dotnet/yarp · error · InvalidOperationException

The transform contains more parameters than expected: {strin

Error message

The transform contains more parameters than expected: {string.Join(';', rawTransform.Keys)}

What it means

Thrown (or recorded) by TransformHelpers.CheckTooManyParameters / TryCheckTooManyParameters when a raw transform dictionary has more keys than the transform expects. Excess keys usually mean a typo'd extra field, a stray comma, or a misunderstanding of which keys belong to a transform. The check version throws InvalidOperationException; the Try version appends to context.Errors for validation.

Source

Thrown at src/ReverseProxy/Transforms/Builder/TransformHelpers.cs:23

using System.Collections.Generic;

namespace Yarp.ReverseProxy.Transforms.Builder;

public static class TransformHelpers
{
    public static void TryCheckTooManyParameters(TransformRouteValidationContext context, IReadOnlyDictionary<string, string> rawTransform, int expected)
    {
        if (rawTransform.Count > expected)
        {
            context.Errors.Add(new InvalidOperationException("The transform contains more parameters than expected: " + string.Join(';', rawTransform.Keys)));
        }
    }

    public static void CheckTooManyParameters(IReadOnlyDictionary<string, string> rawTransform, int expected)
    {
        if (rawTransform.Count > expected)
        {
            throw new InvalidOperationException("The transform contains more parameters than expected: " + string.Join(';', rawTransform.Keys));
        }
    }

    internal static void RemoveAllXForwardedHeaders(TransformBuilderContext context, string prefix)
    {
        context.AddXForwardedFor(prefix + ForwardedTransformFactory.ForKey, ForwardedTransformActions.Remove);
        context.AddXForwardedPrefix(prefix + ForwardedTransformFactory.PrefixKey, ForwardedTransformActions.Remove);
        context.AddXForwardedHost(prefix + ForwardedTransformFactory.HostKey, ForwardedTransformActions.Remove);
        context.AddXForwardedProto(prefix + ForwardedTransformFactory.ProtoKey, ForwardedTransformActions.Remove);
    }

    internal static void RemoveForwardedHeader(TransformBuilderContext context)
    {
        context.RequestTransforms.Add(RequestHeaderForwardedTransform.RemoveTransform);
    }
}

View on GitHub (pinned to bd11867bee)

Solutions

1. Compare the keys in the error to the transform's documented parameter set and remove the extra key.
2. Split a multi-purpose transform object into separate transform entries.
3. Run route config validation (TryCheckTooManyParameters path) during startup to surface these as config errors instead of runtime exceptions.

Example fix

// before
{ "PathRemovePrefix": "/api", "Set": "/new" }
// after
{ "PathRemovePrefix": "/api" }

TransformHelpers.CheckTooManyParameters(rawTransform, expected);
// or for validation context:
TransformHelpers.TryCheckTooManyParameters(context, rawTransform, expected);

static bool HasExpectedKeys(IReadOnlyDictionary<string,string> raw, int expected) =>
    raw.Count <= expected;

When it happens

Trigger: A transform entry includes an unexpected extra key alongside the recognized ones, e.g. `PathRemovePrefix` with an additional `Set` key, or two transform types accidentally merged into one object.

Common situations: Merging transform config by hand and leaving a key from a sibling transform. Typo creating a phantom key. Old config retained a removed parameter after a YARP upgrade.

Related errors

• Unknown transform: {string.Join(';', rawTransform.Keys)}
• Unexpected value for Forwarded: {token}. Expected 'for', 'ho
• {string.Join(";", transformValues.Keys)}
• Unexpected parameters for RequestHeader: {string.Join(';', t
• {string.Join(";", transformValues.Keys)}

AI-assisted analysis of dotnet/yarp@bd11867bee (2026-08-13). Data as JSON: /api/errors/3b9d2bc050d15455. Report an issue: GitHub.