feat(langchain): add a big tool middleware

[christian-bromann]

Add BigToolMiddleware for Dynamic Tool Selection

This PR introduces a new BigToolMiddleware that enables intelligent tool selection for agents with large tool sets (1000+ tools), preventing context window explosion while maintaining functionality.

When agents have access to hundreds or thousands of tools, sending all tool descriptions with every model request can:

• Exhaust context windows (50,000+ tokens for 1000 tools)
• Increase API costs significantly
• Slow down response times
• Reduce available context for actual conversation

Solution

The BigToolMiddleware dynamically selects only the most relevant tools for each query using configurable strategies:

• Keyword Matching: Selects tools based on keyword overlap between query and tool names/descriptions
• Semantic Similarity: Uses text similarity to find the most relevant tools
• Custom Logic: Allows users to implement their own tool selection algorithms

Usage Examples

Basic Keyword Strategy

import { bigToolMiddleware } from "langchain/middleware";

const agent = createAgent({
  model: "openai:gpt-4",
  tools: [...manyTools], // 1000+ tools
  middleware: [
    bigToolMiddleware({
      strategy: "keyword",
      maxTools: 20,
      keywordConfig: {
        keywords: ["file", "database", "api", "search"],
        matchDescriptions: true,
        minMatches: 1
      }
    })
  ]
});

Semantic Similarity Strategy

const semanticMiddleware = bigToolMiddleware({
  strategy: "semantic",
  semanticConfig: {
    threshold: 0.4,
    maxTools: 15
  }
});

Custom Selection Logic

const customMiddleware = bigToolMiddleware({
  strategy: "custom",
  customSelector: async (tools, query, context) => {
    // Your custom logic here
    return tools.filter(tool => someCustomLogic(tool, query)).slice(0, 10);
  }
});

[vercel]

The latest updates on your projects. Learn more about Vercel for GitHub.

Project	Deployment	Preview	Comments	Updated (UTC)
langchainjs-docs	Ready	Preview	Comment	Sep 18, 2025 1:32am

1 Skipped Deployment

Project	Deployment	Preview	Comments	Updated (UTC)
langchainjs-api-refs	Ignored			Sep 18, 2025 1:32am

[changeset-bot]

⚠️ No Changeset found

Latest commit: 0f3cf6a

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

Click here to learn what changesets are, and how to add one.

Click here if you're a maintainer who wants to add a changeset to this PR

[christian-bromann]

Closing in favor of #9050