Content API `browse()` error "model layer expects using `withRelated`" when using include with filter

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Issue Summary

Content API browse() method throws IncorrectUsageError when using include parameter with filter parameter in Ghost 6.0. The error message states "The model layer expects using withRelated", but withRelated is not a documented parameter for the Content API.

Steps to Reproduce

Steps to Reproduce

1. Use Ghost Content API SDK v5.0 against hosted Ghost CMS
2. Call posts.browse() with both filter and include parameters:

   const posts = await ghostContentAPI.posts.browse({
     filter: 'tag:some-tag',
     include: 'tags'
   })

3. Error is thrown

Expected Behavior

According to the Ghost Content API documentation, the include parameter should work with the browse() method to include related resources like tags. This should work regardless of whether a filter parameter is used.

Actual Behavior

The API throws an IncorrectUsageError:

{
    "errors": [
        {
            "message": "Incorrect usage error, cannot list posts.",
            "context": "The model layer expects using `withRelated`.",
            "type": "IncorrectUsageError"
        }
    ]
}

Ghost Version

6.0 (hosted Ghost CMS - SaaS)

Node.js Version

Your version - I don't have visibility to hosted Ghost CMS environment

How did you install Ghost?

Hosted ghost CMS

Database type

MySQL 8 (I assume - again this is hosted Ghost CMS, I don't own this enviroment)

Browser & OS version

Chrome, MacOS 15.1.1

Relevant log / error output

{
    "errors": [
        {
            "message": "Incorrect usage error, cannot list posts.",
            "context": "The model layer expects using `withRelated`.",
            "type": "IncorrectUsageError"
        }
    ]
}

Code of Conduct

• I agree to be friendly and polite to people in this repository

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Additional Testing Attempted

I've attempted a couple of workarounds to rule out potential causes:

1. SDK Version Mismatch: I tried upgrading to Content API SDK v6.0 to match what I believe the hosted Ghost CMS server is running, thinking a version mismatch might be the issue. This didn't resolve the problem - instead, I encountered CORS errors that prevent testing with v6.0. However, based on the error originating from the server's model layer, I have no reason to believe an SDK version change would fix this server-side issue anyway.

2. Using withRelated Parameter: Out of desperation, I also tried using withRelated instead of include, despite it being undocumented for the Content API. As expected, this didn't work - the Content API doesn't recognize withRelated as a valid parameter.

These attempts support a hypothesis that this is a server-side issue that requires a fix from Ghost, not a client-side configuration problem.