Agent Loop: "An item with the same key has already been added. Key: {tool_name}" when using McpClientTool

[imdj360]

Describe the Bug

Description

When adding a McpClientTool to an Agent Loop tools block, the workflow fails at runtime regardless of the key name used — including the designer-generated default MCP_server.

Error:

BadRequest – The request sent to the agent failed with the following error:
'HTTP request failed: 'An item with the same key has already been added. Key: {tool_name}'.'

Plan Type

Standard

Steps to Reproduce the Bug or Issue

Steps to Reproduce

1. Create a Stateful workflow with an Agent action
2. Add a McpClientTool under tools with any key name (e.g. MCP_server, ArithmeticMCP)
3. Trigger the workflow

Expected Behavior

Agent executes and routes calls to MCP tools correctly.

Actual Behavior

Immediate BadRequest — duplicate key error referencing the MCP tool key name.

Key Observations

• Happens with any key name — not just MCP_server
• The key appears exactly once in the workflow JSON — no actual duplicate in the definition
• Suggests the runtime registers the tool name internally before processing the user definition, causing the collision

Workflow JSON (sanitized)

"tools": {
    "ArithmeticMCP": {
        "type": "McpClientTool",
        "kind": "Builtin",
        "inputs": {
            "parameters": {
                "allowedTools": [
                    "wf_arithmetic_add",
                    "wf_arithmetic_sub",
                    "wf_arithmetic_mul",
                    "wf_arithmetic_div",
                    "wf_arithmetic_mod",
                    "wf_arithmetic_pow",
                    "wf_arithmetic_sqrt"
                ]
            },
            "connectionReference": {
                "connectionName": "mcpclient"
            }
        }
    },
    "SendTheFinalResult": { ... }
}

Environment

• Logic Apps Standard on ASP
• Agent Loop with McpClientTool (Builtin kind)
• MCP connection via mcpclient connection reference
• Stateful workflow

Workflow JSON

{
    "definition": {
        "$schema": "https://schema.management.azure.com/providers/Microsoft.Logic/schemas/2016-06-01/workflowdefinition.json#",
        "contentVersion": "1.0.0.0",
        "actions": {
            "Agent": {
                "type": "Agent",
                "inputs": {
                    "parameters": {
                        "deploymentId": "gpt-5-chat",
                        "messages": [
                            {
                                "role": "system",
                                "content": "You are a maths agent that solves arithmetic expressions following BODMAS order (Brackets, Orders/powers, Division, Multiplication, Addition, Subtraction).\n\nYou have access to arithmetic tools via an MCP server — use them for every calculation, do NOT compute in your head.\n\nApproach:\n1. Parse the expression and identify the correct BODMAS evaluation order.\n2. Call the appropriate MCP tool for each sub-expression, in order.\n3. Substitute the result back and continue until fully resolved.\n4. Return a clear step-by-step breakdown showing each tool call and its result, followed by the final answer.\n\nIf the expression is invalid or undefined (e.g. divide by zero), explain why."
                            },
                            {
                                "role": "user",
                                "content": "Solve this expression: @{triggerBody()?['expression']}"
                            }
                        ],
                        "agentModelType": "AzureOpenAI",
                        "agentModelSettings": {
                            "agentHistoryReductionSettings": {
                                "agentHistoryReductionType": "maximumTokenCountReduction",
                                "maximumTokenCount": 128000
                            },
                            "deploymentModelProperties": {
                                "name": "gpt-5-chat",
                                "format": "OpenAI",
                                "version": "2025-10-03"
                            }
                        }
                    },
                    "modelConfigurations": {
                        "model1": {
                            "referenceName": "agent"
                        }
                    }
                },
                "tools": {
                    "ArithmeticMCP": {
                        "type": "McpClientTool",
                        "kind": "Builtin",
                        "inputs": {
                            "parameters": {
                                "allowedTools": [
                                    "wf_arithmetic_add",
                                    "wf_arithmetic_div",
                                    "wf_arithmetic_mod",
                                    "wf_arithmetic_mul",
                                    "wf_arithmetic_pow",
                                    "wf_arithmetic_sqrt",
                                    "wf_arithmetic_sub"
                                ]
                            },
                            "connectionReference": {
                                "connectionName": "mcpclient"
                            }
                        }
                    },
                    "SendTheFinalResult": {
                        "actions": {
                            "Terminate": {
                                "type": "Terminate",
                                "inputs": {
                                    "runStatus": "Succeeded"
                                },
                                "runAfter": {
                                    "NotifyResultToUser": [
                                        "SUCCEEDED"
                                    ]
                                }
                            },
                            "NotifyResultToUser": {
                                "type": "Compose",
                                "inputs": "@agentParameters('MathResult')"
                            }
                        },
                        "description": "Send the final result to the user",
                        "agentParameterSchema": {
                            "type": "object",
                            "properties": {
                                "MathResult": {
                                    "type": "string",
                                    "description": "Result of the Math Equation"
                                }
                            },
                            "required": [
                                "MathResult"
                            ]
                        }
                    }
                },
                "runAfter": {},
                "limit": {
                    "count": 100
                }
            }
        },
        "outputs": {},
        "triggers": {
            "ReqMsg": {
                "type": "Request",
                "kind": "Http",
                "inputs": {
                    "method": "POST",
                    "schema": {
                        "type": "object",
                        "properties": {
                            "expression": {
                                "type": "string"
                            }
                        }
                    }
                }
            }
        }
    },
    "kind": "Stateful"
}

Screenshots or Videos

Additional context

Additional Context

• MCP Server is a Logic Apps Standard workflow exposed as an MCP server
• MCP Server is configured with anonymous authentication
• Auth is not a factor — the error occurs before any MCP call is made, at agent initialization
• The duplicate key exception is thrown by the Agent Loop runtime itself during tool registration, not by the MCP server or connection layer
• Both the Agent Loop and the MCP Server are Logic Apps Standard deployed on App Service Plan(ASP)
• MCP endpoint is reachable and functional when called directly outside the Agent Loop

Intermittent Behavior

• The error is not consistent — some runs fail with the duplicate key error, others succeed with identical configuration
• No code or configuration changes between failing and succeeding runs
• Suggests a race condition in the Agent Loop tool registration phase, where the runtime occasionally registers the MCP tool key twice before the workflow executes
• Failure rate appears random with no clear pattern tied to time, load, or input

[imdj360]

MCP Connector also doesn't support OAuth authorization. Error happens at tool registration, before any MCP call is made

Via Postman

Not able to trigger the workflow:

Works from Postman

[nxtdj]

I have reported the same issue in another gitrepo,

Azure/LogicAppsUX#9136