GPT-5.5 tool calls fail with Backstage MCP pattern schemas

Problem

With Backstage MCP enabled and an OpenAI-backed GPT-5.5 model, requests can fail before tool execution with a generic server_error / response.failed.

Repro context

Backstage MCP exposes tool parameter schemas with many JSON Schema pattern regex constraints, such as entity refs and catalog filters. OpenAI's tool schema validator rejects that shape in practice, while Anthropic/Bedrock accept it.

Observed locally against the real opencode config with Backstage enabled:

before the fix: GPT-5.5 + Backstage MCP crashed with server_error / response.failed
after stripping pattern constraints for @ai-sdk/openai: GPT-5.5 successfully routed to backstage_auth_who-am-i
the resulting NotAllowedError was expected Backstage auth behavior, confirming the model request/tool-routing path no longer crashed

Expected behavior

The model request should accept the tool schemas and proceed to either tool execution or a tool-level auth/error response.

Actual behavior

The model request fails before useful tool execution with a generic server error.

I can reproduce what looks like the same issue with Figma MCP.

Environment:

opencode 1.17.4
macOS 15.7.7
model: openai/gpt-5.5
MCP: Figma enabled

What I tried:

With Figma MCP enabled + GPT-5.5, a session eventually fails with:
Invalid JSON schema: regex lookaround is not supported. Found at $.properties.fileKey.pattern
Once it appears, the session becomes unusable.
Switch the same broken session to GPT-5.4: it works again.
Switch back to GPT-5.5 while Figma MCP is enabled: the same failure returns.
Disable Figma MCP while staying on GPT-5.5: it works again.
GPT-5.4 + Figma MCP disabled also works.

This looks like GPT-5.5/OpenAI schema validation rejecting a Figma MCP tool schema pattern, similar to the Backstage pattern schema issue here.

Closed duplicate with screenshot/context: #31996

Screenshots: