Error Handling

The Chucky SDK provides a comprehensive error hierarchy to help you handle different failure scenarios appropriately.

Error Classes

All SDK errors extend from ChuckyError:

import {
  ChuckyError,
  ConnectionError,
  AuthenticationError,
  BudgetExceededError,
  ConcurrencyLimitError,
  RateLimitError,
  SessionError,
  ToolExecutionError,
  TimeoutError,
  ValidationError,
} from '@chucky.cloud/sdk';

Error Hierarchy

ChuckyError (base)
├── ConnectionError      - WebSocket connection failures
├── AuthenticationError  - Invalid or expired tokens
├── BudgetExceededError  - AI or compute budget exceeded
├── ConcurrencyLimitError - Too many concurrent sessions
├── RateLimitError       - Rate limit hit
├── SessionError         - Session-specific errors
├── ToolExecutionError   - Tool handler failures
├── TimeoutError         - Operation timed out
└── ValidationError      - Invalid input/options

Error Properties

class ChuckyError extends Error {
  readonly code: string;                    // Error code (e.g., 'BUDGET_EXCEEDED')
  readonly details?: Record<string, unknown>; // Additional context
}

Handling Errors

Basic Error Handling

try {
  const result = await client.prompt({ message: 'Hello' });
} catch (error) {
  if (error instanceof ChuckyError) {
    console.error(`Error [${error.code}]:`, error.message);
    console.error('Details:', error.details);
  } else {
    console.error('Unexpected error:', error);
  }
}

Specific Error Types

try {
  const result = await client.prompt({ message: 'Hello' });
} catch (error) {
  if (error instanceof BudgetExceededError) {
    // User ran out of budget
    showUpgradePrompt();
  } else if (error instanceof AuthenticationError) {
    // Token invalid or expired
    await refreshToken();
    retry();
  } else if (error instanceof ConnectionError) {
    // Network issue
    showOfflineMessage();
  } else if (error instanceof RateLimitError) {
    // Too many requests
    await delay(error.details?.retryAfter || 5000);
    retry();
  } else if (error instanceof TimeoutError) {
    // Request took too long
    showTimeoutMessage();
  } else {
    // Unknown error
    showGenericError(error.message);
  }
}

Error Codes

Code	Error Class	HTTP Status	Description
`token_expired`	`AuthenticationError`	401	Token has expired
`user_budget_exhausted`	`BudgetExceededError`	429	User’s per-app budget exceeded
`developer_budget_exhausted`	`BudgetExceededError`	402	Service compute quota exhausted
`concurrent_limit_reached`	`ConcurrencyLimitError`	429	Max concurrent sessions reached
`CONNECTION_ERROR`	`ConnectionError`	-	WebSocket connection failed
`RATE_LIMIT`	`RateLimitError`	429	Too many requests
`SESSION_ERROR`	`SessionError`	-	Session operation failed
`TOOL_EXECUTION_ERROR`	`ToolExecutionError`	-	Tool handler threw error
`TIMEOUT`	`TimeoutError`	-	Operation timed out
`VALIDATION_ERROR`	`ValidationError`	400	Invalid input

Server Error Responses

The server returns errors with both a machine-readable error code and a human-readable message:

{
  "error": "concurrent_limit_reached",
  "message": "Maximum concurrent sessions reached (2/2). Please wait for an existing session to complete before starting a new one.",
  "active": 2,
  "max": 2
}

Error Messages

Error Code	User-Friendly Message
`token_expired`	”Your session token has expired. Please request a new token from the application.”
`user_budget_exhausted`	”You have reached your usage limit for this app. Please contact the app developer to increase your limit.”
`developer_budget_exhausted`	”This service has exhausted its compute quota for the current billing period. Please try again later or contact the service provider.”
`concurrent_limit_reached`	”Maximum concurrent sessions reached (X/Y). Please wait for an existing session to complete before starting a new one.”

Connection Errors

try {
  const client = new ChuckyClient({ token });
  await client.prompt({ message: 'Hello' });
} catch (error) {
  if (error instanceof ConnectionError) {
    // error.details may contain:
    // - code: WebSocket close code
    // - reason: Close reason
    // - wasClean: Whether close was clean

    if (error.details?.code === 1006) {
      // Abnormal closure - network issue
      showNetworkError();
    }
  }
}

Concurrency Errors

try {
  const result = await client.prompt({ message: 'Hello' });
} catch (error) {
  if (error instanceof ConcurrencyLimitError) {
    // error.details may contain:
    // - active: Number of active sessions
    // - max: Maximum allowed sessions
    // - sessions: List of active session info
    // - message: Human-readable error message

    const { active, max, message } = error.details || {};
    console.log(`Concurrency limit: ${active}/${max}`);

    // Show the user-friendly message from the server
    showMessage(message || `Too many sessions (${active}/${max}). Please wait.`);
  }
}

Budget Errors

try {
  const result = await session.send('Complex query...');
} catch (error) {
  if (error instanceof BudgetExceededError) {
    // error.details may contain:
    // - budgetType: 'ai' or 'compute'
    // - used: Amount used
    // - limit: Budget limit
    // - message: Human-readable error message

    const { budgetType, message } = error.details || {};

    // Show the server-provided message (includes helpful context)
    if (message) {
      showMessage(message);
    } else if (budgetType === 'ai') {
      showMessage('You\'ve used your AI budget for this period');
    } else {
      showMessage('Session time limit reached');
    }
  }
}

Session Errors

session.on({
  onError: (error) => {
    if (error instanceof SessionError) {
      // error.details may contain:
      // - sessionId: The session ID
      // - state: Session state when error occurred

      console.error(`Session ${error.details?.sessionId} error:`, error.message);

      // Session may be in error state
      if (session.state === 'error') {
        // Create a new session
        await createNewSession();
      }
    }
  },
});

Tool Execution Errors

Handle errors in tool handlers:

const myTool = tool(
  'risky_operation',
  'Do something that might fail',
  schema,
  async (input) => {
    try {
      const result = await riskyOperation(input);
      return textResult(result);
    } catch (err) {
      // Return error result instead of throwing
      return errorResult(`Operation failed: ${err.message}`);
    }
  }
);

When a tool throws, it becomes a ToolExecutionError:

try {
  await session.send('Use the risky tool');
} catch (error) {
  if (error instanceof ToolExecutionError) {
    // error.details may contain:
    // - toolName: Name of the tool that failed
    // - input: Input that was passed to the tool
    // - originalError: The original error thrown

    console.error(`Tool ${error.details?.toolName} failed:`, error.message);
  }
}

Retry Strategies

Simple Retry

async function withRetry<T>(
  fn: () => Promise<T>,
  maxAttempts = 3,
  delayMs = 1000
): Promise<T> {
  let lastError: Error;

  for (let attempt = 1; attempt <= maxAttempts; attempt++) {
    try {
      return await fn();
    } catch (error) {
      lastError = error as Error;

      // Don't retry certain errors
      if (error instanceof AuthenticationError) throw error;
      if (error instanceof BudgetExceededError) throw error;
      if (error instanceof ValidationError) throw error;

      if (attempt < maxAttempts) {
        await new Promise(r => setTimeout(r, delayMs * attempt));
      }
    }
  }

  throw lastError!;
}

// Usage
const result = await withRetry(() =>
  client.prompt({ message: 'Hello' })
);

Exponential Backoff

async function withExponentialBackoff<T>(
  fn: () => Promise<T>,
  options = { maxAttempts: 5, baseDelayMs: 1000, maxDelayMs: 30000 }
): Promise<T> {
  for (let attempt = 0; attempt < options.maxAttempts; attempt++) {
    try {
      return await fn();
    } catch (error) {
      if (
        error instanceof AuthenticationError ||
        error instanceof BudgetExceededError ||
        error instanceof ValidationError
      ) {
        throw error;
      }

      if (attempt === options.maxAttempts - 1) throw error;

      const delay = Math.min(
        options.baseDelayMs * Math.pow(2, attempt),
        options.maxDelayMs
      );
      const jitter = delay * 0.1 * Math.random();
      await new Promise(r => setTimeout(r, delay + jitter));
    }
  }

  throw new Error('Max attempts reached');
}

Error Recovery Patterns

Graceful Degradation

async function getAnswer(question: string): Promise<string> {
  try {
    const result = await client.prompt({ message: question });
    return result.text || 'No response';
  } catch (error) {
    if (error instanceof BudgetExceededError) {
      return 'You\'ve reached your usage limit. Please try again later.';
    }
    if (error instanceof ConnectionError) {
      return 'Unable to connect. Please check your internet connection.';
    }
    return 'Something went wrong. Please try again.';
  }
}

Circuit Breaker

class CircuitBreaker {
  private failures = 0;
  private lastFailure = 0;
  private state: 'closed' | 'open' | 'half-open' = 'closed';

  constructor(
    private threshold = 5,
    private resetTimeMs = 30000
  ) {}

  async call<T>(fn: () => Promise<T>): Promise<T> {
    if (this.state === 'open') {
      if (Date.now() - this.lastFailure > this.resetTimeMs) {
        this.state = 'half-open';
      } else {
        throw new Error('Circuit breaker is open');
      }
    }

    try {
      const result = await fn();
      this.onSuccess();
      return result;
    } catch (error) {
      this.onFailure();
      throw error;
    }
  }

  private onSuccess() {
    this.failures = 0;
    this.state = 'closed';
  }

  private onFailure() {
    this.failures++;
    this.lastFailure = Date.now();
    if (this.failures >= this.threshold) {
      this.state = 'open';
    }
  }
}

// Usage
const breaker = new CircuitBreaker();

try {
  const result = await breaker.call(() =>
    client.prompt({ message: 'Hello' })
  );
} catch (error) {
  if (error.message === 'Circuit breaker is open') {
    showServiceUnavailable();
  }
}

Logging Errors

function logError(error: Error, context?: Record<string, unknown>) {
  if (error instanceof ChuckyError) {
    console.error({
      type: 'ChuckyError',
      code: error.code,
      message: error.message,
      details: error.details,
      context,
      stack: error.stack,
    });
  } else {
    console.error({
      type: 'UnexpectedError',
      message: error.message,
      context,
      stack: error.stack,
    });
  }
}

Getting Started

Core Concepts

CLI

Advanced

Error Handling

Error Handling

Error Classes

Error Hierarchy

Error Properties

Handling Errors

Basic Error Handling

Specific Error Types

Error Codes

Server Error Responses

Error Messages

Connection Errors

Concurrency Errors

Budget Errors

Session Errors

Tool Execution Errors

Retry Strategies

Simple Retry

Exponential Backoff

Error Recovery Patterns

Graceful Degradation

Circuit Breaker

Logging Errors

Getting Started

Core Concepts

CLI

Advanced

​Error Handling

​Error Classes

​Error Hierarchy

​Error Properties

​Handling Errors

​Basic Error Handling

​Specific Error Types

​Error Codes

​Server Error Responses

​Error Messages

​Connection Errors

​Concurrency Errors

​Budget Errors

​Session Errors

​Tool Execution Errors

​Retry Strategies

​Simple Retry

​Exponential Backoff

​Error Recovery Patterns

​Graceful Degradation

​Circuit Breaker

​Logging Errors

Error Handling

Error Classes

Error Hierarchy

Error Properties

Handling Errors

Basic Error Handling

Specific Error Types

Error Codes

Server Error Responses

Error Messages

Connection Errors

Concurrency Errors

Budget Errors

Session Errors

Tool Execution Errors

Retry Strategies

Simple Retry

Exponential Backoff

Error Recovery Patterns

Graceful Degradation

Circuit Breaker

Logging Errors