Harness Limits, Retry, Fallback, And Rate Limiting

This feature owns policy for stopping runaway agent loops, retrying transient failures, falling back between models/tools, and pacing provider calls.

Source Inspiration

LangChain and LangChain v1 provide related behavior in several places:

• runnable retry: https://github.com/langchain-ai/langchain/blob/master/libs/core/langchain_core/runnables/retry.py
• runnable fallbacks: https://github.com/langchain-ai/langchain/blob/master/libs/core/langchain_core/runnables/fallbacks.py
• in-memory rate limiter: https://github.com/langchain-ai/langchain/blob/master/libs/core/langchain_core/rate_limiters.py
• model and tool retry/fallback middleware: https://github.com/langchain-ai/langchain/tree/master/libs/langchain_v1/langchain/agents/middleware

TinyAgents should centralize these policies so every model call, tool call, agent loop, and graph node can share predictable behavior.

Responsibilities

• Enforce maximum model calls.
• Enforce maximum tool calls.
• Enforce maximum retries.
• Enforce maximum wall-clock duration.
• Enforce per-call timeouts.
• Enforce maximum concurrency.
• Enforce optional recursion depth.
• Enforce graph super-step caps.
• Support cancellation.
• Classify retryable and non-retryable errors.
• Support exponential backoff with jitter.
• Support model fallback chains.
• Support tool fallback or emulator policies.
• Support token-bucket rate limiting.
• Emit wait, retry, fallback, limit, timeout, and cancellation events.

Limit Policy

pub struct RunLimits {
    pub max_model_calls: usize,
    pub max_tool_calls: usize,
    pub max_retries: usize,
    pub max_concurrency: usize,
    pub max_recursion_depth: usize,
    pub max_graph_steps: usize,
    pub wall_clock_timeout: Option<Duration>,
    pub model_call_timeout: Option<Duration>,
    pub tool_call_timeout: Option<Duration>,
}

Limits should fail closed. When a limit is reached, the harness should stop the agent loop and return a classified error that includes the current counters.

For graph-backed runs, max-step failures should include graph name, run id, current super-step, frontier nodes, and the configured cap. The run should be persisted as failed when a checkpointer is configured.

Retry Policy

pub struct RetryPolicy {
    pub max_attempts: usize,
    pub initial_backoff: Duration,
    pub max_backoff: Duration,
    pub jitter: JitterPolicy,
    pub retry_on: RetryClassifier,
}

Retry classifiers should distinguish:

• transport errors
• provider rate-limit errors
• provider 5xx errors
• provider 4xx errors
• authentication errors
• timeout errors
• local validation errors
• structured-output validation errors
• tool validation errors
• tool execution errors
• budget errors
• cancellation

Authentication, budget, schema definition, and cancellation errors should not be retried by default.

Fallback Policy

pub struct ModelFallbackPolicy {
    pub candidates: Vec<ModelName>,
    pub require_same_capabilities: bool,
    pub retry_before_fallback: usize,
}

Fallback selection must honor required capabilities. A request that requires tool calling must not silently fall back to a model without tool calling. Fallback events should include the failed model, selected model, error class, and attempt count.

Rate Limiting

Rate limiting should be separate from LLM token accounting. It paces requests; it does not count prompt tokens.

#[async_trait]
pub trait RateLimiter: Send + Sync {
    async fn acquire(&self, permit: RateLimitPermit) -> Result<RateLimitLease>;
}

pub struct TokenBucketRateLimiter {
    pub requests_per_second: f64,
    pub max_bucket_size: f64,
}

Rate-limit wait time should be emitted as an event and included in timing breakdowns. This avoids hiding queueing latency inside provider latency.

Cancellation

RunContext should carry a cancellation token. Model adapters, tools, stores, streams, retry sleeps, and rate-limit waits must observe it. Cancellation should produce a distinct error class and should not be retried.