Bug: Invalid JSON Schema Generated Due to Unsupported Regex Lookaround in fileKey Pattern On GPT 5.5

[fresp]

Description

Summary

When using OpenCode with an OpenAI-compatible provider, requests fail before reaching the model with the following error:

Invalid JSON schema: regex lookaround is not supported.
Found at $.properties.fileKey.pattern

This appears to be caused by OpenCode generating a JSON Schema containing a regex pattern with lookaround assertions ((?=...), (?!...), (?<=...), or (?<!...)).

Many JSON Schema validators used by OpenAI-compatible providers do not support regex lookarounds, causing schema validation to fail before inference begins.

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Environment

• OpenCode Version: Latest
• Provider: OpenAI-compatible API
• Model: GPT-5.5
• OS: Linux

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Steps to Reproduce

1. Configure OpenCode with an OpenAI-compatible provider.
2. Start a new session.
3. Send a simple prompt:

hai

4. Observe the response.

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Actual Result

OpenCode immediately returns:

Invalid JSON schema: regex lookaround is not supported.
Found at $.properties.fileKey.pattern

No request reaches the model.

---

Expected Result

OpenCode should either:

• Generate a JSON Schema compatible with common Structured Output validators, or
• Detect unsupported schema features and gracefully fall back to a non-structured-output mode.

The request should not fail before model execution.

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Analysis

The error points to:

$.properties.fileKey.pattern

which suggests OpenCode is generating a schema similar to:

{
  "type": "object",
  "properties": {
    "fileKey": {
      "type": "string",
      "pattern": "..."
    }
  }
}

The generated regex appears to contain lookaround assertions.

While lookarounds are supported by some regex engines, they are not universally supported by JSON Schema validators used by OpenAI-compatible APIs and structured output implementations.

As a result, schema validation fails before the request can be processed by the model.

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Suggested Fix

• Avoid generating regex patterns that rely on lookaround assertions.
• Replace lookaround-based validation with schema-compatible alternatives.
• Add a fallback path when structured output schema validation fails.
• Consider validating generated schemas against common OpenAI-compatible implementations before sending requests.

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Additional Context

The issue is reproducible with a simple prompt and appears unrelated to the model itself.

Based on the error message, the failure occurs during schema validation rather than inference.

This may affect any OpenAI-compatible provider that enforces JSON Schema validation and does not support regex lookaround syntax.

Plugins

No response

OpenCode version

1.17.4

Steps to reproduce

No response

Screenshot and/or share link

Operating System

No response

Terminal

No response

[github-actions]

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This issue might be a duplicate of existing issues. Please check:

• GPT-5.5 tool calls fail with Backstage MCP pattern schemas #30553: GPT-5.5 tool calls fail with pattern-based JSON Schema validation — same provider (GPT-5.5), same root cause (schema pattern fields rejected by OpenAI's validator)
• OpenRouter xAI/Grok tool schemas fail when additionalProperties: false is emitted #23704: OpenRouter xAI/Grok tool schemas fail when additionalProperties: false is emitted — same class of issue (provider-specific JSON Schema incompatibility requiring sanitization in ProviderTransform.schema)

If you believe this was flagged incorrectly, please let a maintainer know.

[dkornilove]

started to experience after update to 1.17.4.

upd: not really depends on version, looks like some changes on openai side

[mattbdc]

It works if you downgrade model to 5.4. It seems an issue with 5.5.

[grummans]

@fresp are you using Figma MCP? I have tested, only Figma MCP cause that error.
I think Figma has change the way they handle MCP request.
Update: I tested with another model in Opencode Go (Figma MCP enabled) -> still work
Maybe the issue only cause with GPTs-model when Figma mcp enabled

[github-actions]

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