Function validateContextLength

extensions/cli/src/util/tokenizer.ts:354–393

(params: ValidateContextLengthParams)

 * @returns Validation result with error details if invalid
 */
export function validateContextLength(params: ValidateContextLengthParams): {
  isValid: boolean;
  error?: string;
  inputTokens?: number;
  contextLimit?: number;
  maxTokens?: number;
} {
  const { chatHistory, model, safetyBuffer = 0, systemMessage, tools } = params;

  const inputTokens = countTotalInputTokens({
    chatHistory,
    systemMessage,
    tools,
    model,
  });
  const contextLimit = getModelContextLimit(model);
  const maxTokens = model.defaultCompletionOptions?.maxTokens || 0;

  // If maxTokens is not set, use 35% default reservation for output
  const reservedForOutput =
    maxTokens > 0 ? maxTokens : Math.ceil(contextLimit * 0.35);
  const totalRequired = inputTokens + reservedForOutput + safetyBuffer;

  if (totalRequired > contextLimit) {
    return {
      isValid: false,
      error: `Context length exceeded: input (${inputTokens.toLocaleString()}) + max_tokens (${reservedForOutput.toLocaleString()})${safetyBuffer > 0 ? ` + buffer (${safetyBuffer})` : ""} = ${totalRequired.toLocaleString()} > context_limit (${contextLimit.toLocaleString()})`,
      inputTokens,
      contextLimit,
      maxTokens: reservedForOutput,
    };
  }

  return {
    isValid: true,
    inputTokens,
    contextLimit,
    maxTokens: reservedForOutput,
  };
}