diff --git a/readme-dev.md b/readme-dev.md
index bc147807..2de6d510 100644
--- a/readme-dev.md
+++ b/readme-dev.md
@@ -6,7 +6,8 @@ Set `CODEX_PATH` to run a different Codex binary; versions other than the one sp
 - `CODEX_API_KEY` - API key used when the API-key auth method is selected. Takes precedence over `OPENAI_API_KEY`.
 - `OPENAI_API_KEY` - fallback API key used when the API-key auth method is selected.
 - `CODEX_PATH` - run a specific Codex executable instead of the bundled package dependency.
-- `CODEX_CONFIG` - JSON object merged into the Codex session config.
+- `CODEX_CONFIG` - JSON object merged into the Codex session config. Supported keys include:
+  - `model_reasoning_summary` - controls reasoning summary verbosity per turn. Valid values: `"auto"`, `"concise"`, `"detailed"`, `"none"`. Defaults to `"auto"` when unset. Example: `CODEX_CONFIG='{"model_reasoning_summary":"detailed"}'`.
 - `MODEL_PROVIDER` - model provider to pass to Codex for new sessions.
 - `DEFAULT_AUTH_REQUEST` - ACP auth request JSON used when Codex requires authentication.
 - `INITIAL_AGENT_MODE` - initial mode id: `read-only`, `agent`, or `agent-full-access`.
diff --git a/src/CodexAcpClient.ts b/src/CodexAcpClient.ts
index 2ea875b5..3109340c 100644
--- a/src/CodexAcpClient.ts
+++ b/src/CodexAcpClient.ts
@@ -13,6 +13,7 @@ import type {Disposable} from "vscode-jsonrpc";
 import type {
     ClientInfo,
     ReasoningEffort,
+    ReasoningSummary,
     ServiceTier,
     ServerNotification
 } from "./app-server";
@@ -672,7 +673,7 @@ export class CodexAcpClient {
             input: input,
             approvalPolicy: agentMode.approvalPolicy,
             sandboxPolicy: addAdditionalDirectoriesToSandboxPolicy(agentMode.sandboxPolicy, additionalDirectories),
-            summary: disableSummary ? "none" : "auto",
+            summary: disableSummary ? "none" : (resolveConfiguredSummary(this.config) ?? "auto"),
             effort: effort,
             model: modelId.model,
             serviceTier: serviceTier,
@@ -854,6 +855,16 @@ export type SessionMetadataWithThread = SessionMetadata & {
     thread: Thread,
 }
 
+const validReasoningSummaries: ReadonlySet<string> = new Set<ReasoningSummary>(["auto", "concise", "detailed", "none"]);
+
+function resolveConfiguredSummary(config: JsonObject): ReasoningSummary | undefined {
+    const value = config["model_reasoning_summary"];
+    if (typeof value === "string" && validReasoningSummaries.has(value)) {
+        return value as ReasoningSummary;
+    }
+    return undefined;
+}
+
 function buildPromptItems(prompt: acp.ContentBlock[]): UserInput[] {
     return prompt.map((block): UserInput | null => {
         switch (block.type) {
diff --git a/src/__tests__/CodexACPAgent/CodexAcpClient.test.ts b/src/__tests__/CodexACPAgent/CodexAcpClient.test.ts
index 255d4ab2..802282ea 100644
--- a/src/__tests__/CodexACPAgent/CodexAcpClient.test.ts
+++ b/src/__tests__/CodexACPAgent/CodexAcpClient.test.ts
@@ -11,6 +11,7 @@ import {
     type TestFixture
 } from "../acp-test-utils";
 import type {ServerNotification} from "../../app-server";
+import type {JsonObject} from "../../CodexAcpClient";
 import type {SessionState} from "../../CodexAcpServer";
 import {AgentMode} from "../../AgentMode";
 import type {Model, ReviewStartResponse, ThreadGoal, TurnCompletedNotification, TurnStartParams} from "../../app-server/v2";
@@ -2968,8 +2969,8 @@ describe('ACP server test', { timeout: 40_000 }, () => {
      * Sets up a mock fixture with turnStart/awaitTurnCompleted spied on,
      * and a given session state. Returns the fixture and turnStart spy.
      */
-    function setupPromptFixture(sessionOverrides?: Partial<SessionState>) {
-        const mockFixture = createCodexMockTestFixture();
+    function setupPromptFixture(sessionOverrides?: Partial<SessionState>, options?: { codexConfig?: JsonObject }) {
+        const mockFixture = createCodexMockTestFixture(options);
         const sessionState = createTestSessionState(sessionOverrides);
         const turnStartSpy = vi.spyOn(mockFixture.getCodexAppServerClient(), "turnStart").mockResolvedValue({
             turn: {
@@ -3092,6 +3093,36 @@ describe('ACP server test', { timeout: 40_000 }, () => {
         expect(turnStartSpy).toHaveBeenCalledWith(expect.objectContaining({ summary: "auto" }));
     });
 
+    it ('should honor configured model_reasoning_summary from CODEX_CONFIG', async () => {
+        const { mockFixture, turnStartSpy } = setupPromptFixture({
+            account: { type: "chatgpt", email: "test@example.com", planType: "pro" },
+        }, { codexConfig: { model_reasoning_summary: "detailed" } });
+
+        await mockFixture.getCodexAcpAgent().prompt({ sessionId: "id", prompt: [{ type: "text", text: "test" }] });
+
+        expect(turnStartSpy).toHaveBeenCalledWith(expect.objectContaining({ summary: "detailed" }));
+    });
+
+    it ('should fall back to auto for invalid model_reasoning_summary value', async () => {
+        const { mockFixture, turnStartSpy } = setupPromptFixture({
+            account: { type: "chatgpt", email: "test@example.com", planType: "pro" },
+        }, { codexConfig: { model_reasoning_summary: "verbose" } });
+
+        await mockFixture.getCodexAcpAgent().prompt({ sessionId: "id", prompt: [{ type: "text", text: "test" }] });
+
+        expect(turnStartSpy).toHaveBeenCalledWith(expect.objectContaining({ summary: "auto" }));
+    });
+
+    it ('should force none when disableSummary is true regardless of configured summary', async () => {
+        const { mockFixture, turnStartSpy } = setupPromptFixture({
+            account: { type: "apiKey" },
+        }, { codexConfig: { model_reasoning_summary: "detailed" } });
+
+        await mockFixture.getCodexAcpAgent().prompt({ sessionId: "id", prompt: [{ type: "text", text: "test" }] });
+
+        expect(turnStartSpy).toHaveBeenCalledWith(expect.objectContaining({ summary: "none" }));
+    });
+
     it ('should reject prompt with images when model does not support image input', async () => {
         const { mockFixture } = setupPromptFixture({
             supportedInputModalities: ["text"],
diff --git a/src/__tests__/acp-test-utils.ts b/src/__tests__/acp-test-utils.ts
index 13505155..9c6a3399 100644
--- a/src/__tests__/acp-test-utils.ts
+++ b/src/__tests__/acp-test-utils.ts
@@ -1,6 +1,6 @@
 import * as acp from "@agentclientprotocol/sdk";
 import type {CreateElicitationResponse, McpServerStdio, RequestPermissionResponse} from "@agentclientprotocol/sdk";
-import {CodexAcpClient} from '../CodexAcpClient';
+import {CodexAcpClient, type JsonObject} from '../CodexAcpClient';
 import {CodexAppServerClient, type CodexConnectionEvent} from '../CodexAppServerClient';
 import {startCodexConnection} from "../CodexJsonRpcConnection";
 import {CodexAcpServer, type SessionState} from "../CodexAcpServer";
@@ -84,6 +84,7 @@ export interface ConnectionConfig {
     connection: MessageConnection;
     getExitCode: () => number | null;
     acpConnection?: AcpConnectionConfig;
+    codexConfig?: JsonObject;
 }
 
 export function createBaseTestFixture(config: ConnectionConfig): TestFixture {
@@ -96,7 +97,7 @@ export function createBaseTestFixture(config: ConnectionConfig): TestFixture {
     });
 
     const codexAppServerClient = new CodexAppServerClient(config.connection);
-    const codexAcpClient = new CodexAcpClient(codexAppServerClient);
+    const codexAcpClient = new CodexAcpClient(codexAppServerClient, config.codexConfig);
     const codexAcpAgent = new CodexAcpServer(acpConnection, codexAcpClient, undefined, config.getExitCode);
 
     const transportEvents: CodexConnectionEvent[] = [];
@@ -254,7 +255,7 @@ export interface CodexMockTestFixture extends TestFixture {
  * Provides `sendServerRequest()` to simulate server-initiated requests (e.g., approval requests).
  * Provides `setPermissionResponse()` to control ACP permission dialog responses.
  */
-export function createCodexMockTestFixture(): CodexMockTestFixture {
+export function createCodexMockTestFixture(options?: { codexConfig?: JsonObject }): CodexMockTestFixture {
     let unhandledNotificationHandler: ((notification: any) => void) | null = null;
     const requestHandlers = new Map<string, (params: unknown) => Promise<unknown>>();
 
@@ -306,7 +307,8 @@ export function createCodexMockTestFixture(): CodexMockTestFixture {
             connection: acpConnection,
             events: acpConnectionEvents,
             eventHandlers: acpEventHandlers,
-        }
+        },
+        ...(options?.codexConfig != null ? { codexConfig: options.codexConfig } : {}),
     });
 
     return {