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Fleet Prime Agent has three layers of capability, ordered from most built-in to most flexible.

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Built-in tools

Every session ships with:
  • ipython — a persistent Python kernel provisioned by IpythonKernelProvisioner.
  • bash / shell — run a shell command.
  • edit and file operations — read, write, patch files with structured diffs.
  • grep and find — search the working tree.
The older built-in names read, write, grep, find, ls were removed and no longer exist as separate tools; the current tools cover their use cases. Every tool implements the AgentTool interface (execute, prepareArguments) plus optional beforeToolCall / afterToolCall hooks that can block or override results. See Tool cards for what they look like in a run.

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Skills

Skills are executable packages the agent loads automatically. Two flavors:
  • MarkdownSkill — a markdown file (SKILL.md) that describes a workflow the agent should follow when a matching request comes in.
  • PythonSkill — an importable Python module that adds real behavior.
Skills load with loadSkills and loadSkillsFromDir. Under the hood a skill registers as an extension: it adds tools, slash commands, and shortcuts to the session. The built-in skill creator turns any recurring workflow you notice yourself repeating into a personal or project-level skill. See Skills guide.

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Extensions

Extensions are the general escape hatch. Any package that implements the ExtensionRuntime / ExtensionAPI interface can register:
  • New tools
  • New slash commands
  • Shortcuts
  • UI extensions consumed by the web adapter through ExtensionUIContext
Extensions must degrade locally: an optional feature failing to load can’t block agent startup or session attach. When an extension changes its public surface, both the runtime and the web adapter are updated in lockstep. See Extensions guide.

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MCP is a special case

Model Context Protocol servers are exposed to the agent as extensions. The runtime treats each registered MCP tool as a normal AgentTool — you get MCP interop without any special code path on the agent side. See MCP guide.
Last modified on August 17, 2026