fix(acp): preserve runtime option clears (#101044)

* fix(acp): preserve runtime option clears

* refactor(acp): simplify runtime option clears

---------

Co-authored-by: Peter Steinberger <steipete@gmail.com>

src/acp/control-plane/runtime-options.test.ts

@@ -1,6 +1,55 @@
 /** Tests runtime config-option serialization against advertised backend keys. */
 import { describe, expect, it } from "vitest";
-import { buildRuntimeConfigOptionPairs } from "./runtime-options.js";
+import type { AcpSessionRuntimeOptions } from "../../config/sessions/types.js";
+import { buildRuntimeConfigOptionPairs, mergeRuntimeOptions } from "./runtime-options.js";
+
+describe("mergeRuntimeOptions", () => {
+  it("clears top-level options when a patch explicitly sets them to undefined", () => {
+    const patch = {
+      runtimeMode: undefined,
+      model: undefined,
+      thinking: undefined,
+      cwd: undefined,
+      permissionProfile: undefined,
+      timeoutSeconds: undefined,
+    } as Partial<AcpSessionRuntimeOptions>;
+
+    expect(
+      mergeRuntimeOptions({
+        current: {
+          runtimeMode: "plan",
+          model: "claude-sonnet-4.6",
+          thinking: "high",
+          cwd: "/tmp/project",
+          permissionProfile: "trusted",
+          timeoutSeconds: 120,
+        },
+        patch,
+      }),
+    ).toEqual({});
+  });
+
+  it("clears backend extras when a patch explicitly clears them", () => {
+    expect(
+      mergeRuntimeOptions({
+        current: {
+          model: "claude-sonnet-4.6",
+          backendExtras: { provider: "anthropic", profile: "work" },
+        },
+        patch: { backendExtras: undefined } as Partial<AcpSessionRuntimeOptions>,
+      }),
+    ).toEqual({ model: "claude-sonnet-4.6" });
+  });
+
+  it("keeps merging backend extras when a patch provides new entries", () => {
+    expect(
+      mergeRuntimeOptions({
+        current: { backendExtras: { provider: "anthropic" } },
+        patch: { backendExtras: { profile: "work" } },
+      }),
+    ).toEqual({ backendExtras: { provider: "anthropic", profile: "work" } });
+  });
+});
 
 describe("buildRuntimeConfigOptionPairs timeout advertisement", () => {
   it("omits the timeout pair when advertised keys exclude every timeout alias", () => {

src/acp/control-plane/runtime-options.ts

@@ -277,15 +277,13 @@ export function mergeRuntimeOptions(params: {
   patch?: Partial<AcpSessionRuntimeOptions>;
 }): AcpSessionRuntimeOptions {
   const current = normalizeRuntimeOptions(params.current);
-  const patch = normalizeRuntimeOptions(validateRuntimeOptionPatch(params.patch));
-  const mergedExtras = {
-    ...current.backendExtras,
-    ...patch.backendExtras,
-  };
+  const patch = validateRuntimeOptionPatch(params.patch);
   return normalizeRuntimeOptions({
     ...current,
     ...patch,
-    ...(Object.keys(mergedExtras).length > 0 ? { backendExtras: mergedExtras } : {}),
+    ...(patch.backendExtras
+      ? { backendExtras: { ...current.backendExtras, ...patch.backendExtras } }
+      : {}),
   });
 }