fix(model): preserve [1m] tag for the 'best' alias (#1671)

parseUserSpecifiedModel appended the [1m] (1M-context) tag for the opus,
sonnet, and haiku aliases but not for 'best'. Since 'best' resolves to the
same model as 'opus' (getDefaultOpusModel), 'best[1m]' silently dropped the
1M-context request while 'opus[1m]' kept it — so a user pinning 'best[1m]'
lost the larger context window.

Append the tag for 'best' as well, matching the other aliases. Add a
relational regression test (best[1m] tracks opus[1m], tag is case-insensitive
and not duplicated).

src/utils/model/model.ts

@@ -760,7 +760,7 @@ export function parseUserSpecifiedModel(
       case 'opus':
         return getDefaultOpusModel() + (has1mTag ? '[1m]' : '')
       case 'best':
-        return getBestModel()
+        return getBestModel() + (has1mTag ? '[1m]' : '')
       default:
     }
   }

src/utils/model/parseUserSpecifiedModel.bestTag.test.ts

@@ -0,0 +1,30 @@
+import { describe, expect, test } from 'bun:test'
+import { parseUserSpecifiedModel } from './model.js'
+
+// Regression: the `best` alias dropped the `[1m]` (1M-context) tag while the
+// other aliases (opus/sonnet/haiku) preserved it. `best` resolves to the same
+// model as `opus`, so `best[1m]` should behave exactly like `opus[1m]` and keep
+// the 1M tag. Assertions are relational so they don't pin a specific model id.
+describe('parseUserSpecifiedModel — best alias 1M tag', () => {
+  test('best[1m] preserves the [1m] tag, matching the opus alias', () => {
+    const best = parseUserSpecifiedModel('best')
+    const best1m = parseUserSpecifiedModel('best[1m]')
+
+    expect(best1m).toBe(`${best}[1m]`)
+    expect(best1m.endsWith('[1m]')).toBe(true)
+  })
+
+  test('best and best[1m] track the opus alias exactly', () => {
+    expect(parseUserSpecifiedModel('best')).toBe(parseUserSpecifiedModel('opus'))
+    expect(parseUserSpecifiedModel('best[1m]')).toBe(
+      parseUserSpecifiedModel('opus[1m]'),
+    )
+  })
+
+  test('the tag is case-insensitive and not duplicated', () => {
+    const best1m = parseUserSpecifiedModel('best[1m]')
+    expect(parseUserSpecifiedModel('BEST[1M]')).toBe(best1m)
+    // exactly one trailing [1m], no doubling
+    expect(best1m.match(/\[1m]/gi)?.length).toBe(1)
+  })
+})