diff --git a/services/web/scripts/lezer-latex/random.mjs b/services/web/scripts/lezer-latex/random.mjs
new file mode 100644
index 0000000000..0c07468b59
--- /dev/null
+++ b/services/web/scripts/lezer-latex/random.mjs
@@ -0,0 +1,19 @@
+// Super quick and dirty LCG PRNG
+
+const m = 0xffffffff
+let X = Math.floor(Math.random() * (m - 1))
+const a = 16807
+const c = 0
+
+// Should probably be a large-ish number
+export function seed(i) {
+  if (i < 0) {
+    throw new Error('Seed must be a positive integer')
+  }
+  X = i & m
+}
+
+export function random() {
+  X = (a * X + c) % m
+  return X / m
+}
diff --git a/services/web/scripts/lezer-latex/test-incremental-parser.mjs b/services/web/scripts/lezer-latex/test-incremental-parser.mjs
index 11cef79682..a32894e16e 100644
--- a/services/web/scripts/lezer-latex/test-incremental-parser.mjs
+++ b/services/web/scripts/lezer-latex/test-incremental-parser.mjs
@@ -5,10 +5,16 @@ import * as path from 'path'
 import { fileURLToPath } from 'url'
 import { TreeFragment } from '@lezer/common'
 import minimist from 'minimist'
+import { seed, random } from './random.mjs'
 
 const argv = minimist(process.argv.slice(2))
 const NUMBER_OF_OPS = argv.ops || 1000
 const CSV_OUTPUT = argv.csv || false
+const SEED = argv.seed
+
+if (SEED) {
+  seed(SEED)
+}
 
 const __dirname = path.dirname(fileURLToPath(import.meta.url))
 
@@ -108,13 +114,13 @@ function writeTextAt(document, position, text) {
 
 function randomInsertions(document, num) {
   return timedChanges(document, num, currentDoc =>
-    insertAt(currentDoc, Math.floor(Math.random() * currentDoc.length), 'a')
+    insertAt(currentDoc, Math.floor(random() * currentDoc.length), 'a')
   )
 }
 
 function randomDeletions(document, num) {
   return timedChanges(document, num, currentDoc =>
-    deleteAt(currentDoc, Math.floor(Math.random() * currentDoc.length), 1)
+    deleteAt(currentDoc, Math.floor(random() * currentDoc.length), 1)
   )
 }