diff --git a/changelog.d/fixes/11872-tunnel-mitm-error-sanitization.md b/changelog.d/fixes/11872-tunnel-mitm-error-sanitization.md
new file mode 100644
index 0000000000..f402b9c8a3
--- /dev/null
+++ b/changelog.d/fixes/11872-tunnel-mitm-error-sanitization.md
@@ -0,0 +1 @@
+- **fix(api):** Stop the tunnel and MITM routes from returning raw child-process error text, which disclosed host paths, binary install locations and Tailscale `tskey-*` credentials in a body some of these routes serve to non-loopback callers, and return a real 400 instead of a framework 500 when a tunnel request body fails validation.
diff --git a/src/app/api/settings/mitm/route.ts b/src/app/api/settings/mitm/route.ts
index bbd5622455..72c8c7d1e5 100644
--- a/src/app/api/settings/mitm/route.ts
+++ b/src/app/api/settings/mitm/route.ts
@@ -5,6 +5,7 @@ import path from "path";
 import { z } from "zod";
 import { NextResponse } from "next/server";
 import { requireManagementAuth } from "@/lib/api/requireManagementAuth";
+import { toPublicSafeTunnelError } from "@/lib/api/publicSafeTunnelError";
 import { resolveApiKey } from "@/shared/services/apiKeyResolver";
 import { resolveMitmDataDir } from "@/mitm/dataDir";
 import { KIRO_MITM_PROFILE } from "@/mitm/targets/kiro";
@@ -180,8 +181,10 @@ export async function GET(request: Request) {
 
     return NextResponse.json(await buildMitmResponse());
   } catch (error) {
-    const message = error instanceof Error ? error.message : "Failed to load MITM settings";
-    return NextResponse.json({ error: message }, { status: 500 });
+    return NextResponse.json(
+      toPublicSafeTunnelError(error, "Failed to load the MITM settings.", "settings/mitm GET"),
+      { status: 500 }
+    );
   }
 }
 
@@ -236,8 +239,10 @@ export async function PUT(request: Request) {
 
     return NextResponse.json(await buildMitmResponse());
   } catch (error) {
-    const message = error instanceof Error ? error.message : "Failed to update MITM settings";
-    return NextResponse.json({ error: message }, { status: 500 });
+    return NextResponse.json(
+      toPublicSafeTunnelError(error, "Failed to update the MITM settings.", "settings/mitm PUT"),
+      { status: 500 }
+    );
   }
 }
 
@@ -274,8 +279,13 @@ export async function POST(request: Request) {
 
     return NextResponse.json(await buildMitmResponse());
   } catch (error) {
-    const message =
-      error instanceof Error ? error.message : "Failed to regenerate MITM certificate";
-    return NextResponse.json({ error: message }, { status: 500 });
+    return NextResponse.json(
+      toPublicSafeTunnelError(
+        error,
+        "Failed to regenerate the MITM certificate.",
+        "settings/mitm POST"
+      ),
+      { status: 500 }
+    );
   }
 }
diff --git a/src/app/api/tunnels/cloudflared/route.ts b/src/app/api/tunnels/cloudflared/route.ts
index 14f78fbdf5..b0aa08abbc 100644
--- a/src/app/api/tunnels/cloudflared/route.ts
+++ b/src/app/api/tunnels/cloudflared/route.ts
@@ -1,12 +1,17 @@
 import { NextRequest, NextResponse } from "next/server";
 import { z } from "zod";
 import { isAuthenticated } from "@/shared/utils/apiAuth";
-import { isValidationFailure, validateBody } from "@/shared/validation/helpers";
+import {
+  formatValidationMessage,
+  isValidationFailure,
+  validateBody,
+} from "@/shared/validation/helpers";
 import {
   getCloudflaredTunnelStatus,
   startCloudflaredTunnel,
   stopCloudflaredTunnel,
 } from "@/lib/cloudflaredTunnel";
+import { toPublicSafeTunnelError } from "@/lib/api/publicSafeTunnelError";
 
 export const dynamic = "force-dynamic";
 
@@ -28,9 +33,11 @@ export async function GET(request: NextRequest) {
     return NextResponse.json(status);
   } catch (error) {
     return NextResponse.json(
-      {
-        error: error instanceof Error ? error.message : "Failed to load cloudflared tunnel status",
-      },
+      toPublicSafeTunnelError(
+        error,
+        "Failed to load the cloudflared tunnel status.",
+        "tunnels/cloudflared GET"
+      ),
       { status: 500 }
     );
   }
@@ -50,7 +57,10 @@ export async function POST(request: NextRequest) {
 
   const validation = validateBody(actionSchema, rawBody);
   if (isValidationFailure(validation)) {
-    return validation.response;
+    // validateBody() returns { success, error } — it has no `response` field, so
+    // the previous `return validation.response` returned undefined and Next
+    // answered with a framework 500 instead of this 400.
+    return NextResponse.json({ error: formatValidationMessage(validation.error) }, { status: 400 });
   }
 
   const parsed = validation.data;
@@ -66,9 +76,11 @@ export async function POST(request: NextRequest) {
     });
   } catch (error) {
     return NextResponse.json(
-      {
-        error: error instanceof Error ? error.message : "Failed to update cloudflared tunnel",
-      },
+      toPublicSafeTunnelError(
+        error,
+        "Failed to update the cloudflared tunnel.",
+        "tunnels/cloudflared POST"
+      ),
       { status: 500 }
     );
   }
diff --git a/src/app/api/tunnels/ngrok/route.ts b/src/app/api/tunnels/ngrok/route.ts
index 5fad841674..1ad8824d35 100644
--- a/src/app/api/tunnels/ngrok/route.ts
+++ b/src/app/api/tunnels/ngrok/route.ts
@@ -1,8 +1,13 @@
 import { NextRequest, NextResponse } from "next/server";
 import { z } from "zod";
 import { isAuthenticated } from "@/shared/utils/apiAuth";
-import { isValidationFailure, validateBody } from "@/shared/validation/helpers";
+import {
+  formatValidationMessage,
+  isValidationFailure,
+  validateBody,
+} from "@/shared/validation/helpers";
 import { getNgrokTunnelStatus, startNgrokTunnel, stopNgrokTunnel } from "@/lib/ngrokTunnel";
+import { toPublicSafeTunnelError } from "@/lib/api/publicSafeTunnelError";
 
 export const dynamic = "force-dynamic";
 
@@ -25,9 +30,11 @@ export async function GET(request: NextRequest) {
     return NextResponse.json(status);
   } catch (error) {
     return NextResponse.json(
-      {
-        error: error instanceof Error ? error.message : "Failed to load ngrok tunnel status",
-      },
+      toPublicSafeTunnelError(
+        error,
+        "Failed to load the ngrok tunnel status.",
+        "tunnels/ngrok GET"
+      ),
       { status: 500 }
     );
   }
@@ -47,7 +54,10 @@ export async function POST(request: NextRequest) {
 
   const validation = validateBody(actionSchema, rawBody);
   if (isValidationFailure(validation)) {
-    return validation.response;
+    // validateBody() returns { success, error } — it has no `response` field, so
+    // the previous `return validation.response` returned undefined and Next
+    // answered with a framework 500 instead of this 400.
+    return NextResponse.json({ error: formatValidationMessage(validation.error) }, { status: 400 });
   }
 
   const parsed = validation.data;
@@ -65,9 +75,7 @@ export async function POST(request: NextRequest) {
     });
   } catch (error) {
     return NextResponse.json(
-      {
-        error: error instanceof Error ? error.message : "Failed to update ngrok tunnel",
-      },
+      toPublicSafeTunnelError(error, "Failed to update the ngrok tunnel.", "tunnels/ngrok POST"),
       { status: 500 }
     );
   }
diff --git a/src/app/api/tunnels/tailscale/check/route.ts b/src/app/api/tunnels/tailscale/check/route.ts
index 864c396be4..acdf4def34 100644
--- a/src/app/api/tunnels/tailscale/check/route.ts
+++ b/src/app/api/tunnels/tailscale/check/route.ts
@@ -1,5 +1,6 @@
 import { NextResponse } from "next/server";
 import { getTailscaleCheckStatus } from "@/lib/tailscaleTunnel";
+import { toPublicSafeTunnelError } from "@/lib/api/publicSafeTunnelError";
 import { requireTailscaleAuth } from "../routeUtils";
 
 export const dynamic = "force-dynamic";
@@ -13,9 +14,11 @@ export async function GET(request: Request) {
     return NextResponse.json(status);
   } catch (error) {
     return NextResponse.json(
-      {
-        error: error instanceof Error ? error.message : "Failed to check Tailscale state",
-      },
+      toPublicSafeTunnelError(
+        error,
+        "Failed to check the Tailscale state.",
+        "tunnels/tailscale/check GET"
+      ),
       { status: 500 }
     );
   }
diff --git a/src/app/api/tunnels/tailscale/disable/route.ts b/src/app/api/tunnels/tailscale/disable/route.ts
index 858e337037..a5f4cbd303 100644
--- a/src/app/api/tunnels/tailscale/disable/route.ts
+++ b/src/app/api/tunnels/tailscale/disable/route.ts
@@ -1,5 +1,6 @@
 import { NextResponse } from "next/server";
 import { disableTailscaleTunnel } from "@/lib/tailscaleTunnel";
+import { toPublicSafeTunnelError } from "@/lib/api/publicSafeTunnelError";
 import { parseOptionalJsonBody, requireTailscaleAuth, tailscaleSudoSchema } from "../routeUtils";
 
 export const dynamic = "force-dynamic";
@@ -16,9 +17,11 @@ export async function POST(request: Request) {
     return NextResponse.json(result);
   } catch (error) {
     return NextResponse.json(
-      {
-        error: error instanceof Error ? error.message : "Failed to disable Tailscale Funnel",
-      },
+      toPublicSafeTunnelError(
+        error,
+        "Failed to disable the Tailscale Funnel.",
+        "tunnels/tailscale/disable POST"
+      ),
       { status: 500 }
     );
   }
diff --git a/src/app/api/tunnels/tailscale/enable/route.ts b/src/app/api/tunnels/tailscale/enable/route.ts
index 7918814de2..9d117fd357 100644
--- a/src/app/api/tunnels/tailscale/enable/route.ts
+++ b/src/app/api/tunnels/tailscale/enable/route.ts
@@ -1,5 +1,6 @@
 import { NextResponse } from "next/server";
 import { enableTailscaleTunnel } from "@/lib/tailscaleTunnel";
+import { toPublicSafeTunnelError } from "@/lib/api/publicSafeTunnelError";
 import { parseOptionalJsonBody, requireTailscaleAuth, tailscaleEnableSchema } from "../routeUtils";
 
 export const dynamic = "force-dynamic";
@@ -16,9 +17,11 @@ export async function POST(request: Request) {
     return NextResponse.json(result);
   } catch (error) {
     return NextResponse.json(
-      {
-        error: error instanceof Error ? error.message : "Failed to enable Tailscale Funnel",
-      },
+      toPublicSafeTunnelError(
+        error,
+        "Failed to enable the Tailscale Funnel.",
+        "tunnels/tailscale/enable POST"
+      ),
       { status: 500 }
     );
   }
diff --git a/src/app/api/tunnels/tailscale/install/route.ts b/src/app/api/tunnels/tailscale/install/route.ts
index b5f747eeb2..1cecac4b26 100644
--- a/src/app/api/tunnels/tailscale/install/route.ts
+++ b/src/app/api/tunnels/tailscale/install/route.ts
@@ -1,5 +1,6 @@
 import { NextResponse } from "next/server";
 import { getTailscaleTunnelStatus, installTailscale } from "@/lib/tailscaleTunnel";
+import { toPublicSafeTunnelError } from "@/lib/api/publicSafeTunnelError";
 import { parseOptionalJsonBody, requireTailscaleAuth, tailscaleSudoSchema } from "../routeUtils";
 
 export const dynamic = "force-dynamic";
@@ -31,9 +32,14 @@ export async function POST(request: Request) {
             status: await getTailscaleTunnelStatus(),
           });
         } catch (error) {
-          pushEvent("error", {
-            error: error instanceof Error ? error.message : "Failed to install Tailscale",
-          });
+          pushEvent(
+            "error",
+            toPublicSafeTunnelError(
+              error,
+              "Failed to install Tailscale.",
+              "tunnels/tailscale/install POST"
+            )
+          );
         } finally {
           controller.close();
         }
diff --git a/src/app/api/tunnels/tailscale/login/route.ts b/src/app/api/tunnels/tailscale/login/route.ts
index 2606430723..09d9e1ec26 100644
--- a/src/app/api/tunnels/tailscale/login/route.ts
+++ b/src/app/api/tunnels/tailscale/login/route.ts
@@ -1,5 +1,6 @@
 import { NextResponse } from "next/server";
 import { startTailscaleLogin } from "@/lib/tailscaleTunnel";
+import { toPublicSafeTunnelError } from "@/lib/api/publicSafeTunnelError";
 import { parseOptionalJsonBody, requireTailscaleAuth, tailscaleLoginSchema } from "../routeUtils";
 
 export const dynamic = "force-dynamic";
@@ -16,9 +17,11 @@ export async function POST(request: Request) {
     return NextResponse.json(result);
   } catch (error) {
     return NextResponse.json(
-      {
-        error: error instanceof Error ? error.message : "Failed to start Tailscale login",
-      },
+      toPublicSafeTunnelError(
+        error,
+        "Failed to start the Tailscale login.",
+        "tunnels/tailscale/login POST"
+      ),
       { status: 500 }
     );
   }
diff --git a/src/app/api/tunnels/tailscale/route.ts b/src/app/api/tunnels/tailscale/route.ts
index 2adf374ae8..dcba14d660 100644
--- a/src/app/api/tunnels/tailscale/route.ts
+++ b/src/app/api/tunnels/tailscale/route.ts
@@ -1,5 +1,6 @@
 import { NextResponse } from "next/server";
 import { getTailscaleTunnelStatus } from "@/lib/tailscaleTunnel";
+import { toPublicSafeTunnelError } from "@/lib/api/publicSafeTunnelError";
 import { requireTailscaleAuth } from "./routeUtils";
 
 export const dynamic = "force-dynamic";
@@ -13,9 +14,11 @@ export async function GET(request: Request) {
     return NextResponse.json(status);
   } catch (error) {
     return NextResponse.json(
-      {
-        error: error instanceof Error ? error.message : "Failed to load Tailscale status",
-      },
+      toPublicSafeTunnelError(
+        error,
+        "Failed to load the Tailscale status.",
+        "tunnels/tailscale GET"
+      ),
       { status: 500 }
     );
   }
diff --git a/src/app/api/tunnels/tailscale/routeUtils.ts b/src/app/api/tunnels/tailscale/routeUtils.ts
index b6df5118e5..426280a235 100644
--- a/src/app/api/tunnels/tailscale/routeUtils.ts
+++ b/src/app/api/tunnels/tailscale/routeUtils.ts
@@ -1,7 +1,11 @@
 import { NextResponse } from "next/server";
 import { z } from "zod";
 import { isAuthenticated } from "@/shared/utils/apiAuth";
-import { isValidationFailure, validateBody } from "@/shared/validation/helpers";
+import {
+  formatValidationMessage,
+  isValidationFailure,
+  validateBody,
+} from "@/shared/validation/helpers";
 
 export const tailscaleEnableSchema = z.object({
   sudoPassword: z.string().optional(),
@@ -34,7 +38,15 @@ export async function parseOptionalJsonBody<T extends z.ZodTypeAny>(request: Req
 
   const validation = validateBody(schema, rawBody);
   if (isValidationFailure(validation)) {
-    return { response: validation.response };
+    // validateBody() returns { success, error } — it has no `response` field, so
+    // the previous `{ response: validation.response }` handed every caller an
+    // `undefined` response and Next answered with a framework 500 instead of a 400.
+    return {
+      response: NextResponse.json(
+        { error: formatValidationMessage(validation.error) },
+        { status: 400 }
+      ),
+    };
   }
 
   return { data: validation.data };
diff --git a/src/app/api/tunnels/tailscale/start-daemon/route.ts b/src/app/api/tunnels/tailscale/start-daemon/route.ts
index b3b1208834..7d65bda68f 100644
--- a/src/app/api/tunnels/tailscale/start-daemon/route.ts
+++ b/src/app/api/tunnels/tailscale/start-daemon/route.ts
@@ -1,5 +1,6 @@
 import { NextResponse } from "next/server";
 import { getTailscaleTunnelStatus, startTailscaleDaemon } from "@/lib/tailscaleTunnel";
+import { toPublicSafeTunnelError } from "@/lib/api/publicSafeTunnelError";
 import { parseOptionalJsonBody, requireTailscaleAuth, tailscaleSudoSchema } from "../routeUtils";
 
 export const dynamic = "force-dynamic";
@@ -19,9 +20,11 @@ export async function POST(request: Request) {
     });
   } catch (error) {
     return NextResponse.json(
-      {
-        error: error instanceof Error ? error.message : "Failed to start the Tailscale daemon",
-      },
+      toPublicSafeTunnelError(
+        error,
+        "Failed to start the Tailscale daemon.",
+        "tunnels/tailscale/start-daemon POST"
+      ),
       { status: 500 }
     );
   }
diff --git a/src/lib/api/publicSafeTunnelError.ts b/src/lib/api/publicSafeTunnelError.ts
new file mode 100644
index 0000000000..b2752f85aa
--- /dev/null
+++ b/src/lib/api/publicSafeTunnelError.ts
@@ -0,0 +1,96 @@
+/**
+ * Public-safe error bodies for the tunnel and MITM management routes.
+ *
+ * Hard Rule #12 forbids returning a raw `err.message` in an HTTP body, and
+ * `sanitizeErrorMessage()` is the repo's general answer. It is not enough here:
+ * it only rewrites tokens that look like an absolute path ending in a *source*
+ * extension (`ts|tsx|js|jsx|mjs|cjs` — see `SOURCE_EXT` in
+ * open-sse/utils/error.ts), so the three leak shapes these routes actually
+ * produce all survive it verbatim:
+ *
+ *   - config/state paths:  `ENOENT ... open '/home/<user>/.omniroute/data/tunnels.json'`
+ *   - binary paths:        `spawn /usr/local/bin/cloudflared ENOENT`
+ *   - Tailscale auth keys: `invalid key tskey-auth-kMn3Qz7RtY-9fVbXsPq2LdWc`
+ *
+ * These come from child processes (`cloudflared`, `tailscale`, `tailscaled`,
+ * `ngrok`) and from filesystem I/O on the operator's home directory, so the raw
+ * message discloses the host layout, the install location, the OS account name
+ * and — for Tailscale — a live credential.
+ *
+ * Reachability is not uniform. `/api/tunnels/ngrok` (both methods),
+ * `/api/tunnels/tailscale` and `/api/tunnels/tailscale/check` are NOT in
+ * `LOCAL_ONLY_API_PREFIXES`, and `GET /api/tunnels/cloudflared` is explicitly
+ * exempted through `LOCAL_ONLY_API_GET_EXEMPTIONS` (#11531), so those bodies can
+ * reach a non-loopback caller. The remaining tunnel routes and
+ * `/api/settings/mitm` are loopback-gated; they are covered here for one
+ * consistent contract, not because they are remotely reachable.
+ *
+ * The contract deliberately does NOT try to scrub the upstream text. It returns
+ * a fixed operator-facing sentence plus a coarse machine-readable `reason`, and
+ * logs the real error server-side where the operator can still read it.
+ */
+
+/** Coarse classification a client can branch on without seeing host details. */
+export type PublicSafeTunnelErrorReason =
+  "not_installed" | "permission_denied" | "already_running" | "timeout" | "network" | "unknown";
+
+export interface PublicSafeTunnelErrorBody {
+  error: string;
+  reason: PublicSafeTunnelErrorReason;
+}
+
+/** Classify without echoing: only the reason label ever reaches the client. */
+export function classifyTunnelError(error: unknown): PublicSafeTunnelErrorReason {
+  const raw = (error instanceof Error ? error.message : String(error ?? "")).toLowerCase();
+  if (!raw) return "unknown";
+  if (raw.includes("enoent") || raw.includes("not found") || raw.includes("not installed")) {
+    return "not_installed";
+  }
+  if (
+    raw.includes("eacces") ||
+    raw.includes("eperm") ||
+    raw.includes("permission denied") ||
+    raw.includes("sudo") ||
+    raw.includes("must be run as root")
+  ) {
+    return "permission_denied";
+  }
+  if (
+    raw.includes("eaddrinuse") ||
+    raw.includes("already running") ||
+    raw.includes("already in use")
+  ) {
+    return "already_running";
+  }
+  if (raw.includes("etimedout") || raw.includes("timed out") || raw.includes("timeout")) {
+    return "timeout";
+  }
+  if (
+    raw.includes("econnrefused") ||
+    raw.includes("econnreset") ||
+    raw.includes("enotfound") ||
+    raw.includes("network")
+  ) {
+    return "network";
+  }
+  return "unknown";
+}
+
+/**
+ * Build the 500 body for a tunnel/MITM route.
+ *
+ * @param error    The caught value. Never echoed.
+ * @param fallback Operator-facing sentence describing what failed. Must be a
+ *                 literal owned by the route — never derived from `error`.
+ * @param context  Short route label used only for the server-side log line.
+ */
+export function toPublicSafeTunnelError(
+  error: unknown,
+  fallback: string,
+  context: string
+): PublicSafeTunnelErrorBody {
+  // The operator still needs the real cause; it belongs in the server log, not
+  // in a body that may cross a tunnel.
+  console.error(`[${context}]`, error);
+  return { error: fallback, reason: classifyTunnelError(error) };
+}
diff --git a/tests/unit/tunnel-routes-error-sanitization.test.ts b/tests/unit/tunnel-routes-error-sanitization.test.ts
new file mode 100644
index 0000000000..2e5036e9ea
--- /dev/null
+++ b/tests/unit/tunnel-routes-error-sanitization.test.ts
@@ -0,0 +1,268 @@
+/**
+ * OSS-051 — the tunnel and MITM management routes returned raw `error.message`.
+ *
+ * Two independent defects, both in `src/app/api/tunnels/**` and
+ * `src/app/api/settings/mitm/route.ts`:
+ *
+ * 1. Fourteen catch blocks answered with
+ *    `{ error: error instanceof Error ? error.message : "<fallback>" }`. Those
+ *    errors come from child processes (cloudflared / tailscale / tailscaled /
+ *    ngrok) and from filesystem I/O under the operator's home directory, so the
+ *    body disclosed host layout, binary install paths, the OS account name and —
+ *    for Tailscale — live `tskey-*` credentials. Hard Rule #12 forbids this.
+ *
+ *    `sanitizeErrorMessage()` alone does not close it: it only rewrites tokens
+ *    that look like an absolute path ending in a *source* extension (SOURCE_EXT
+ *    in open-sse/utils/error.ts), so `.json` state paths, extension-less binary
+ *    paths and `tskey-*` keys all survive it verbatim. The first test below pins
+ *    that, so the reason this module exists stays visible.
+ *
+ * 2. `validateBody()` returns `{ success, error }` and has NO `response` field
+ *    (`validatedJsonBody()` is the helper that has one). Three call sites did
+ *    `return validation.response`, so a failed body validation returned
+ *    `undefined` and Next answered with a framework 500 instead of a 400.
+ *
+ * Reachability is not uniform, and the tests say so: `/api/tunnels/ngrok`,
+ * `/api/tunnels/tailscale` and `/api/tunnels/tailscale/check` are not in
+ * `LOCAL_ONLY_API_PREFIXES`, and `GET /api/tunnels/cloudflared` is exempted via
+ * `LOCAL_ONLY_API_GET_EXEMPTIONS` (#11531); the rest are loopback-gated.
+ */
+
+import assert from "node:assert/strict";
+import fs from "node:fs";
+import os from "node:os";
+import path from "node:path";
+import test from "node:test";
+import type { NextRequest } from "next/server";
+
+const TEST_DATA_DIR = fs.mkdtempSync(path.join(os.tmpdir(), "omniroute-tunnel-sanitize-"));
+const ORIGINAL_DATA_DIR = process.env.DATA_DIR;
+process.env.DATA_DIR = TEST_DATA_DIR;
+process.env.API_KEY_SECRET = process.env.API_KEY_SECRET || "tunnel-sanitize-test-secret";
+
+const core = await import("../../src/lib/db/core.ts");
+const { sanitizeErrorMessage } = await import("../../open-sse/utils/error.ts");
+const { classifyTunnelError, toPublicSafeTunnelError } =
+  await import("../../src/lib/api/publicSafeTunnelError.ts");
+const ngrokRoute = await import("../../src/app/api/tunnels/ngrok/route.ts");
+const cloudflaredRoute = await import("../../src/app/api/tunnels/cloudflared/route.ts");
+const tailscaleEnableRoute = await import("../../src/app/api/tunnels/tailscale/enable/route.ts");
+
+test.after(() => {
+  core.resetDbInstance();
+  fs.rmSync(TEST_DATA_DIR, { recursive: true, force: true });
+  if (ORIGINAL_DATA_DIR === undefined) delete process.env.DATA_DIR;
+  else process.env.DATA_DIR = ORIGINAL_DATA_DIR;
+});
+
+/** The exact leak shapes these routes produce in the field. */
+const LEAKS = [
+  {
+    label: "config/state path (.json)",
+    message:
+      "ENOENT: no such file or directory, open '/home/operator/.omniroute/data/tunnels.json'",
+    secrets: ["/home/operator", "tunnels.json"],
+  },
+  {
+    label: "binary path (no extension)",
+    message: "spawn /usr/local/bin/cloudflared ENOENT",
+    secrets: ["/usr/local/bin/cloudflared"],
+  },
+  {
+    label: "tailscale auth key",
+    message: "tailscale up failed: invalid key tskey-auth-kMn3Qz7RtY-9fVbXsPq2LdWc",
+    secrets: ["tskey-auth-kMn3Qz7RtY-9fVbXsPq2LdWc"],
+  },
+  {
+    label: "daemon state path",
+    message:
+      "Command failed: /opt/omniroute/bin/tailscaled --state=/var/lib/tailscale/tailscaled.state",
+    secrets: ["/opt/omniroute/bin/tailscaled", "/var/lib/tailscale"],
+  },
+  {
+    label: "windows config path",
+    message:
+      "listen EADDRINUSE: address already in use 0.0.0.0:41641 (config C:\\Users\\operator\\AppData\\omniroute\\ngrok.yml)",
+    secrets: ["C:\\Users\\operator", "ngrok.yml"],
+  },
+] as const;
+
+/** Run `fn` with console.error captured, so the helper's log does not spam output. */
+async function withSilencedConsoleError<T>(fn: () => T | Promise<T>): Promise<[T, unknown[][]]> {
+  const original = console.error;
+  const calls: unknown[][] = [];
+  console.error = (...args: unknown[]) => {
+    calls.push(args);
+  };
+  try {
+    return [await fn(), calls];
+  } finally {
+    console.error = original;
+  }
+}
+
+// ── Why a dedicated module: sanitizeErrorMessage does not cover these ───────
+
+test("sanitizeErrorMessage alone leaves every tunnel leak shape intact", () => {
+  for (const leak of LEAKS) {
+    const out = sanitizeErrorMessage(leak.message);
+    const stillLeaks = leak.secrets.some((s) => out.includes(s));
+    assert.ok(
+      stillLeaks,
+      `${leak.label}: sanitizeErrorMessage unexpectedly covers this now — if the ` +
+        `shared sanitizer grew to handle it, simplify publicSafeTunnelError accordingly. Got: ${out}`
+    );
+  }
+});
+
+// ── The public-safe contract ───────────────────────────────────────────────
+
+test("toPublicSafeTunnelError never echoes the upstream message", async () => {
+  for (const leak of LEAKS) {
+    const [body] = await withSilencedConsoleError(() =>
+      toPublicSafeTunnelError(new Error(leak.message), "Failed to update the tunnel.", "test")
+    );
+    assert.equal(body.error, "Failed to update the tunnel.", `${leak.label}: fallback replaced`);
+    for (const secret of leak.secrets) {
+      assert.ok(!body.error.includes(secret), `${leak.label}: leaked ${secret}`);
+      assert.ok(
+        !JSON.stringify(body).includes(secret),
+        `${leak.label}: leaked ${secret} elsewhere in the body`
+      );
+    }
+    assert.equal(typeof body.reason, "string");
+  }
+});
+
+test("toPublicSafeTunnelError logs the real error server-side", async () => {
+  const [, calls] = await withSilencedConsoleError(() =>
+    toPublicSafeTunnelError(new Error("spawn /usr/local/bin/cloudflared ENOENT"), "nope", "ctx")
+  );
+  assert.equal(calls.length, 1, "operator must still get the real cause in the server log");
+  assert.equal(calls[0][0], "[ctx]");
+});
+
+test("classifyTunnelError maps causes without echoing them", () => {
+  assert.equal(classifyTunnelError(new Error("spawn cloudflared ENOENT")), "not_installed");
+  assert.equal(classifyTunnelError(new Error("EACCES: permission denied")), "permission_denied");
+  assert.equal(classifyTunnelError(new Error("listen EADDRINUSE")), "already_running");
+  assert.equal(classifyTunnelError(new Error("connect ETIMEDOUT")), "timeout");
+  assert.equal(classifyTunnelError(new Error("connect ECONNREFUSED")), "network");
+  assert.equal(classifyTunnelError(new Error("something else entirely")), "unknown");
+  assert.equal(classifyTunnelError(undefined), "unknown");
+});
+
+// ── Defect 1, end to end on a remotely reachable route ─────────────────────
+
+function makeRequest(url: string, init?: RequestInit): NextRequest {
+  return new Request(url, init) as unknown as NextRequest;
+}
+
+test("GET /api/tunnels/ngrok does not leak a host path in its 500 body", async () => {
+  // getNgrokTunnelStatus() reads globalThis.__ngrokListener and then calls
+  // getTunnelApiUrl(currentUrl) OUTSIDE its try/catch, so a listener whose url()
+  // yields an object with a throwing `replace` reproduces a real 500 here.
+  const LEAK = "/home/operator/.omniroute/data/tunnels.json";
+  const g = globalThis as unknown as { __ngrokListener?: unknown };
+  const previous = g.__ngrokListener;
+  g.__ngrokListener = {
+    url: () => ({
+      replace: () => {
+        throw new Error(`ENOENT: no such file or directory, open '${LEAK}'`);
+      },
+    }),
+  };
+
+  try {
+    const [res] = await withSilencedConsoleError(() =>
+      ngrokRoute.GET(makeRequest("http://localhost/api/tunnels/ngrok"))
+    );
+    assert.equal(res.status, 500);
+    const body = (await res.json()) as { error?: unknown; reason?: unknown };
+    assert.equal(typeof body.error, "string", "dashboard reads data.error as a string");
+    const text = JSON.stringify(body);
+    assert.ok(!text.includes(LEAK), `body leaked the state path: ${text}`);
+    assert.ok(!text.includes("/home/operator"), `body leaked the home directory: ${text}`);
+    assert.equal(body.reason, "not_installed");
+  } finally {
+    if (previous === undefined) delete g.__ngrokListener;
+    else g.__ngrokListener = previous;
+  }
+});
+
+// ── Defect 2: validateBody has no `response` field ─────────────────────────
+
+test("POST /api/tunnels/ngrok answers 400 (not a framework 500) on an invalid body", async () => {
+  const res = await ngrokRoute.POST(
+    makeRequest("http://localhost/api/tunnels/ngrok", {
+      method: "POST",
+      headers: { "content-type": "application/json" },
+      body: JSON.stringify({ action: "not-a-valid-action" }),
+    })
+  );
+  assert.ok(res, "handler must return a Response, not undefined");
+  assert.equal(res.status, 400);
+  const body = (await res.json()) as { error?: unknown };
+  assert.equal(typeof body.error, "string");
+  assert.ok((body.error as string).length > 0);
+});
+
+test("POST /api/tunnels/cloudflared answers 400 (not a framework 500) on an invalid body", async () => {
+  const res = await cloudflaredRoute.POST(
+    makeRequest("http://localhost/api/tunnels/cloudflared", {
+      method: "POST",
+      headers: { "content-type": "application/json" },
+      body: JSON.stringify({ action: "not-a-valid-action" }),
+    })
+  );
+  assert.ok(res, "handler must return a Response, not undefined");
+  assert.equal(res.status, 400);
+  assert.equal(typeof ((await res.json()) as { error?: unknown }).error, "string");
+});
+
+test("POST /api/tunnels/tailscale/enable answers 400 (not a framework 500) on an out-of-range port", async () => {
+  const res = await tailscaleEnableRoute.POST(
+    makeRequest("http://localhost/api/tunnels/tailscale/enable", {
+      method: "POST",
+      headers: { "content-type": "application/json" },
+      body: JSON.stringify({ port: 999999 }),
+    })
+  );
+  assert.ok(res, "handler must return a Response, not undefined");
+  assert.equal(res.status, 400);
+  const body = (await res.json()) as { error?: unknown };
+  assert.equal(typeof body.error, "string");
+  assert.match(body.error as string, /port/i, "the 400 should name the offending field");
+});
+
+// ── Regression sweep over the whole surface this PR covers ────────────────
+
+test("no tunnel or MITM route echoes a raw error.message any more", () => {
+  const roots = ["src/app/api/tunnels", "src/app/api/settings/mitm"];
+  const offenders: string[] = [];
+
+  const walk = (dir: string) => {
+    for (const entry of fs.readdirSync(dir, { withFileTypes: true })) {
+      const full = path.join(dir, entry.name);
+      if (entry.isDirectory()) {
+        walk(full);
+        continue;
+      }
+      if (!entry.name.endsWith(".ts")) continue;
+      for (const [i, line] of fs.readFileSync(full, "utf8").split("\n").entries()) {
+        // The comments this PR adds mention the old pattern by name; only flag code.
+        if (line.trimStart().startsWith("//")) continue;
+        if (/\b\w+ instanceof Error \? \w+\.message\b/.test(line)) {
+          offenders.push(`${full}:${i + 1}`);
+        }
+      }
+    }
+  };
+
+  for (const root of roots) walk(path.resolve(process.cwd(), root));
+  assert.deepEqual(
+    offenders,
+    [],
+    `Hard Rule #12: these lines put a raw error.message in a response body:\n${offenders.join("\n")}`
+  );
+});