diff --git a/src/web/routes/session-routes.ts b/src/web/routes/session-routes.ts
index 2121313b..2bcd85a6 100644
--- a/src/web/routes/session-routes.ts
+++ b/src/web/routes/session-routes.ts
@@ -4501,6 +4501,23 @@ export function registerSessionRoutes(
       const cmContextLength = resolveCustomModelContextLength(cmEndpoint, customModel.modelId);
       const cmSlotCount = cmEndpoint.modelSlotCounts?.[customModel.modelId];
 
+      // See the dedicated route: refused on the pure build, before either confirmation, so
+      // nobody agrees to evict a model for an id that could never launch, and before anything
+      // is written.
+      const cmModelSpec = cmEntry.launch.params.model;
+      const cmInjection = buildCustomModelInjection(cmEntry, cmEndpoint, customModel.modelId, cmContextLength);
+      const cmForcedLaunchModel = cmInjection.kind === 'unsupported' ? undefined : cmInjection.launchModel;
+      if (
+        cmForcedLaunchModel !== undefined &&
+        cmModelSpec?.type === 'token' &&
+        !matchesPattern(cmModelSpec.pattern, cmForcedLaunchModel)
+      ) {
+        return createErrorResponse(
+          ApiErrorCode.INVALID_INPUT,
+          `Model id ${JSON.stringify(customModel.modelId)} cannot be passed to ${mode} on its command line`
+        );
+      }
+
       // See the dedicated route's own comment for the full reasoning: some CLIs' own fixed
       // overhead can exceed a small enough real context on the very first message,
       // regardless of contextLengthVar. Warn before creating a session that's certain to
@@ -4571,19 +4588,6 @@ export function registerSessionRoutes(
       if (!cmApplied) {
         return createErrorResponse(ApiErrorCode.OPERATION_FAILED, `${mode} has no known custom-model mechanism`);
       }
-      const cmModelSpec = cmEntry.launch.params.model;
-      if (
-        cmApplied.launchModel !== undefined &&
-        cmModelSpec?.type === 'token' &&
-        !matchesPattern(cmModelSpec.pattern, cmApplied.launchModel)
-      ) {
-        removeConfigDir(cmApplied.configDir);
-        return createErrorResponse(
-          ApiErrorCode.INVALID_INPUT,
-          `Model id ${JSON.stringify(customModel.modelId)} cannot be passed to ${mode} on its command line`
-        );
-      }
-
       qsCustomModelEnvOverrides = { ...qsGatedEnvOverrides, ...cmApplied.envOverrides };
       qsCustomModelAppliedEnvOverrides = cmApplied.envOverrides;
       qsCustomModelLaunchModel = cmApplied.launchModel;
diff --git a/test/routes/quick-start-custom-model.test.ts b/test/routes/quick-start-custom-model.test.ts
index 71e6166c..6a5aca2c 100644
--- a/test/routes/quick-start-custom-model.test.ts
+++ b/test/routes/quick-start-custom-model.test.ts
@@ -21,7 +21,7 @@ import { describe, it, expect, beforeEach, afterEach, vi } from 'vitest';
 import Fastify, { type FastifyInstance } from 'fastify';
 import fastifyCookie from '@fastify/cookie';
 import { rm, readFile } from 'node:fs/promises';
-import { existsSync } from 'node:fs';
+import { existsSync, readdirSync } from 'node:fs';
 import { join } from 'node:path';
 import { createMockRouteContext, safeRmHomeTree, type MockRouteContext } from '../mocks/index.js';
 import { installRouteErrorHandler } from '../../src/web/route-error-handler.js';
@@ -195,7 +195,9 @@ describe('POST /api/quick-start: customModel (one-shot custom-model launch)', ()
     expect(res.json().errorCode).toBe('OPERATION_FAILED');
   });
 
-  it('refuses a model id the CLI cannot carry on its command line, cleaning up any written config dir', async () => {
+  it('refuses a model id the CLI cannot carry on its command line, before writing anything', async () => {
+    const dirs = join(getDataDir(), 'custom-model-configs');
+    const before = existsSync(dirs) ? readdirSync(dirs).length : 0;
     const res = await quickStart({
       caseName: 'cm-badmodel',
       mode: 'pi',
@@ -203,6 +205,7 @@ describe('POST /api/quick-start: customModel (one-shot custom-model launch)', ()
     });
     expect(res.json().success).toBe(false);
     expect(res.json().errorCode).toBe('INVALID_INPUT');
+    expect(existsSync(dirs) ? readdirSync(dirs).length : 0).toBe(before);
   });
 
   it('refuses customModel for a remote case', async () => {
@@ -229,6 +232,21 @@ describe('POST /api/quick-start: customModel (one-shot custom-model launch)', ()
       });
     }
 
+    it('refuses a model id the CLI cannot launch before asking anyone to evict a model', async () => {
+      const other = ctx.sessions.get('test-session-1')!;
+      (other as unknown as { customModel: unknown }).customModel = { endpointId: 'ep1', modelId: 'llama3' };
+      mockRunning([{ model: 'llama3', state: 'ready' }]);
+
+      const res = await quickStart({
+        caseName: 'cm-badmodel-swap',
+        mode: 'codex',
+        customModel: { endpointId: 'ep1', modelId: 'org/model@q4_k_m' },
+      });
+
+      expect(res.json().requiresConfirmation).toBeUndefined();
+      expect(res.json().errorCode).toBe('INVALID_INPUT');
+    });
+
     it('asks for confirmation instead of launching when another live session is using the currently loaded model', async () => {
       const other = ctx.sessions.get('test-session-1')!;
       (other as unknown as { customModel: unknown }).customModel = { endpointId: 'ep1', modelId: 'llama3' };