diff --git a/desktop/src-tauri/src/managed_agents/restore.rs b/desktop/src-tauri/src/managed_agents/restore.rs
index 1910620159..f485d4a7ca 100644
--- a/desktop/src-tauri/src/managed_agents/restore.rs
+++ b/desktop/src-tauri/src/managed_agents/restore.rs
@@ -330,10 +330,14 @@ pub async fn restore_managed_agents_on_launch(
                                             // mid-turn session is not resumed by an
                                             // eager child — and silently reintroduces
                                             // N idle brains on every launch.
+                                            // Configured spelling, not
+                                            // `key.relay_url` — Host-derived
+                                            // tenancy (see
+                                            // `child_connect_relay_url`).
                                             spawn_agent_child(
                                                 app,
                                                 record,
-                                                &key.relay_url,
+                                                &relay_url,
                                                 true,
                                                 owner_hex_ref,
                                             )
diff --git a/desktop/src-tauri/src/managed_agents/runtime.rs b/desktop/src-tauri/src/managed_agents/runtime.rs
index f3b4cb67fd..b1d2d29757 100644
--- a/desktop/src-tauri/src/managed_agents/runtime.rs
+++ b/desktop/src-tauri/src/managed_agents/runtime.rs
@@ -445,10 +445,36 @@ pub(crate) fn configure_runtime_cli(
     }
 }
 
+/// Relay URL for the spawned harness's `BUZZ_RELAY_URL` and git-credential
+/// scope: always the caller's *configured* URL, never the canonical
+/// runtime-key form. Relay tenancy is derived from the HTTP Host header, so
+/// the canonical loopback rewrite (`ws://localhost:3000` →
+/// `ws://127.0.0.1:3000`) names a different — empty — community than the one
+/// the desktop wrote the agent's channel memberships into; a harness handed
+/// the canonical form discovers 0 channels and never sees a mention. The
+/// canonical form stays correct for identity only: runtime-map keys,
+/// receipts, log paths, and the spawn-config hash (which `status_for`
+/// recomputes from `key.relay_url`).
+pub(crate) fn child_connect_relay_url<'a>(
+    configured: &'a str,
+    key: &ManagedAgentRuntimeKey,
+) -> &'a str {
+    debug_assert!(
+        buzz_core_pkg::relay::normalize_relay_url(configured)
+            .is_ok_and(|canonical| canonical == key.relay_url),
+        "configured relay URL must canonicalize to the runtime key's relay"
+    );
+    configured
+}
+
 /// Spawn an agent process without holding any locks on records or runtimes.
 /// Returns the child process and log path on success. The caller is responsible
 /// for updating `ManagedAgentRecord` fields and inserting into the runtimes map.
 ///
+/// `relay_url`: the *configured* pair relay (workspace override / start
+/// request), not a canonical runtime-key round-trip — see
+/// `child_connect_relay_url`.
+///
 /// `owner_hex`: the workspace owner's pubkey, used as a fallback for legacy
 /// records that have no NIP-OA `auth_tag`. See `build_respond_to_env`.
 pub fn spawn_agent_child(
@@ -545,9 +571,11 @@ pub fn spawn_agent_child(
         .map(|p| p.display().to_string())
         .unwrap_or_else(|| effective_command.clone());
 
-    // The caller supplies the explicit canonical pair relay. This is the only
-    // relay this child may connect to, regardless of the record/workspace default.
-    let effective_relay_url = runtime_key.relay_url.clone();
+    // The caller supplies the explicit pair relay. This is the only relay this
+    // child may connect to, regardless of the record/workspace default — in the
+    // configured spelling, because the Host header decides which community the
+    // child lands in (see `child_connect_relay_url`).
+    let child_relay_url = child_connect_relay_url(relay_url, &runtime_key);
 
     // Augment PATH for DMG launches so child processes can find:
     //   - bundled CLI via ~/.local/bin symlink
@@ -578,7 +606,7 @@ pub fn spawn_agent_child(
     }
     command.env("RUST_LOG", child_rust_log_filter());
     command.env("BUZZ_PRIVATE_KEY", &record.private_key_nsec);
-    command.env("BUZZ_RELAY_URL", &effective_relay_url);
+    command.env("BUZZ_RELAY_URL", child_relay_url);
     command.env("BUZZ_ACP_LAZY_POOL", if lazy { "true" } else { "false" });
     command.env("BUZZ_ACP_AGENT_COMMAND", &resolved_agent_command);
     command.env("BUZZ_ACP_AGENT_ARGS", agent_args.join(","));
@@ -826,7 +854,7 @@ pub fn spawn_agent_child(
     //
     // NOSTR_PRIVATE_KEY mirrors BUZZ_PRIVATE_KEY — keep in sync.
     if let Some(cred_helper) = resolve_command("git-credential-nostr") {
-        let relay_http_url = crate::relay::relay_http_base_url(&effective_relay_url);
+        let relay_http_url = crate::relay::relay_http_base_url(child_relay_url);
 
         command.env("NOSTR_PRIVATE_KEY", &record.private_key_nsec);
         command.env("GIT_TERMINAL_PROMPT", "0");
@@ -914,13 +942,14 @@ pub fn spawn_agent_child(
 
     // Stamp the effective spawn config so the summary builder can flag
     // needs_restart when disk state drifts from what this process runs.
-    // `effective_relay_url` is already resolved, and resolution is idempotent,
-    // so it serves as the workspace-relay input here.
+    // Hash the *canonical* relay form: `status_for` recomputes this hash from
+    // `key.relay_url`, so hashing the configured spelling here would read as
+    // permanent drift and flag a spurious restart on every status pass.
     let spawn_config_hash = super::spawn_hash::spawn_config_hash(
         record,
         &personas,
         &teams,
-        &effective_relay_url,
+        &runtime_key.relay_url,
         &global,
     );
 
@@ -1003,7 +1032,9 @@ pub fn start_managed_agent_process(
     // Scalar PIDs are migration-only and never establish pair liveness.
     record.runtime_pid = None;
 
-    let mut process = spawn_agent_child(app, record, &key.relay_url, false, owner_hex)?;
+    // Configured spelling, not `key.relay_url` — Host-derived tenancy (see
+    // `child_connect_relay_url`).
+    let mut process = spawn_agent_child(app, record, &relay_url, false, owner_hex)?;
     let now = now_iso();
     let receipt = super::ManagedAgentRuntimeReceipt {
         key: key.clone(),
diff --git a/desktop/src-tauri/src/managed_agents/runtime/tests.rs b/desktop/src-tauri/src/managed_agents/runtime/tests.rs
index 3f6ee996f6..8d319e3804 100644
--- a/desktop/src-tauri/src/managed_agents/runtime/tests.rs
+++ b/desktop/src-tauri/src/managed_agents/runtime/tests.rs
@@ -1314,3 +1314,31 @@ fn restart_eligible_false_when_orphan_has_no_drift() {
 fn restart_eligible_false_when_non_orphan_has_no_drift() {
     assert!(!super::restart_eligible(false, false, false));
 }
+
+// ── child_connect_relay_url tests ───────────────────────────────────────
+
+#[test]
+fn child_connect_relay_url_preserves_configured_loopback_authority() {
+    // Regression: relay tenancy is Host-header-derived, so handing the child
+    // the canonical runtime-key form (localhost → 127.0.0.1) parks every
+    // spawned agent in a different, empty community than the desktop's
+    // ws://localhost:3000 workspace — the harness logs "discovered 0
+    // channel(s)" and never sees a mention.
+    let key = super::ManagedAgentRuntimeKey::new("a".repeat(64), "ws://localhost:3000")
+        .expect("valid runtime key");
+    assert_eq!(key.relay_url, "ws://127.0.0.1:3000");
+    assert_eq!(
+        super::child_connect_relay_url("ws://localhost:3000", &key),
+        "ws://localhost:3000",
+    );
+}
+
+#[test]
+fn child_connect_relay_url_is_identity_for_non_loopback_relays() {
+    let key = super::ManagedAgentRuntimeKey::new("a".repeat(64), "wss://relay.example.com")
+        .expect("valid runtime key");
+    assert_eq!(
+        super::child_connect_relay_url("wss://relay.example.com", &key),
+        "wss://relay.example.com",
+    );
+}
diff --git a/desktop/src-tauri/src/managed_agents/runtime_commands.rs b/desktop/src-tauri/src/managed_agents/runtime_commands.rs
index c0e55184b1..b91d012031 100644
--- a/desktop/src-tauri/src/managed_agents/runtime_commands.rs
+++ b/desktop/src-tauri/src/managed_agents/runtime_commands.rs
@@ -272,7 +272,13 @@ fn start_pair(
         .get_mut(&key)
         .is_some_and(|runtime| runtime.child.try_wait().ok().flatten().is_none())
     {
-        let status = status_for(&app, record, &key, runtimes.get(&key), None);
+        let status = status_for(
+            &app,
+            record,
+            &key,
+            runtimes.get(&key),
+            Some(relay_url.clone()),
+        );
         return Ok(status);
     }
     runtimes.remove(&key);
@@ -283,7 +289,9 @@ fn start_pair(
         .lock()
         .ok()
         .map(|keys| keys.public_key().to_hex());
-    let mut process = spawn_agent_child(&app, record, &key.relay_url, lazy, owner.as_deref())?;
+    // Configured spelling, not `key.relay_url` — Host-derived tenancy (see
+    // `child_connect_relay_url`).
+    let mut process = spawn_agent_child(&app, record, &relay_url, lazy, owner.as_deref())?;
     let now = crate::util::now_iso();
     let receipt = ManagedAgentRuntimeReceipt {
         key: key.clone(),
@@ -302,7 +310,13 @@ fn start_pair(
     record.last_stopped_at = None;
     record.last_error = None;
     runtimes.insert(key.clone(), ManagedAgentPairRuntime::starting(process));
-    let status = status_for(&app, record, &key, runtimes.get(&key), None);
+    let status = status_for(
+        &app,
+        record,
+        &key,
+        runtimes.get(&key),
+        Some(relay_url.clone()),
+    );
     drop(runtimes);
     save_managed_agents(&app, &records)?;
     emit_status(&app, &status);
@@ -403,7 +417,10 @@ async fn probe_agent_relay_access(
     let key = ManagedAgentRuntimeKey::new(record.pubkey.clone(), &requested_relay_url)?;
     let keys = nostr::Keys::parse(record.private_key_nsec.trim())
         .map_err(|error| format!("invalid managed-agent key: {error}"))?;
-    let api_base = crate::relay::relay_http_base_url(&key.relay_url);
+    // Probe over the requested authority, not the canonical key form — the
+    // Host header picks the community, and the probe must hit the same one
+    // the child will join (see `child_connect_relay_url`).
+    let api_base = crate::relay::relay_http_base_url(&requested_relay_url);
     tokio::time::timeout(
         std::time::Duration::from_secs(10),
         crate::relay::query_relay_at_with_keys(
@@ -502,9 +519,11 @@ pub async fn reconcile_managed_agent_runtimes(
         for probe in probes {
             match probe {
                 Ok((record, key, requested)) => {
+                    // Configured spelling, not `key.relay_url` — Host-derived
+                    // tenancy (see `child_connect_relay_url`).
                     match start_pair(
                         record.pubkey.clone(),
-                        key.relay_url.clone(),
+                        requested.clone(),
                         true,
                         Some(&record.updated_at),
                         app.clone(),