diff --git a/.env.example b/.env.example
index 31c8b0883e..3ce1260f97 100644
--- a/.env.example
+++ b/.env.example
@@ -167,7 +167,21 @@ GITTENSORY_REVIEW_DRAFT=false
 #                                            #   EXACT public URL operators browse to (scheme + host [+ port]).
 #                                            #   Deriving it from the request Host header would let an attacker
 #                                            #   redirect the App-creation callback, so it must be set explicitly.
-#                                            #   Not needed once the App credentials are configured.
+#                                            #   Stays load-bearing after setup too: this same origin is embedded
+#                                            #   as the <img src> for every visual-capture screenshot posted to a
+#                                            #   PR comment (see PUBLIC_SITE_ORIGIN below), so it must be
+#                                            #   reachable from GitHub's own servers, not just this host or your
+#                                            #   tailnet. See PUBLIC_ORIGIN_ACKNOWLEDGED below.
+# PUBLIC_SITE_ORIGIN=https://your-fork.example.com  # "before" screenshots in the visual-capture PR comment
+#                                            #   table render THIS origin's production pages (vs. PUBLIC_API_ORIGIN,
+#                                            #   which serves the screenshot images themselves). Unset ⇒ every
+#                                            #   "Before (production)" cell is blank, not an error — visual capture
+#                                            #   degrades to after-only, never fails the review.
+# PUBLIC_ORIGIN_ACKNOWLEDGED=false            # silences the boot-time warning when PUBLIC_API_ORIGIN/
+#                                            #   PUBLIC_SITE_ORIGIN look like a private/internal hostname (e.g. a
+#                                            #   bare Tailscale MagicDNS `*.ts.net` address) once you've confirmed
+#                                            #   it's genuinely public (Funnel enabled, a reverse proxy in front of
+#                                            #   it, etc.). Default false (warning shown).
 # SELFHOST_SETUP_TOKEN=change-this-long-random-value  # REQUIRED to unlock the first-run /setup wizard. Without it
 #                                            #   /setup returns 400; with it, enter the token in the browser form
 #                                            #   or send an x-setup-token / Bearer header. Never put this token in
diff --git a/.env.selfhost.example b/.env.selfhost.example
index 09e3120c42..95220217a4 100644
--- a/.env.selfhost.example
+++ b/.env.selfhost.example
@@ -27,8 +27,12 @@ GITTENSOR_REGISTRY_URL=https://example.invalid/registry.json
 # GITTENSORY_MCP_TOKEN=           # shared MCP bearer token
 # INTERNAL_JOB_TOKEN=             # gates internal-only routes
 
-# REQUIRED once, for the first-run /setup wizard that creates the GitHub App manifest. Not needed
-# after the App credentials above are filled in.
+# REQUIRED for the first-run /setup wizard that creates the GitHub App manifest, AND required ongoing:
+# this exact origin is embedded as the <img src> for every visual-capture screenshot posted to a PR
+# comment, so it MUST be reachable from GitHub's own servers, not just from this machine or your
+# tailnet — a private/internal hostname here (e.g. a Tailscale MagicDNS `*.ts.net` address with no
+# Funnel) makes every screenshot render as a broken image with no other warning besides this instance's
+# own boot log (see PUBLIC_ORIGIN_ACKNOWLEDGED below).
 PUBLIC_API_ORIGIN=https://reviews.example.com
 # SELFHOST_SETUP_TOKEN=           # generate a real random value the same way as above before setting this
 
diff --git a/apps/gittensory-ui/src/lib/selfhost-env-reference.ts b/apps/gittensory-ui/src/lib/selfhost-env-reference.ts
index 179ae5dce9..482340970b 100644
--- a/apps/gittensory-ui/src/lib/selfhost-env-reference.ts
+++ b/apps/gittensory-ui/src/lib/selfhost-env-reference.ts
@@ -325,6 +325,14 @@ export const SELFHOST_ENV_REFERENCE_ROWS: SelfHostEnvReferenceRow[] = [
     name: "PUBLIC_API_ORIGIN",
     firstReference: "src/selfhost/preflight.ts",
   },
+  {
+    name: "PUBLIC_ORIGIN_ACKNOWLEDGED",
+    firstReference: "src/server.ts",
+  },
+  {
+    name: "PUBLIC_SITE_ORIGIN",
+    firstReference: "src/server.ts",
+  },
   {
     name: "QDRANT_API_KEY",
     firstReference: "src/selfhost/qdrant-vectorize.ts",
@@ -486,6 +494,8 @@ export const SELFHOST_ENV_REFERENCE_MARKDOWN = [
   "| `PGVECTOR_ENABLED` | `src/server.ts` |",
   "| `PORT` | `src/server.ts` |",
   "| `PUBLIC_API_ORIGIN` | `src/selfhost/preflight.ts` |",
+  "| `PUBLIC_ORIGIN_ACKNOWLEDGED` | `src/server.ts` |",
+  "| `PUBLIC_SITE_ORIGIN` | `src/server.ts` |",
   "| `QDRANT_API_KEY` | `src/selfhost/qdrant-vectorize.ts` |",
   "| `QDRANT_DIM` | `src/selfhost/qdrant-vectorize.ts` |",
   "| `QDRANT_URL` | `src/server.ts` |",
diff --git a/src/selfhost/health.ts b/src/selfhost/health.ts
index b02b6b7b8e..b9daaf0e04 100644
--- a/src/selfhost/health.ts
+++ b/src/selfhost/health.ts
@@ -155,3 +155,78 @@ export function sqliteBackupAdvisory(opts: { usingSqlite: boolean; backupAcknowl
 export function backupAcknowledgedGaugeValue(opts: { usingSqlite: boolean; backupAcknowledged: boolean }): 0 | 1 {
   return sqliteBackupAdvisory(opts) === null ? 1 : 0;
 }
+
+// Hostnames that are NEVER reachable from the public internet, regardless of deployment — loopback, RFC1918
+// private ranges, and the mDNS/internal-DNS conventions no public resolver honors. Deliberately excludes
+// Tailscale's own *.ts.net MagicDNS suffix from this "never" list: a node with Funnel explicitly enabled DOES
+// serve that exact hostname over public HTTPS, so *.ts.net gets its own softer check below rather than being
+// treated as definitely non-public.
+function isDefinitelyPrivateHostname(hostname: string): boolean {
+  const host = hostname.toLowerCase();
+  // URL.hostname keeps the brackets on an IPv6 literal ("[::1]", not "::1") — compare against the bracketed
+  // form, not the bare address.
+  if (host === "localhost" || host === "0.0.0.0" || host === "[::1]") return true;
+  if (host.endsWith(".local") || host.endsWith(".internal")) return true;
+  const ipv4 = /^(\d{1,3})\.(\d{1,3})\.\d{1,3}\.\d{1,3}$/.exec(host);
+  if (ipv4) {
+    const first = Number(ipv4[1]);
+    const second = Number(ipv4[2]);
+    if (first === 127 || first === 10) return true;
+    if (first === 192 && second === 168) return true;
+    if (first === 172 && second >= 16 && second <= 31) return true;
+  }
+  return false;
+}
+
+/** True when `origin` parses as a URL whose hostname is a well-known non-public pattern (loopback, RFC1918,
+ *  mDNS/`.internal`), OR a bare Tailscale MagicDNS hostname (`*.ts.net`) — publicly reachable ONLY when the
+ *  operator has explicitly enabled Tailscale Funnel for that node, which this check has no way to observe.
+ *  An unparseable value is this function's job to reject as "not a URL at all", not "looks non-public" — but
+ *  that is a distinct, well-formedness problem this advisory doesn't cover (PUBLIC_API_ORIGIN's own
+ *  first-run-setup preflight check already validates that shape; nothing else currently reads
+ *  PUBLIC_SITE_ORIGIN at boot), so it's silently false here rather than double-reported. */
+function looksNonPublic(origin: string): boolean {
+  try {
+    const { hostname } = new URL(origin);
+    return isDefinitelyPrivateHostname(hostname) || hostname.toLowerCase().endsWith(".ts.net");
+  } catch {
+    return false;
+  }
+}
+
+/** Boot-time advisory (JSONbored/gittensory PR #4180's live bug): `PUBLIC_API_ORIGIN`/`PUBLIC_SITE_ORIGIN` get
+ *  embedded VERBATIM as `<img src>` in the public "Visual preview" PR comment table (see
+ *  `src/review/visual/capture.ts`) — a value GitHub's own servers, not this instance, must be able to fetch.
+ *  `PUBLIC_API_ORIGIN`'s existing preflight check (see `isBareHttpsOrigin` above) only confirms it's a
+ *  WELL-FORMED bare https origin — a private tailnet hostname like `https://node.example.ts.net` passes that
+ *  check fine while still being completely unfetchable by GitHub, so every screenshot silently renders as a
+ *  broken image with no operator-visible signal until a human notices. This never hard-fails boot (unlike
+ *  `assertSelfHostPreflight`'s checks): a false positive here (e.g. a legitimately Funnel-exposed `*.ts.net`
+ *  node) would only be a degraded review feature, not a security or data-loss risk, so — mirroring
+ *  {@link sqliteBackupAdvisory}'s own acknowledgment escape hatch — it warns rather than blocks, and the
+ *  operator can silence it with `PUBLIC_ORIGIN_ACKNOWLEDGED=true` once they've confirmed the origin really is
+ *  public (Funnel enabled, a reverse proxy in front of it, etc.). Neither var set is left alone: visual
+ *  capture degrades to dash cells in that case (see capture.ts), not a broken image, so there's nothing to warn
+ *  about until an operator sets one of these to something that looks wrong. */
+export function publicOriginReachabilityAdvisory(opts: {
+  publicApiOrigin: string | undefined;
+  publicSiteOrigin: string | undefined;
+  acknowledged: boolean;
+}): string | null {
+  if (opts.acknowledged) return null;
+  const suspect = [opts.publicApiOrigin, opts.publicSiteOrigin]
+    .filter((value): value is string => Boolean(value?.trim()))
+    .find((value) => looksNonPublic(value));
+  if (!suspect) return null;
+  return `PUBLIC_API_ORIGIN/PUBLIC_SITE_ORIGIN includes "${suspect}", which looks like a private/internal hostname — GitHub's servers cannot fetch it, so visual-capture screenshots embedded in PR comments will render as broken images. Set it to a real publicly-reachable origin (or, if this host has Tailscale Funnel enabled and IS genuinely public, set PUBLIC_ORIGIN_ACKNOWLEDGED=true to silence this warning).`;
+}
+
+/** Prometheus gauge value mirroring {@link publicOriginReachabilityAdvisory}: 1 when no suspect origin is
+ *  configured (or the operator acknowledged it), 0 when the advisory would fire. */
+export function publicOriginAcknowledgedGaugeValue(opts: {
+  publicApiOrigin: string | undefined;
+  publicSiteOrigin: string | undefined;
+  acknowledged: boolean;
+}): 0 | 1 {
+  return publicOriginReachabilityAdvisory(opts) === null ? 1 : 0;
+}
diff --git a/src/server.ts b/src/server.ts
index 44198a8f90..efedbc5e03 100644
--- a/src/server.ts
+++ b/src/server.ts
@@ -47,6 +47,8 @@ import {
   buildHealthBody,
   codexAuthReadinessProbe,
   githubAppReadinessProbe,
+  publicOriginAcknowledgedGaugeValue,
+  publicOriginReachabilityAdvisory,
   readiness,
   sqliteBackupAdvisory,
   type ReadinessProbe,
@@ -392,6 +394,24 @@ async function main(): Promise<void> {
       }),
     );
 
+  // Public-origin advisory (JSONbored/gittensory#4180): warn LOUDLY at boot if PUBLIC_API_ORIGIN/
+  // PUBLIC_SITE_ORIGIN look like a private/internal hostname, so an operator doesn't run for weeks with every
+  // visual-capture screenshot silently rendering as a broken image in public PR comments.
+  const publicOriginOpts = {
+    publicApiOrigin: process.env.PUBLIC_API_ORIGIN,
+    publicSiteOrigin: process.env.PUBLIC_SITE_ORIGIN,
+    acknowledged: process.env.PUBLIC_ORIGIN_ACKNOWLEDGED === "true",
+  };
+  const publicOriginAdvisory = publicOriginReachabilityAdvisory(publicOriginOpts);
+  if (publicOriginAdvisory)
+    console.warn(
+      JSON.stringify({
+        level: "warn",
+        event: "selfhost_public_origin_advisory",
+        message: publicOriginAdvisory,
+      }),
+    );
+
   const applied = await runSelfHostMigrations(
     backend.db,
     process.env.MIGRATIONS_DIR ?? "migrations",
@@ -701,6 +721,7 @@ async function main(): Promise<void> {
     Math.floor((Date.now() - startedAt) / 1000),
   );
   gauge("gittensory_backup_acknowledged", () => backupAcknowledgedGaugeValue(sqliteBackupOpts));
+  gauge("gittensory_public_origin_acknowledged", () => publicOriginAcknowledgedGaugeValue(publicOriginOpts));
   // Pre-initialize job counters to 0 so they appear in the first Prometheus scrape (lazy counters
   // created on first use would otherwise cause "No data" in Grafana until the first job event).
   for (const c of [
diff --git a/test/unit/selfhost-health.test.ts b/test/unit/selfhost-health.test.ts
index 65a386a483..9f8a394feb 100644
--- a/test/unit/selfhost-health.test.ts
+++ b/test/unit/selfhost-health.test.ts
@@ -9,6 +9,8 @@ import {
   buildHealthBody,
   codexAuthReadinessProbe,
   githubAppReadinessProbe,
+  publicOriginAcknowledgedGaugeValue,
+  publicOriginReachabilityAdvisory,
   readiness,
   sqliteBackupAdvisory,
 } from "../../src/selfhost/health";
@@ -219,6 +221,91 @@ describe("backupAcknowledgedGaugeValue (#2089)", () => {
   });
 });
 
+describe("publicOriginReachabilityAdvisory (#4180)", () => {
+  it("is silent when acknowledged, regardless of how bad the origin looks", () => {
+    expect(
+      publicOriginReachabilityAdvisory({
+        publicApiOrigin: "https://node.raccoon-bushi.ts.net",
+        publicSiteOrigin: undefined,
+        acknowledged: true,
+      }),
+    ).toBeNull();
+  });
+
+  it("is silent when neither origin is set, or both are ordinary public https origins", () => {
+    expect(publicOriginReachabilityAdvisory({ publicApiOrigin: undefined, publicSiteOrigin: undefined, acknowledged: false })).toBeNull();
+    expect(publicOriginReachabilityAdvisory({ publicApiOrigin: "  ", publicSiteOrigin: undefined, acknowledged: false })).toBeNull();
+    expect(
+      publicOriginReachabilityAdvisory({
+        publicApiOrigin: "https://reviews.example.com",
+        publicSiteOrigin: "https://example.com",
+        acknowledged: false,
+      }),
+    ).toBeNull();
+  });
+
+  it("is silent on an unparseable value — a well-formedness problem, not this advisory's job", () => {
+    expect(
+      publicOriginReachabilityAdvisory({ publicApiOrigin: "not a url", publicSiteOrigin: undefined, acknowledged: false }),
+    ).toBeNull();
+  });
+
+  it("regression: warns on the exact PR #4180 shape — a bare Tailscale MagicDNS origin", () => {
+    const message = publicOriginReachabilityAdvisory({
+      publicApiOrigin: "https://edge-us-01.raccoon-bushi.ts.net",
+      publicSiteOrigin: undefined,
+      acknowledged: false,
+    });
+    expect(message).toMatch(/edge-us-01\.raccoon-bushi\.ts\.net/);
+    expect(message).toMatch(/PUBLIC_ORIGIN_ACKNOWLEDGED/);
+  });
+
+  it("checks PUBLIC_SITE_ORIGIN too, not just PUBLIC_API_ORIGIN", () => {
+    expect(
+      publicOriginReachabilityAdvisory({
+        publicApiOrigin: "https://reviews.example.com",
+        publicSiteOrigin: "http://localhost:3000",
+        acknowledged: false,
+      }),
+    ).toMatch(/localhost/);
+  });
+
+  it("flags loopback, RFC1918 ranges, mDNS, and .internal — but not a normal public IP or domain", () => {
+    const bad = [
+      "http://localhost",
+      "http://0.0.0.0",
+      "http://[::1]",
+      "https://node.local",
+      "https://svc.internal",
+      "http://127.0.0.1",
+      "http://10.0.0.5",
+      "http://192.168.1.1",
+      "http://172.20.0.5",
+    ];
+    for (const publicApiOrigin of bad) {
+      expect(publicOriginReachabilityAdvisory({ publicApiOrigin, publicSiteOrigin: undefined, acknowledged: false })).not.toBeNull();
+    }
+    const fine = ["https://8.8.8.8", "http://172.15.0.5", "http://172.32.0.5", "https://reviews.example.com"];
+    for (const publicApiOrigin of fine) {
+      expect(publicOriginReachabilityAdvisory({ publicApiOrigin, publicSiteOrigin: undefined, acknowledged: false })).toBeNull();
+    }
+  });
+});
+
+describe("publicOriginAcknowledgedGaugeValue (#4180)", () => {
+  it("mirrors the advisory: 0 only when a suspect origin is unacknowledged", () => {
+    expect(
+      publicOriginAcknowledgedGaugeValue({ publicApiOrigin: "https://node.ts.net", publicSiteOrigin: undefined, acknowledged: false }),
+    ).toBe(0);
+    expect(
+      publicOriginAcknowledgedGaugeValue({ publicApiOrigin: "https://node.ts.net", publicSiteOrigin: undefined, acknowledged: true }),
+    ).toBe(1);
+    expect(
+      publicOriginAcknowledgedGaugeValue({ publicApiOrigin: "https://reviews.example.com", publicSiteOrigin: undefined, acknowledged: false }),
+    ).toBe(1);
+  });
+});
+
 describe("readiness (#982)", () => {
   afterEach(() => {
     vi.restoreAllMocks();