+    return `${filename.slice(0, -suffix.length)}${replacement}`;
+  }
+  return filename;
+}
+
+/**
+ * @param {string} filename
+ * @returns {boolean}
+ */
+function isDeclaration(filename) {
+  return (
+    filename.endsWith('.d.ts') ||
+    filename.endsWith('.d.mts') ||
+    filename.endsWith('.d.cts')
+  );
+}
+
+/**
+ * @param {string} filename
+ * @returns {boolean}
+ */
+function isDeclarationMap(filename) {
+  return (
+    filename.endsWith('.d.ts.map') ||
+    filename.endsWith('.d.mts.map') ||
+    filename.endsWith('.d.cts.map')
+  );
+}
+
+/**
+ * @param {string} contents
+ * @param {string} source the declaration being converted, for error messages
+ * @returns {string}
+ */
+function convertDeclaration(contents, source) {
+  return contents
+    .replace(
+      REFERENCE_PATH,
+      (_match, prefix, _quote, target, suffix) =>
+        `${prefix}${toCjsFilename(target)}${suffix}`
+    )
+    .replace(
+      RELATIVE_SPECIFIER,
+      (_match, quote, specifier) =>
+        `${quote}${toCjsSpecifier(specifier, source)}${quote}`
+    )
+    .replace(
+      SOURCE_MAPPING_URL,
+      (_match, prefix, url) => `${prefix}${toCjsFilename(url)}`
+    );
+}
+
+/**
+ * `sources` stay correct because `dist/cjs-types` sits at the same depth as
+ * `dist/types`; only the name of the declaration the map describes changes.
+ *
+ * `mappings` are left alone. Rewriting `.ts` to `.cjs` lengthens a specifier by
+ * one character, so columns after it on that line are off by one; that is
+ * cheaper to live with than re-emitting the map.
+ *
+ * @param {string} contents
+ * @returns {string}
+ */
+function convertDeclarationMap(contents) {
+  const map = JSON.parse(contents);
+  if (typeof map.file === 'string') {
+    map.file = toCjsFilename(map.file);
+  }
+  return `${JSON.stringify(map)}\n`;
+}
+
+/**
+ * @param {string} dir
+ * @returns {Promise<string[]>} paths relative to `dir`
+ */
+async function walk(dir) {
+  const entries = await readdir(dir, {recursive: true, withFileTypes: true});
+  return entries
+    .filter((entry) => entry.isFile())
+    .map((entry) =>
+      path.relative(dir, path.join(entry.parentPath, entry.name))
+    );
+}
+
+/**
+ * @param {string} projectRoot
+ */
+async function main(projectRoot) {
+  const typesDir = path.join(projectRoot, 'dist', 'types');
+  const cjsTypesDir = path.join(projectRoot, 'dist', 'cjs-types');
+
+  const relativePaths = await walk(typesDir);
+  if (relativePaths.length === 0) {
+    throw new Error(
+      `${typesDir} is empty; every package's require condition points at dist/cjs-types/index.d.cts`
+    );
+  }
+
+  // Anything left in `dist/cjs-types` from a previous run would otherwise
+  // survive a rename or deletion in `dist/types`.
+  await rm(cjsTypesDir, {force: true, recursive: true});
+
+  /** @type {Map<string, string>} */
+  const claimed = new Map();
+
+  for (const relativePath of relativePaths) {
+    const from = path.join(typesDir, relativePath);
+    const to = path.join(
+      cjsTypesDir,
+      path.join(
+        path.dirname(relativePath),
+        toCjsFilename(path.basename(relativePath))
+      )
+    );
+
+    const previous = claimed.get(to);
+    if (previous) {
+      throw new Error(
+        `${previous} and ${relativePath} both convert to ${path.relative(projectRoot, to)}`
+      );
+    }
+    claimed.set(to, relativePath);
+
+    await mkdir(path.dirname(to), {recursive: true});
+
+    const filename = path.basename(relativePath);
+    if (isDeclaration(filename)) {
+      await writeFile(
+        to,
+        convertDeclaration(await readFile(from, 'utf8'), relativePath)
+      );
+    } else if (isDeclarationMap(filename)) {
+      await writeFile(to, convertDeclarationMap(await readFile(from, 'utf8')));
+    } else {
+      await cp(from, to);
+    }
+  }
+}
+
+const [projectRoot] = process.argv.slice(2);
+if (!projectRoot) {
+  process.stderr.write(
+    `usage: ${path.basename(process.argv[1])} <projectRoot>\n`
+  );
+  process.exit(1);
+}
+
+await main(projectRoot);