---
name: fail-fast
description: >-
  Detect and halt on precondition failures before expensive operations begin. Validate inputs, permissions, and state upfront in CI/CD and automation workflows.
---

# Fail Fast

## When to Use This Skill

Fail fast is a design pattern that validates preconditions before executing expensive or irreversible operations. When validation fails, the system immediately reports the error rather than proceeding and failing later in an unpredictable state.

```mermaid
flowchart TD
    subgraph validation[Validation Phase]
        A[Request] --> B{Preconditions Met?}
    end

    subgraph execution[Execution Phase]
        C[Execute Operation]
        D[Success]
    end

    subgraph failure[Failure Phase]
        E[Report Error]
        F[No Side Effects]
    end

    B -->|Yes| C
    C --> D
    B -->|No| E
    E --> F

    %% Ghostty Hardcore Theme
    style A fill:#65d9ef,color:#1b1d1e
    style B fill:#fd971e,color:#1b1d1e
    style C fill:#a7e22e,color:#1b1d1e
    style D fill:#a7e22e,color:#1b1d1e
    style E fill:#f92572,color:#1b1d1e
    style F fill:#f92572,color:#1b1d1e

```

The key insight: **fail before you start, not in the middle**.

---


## When to Apply

| Scenario | Apply Fail Fast? | Reasoning |
| ---------- | ------------------ | ----------- |
| Invalid user input | Yes | User error, report immediately |
| Missing required config | Yes | Can't continue safely |
| Insufficient permissions | Yes | Operation will fail anyway |
| Resource allocation failure | Yes | Partial allocation is worse |
| Network timeout | No | Use [graceful degradation](../graceful-degradation/index.md) |
| Cache miss | No | Expensive path still works |

**Decision rule**: Fail fast on **precondition failures**. Degrade gracefully on **runtime failures**.

---


## Implementation

### GitHub Actions Workflow Validation


*See [examples.md](examples.md) for detailed code examples.*

### Go Function with Precondition Validation


*See [examples.md](examples.md) for detailed code examples.*

---


## Techniques


### Fail Fast Techniques

Comprehensive techniques for implementing fail fast patterns:

### [Early Termination](techniques/early-termination.md)

Stop execution immediately when errors occur:

- Shell strict mode (`set -euo pipefail`)
- GitHub Actions matrix fail-fast
- Go error propagation
- Circuit breakers

### [Strict Mode Execution](techniques/strict-mode.md)

Enable strictest validation and error detection:

- Shell/TypeScript/Go strict modes
- Linter enforcement
- Schema validation

### [Assertion Patterns](techniques/assertions.md)

Validate assumptions and fail if they're wrong:

- Runtime assertions
- Contract validation (pre/post conditions)
- Invariant checks
- Type guards

### [Error Escalation](techniques/error-escalation.md)

Determine when to throw vs return, panic vs recover:

- Throw vs return error
- Error aggregation vs first-error-wins
- Panic vs recoverable errors
- Exit codes

### [Timeout Enforcement](techniques/timeouts.md)

Prevent operations from running indefinitely:

- Operation timeouts
- Job timeouts
- Circuit breaker timeouts
- Deadlock detection

---

*See [reference.md](reference.md) for additional techniques and detailed examples.*


## Comparison

These patterns are **complementary**, not contradictory:


*See [examples.md](examples.md) for detailed code examples.*

| Error Type | Pattern | Example |
| ------------ | --------- | --------- |
| Missing config | Fail Fast | Can't start without database URL |
| Database timeout | Graceful Degradation | Retry, then use cache |
| Invalid input | Fail Fast | Reject malformed request |
| API unavailable | Graceful Degradation | Use backup endpoint |
| Insufficient permissions | Fail Fast | Don't attempt forbidden operation |
| Rate limited | Graceful Degradation | Exponential backoff |

---


## Anti-Patterns to Avoid

### 1. Late Validation

Validating after side effects have occurred.


*See [examples.md](examples.md) for detailed code examples.*

### 2. Swallowing Errors

Continuing despite failures.


*See [examples.md](examples.md) for detailed code examples.*

### 3. Partial Execution

Executing some operations before checking all preconditions.


*See [examples.md](examples.md) for detailed code examples.*

### 4. Vague Error Messages

Failing fast but not explaining why.

```bash
# Bad: unhelpful error
[ -f "$CONFIG" ] || exit 1

# Good: actionable error message
[ -f "$CONFIG" ] || { echo "Config file not found: $CONFIG. Create it from config.example.yml"; exit 1; }
```

---


## Examples

See [examples.md](examples.md) for code examples.


## Full Reference

See [reference.md](reference.md) for complete documentation.
## References

- [Source Documentation](https://adaptive-enforcement-lab.com/patterns/error-handling/)
- [AEL Patterns](https://adaptive-enforcement-lab.com/patterns/)