diff --git a/packages/server/src/server/agent/providers/acp-agent.ts b/packages/server/src/server/agent/providers/acp-agent.ts
index caff01eb0..c2090540d 100644
--- a/packages/server/src/server/agent/providers/acp-agent.ts
+++ b/packages/server/src/server/agent/providers/acp-agent.ts
@@ -107,6 +107,11 @@ const ACP_CLIENT_CAPABILITIES: ACPClientCapabilities = {
 
 const COPILOT_AUTOPILOT_MODE = "https://agentclientprotocol.com/protocol/session-modes#autopilot";
 
+// Suppress interactive auth side-effects (e.g. Gemini CLI opening a Google
+// sign-in URL in the browser) when probing an ACP agent for models/modes.
+// NO_BROWSER is honored by Gemini CLI; other ACP agents ignore it.
+const PROBE_ENV: Record<string, string> = { NO_BROWSER: "true" };
+
 type ACPAgentClientOptions = {
   provider: string;
   logger: Logger;
@@ -391,7 +396,7 @@ export class ACPAgentClient implements AgentClient {
 
   async listModels(options?: ListModelsOptions): Promise<AgentModelDefinition[]> {
     const cwd = options?.cwd ?? process.cwd();
-    const probe = await this.spawnProcess(undefined);
+    const probe = await this.spawnProcess(PROBE_ENV);
     try {
       const response = await probe.connection.newSession({
         cwd,
@@ -411,7 +416,7 @@ export class ACPAgentClient implements AgentClient {
 
   async listModes(options?: ListModesOptions): Promise<AgentMode[]> {
     const cwd = options?.cwd ?? process.cwd();
-    const probe = await this.spawnProcess(undefined);
+    const probe = await this.spawnProcess(PROBE_ENV);
     try {
       const response = await probe.connection.newSession({
         cwd,
@@ -432,7 +437,7 @@ export class ACPAgentClient implements AgentClient {
   async listPersistedAgents(
     options?: ListPersistedAgentsOptions,
   ): Promise<PersistedAgentDescriptor[]> {
-    const probe = await this.spawnProcess(undefined);
+    const probe = await this.spawnProcess(PROBE_ENV);
     try {
       if (!probe.initialize.agentCapabilities?.sessionCapabilities?.list) {
         return [];