pruneMessages()
The pruneMessages
function is used to prune or filter an array of ModelMessage
objects. This is useful for reducing message context (to save tokens), removing intermediate reasoning, or trimming tool calls and empty messages before sending to an LLM.
app/api/chat/route.ts
import { pruneMessages, streamText } from 'ai';
export async function POST(req: Request) { const { messages } = await req.json();
const prunedMessages = pruneMessages({ messages, reasoning: 'before-last-message', toolCalls: 'before-last-2-messages', emptyMessages: 'remove', });
const result = streamText({ model: 'openai/gpt-4o', messages: prunedMessages, });
return result.toUIMessageStreamResponse();}
Import
import { pruneMessages } from "ai"
API Signature
Parameters
messages:
ModelMessage[]
An array of ModelMessage objects to prune.
reasoning:
'all' | 'before-last-message' | 'none'
How to remove reasoning content from assistant messages. Default: "none".
toolCalls:
'all' | 'before-last-message' | 'before-last-${number}-messages' | 'none' | PruneToolCallsOption[]
How to prune tool call/results/approval content. Can specify strategy or a list with tools.
emptyMessages:
'keep' | 'remove'
Whether to keep or remove messages whose content is empty after pruning. Default: "remove".
Returns
An array of ModelMessage
objects, pruned according to the provided options.
ModelMessage[]:
Array
The pruned list of ModelMessage objects
Example Usage
import { pruneMessages } from 'ai';
const pruned = pruneMessages({ messages, reasoning: 'all', // Remove all reasoning parts toolCalls: 'before-last-message', // Remove tool calls except those in the last message});
Pruning Options
- reasoning: Removes reasoning parts from assistant messages. Use
'all'
to remove all,'before-last-message'
to keep reasoning in the last message, or'none'
to retain all reasoning. - toolCalls: Prune tool-call, tool-result, and tool-approval chunks from assistant/tool messages. Options include:
'all'
: Prune all such content.'before-last-message'
: Prune except in the last message.before-last-N-messages
: Prune except in the last N messages.'none'
: Do not prune.- Or provide an array for per-tool fine control.
- emptyMessages: Set to
'remove'
(default) to exclude messages that have no content after pruning.
Tip:
pruneMessages
is typically used prior to sending a context window to an LLM to reduce message/token count, especially after a series of tool-calls and approvals.
For advanced usage and the full list of possible message parts, see ModelMessage
and pruneMessages
implementation.