Tool Execution

Overview

When an LLM returns tool calls in its response, ModRouter does not automatically execute them. Instead, your application explicitly calls the tool execution API, giving you full control over:

Which tool calls to execute
User approval workflows
Security validation
Audit logging

The basic flow is: Chat Request → Review Tool Calls → Execute Tools → Continue Conversation.

Authentication

The /v1/mcp/tool/execute endpoint uses the same authentication as other inference endpoints like /v1/chat/completions:

Auth Configuration	Behavior
`disable_auth_on_inference: true`	No auth required
`disable_auth_on_inference: false`	Auth required

Virtual keys and authentication are independent layers that work together. For details on how to use virtual keys with authentication, see Authentication and virtual keys.

End-to-End Example

Gateway
Go SDK

Step 1: Send Chat Request

export MODROUTER_BASE_URL="https://api.modrouter.online"
export MODROUTER_API_KEY="<your-modrouter-virtual-key>"

curl -X POST "https://api.modrouter.online/v1/chat/completions" \
  -H "Authorization: Bearer ${MODROUTER_API_KEY}" \
  -H "Content-Type: application/json" \
  -d '{
    "model": "openai/gpt-4o",
    "messages": [
      {
        "role": "user",
        "content": "List files in the current directory"
      }
    ]
  }'

Response with tool calls:

{
  "id": "chatcmpl-abc123",
  "choices": [{
    "index": 0,
    "message": {
      "role": "assistant",
      "content": null,
      "tool_calls": [{
        "id": "call_xyz789",
        "type": "function",
        "function": {
          "name": "filesystem_list_directory",
          "arguments": "{\"path\": \".\"}"
        }
      }]
    },
    "finish_reason": "tool_calls"
  }]
}

Tool names are prefixed with the MCP client name (e.g., filesystem_list_directory). This ensures uniqueness across multiple MCP clients.

Step 2: Execute the Tool

The request body matches the tool call object from the response:

curl -X POST "https://api.modrouter.online/v1/mcp/tool/execute" \
  -H "Authorization: Bearer ${MODROUTER_API_KEY}" \
  -H "Content-Type: application/json" \
  -d '{
    "id": "call_xyz789",
    "type": "function",
    "function": {
      "name": "filesystem_list_directory",
      "arguments": "{\"path\": \".\"}"
    }
  }'

Tool result response:

{
  "role": "tool",
  "content": "[\"config.json\", \"main.go\", \"README.md\"]",
  "tool_call_id": "call_xyz789"
}

Step 3: Continue the Conversation

Assemble the full conversation history and continue:

curl -X POST "https://api.modrouter.online/v1/chat/completions" \
  -H "Authorization: Bearer ${MODROUTER_API_KEY}" \
  -H "Content-Type: application/json" \
  -d '{
    "model": "openai/gpt-4o",
    "messages": [
      {
        "role": "user",
        "content": "List files in the current directory"
      },
      {
        "role": "assistant",
        "content": null,
        "tool_calls": [{
          "id": "call_xyz789",
          "type": "function",
          "function": {
            "name": "filesystem_list_directory",
            "arguments": "{\"path\": \".\"}"
          }
        }]
      },
      {
        "role": "tool",
        "content": "[\"config.json\", \"main.go\", \"README.md\"]",
        "tool_call_id": "call_xyz789"
      }
    ]
  }'

Final response:

{
  "choices": [{
    "message": {
      "role": "assistant",
      "content": "The current directory contains 3 files:\n\n1. **config.json** - Configuration file\n2. **main.go** - Go source file\n3. **README.md** - Documentation"
    },
    "finish_reason": "stop"
  }]
}

package main

import (
    "context"
    "fmt"

    bifrost "github.com/maximhq/bifrost/core"
    "github.com/maximhq/bifrost/core/schemas"
)

func main() {
    // Initialize ModRouter with MCP (see Connecting to Servers)
    var config schemas.BifrostConfig
    client, err := bifrost.Init(context.Background(), config)
    if err != nil {
        panic(err)
    }

    // Step 1: Send initial request
    firstMessage := schemas.ChatMessage{
        Role: schemas.ChatMessageRoleUser,
        Content: &schemas.ChatMessageContent{
            ContentStr: bifrost.Ptr("List files in the current directory"),
        },
    }

    request := &schemas.BifrostChatRequest{
        Provider: schemas.OpenAI,
        Model:    "gpt-4o",
        Input:    []schemas.ChatMessage{firstMessage},
    }

    ctx := schemas.NewBifrostContext(context.Background(), schemas.NoDeadline)
    response, err := client.ChatCompletionRequest(ctx, request)
    if err != nil {
        panic(err)
    }

    // Build conversation history
    history := []schemas.ChatMessage{firstMessage}

    // Step 2: Process tool calls
    choice := response.Choices[0]
    assistantMessage := choice.ChatNonStreamResponseChoice.Message
    history = append(history, *assistantMessage)

    if assistantMessage.ChatAssistantMessage != nil {
        for i := range assistantMessage.ChatAssistantMessage.ToolCalls {
            toolCall := &assistantMessage.ChatAssistantMessage.ToolCalls[i]
            fmt.Printf("Tool requested: %s\n", *toolCall.Function.Name)

            // YOUR APPROVAL LOGIC HERE
            // - Validate arguments
            // - Check permissions
            // - Get user confirmation if needed

            // Step 3: Execute the tool
            toolResult, err := client.ExecuteChatMCPTool(ctx, toolCall)
            if err != nil {
                fmt.Printf("Tool execution failed: %v\n", err)
                continue
            }

            fmt.Printf("Tool result: %s\n", *toolResult.Content.ContentStr)
            history = append(history, *toolResult)
        }
    }

    // Step 4: Continue conversation with results
    finalRequest := &schemas.BifrostChatRequest{
        Provider: schemas.OpenAI,
        Model:    "gpt-4o",
        Input:    history,
    }

    finalResponse, err := client.ChatCompletionRequest(ctx, finalRequest)
    if err != nil {
        panic(err)
    }

    finalMessage := finalResponse.Choices[0].ChatNonStreamResponseChoice.Message
    fmt.Printf("Final response: %s\n", *finalMessage.Content.ContentStr)
}

Response Formats

ModRouter supports two API formats for tool execution:

Chat Format (Default)

Use ?format=chat or omit the parameter:

POST /v1/mcp/tool/execute?format=chat

Request:

{
  "id": "call_xyz789",
  "type": "function",
  "function": {
    "name": "filesystem_read_file",
    "arguments": "{\"path\": \"config.json\"}"
  }
}

Response:

{
  "role": "tool",
  "content": "{\"key\": \"value\"}",
  "tool_call_id": "call_xyz789"
}

Responses Format

Use ?format=responses for the Responses API format:

POST /v1/mcp/tool/execute?format=responses

Request:

{
  "type": "function_call_output",
  "call_id": "call_xyz789",
  "name": "filesystem_read_file",
  "arguments": "{\"path\": \"config.json\"}"
}

Response:

{
  "type": "function_call_output",
  "call_id": "call_xyz789",
  "output": "{\"key\": \"value\"}"
}

Multiple Tool Calls

LLMs often request multiple tools in a single response. Execute them in sequence or parallel:

Sequential
Parallel

for _, toolCall := range *response.Choices[0].Message.ToolCalls {
    result, err := client.ExecuteChatMCPTool(ctx, toolCall)
    if err != nil {
        // Handle error
        continue
    }
    history = append(history, *result)
}

toolCalls := *response.Choices[0].Message.ToolCalls
results := make([]*schemas.ChatMessage, len(toolCalls))
var wg sync.WaitGroup

for i, toolCall := range toolCalls {
    wg.Add(1)
    go func(idx int, tc schemas.ChatAssistantMessageToolCall) {
        defer wg.Done()
        result, err := client.ExecuteChatMCPTool(ctx, tc)
        if err == nil {
            results[idx] = result
        }
    }(i, toolCall)
}

wg.Wait()

for _, result := range results {
    if result != nil {
        history = append(history, *result)
    }
}

Error Handling

Tool execution can fail for various reasons:

result, err := client.ExecuteChatMCPTool(ctx, toolCall)
if err != nil {
    switch {
    case errors.Is(err, context.DeadlineExceeded):
        // Tool execution timed out
    case strings.Contains(err.Error(), "tool not found"):
        // Tool doesn't exist or client disconnected
    case strings.Contains(err.Error(), "not allowed"):
        // Tool filtered out by configuration
    default:
        // Other execution error
    }
}

Gateway error responses:

{
  "error": {
    "type": "tool_execution_error",
    "message": "Tool 'filesystem_delete_file' is not allowed for this request"
  }
}

Copy-Pastable Responses

Tool execution responses are designed to be directly appended to your conversation history:

// Tool result is already in the correct format
toolResult, _ := client.ExecuteChatMCPTool(ctx, toolCall)

// Just append it directly
history = append(history, *toolResult)

The response includes:

Correct role field ("tool")
Matching tool_call_id for correlation
Properly formatted content

Next Steps

Agent Mode

Enable autonomous tool execution with auto-approval

Tool Filtering

Control which tools are available per request

CLI Agents & Editors

MCP Gateway

Overview

Authentication

End-to-End Example

Step 1: Send Chat Request

Step 2: Execute the Tool

Step 3: Continue the Conversation

Response Formats

Chat Format (Default)

Responses Format

Multiple Tool Calls

Error Handling

Copy-Pastable Responses

Next Steps

Agent Mode

Tool Filtering

​Overview

​Authentication

​End-to-End Example

​Step 1: Send Chat Request

​Step 2: Execute the Tool

​Step 3: Continue the Conversation

​Response Formats

​Chat Format (Default)

​Responses Format

​Multiple Tool Calls

​Error Handling

​Copy-Pastable Responses

​Next Steps

Agent Mode

Tool Filtering

Overview

Authentication

End-to-End Example

Step 1: Send Chat Request

Step 2: Execute the Tool

Step 3: Continue the Conversation

Response Formats

Chat Format (Default)

Responses Format

Multiple Tool Calls

Error Handling

Copy-Pastable Responses

Next Steps