# How Switchyard Handles Context Window Exceeded Errors from Upstream LLM Providers

> Learn how Switchyard effectively handles context window exceeded errors from upstream LLM providers, classifying and propagating these errors for robust application development.

- Repository: [NVIDIA-NeMo/Switchyard](https://github.com/NVIDIA-NeMo/Switchyard)
- Tags: how-to-guide
- Published: 2026-08-15

---

**Switchyard detects upstream "context window exceeded" errors in its Rust HTTP client, classifies them as `LlmClientError::ContextWindowExceeded`, propagates them through the routing layer for optional fallback, and exposes them to Python code as a catchable `ContextWindowExceededError` exception.**

NVIDIA-NeMo's Switchyard is an inference routing engine that normalizes error handling across diverse LLM providers. One critical failure mode it handles is the **context window exceeded** error—when a prompt or completion exceeds a model's maximum token limit. This article explains how Switchyard detects, propagates, and surfaces these errors from the low-level Rust client up to Python applications.

## Detection in the LLM Client Layer

The detection occurs in [`crates/libsy-llm-client/src/client.rs`](https://github.com/NVIDIA-NeMo/Switchyard/blob/main/crates/libsy-llm-client/src/client.rs) within the HTTP response handler. When an upstream provider returns `400 Bad Request`, Switchyard checks whether the response body matches a provider-specific overflow signature.

```rust
// From crates/libsy-llm-client/src/client.rs lines 55-60
if status == reqwest::StatusCode::BAD_REQUEST && backend.is_context_overflow(&body) {
    LlmClientError::ContextWindowExceeded {
        model: model.clone(),
        message: body,
    }
} else {
    LlmClientError::UpstreamHttp { status: status.as_u16(), body }
}

```

The `backend.is_context_overflow(&body)` method abstracts provider-specific heuristics—different vendors use varying error message formats, but Switchyard normalizes them into a single error variant.

## Propagation Through the Routing Layer

Once classified, the `ContextWindowExceeded` error travels up the call stack. In [`crates/libsy-llm-client/src/run.rs`](https://github.com/NVIDIA-NeMo/Switchyard/blob/main/crates/libsy-llm-client/src/run.rs), the routing logic recognizes this error as a **fallback trigger**:

```rust
// Lines 263-266 — error converted to RoutingFallbackReason::ContextWindow

```

This allows Switchyard's routing algorithm to optionally retry the request against a different model with a larger context window, or apply a fallback policy configured by the operator.

## Mapping to HTTP Responses in the Switchyard Server

The server component converts internal Rust errors into standardized HTTP responses. In [`crates/switchyard-server/src/lib.rs`](https://github.com/NVIDIA-NeMo/Switchyard/blob/main/crates/switchyard-server/src/lib.rs) (lines 909-914), the `ContextWindowExceeded` variant maps to:

```rust
LlmClientError::ContextWindowExceeded { message, .. } => error_response(
    StatusCode::BAD_REQUEST,
    message,
    "invalid_request_error",
    "context_length_exceeded",
),

```

This produces a **400 Bad Request** with error code `"context_length_exceeded"`, giving API consumers a predictable, documented error format regardless of which upstream provider triggered the overflow.

## Python API Exposure via switchyard-py

Switchyard exposes the error to Python through PyO3 bindings. The exception hierarchy is defined in [`crates/switchyard-py/src/errors.rs`](https://github.com/NVIDIA-NeMo/Switchyard/blob/main/crates/switchyard-py/src/errors.rs) (lines 11-12):

```rust
// ContextWindowExceededError defined as subclass of RuntimeError

```

And re-exported for users in [`switchyard/libsy/__init__.py`](https://github.com/NVIDIA-NeMo/Switchyard/blob/main/switchyard/libsy/__init__.py) (lines 6-9), enabling direct exception handling:

```python
from switchyard.libsy import Algorithm, ContextWindowExceededError

alg = Algorithm(...)
try:
    response = await alg.call({"prompt": "Very long text …"})
except ContextWindowExceededError as e:
    print("Context window was exceeded:", e)
    # Optionally retry with a smaller prompt or a different model

```

## Provider-Specific Overflow Detection

The `backend.is_context_overflow()` abstraction handles heterogeneity across LLM APIs. While the source analysis doesn't detail every provider's signature, the pattern suggests Switchyard maintains provider-specific detection logic without leaking those details to higher layers. This normalization ensures that:

- OpenAI's `"context_length_exceeded"` errors
- Anthropic's analogous limits
- Other providers' custom formats

All resolve to the same internal error type and user-facing exception.

## Error Handling Flow Summary

| Stage | Component | Key File | Output |
|-------|-----------|----------|--------|
| Detection | LLM HTTP client | [`crates/libsy-llm-client/src/client.rs`](https://github.com/NVIDIA-NeMo/Switchyard/blob/main/crates/libsy-llm-client/src/client.rs) | `LlmClientError::ContextWindowExceeded` |
| Routing decision | Request router | [`crates/libsy-llm-client/src/run.rs`](https://github.com/NVIDIA-NeMo/Switchyard/blob/main/crates/libsy-llm-client/src/run.rs) | `RoutingFallbackReason::ContextWindow` |
| HTTP serialization | Server | [`crates/switchyard-server/src/lib.rs`](https://github.com/NVIDIA-NeMo/Switchyard/blob/main/crates/switchyard-server/src/lib.rs) | 400 + `"context_length_exceeded"` |
| Python exposure | PyO3 bindings | [`crates/switchyard-py/src/errors.rs`](https://github.com/NVIDIA-NeMo/Switchyard/blob/main/crates/switchyard-py/src/errors.rs) | `ContextWindowExceededError` |

## Summary

- **Detection happens early** in [`client.rs`](https://github.com/NVIDIA-NeMo/Switchyard/blob/main/client.rs) by matching HTTP 400 responses against provider-specific overflow patterns.
- **Classification is uniform**—all providers converge on `LlmClientError::ContextWindowExceeded`.
- **Routing can react** to context window errors as a specific fallback reason for model retry.
- **HTTP responses are standardized** with code `"context_length_exceeded"` regardless of upstream.
- **Python users catch `ContextWindowExceededError`** directly from the `switchyard.libsy` module.

## Frequently Asked Questions

### How do I catch context window errors in Python when using Switchyard?

Import `ContextWindowExceededError` from `switchyard.libsy` and wrap your inference calls in a try/except block. The exception inherits from `RuntimeError` and carries the original error message from the upstream provider.

### Can Switchyard automatically retry with a larger context model?

Yes. The routing layer in [`run.rs`](https://github.com/NVIDIA-NeMo/Switchyard/blob/main/run.rs) converts `ContextWindowExceeded` to `RoutingFallbackReason::ContextWindow`, enabling configured fallback policies to attempt alternate models. This behavior depends on your routing configuration.

### What HTTP status code does Switchyard return for context window errors?

The server returns **400 Bad Request** with a JSON error body containing `"code": "context_length_exceeded"`. This matches OpenAI's error format for compatibility with existing client libraries.

### Where is the Python exception actually defined?

The Rust struct is defined in [`crates/switchyard-py/src/errors.rs`](https://github.com/NVIDIA-NeMo/Switchyard/blob/main/crates/switchyard-py/src/errors.rs) using PyO3's `create_exception!` macro, then re-exported through [`switchyard/libsy/__init__.py`](https://github.com/NVIDIA-NeMo/Switchyard/blob/main/switchyard/libsy/__init__.py) for clean Python imports.