Inngest Middleware

---
name: inngest-middleware
description: Use when adding cross-cutting concerns to durable functions — structured logging or tracing across all functions, error tracking with Sentry, payload encryption for sensitive data, dependency injection of clients (DB, Stripe, etc.) into function handlers, custom telemetry, or behavior that should apply uniformly across many functions. Covers Inngest middleware lifecycle, creating custom middleware, dependencyInjectionMiddleware, @inngest/middleware-encryption, @inngest/middleware-sentry, and custom middleware patterns.
---

# Inngest Middleware

Master Inngest middleware to handle cross-cutting concerns like logging, error tracking, dependency injection, and data transformation. Middleware runs at key points in the function lifecycle, enabling powerful patterns for observability and shared functionality.

> **These skills are focused on TypeScript.** For Python or Go, refer to the [Inngest documentation](https://www.inngest.com/llms.txt) for language-specific guidance. Core concepts apply across all languages.

> **Note:** The middleware system was significantly rewritten in v4. The lifecycle hooks documented here reflect the v4 API. If migrating from v3, consult the [migration guide](https://www.inngest.com/docs-markdown/reference/typescript/v4/migrations/v3-to-v4) for details on breaking changes.

> **⚠ For Realtime use the `inngest-realtime` skill, NOT this one.** Inngest v3 used `realtimeMiddleware()` from `@inngest/realtime` to inject a `publish` arg into function handlers. **v4 ships realtime natively** — `step.realtime.publish` is built-in, no middleware required. Do NOT install `@inngest/realtime` on a v4 project (it's a v3-era package and produces `TypeError: Cls is not a constructor` at runtime). See the `inngest-realtime` skill for the v4 pattern.

## What is Middleware?

Middleware allows code to run at various points in an Inngest client's lifecycle - during function execution, event sending, and more. Think of middleware as hooks into the Inngest execution pipeline.

**When to use middleware:**

- **Observability:** Add logging, tracing, or metrics
- **Dependency injection:** Share client instances across functions
- **Data transformation:** Encrypt/decrypt, validate, or enrich data
- **Error handling:** Custom error tracking and alerting
- **Authentication:** Validate user context or permissions

## Middleware Lifecycle

Middleware can be registered at **client-level** (affects all functions) or **function-level** (affects specific functions).

### Execution Order

```typescript
const inngest = new Inngest({
  id: "my-app",
  middleware: [
    loggingMiddleware, // Runs 1st
    errorMiddleware, // Runs 2nd
  ],
});

inngest.createFunction(
  {
    id: "example",
    middleware: [
      authMiddleware, // Runs 3rd
      metricsMiddleware, // Runs 4th
    ],
    triggers: [{ event: "test" }],
  },
  async () => {
    /* function code */
  },
);
```

**Order matters:** Client middleware runs first, then function middleware, in the order specified.

## Creating Custom Middleware

### Basic Middleware Structure

```typescript
import { InngestMiddleware } from "inngest";

const loggingMiddleware = new InngestMiddleware({
  name: "Logging Middleware",
  init() {
    // Setup phase - runs when client initializes
    const logger = setupLogger();

    return {
      // Function execution lifecycle
      // Note: `fn` is loosely typed in middleware generics; fn.id works at runtime
      onFunctionRun({ ctx, fn }) {
        return {
          beforeExecution() {
            logger.info("Function starting", {
              functionId: fn.id,
              eventName: ctx.event.name,
              runId: ctx.runId,
            });
          },

          afterExecution() {
            logger.info("Function completed", {
              functionId: fn.id,
              runId: ctx.runId,
            });
          },

          transformOutput({ result }) {
            // Log function output
            logger.debug("Function output", {
              functionId: fn.id,
              output: result.data,
            });

            // Return unmodified result
            return { result };
          },
        };
      },

      // Event sending lifecycle
      onSendEvent() {
        return {
          transformInput({ payloads }) {
            logger.info("Sending events", {
              count: payloads.length,
              events: payloads.map((p) => p.name),
            });

            // Spread to convert readonly array to mutable array
            return { payloads: [...payloads] };
          },
        };
      },
    };
  },
});
```

### Python Implementation

Python middleware follows a similar pattern. See [Dependency Injection Reference](./references/dependency-injection.md) for complete Python examples.

````

## Dependency Injection

Share expensive or stateful clients across all functions. **See [Dependency Injection Reference](./references/dependency-injection.md) for detailed patterns.**

### Quick Example - Built-in DI

```typescript
import { dependencyInjectionMiddleware } from "inngest";

const inngest = new Inngest({
  id: 'my-app',
  middleware: [
    dependencyInjectionMiddleware({
      openai: new OpenAI(),
      db: new PrismaClient(),
    }),
  ],
});

// Functions automatically get injected dependencies
inngest.createFunction(
  { id: "ai-summary", triggers: [{ event: "document/uploaded" }] },
  async ({ event, openai, db }) => {
    // Dependencies available in function context
    const summary = await openai.chat.completions.create({
      messages: [{ role: "user", content: event.data.content }],
      model: "gpt-4",
    });

    await db.document.update({
      where: { id: event.data.documentId },
      data: { summary: summary.choices[0].message.content }
    });
  }
);
````

## Middleware Packages

Beyond `dependencyInjectionMiddleware` (built-in, shown above), Inngest provides official middleware as **separate packages**. **See [Middleware Reference](./references/built-in-middleware.md) for complete details.**

### Encryption Middleware

```bash
npm install @inngest/middleware-encryption
```

```typescript
import { encryptionMiddleware } from "@inngest/middleware-encryption";

const inngest = new Inngest({
  id: "my-app",
  middleware: [
    encryptionMiddleware({
      key: process.env.ENCRYPTION_KEY,
    }),
  ],
});
```

Automatically encrypts all step data, function output, and event `data.encrypted` field. Supports key rotation via `fallbackDecryptionKeys`.

### Sentry Error Tracking

```bash
npm install @inngest/middleware-sentry
```

```typescript
import * as Sentry from "@sentry/node";
import { sentryMiddleware } from "@inngest/middleware-sentry";

Sentry.init({
  /* your Sentry config */
});

const inngest = new Inngest({
  id: "my-app",
  middleware: [sentryMiddleware()],
});
```

Captures exceptions, adds tracing to each function run, and includes function ID and event names as context. Requires `@sentry/*@>=8.0.0`.

## Common Middleware Patterns

### Metrics and Performance Tracking

```typescript
const metricsMiddleware = new InngestMiddleware({
  name: "Metrics Tracking",
  init() {
    return {
      onFunctionRun({ ctx, fn }) {
        let startTime: number;

        return {
          beforeExecution() {
            startTime = Date.now();
            metrics.increment("inngest.step.started", {
              function: fn.id,
              event: ctx.event.name,
            });
          },

          afterExecution() {
            const duration = Date.now() - startTime;
            metrics.histogram("inngest.step.duration", duration, {
              function: fn.id,
              event: ctx.event.name,
            });
          },

          transformOutput({ result }) {
            const status = result.error ? "error" : "success";
            metrics.increment("inngest.step.completed", {
              function: fn.id,
              status: status,
            });

            return { result };
          },
        };
      },
    };
  },
});
```

### Advanced Patterns

**Authentication:** Validate tokens and inject user context
**Conditional logic:** Apply middleware based on event type or function
**Circuit breakers:** Prevent cascading failures from external services

### Configuration-Based Middleware

Create reusable middleware with configuration options for different environments and use cases. See reference documentation for complete examples.

## Best Practices

### Design Principles

1. **Keep middleware focused:** One concern per middleware
2. **Handle errors gracefully:** Don't let middleware crash functions
3. **Consider performance:** Middleware runs on every execution
4. **Use proper typing:** Let TypeScript infer middleware types
5. **Test thoroughly:** Middleware affects all functions that use it

### Common Use Cases to Implement

- **Retry logic** for transient failures
- **Circuit breakers** for external service calls
- **Request/response logging** for debugging
- **User context enrichment** from external sources
- **Feature flags** for gradual rollouts
- **Custom authentication** and authorization checks

### Error Handling in Middleware

```typescript
const robustMiddleware = new InngestMiddleware({
  name: "Robust Middleware",
  init() {
    return {
      onFunctionRun({ ctx, fn }) {
        return {
          transformOutput({ result }) {
            try {
              // Your middleware logic here
              return performTransformation(result);
            } catch (middlewareError) {
              // Log error but don't break the function
              console.error("Middleware error:", middlewareError);

              // Return original result on middleware failure
              return { result };
            }
          },
        };
      },
    };
  },
});
```

### Testing Middleware

Use Inngest's testing utilities (`createMockContext`, `createMockFunction`) to unit test middleware behavior.

**For complete implementation examples and advanced patterns, see:**

- [Dependency Injection Reference](./references/dependency-injection.md)
- [Built-in Middleware Reference](./references/built-in-middleware.md)

## This Repository

These upstream Inngest instructions are vendored for agent tooling and
integration work in this monorepo.

## Repository Triggers

Use this skill when `inngest-middleware` matches the current Inngest task. If the
right skill is unclear, start with `docs/ai/skills/inngest/SKILL.md`.

## Repository Workflow

1. Confirm whether the request is agent-tooling guidance or product runtime
   integration.
2. Use `inngest-brownfield-audit` before changing existing app workflows or
   fragile background work.
3. Follow this upstream guidance under OpenSpec, root `AGENTS.md`, repo
   rulebooks, framework docs, and runtime evidence.
4. Keep runtime packages, app code, migrations, and `INNGEST_*` env
   requirements out of agent-tooling-only changes.

## Repository Checklist

- [ ] The task has explicit product-runtime scope before adding Inngest app code
      or dependencies.
- [ ] Existing workflows were audited before introducing or changing durable
      workflow behavior.
- [ ] Any MCP usage is backed by a running Inngest dev server on the configured
      port.
- [ ] Upstream source and license attribution remain documented in
      `docs/ai/skills/inngest/references/upstream.md`.