diff --git a/extensions/ext-bundler-esbuild/src/es-module-lexer.ts b/extensions/ext-bundler-esbuild/src/es-module-lexer.ts
index b937826fcc..29cbe14db1 100644
--- a/extensions/ext-bundler-esbuild/src/es-module-lexer.ts
+++ b/extensions/ext-bundler-esbuild/src/es-module-lexer.ts
@@ -9,7 +9,10 @@
  */
 
 import type { ImportSpecifier, ModuleLexer } from "veryfront/extensions/bundler";
-import { init, parse } from "npm:es-module-lexer@2.3.1";
+// Bare specifier, mapped to the pinned npm package by this extension's
+// deno.json -- as `esbuild` is. Writing the `npm:` specifier inline instead
+// resolves only under Deno, which is what broke `deno task test:bun`.
+import { init, parse } from "es-module-lexer";
 
 /** es-module-lexer-backed {@link ModuleLexer} implementation. */
 export class EsModuleLexer implements ModuleLexer {
diff --git a/tests/bun/run-tests.mjs b/tests/bun/run-tests.mjs
index 84c447a568..f20b688e39 100644
--- a/tests/bun/run-tests.mjs
+++ b/tests/bun/run-tests.mjs
@@ -5,6 +5,7 @@ import os from "node:os";
 import { readFileSync } from "node:fs";
 import { filterTestFiles, listTestFiles, splitIntoShards } from "../test-file-utils.mjs";
 import { ensureNpmNodeModulesLinks } from "../ensure-npm-links.mjs";
+import { DENO_ONLY_TESTS } from "../deno-only-tests.mjs";
 
 function resolveConcurrency(envKeys) {
   for (const key of envKeys) {
@@ -45,6 +46,8 @@ const includePatterns = (process.env.BUN_TEST_INCLUDE || process.env.VF_TEST_INC
   .map((value) => value.trim())
   .filter(Boolean);
 const runtimeIncompatibleTests = [
+  // Files the `Deno.`-in-source heuristic below cannot see; see the shared list.
+  ...DENO_ONLY_TESTS,
   "src/config/env.test.ts",
   "src/proxy/handler.test.ts",
   "src/proxy/oauth-client.test.ts",
diff --git a/tests/deno-only-tests.mjs b/tests/deno-only-tests.mjs
new file mode 100644
index 0000000000..c3659238e4
--- /dev/null
+++ b/tests/deno-only-tests.mjs
@@ -0,0 +1,25 @@
+/**
+ * Test files that only make sense under `deno test`, shared by the Node and Bun
+ * runners.
+ *
+ * Both runners already drop a file whose own source mentions `Deno.`, which
+ * covers almost everything. It does not cover a file whose Deno usage lives in
+ * a helper it imports -- the heuristic reads one file, not the module graph --
+ * and it says nothing about files that are Deno-only by *subject* rather than
+ * by which API they happen to call.
+ *
+ * The pair below is both. It asserts a property of `deno test --parallel`
+ * itself: that test files sharing one process do not share a working directory.
+ * Node and Bun give each file its own process, so there is no property there to
+ * assert even if the APIs existed.
+ *
+ * Kept here rather than duplicated in each runner so the two cannot drift, and
+ * so it can be tested -- see ./runtime-test-filters.test.ts.
+ *
+ * @module tests/deno-only-tests
+ */
+
+/** Glob patterns for tests that must not run outside Deno. */
+export const DENO_ONLY_TESTS = [
+  "src/testing/cwd-exclusion-*.test.ts",
+];
diff --git a/tests/node/run-tests.mjs b/tests/node/run-tests.mjs
index 89f0d06a2e..c98c6ad66d 100644
--- a/tests/node/run-tests.mjs
+++ b/tests/node/run-tests.mjs
@@ -5,6 +5,7 @@ import os from "node:os";
 import { readFileSync } from "node:fs";
 import { filterTestFiles, listTestFiles, splitIntoShards } from "../test-file-utils.mjs";
 import { ensureNpmNodeModulesLinks } from "../ensure-npm-links.mjs";
+import { DENO_ONLY_TESTS } from "../deno-only-tests.mjs";
 
 function resolveConcurrency(envKeys) {
   for (const key of envKeys) {
@@ -43,8 +44,14 @@ const includePatterns = (process.env.NODE_TEST_INCLUDE || process.env.VF_TEST_IN
   .split(",")
   .map((value) => value.trim())
   .filter(Boolean);
-// Exclude Deno-specific test files that use Deno.test directly
-const denoOnlyTests = ["src/issues/**", "src/cache/backend.test.ts"];
+// Exclude Deno-specific test files that use Deno.test directly. Files the
+// `Deno.`-in-source heuristic below cannot see live in ./deno-only-tests.mjs,
+// shared with the Bun runner.
+const denoOnlyTests = [
+  "src/issues/**",
+  "src/cache/backend.test.ts",
+  ...DENO_ONLY_TESTS,
+];
 const runtimeIncompatibleTests = [
   "src/proxy/handler.test.ts",
   "src/proxy/oauth-client.test.ts",
diff --git a/tests/runtime-test-filters.test.ts b/tests/runtime-test-filters.test.ts
new file mode 100644
index 0000000000..f559354173
--- /dev/null
+++ b/tests/runtime-test-filters.test.ts
@@ -0,0 +1,61 @@
+/**
+ * The Node and Bun runners must keep the Deno-only tests out, and everything
+ * else in.
+ *
+ * Both halves matter. A filter that excludes too little lets
+ * `src/testing/cwd-exclusion-*.test.ts` run on a runtime without `Deno.chdir`,
+ * which is the regression this list was added for. A filter that excludes too
+ * much silently shrinks the suite, which nothing else would notice.
+ *
+ * The list is easy to break by accident: renaming those files, or moving them
+ * out of `src/testing/`, leaves a pattern matching nothing and the runner fails
+ * again the next time someone runs `deno task test:node`. Neither task runs in
+ * CI, so this is the only thing standing between that and a surprised human.
+ *
+ * @module tests/runtime-test-filters
+ */
+
+import { describe, it } from "#veryfront/testing/bdd.ts";
+import { assert, assertEquals } from "#veryfront/testing/assert.ts";
+import { DENO_ONLY_TESTS } from "./deno-only-tests.mjs";
+import { filterTestFiles } from "./test-file-utils.mjs";
+
+/** The files the shared list exists to exclude. */
+const DENO_ONLY_FILES = [
+  "src/testing/cwd-exclusion-a.test.ts",
+  "src/testing/cwd-exclusion-b.test.ts",
+];
+
+/** Ordinary tests, including neighbours of the excluded pair. */
+const ELIGIBLE_FILES = [
+  "src/testing/cwd.test.ts",
+  "src/testing/isolation.test.ts",
+  "src/errors/error-registry.test.ts",
+  "cli/router.test.ts",
+];
+
+describe("runtime test filters", () => {
+  it("excludes the Deno-only tests from non-Deno runners", () => {
+    const kept = filterTestFiles(DENO_ONLY_FILES, { exclude: DENO_ONLY_TESTS });
+
+    assertEquals(kept, [], "these cannot run without Deno.chdir");
+  });
+
+  it("keeps every other test eligible", () => {
+    const kept = filterTestFiles(ELIGIBLE_FILES, { exclude: DENO_ONLY_TESTS });
+
+    // `cwd.test.ts` sits beside the excluded pair and starts with the same
+    // three letters, so an over-broad pattern would take it too.
+    assertEquals(kept, ELIGIBLE_FILES, "the filter must not shrink the suite");
+  });
+
+  it("matches files that actually exist", async () => {
+    // A renamed or moved file leaves a pattern matching nothing, and the
+    // exclusion silently stops working. Cheaper to catch here than in a failing
+    // `test:node` run.
+    for (const path of DENO_ONLY_FILES) {
+      const stat = await Deno.stat(new URL(`../${path}`, import.meta.url));
+      assert(stat.isFile, `${path} is named in the exclusion list but is missing`);
+    }
+  });
+});