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fastmcp.experimental.transforms.code_mode

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Classes

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SandboxProvider

Interface for executing LLM-generated Python code in a sandbox. WARNING: The code parameter passed to run contains untrusted, LLM-generated Python. Implementations MUST execute it in an isolated sandbox — never with plain exec(). Use MontySandboxProvider (backed by pydantic-monty) for production workloads. Methods:

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run 

run(self, code: str) -> Any

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MontySandboxProvider

Sandbox provider backed by pydantic-monty. Args:
  • limits: Resource limits for sandbox execution. Supported keys: max_duration_secs (float), max_allocations (int), max_memory (int), max_recursion_depth (int), gc_interval (int). All are optional; omit a key to leave that limit uncapped.
Methods:

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run 

run(self, code: str) -> Any
Discovery tool factory that searches the catalog by query. Args:
  • search_fn: Async callable (tools, query) -> matching_tools. Defaults to BM25 ranking.
  • name: Name of the synthetic tool exposed to the LLM.
  • default_detail: Default detail level for search results. "brief" returns tool names and descriptions only. "detailed" returns compact markdown with parameter schemas. "full" returns complete JSON tool definitions.
  • default_limit: Maximum number of results to return. The LLM can override this per call. None means no limit.

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GetSchemas

Discovery tool factory that returns schemas for tools by name. Args:
  • name: Name of the synthetic tool exposed to the LLM.
  • default_detail: Default detail level for schema results. "brief" returns tool names and descriptions only. "detailed" renders compact markdown with parameter names, types, and required markers. "full" returns the complete JSON schema.

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GetTags

Discovery tool factory that lists tool tags from the catalog. Reads tool.tags from the catalog and groups tools by tag. Tools without tags appear under "untagged". Args:
  • name: Name of the synthetic tool exposed to the LLM.
  • default_detail: Default detail level. "brief" returns tag names with tool counts. "full" lists all tools under each tag.

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ListTools

Discovery tool factory that lists all tools in the catalog. Args:
  • name: Name of the synthetic tool exposed to the LLM.
  • default_detail: Default detail level. "brief" returns tool names and one-line descriptions. "detailed" returns compact markdown with parameter schemas. "full" returns the complete JSON schema.

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CodeMode

Transform that collapses all tools into discovery + execute meta-tools. Discovery tools are composable via the discovery_tools parameter. Each is a callable that receives catalog access and returns a Tool. By default, Search and GetSchemas are included for progressive disclosure: search finds candidates, get_schema retrieves parameter details, and execute runs code. The execute tool is always present and provides a sandboxed Python environment with call_tool(name, params) in scope. Methods:

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transform_tools 

transform_tools(self, tools: Sequence[Tool]) -> Sequence[Tool]

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get_tool 

get_tool(self, name: str, call_next: GetToolNext) -> Tool | None