diff --git a/apps/server/src/serverSettings.test.ts b/apps/server/src/serverSettings.test.ts
index f26fece246..c0aec009a0 100644
--- a/apps/server/src/serverSettings.test.ts
+++ b/apps/server/src/serverSettings.test.ts
@@ -104,6 +104,43 @@ it.layer(NodeServices.layer)("server settings", (it) => {
     }).pipe(Effect.provide(makeServerSettingsLayer())),
   );
 
+  it.effect("preserves model when switching providers via textGenerationModelSelection", () =>
+    Effect.gen(function* () {
+      const serverSettings = yield* ServerSettingsService;
+
+      // Start with Claude text generation selection
+      yield* serverSettings.updateSettings({
+        textGenerationModelSelection: {
+          provider: "claudeAgent",
+          model: "claude-sonnet-4-6",
+          options: {
+            effort: "high",
+          },
+        },
+      });
+
+      // Switch to Codex — the stale Claude "effort" in options must not
+      // cause the update to lose the selected model.
+      const next = yield* serverSettings.updateSettings({
+        textGenerationModelSelection: {
+          provider: "codex",
+          model: "gpt-5.4",
+          options: {
+            reasoningEffort: "high",
+          },
+        },
+      });
+
+      assert.deepEqual(next.textGenerationModelSelection, {
+        provider: "codex",
+        model: "gpt-5.4",
+        options: {
+          reasoningEffort: "high",
+        },
+      });
+    }).pipe(Effect.provide(makeServerSettingsLayer())),
+  );
+
   it.effect("trims provider path settings when updates are applied", () =>
     Effect.gen(function* () {
       const serverSettings = yield* ServerSettingsService;
diff --git a/apps/server/src/serverSettings.ts b/apps/server/src/serverSettings.ts
index f638e7fdfa..d5df84724e 100644
--- a/apps/server/src/serverSettings.ts
+++ b/apps/server/src/serverSettings.ts
@@ -27,6 +27,7 @@ import {
   FileSystem,
   Layer,
   Path,
+  Equal,
   PubSub,
   Ref,
   Schema,
@@ -130,6 +131,9 @@ function resolveTextGenerationProvider(settings: ServerSettings): ServerSettings
   };
 }
 
+// Values under these keys are compared as a whole — never stripped field-by-field.
+const ATOMIC_SETTINGS_KEYS: ReadonlySet<string> = new Set(["textGenerationModelSelection"]);
+
 function stripDefaultServerSettings(current: unknown, defaults: unknown): unknown | undefined {
   if (Array.isArray(current) || Array.isArray(defaults)) {
     return JSON.stringify(current) === JSON.stringify(defaults) ? undefined : current;
@@ -146,9 +150,15 @@ function stripDefaultServerSettings(current: unknown, defaults: unknown): unknow
     const next: Record<string, unknown> = {};
 
     for (const key of Object.keys(currentRecord)) {
-      const stripped = stripDefaultServerSettings(currentRecord[key], defaultsRecord[key]);
-      if (stripped !== undefined) {
-        next[key] = stripped;
+      if (ATOMIC_SETTINGS_KEYS.has(key)) {
+        if (!Equal.equals(currentRecord[key], defaultsRecord[key])) {
+          next[key] = currentRecord[key];
+        }
+      } else {
+        const stripped = stripDefaultServerSettings(currentRecord[key], defaultsRecord[key]);
+        if (stripped !== undefined) {
+          next[key] = stripped;
+        }
       }
     }