diff --git a/packages/server/src/server/agent/providers/acp-agent.test.ts b/packages/server/src/server/agent/providers/acp-agent.test.ts
index c8a24085f9..ae7ede3579 100644
--- a/packages/server/src/server/agent/providers/acp-agent.test.ts
+++ b/packages/server/src/server/agent/providers/acp-agent.test.ts
@@ -1956,6 +1956,24 @@ describe("ACPAgentSession options against an ACP agent", () => {
       ]),
     );
   });
+
+  test("a select that appears after startup is reported with its native value", async () => {
+    const { session, optionEvents } = await openSession({
+      scenario: {
+        options: [thoughtOption("low")],
+        adds: { thought: [copilotAgentConfigOption("Probe Agent")] },
+      },
+    });
+    expect(session.openOptionsPatch).toEqual({});
+
+    await session.setThinkingOption("high");
+
+    await vi.waitFor(() =>
+      expect(optionEvents).toEqual([
+        { type: "options_changed", provider: "copilot", patch: { agent: "Probe Agent" } },
+      ]),
+    );
+  });
 });
 
 describe("deriveModelDefinitionsFromACP", () => {
diff --git a/packages/server/src/server/agent/providers/acp-agent.ts b/packages/server/src/server/agent/providers/acp-agent.ts
index f48ae85361..1055a56a2c 100644
--- a/packages/server/src/server/agent/providers/acp-agent.ts
+++ b/packages/server/src/server/agent/providers/acp-agent.ts
@@ -2467,12 +2467,10 @@ export class ACPAgentSession implements AgentSession, ACPClient {
     return values;
   }
 
-  /** The selects whose value is not what `expected` says; selects it does not name are skipped. */
+  /** The selects whose value is not what `expected` says, including ones it does not name. */
   private featureDifferences(expected: Record<string, unknown>): Record<string, string> {
     return Object.fromEntries(
-      Object.entries(this.featureValues()).filter(
-        ([id, value]) => id in expected && expected[id] !== value,
-      ),
+      Object.entries(this.featureValues()).filter(([id, value]) => expected[id] !== value),
     );
   }
 
@@ -3130,11 +3128,7 @@ export class ACPAgentSession implements AgentSession, ACPClient {
     }
     // The options are the state, so every select the session opened with is reported, including
     // ones nobody set (a native default is stored like any other value).
-    this.openOptionsPatch = Object.fromEntries(
-      Object.entries(this.featureValues()).filter(
-        ([id, value]) => configuredFeatureValues[id] !== value,
-      ),
-    );
+    this.openOptionsPatch = this.featureDifferences(configuredFeatureValues);
   }
 
   /**
diff --git a/packages/server/src/server/agent/providers/test-utils/fake-acp-agent.mjs b/packages/server/src/server/agent/providers/test-utils/fake-acp-agent.mjs
index 1ea77896f2..e284a326da 100644
--- a/packages/server/src/server/agent/providers/test-utils/fake-acp-agent.mjs
+++ b/packages/server/src/server/agent/providers/test-utils/fake-acp-agent.mjs
@@ -5,6 +5,7 @@
 //            "notify-then-reject" does the same and then fails the write
 //   moves:   { [configId]: { [id]: value } } writes that also move another select, announced
 //            by a config_option_update after the write is answered
+//   adds:    { [configId]: option[] } writes that make more selects appear, announced the same way
 import { Readable, Writable } from "node:stream";
 import { AgentSideConnection, ndJsonStream } from "@agentclientprotocol/sdk";
 
@@ -39,8 +40,10 @@ export const connection = new AgentSideConnection(
         return { configOptions: options };
       }
       const moves = scenario.moves?.[configId];
-      if (moves) {
-        for (const [id, moved] of Object.entries(moves)) set(id, moved);
+      const adds = scenario.adds?.[configId];
+      if (moves || adds) {
+        for (const [id, moved] of Object.entries(moves ?? {})) set(id, moved);
+        options = [...options, ...(adds ?? [])];
         setTimeout(() => void announce(), 20);
       }
       return { configOptions: options };