[Bug] IChatClient tool conversion produces invalid JSON Schema - properties not nested correctly

[mattbrailsford]

Description

When using AsIChatClient() with AIFunction tools, the SDK produces invalid JSON Schema for the input_schema field. Property definitions are placed at the top level instead of being nested under "properties", causing Anthropic's API to reject the request with:

tools.0.custom.input_schema: JSON schema is invalid. It must match JSON Schema draft 2020-12

Root Cause

In AnthropicClientExtensions.cs (lines ~580-620), the InputSchema is constructed incorrectly:

Dictionary<string, JsonElement> properties = [];
// ... populates properties from inputSchema ...

(createdTools ??= []).Add(
    new Tool()
    {
        Name = af.Name,
        Description = af.Description,
        InputSchema = new(properties) { Required = required },  // ← Bug here
    }
);

The InputSchema(IReadOnlyDictionary<string, JsonElement> rawData) constructor sets rawData directly to _rawData:

public InputSchema(IReadOnlyDictionary<string, JsonElement> rawData)
{
    this._rawData = [.. rawData];  // Properties end up at top level!
    this.Type = JsonSerializer.Deserialize<JsonElement>("\"object\"");
}

Actual Output (Invalid)

{
  "format": { "type": "string", "enum": ["iso", "locale", "unix"] },
  "type": "object",
  "required": []
}

Expected Output (Valid)

{
  "type": "object",
  "properties": {
    "format": { "type": "string", "enum": ["iso", "locale", "unix"] }
  },
  "required": []
}

Suggested Fix

Change the tool conversion code to use the default constructor with property initializers:

InputSchema = new InputSchema() { Properties = properties, Required = required }

Instead of:

InputSchema = new(properties) { Required = required }

Reproduction Steps

1. Create an AIFunction with a simple JSON schema
2. Use client.AsIChatClient() to get an IChatClient
3. Pass the AIFunction as a tool in ChatOptions.Tools
4. Call GetResponseAsync or GetStreamingResponseAsync
5. Observe the 400 error from Anthropic's API

Environment

• Anthropic SDK Version: 12.0.1
• Microsoft.Extensions.AI Version: 10.0.1
• .NET Version: .NET 10.0

Example Code

// Simple AIFunction with schema
public class MyTool : AIFunction
{
    public override string Name => "getCurrentTime";
    public override string Description => "Get the current time";
    public override JsonElement JsonSchema => JsonSerializer.SerializeToElement(new
    {
        type = "object",
        properties = new
        {
            format = new { type = "string", enum = new[] { "iso", "locale", "unix" } }
        }
    });
    
    protected override ValueTask<object?> InvokeCoreAsync(
        AIFunctionArguments arguments, CancellationToken cancellationToken)
        => ValueTask.FromResult<object?>("result");
}

// Usage
var client = new AnthropicClient(apiKey);
var chatClient = client.AsIChatClient("claude-sonnet-4-20250514");

var options = new ChatOptions { Tools = [new MyTool()] };
var response = await chatClient.GetResponseAsync(messages, options); // ← Fails with 400

[davidames]

I noticed this error started yesterday, after working for a few weeks in prod. I suspect that validation has been tightened on the API end recently?

[stephentoub]

The InputSchema ctor used to accept "properties". That changed in 75a2cce#diff-79ec764df52aaa105c06a11788463cf56f9aa0f6fcfb87d49e433f4b78d1654bL274

[TomerAberbach]

That commit only changes the name of the raw data of the model so that it doesn't clash with common JSON field names. So now the C# property for the "properties" JSON field is now Properties instead of Properties1

So I don't think any behavior what changed in that commit? Am I missing something @stephentoub?

[stephentoub]

No behavior changed. I was pointing out why it was written the way it was, because the ctor accepted "properties".

[TomerAberbach]

Ah got it, so it's true that we need to update AnthropicClientExtensions.cs right?

[stephentoub]

Yup

[Ewoofcoding]

I faced a similar issue and I think I solved it. Can someone please check and confirm what I did.

The AsIChatClient() extension method generates invalid JSON schemas that violate JSON Schema Draft 2020-12, causing the Anthropic API to reject tool calls with:

tools.X.custom.input_schema: JSON schema is invalid.
It must match JSON Schema draft 2020-12

Root Cause

File: src/Anthropic/AnthropicClientExtensions.cs
Lines: ~843-850

The InputSchema constructor treats the properties dictionary as raw top-level data instead of nesting them under a "properties" key.

Buggy Code

case AIFunctionDeclaration af:
    Dictionary<string, JsonElement> properties = [];
    List<string> required = [];
    // ... extract properties and required ...
    
    (createdTools ??= []).Add(
        new Tool()
        {
            Name = af.Name,
            Description = af.Description,
            InputSchema = new(properties) { Required = required },  //  BUG
        }
    );
The Problem
What Gets Sent to Anthropic (INVALID )

{
  "rootPath": { "type": "string" },
  "pattern": { "type": "string" },
  "maxResults": { "type": "integer", "default": 100 },
  "type": "object",
  "required": ["rootPath", "pattern"]
}
Issue: Properties are at the top level, violating the JSON Schema spec.

What Should Be Sent (VALID)

{
  "type": "object",
  "properties": {
    "rootPath": { "type": "string" },
    "pattern": { "type": "string" },
    "maxResults": { "type": "integer", "default": 100 }
  },
  "required": ["rootPath", "pattern"]
}
Per [JSON Schema Draft 2020-12](https://json-schema.org/draft/2020-12/json-schema-core.html#name-properties), the "properties" keyword must contain an object of property schemas, properly nested within the parent schema object.

The Fix
Replace the buggy code with:


case AIFunctionDeclaration af:
    Dictionary<string, JsonElement> properties = [];
    List<string> required = [];
    JsonElement inputSchema = af.JsonSchema;
    
    if (inputSchema.ValueKind is JsonValueKind.Object)
    {
        if (inputSchema.TryGetProperty("properties", out JsonElement propsElement)
            && propsElement.ValueKind is JsonValueKind.Object)
        {
            foreach (JsonProperty p in propsElement.EnumerateObject())
            {
                properties[p.Name] = p.Value;
            }
        }

        if (inputSchema.TryGetProperty("required", out JsonElement reqElement)
            && reqElement.ValueKind is JsonValueKind.Array)
        {
            foreach (JsonElement r in reqElement.EnumerateArray())
            {
                if (r.ValueKind is JsonValueKind.String
                    && r.GetString() is { } s
                    && !string.IsNullOrWhiteSpace(s))
                {
                    required.Add(s);
                }
            }
        }
    }

    // FIX: Properly nest properties under "properties" key
    var inputSchemaData = new Dictionary<string, JsonElement>();
    inputSchemaData["type"] = JsonSerializer.SerializeToElement("object");
    if (properties.Count > 0)
    {
        inputSchemaData["properties"] = JsonSerializer.SerializeToElement(properties);
    }
    if (required.Count > 0)
    {
        inputSchemaData["required"] = JsonSerializer.SerializeToElement(required);
    }

    (createdTools ??= []).Add(
        new Tool()
        {
            Name = af.Name,
            Description = af.Description,
            InputSchema = InputSchema.FromRawUnchecked(inputSchemaData),  //  FIXED
        }
    );
    break;

Verification
I've verified this fix by switching to a local project reference, applying the fix, testing with multiple AIFunctions and confirming all schemas now pass Anthropic's validation. Can someone confirm the fix is right and satisfactory?