Function Calling

Tool use with LeapFunction — same API on every platform, with Hermes and Pythonic parsers.

Function calling lets the model invoke predefined functions provided by your app — query an API, run a calculation, fetch external state. Register LeapFunction definitions on the Conversation, run generation as usual, and the model’s tool-call tokens come back as MessageResponse.functionCalls.

Not all models support function calling. Check the model card on Hugging Face before assuming a checkpoint supports tool use.

Vision and audio-capable models require companion files. Bundles embed these references; GGUF checkpoints expect siblings such as mmproj-*.gguf (vision) and audio decoder/tokenizer files. When detected, you can attach ChatMessageContent.image / .audio parts to your tool responses as well as user messages.

Register functions on the conversation

Conversation.registerFunction takes a LeapFunction instance describing name, parameters, and purpose.

Swift (iOS / macOS)

The Kotlin LeapFunction / LeapFunctionParameter constructors carry @ObjCName annotations on description:, so the Swift labels are functionDescription: and parameterDescription:. LeapFunctionParameter’s optional parameter has no Swift default — pass optional: false for required parameters.

conversation.registerFunction(
  function: LeapFunction(
    name: "get_weather",
    functionDescription: "Query the weather of a city",
    parameters: [
      LeapFunctionParameter(
        name: "city",
        type: LeapFunctionParameterType.LeapStr(enumValues: nil, description: nil),
        parameterDescription: "The city to query weather for",
        optional: false
      ),
      LeapFunctionParameter(
        name: "unit",
        type: LeapFunctionParameterType.LeapStr(
          enumValues: ["celsius", "fahrenheit"],
          description: nil
        ),
        parameterDescription: "Temperature unit (celsius or fahrenheit)",
        optional: false
      ),
    ]
  )
)

Kotlin (all platforms)

val conversation = modelRunner.createConversation("You are a helpful assistant.")

conversation.registerFunction(
  LeapFunction(
    name = "get_weather",
    description = "Get the weather forecast of a city",
    parameters = listOf(
      LeapFunctionParameter(
        name = "city",
        type = LeapFunctionParameterType.LeapStr(),
        description = "The city name",
      ),
      LeapFunctionParameter(
        name = "unit",
        type = LeapFunctionParameterType.LeapStr(
          enumValues = listOf("celsius", "fahrenheit")
        ),
        description = "Temperature unit",
      ),
    ),
  ),
)

Use normal identifiers — letters, underscores, and digits (not starting with a digit). Most models trained for tool use recognize that shape.

The Kotlin parameter type classes are named with a Leap prefix (LeapStr, LeapNum, LeapInt, LeapBool, LeapArr, LeapObj, LeapNull) to avoid collisions with Kotlin’s built-in String, Number, Int, Boolean, etc. The Swift bindings expose the same names — there are no separate .string(...) / .number(...) aliases; SKIE preserves the Kotlin nested-class names.

Handle the response

Function calls arrive as the MessageResponse.FunctionCalls variant on both platforms, wrapping a list of LeapFunctionCall payloads.

Swift (iOS / macOS)

LeapFunctionCall is a Kotlin data class bridged into Swift. arguments is a Kotlin Map<String, Any?> exposed as Swift [String: Any] (the ObjC bridge collapses Any? to non-optional id):

public class LeapFunctionCall {
  public var name: String
  public var arguments: [String: Any]
}

let userMessage = ChatMessage(role: .user, textContent: "What's the weather in NYC?")

for try await response in conversation.generateResponse(message: userMessage) {
    switch onEnum(of: response) {
    case .functionCalls(let payload):
        for call in payload.functionCalls {
            print("Function call: \(call.name) \(call.arguments)")
            // dispatch to your tool implementation
        }
    case .chunk, .reasoningChunk, .audioSample, .complete:
        break
    case .error(let err):
        print("Generation failed: \(err.message)")
    }
}

Kotlin (all platforms)

data class LeapFunctionCall(
    val name: String,
    val arguments: Map<String, Any?>,
)

conversation.generateResponse(userMessage).onEach { response ->
    when (response) {
        is MessageResponse.Chunk -> { /* text chunk */ }
        is MessageResponse.FunctionCalls -> {
            response.functionCalls.forEach { call ->
                Log.d(TAG, "Call ${call.name} with ${call.arguments}")
                // dispatch to your tool implementation
            }
        }
        is MessageResponse.Complete -> {
            // Tool calls are also surfaced on the assembled assistant message:
            response.fullMessage.functionCalls?.forEach { /* ... */ }
        }
        is MessageResponse.ReasoningChunk -> {}
        is MessageResponse.AudioSample -> {}
        is MessageResponse.Error -> Log.e(TAG, "Generation failed: ${response.message}", response.throwable)
    }
}.collect()

Tool calls are also attached to the final assistant message on the Complete event — useful if you’d rather batch-process tool calls once generation finishes.

The model may emit missing or invalid arguments — defensively validate the arguments map against your tool’s expected shape before dispatching.

Sending tool results back

Append the tool’s output as a tool-role message and continue the conversation. Both platforms use the same shape — see also Quick Start → Add tool results back to history.

Swift (iOS / macOS)

let toolMessage = ChatMessage(
  role: .tool,
  textContent: #"{"temperature":72,"conditions":"sunny"}"#
)

let updatedHistory = conversation.history + [toolMessage]
let nextConversation = conversation.modelRunner.createConversationFromHistory(history: updatedHistory)
// Continue generation against `nextConversation`.

Kotlin (all platforms)

val toolMessage = ChatMessage(
    role = ChatMessage.Role.TOOL,
    content = listOf(
        ChatMessageContent.Text("""{"temperature":72,"conditions":"sunny"}""")
    )
)

val updatedHistory = conversation.history + toolMessage
val nextConversation = modelRunner.createConversationFromHistory(updatedHistory)

Then call generateResponse(...) on the new conversation to get the model’s tool-aware reply.

LeapFunction

Swift (iOS / macOS)

Both types are Kotlin data classes bridged into Swift. @ObjCName annotations rename the description parameter on the Swift inits to functionDescription: / parameterDescription:.

public class LeapFunction {
  public var name: String
  public var functionDescription: String       // ObjC-renamed from Kotlin `description`
  public var parameters: [LeapFunctionParameter]

  public init(name: String, functionDescription: String, parameters: [LeapFunctionParameter])
}

public class LeapFunctionParameter {
  public var name: String
  public var type: LeapFunctionParameterType
  public var parameterDescription: String      // ObjC-renamed from Kotlin `description`
  public var optional: Bool

  public init(
    name: String,
    type: LeapFunctionParameterType,
    parameterDescription: String,
    optional: Bool                              // no default in Swift — pass `false` for required
  )
}

Kotlin (all platforms)

data class LeapFunction(
    val name: String,
    val description: String,
    val parameters: List<LeapFunctionParameter>,
)

data class LeapFunctionParameter(
    val name: String,
    val type: LeapFunctionParameterType,
    val description: String,
    val optional: Boolean = false,
)

Parameter types

LeapFunctionParameterType is translated into JSON Schema for the model. The same primitive set is exposed on both platforms.

Swift (iOS / macOS)

LeapFunctionParameterType is a Kotlin sealed class. SKIE generates an onEnum(of:)-compatible enum view, but the constructors you use to build instances keep the Kotlin nested-class names — there is no .string(...) / .number(...) alias.

// Direct constructors (use these to build parameter types):
LeapFunctionParameterType.LeapStr(enumValues: [String]?, description: String?)
LeapFunctionParameterType.LeapNum(enumValues: [NSNumber]?, description: String?)
LeapFunctionParameterType.LeapInt(enumValues: [KotlinInt]?, description: String?)
LeapFunctionParameterType.LeapBool(description: String?)
LeapFunctionParameterType.LeapArr(itemType: LeapFunctionParameterType, description: String?)
LeapFunctionParameterType.LeapObj(
  properties: [String: LeapFunctionParameterType],
  required: [String],
  description: String?
)
LeapFunctionParameterType.LeapNull()   // no description parameter

• LeapStr / LeapNum / LeapInt accept enumValues to constrain valid values.
• LeapArr has itemType describing the element type.
• LeapObj has properties: [String: LeapFunctionParameterType] and required: [String].
• The nested description is overridden when the type is used directly as LeapFunctionParameter.type; it’s only consulted when the type is used inside LeapArr.itemType or LeapObj.properties.

Kotlin (all platforms)

LeapFunctionParameterType.LeapStr(enumValues: List<String>? = null, description: String? = null)
LeapFunctionParameterType.LeapNum(enumValues: List<Number>? = null, description: String? = null)
LeapFunctionParameterType.LeapInt(enumValues: List<Int>? = null, description: String? = null)
LeapFunctionParameterType.LeapBool(description: String? = null)
LeapFunctionParameterType.LeapArr(itemType: LeapFunctionParameterType, description: String? = null)
LeapFunctionParameterType.LeapObj(
    properties: Map<String, LeapFunctionParameterType>,
    required: List<String> = listOf(),
    description: String? = null,
)
LeapFunctionParameterType.LeapNull()

The nested description is overridden when the type is used directly as LeapFunctionParameter.type; it’s only consulted when the type is used inside LeapArr.itemType or LeapObj.properties.

Example: array + enum parameters

Swift (iOS / macOS)

LeapFunction(
  name: "get_weather",
  functionDescription: "Query the weather of cities",
  parameters: [
    LeapFunctionParameter(
      name: "cities",
      type: LeapFunctionParameterType.LeapArr(
        itemType: LeapFunctionParameterType.LeapStr(enumValues: nil, description: nil),
        description: nil
      ),
      parameterDescription: "Names of the cities to query weather for",
      optional: false
    ),
    LeapFunctionParameter(
      name: "unit",
      type: LeapFunctionParameterType.LeapStr(
        enumValues: ["celsius", "fahrenheit"],
        description: nil
      ),
      parameterDescription: "Temperature unit",
      optional: false
    ),
  ]
)

Kotlin (all platforms)

LeapFunction(
  name = "get_weather",
  description = "Get the weather forecast of cities",
  parameters = listOf(
    LeapFunctionParameter(
      name = "cities",
      type = LeapFunctionParameterType.LeapArr(
        itemType = LeapFunctionParameterType.LeapStr()
      ),
      description = "City names to query",
    ),
    LeapFunctionParameter(
      name = "temperature_unit",
      type = LeapFunctionParameterType.LeapStr(
        enumValues = listOf("Fahrenheit", "Celsius", "Kelvin")
      ),
      description = "Units for temperature",
    ),
  ),
)

Function call parser

Different models emit tool-call tokens in different shapes. The parser translates those tokens into LeapFunctionCall values. The default LFMFunctionCallParser handles Liquid Foundation Model (LFM2) Pythonic-style control tokens (<|tool_call_start|> ... <|tool_call_end|>). For Qwen3 and other models using the Hermes function-calling format, use HermesFunctionCallParser.

Swift (iOS / macOS)

var options = GenerationOptions()
options.functionCallParser = HermesFunctionCallParser()

for try await response in conversation.generateResponse(
  message: userMessage,
  generationOptions: options
) {
  // ...
}

Kotlin (all platforms)

val options = GenerationOptions.build {
    functionCallParser = HermesFunctionCallParser()
}

conversation.generateResponse(userMessage, options).onEach { /* ... */ }.collect()

Pass null / nil as the parser to disable tool-call parsing entirely — the raw tool-call text is then surfaced as ordinary Chunks.