diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index be1d3e4465..706b6c22f9 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -16,6 +16,10 @@ permissions:
   contents: read
 
 env:
+  # CI must never mutate the runner's OS trust store (2026-07-05: a cert-flow
+  # test installed a fake PEM on a persistent self-hosted runner and broke all
+  # system TLS). Belt-and-suspenders with tests/_setup/isolateDataDir.ts.
+  OMNIROUTE_SKIP_SYSTEM_TRUST: "1"
   CI_NODE_VERSION: "24"
   CI_NODE_24_VERSION: "24"
   CI_NODE_26_VERSION: "26"
diff --git a/.github/workflows/nightly-release-green.yml b/.github/workflows/nightly-release-green.yml
index c1d5128a74..646c741811 100644
--- a/.github/workflows/nightly-release-green.yml
+++ b/.github/workflows/nightly-release-green.yml
@@ -31,6 +31,9 @@ concurrency:
   group: nightly-release-green
   cancel-in-progress: true
 
+env:
+  OMNIROUTE_SKIP_SYSTEM_TRUST: "1"
+
 jobs:
   release-green:
     name: Validate active release branch
diff --git a/.github/workflows/quality.yml b/.github/workflows/quality.yml
index fda787cfb9..f5c0aa9967 100644
--- a/.github/workflows/quality.yml
+++ b/.github/workflows/quality.yml
@@ -14,6 +14,10 @@ permissions:
   contents: read
 
 env:
+  # CI must never mutate the runner's OS trust store (2026-07-05: a cert-flow
+  # test installed a fake PEM on a persistent self-hosted runner and broke all
+  # system TLS). Belt-and-suspenders with tests/_setup/isolateDataDir.ts.
+  OMNIROUTE_SKIP_SYSTEM_TRUST: "1"
   CI_NODE_VERSION: "24"
 
 jobs:
diff --git a/src/mitm/cert/install.ts b/src/mitm/cert/install.ts
index 74dcf62d45..f6871b0bd2 100644
--- a/src/mitm/cert/install.ts
+++ b/src/mitm/cert/install.ts
@@ -184,6 +184,11 @@ export async function installCert(sudoPassword: string, certPath: string): Promi
     return;
   }
 
+  if (process.env.OMNIROUTE_SKIP_SYSTEM_TRUST === "1") {
+    console.log("[cert] OMNIROUTE_SKIP_SYSTEM_TRUST=1 — skipping OS trust-store mutation");
+    return;
+  }
+
   if (IS_WIN) {
     await installCertWindows(certPath);
   } else if (IS_MAC) {
@@ -370,6 +375,11 @@ export async function uninstallCert(sudoPassword: string, certPath: string): Pro
     return;
   }
 
+  if (process.env.OMNIROUTE_SKIP_SYSTEM_TRUST === "1") {
+    console.log("[cert] OMNIROUTE_SKIP_SYSTEM_TRUST=1 — skipping OS trust-store mutation");
+    return;
+  }
+
   if (IS_WIN) {
     await uninstallCertWindows();
   } else if (IS_MAC) {
diff --git a/src/mitm/tproxy/caTrust.ts b/src/mitm/tproxy/caTrust.ts
index 7d1cea8655..be7323ef2c 100644
--- a/src/mitm/tproxy/caTrust.ts
+++ b/src/mitm/tproxy/caTrust.ts
@@ -78,6 +78,10 @@ export async function installTproxyCa(
   sudoPassword = "",
   deps: Partial<CaTrustDeps> = {}
 ): Promise<void> {
+  if (process.env.OMNIROUTE_SKIP_SYSTEM_TRUST === "1" && deps.run === undefined) {
+    console.log("[tproxy-ca] OMNIROUTE_SKIP_SYSTEM_TRUST=1 — skipping OS trust-store mutation");
+    return;
+  }
   const d = { ...realDeps, ...deps };
   if (d.platform() !== "linux") {
     throw new Error("TPROXY CA trust install is Linux-only.");
@@ -103,6 +107,10 @@ export async function uninstallTproxyCa(
   sudoPassword = "",
   deps: Partial<CaTrustDeps> = {}
 ): Promise<void> {
+  if (process.env.OMNIROUTE_SKIP_SYSTEM_TRUST === "1" && deps.run === undefined) {
+    console.log("[tproxy-ca] OMNIROUTE_SKIP_SYSTEM_TRUST=1 — skipping OS trust-store mutation");
+    return;
+  }
   const d = { ...realDeps, ...deps };
   if (d.platform() !== "linux") return;
   const cfg = d.certConfig();
diff --git a/tests/_setup/isolateDataDir.ts b/tests/_setup/isolateDataDir.ts
index f13db04863..e3d943748b 100644
--- a/tests/_setup/isolateDataDir.ts
+++ b/tests/_setup/isolateDataDir.ts
@@ -34,3 +34,10 @@ if (!process.env.DATA_DIR) {
     }
   });
 }
+
+// System-trust guard: the suite must NEVER mutate the OS trust store. On a
+// persistent self-hosted runner the cert-flow integration test installed a fake
+// 105-byte PEM into /usr/local/share/ca-certificates and update-ca-certificates
+// baked it into the bundle, breaking ALL system TLS on the VM (2026-07-05).
+// installCert/uninstallCert/installTproxyCa/uninstallTproxyCa no-op under this.
+process.env.OMNIROUTE_SKIP_SYSTEM_TRUST = "1";
diff --git a/tests/unit/system-trust-test-guard.test.ts b/tests/unit/system-trust-test-guard.test.ts
new file mode 100644
index 0000000000..969c3588f8
--- /dev/null
+++ b/tests/unit/system-trust-test-guard.test.ts
@@ -0,0 +1,42 @@
+// Guard: the test suite must NEVER touch the OS trust store. On 2026-07-05 the
+// integration test "POST /cert: installs trust when cert exists" ran the REAL
+// install path on a persistent self-hosted runner and wrote a 105-byte fake PEM
+// into /usr/local/share/ca-certificates — update-ca-certificates then baked the
+// invalid entry into ca-certificates.crt and broke ALL system TLS on the VM
+// (curl error 77, apt cert failures, corrupted artifact downloads). Hosted
+// runners are ephemeral, so the same write went unnoticed for months.
+//
+// OMNIROUTE_SKIP_SYSTEM_TRUST=1 (set globally in tests/_setup/isolateDataDir.ts)
+// makes installCert/uninstallCert no-ops before any filesystem/spawn work.
+import { test } from "node:test";
+import assert from "node:assert/strict";
+import fs from "node:fs";
+import os from "node:os";
+import path from "node:path";
+
+import { installCert } from "../../src/mitm/cert/install.ts";
+
+test("isolateDataDir setup exports the system-trust guard for every test process", () => {
+  assert.equal(process.env.OMNIROUTE_SKIP_SYSTEM_TRUST, "1");
+});
+
+test("installCert under the guard skips the OS mutation but keeps input contracts", async () => {
+  // Contract preserved: a missing cert file still throws (agent-bridge fallback
+  // #4546 depends on it to build the environment-skip result).
+  await assert.rejects(() => installCert("", "/nonexistent/omniroute-guard-test.pem"));
+
+  // With a REAL (fake-content) cert file, the un-guarded path would go on to
+  // sudo/update-ca-certificates — under the guard it must resolve without
+  // mutating the OS trust store (this exact write bricked the VM's TLS).
+  const dir = fs.mkdtempSync(path.join(os.tmpdir(), "omniroute-trust-guard-"));
+  const pem = path.join(dir, "omniroute-guard-test.pem");
+  fs.writeFileSync(
+    pem,
+    "-----BEGIN CERTIFICATE-----\nMIIBpDCCAQ2gAwIBAgIUFakeGuardCertXX==\n-----END CERTIFICATE-----\n"
+  );
+  try {
+    await installCert("", pem);
+  } finally {
+    fs.rmSync(dir, { recursive: true, force: true });
+  }
+});