Array schema missing 'items' error when using OpenAI plugin with Tool (Function Calling)

[codeMonkey-shin]

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Dify version

1.11.4

Cloud or Self Hosted

Self Hosted (Docker)

Steps to reproduce

When using a custom tool (img to pdf conversion tool) with the OpenAI plugin, the following error occurs:

array schema missing items., 'type': 'invalid_request_error', 
'param': 'tools[3].function.parameters', 
'code': 'invalid_function_parameters'

The same tool works perfectly fine with other model plugins, so the issue appears to be specific to the OpenAI plugin.

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Steps to Reproduce

1. Create a custom tool that includes an array type parameter in its schema
2. Set the app to use the OpenAI plugin (e.g., gpt-4.1, gpt-5.2
3. Run the workflow or agent
4. Error is thrown at tool call stage

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Possible Cause

It seems OpenAI has recently updated their API requirements for function calling schemas — specifically, array type parameters now require an items field to be explicitly defined.

This may require Dify to validate or auto-complete array schemas before passing them to the OpenAI API.

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Environment

• Model Provider: OpenAI plugin
• Works on: ✅ Google Gemini plugin
• Works on: ✅ Anthropic plugin
• Works on: ✅ OpenAI Compatible plugin
• Fails on: ❌ OpenAI plugin

✔️ Expected Behavior

The tool should work with the OpenAI plugin as it does with other model plugins.

❌ Actual Behavior

OpenAI API returns invalid_function_parameters error, stating that the array schema is missing the items field.

[dosubot]

This is a known issue that has been reported before. The root cause is that OpenAI's function calling API now strictly requires an items field for every array parameter in the schema, and Dify passes the schema directly to OpenAI without auto-completing missing items fields.

Workaround: Update your custom tool's schema to explicitly include items for all array parameters. For example:

parameters:
  - name: images
    type: array
    items:
      type: string
    description: List of image URLs

For nested arrays, each level needs its own items definition.

This has been discussed in issue #27993, which covers the same validation error. The schema construction happens in base_agent_runner.py where array parameters use the input_schema field directly rather than being auto-generated.

A potential enhancement would be for Dify to validate or auto-complete array schemas with default items before passing them to OpenAI's API.

To reply, just mention @dosu.

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