microsoft/semantic-kernel · error · KernelException

No argument '{variable.Value.ArgumentName ?? variableName}'

Error message

No argument '{variable.Value.ArgumentName ?? variableName}' provided for the '{variableName}' server variable of the operation - '{this.Id}'.

What it means

Thrown by GetServerUrl when resolving a templated server URL: a server variable (e.g. {username} in 'https://{username}.example.com') has no valid argument under its ArgumentName or Name and no Default value defined in the spec. Without a value the URL cannot be constructed, so the call fails before any network activity.

Source

Thrown at dotnet/src/Functions/Functions.OpenApi/Model/RestApiOperation.cs:421

                {
                    serverUrlString = serverUrlString.Replace($"{{{variableName}}}", Uri.EscapeDataString(argStrValue));
                }
                // Try to get the variable value by the variable name.
                else if (arguments.TryGetValue(variableName, out value) &&
                    value is string { } strValue &&
                    variable.Value.IsValid(strValue))
                {
                    serverUrlString = serverUrlString.Replace($"{{{variableName}}}", Uri.EscapeDataString(strValue));
                }
                // Use the default value if no argument is provided.
                else if (variable.Value.Default is not null)
                {
                    serverUrlString = serverUrlString.Replace($"{{{variableName}}}", variable.Value.Default);
                }
                // Throw an exception if there's no value for the variable.
                else
                {
                    throw new KernelException($"No argument '{variable.Value.ArgumentName ?? variableName}' provided for the '{variableName}' server variable of the operation - '{this.Id}'.");
                }
            }
        }
        else
        {
            serverUrlString =
                apiHostUrl?.AbsoluteUri ??
                throw new InvalidOperationException($"Server url is not defined for operation {this.Id}");
        }

        // Make sure base url ends with trailing slash
        if (!serverUrlString.EndsWith("/", StringComparison.OrdinalIgnoreCase))
        {
            serverUrlString += "/";
        }

        return new Uri(serverUrlString);
    }

View on GitHub (pinned to c028a0c7dc)

Solutions

1. Supply the server variable value in KernelArguments under the variable's name (or its sanitized ArgumentName).
2. If the variable has an allowed set, provide one of the enum values; otherwise add a 'default' to servers[].variables in the spec.
3. Pre-resolve the server by passing OpenApiFunctionExecutionParameters.ServerUrlOverride with the variables already substituted, bypassing template resolution.

Example fix

// before - server variable 'region' has no value and no default
"servers": [ { "url": "https://{region}.api.com", "variables": { "region": { "enum": ["us","eu"] } } } ]

// after - supply the region argument at invocation
var args = new KernelArguments { ["region"] = "us" };
var result = await kernel.InvokeAsync(plugin["op"], args);

foreach (var server in operation.Servers)
    foreach (var kv in server.Variables)
    {
        var key = kv.Value.ArgumentName ?? kv.Key;
        var hasVal = arguments.TryGetValue(key, out var v) && v is string s && kv.Value.IsValid(s);
        if (!hasVal && kv.Value.Default is null)
            throw new ArgumentException($"Provide server variable '{key}' for operation '{operation.Id}'.");
    }

static bool HasAllServerVariables(RestApiOperation op, IDictionary<string,object?> args)
    => op.Servers.SelectMany(s => s.Variables).All(kv =>
       { var key = kv.Value.ArgumentName ?? kv.Key;
         return (args.TryGetValue(key, out var v) && v is string s && kv.Value.IsValid(s)) || kv.Value.Default is not null; });

try { var url = operation.BuildOperationUrl(arguments); }
catch (KernelException ex) when (ex.Message.Contains("server variable of the operation"))
{ logger.LogWarning(ex, "Missing server variable; supply it or add a default."); throw; }

When it happens

Trigger: An OpenAPI document whose servers[].url uses template variables (curly braces) and the invocation does not supply a value for one of them, and that variable has no 'default' in servers[].variables. The lookup also validates the value via variable.IsValid (enum pattern), so an invalid enum value falls through to this same throw.

Common situations: A spec with per-tenant or per-region server templates where the caller forgets to pass the variable; an enum-constrained variable where the supplied value does not match an allowed entry; a spec whose variable has no default and the agent does not know to supply it.

Related errors

• The operation path resolves to '{requestAuthority}', which d
• No argument '{parameter.ArgumentName ?? parameter.Name}' is
• Server url is not defined for operation {this.Id}
• Payload parameters cannot be retrieved from the '{operation.
• The operation path resolves to '{requestPath}', which is out

AI-assisted analysis of microsoft/semantic-kernel@c028a0c7dc (2026-08-13). Data as JSON: /api/errors/ef3c8ea192df735f. Report an issue: GitHub.