diff --git a/tests/_setup/isolateDataDir.ts b/tests/_setup/isolateDataDir.ts
index b1add14b89..37cb154822 100644
--- a/tests/_setup/isolateDataDir.ts
+++ b/tests/_setup/isolateDataDir.ts
@@ -47,6 +47,14 @@ if (!process.env.DATA_DIR) {
 // installCert/uninstallCert/installTproxyCa/uninstallTproxyCa no-op under this.
 process.env.OMNIROUTE_SKIP_SYSTEM_TRUST = "1";
 
+// Browser-spawn guard: the Adobe Firefly session warm (adobeFireflySession.ts)
+// spawns the SYSTEM Chrome with --remote-debugging-port whenever a test reaches it
+// without a valid user JWT — which any mocked-fetch test does by construction.
+// Per-call-site allowBrowserRefresh/tryBrowser flags are not enough: the warm is also
+// reachable indirectly via client/handler paths, so the guard must be global.
+// ||= (not =) so a browser-path integration test can still opt back in.
+process.env.ADOBE_FIREFLY_BROWSER_REFRESH ||= "0";
+
 // DNS-write guard: the suite must NEVER mutate /etc/hosts. Tests that exercise
 // the real MITM path call addDNSEntries(); this env var makes it a no-op.
 process.env.OMNIROUTE_SKIP_DNS_WRITE = "1";
diff --git a/tests/unit/adobe-firefly-test-browser-guard.test.ts b/tests/unit/adobe-firefly-test-browser-guard.test.ts
new file mode 100644
index 0000000000..8f41f0d745
--- /dev/null
+++ b/tests/unit/adobe-firefly-test-browser-guard.test.ts
@@ -0,0 +1,56 @@
+/**
+ * Regression guard: the unit suite must never launch a real browser.
+ *
+ * The Adobe Firefly session warm (adobeFireflySession.ts::shouldWarm) spawns the SYSTEM
+ * browser with --remote-debugging-port whenever a test reaches it without a valid user
+ * JWT — which any mocked-fetch test does by construction. Per-call-site
+ * `allowBrowserRefresh: false` / `tryBrowser: false` is NOT enough: the warm is also
+ * reachable indirectly via client/handler paths, so the guard must be global.
+ */
+import { test } from "node:test";
+import assert from "node:assert";
+import { spawnSync } from "node:child_process";
+import { adobeFireflyBrowserEnabled } from "../../open-sse/services/adobeFireflySession.ts";
+
+test("test setup disables Adobe Firefly browser warm", () => {
+  assert.equal(
+    process.env.ADOBE_FIREFLY_BROWSER_REFRESH,
+    "0",
+    "tests/_setup/isolateDataDir.ts must set ADOBE_FIREFLY_BROWSER_REFRESH=0 so the suite " +
+      "never launches the system Chrome at firefly.adobe.com"
+  );
+  assert.equal(
+    adobeFireflyBrowserEnabled(),
+    false,
+    "adobeFireflyBrowserEnabled() must be false under the test setup"
+  );
+});
+
+/**
+ * Behavioral check of the ||= semantics, in a fresh process (this test's own env was
+ * already written by the setup, so the guard can't be re-observed in-process):
+ *   - with the var unset, the setup fills in "0" (browser disabled by default);
+ *   - with the var preset to "1", the setup leaves it alone — a browser-path
+ *     integration test can still opt back in.
+ */
+function runSetupWithEnv(value: string | undefined): string {
+  const env = { ...process.env } as Record<string, string | undefined>;
+  if (value === undefined) delete env.ADOBE_FIREFLY_BROWSER_REFRESH;
+  else env.ADOBE_FIREFLY_BROWSER_REFRESH = value;
+  const res = spawnSync(
+    process.execPath,
+    [
+      "--import", "tsx/esm",
+      "--import", "./tests/_setup/isolateDataDir.ts",
+      "-e", "process.stdout.write(String(process.env.ADOBE_FIREFLY_BROWSER_REFRESH))",
+    ],
+    { env, encoding: "utf8" }
+  );
+  assert.equal(res.status, 0, `setup subprocess failed: ${res.stderr}`);
+  return res.stdout;
+}
+
+test("setup disables the browser by default but preserves an explicit opt-in", () => {
+  assert.equal(runSetupWithEnv(undefined), "0", "unset must become 0 (browser off)");
+  assert.equal(runSetupWithEnv("1"), "1", "preset 1 must survive (integration tests can opt in)");
+});