diff --git a/js/test/helpers.js b/js/test/helpers.js
index 702e11d..15128cc 100644
--- a/js/test/helpers.js
+++ b/js/test/helpers.js
@@ -1,9 +1,6 @@
 import { readFileSync } from 'node:fs'
 import { fileURLToPath } from 'node:url'
 import { dirname, join } from 'node:path'
-import { states, classicActions, render } from '../src/app.js'
-import { map as mapStream } from '../src/stream.js'
-import { map as mapIO, randomSeed } from '../src/io.js'
 
 const here = dirname(fileURLToPath(import.meta.url))
 
@@ -20,6 +17,3 @@ export const freeze = (x, seen = new WeakSet()) => {
   }
   return x
 }
-
-/** The m09 program built from classicActions (ticks every 150 ms). The shipped `main` switches to speedActions in m10. */
-export const classicMain = mapIO(randomSeed, (seed) => mapStream(states(seed, classicActions), render))
diff --git a/js/test/m09-app.test.js b/js/test/m09-app.test.js
index 15531f5..4717347 100644
--- a/js/test/m09-app.test.js
+++ b/js/test/m09-app.test.js
@@ -6,7 +6,8 @@ import { trace } from '../src/io.js'
 import { view } from '../src/view.js'
 import { newGame } from '../src/spawn.js'
 import { Action } from '../src/model.js'
-import { freeze, classicMain } from './helpers.js'
+import { freeze } from './helpers.js'
+import { classicMain } from './m09-helpers.js'
 
 const time = (t) => ({ tag: 'Time', t })
 const down = (key) => ({ tag: 'KeyDown', key })
diff --git a/js/test/m09-helpers.js b/js/test/m09-helpers.js
new file mode 100644
index 0000000..c60a290
--- /dev/null
+++ b/js/test/m09-helpers.js
@@ -0,0 +1,6 @@
+import { states, classicActions, render } from '../src/app.js'
+import { map as mapStream } from '../src/stream.js'
+import { map as mapIO, randomSeed } from '../src/io.js'
+
+/** The m09 program built from classicActions (ticks every 150 ms). The shipped `main` switches to speedActions in m10. */
+export const classicMain = mapIO(randomSeed, (seed) => mapStream(states(seed, classicActions), render))
diff --git a/js/test/m09-runtime.test.js b/js/test/m09-runtime.test.js
index 7de0395..3cce7fd 100644
--- a/js/test/m09-runtime.test.js
+++ b/js/test/m09-runtime.test.js
@@ -4,7 +4,7 @@ import { runIO, runApp } from '../src/runtime.js'
 import { pure, draw, randomSeed, map, sequence } from '../src/io.js'
 import { view } from '../src/view.js'
 import { newGame, tick } from '../src/spawn.js'
-import { classicMain } from './helpers.js'
+import { classicMain } from './m09-helpers.js'
 
 // A fake platform: records every call and lets the test fire events by hand.
 const fakePlatform = (seed = 42) => {
diff --git a/js/test/purity.test.js b/js/test/purity.test.js
index 8b78267..cde3355 100644
--- a/js/test/purity.test.js
+++ b/js/test/purity.test.js
@@ -73,3 +73,7 @@ jsFiles(srcDir)
   .forEach((f) =>
     test(`purity: ${f.slice(srcDir.length + 1)} has no banned constructs`, () =>
       assert.deepEqual(violations(readFileSync(f, 'utf8')), [])))
+
+// helpers.js is loaded by every test file; importing src/ there would make all tests fail at load time against the stubs.
+test('test/helpers.js imports nothing from src/', () =>
+  assert.doesNotMatch(readFileSync(join(dirname(fileURLToPath(import.meta.url)), 'helpers.js'), 'utf8'), /from\s+['"][^'"]*src\//))