fix: disable session_config in JB IDEs v2026.1

src/CodexAcpServer.ts

@@ -42,6 +42,7 @@ import {
     resolveFastServiceTier,
 } from "./FastModeConfig";
 import packageJson from "../package.json";
+import {isJetBrains2026_1Client} from "./JBUtils";
 
 export interface SessionState {
     sessionId: string,
@@ -90,6 +91,7 @@ export class CodexAcpServer implements acp.Agent {
     private readonly defaultAuthRequest: CodexAuthRequest | null;
     private readonly getExitCode: () => number | null;
     private readonly availableCommands: CodexCommands;
+    private clientInfo: acp.Implementation | null;
 
     private readonly sessions: Map<string, SessionState>;
     private readonly pendingMcpStartupSessions: Map<string, PendingMcpStartupSession>;
@@ -116,6 +118,7 @@ export class CodexAcpServer implements acp.Agent {
         this.codexAcpClient = codexAcpClient;
         this.defaultAuthRequest = defaultAuthRequest ?? null;
         this.getExitCode = getExitCode ?? (() => null);
+        this.clientInfo = null;
         this.availableCommands = new CodexCommands(
             connection,
             codexAcpClient,
@@ -127,6 +130,7 @@ export class CodexAcpServer implements acp.Agent {
         _params: acp.InitializeRequest,
     ): Promise<acp.InitializeResponse> {
         logger.log("Initialize request received");
+        this.clientInfo = _params.clientInfo ?? null;
         await this.runWithProcessCheck(() => this.codexAcpClient.initialize(_params));
         return {
             protocolVersion: acp.PROTOCOL_VERSION,
@@ -382,7 +386,7 @@ export class CodexAcpServer implements acp.Agent {
         return {
             models: modelState,
             modes: modeState,
-            configOptions: this.createSessionConfigOptions(this.getSessionState(sessionId)),
+            ...this.createSessionConfigOptionsResponse(this.getSessionState(sessionId)),
         };
     }
 
@@ -398,7 +402,7 @@ export class CodexAcpServer implements acp.Agent {
         return {
             models: modelState,
             modes: modeState,
-            configOptions: this.createSessionConfigOptions(this.getSessionState(sessionId)),
+            ...this.createSessionConfigOptionsResponse(this.getSessionState(sessionId)),
         };
     }
 
@@ -458,7 +462,7 @@ export class CodexAcpServer implements acp.Agent {
             sessionId: sessionId,
             models: modelState,
             modes: modeState,
-            configOptions: this.createSessionConfigOptions(this.getSessionState(sessionId)),
+            ...this.createSessionConfigOptionsResponse(this.getSessionState(sessionId)),
         };
     }
 
@@ -567,6 +571,22 @@ export class CodexAcpServer implements acp.Agent {
         ];
     }
 
+    private createSessionConfigOptionsResponse(sessionState: SessionState): {
+        configOptions?: Array<acp.SessionConfigOption>;
+    } {
+        if (!this.isSessionConfigEnabled()) {
+            return {};
+        }
+        return {
+            configOptions: this.createSessionConfigOptions(sessionState),
+        };
+    }
+
+    private isSessionConfigEnabled(): boolean {
+        // Temporarily disabled for JB IDEs 2026.1 due to issues in session_config (LLM-28118)
+        return !isJetBrains2026_1Client(this.clientInfo);
+    }
+
     private publishAvailableCommandsAsync(sessionId: string) {
         void this.availableCommands.publish(sessionId);
     }

src/JBUtils.ts

@@ -0,0 +1,12 @@
+import type * as acp from "@agentclientprotocol/sdk";
+
+export function isJetBrains2026_1Client(clientInfo: acp.Implementation | null): boolean {
+    if (!clientInfo) {
+        return false;
+    }
+
+    const platform = clientInfo._meta?.["platform"];
+    const isIntelliJPlatform = platform === "intellij";
+    const isJetBrainsClient = clientInfo.name.startsWith("JetBrains");
+    return (isIntelliJPlatform || isJetBrainsClient) && clientInfo.version.startsWith("2026.1");
+}

src/__tests__/CodexACPAgent/fast-mode-config.test.ts

@@ -1,4 +1,5 @@
 import {describe, expect, it, vi} from "vitest";
+import * as acp from "@agentclientprotocol/sdk";
 import {
     createCodexMockTestFixture,
     createTestModel,
@@ -13,7 +14,10 @@ import {
 } from "../../FastModeConfig";
 
 describe("Fast mode session config", () => {
-    async function createSession(currentServiceTier: "fast" | "flex" | null = null) {
+    async function createSession(
+        currentServiceTier: "fast" | "flex" | null = null,
+        clientInfo: acp.Implementation | null = null
+    ) {
         const fixture = createCodexMockTestFixture();
         const codexAcpAgent = fixture.getCodexAcpAgent();
         const codexAcpClient = fixture.getCodexAcpClient();
@@ -31,6 +35,11 @@ describe("Fast mode session config", () => {
             currentServiceTier,
         });
 
+        await codexAcpAgent.initialize({
+            protocolVersion: acp.PROTOCOL_VERSION,
+            clientInfo,
+        });
+
         const response = await codexAcpAgent.newSession({cwd: "/test/cwd", mcpServers: []});
         return {fixture, codexAcpAgent, codexAcpClient, response};
     }
@@ -58,6 +67,55 @@ describe("Fast mode session config", () => {
         expect(codexAcpAgent.getSessionState("session-id").fastModeEnabled).toBe(true);
     });
 
+    it("omits Fast mode config options for JetBrains 2026.1 IntelliJ clients", async () => {
+        const {response} = await createSession(null, {
+            name: "JetBrains.WebStorm",
+            version: "2026.1.1",
+            title: "WebStorm 2026.1.1",
+            _meta: {
+                platform: "intellij",
+            },
+        });
+
+        expect(response.configOptions).toBeUndefined();
+    });
+
+    it("omits Fast mode config options for JetBrains 2026.1 clients by name", async () => {
+        const {response} = await createSession(null, {
+            name: "JetBrains.IDE",
+            version: "2026.1",
+            title: "JetBrains IDE",
+        });
+
+        expect(response.configOptions).toBeUndefined();
+    });
+
+    it("keeps Fast mode config options for JetBrains clients outside 2026.1", async () => {
+        const {response} = await createSession(null, {
+            name: "JetBrains.WebStorm",
+            version: "2026.2.0",
+            title: "WebStorm 2026.2.0",
+            _meta: {
+                platform: "intellij",
+            },
+        });
+
+        expect(response.configOptions).toEqual([createFastModeConfigOption(false)]);
+    });
+
+    it("keeps Fast mode config options for non-JetBrains 2026.1 clients", async () => {
+        const {response} = await createSession(null, {
+            name: "VSCode",
+            version: "2026.1.1",
+            title: "VS Code",
+            _meta: {
+                platform: "vscode",
+            },
+        });
+
+        expect(response.configOptions).toEqual([createFastModeConfigOption(false)]);
+    });
+
     it("toggles Fast mode through session config options", async () => {
         const {codexAcpAgent} = await createSession();