diff --git a/cli/src/codex/codexLocal.test.ts b/cli/src/codex/codexLocal.test.ts
index 2d251c27..3c8654ef 100644
--- a/cli/src/codex/codexLocal.test.ts
+++ b/cli/src/codex/codexLocal.test.ts
@@ -92,4 +92,25 @@ describe('codexLocal', () => {
         expect(hookArg).toContain('{ hooks = [{ type = "command", command = "');
         expect(args).toContain("mcp_servers.hapi.args=['mcp','--url','http://127.0.0.1:63995/']");
     });
+
+    it('passes reasoning effort through Codex config instead of an unsupported CLI flag', async () => {
+        const controller = new AbortController();
+
+        await codexLocal({
+            abort: controller.signal,
+            sessionId: 'codex-session-1',
+            path: '/workspace/project',
+            modelReasoningEffort: 'high',
+            onSessionFound: vi.fn()
+        });
+
+        expect(spawnWithTerminalGuardMock).toHaveBeenCalledOnce();
+        const spawnOptions = spawnWithTerminalGuardMock.mock.calls[0][0] as {
+            args: string[];
+        };
+
+        expect(spawnOptions.args).toContain('-c');
+        expect(spawnOptions.args).toContain('model_reasoning_effort="high"');
+        expect(spawnOptions.args).not.toContain('--model-reasoning-effort');
+    });
 });
diff --git a/cli/src/codex/codexLocal.ts b/cli/src/codex/codexLocal.ts
index 60e46f0c..8b458c62 100644
--- a/cli/src/codex/codexLocal.ts
+++ b/cli/src/codex/codexLocal.ts
@@ -3,7 +3,8 @@ import { spawnWithTerminalGuard } from '@/utils/spawnWithTerminalGuard';
 import {
     buildMcpServerConfigArgs,
     buildDeveloperInstructionsArg,
-    buildSessionStartHookConfigArgs
+    buildSessionStartHookConfigArgs,
+    buildModelReasoningEffortConfigArgs
 } from './utils/codexMcpConfig';
 import { codexSystemPrompt } from './utils/systemPrompt';
 import type { ReasoningEffort } from './appServerTypes';
@@ -54,7 +55,7 @@ export async function codexLocal(opts: {
     }
 
     if (opts.modelReasoningEffort) {
-        args.push('--model-reasoning-effort', opts.modelReasoningEffort);
+        args.push(...buildModelReasoningEffortConfigArgs(opts.modelReasoningEffort));
     }
 
     if (opts.sandbox) {
diff --git a/cli/src/codex/utils/appServerConfig.test.ts b/cli/src/codex/utils/appServerConfig.test.ts
index 7f2eaecb..f06d272e 100644
--- a/cli/src/codex/utils/appServerConfig.test.ts
+++ b/cli/src/codex/utils/appServerConfig.test.ts
@@ -130,6 +130,7 @@ describe('appServerConfig', () => {
             mode: 'default',
             settings: {
                 model: 'o3',
+                reasoning_effort: 'high',
                 developer_instructions: withCollaborationInstructions(codexSystemPrompt)
             }
         });
@@ -155,6 +156,7 @@ describe('appServerConfig', () => {
             mode: 'default',
             settings: {
                 model: 'gpt-5.3-codex-spark',
+                reasoning_effort: 'high',
                 developer_instructions: withCollaborationInstructions(codexSystemPrompt)
             }
         });
@@ -236,6 +238,7 @@ describe('appServerConfig', () => {
             mode: 'plan',
             settings: {
                 model: 'o3',
+                reasoning_effort: 'high',
                 developer_instructions: withCollaborationInstructions(codexSystemPrompt)
             }
         });
@@ -255,6 +258,7 @@ describe('appServerConfig', () => {
             mode: 'plan',
             settings: {
                 model: 'o3',
+                reasoning_effort: null,
                 developer_instructions: withCollaborationInstructions(`${codexSystemPrompt}\n\nOnly respond in Chinese.`)
             }
         });
@@ -299,6 +303,7 @@ describe('appServerConfig', () => {
             mode: 'default',
             settings: {
                 model: 'o3',
+                reasoning_effort: null,
                 developer_instructions: withCollaborationInstructions(codexSystemPrompt)
             }
         });
@@ -319,6 +324,7 @@ describe('appServerConfig', () => {
             mode: 'default',
             settings: {
                 model: 'o3',
+                reasoning_effort: null,
                 developer_instructions: withCollaborationInstructions(codexSystemPrompt)
             }
         });
@@ -338,6 +344,7 @@ describe('appServerConfig', () => {
             mode: 'default',
             settings: {
                 model: 'gpt-5',
+                reasoning_effort: null,
                 developer_instructions: withCollaborationInstructions(codexSystemPrompt)
             }
         });
diff --git a/cli/src/codex/utils/appServerConfig.ts b/cli/src/codex/utils/appServerConfig.ts
index f7f785fa..375d5a80 100644
--- a/cli/src/codex/utils/appServerConfig.ts
+++ b/cli/src/codex/utils/appServerConfig.ts
@@ -168,9 +168,10 @@ export function buildTurnStartParams(args: {
         ? undefined
         : args.mode?.collaborationMode;
     const model = args.overrides?.model ?? args.mode?.model;
+    const modelReasoningEffort = args.mode?.modelReasoningEffort;
 
-    if (args.mode?.modelReasoningEffort) {
-        params.effort = args.mode.modelReasoningEffort;
+    if (modelReasoningEffort) {
+        params.effort = modelReasoningEffort;
         if (!collaborationMode && supportsReasoningSummary(model)) {
             params.summary = 'detailed';
         }
@@ -185,6 +186,7 @@ export function buildTurnStartParams(args: {
             mode: collaborationMode,
             settings: {
                 model,
+                reasoning_effort: modelReasoningEffort ?? null,
                 developer_instructions: appendCollaborationInstructions(developerInstructions)
             }
         };
diff --git a/cli/src/codex/utils/codexMcpConfig.ts b/cli/src/codex/utils/codexMcpConfig.ts
index bdf17c1b..1cb04531 100644
--- a/cli/src/codex/utils/codexMcpConfig.ts
+++ b/cli/src/codex/utils/codexMcpConfig.ts
@@ -151,3 +151,7 @@ export function buildDeveloperInstructionsArg(instructions: string): string[] {
     const escaped = escapeTomlString(instructions);
     return ['-c', `developer_instructions="${escaped}"`];
 }
+
+export function buildModelReasoningEffortConfigArgs(effort: string): string[] {
+    return ['-c', `model_reasoning_effort="${escapeTomlString(effort)}"`];
+}