Validation error with Copilot Agent: "tool parameters array type must have items"

[ar90n]

Thanks for your great project. When I used this tool, I met an issue as following.

Description

When using Copilot Agent, the following validation error occurs:

Failed to validate tool f1e_create_container: Error: tool parameters array type must have items

Workaround

This issue no longer occurs with the following workaround:

🔗 ar90n@4d55aa9

This fix adds an empty items field next to prefixItems, which resolves the issue in environments with strict JSON Schema validation.

Cause

The schema generated for tuple[str, int] includes only prefixItems without an items field, which violates the expectations of some JSON Schema validators (such as the one used by Copilot Agent).

Proposal

Would it be okay for me to open a PR that adds items: {} where prefixItems is used?

If you're okay with that, feel free to assign this issue to me.

Thanks for the great project!

[ckreiling]

Thank you for your well-written & detailed issue description!

Before submitting a PR I'd like to understand a little more:

• What version of JsonSchema is the tool using to validate? According to up-to-date JsonSchema docs (link), it seems the lack of items in a schema of type "array" is valid, which tells me it's a buggy or maybe outdated validator...
• Can we perhaps use the GenerateJsonSchema customizations provided by Pydantic? Docs link. I can't find evidence in the documentation that overriding the model_json_schema() method is recommended but I'm happy to be corrected here

Nonetheless I left a small suggestion on your linked commit. Feel free to submit a PR with those changes for us to iterate on a solution here

[ar90n]

@ckreiling
Thanks for yoru thorough review.

What version of JsonSchema is the tool using to validate? According to up-to-date JsonSchema docs (link), it seems the lack of items in a schema of type "array" is valid, which tells me it's a buggy or maybe outdated validator...

Unfortunately, I don’t know which version of JsonSchema Copilot Agent uses for validation, since its source code is not public. I completely agree with your point—the JsonSchema generated by mcp-server-docker should be valid according to the official documentation.
However, when I tried using it with GitHub Copilot Chat (version 0.27.1), I got the following error:

Can we perhaps use the GenerateJsonSchema customizations provided by Pydantic? Docs link. I can't find evidence in the documentation that overriding the model_json_schema() method is recommended but I'm happy to be corrected here

Thank you for the suggestion. I’m not very experienced with Pydantic, so I went with a straightforward workaround for now.
I’ll update my code to use GenerateJsonSchema as you suggested and create PR later.

Thanks again for your feedback.

[ckreiling]

Thanks for addressing my questions in such a short amount of time. Totally understand that the closed source nature of the tool makes it difficult to debug... I want to make sure we comply with the JsonSchema spec and still satisfy Copilot Agent, which I think we can still do with minimal effort.

Keeping this open to track this issue as I'm sure you aren't the only one! I'll be happy to review your PR using GenerateJsonSchema as soon as it's ready. Thank you again!

[hjed]

Hey folks - I get the same error trying to run this with augment

Failed to start the MCP server. {"command":"uvx mcp-server-docker","args":[],"error":"Invalid schema for tool create_container: strict mode: unknown keyword: \"prefixItems\"","stderr":""}
```

[shqear93]

I see similar issue using Augment on VSCode:

2025-07-29 01:15:49.147 [info] 'AgentWebviewMessageHandler' Retrieved swarm mode settings
2025-07-29 01:15:50.139 [error] 'McpHost' Invalid schema for tool create_container: strict mode: unknown keyword: "prefixItems"
2025-07-29 01:15:50.140 [error] 'McpHost' Invalid schema for tool run_container: strict mode: unknown keyword: "prefixItems"
2025-07-29 01:15:50.140 [error] 'McpHost' Invalid schema for tool recreate_container: strict mode: unknown keyword: "prefixItems"
2025-07-29 01:15:50.140 [error] 'McpHost' Failed to connect to MCP server "Docker"
  Command: uvx mcp-server-docker
  Args: 
  Error: Invalid schema for tool create_container: strict mode: unknown keyword: "prefixItems"
  Stderr: 
2025-07-29 01:15:50.140 [error] 'AugmentExtension' MCP tool startup error: %s