diff --git a/src/cliproxy/executor/__tests__/launch-settings.test.ts b/src/cliproxy/executor/__tests__/launch-settings.test.ts
index 6080ab27..0b313b84 100644
--- a/src/cliproxy/executor/__tests__/launch-settings.test.ts
+++ b/src/cliproxy/executor/__tests__/launch-settings.test.ts
@@ -57,6 +57,30 @@ describe('buildLaunchSettingsOverlay', () => {
     expect(env.ANTHROPIC_AUTH_TOKEN).toBe('ccs-internal-managed');
   });
 
+  it('overlays the extended-context extra model keys so a saved bare value cannot clobber --1m/--no-1m', () => {
+    writePersisted({
+      env: {
+        ANTHROPIC_BASE_URL: 'http://127.0.0.1:8317/api/provider/claude',
+        ANTHROPIC_MODEL: 'claude-opus-5[1m]',
+        ANTHROPIC_DEFAULT_MODEL: 'claude-opus-5[1m]',
+        CLAUDE_CODE_SUBAGENT_MODEL: 'claude-sonnet-5[1m]',
+      },
+    });
+
+    const { settings, changed } = buildLaunchSettingsOverlay(settingsPath, {
+      ANTHROPIC_BASE_URL: 'http://127.0.0.1:8317/api/provider/claude',
+      ANTHROPIC_MODEL: 'claude-opus-5',
+      ANTHROPIC_DEFAULT_MODEL: 'claude-opus-5',
+      CLAUDE_CODE_SUBAGENT_MODEL: 'claude-sonnet-5',
+    } as NodeJS.ProcessEnv);
+
+    expect(changed).toBe(true);
+    const env = settings.env as Record<string, string>;
+    expect(env.ANTHROPIC_MODEL).toBe('claude-opus-5');
+    expect(env.ANTHROPIC_DEFAULT_MODEL).toBe('claude-opus-5');
+    expect(env.CLAUDE_CODE_SUBAGENT_MODEL).toBe('claude-sonnet-5');
+  });
+
   it('preserves non-env settings (permissions, hooks, etc.)', () => {
     writePersisted({
       env: { ANTHROPIC_BASE_URL: 'http://127.0.0.1:8317/api/provider/codex' },
diff --git a/src/cliproxy/executor/launch-settings.ts b/src/cliproxy/executor/launch-settings.ts
index 5e33bf7e..aadbd96e 100644
--- a/src/cliproxy/executor/launch-settings.ts
+++ b/src/cliproxy/executor/launch-settings.ts
@@ -26,6 +26,7 @@ import * as os from 'os';
 import * as path from 'path';
 
 import { ANTHROPIC_MODEL_ENV_KEYS, ANTHROPIC_ROUTING_ENV_KEYS } from '../../utils/shell-executor';
+import { EXTRA_EXTENDED_CONTEXT_MODEL_ENV_KEYS } from '../../shared/extended-context-utils';
 
 // SIBLING HELPER: src/utils/openai-compat-launch-settings.ts solves the same
 // "persisted --settings env clobbers runtime routing env" problem by STRIPPING
@@ -38,9 +39,15 @@ import { ANTHROPIC_MODEL_ENV_KEYS, ANTHROPIC_ROUTING_ENV_KEYS } from '../../util
  * Environment keys that control provider routing/model selection and are read
  * by Claude from the settings `env` block. These must reflect the resolved
  * proxy-chain environment, not the persisted on-disk values. Reuses the
- * canonical routing/model key lists from shell-executor.
+ * canonical routing/model key lists from shell-executor, plus the extra model
+ * keys the [1m] preference manages (subagent + startup default): a saved bare
+ * value there would otherwise clobber the resolved --1m/--no-1m result.
  */
-const ROUTING_ENV_KEYS = [...ANTHROPIC_ROUTING_ENV_KEYS, ...ANTHROPIC_MODEL_ENV_KEYS];
+const ROUTING_ENV_KEYS = [
+  ...ANTHROPIC_ROUTING_ENV_KEYS,
+  ...ANTHROPIC_MODEL_ENV_KEYS,
+  ...EXTRA_EXTENDED_CONTEXT_MODEL_ENV_KEYS,
+];
 
 function isRecord(value: unknown): value is Record<string, unknown> {
   return typeof value === 'object' && value !== null && !Array.isArray(value);
diff --git a/src/shared/extended-context-utils.ts b/src/shared/extended-context-utils.ts
index 8c717720..ebed49c9 100644
--- a/src/shared/extended-context-utils.ts
+++ b/src/shared/extended-context-utils.ts
@@ -19,8 +19,15 @@ export type AnthropicModelEnvKey = (typeof ANTHROPIC_MODEL_ENV_KEYS)[number];
  * model id, so the extended-context preference has to cover them too. Kept
  * separate from ANTHROPIC_MODEL_ENV_KEYS, which also drives routing, model-id
  * normalization and profile validation.
+ *
+ * ANTHROPIC_DEFAULT_MODEL is Claude Code's lowest-priority startup model
+ * (after --model, ANTHROPIC_MODEL and the settings `model` field), so it needs
+ * the same [1m] treatment whenever a profile carries it.
  */
-export const EXTRA_EXTENDED_CONTEXT_MODEL_ENV_KEYS = ['CLAUDE_CODE_SUBAGENT_MODEL'] as const;
+export const EXTRA_EXTENDED_CONTEXT_MODEL_ENV_KEYS = [
+  'CLAUDE_CODE_SUBAGENT_MODEL',
+  'ANTHROPIC_DEFAULT_MODEL',
+] as const;
 
 /** Every model env key the [1m] preference is applied to. */
 export const EXTENDED_CONTEXT_MODEL_ENV_KEYS = [
diff --git a/tests/unit/shared/extended-context-utils.test.ts b/tests/unit/shared/extended-context-utils.test.ts
index a65af8b1..c1ce474a 100644
--- a/tests/unit/shared/extended-context-utils.test.ts
+++ b/tests/unit/shared/extended-context-utils.test.ts
@@ -28,13 +28,16 @@ describe('Anthropic model env keys', () => {
 });
 
 describe('extended-context model env keys', () => {
-  it('covers the Anthropic tiers plus the subagent model key', () => {
+  it('covers the Anthropic tiers plus the subagent and startup-default model keys', () => {
     expect([...EXTENDED_CONTEXT_MODEL_ENV_KEYS]).toEqual([
       ...ANTHROPIC_MODEL_ENV_KEYS,
       'CLAUDE_CODE_SUBAGENT_MODEL',
+      'ANTHROPIC_DEFAULT_MODEL',
     ]);
     expect(isExtendedContextModelEnvKey('CLAUDE_CODE_SUBAGENT_MODEL')).toBe(true);
+    expect(isExtendedContextModelEnvKey('ANTHROPIC_DEFAULT_MODEL')).toBe(true);
     expect(isAnthropicModelEnvKey('CLAUDE_CODE_SUBAGENT_MODEL')).toBe(false);
+    expect(isAnthropicModelEnvKey('ANTHROPIC_DEFAULT_MODEL')).toBe(false);
   });
 });
 
@@ -64,6 +67,7 @@ describe('applyExtendedContextPreferenceToAnthropicModels', () => {
     const env = applyExtendedContextPreferenceToAnthropicModels(
       {
         ANTHROPIC_MODEL: 'claude-opus-5[1m]',
+        ANTHROPIC_DEFAULT_MODEL: 'claude-opus-5[1m]',
         ANTHROPIC_DEFAULT_FABLE_MODEL: 'claude-fable-5-1[1m]',
         CLAUDE_CODE_SUBAGENT_MODEL: 'claude-fable-5-1[1m]',
       },
@@ -72,11 +76,21 @@ describe('applyExtendedContextPreferenceToAnthropicModels', () => {
 
     expect(env).toEqual({
       ANTHROPIC_MODEL: 'claude-opus-5',
+      ANTHROPIC_DEFAULT_MODEL: 'claude-opus-5',
       ANTHROPIC_DEFAULT_FABLE_MODEL: 'claude-fable-5-1',
       CLAUDE_CODE_SUBAGENT_MODEL: 'claude-fable-5-1',
     });
   });
 
+  it('suffixes the startup-default model key when the preference is on', () => {
+    const env = applyExtendedContextPreferenceToAnthropicModels(
+      { ANTHROPIC_DEFAULT_MODEL: 'claude-opus-5' },
+      true
+    );
+
+    expect(env).toEqual({ ANTHROPIC_DEFAULT_MODEL: 'claude-opus-5[1m]' });
+  });
+
   it('honors a caller compatibility predicate', () => {
     const env = applyExtendedContextPreferenceToAnthropicModels(
       {