npm package: @sisu-ai/mw-error-boundary
Description: Prevent pipeline crashes by catching middleware errors and returning controlled fallbacks.
Latest version: 12.0.0

# @sisu-ai/mw-error-boundary

Prevent pipeline crashes by catching middleware errors and returning controlled fallbacks.

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## Setup
```bash
npm i @sisu-ai/mw-error-boundary
```

## Exports
- `errorBoundary(onError: (err, ctx, next) => Promise<void>)`
  - Call early in your stack to ensure all downstream errors are caught.
  - Inside `onError`, you typically:
    - log the error (`ctx.log.error(err)`),
    - push a friendly assistant message to `ctx.messages`,
    - optionally write to `ctx.stream` if you’re mid‑stream.

## What It Does
- Wraps the rest of your pipeline in a `try/catch`.
- Invokes your handler with the error and context, so you can log and produce a user‑friendly fallback.
- Prevents crashes from bubbling to the caller while keeping control in your app.

## How It Works
`errorBoundary(onError)` returns middleware that does:

```ts
try {
  await next();
} catch (err) {
  await onError(err, ctx, async () => {}); // next is a no-op in error state
}
```

Your `onError` receives `(err, ctx, next)` where `next` is a no‑op to signal the boundary is terminal for that request.

## Usage
```ts
import { Agent } from '@sisu-ai/core';
import { errorBoundary } from '@sisu-ai/mw-error-boundary';

const app = new Agent()
  .use(errorBoundary(async (err, ctx) => {
    ctx.log.error(err);
    // If streaming UI, consider writing to the stream instead/as well.
    ctx.messages.push({ role: 'assistant', content: 'Sorry, something went wrong.' });
  }))
  // ... other middleware
```

### Placement & Ordering
- Put `errorBoundary` at or near the top to catch as much as possible.
- Combine with tracing/usage middleware as needed; if placed before them, those middlewares may not observe the error.
- If you use a server adapter, ensure it wraps request handling so per‑request errors are isolated.

## When To Use
- Any production app to avoid unhandled rejections crashing the process.
- CLIs and demos where you want graceful failure and a helpful message.
- Around tool‑calling loops where third‑party tools may throw.

## When Not To Use
- If you want errors to propagate to an outer boundary (e.g., framework handler) and be handled there.
- Highly controlled test scenarios where you prefer tests to fail fast instead of being swallowed.

## Notes & Gotchas
- The boundary swallows the error after `onError` runs; rethrow inside `onError` if you want upstream handling.
- Be careful not to leak secrets when logging errors — consider the redacting logger `createRedactingLogger` from `@sisu-ai/core`.
- If you are streaming tokens, write an error notice to `ctx.stream` and call `ctx.stream.end()` to close the client stream cleanly.
- Keep `onError` fast and robust; avoid throwing inside it.


# Community & Support

Discover what you can do through examples or documentation. Check it out at https://github.com/finger-gun/sisu. Example projects live under [`examples/`](https://github.com/finger-gun/sisu/tree/main/examples) in the repo.

- [Code of Conduct](https://github.com/finger-gun/sisu/blob/main/CODE_OF_CONDUCT.md)
- [Contributing Guide](https://github.com/finger-gun/sisu/blob/main/CONTRIBUTING.md)
- [License](https://github.com/finger-gun/sisu/blob/main/LICENSE)
- [Report a Bug](https://github.com/finger-gun/sisu/issues/new?template=bug_report.md)
- [Request a Feature](https://github.com/finger-gun/sisu/issues/new?template=feature_request.md)

---

## Documentation

**Core** — [Package docs](packages/core/README.md) · [Error types](packages/core/ERROR_TYPES.md)

**Adapters** — [OpenAI](packages/adapters/openai/README.md) · [Anthropic](packages/adapters/anthropic/README.md) · [Ollama](packages/adapters/ollama/README.md)

<details>
<summary>All middleware packages</summary>

- [@sisu-ai/mw-agent-run-api](packages/middleware/agent-run-api/README.md)
- [@sisu-ai/mw-context-compressor](packages/middleware/context-compressor/README.md)
- [@sisu-ai/mw-control-flow](packages/middleware/control-flow/README.md)
- [@sisu-ai/mw-conversation-buffer](packages/middleware/conversation-buffer/README.md)
- [@sisu-ai/mw-cors](packages/middleware/cors/README.md)
- [@sisu-ai/mw-error-boundary](packages/middleware/error-boundary/README.md)
- [@sisu-ai/mw-guardrails](packages/middleware/guardrails/README.md)
- [@sisu-ai/mw-invariants](packages/middleware/invariants/README.md)
- [@sisu-ai/mw-orchestration](packages/middleware/orchestration/README.md)
- [@sisu-ai/mw-rag](packages/middleware/rag/README.md)
- [@sisu-ai/mw-react-parser](packages/middleware/react-parser/README.md)
- [@sisu-ai/mw-register-tools](packages/middleware/register-tools/README.md)
- [@sisu-ai/mw-tool-calling](packages/middleware/tool-calling/README.md)
- [@sisu-ai/mw-trace-viewer](packages/middleware/trace-viewer/README.md)
- [@sisu-ai/mw-usage-tracker](packages/middleware/usage-tracker/README.md)
</details>

<details>
<summary>All tool packages</summary>

- [@sisu-ai/tool-aws-s3](packages/tools/aws-s3/README.md)
- [@sisu-ai/tool-azure-blob](packages/tools/azure-blob/README.md)
- [@sisu-ai/tool-extract-urls](packages/tools/extract-urls/README.md)
- [@sisu-ai/tool-github-projects](packages/tools/github-projects/README.md)
- [@sisu-ai/tool-rag](packages/tools/rag/README.md)
- [@sisu-ai/tool-summarize-text](packages/tools/summarize-text/README.md)
- [@sisu-ai/tool-terminal](packages/tools/terminal/README.md)
- [@sisu-ai/tool-web-fetch](packages/tools/web-fetch/README.md)
- [@sisu-ai/tool-web-search-duckduckgo](packages/tools/web-search-duckduckgo/README.md)
- [@sisu-ai/tool-web-search-google](packages/tools/web-search-google/README.md)
- [@sisu-ai/tool-web-search-openai](packages/tools/web-search-openai/README.md)
- [@sisu-ai/tool-wikipedia](packages/tools/wikipedia/README.md)
</details>

<details>
<summary>All RAG packages</summary>

- [@sisu-ai/rag-core](packages/rag/core/README.md)
</details>

<details>
<summary>All vector packages</summary>

- [@sisu-ai/vector-core](packages/vector/core/README.md)
- [@sisu-ai/vector-chroma](packages/vector/chroma/README.md)
</details>

<details>
<summary>All examples</summary>

**Anthropic** — [hello](examples/anthropic-hello/README.md) · [control-flow](examples/anthropic-control-flow/README.md) · [stream](examples/anthropic-stream/README.md) · [weather](examples/anthropic-weather/README.md)

**Ollama** — [hello](examples/ollama-hello/README.md) · [stream](examples/ollama-stream/README.md) · [vision](examples/ollama-vision/README.md) · [weather](examples/ollama-weather/README.md) · [web-search](examples/ollama-web-search/README.md)

**OpenAI** — [hello](examples/openai-hello/README.md) · [weather](examples/openai-weather/README.md) · [stream](examples/openai-stream/README.md) · [vision](examples/openai-vision/README.md) · [reasoning](examples/openai-reasoning/README.md) · [react](examples/openai-react/README.md) · [control-flow](examples/openai-control-flow/README.md) · [branch](examples/openai-branch/README.md) · [parallel](examples/openai-parallel/README.md) · [graph](examples/openai-graph/README.md) · [orchestration](examples/openai-orchestration/README.md) · [orchestration-adaptive](examples/openai-orchestration-adaptive/README.md) · [guardrails](examples/openai-guardrails/README.md) · [error-handling](examples/openai-error-handling/README.md) · [rag-chroma](examples/openai-rag-chroma/README.md) · [web-search](examples/openai-web-search/README.md) · [web-fetch](examples/openai-web-fetch/README.md) · [wikipedia](examples/openai-wikipedia/README.md) · [terminal](examples/openai-terminal/README.md) · [github-projects](examples/openai-github-projects/README.md) · [server](examples/openai-server/README.md) · [aws-s3](examples/openai-aws-s3/README.md) · [azure-blob](examples/openai-azure-blob/README.md)
</details>

---

## Contributing

We build Sisu in the open. Contributions welcome.

[Contributing Guide](CONTRIBUTING.md) · [Report a Bug](https://github.com/finger-gun/sisu/issues/new?template=bug_report.md) · [Request a Feature](https://github.com/finger-gun/sisu/issues/new?template=feature_request.md) · [Code of Conduct](CODE_OF_CONDUCT.md)

---

<div align="center">

**[Star on GitHub](https://github.com/finger-gun/sisu)** if Sisu helps you build better agents.

*Quiet, determined, relentlessly useful.*

[Apache 2.0 License](LICENSE)

</div>