vercel/ai · error · UnsupportedFunctionalityError

Multiple system messages that are separated by user/assistan

Error message

Multiple system messages that are separated by user/assistant messages

What it means

convertToAmazonBedrockChatMessages only supports a system prompt as the very first message; Bedrock's Converse-style payload cannot represent a system message appearing after user/assistant turns. When a system message follows other messages, an UnsupportedFunctionalityError is thrown.

Source

Thrown at packages/amazon-bedrock/src/convert-to-amazon-bedrock-chat-messages.ts:174

  messages: AmazonBedrockMessages;
}> {
  const blocks = groupIntoBlocks(prompt);

  let system: AmazonBedrockSystemMessages = [];
  const messages: AmazonBedrockMessages = [];

  let documentCounter = 0;
  const generateDocumentName = () => `document-${++documentCounter}`;

  for (let i = 0; i < blocks.length; i++) {
    const block = blocks[i];
    const isLastBlock = i === blocks.length - 1;
    const type = block.type;

    switch (type) {
      case 'system': {
        if (messages.length > 0) {
          throw new UnsupportedFunctionalityError({
            functionality:
              'Multiple system messages that are separated by user/assistant messages',
          });
        }

        for (const message of block.messages) {
          system.push({ text: message.content });
          const cachePoint = getCachePoint(message.providerOptions);
          if (cachePoint) {
            system.push(cachePoint);
          }
        }
        break;
      }

      case 'user': {
        // combines all user and tool messages in this block into a single message:
        const amazonBedrockContent: AmazonBedrockUserMessage['content'] = [];

View on GitHub (pinned to 69428b1f8b)

Solutions

1. Move all system content into a single system message at the start of the message array.
2. Merge multiple/mid-conversation system text into the first system message or prepend it to the following user message.
3. If a mid-conversation reminder is needed, include it as part of the user message content instead.

Example fix

// before
messages: [
  { role: 'user', content: 'Hi' },
  { role: 'system', content: 'Be terse' },
]
// after
messages: [
  { role: 'system', content: 'Be terse' },
  { role: 'user', content: 'Hi' },
]

function assertSystemFirst(messages) {
  let seenOther = false;
  for (const m of messages) {
    if (m.role === 'system') {
      if (seenOther) throw new Error('System message must be the first message for Bedrock.');
    } else {
      seenOther = true;
    }
  }
}

import { UnsupportedFunctionalityError } from '@ai-sdk/provider';
try {
  await generateText({ model: bedrock(modelId), messages });
} catch (error) {
  if (UnsupportedFunctionalityError.isInstance(error) && error.functionality.includes('system messages')) {
    // reorder messages: hoist system prompt to position 0
  } else {
    throw error;
  }
}

When it happens

Trigger: Building a prompt array where a message with role 'system' appears after any user or assistant message — e.g. [user, system] or [user, assistant, system] — then passing it to generateText/streamText with a Bedrock model.

Common situations: Dynamically appending a system prompt mid-conversation; merging conversation histories where a reminder system message got inserted later; porting code from providers that allow interleaved system messages.

Related errors

• Unsupported task type: ${taskType}
• URL-based images are not supported for Amazon Bedrock image
• AI_UnsupportedFunctionalityError
• file parts with provider references
• File URL data

AI-assisted analysis of vercel/ai@69428b1f8b (2026-08-30). Data as JSON: /api/errors/8972a84abd9391d0. Report an issue: GitHub.