> ## Documentation Index > Fetch the complete documentation index at: https://gofastmcp.com/llms.txt > Use this file to discover all available pages before exploring further. # prefab_payload # `fastmcp.server.providers.prefab_payload` Late-bound tool names in Prefab UI payloads. A Prefab UI is serialized during the entry tool's call, deep inside whatever composition the server happens to have. At that moment nothing knows what the backend tools will be *called* by the time the payload reaches a host: every layer above may rename them, and the outermost layer's names are the only ones a client can actually invoke. So the payload leaves the app addressed by identity — `_`, stable everywhere — and every FastMCP server rewrites those references on the way out to whatever it lists that tool as. Servers rewrite innermost-first, so the edge writes last and wins. Rewriting a name in place would destroy the identity for the next layer up, so the payload carries a name-to-identity map under `_meta.fastmcp.toolNames`. Each layer resolves through the map and updates it. The action objects keep the exact shape `prefab_ui` defines — only the value of `tool` changes, and only ever to another valid tool name. Renderers read `_meta` already and ignore keys they don't recognize, so this needs no renderer change. ## Functions ### `payload_has_identities` ```python theme={"theme":{"light":"snazzy-light","dark":"dark-plus"}} payload_has_identities(payload: Any) -> bool ``` Cheap guard: does this payload carry tool references worth rewriting? Runs on every tool result, so it must not walk the tree. ### `annotate_payload_identities` ```python theme={"theme":{"light":"snazzy-light","dark":"dark-plus"}} annotate_payload_identities(payload: dict[str, Any]) -> dict[str, Any] ``` Record the identity-addressed form of each reference, at serialization. References start out as `_`, so the map begins as an identity map to itself. Once a later layer rewrites a name, this is the only remaining route back: it carries both what the reference points at and the address any server can fall back to. ### `rewrite_payload_tool_names` ```python theme={"theme":{"light":"snazzy-light","dark":"dark-plus"}} rewrite_payload_tool_names(payload: Any, resolve: IdentityResolver) -> Any ``` Re-address a payload's tool references to this server's own names. Mutates in place and returns the payload. A reference this server cannot resolve is restored to its identity-addressed form rather than left as-is. Leaving it would strand whatever name an inner server chose — a name that is correct there and meaningless here — and, unlike the identity form, a stranded name has no route back. Restoring keeps the reference resolvable by the dispatcher, or by any server further out with a better view.