diff --git a/desktop/src/features/agents/AGENTS.md b/desktop/src/features/agents/AGENTS.md
index 1ab4ddf65..f7cc56a69 100644
--- a/desktop/src/features/agents/AGENTS.md
+++ b/desktop/src/features/agents/AGENTS.md
@@ -96,6 +96,8 @@ 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.
+   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
    runtime-less deployment or provisioning path. Definition-to-instance starts
    use the stricter `resolveStartRuntimeForDefinition` wrapper; call one of
diff --git a/desktop/src/features/agents/ui/AgentDefinitionDialog.tsx b/desktop/src/features/agents/ui/AgentDefinitionDialog.tsx
index 888ea9fc1..4557de3d7 100644
--- a/desktop/src/features/agents/ui/AgentDefinitionDialog.tsx
+++ b/desktop/src/features/agents/ui/AgentDefinitionDialog.tsx
@@ -68,7 +68,7 @@ import {
   usePersonaModelDiscovery,
 } from "./usePersonaModelDiscovery";
 import { useBakedBuildEnvKeysQuery, useRuntimeFileConfigQuery } from "../hooks";
-import { useAgentDialogDefaults } from "./useAgentDialogDefaults";
+import { useDefinitionAgentDialogDefaults } from "./useAgentDialogDefaults";
 import { AgentDefaultsDialog } from "./AgentDefaultsDialog";
 import { AgentHarnessField } from "./AgentHarnessField";
 import {
@@ -164,7 +164,7 @@ export function AgentDefinitionDialog({
       model: inheritedModelDefault,
     },
     inheritedEnvVars: inheritedEnvVarsForAdvanced,
-  } = useAgentDialogDefaults({ open });
+  } = useDefinitionAgentDialogDefaults(initialValues, open);
   const selectableRuntimes = useSelectableAcpRuntimes(runtimes);
   const defaultRuntime = getDefaultPersonaRuntime(
     selectableRuntimes,
diff --git a/desktop/src/features/agents/ui/AgentInstanceEditDialog.tsx b/desktop/src/features/agents/ui/AgentInstanceEditDialog.tsx
index 52edfcf18..1f8e140f4 100644
--- a/desktop/src/features/agents/ui/AgentInstanceEditDialog.tsx
+++ b/desktop/src/features/agents/ui/AgentInstanceEditDialog.tsx
@@ -367,7 +367,11 @@ export function AgentInstanceEditDialog({
       model: inheritedModelDefault,
     },
     inheritedEnvVars: inheritedEnvVarsForAdvanced,
-  } = useAgentDialogDefaults({ inheritedEnvVars, open });
+  } = useAgentDialogDefaults({
+    configScope: "existing",
+    inheritedEnvVars,
+    open,
+  });
 
   // Runtime/provider-required credential state, derived from the PROSPECTIVE
   // post-submit runtime — see the hook for the inherit-transition rationale.
diff --git a/desktop/src/features/agents/ui/agentConfigOptions.test.mjs b/desktop/src/features/agents/ui/agentConfigOptions.test.mjs
index ffa680006..ed71f6477 100644
--- a/desktop/src/features/agents/ui/agentConfigOptions.test.mjs
+++ b/desktop/src/features/agents/ui/agentConfigOptions.test.mjs
@@ -207,6 +207,20 @@ test("reconcilePreferredRuntimeFallback updates a hidden saved default", () => {
   assert.equal(reconcilePreferredRuntimeFallback(config, "goose"), config);
 });
 
+test("reconcilePreferredRuntimeFallback persists an unsaved displayed fallback", () => {
+  const config = {
+    env_vars: { SHARED: "kept" },
+    provider: "relay-mesh",
+    model: "auto",
+    preferred_runtime: null,
+  };
+
+  assert.deepEqual(reconcilePreferredRuntimeFallback(config, "buzz-agent"), {
+    ...config,
+    preferred_runtime: "buzz-agent",
+  });
+});
+
 // ── getPersonaModelOptions — codex/claude do not use global provider ──────────
 //
 // The discovery call in AgentDefinitionDialog passes
diff --git a/desktop/src/features/agents/ui/agentConfigOptions.tsx b/desktop/src/features/agents/ui/agentConfigOptions.tsx
index 120098c16..2eecb3671 100644
--- a/desktop/src/features/agents/ui/agentConfigOptions.tsx
+++ b/desktop/src/features/agents/ui/agentConfigOptions.tsx
@@ -199,21 +199,25 @@ export function resetConfigForHarnessChange(
 }
 
 /**
- * Align a stale or hidden saved preference with the fallback shown by a
- * runtime selector, clearing values that are not portable across harnesses.
+ * Align a missing, stale, or hidden saved preference with the shown fallback.
+ * A missing preference adopts the current context without clearing its draft;
+ * switching away from a saved harness clears values that are not portable.
  */
 export function reconcilePreferredRuntimeFallback(
   config: GlobalAgentConfig,
   fallbackRuntimeId: string | null,
 ): GlobalAgentConfig {
-  if (
-    !config.preferred_runtime ||
-    !fallbackRuntimeId ||
-    config.preferred_runtime === fallbackRuntimeId
-  ) {
+  if (!fallbackRuntimeId || config.preferred_runtime === fallbackRuntimeId) {
     return config;
   }
 
+  if (!config.preferred_runtime) {
+    return {
+      ...config,
+      preferred_runtime: fallbackRuntimeId,
+    };
+  }
+
   return resetConfigForHarnessChange(config, fallbackRuntimeId);
 }
 
diff --git a/desktop/src/features/agents/ui/useAgentDialogDefaults.test.mjs b/desktop/src/features/agents/ui/useAgentDialogDefaults.test.mjs
new file mode 100644
index 000000000..12e0a02b2
--- /dev/null
+++ b/desktop/src/features/agents/ui/useAgentDialogDefaults.test.mjs
@@ -0,0 +1,42 @@
+import assert from "node:assert/strict";
+import test from "node:test";
+
+import {
+  agentDefinitionConfigScope,
+  resolveAgentDialogGlobalConfig,
+} from "./useAgentDialogDefaults.ts";
+
+const persistedConfig = {
+  env_vars: { SHARED: "kept" },
+  provider: "relay-mesh",
+  model: "auto",
+  preferred_runtime: "buzz-agent",
+};
+
+test("create-mode defaults mask values owned by a hidden harness", () => {
+  assert.deepEqual(
+    resolveAgentDialogGlobalConfig(persistedConfig, "implicit", ["buzz-agent"]),
+    {
+      env_vars: { SHARED: "kept" },
+      provider: null,
+      model: null,
+      preferred_runtime: null,
+    },
+  );
+});
+
+test("existing edit defaults preserve values owned by a hidden harness", () => {
+  assert.equal(
+    resolveAgentDialogGlobalConfig(persistedConfig, "existing", ["buzz-agent"]),
+    persistedConfig,
+  );
+});
+
+test("definition dialogs select config scope from create versus edit values", () => {
+  assert.equal(agentDefinitionConfigScope(null), "implicit");
+  assert.equal(agentDefinitionConfigScope({ displayName: "New" }), "implicit");
+  assert.equal(
+    agentDefinitionConfigScope({ id: "existing", displayName: "Existing" }),
+    "existing",
+  );
+});
diff --git a/desktop/src/features/agents/ui/useAgentDialogDefaults.ts b/desktop/src/features/agents/ui/useAgentDialogDefaults.ts
index 5ede9558f..f9fbbd31c 100644
--- a/desktop/src/features/agents/ui/useAgentDialogDefaults.ts
+++ b/desktop/src/features/agents/ui/useAgentDialogDefaults.ts
@@ -1,18 +1,57 @@
 import * as React from "react";
 
+import type {
+  CreatePersonaInput,
+  GlobalAgentConfig,
+  UpdatePersonaInput,
+} from "@/shared/api/types";
 import { useBakedBuildEnvQuery } from "../hooks";
+import {
+  maskDisabledAcpRuntimePreference,
+  useDisabledAcpRuntimeIds,
+} from "../lib/runtimeVisibilityPreference";
 import { useGlobalAgentConfig } from "../useGlobalAgentConfig";
 import { BUZZ_AGENT_THINKING_EFFORT } from "./buzzAgentConfig";
 import { getInheritedAgentDefaults } from "./bakedEnvHelpers";
 
+export type AgentDialogConfigScope = "existing" | "implicit";
+
+export function agentDefinitionConfigScope(
+  initialValues: CreatePersonaInput | UpdatePersonaInput | null,
+): AgentDialogConfigScope {
+  return initialValues && "id" in initialValues ? "existing" : "implicit";
+}
+
+export function resolveAgentDialogGlobalConfig(
+  persistedConfig: GlobalAgentConfig,
+  configScope: AgentDialogConfigScope,
+  disabledRuntimeIds: readonly string[],
+): GlobalAgentConfig {
+  return configScope === "implicit"
+    ? maskDisabledAcpRuntimePreference(persistedConfig, disabledRuntimeIds)
+    : persistedConfig;
+}
+
 export function useAgentDialogDefaults({
+  configScope,
   inheritedEnvVars = {},
   open,
 }: {
+  configScope: AgentDialogConfigScope;
   inheritedEnvVars?: Record<string, string>;
   open: boolean;
 }) {
-  const { globalConfig } = useGlobalAgentConfig();
+  const { globalConfig: persistedConfig } = useGlobalAgentConfig();
+  const disabledRuntimeIds = useDisabledAcpRuntimeIds();
+  const globalConfig = React.useMemo(
+    () =>
+      resolveAgentDialogGlobalConfig(
+        persistedConfig,
+        configScope,
+        disabledRuntimeIds,
+      ),
+    [configScope, disabledRuntimeIds, persistedConfig],
+  );
   const { data: bakedEnv } = useBakedBuildEnvQuery({ enabled: open });
   const inheritedDefaults = getInheritedAgentDefaults(globalConfig, bakedEnv);
   const effectiveInheritedEnvVars = React.useMemo(
@@ -31,3 +70,13 @@ export function useAgentDialogDefaults({
     inheritedEnvVars: effectiveInheritedEnvVars,
   };
 }
+
+export function useDefinitionAgentDialogDefaults(
+  initialValues: CreatePersonaInput | UpdatePersonaInput | null,
+  open: boolean,
+) {
+  return useAgentDialogDefaults({
+    configScope: agentDefinitionConfigScope(initialValues),
+    open,
+  });
+}