diff --git a/src/javascript/sourceMaps/SourceMapAdmission.test.ts b/src/javascript/sourceMaps/SourceMapAdmission.test.ts
new file mode 100644
index 0000000000..f7ade1288b
--- /dev/null
+++ b/src/javascript/sourceMaps/SourceMapAdmission.test.ts
@@ -0,0 +1,51 @@
+import { expect, it } from "vitest";
+
+import { inspectSourceMapEnvelope } from "../../domain/sourceMapEnvelope.js";
+import { inspectSourceMapValue } from "./SourceMapFormat.js";
+import { traceSourceMap } from "./TraceSourceMap.js";
+
+const leaf = {
+  version: 3,
+  sources: ["original.ts"],
+  sourcesContent: ["hello"],
+  mappings: "AAAA",
+};
+const indexed = (map: unknown) => ({
+  version: 3,
+  sections: [{ offset: { line: 0, column: 0 }, map }],
+});
+
+it.each([2, null, undefined, "3"])(
+  "rejects nested indexed version %s consistently",
+  (version) => {
+    const map = indexed({ ...indexed(leaf), version });
+    expect(inspectSourceMapEnvelope(map)).toMatchObject({
+      status: "malformed",
+      invalid_sections: [{ section_path: [0] }],
+    });
+    for (const profile of [undefined, "browser-collection"] as const)
+      expect(() =>
+        inspectSourceMapValue(map, profile === undefined ? {} : { profile }),
+      ).toThrow(expect.objectContaining({ reason: "format" }));
+    expect(() =>
+      traceSourceMap(JSON.stringify(map), "https://example.test/main.js.map", {
+        line: 1,
+        column: 0,
+      }),
+    ).toThrow(expect.objectContaining({ reason: "format" }));
+  },
+);
+
+it("admits numeric version 3 through the same consumers", () => {
+  const map = indexed(indexed(leaf));
+  expect(inspectSourceMapEnvelope(map)).toMatchObject({ status: "included" });
+  expect(
+    inspectSourceMapValue(map, { profile: "browser-collection" }).leaves,
+  ).toHaveLength(1);
+  expect(
+    traceSourceMap(JSON.stringify(map), "https://example.test/main.js.map", {
+      line: 1,
+      column: 0,
+    }).matches,
+  ).toMatchObject([{ section_path: [0, 0], reported_source: "original.ts" }]);
+});