diff --git a/lib/index.d.ts b/lib/index.d.ts
index 64a127e..9357596 100644
--- a/lib/index.d.ts
+++ b/lib/index.d.ts
@@ -5,6 +5,8 @@
  *   Defaults to true if peggy$debugger is set on any start, otherwise false.
  * @prop {boolean} [noGenerate] Do not generate a file, only run tests on the
  *   original.
+ * @prop {boolean} [noOriginal] Do not run tests on the original code, only
+ *   on the generated code.
  */
 /**
  * Test the basic functionality of a Peggy grammar, to make coverage easier.
@@ -20,6 +22,8 @@ export function testPeggy<T>(grammarUrl: URL | string, starts: PeggyTestOptions<
 export type TestCounts = {
     valid: number;
     invalid: number;
+    grammarPath: string;
+    modifiedPath: string;
 };
 export type ExtraParserOptions = {
     /**
@@ -86,4 +90,9 @@ export type TestPeggyOptions = {
      * original.
      */
     noGenerate?: boolean | undefined;
+    /**
+     * Do not run tests on the original code, only
+     * on the generated code.
+     */
+    noOriginal?: boolean | undefined;
 };
diff --git a/lib/index.js b/lib/index.js
index 21bc397..57416d3 100644
--- a/lib/index.js
+++ b/lib/index.js
@@ -11,11 +11,14 @@ import fs from "node:fs/promises";
 import path from "node:path";
 
 const INVALID = "\uffff";
+let counter = 0;
 
 /**
  * @typedef {object} TestCounts
  * @prop {number} valid
  * @prop {number} invalid
+ * @prop {string} grammarPath
+ * @prop {string} modifiedPath
  */
 
 /**
@@ -176,6 +179,8 @@ function checkParserStarts(grammar, starts, modified, counts) {
  *   Defaults to true if peggy$debugger is set on any start, otherwise false.
  * @prop {boolean} [noGenerate] Do not generate a file, only run tests on the
  *   original.
+ * @prop {boolean} [noOriginal] Do not run tests on the original code, only
+ *   on the generated code.
  */
 
 /**
@@ -189,51 +194,67 @@ function checkParserStarts(grammar, starts, modified, counts) {
  * @returns {Promise<TestCounts>}
  */
 export async function testPeggy(grammarUrl, starts, opts) {
-  /** @type {TestCounts} */
-  const counts = {
-    valid: 0,
-    invalid: 0,
-  };
-
   if (!(typeof grammarUrl === "string") && !(grammarUrl instanceof URL)) {
     throw new TypeError("Invalid grammarUrl");
   }
+
   let grammarPath = String(grammarUrl);
   if (grammarPath.startsWith("file:")) {
     grammarPath = fileURLToPath(grammarPath);
   }
-  const grammar = /** @type {Parser} */ (
-    await import(grammarPath)
-  );
-  // @ts-ignore
-  ok(grammar);
-  ok(grammar.parse);
-  ok(typeof grammar.parse, "function");
-  ok(grammar.StartRules);
-  ok(Array.isArray(grammar.StartRules));
-  ok(grammar.StartRules.length > 0);
-  ok(grammar.SyntaxError);
-  equal(typeof grammar.SyntaxError, "function");
 
-  // @ts-expect-error null is not valid input
-  throws(() => grammar.parse(null));
-  throws(() => grammar.parse("", {
-    startRule: "__ INVALID __",
-  }));
+  // Can't use @peggyjs/from-mem since c8 can't see into those modules for
+  // coverage.  Maybe file a bug on c8? Given that, the modified file MUST be
+  // written to the same directory, with the same file name, so that `import`
+  // or `require` of relative paths or npm modules will work as expected.
+  const gp = path.parse(grammarPath);
+  // @ts-expect-error This TS error is bogus.
+  delete gp.base;
+  gp.name += `___TEST-${process.pid}-${counter++}`;
+  const modifiedPath = path.format(gp);
+
+  /** @type {TestCounts} */
+  const counts = {
+    valid: 0,
+    invalid: 0,
+    grammarPath,
+    modifiedPath,
+  };
+
+  if (!opts?.noOriginal) {
+    const grammar = /** @type {Parser} */ (
+      await import(grammarPath)
+    );
+    // @ts-ignore
+    ok(grammar);
+    ok(grammar.parse);
+    ok(typeof grammar.parse, "function");
+    ok(grammar.StartRules);
+    ok(Array.isArray(grammar.StartRules));
+    ok(grammar.StartRules.length > 0);
+    ok(grammar.SyntaxError);
+    equal(typeof grammar.SyntaxError, "function");
 
-  for (const startRule of grammar.StartRules) {
     // @ts-expect-error null is not valid input
-    throws(() => grammar.parse(null, {
-      startRule,
+    throws(() => grammar.parse(null));
+    throws(() => grammar.parse("", {
+      startRule: "__ INVALID __",
     }));
-  }
-  checkParserStarts(grammar, starts, false, counts);
 
-  const grammarJs = await fs.readFile(grammarPath, "utf8");
-  ok(grammarJs);
-  equal(typeof grammarJs, "string");
+    for (const startRule of grammar.StartRules) {
+      // @ts-expect-error null is not valid input
+      throws(() => grammar.parse(null, {
+        startRule,
+      }));
+    }
+    checkParserStarts(grammar, starts, false, counts);
+  }
 
   if (!opts?.noGenerate) {
+    const grammarJs = await fs.readFile(grammarPath, "utf8");
+    ok(grammarJs);
+    equal(typeof grammarJs, "string");
+
     // Approach: generate a new file next to the existing grammar file, with
     // test code injected just before the parser runs.  Source map information
     // embedded in the new file will make coverage show up on the original file.
@@ -242,11 +263,11 @@ export async function testPeggy(grammarUrl, starts, opts) {
     for (const line of grammarJs.split(/(?<=\n)/)) {
       if (/^\s*peg\$result = peg\$startRuleFunction\(\);/.test(line)) {
         src.add(`\
+  //#region Inserted by @peggyjs/coverage
   (() => {
     if (options.peg$debugger) {
       debugger;
     }
-    // Inserted by @peggyjs/coverage
     if (options.peg$startRuleFunction) {
       peg$startRuleFunction = eval(options.peg$startRuleFunction); // Ew.
     }
@@ -293,27 +314,18 @@ export async function testPeggy(grammarUrl, starts, opts) {
     err.format([{ source: 'source', text: 'a\\nb' }]);
 
     peg$buildStructuredError([
-      { type: 'other', description: 'one' },
-      { type: 'any' },
+      peg$otherExpectation('one'),
+      peg$anyExpectation(),
     ], "", loc);
 
     peg$buildStructuredError([
-        { type: 'other', description: 'one' },
-        { type: 'any' },
-        { type: 'literal', text: 'b', ignoreCase: false },
-        { type: 'literal', text: 'b', ignoreCase: true },
-        {
-          type: 'class',
-          parts: [ [ 'a', 'b' ], '\x7f' ],
-          inverted: true,
-          ignoreCase: false
-        },
-        {
-          type: 'class',
-          parts: [ 'a' ],
-          inverted: false,
-          ignoreCase: false
-        },
+        peg$literalExpectation('b', false),
+        peg$literalExpectation('b', true),
+        peg$classExpectation([ [ 'a', 'b' ], '\\x7f' ], true, false),
+        peg$classExpectation([ 'a' ], false, false),
+        peg$anyExpectation(),
+        peg$endExpectation(),
+        peg$otherExpectation('one'),
       ], "", loc);
     peg$padEnd("foo", 2);
     const oldMax = peg$maxFailPos;
@@ -321,6 +333,7 @@ export async function testPeggy(grammarUrl, starts, opts) {
     peg$fail();
     peg$maxFailPos = oldMax;
   })();
+  //#endregion
 
 `);
       }
@@ -333,23 +346,13 @@ export async function testPeggy(grammarUrl, starts, opts) {
     const sm = (opts && Object.prototype.hasOwnProperty.call(opts, "noMap"))
       ? !opts.noMap
       : starts.every(s => !s.options?.peg$debugger);
-    const start = "//# "; // c8: THIS file is not mapped.
+    const start = "//#"; // c8: THIS file is not mapped.
     if (sm) {
       code += `
-  ${start}sourceMappingURL=data:application/json;charset=utf-8;base64,${map}
-  `;
+${start} sourceMappingURL=data:application/json;charset=utf-8;base64,${map}
+`;
     }
 
-    // Can't use @peggyjs/from-mem since c8 can't see into those modules for
-    // coverage.  Maybe file a bug on c8? Given that, the modified file MUST be
-    // written to the same directory, with the same file name, so that `import`
-    // or `require` of relative paths or npm modules will work as expected.
-    const gp = path.parse(grammarPath);
-    // @ts-expect-error This TS error is bogus.
-    delete gp.base;
-    gp.name += `___TEST-${process.pid}`;
-    const modifiedPath = path.format(gp);
-
     await fs.writeFile(modifiedPath, code);
     try {
       const agrammar = /** @type {Parser} */ (
diff --git a/package.json b/package.json
index 1ad3016..9821be0 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
   "name": "@peggyjs/coverage",
-  "version": "1.1.0",
+  "version": "1.1.1",
   "decription": "Generate better code coverage for Peggy grammars",
   "main": "lib/index.js",
   "type": "module",
diff --git a/test/index.test.js b/test/index.test.js
index 2de8f2b..502e103 100644
--- a/test/index.test.js
+++ b/test/index.test.js
@@ -1,7 +1,14 @@
-import { deepEqual, rejects } from "node:assert";
+import { deepEqual, equal, rejects } from "node:assert";
 import test from "node:test";
 import { testPeggy } from "../lib/index.js";
 
+function cleanCounts(counts) {
+  equal(typeof counts.grammarPath, "string");
+  delete counts.grammarPath;
+  equal(typeof counts.modifiedPath, "string");
+  delete counts.modifiedPath;
+}
+
 test("test peggy coverage", async() => {
   const counts = await testPeggy(new URL("minimal.js", import.meta.url), [
     {
@@ -51,6 +58,8 @@ test("test peggy coverage", async() => {
       },
     },
   ]);
+
+  cleanCounts(counts);
   deepEqual(counts, {
     valid: 10,
     invalid: 8,
@@ -67,6 +76,7 @@ test("noGenerate", async() => {
     noGenerate: true,
   });
 
+  cleanCounts(counts);
   deepEqual(counts, {
     valid: 1,
     invalid: 1,
@@ -83,6 +93,7 @@ test("noMap", async() => {
     noMap: true,
   });
 
+  cleanCounts(counts);
   deepEqual(counts, {
     valid: 2,
     invalid: 2,