diff --git a/src/auth/resume-lane-warning.ts b/src/auth/resume-lane-warning.ts
new file mode 100644
index 00000000..ca59217c
--- /dev/null
+++ b/src/auth/resume-lane-warning.ts
@@ -0,0 +1,64 @@
+import * as path from 'path';
+import { info, warn } from '../utils/ui';
+import {
+  parseResumeFlagIntent,
+  resolveRuntimePlainCcsResumeLane,
+  type ResumeLaneSummary,
+} from './resume-lane-diagnostics';
+
+interface ResumeLaneWarningDependencies {
+  resolvePlainLane?: () => Promise<ResumeLaneSummary>;
+  log?: (message: string) => void;
+  debug?: boolean;
+}
+
+export async function maybeWarnAboutResumeLaneMismatch(
+  profileName: string,
+  accountConfigDir: string,
+  args: string[],
+  deps: ResumeLaneWarningDependencies = {}
+): Promise<void> {
+  const resumeIntent = parseResumeFlagIntent(args);
+  if (!resumeIntent) {
+    return;
+  }
+
+  const log = deps.log ?? console.error;
+
+  try {
+    const plainLane = await (deps.resolvePlainLane ?? resolveRuntimePlainCcsResumeLane)();
+    if (path.resolve(plainLane.configDir) === path.resolve(accountConfigDir)) {
+      return;
+    }
+
+    log(
+      warn(
+        `Resume for account "${profileName}" will search that account lane, not the current plain ccs lane.`
+      )
+    );
+    log(info(`  Account lane: ${accountConfigDir}`));
+    log(info(`  Plain ccs lane: ${plainLane.label} (${plainLane.configDir})`));
+    if (resumeIntent.explicitSessionId) {
+      log(
+        info(
+          '  This explicit session ID may have been created in a different lane, so Claude may not find it here.'
+        )
+      );
+    }
+    log(info('  Recover the original lane first: ccs -r'));
+    log(info('  Back it up before changing setup: ccs auth backup default'));
+    log(
+      info(`  For future work, align plain ccs with this account: ccs auth default ${profileName}`)
+    );
+    log('');
+  } catch (error) {
+    log(
+      warn(
+        'Resume lane guidance skipped because diagnostics failed; continuing with the account lane.'
+      )
+    );
+    if (deps.debug ?? Boolean(process.env.CCS_DEBUG)) {
+      log(info(`  Diagnostic error: ${(error as Error).message}`));
+    }
+  }
+}
diff --git a/src/ccs.ts b/src/ccs.ts
index 9618da71..2806235f 100644
--- a/src/ccs.ts
+++ b/src/ccs.ts
@@ -1,7 +1,6 @@
 import './utils/fetch-proxy-setup';
 
 import * as fs from 'fs';
-import * as path from 'path';
 import { detectClaudeCli } from './utils/claude-detector';
 import {
   getSettingsPath,
@@ -65,10 +64,7 @@ import { tryHandleRootCommand } from './commands/root-command-router';
 // Import extracted utility functions
 import { execClaude } from './utils/shell-executor';
 import { isDeprecatedGlmtProfileName, normalizeDeprecatedGlmtEnv } from './utils/glmt-deprecation';
-import {
-  parseResumeFlagIntent,
-  resolveRuntimePlainCcsResumeLane,
-} from './auth/resume-lane-diagnostics';
+import { maybeWarnAboutResumeLaneMismatch } from './auth/resume-lane-warning';
 
 // Import target adapter system
 import {
@@ -264,43 +260,6 @@ function shouldPassthroughNativeCodexFlagCommand(args: string[]): boolean {
   return getNativeCodexPassthroughArgs(args) !== null;
 }
 
-async function maybeWarnAboutResumeLaneMismatch(
-  profileName: string,
-  accountConfigDir: string,
-  args: string[]
-): Promise<void> {
-  const resumeIntent = parseResumeFlagIntent(args);
-  if (!resumeIntent) {
-    return;
-  }
-
-  const plainLane = await resolveRuntimePlainCcsResumeLane();
-  if (path.resolve(plainLane.configDir) === path.resolve(accountConfigDir)) {
-    return;
-  }
-
-  console.error(
-    warn(
-      `Resume for account "${profileName}" will search that account lane, not the current plain ccs lane.`
-    )
-  );
-  console.error(info(`  Account lane: ${accountConfigDir}`));
-  console.error(info(`  Plain ccs lane: ${plainLane.label} (${plainLane.configDir})`));
-  if (resumeIntent.explicitSessionId) {
-    console.error(
-      info(
-        `  This explicit session ID may have been created in a different lane, so Claude may not find it here.`
-      )
-    );
-  }
-  console.error(info(`  Recover the original lane first: ccs -r`));
-  console.error(info(`  Back it up before changing setup: ccs auth backup default`));
-  console.error(
-    info(`  For future work, align plain ccs with this account: ccs auth default ${profileName}`)
-  );
-  console.error('');
-}
-
 function getNativeCodexPassthroughArgs(args: string[]): string[] | null {
   const targetArgs = stripTargetFlag(args);
   if (resolveTargetType(args) !== 'codex' || targetArgs.length === 0) {
diff --git a/tests/unit/auth/resume-lane-warning.test.ts b/tests/unit/auth/resume-lane-warning.test.ts
new file mode 100644
index 00000000..49eb3064
--- /dev/null
+++ b/tests/unit/auth/resume-lane-warning.test.ts
@@ -0,0 +1,53 @@
+import { describe, expect, it } from 'bun:test';
+import { maybeWarnAboutResumeLaneMismatch } from '../../../src/auth/resume-lane-warning';
+
+describe('resume lane warning', () => {
+  it('prints guidance when the plain ccs lane differs from the account lane', async () => {
+    const logs: string[] = [];
+
+    await maybeWarnAboutResumeLaneMismatch('work', '/tmp/account-lane', ['--resume'], {
+      log: (message) => logs.push(message),
+      resolvePlainLane: async () => ({
+        kind: 'native',
+        label: 'native Claude lane',
+        configDir: '/tmp/native-lane',
+        projectCount: 12,
+      }),
+    });
+
+    expect(logs[0]).toContain('Resume for account "work" will search that account lane');
+    expect(logs).toContain('[i]   Account lane: /tmp/account-lane');
+    expect(logs).toContain('[i]   Plain ccs lane: native Claude lane (/tmp/native-lane)');
+    expect(logs).toContain('[i]   Recover the original lane first: ccs -r');
+  });
+
+  it('does not log anything when resume is not requested', async () => {
+    const logs: string[] = [];
+
+    await maybeWarnAboutResumeLaneMismatch('work', '/tmp/account-lane', ['hello'], {
+      log: (message) => logs.push(message),
+      resolvePlainLane: async () => {
+        throw new Error('should not be called');
+      },
+    });
+
+    expect(logs).toHaveLength(0);
+  });
+
+  it('swallows diagnostic failures and keeps the warning path non-fatal', async () => {
+    const logs: string[] = [];
+
+    await expect(
+      maybeWarnAboutResumeLaneMismatch('work', '/tmp/account-lane', ['-r'], {
+        debug: true,
+        log: (message) => logs.push(message),
+        resolvePlainLane: async () => {
+          throw new Error('broken config');
+        },
+      })
+    ).resolves.toBeUndefined();
+
+    expect(logs[0]).toContain('Resume lane guidance skipped because diagnostics failed');
+    expect(logs[1]).toContain('Diagnostic error: broken config');
+  });
+});