Resolving Inconsistent Schema In MCP Tools For String Arrays

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Hey everyone! Today, we're diving deep into a quirky inconsistency we've found within the Model Context Protocol (MCP) tools, specifically when dealing with different array types. It's a bit of a technical journey, but stick with me, and we'll unravel this puzzle together.

Understanding the Inconsistency in MCP Tools Schema

When working with MCP tools and schemas, one expects a certain level of consistency, especially when dealing with arrays. The core issue arises when a tool returns an array of different types. Ideally, the schema for the array itself should remain consistent, with only the type of element within the array changing. However, this isn't the case when comparing the responses for string[] and other object arrays like Animal[].

Let's break down the specific problem. Imagine you have a method that returns an array of animals (Animal[]). The response you get from the MCP tool might look something like this:

{
 "content": [
 {
 "type": "text",
 "text": "[{\"name\":\"Bear\",\"weight\":500},{\"name\":\"Elephant\",\"weight\":6000}]"
 }
 ],
 "isError": false
}

Notice how the content array has a single element, and the actual array of animals is represented as a JSON-formatted string within the text field. This is how we get an array of Animal objects represented in the response.

Now, let's consider a method that returns an array of strings (string[]). The response looks quite different:

{
 "content": [
 {
 "type": "text",
 "text": "Bear"
 },
 {
 "type": "text",
 "text": "Elephant"
 }
 ]
}

Here, the content array contains two elements, each being a string. There's no JSON-formatted string encapsulating the array. The inconsistency is subtle but significant: for Animal[], we get a content array with one element containing a JSON string, while for string[], we get a content array with multiple elements, each being a string. This divergence in structure can lead to confusion and extra work when processing responses.

This inconsistency stems from how the MCP server library handles IEnumerable<string>. Stephen Toub pinpointed the special casing in the MCP server library that leads to this behavior, specifically in the AIFunctionMcpServerTool.cs file. Let's explore the root cause in more detail.

Root Cause Analysis: Special Casing of IEnumerable<string>

To truly understand why this inconsistency exists, we need to delve into the code. The culprit lies within the MCP server library, where IEnumerable<string> is treated differently from other enumerable types. This special handling, while perhaps intended to simplify certain scenarios, introduces the inconsistency we're discussing.

Specifically, the issue arises in the AIFunctionMcpServerTool.cs file within the csharp-sdk. The relevant code snippet identified by @stephentoub is:

// [Link to GitHub code]

This section of the code demonstrates that when the return type is IEnumerable<string>, the server directly serializes each string element into the content array. This contrasts with the behavior for other array types, such as Animal[], where the entire array is serialized into a single JSON string within the text field.

Why does this special casing exist? It's possible that the initial design aimed to simplify the handling of string arrays, assuming they would be a common return type. By directly embedding the strings in the content array, the server avoids an extra layer of JSON serialization. However, this optimization comes at the cost of consistency. The differing schema for string[] responses requires developers to write conditional logic to handle the two cases separately, increasing complexity and the potential for errors.

The impact of this inconsistency extends beyond mere inconvenience. It affects the predictability of the MCP tool responses. Developers must be acutely aware of the return type to correctly parse the content. This adds cognitive load and can lead to brittle code that breaks if the return type changes. Furthermore, it violates the principle of least astonishment, as the behavior is not what one would intuitively expect.

To illustrate the issue further, let's revisit the example code provided in the original discussion.

Demonstrating the Issue with a Repro Server

To clearly demonstrate this inconsistency, a simple reproduction server was created. This server defines two methods: GetAnimals which returns an Animal[], and GetAnimalStrings which returns a string[]. Let's examine the code:

[McpServerToolType]
public class TimeTool
{
 [McpServerTool, Description("Gets a list of interesting animals.")]
 public static Animal[] GetAnimals(
 IMcpServer server,
 CancellationToken cancellationToken)
 {
 return [new Animal("Bear", 500), new Animal("Elephant", 6000)];
 }

 [McpServerTool, Description("Gets a list of interesting animals.")]
 public static string[] GetAnimalStrings(
 IMcpServer server,
 CancellationToken cancellationToken)
 {
 return ["Bear", "Elephant"];
 }

 public record Animal(string Name, int Weight);
}

This code snippet clearly shows two methods designed to return different types of arrays. When these methods are called via the MCP tool, the responses highlight the inconsistency we've been discussing. The GetAnimals method returns a JSON response where the array is stringified, while the GetAnimalStrings method returns an array of strings directly within the content.

This simple server effectively showcases the problem. It underscores the need for a unified approach to handling array responses within the MCP framework. The next logical step is to explore potential solutions that can address this inconsistency.

Potential Solutions for a Consistent Schema

Okay, guys, now that we've thoroughly dissected the problem, let's brainstorm some solutions. The goal here is to create a consistent schema for array responses in MCP tools, regardless of the element type. This will simplify development, reduce the risk of errors, and make the system more predictable.

Here are a few approaches we could consider:

  1. Standardize on JSON Stringification: One straightforward solution is to consistently stringify all array responses into a JSON string within the text field, just like the current behavior for Animal[]. This would mean that the content array would always have a single element, and the actual array data would be accessible by parsing the JSON string. This approach offers simplicity and uniformity. It eliminates the special casing for string[] and ensures that all array responses follow the same structure. However, it does introduce an extra step of JSON parsing on the client-side, which could have a slight performance impact.
  2. Always Return Arrays in content: Another approach is to always return arrays as direct elements within the content array, similar to the current behavior for string[]. For non-string types, this would involve serializing each element of the array into a JSON object and placing these objects directly into the content array. This approach would avoid the double serialization of JSON within JSON. This method might require more complex serialization logic on the server side, especially for complex objects. However, it could lead to more efficient client-side processing, as the client wouldn't need to parse a JSON string.
  3. Introduce a data Field: A third option is to introduce a new field, perhaps called data, specifically for returning structured data like arrays. The content field could then be reserved for metadata or other contextual information. This approach would provide a clear separation between the data and metadata, making the response structure more organized. It would also allow for future extensions without modifying the existing content structure. This might require more significant changes to the MCP schema and could impact existing tools and clients. However, it could lead to a cleaner and more maintainable architecture in the long run.

Each of these solutions has its trade-offs. The best approach will depend on factors such as performance requirements, backward compatibility concerns, and the overall design goals of the MCP framework. Let's explore a potential implementation strategy for the chosen solution.

Implementation Strategy and Next Steps

So, we've identified the problem, analyzed the root cause, and brainstormed potential solutions. Now, let's think about how we might actually implement a fix and what the next steps should be.

First and foremost, a decision needs to be made on which solution to pursue. This decision should be based on a careful evaluation of the trade-offs we discussed earlier. Factors to consider include:

  • Performance: How will each solution impact the speed of serialization and deserialization?
  • Backward Compatibility: Will the change break existing tools or clients?
  • Complexity: How difficult will each solution be to implement and maintain?
  • Maintainability: Which solution will have a better architecture in the long run?

Once a solution is chosen, a detailed implementation plan should be developed. This plan should outline the specific code changes required, the testing strategy, and the deployment process. Given the nature of the inconsistency, it's crucial to have a robust testing strategy in place. This should include unit tests to verify the correctness of the new code, as well as integration tests to ensure that the fix doesn't introduce any regressions. It should cover various data types and array sizes to ensure consistency across different scenarios.

The implementation itself would likely involve modifying the AIFunctionMcpServerTool.cs file, where the special casing for IEnumerable<string> currently exists. The code should be updated to serialize array responses consistently, regardless of the element type. This might involve removing the special case for strings and implementing a generic serialization method for all arrays.

Finally, communication with the MCP community is essential. Any changes to the schema or response format should be clearly communicated to developers who use the MCP tools. This will help them adapt their code and avoid any unexpected issues. We should create clear and concise documentation explaining the changes and provide examples of how to handle the new response format. Consider creating a migration guide to assist developers in updating their code.

By following these steps, we can address the inconsistency in MCP tool schema and create a more robust and predictable system for everyone. It’s a journey of continuous improvement, and your input and feedback are invaluable in shaping the future of MCP.

Conclusion: Towards a More Consistent MCP Ecosystem

In conclusion, the inconsistency in how MCP tools handle string[] versus other array types is a significant issue that needs addressing. By understanding the root cause – the special casing of IEnumerable<string> in the server library – we can begin to implement effective solutions. Whether it's standardizing on JSON stringification, consistently returning arrays in content, or introducing a dedicated data field, the goal is to create a more uniform and predictable experience for developers. By prioritizing consistency, we reduce the cognitive load on developers, minimize the risk of errors, and ultimately foster a more robust and user-friendly MCP ecosystem.

#Repair Input Keyword What is the inconsistency in MCP tools schema for string[] vs SomethingElse[]?

Fixing MCP Tools Inconsistent Schema for string[] vs Other Arrays