diff --git a/desktop/src/features/agents/AGENTS.md b/desktop/src/features/agents/AGENTS.md
index 7a226b8a5..54361cf66 100644
--- a/desktop/src/features/agents/AGENTS.md
+++ b/desktop/src/features/agents/AGENTS.md
@@ -96,9 +96,11 @@ 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.
-   Runtime-less agent starts and team deploys must filter through the same
-   visible-runtime set; definitions already pinned to a hidden runtime remain
-   runnable.
+   `resolveStartRuntimeForDefinition` is the shared boundary that filters every
+   runtime-less start through the device's visible-runtime set; call it instead
+   of duplicating default selection in a start surface. Team deploys use the
+   same visible-runtime set. Definitions already pinned to a hidden runtime
+   remain runnable.
 10. **The defaults modal is progressively disclosed.** An unset global config
    starts on the Buzz Agent-first deployment fallback and carries that visible
    harness into the next saved edit. The `progressive-defaults` disclosure
diff --git a/desktop/src/features/agents/lib/instanceInputForDefinition.test.mjs b/desktop/src/features/agents/lib/instanceInputForDefinition.test.mjs
index 4496b01ce..a4588420e 100644
--- a/desktop/src/features/agents/lib/instanceInputForDefinition.test.mjs
+++ b/desktop/src/features/agents/lib/instanceInputForDefinition.test.mjs
@@ -331,6 +331,28 @@ test("item-13: goose-only available — persona with no runtime resolves goose",
   assert.deepEqual(warnings, []);
 });
 
+test("runtime-less starts exclude disabled runtimes at the shared resolver", () => {
+  const { runtime, warnings } = resolveStartRuntimeForDefinition(
+    persona({ runtime: undefined }),
+    [gooseRuntime, buzzAgentRuntime],
+    "buzz-agent",
+    ["buzz-agent"],
+  );
+  assert.equal(runtime.id, "goose");
+  assert.deepEqual(warnings, []);
+});
+
+test("explicit definitions can still start on a hidden runtime", () => {
+  const { runtime, warnings } = resolveStartRuntimeForDefinition(
+    persona({ runtime: "buzz-agent" }),
+    [gooseRuntime, buzzAgentRuntime],
+    null,
+    ["buzz-agent"],
+  );
+  assert.equal(runtime.id, "buzz-agent");
+  assert.deepEqual(warnings, []);
+});
+
 test("item-13: no runtimes available — refuses with actionable error", () => {
   assert.throws(
     () => resolveStartRuntimeForDefinition(persona({ runtime: undefined }), []),
diff --git a/desktop/src/features/agents/lib/instanceInputForDefinition.ts b/desktop/src/features/agents/lib/instanceInputForDefinition.ts
index e5012e467..73b24d165 100644
--- a/desktop/src/features/agents/lib/instanceInputForDefinition.ts
+++ b/desktop/src/features/agents/lib/instanceInputForDefinition.ts
@@ -9,6 +9,10 @@ import {
   resolvePersonaRuntime,
   type ResolvePersonaRuntimeResult,
 } from "./resolvePersonaRuntime";
+import {
+  getDisabledAcpRuntimeIdsSnapshot,
+  runtimesForImplicitAcpSelection,
+} from "./runtimeVisibilityPreference";
 import {
   resolveManagedAgentAvatarUrl,
   type UploadMediaBytes,
@@ -46,13 +50,22 @@ export function resolveStartRuntimeForDefinition(
   persona: AgentPersona,
   runtimes: readonly AcpRuntime[],
   preferredRuntimeId?: string | null,
+  disabledRuntimeIds: readonly string[] = getDisabledAcpRuntimeIdsSnapshot(),
 ): { runtime: AcpRuntime; warnings: string[] } {
+  const selectableRuntimes = runtimesForImplicitAcpSelection(
+    runtimes,
+    disabledRuntimeIds,
+    persona.runtime,
+  );
   // Use the buzz-agent-first preference (buzz-agent → goose → first available)
   // so a freshly installed goose never beats the bundled buzz-agent sidecar
   // for runtime-less personas (item 13 regression guard).
-  const defaultRuntime = getDefaultPersonaRuntime(runtimes, preferredRuntimeId);
+  const defaultRuntime = getDefaultPersonaRuntime(
+    selectableRuntimes,
+    preferredRuntimeId,
+  );
   const { runtime, warnings, isOverridden }: ResolvePersonaRuntimeResult =
-    resolvePersonaRuntime(persona.runtime, runtimes, defaultRuntime);
+    resolvePersonaRuntime(persona.runtime, selectableRuntimes, defaultRuntime);
 
   if (!runtime) {
     throw new Error("No available runtime found for this agent.");
diff --git a/desktop/src/features/agents/lib/runtimeVisibilityPreference.ts b/desktop/src/features/agents/lib/runtimeVisibilityPreference.ts
index 361259b39..071176ead 100644
--- a/desktop/src/features/agents/lib/runtimeVisibilityPreference.ts
+++ b/desktop/src/features/agents/lib/runtimeVisibilityPreference.ts
@@ -143,7 +143,7 @@ export function maskDisabledAcpRuntimePreference<
   };
 }
 
-function getDisabledRuntimeIdsSnapshot(): readonly string[] {
+export function getDisabledAcpRuntimeIdsSnapshot(): readonly string[] {
   const storage = getLocalStorage();
   if (!storage) return EMPTY_DISABLED_RUNTIME_IDS;
 
@@ -191,7 +191,7 @@ export function setAcpRuntimeEnabled(
   const storage = getLocalStorage();
   if (!storage) return false;
 
-  const current = getDisabledRuntimeIdsSnapshot();
+  const current = getDisabledAcpRuntimeIdsSnapshot();
   const next = nextDisabledAcpRuntimeIds(current, runtimeId, enabled);
   if (next === current) return true;
 
@@ -211,7 +211,7 @@ export function setAcpRuntimeEnabled(
 export function useDisabledAcpRuntimeIds(): readonly string[] {
   return React.useSyncExternalStore(
     subscribeToRuntimeVisibility,
-    getDisabledRuntimeIdsSnapshot,
+    getDisabledAcpRuntimeIdsSnapshot,
     () => EMPTY_DISABLED_RUNTIME_IDS,
   );
 }
diff --git a/desktop/src/features/agents/ui/useManagedAgentActions.ts b/desktop/src/features/agents/ui/useManagedAgentActions.ts
index b4f216dc8..e1c2e9c9f 100644
--- a/desktop/src/features/agents/ui/useManagedAgentActions.ts
+++ b/desktop/src/features/agents/ui/useManagedAgentActions.ts
@@ -35,14 +35,9 @@ import {
   buildInstanceInputForDefinition,
   resolveStartRuntimeForDefinition,
 } from "../lib/instanceInputForDefinition";
-import {
-  runtimesForImplicitAcpSelection,
-  useDisabledAcpRuntimeIds,
-} from "../lib/runtimeVisibilityPreference";
 
 export function useManagedAgentActions() {
   const { globalConfig } = useGlobalAgentConfig();
-  const disabledRuntimeIds = useDisabledAcpRuntimeIds();
   const relayAgentsQuery = useRelayAgentsQuery();
   const managedAgentsQuery = useManagedAgentsQuery();
   const [shouldLoadChannels, setShouldLoadChannels] = React.useState(false);
@@ -197,11 +192,7 @@ export function useManagedAgentActions() {
     setPersonaStartPending(persona.id, true);
     clearFeedback();
     try {
-      const runtimes = runtimesForImplicitAcpSelection(
-        await availableRuntimesForStart(availableRuntimesQuery),
-        disabledRuntimeIds,
-        persona.runtime,
-      );
+      const runtimes = await availableRuntimesForStart(availableRuntimesQuery);
       const { runtime, warnings } = resolveStartRuntimeForDefinition(
         persona,
         runtimes,