diff --git a/src/CodexAcpClient.ts b/src/CodexAcpClient.ts
index 7040057f..adc349fe 100644
--- a/src/CodexAcpClient.ts
+++ b/src/CodexAcpClient.ts
@@ -773,9 +773,13 @@ export class CodexAcpClient {
             return configWithWorkspaceRoots;
         }
 
+        const configuredMcpServers = configWithWorkspaceRoots["mcp_servers"];
         return {
             ...configWithWorkspaceRoots,
-            "mcp_servers": Object.fromEntries(serversToConfigure.map(mcp => [mcp.name, this.createMcpSeverConfig(mcp.server)])),
+            "mcp_servers": {
+                ...(isJsonObject(configuredMcpServers) ? configuredMcpServers : {}),
+                ...Object.fromEntries(serversToConfigure.map(mcp => [mcp.name, this.createMcpSeverConfig(mcp.server)])),
+            },
         };
     }
 
diff --git a/src/__tests__/CodexACPAgent/mcp-config-overrides.test.ts b/src/__tests__/CodexACPAgent/mcp-config-overrides.test.ts
new file mode 100644
index 00000000..3316b2e5
--- /dev/null
+++ b/src/__tests__/CodexACPAgent/mcp-config-overrides.test.ts
@@ -0,0 +1,126 @@
+import {afterEach, beforeEach, describe, expect, it, vi} from "vitest";
+import type {McpServer} from "@agentclientprotocol/sdk";
+import {CodexAcpClient} from "../../CodexAcpClient";
+import type {CodexAppServerClient} from "../../CodexAppServerClient";
+import type {ConfigReadResponse, ThreadStartResponse} from "../../app-server/v2";
+import {createCodexMockTestFixture, createTestModel} from "../acp-test-utils";
+
+describe("MCP overrides from CODEX_CONFIG", () => {
+    let appServerClient: CodexAppServerClient;
+    const browserOverrides = {
+        env: {
+            NODE_USE_ENV_PROXY: "1",
+            HTTPS_PROXY: "http://127.0.0.1:7890",
+        },
+    };
+    const docsServer: McpServer = {
+        type: "http",
+        name: "docs",
+        url: "http://127.0.0.1:3000/mcp",
+        headers: [],
+    };
+    const docsConfig = {url: docsServer.url, http_headers: {}};
+
+    beforeEach(() => {
+        vi.stubEnv("DISABLE_MCP_CONFIG_FILTERING", "false");
+        appServerClient = createCodexMockTestFixture().getCodexAppServerClient();
+        vi.spyOn(appServerClient, "configRead").mockResolvedValue({
+            config: {
+                mcp_servers: {
+                    node_repl: {command: "node", args: ["browser-mcp.js"]},
+                },
+            },
+            origins: {},
+            layers: [],
+        } as unknown as ConfigReadResponse);
+        vi.spyOn(appServerClient, "threadStart").mockResolvedValue({
+            thread: {id: "thread-id"},
+            model: "gpt-5",
+            reasoningEffort: "medium",
+            serviceTier: null,
+        } as ThreadStartResponse);
+        vi.spyOn(appServerClient, "listModels").mockResolvedValue({
+            data: [createTestModel({id: "gpt-5"})],
+            nextCursor: null,
+        });
+    });
+
+    afterEach(() => {
+        vi.unstubAllEnvs();
+        vi.restoreAllMocks();
+    });
+
+    it("preserves env-only overrides when the session supplies an unrelated MCP server", async () => {
+        const client = new CodexAcpClient(appServerClient, {
+            mcp_servers: {node_repl: browserOverrides},
+        });
+
+        await client.newSession({cwd: "", mcpServers: [docsServer]});
+
+        expect(vi.mocked(appServerClient.threadStart).mock.calls[0]![0].config?.["mcp_servers"]).toEqual({
+            node_repl: browserOverrides,
+            docs: docsConfig,
+        });
+    });
+
+    it("does not carry session MCP entries into later sessions", async () => {
+        const config = {mcp_servers: {node_repl: structuredClone(browserOverrides)}};
+        const client = new CodexAcpClient(appServerClient, config);
+
+        await client.newSession({cwd: "", mcpServers: [docsServer]});
+        await client.newSession({cwd: "", mcpServers: []});
+
+        expect(config).toEqual({mcp_servers: {node_repl: browserOverrides}});
+        expect(vi.mocked(appServerClient.threadStart).mock.calls[1]![0].config?.["mcp_servers"]).toEqual({
+            node_repl: browserOverrides,
+        });
+    });
+
+    it("replaces a same-name startup entry without mixing incompatible transports", async () => {
+        const client = new CodexAcpClient(appServerClient, {
+            mcp_servers: {docs: docsConfig},
+        });
+
+        await client.newSession({
+            cwd: "",
+            mcpServers: [{name: "docs", command: "node", args: ["docs-mcp.js"], env: []}],
+        });
+
+        expect(vi.mocked(appServerClient.threadStart).mock.calls[0]![0].config?.["mcp_servers"]).toEqual({
+            docs: {command: "node", args: ["docs-mcp.js"], env: {}},
+        });
+    });
+
+    it("preserves overrides while filtering conflicts with Codex-configured servers", async () => {
+        const client = new CodexAcpClient(appServerClient, {
+            mcp_servers: {node_repl: browserOverrides},
+        });
+
+        await client.newSession({
+            cwd: "",
+            mcpServers: [
+                {name: "node_repl", command: "node", args: ["other-browser.js"], env: []},
+                docsServer,
+            ],
+        });
+
+        expect(vi.mocked(appServerClient.threadStart).mock.calls[0]![0].config?.["mcp_servers"]).toEqual({
+            node_repl: browserOverrides,
+            docs: docsConfig,
+        });
+    });
+
+    it.each([
+        {mcpConfig: null},
+        {mcpConfig: "invalid"},
+        {mcpConfig: ["invalid"]},
+    ])("does not merge a non-object MCP config: $mcpConfig", async ({mcpConfig}) => {
+        const client = new CodexAcpClient(appServerClient, {mcp_servers: mcpConfig});
+
+        await client.newSession({cwd: "", mcpServers: [docsServer]});
+
+        expect(vi.mocked(appServerClient.threadStart).mock.calls[0]![0].config?.["mcp_servers"]).toEqual({
+            docs: docsConfig,
+        });
+    });
+});