/library/
├── articles/     # Saved articles as markdown
├── notes/        # Agent-generated or user notes
└── context.md    # Library statistics and state

// When user saves an article
const sprite = await getUserSprite(userId);
await sprite.saveArticle(markdown, filename);

// The sprite is their persistent file system
// I never manage containers - Sprites handles that

Using list_files
Using read_file
Using read_file
Using read_file
Using read_file
Using read_file
Using write_file

static async getOrCreateForUser(user: User): Promise<UserSprite> {
  if (user.spriteName) {
    return new UserSprite(user.spriteName);
  }

  // Create new sprite for this user
  const spriteName = `readlater-${user.id.toLowerCase()}`;
  await spritesClient.createSprite(spriteName);

  // Initialize their file system
  const userSprite = new UserSprite(spriteName);
  await userSprite.exec("mkdir -p /library/articles /library/notes");
  await userSprite.writeFile("/library/context.md", INITIAL_CONTEXT);

  return userSprite;
}

function createSystemPrompt(context: string): string {
  return `You are a helpful reading assistant for a read-later app.

## Current Library State
${context}

## Your Environment
The user's library is at /library with:
- /library/articles/ - Saved articles as markdown
- /library/notes/ - Notes created by user or agent
- /library/context.md - Library statistics

## Your Capabilities
- read_file: Read file contents
- write_file: Create or update files
- list_files: List files in a directory
- search: Search across all files
- delete_file: Delete a file
`;
}

while (continueLoop && iteration < maxIterations) {
  const response = await anthropic.messages.create({
    model: "claude-sonnet-4-20250514",
    tools: TOOLS,
    messages,
  });

  // If the agent used tools, execute them
  for (const toolUse of toolUses) {
    const result = await executeTool(sprite, toolUse.name, toolUse.input);
    // Feed result back to agent
  }

  // If no tools used, agent is done
  if (toolUses.length === 0) {
    continueLoop = false;
  }
}

async function executeTool(sprite: UserSprite, toolName: string, input: any) {
  switch (toolName) {
    case "read_file":
      return await sprite.readFile(input.path);

    case "write_file":
      await sprite.writeFile(input.path, input.content);
      return `File written to ${input.path}`;

    case "list_files":
      return await sprite.exec(`ls -1 '${input.directory}'`);

    case "search":
      return await sprite.exec(`grep -r -i '${input.query}' /library`);
  }
}

---
url: https://fly.io/blog/code-and-let-live/
title: Code And Let Live
source: fly.io
saved_at: 2026-01-11T10:30:00Z
word_count: 2400
reading_time: 10
---

# Code And Let Live

Kurt Mackey argues that AI agents need durable computers...