Collapsing

Collapsing is the system that organizes toolkits into a hierarchical structure. Instead of exposing every function to the model simultaneously, toolkits are grouped and presented as high-level "skill" containers. The model first selects a container, then the functions inside it expand for use.

This reduces prompt length and helps models with large tool sets navigate functions more effectively.

Collapsing is enabled by default for all C# toolkits registered via AgentBuilder.

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Configuration

var config = new AgentConfig
{
    Collapsing = new CollapsingConfig
    {
        Enabled = true,
        SkillInstructionMode = SkillInstructionMode.Both   // default
    }
};

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Properties

Property	Type	Default	Description
Enabled	bool	true	Enable collapsing for C# toolkits
CollapseClientTools	bool	false	Also collapse client-side tools (browser/remote)
MaxFunctionNamesInDescription	int	10	Max function names to list in the auto-generated container description
SkillInstructionMode	SkillInstructionMode	Both	Where to inject skill expansion instructions
PersistSystemPromptInjections	bool	false	Keep injected skill instructions across turns (not just the current one)
EnableErrorRecovery	bool	true	Auto-expand a collapsed toolkit if the model calls a function that should be inside it
NeverCollapse	HashSet<string>?	null	Toolkit names that should never be collapsed — always fully expanded
MCPServerInstructions	Dictionary<string, string>?	null	Custom instructions to inject after a specific MCP server container expands
ClientToolsInstructions	string?	null	Instructions to inject after the ClientTools container expands

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SkillInstructionMode

Value	Description
PromptMiddlewareOnly	Inject skill instructions only via prompt middleware — keeps ChatOptions clean.
Both	Inject via both prompt middleware and directly into ChatOptions (default)

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Examples

Disable collapsing for a specific toolkit

Collapsing = new CollapsingConfig
{
    NeverCollapse = new HashSet<string> { "FileToolkit", "CalculatorToolkit" }
}

These toolkits will always have all their functions directly visible to the model.

Per-server MCP instructions

Collapsing = new CollapsingConfig
{
    MCPServerInstructions = new Dictionary<string, string>
    {
        ["my-database-server"] = "Always filter results by the current user's tenant ID.",
        ["my-search-server"] = "Prefer results from the last 6 months."
    }
}

Disable collapsing entirely

Collapsing = new CollapsingConfig { Enabled = false }

All functions are exposed to the model at all times. Suitable for agents with a small number of tools.

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JSON Example

{
    "Collapsing": {
        "Enabled": true,
        "CollapseClientTools": false,
        "MaxFunctionNamesInDescription": 10,
        "SkillInstructionMode": "Both",
        "PersistSystemPromptInjections": false,
        "EnableErrorRecovery": true,
        "NeverCollapse": ["CalculatorToolkit"],
        "ClientToolsInstructions": null
    }
}

MCPServerInstructions can also be expressed as a JSON object.

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See Also

• Agent Config
• Tools Overview