Preloading Toolspublic
Last verified 21 Sep 2026
Action Gateway gives applications and AI agents governed access to tens of thousands of tools through a managed MCP endpoint or SDK. See What You Can Build for example use cases of Action Gateway with Harness Runtime.
Preloading adds selected Action Gateway tool definitions to a session’s MCP tool list alongside the meta tools. The client receives the tools’ names, descriptions, and input schemas, so the model can call them directly without first using action_search.
Without preloading, the agent uses Tool Search to find relevant tools from the broader set the session can access. Search lets the agent choose tools for the current task without loading all their definitions into model context in advance.
Choosing Between Search and Preloading
| Approach | When to use it | Tradeoff |
|---|---|---|
| Tool search | Tasks vary, or the agent needs to discover which provider and tool best fit the request. | Requires a discovery call, but loads only the definitions returned by that search. |
| Preloading | A workflow regularly uses a small, known set of tools. | Avoids discovery for those tools, but their definitions occupy model context even when unused. |
| Both | A workflow has a few common operations and sometimes needs other tools. | Preload the common tools and use search for the rest. |
For example, a support assistant that regularly looks up an issue can preload that lookup tool. If the same assistant occasionally needs a tool from another provider, it can search for a suitable tool when that task arises. A research assistant handling varied requests may benefit more from searching than from loading a large fixed tool list.
Preloading can remove a discovery round trip for known tools. It does not make a tool execute faster or reduce its invocation charges. Loading more definitions can increase the model’s input-token usage, so preload only the tools the workload is likely to need.
Session Configuration
Configure preloads when creating a session. Enable Preload for individual tools or versioned Toolbelts in the Control Panel. In the API, use the config.preloadTools array. Preloading a Toolbelt exposes its member tools, not a single tool that executes the whole group.
Preloading does not change tool selection or permissions. Tools outside the session’s selection, unavailable tools, and tools denied by policy are not exposed as usable preloads. Other permitted tools can still be found through search even when they are not preloaded.
Create a replacement session to change the preload configuration. See Preload Frequently Used Tools.
Tool Definitions and Results
Preloading controls the definitions supplied to the model. Output views separately control which fields tool calls return. Combine a small preload set with focused output views when a known workflow needs both predictable tools and concise results.