deepagents-filesystem (JavaScript/TypeScript)

Overview

FilesystemMiddleware solves context engineering challenges by providing file operations through a pluggable backend system. Tools include: ls, read_file, write_file, edit_file, glob, grep.

Backend Types

StateBackend (Default)

Ephemeral storage in agent state - persists within a thread only.

import { createDeepAgent } from "deepagents";

const agent = await createDeepAgent({});
// Default StateBackend - files exist only within thread

FilesystemBackend (Local Disk)

import { createDeepAgent, FilesystemBackend } from "deepagents";

const agent = await createDeepAgent({
  backend: new FilesystemBackend({
    rootDir: ".",
    virtualMode: true  // Security: restrict paths
  })
});

StoreBackend (Persistent)

import { createDeepAgent, StoreBackend } from "deepagents";
import { InMemoryStore } from "@langchain/langgraph";

const store = new InMemoryStore();

const agent = await createDeepAgent({
  backend: (config) => new StoreBackend(config),
  store
});

CompositeBackend (Hybrid)

import { createDeepAgent, CompositeBackend, StateBackend, StoreBackend } from "deepagents";
import { InMemoryStore } from "@langchain/langgraph";

const store = new InMemoryStore();

const agent = await createDeepAgent({
  backend: (config) => new CompositeBackend(
    new StateBackend(config),
    { "/memories/": new StoreBackend(config) }
  ),
  store
});

Decision Table

Use Case	Backend	Why
Temporary files	StateBackend	Default, no setup
Local development	FilesystemBackend	Direct disk access
Cross-session memory	StoreBackend	Persists across threads
Hybrid storage	CompositeBackend	Mix ephemeral + persistent

Code Examples

Example 1: Managing Large Context

const agent = await createDeepAgent({});

const result = await agent.invoke({
  messages: [{
    role: "user",
    content: "Search for TypeScript best practices and save results for analysis"
  }]
});
// Agent: search -> write_file -> compact context -> read_file when needed

Example 2: Long-term Memory

import { createDeepAgent, CompositeBackend, StateBackend, StoreBackend } from "deepagents";
import { InMemoryStore } from "@langchain/langgraph";

const store = new InMemoryStore();

const agent = await createDeepAgent({
  backend: (config) => new CompositeBackend(
    new StateBackend(config),
    { "/memories/": new StoreBackend(config) }
  ),
  store
});

// Thread 1: Save preferences
await agent.invoke({
  messages: [{ role: "user", content: "Save my preference: concise explanations to /memories/prefs.txt" }]
}, { configurable: { thread_id: "thread-1" } });

// Thread 2: Access preferences
await agent.invoke({
  messages: [{ role: "user", content: "Read my preferences and explain async/await" }]
}, { configurable: { thread_id: "thread-2" } });

Example 3: Custom Tool Descriptions

import { createAgent, createFilesystemMiddleware } from "langchain";

const agent = createAgent({
  model: "claude-sonnet-4-5-20250929",
  middleware: [
    createFilesystemMiddleware({
      systemPrompt: "Save intermediate results to /workspace/",
      customToolDescriptions: {
        read_file: "Read files you've previously written. Use offset/limit for large files.",
        write_file: "Save data to avoid context overflow.",
      }
    }),
  ],
});

Boundaries

What Agents CAN Configure

✅ Backend type and configuration
✅ Custom tool descriptions
✅ File paths and organization
✅ Human-in-the-loop for file operations

What Agents CANNOT Configure

❌ Tool names
❌ Disable filesystem tools
❌ Access outside virtual_mode restrictions

Gotchas

1. StateBackend Files Don't Persist Across Threads

// ❌ Files lost when thread changes
await agent.invoke({messages: [{role: "user", content: "Write /notes.txt"}]}, 
  {configurable: {thread_id: "thread-1"}});
await agent.invoke({messages: [{role: "user", content: "Read /notes.txt"}]}, 
  {configurable: {thread_id: "thread-2"}});
// File not found!

// ✅ Use same thread_id OR StoreBackend

2. FilesystemBackend Security

// ❌ Insecure
new FilesystemBackend({ rootDir: "/project", virtualMode: false })

// ✅ Secure
new FilesystemBackend({ rootDir: "/project", virtualMode: true })

3. StoreBackend Requires Store

// ❌ Missing store
await createDeepAgent({ backend: (config) => new StoreBackend(config) });

// ✅ Provide store
await createDeepAgent({
  backend: (config) => new StoreBackend(config),
  store: new InMemoryStore()
});

Full Documentation

• Filesystem Middleware
• Backends Guide
• Long-term Memory