diff --git a/desktop/src-tauri/src/managed_agents/global_config/mod.rs b/desktop/src-tauri/src/managed_agents/global_config/mod.rs
index 445e89779..be3ec97b6 100644
--- a/desktop/src-tauri/src/managed_agents/global_config/mod.rs
+++ b/desktop/src-tauri/src/managed_agents/global_config/mod.rs
@@ -214,6 +214,8 @@ pub fn save_global_agent_config(app: &AppHandle, config: &GlobalAgentConfig) ->
 /// command is pinned on the record, and provider/model defaults belonging to a
 /// different preferred runtime must not cross the harness boundary. Explicitly
 /// configured definitions and standalone agents keep normal global inheritance.
+/// Configs written before `preferred_runtime` existed implicitly belong to
+/// Buzz Agent.
 pub(crate) fn global_model_provider_for_record<'a>(
     record: &ManagedAgentRecord,
     personas: &[AgentDefinition],
@@ -235,17 +237,17 @@ pub(crate) fn global_model_provider_for_record<'a>(
         return global_values;
     }
 
-    let Some(preferred_runtime) = global
+    let preferred_runtime = global
         .preferred_runtime
         .as_deref()
         .and_then(crate::managed_agents::known_acp_runtime)
-    else {
-        return global_values;
-    };
+        .or_else(|| crate::managed_agents::known_acp_runtime("buzz-agent"));
     let selected_command = crate::managed_agents::record_agent_command(record, personas);
     let selected_runtime = crate::managed_agents::known_acp_runtime(&selected_command);
 
-    if selected_runtime.is_some_and(|selected| preferred_runtime.id == selected.id) {
+    if selected_runtime.is_some_and(|selected| {
+        preferred_runtime.is_some_and(|preferred| preferred.id == selected.id)
+    }) {
         global_values
     } else {
         (None, None)
diff --git a/desktop/src-tauri/src/managed_agents/global_config/tests.rs b/desktop/src-tauri/src/managed_agents/global_config/tests.rs
index a5e6d35d9..8dc2296f4 100644
--- a/desktop/src-tauri/src/managed_agents/global_config/tests.rs
+++ b/desktop/src-tauri/src/managed_agents/global_config/tests.rs
@@ -491,6 +491,25 @@ fn runtime_fallback_does_not_inherit_defaults_from_a_different_preferred_runtime
     );
 }
 
+#[test]
+fn legacy_runtime_fallback_does_not_inherit_implicit_buzz_agent_defaults() {
+    let mut record = bare_record();
+    record.persona_id = Some("p1".to_string());
+    record.agent_command_override = Some("goose".to_string());
+    let personas = vec![persona("p1", None, None)];
+    let global = GlobalAgentConfig {
+        model: Some("auto".to_string()),
+        provider: Some("relay-mesh".to_string()),
+        preferred_runtime: None,
+        ..Default::default()
+    };
+
+    assert_eq!(
+        resolve_effective_model_provider(&record, &personas, &global),
+        (None, None)
+    );
+}
+
 #[test]
 fn default_command_fallback_does_not_inherit_defaults_from_another_preferred_runtime() {
     let mut record = bare_record();
diff --git a/desktop/src/features/agents/AGENTS.md b/desktop/src/features/agents/AGENTS.md
index 2c41a4787..33b23a530 100644
--- a/desktop/src/features/agents/AGENTS.md
+++ b/desktop/src/features/agents/AGENTS.md
@@ -96,6 +96,9 @@ with a TypeScript lookup table or an id comparison in a component.
    editor persists its visible fallback on the next save. Its dependent
    provider/model defaults are also ignored for new implicit fallback agents,
    without changing the persisted configuration used by existing agents.
+   Legacy global defaults with no saved preferred runtime are treated as
+   Buzz Agent-owned for this masking boundary, so hiding Buzz Agent cannot
+   leak its provider/model into another implicit fallback.
    Create-mode dialogs use the implicit masked config; existing definition and
    instance edit dialogs use the raw persisted config.
    `resolvePersonaRuntime` is the shared visibility boundary for every
diff --git a/desktop/src/features/agents/lib/runtimeVisibilityPreference.test.mjs b/desktop/src/features/agents/lib/runtimeVisibilityPreference.test.mjs
index 9bf801493..16ec7ff4c 100644
--- a/desktop/src/features/agents/lib/runtimeVisibilityPreference.test.mjs
+++ b/desktop/src/features/agents/lib/runtimeVisibilityPreference.test.mjs
@@ -118,3 +118,19 @@ test("a disabled saved runtime and its dependent defaults are masked", () => {
   });
   assert.equal(maskDisabledAcpRuntimePreference(config, ["claude"]), config);
 });
+
+test("legacy defaults without a saved runtime are owned by buzz-agent", () => {
+  const config = {
+    env_vars: {},
+    provider: "relay-mesh",
+    model: "auto",
+    preferred_runtime: null,
+  };
+
+  assert.deepEqual(maskDisabledAcpRuntimePreference(config, ["buzz-agent"]), {
+    ...config,
+    provider: null,
+    model: null,
+  });
+  assert.equal(maskDisabledAcpRuntimePreference(config, ["goose"]), config);
+});
diff --git a/desktop/src/features/agents/lib/runtimeVisibilityPreference.ts b/desktop/src/features/agents/lib/runtimeVisibilityPreference.ts
index 040831c28..45e53e614 100644
--- a/desktop/src/features/agents/lib/runtimeVisibilityPreference.ts
+++ b/desktop/src/features/agents/lib/runtimeVisibilityPreference.ts
@@ -166,7 +166,8 @@ export function visibleAcpRuntimeSeedForCreate<T extends { id: string }>(
  * The persisted config is left untouched until the user next saves defaults;
  * existing agents keep their configuration while new consumers immediately
  * fall back through the normal runtime selection path without carrying a
- * provider or model selected for the hidden harness.
+ * provider or model selected for the hidden harness. Configs written before
+ * preferred_runtime existed implicitly belong to Buzz Agent.
  */
 export function maskDisabledAcpRuntimePreference<
   T extends {
@@ -176,11 +177,11 @@ export function maskDisabledAcpRuntimePreference<
   },
 >(config: T, disabledRuntimeIds: readonly string[]): T {
   const preferredRuntime = config.preferred_runtime;
+  const implicitLegacyRuntime = preferredRuntime?.trim() || "buzz-agent";
   if (
-    !preferredRuntime ||
     !disabledRuntimeIds
       .map(normalizeRuntimeId)
-      .includes(normalizeRuntimeId(preferredRuntime))
+      .includes(normalizeRuntimeId(implicitLegacyRuntime))
   ) {
     return config;
   }
diff --git a/desktop/src/features/onboarding/welcomeKickoff.test.mjs b/desktop/src/features/onboarding/welcomeKickoff.test.mjs
index be09d82ad..c21641b12 100644
--- a/desktop/src/features/onboarding/welcomeKickoff.test.mjs
+++ b/desktop/src/features/onboarding/welcomeKickoff.test.mjs
@@ -57,6 +57,40 @@ test("welcome readiness ignores a hidden logged-in runtime", () => {
   );
 });
 
+test("welcome readiness evaluates the runtime selected by the visible fallback", () => {
+  const runtimes = [
+    {
+      id: "buzz-agent",
+      label: "Buzz Agent",
+      availability: "available",
+      authStatus: { status: "not_applicable" },
+    },
+    {
+      id: "goose",
+      label: "Goose",
+      availability: "available",
+      authStatus: { status: "not_applicable" },
+    },
+    {
+      id: "claude",
+      label: "Claude",
+      availability: "available",
+      authStatus: { status: "logged_in" },
+    },
+  ];
+  const globalConfig = {
+    env_vars: {},
+    provider: null,
+    model: null,
+    preferred_runtime: "buzz-agent",
+  };
+
+  assert.deepEqual(
+    resolveWelcomeAgentReadiness(runtimes, globalConfig, ["buzz-agent"]),
+    { ready: false },
+  );
+});
+
 test("welcome provisioning requires an enabled available runtime", () => {
   const runtimes = [
     {
diff --git a/desktop/src/features/onboarding/welcomeKickoff.ts b/desktop/src/features/onboarding/welcomeKickoff.ts
index bba48bf0d..fad049eeb 100644
--- a/desktop/src/features/onboarding/welcomeKickoff.ts
+++ b/desktop/src/features/onboarding/welcomeKickoff.ts
@@ -9,6 +9,7 @@ import {
   filterEnabledAcpRuntimes,
   useDisabledAcpRuntimeIds,
 } from "@/features/agents/lib/runtimeVisibilityPreference";
+import { getDefaultPersonaRuntime } from "@/features/agents/lib/resolvePersonaRuntime";
 import { clearActiveTurnsForAgentOnStop } from "@/features/agents/managedAgentRuntimeHooks";
 import { useImplicitGlobalAgentConfig } from "@/features/agents/useGlobalAgentConfig";
 import { useCommunities } from "@/features/communities/useCommunities";
@@ -61,9 +62,23 @@ export function resolveWelcomeAgentReadiness(
   globalConfig: GlobalAgentConfig,
   disabledRuntimeIds: readonly string[],
 ) {
+  const visibleRuntimes = filterEnabledAcpRuntimes(
+    runtimes,
+    disabledRuntimeIds,
+  );
+  const selectedRuntime = getDefaultPersonaRuntime(
+    visibleRuntimes,
+    globalConfig.preferred_runtime,
+  );
+  if (!selectedRuntime) return { ready: false } as const;
+
   return resolveAgentReadiness(
-    filterEnabledAcpRuntimes(runtimes, disabledRuntimeIds),
-    globalConfig,
+    visibleRuntimes,
+    {
+      ...globalConfig,
+      preferred_runtime: selectedRuntime.id,
+    },
+    "preferred",
   );
 }