Model

Model-level middleware. Automatic retry with exponential backoff for transient failures, and complexity-based routing to optimize cost.

model.retry()

Wraps LLM calls with exponential backoff for transient failures (rate limits, network errors).

function modelRetry(config?: RetryConfig): Middleware

// Default: 2 retries, 1000ms initial delay
agent.use(model.retry())

// Custom: 3 retries, 500ms initial delay
agent.use(model.retry({ maxRetries: 3, initialDelayMs: 500 }))

Config options:

Option	Type	Default	Description
maxRetries	number	2	Maximum retry attempts
initialDelayMs	number	1000	Initial delay (doubles each retry)

Hooks: model — wraps next() in a retry loop.

Retryable errors: RateLimitError, NetworkError. Non-retryable: AuthenticationError, ContentFilterError. For RateLimitError with retryAfter, the delay is the greater of the backoff delay and retryAfter * 1000.

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model.router()

Routes model calls by complexity, switching between cheaper and more capable models automatically. Saves 60-90% on LLM costs for mixed-complexity workloads.

function modelRouter(config: ModelRouterConfig): Middleware

agent.use(model.router({
  routes: {
    simple:  "anthropic/claude-haiku-4-5",
    medium:  "anthropic/claude-sonnet-4-6",
    complex: "anthropic/claude-opus-4-6",
  },
}))

// Custom classifier
agent.use(model.router({
  routes: { simple: "...", medium: "...", complex: "..." },
  classify: (ctx) => {
    if (ctx.toolDefs.length > 3) return "complex"
    return "simple"
  },
}))

Default classification heuristic:

• simple: < 500 estimated tokens AND no tools defined
• complex: > 2000 tokens OR 5+ tools available
• medium: everything else

Config options:

Option	Type	Default	Description
routes	Record<ComplexityTier, string>	required	Model ID for each tier. ComplexityTier is "simple" | "medium" | "complex".
classify	(ctx: ModelContext) => ComplexityTier	built-in heuristic	Custom classifier function
tokenCounter	TokenCounter	chars/4	Token counter for input complexity estimation

Hooks: model — classifies and calls ctx.setModel() before next().