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apps/web/src/state/maxTokens.ts

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+import type { AppConfig } from '../types';
+import litellmData from './litellm-models.json';
+
+// Per-model output cap, used to default `max_tokens` so users on supported
+// models don't have to find Settings to avoid mid-stream truncation.
+//
+// Source of truth: vendored slice of BerriAI/litellm's
+// model_prices_and_context_window.json (MIT). Regenerate with:
+//   node --experimental-strip-types scripts/sync-litellm-models.ts
+//
+// Anything LiteLLM doesn't track (or where its value is wrong for our
+// usage) goes in OVERRIDES; unknown models fall through to FALLBACK.
+export const FALLBACK_MAX_TOKENS = 8192;
+
+// Bounds the user can express via the Settings override. Source of truth
+// for both the UI input attributes and runtime validation in
+// `effectiveMaxTokens`, so a stale or hand-edited localStorage value
+// can't sneak past the UI's promise.
+export const MIN_MAX_TOKENS = 1024;
+export const MAX_MAX_TOKENS = 200000;
+
+const LITELLM_MODELS = litellmData.models as Record<string, number>;
+
+const OVERRIDES: Record<string, number> = {
+  // LiteLLM lists MiMo via OpenRouter and Novita aliases (16k / 32k) but
+  // not the canonical `mimo-v2.5-pro` id we hand to Xiaomi's direct API.
+  // 32k matches what issue #29 reports as the working ceiling.
+  'mimo-v2.5-pro': 32768,
+};
+
+export function modelMaxTokensDefault(model: string): number {
+  return OVERRIDES[model] ?? LITELLM_MODELS[model] ?? FALLBACK_MAX_TOKENS;
+}
+
+function isValidOverride(value: number | undefined): value is number {
+  return (
+    typeof value === 'number' &&
+    Number.isInteger(value) &&
+    value >= MIN_MAX_TOKENS &&
+    value <= MAX_MAX_TOKENS
+  );
+}
+
+export function effectiveMaxTokens(cfg: Pick<AppConfig, 'maxTokens' | 'model'>): number {
+  // Out-of-range or non-integer overrides (stale localStorage, hand-edited
+  // config, future schema drift) fall back to the model default rather
+  // than silently shipping an invalid `max_tokens` upstream.
+  if (isValidOverride(cfg.maxTokens)) return cfg.maxTokens;
+  return modelMaxTokensDefault(cfg.model);
+}