diff --git a/.changeset/curious-foxes-fly.md b/.changeset/curious-foxes-fly.md
new file mode 100644
index 000000000..e55a1098d
--- /dev/null
+++ b/.changeset/curious-foxes-fly.md
@@ -0,0 +1,7 @@
+---
+type: Removed
+pr: 4179
+---
+**Dropped the `test:mutation:since` npm script** — it passed `--since`, which Stryker 9.x does not accept (`error: unknown option '--since'`), so it could not run at all. Nothing invoked it: the mutation gate runs the per-module matrix from `scripts/mutation-matrix.cjs` instead, so no workflow regresses. To see which modules a change puts in scope, run `node scripts/mutation-matrix.cjs --base origin/next --print`. (#4106)
+
+<!-- docs-exempt: contributor tooling only — no docs/ page or CONTRIBUTING.md section referenced this script (grep for `test:mutation` across docs/ and CONTRIBUTING.md returns nothing), and it could never run, so there is no documented behaviour to update. -->
diff --git a/package.json b/package.json
index f29bb1242..6d0f5db0c 100644
--- a/package.json
+++ b/package.json
@@ -155,7 +155,6 @@
     "test:coverage:report": "c8 report --reporter text --reporter json-summary --merge-async --include 'gsd-core/bin/lib/**/*.cjs' --exclude 'tests/**' --all && node scripts/check-coverage-gate.cjs",
     "test:coverage:all": "npm run test:coverage",
     "test:mutation": "stryker run",
-    "test:mutation:since": "stryker run --incremental --since origin/next",
     "gen:features": "node scripts/gen-features.cjs",
     "gen:state-md-docs": "node scripts/gen-state-md-docs.cjs"
   },
diff --git a/tests/repo-invariants.test.cjs b/tests/repo-invariants.test.cjs
index 71ecd30e4..822b94163 100644
--- a/tests/repo-invariants.test.cjs
+++ b/tests/repo-invariants.test.cjs
@@ -431,3 +431,76 @@ test('--cwd pointing at a non-existent path fails uniformly across families', ()
 });
   });
 }
+
+
+// ────────────────────────────────────────────────────────────────────────
+// #4106 — every Stryker flag in package.json is one Stryker actually accepts
+// ────────────────────────────────────────────────────────────────────────
+{
+  const { describe: __d, test: __t } = require('node:test');
+  const __assert = require('node:assert/strict');
+  const __fs = require('node:fs');
+  const __path = require('node:path');
+
+  const __ROOT = __path.resolve(__dirname, '..');
+  const __PKG = require(__path.join(__ROOT, 'package.json'));
+  const __SCHEMA = __path.join(
+    __ROOT, 'node_modules', '@stryker-mutator', 'core', 'schema', 'stryker-schema.json',
+  );
+
+  // Stryker's CLI options are generated from its own options schema, so the schema's
+  // property names ARE the accepted long flags. Three commander-level flags have no
+  // schema entry because they are not run options.
+  const __CLI_ONLY = new Set(['files', 'help', 'version']);
+
+  /** Long flags a `stryker …` npm script passes, normalised (`--mutate=x` → `mutate`). */
+  function __strykerFlags(script) {
+    if (!/(^|\s)(npx\s+)?stryker(\s|$)/.test(script)) return null;
+    return script
+      .split(/\s+/)
+      .filter((tok) => tok.startsWith('--'))
+      .map((tok) => tok.slice(2).split('=')[0]);
+  }
+
+  __d('package.json Stryker scripts pass only flags Stryker accepts (#4106)', () => {
+    const accepted = new Set([
+      ...Object.keys(JSON.parse(__fs.readFileSync(__SCHEMA, 'utf-8')).properties || {}),
+      ...__CLI_ONLY,
+    ]);
+
+    // Guard: a moved/renamed schema would otherwise reject every flag for the wrong reason.
+    __assert.ok(
+      accepted.size > 20,
+      `no options schema at ${__SCHEMA} — @stryker-mutator/core may have moved it`,
+    );
+
+    __t('the checker rejects the flag this bug shipped', () => {
+      // `--since` is what `test:mutation:since` carried; Stryker 9.x exits with
+      // "error: unknown option '--since'". Pinning it here keeps this suite honest
+      // if every real invocation later drops its flags.
+      const flags = __strykerFlags('stryker run --incremental --since origin/next');
+      __assert.deepEqual(flags, ['incremental', 'since']);
+      __assert.equal(accepted.has('incremental'), true);
+      __assert.equal(accepted.has('since'), false, '--since is not a Stryker option');
+    });
+
+    __t('every stryker script in package.json is runnable', () => {
+      const offenders = [];
+      let scanned = 0;
+      for (const [name, script] of Object.entries(__PKG.scripts || {})) {
+        const flags = __strykerFlags(script);
+        if (flags === null) continue;
+        scanned += 1;
+        for (const flag of flags) {
+          if (!accepted.has(flag)) offenders.push(`${name}: --${flag}`);
+        }
+      }
+      __assert.ok(scanned > 0, 'no stryker script found — the sweep matched nothing');
+      __assert.deepEqual(
+        offenders,
+        [],
+        `package.json scripts pass flags Stryker does not accept: ${offenders.join(', ')}`,
+      );
+    });
+  });
+}