diff --git a/docs/systems/federation.md b/docs/systems/federation.md
index b983b95b..318b5fc5 100644
--- a/docs/systems/federation.md
+++ b/docs/systems/federation.md
@@ -1427,6 +1427,10 @@ DM channel hard-delete cascades: reactions, embeds, attachments (DB rows + disk
 
 `startFederationWorkers()` in `utils/federationWorker.ts:1211` starts: outbox-delivery tick, federated-call sentinel, file-replication ticks, health-check ticks, and the storage janitor. The whole bundle is gated by `process.env.DISABLE_FEDERATION_WORKERS` matching `'1'` or `'true'` at `index.ts:171-176` (envBool semantics). Tests set `DISABLE_FEDERATION_WORKERS=1` to silence cross-instance background traffic during setup.
 
+### Public Origin Override
+
+`PUBLIC_ORIGIN` env (read via `config.publicOrigin`, consumed by `getOurOrigin()` in `utils/federationAuth.ts`) overrides the federation transport URL verbatim, taking precedence over the default `https://${DOMAIN}`. When unset, behaviour is unchanged. Intended for reverse-proxy / dev-without-TLS deployments where the public origin must be advertised explicitly (typically `http://...`) and differs from the bare `DOMAIN` value used for federated identity. The integration test harness does NOT use this override — see `seedPeer.ts` for why localhost-port instances cannot collapse to a single peer row.
+
 ### Test-Only Routes
 
 `POST /api/admin/test/seed-peer` directly inserts a `federation_peers` row, skipping the multi-step peer handshake. Strictly gated: `NODE_ENV='test'` AND `ENABLE_TEST_ROUTES='1'` together; returns 404 in any other configuration. Used exclusively by the two-instance integration harness in `packages/server/test/`. Validates `origin` (must be http(s) URL), `hmacSecret` (≥32 chars), and `status` (must be one of `'active'`, `'pending'`, `'awaiting_approval'`, `'rejected'`, `'revoked'`, `'needs_attention'`, `'unreachable'`, `'accepted'`).
diff --git a/packages/server/src/config.ts b/packages/server/src/config.ts
index 0ebe7042..51642fbd 100644
--- a/packages/server/src/config.ts
+++ b/packages/server/src/config.ts
@@ -34,12 +34,27 @@ function envBool(key: string, defaultValue: boolean): boolean {
   return value === 'true' || value === '1';
 }
 
+// PUBLIC_ORIGIN overrides the federation transport URL returned by getOurOrigin().
+// Used by integration test harnesses that bind to 127.0.0.1:<ephemeral> and by
+// reverse-proxy setups where federation must advertise an http:// origin (the
+// proxy terminates TLS upstream). When unset, getOurOrigin() falls back to
+// https://${DOMAIN} for production safety.
+const publicOrigin = envOptional('PUBLIC_ORIGIN');
+if (publicOrigin !== undefined) {
+  if (!/^https?:\/\//i.test(publicOrigin)) {
+    throw new Error(
+      `PUBLIC_ORIGIN must start with http:// or https:// — got: ${publicOrigin}`
+    );
+  }
+}
+
 export const config = {
   port: envInt('PORT', 3000),
   host: env('HOST', '0.0.0.0'),
   jwtSecret: env('JWT_SECRET'),
   jwtExpiresIn: env('JWT_EXPIRES_IN', '30d'),
   domain: envOptional('DOMAIN'),
+  publicOrigin,
 
   livekit: {
     url: envOptional('LIVEKIT_URL'),
diff --git a/packages/server/src/utils/federationAuth.ts b/packages/server/src/utils/federationAuth.ts
index c80d05e1..50580089 100644
--- a/packages/server/src/utils/federationAuth.ts
+++ b/packages/server/src/utils/federationAuth.ts
@@ -174,9 +174,21 @@ export function parseFederationHeaders(
 
 /**
  * Return the canonical origin URL for this instance.
- * Uses DOMAIN env var for production, falls back to localhost for dev.
+ *
+ * Resolution order:
+ *   1. `PUBLIC_ORIGIN` env (verbatim, trailing slash stripped) — overrides everything.
+ *      Used by integration test harnesses that bind to 127.0.0.1:<ephemeral> and by
+ *      reverse-proxy / dev-without-TLS setups where federation must advertise an
+ *      explicit origin distinct from the public DOMAIN. The override is the URL
+ *      transport layer; identity (homeInstance) still derives from DOMAIN.
+ *   2. `https://${DOMAIN}` — production default.
+ *   3. `http://localhost:${PORT}` — dev fallback when DOMAIN is unset.
  */
 export function getOurOrigin(): string {
+  const override = config.publicOrigin;
+  if (override && override.trim()) {
+    return override.trim().replace(/\/$/, '');
+  }
   if (config.domain) {
     return `https://${config.domain}`;
   }