[Bug] OpenCode Go: Kimi K2.6 rejects tool schemas with if/then/else conditionals

Description

OpenCode Go does not transform tool schemas for Moonshot AI models (kimi-k2.6, kimi-k2.5), causing 400 Bad Request errors when any client sends standard OpenAI-compatible tool definitions that contain conditional schemas.

The error occurs at the Moonshot API validation layer before the request reaches the model.

Environment

• Provider: OpenCode Go (https://opencode.ai/zen/go/v1)
• Model: kimi-k2.6 (also kimi-k2.5)
• Client: pi (earendil-works/pi) v0.79.1 via litellm 1.85.0
• API mode: chat_completions

Error Message

400 litellm.BadRequestError: OpenAIException - Error from provider: 
Conflict in schema definitions for key 'if'. 
Previous: {'required': ['expand']}, New: {'required': ['collect']}

The error is wrapped by the Vercel AI SDK as AI_APICallError: Error from provider: Provider returned error in some clients (see #23665).

Root Cause

Moonshot AI's JSON Schema validator is stricter than standard OpenAI:

• It does not accept if/then/else conditional schemas in tool definitions
• It does not accept description after $ref expansion
• It does not accept properties.variantOptions patterns
• It expects a "moonshot flavored JSON schema" (quoted from their error messages)

Comparison with Other Models (Same Provider)

Expected Behavior

OpenCode Go should transform or sanitize tool schemas for Kimi models before forwarding to Moonshot AI, similar to how it handles thinking/reasoning mode conversions (see #5169 fix).

Suggested Fix

1. Provider-side schema normalization: When model starts with kimi-*, transform the incoming OpenAI tool schemas to Moonshot-compatible format:

   • Remove if/then/else blocks (flatten to anyOf/oneOf if possible)
   • Remove description after $ref
   • Strip variantOptions patterns
   • Simplify nested required arrays

2. Or: Document the limitation and provide a client-side workaround option

Related Issues

• [Bug]: OpenCode Go + Kimi K2.6/K2.5 returns 'Provider returned error' on CLI 1.14.20 #23887 — "OpenCode Go + Kimi K2.6/K2.5 returns 'Provider returned error'"
• The provider error is caused by tool schema incompatibility, not by prompt failure. #24158 — "tool schema incompatibility with Moonshot AI"
• KIMI K2.6 showing error in Opencode GO #27054 — "KIMI K2.6 showing error in Opencode GO"
• "Provider returned error" hides actual Kimi/Moonshot API errors | Opencode GO | kimi k2.6 #23665 — "Provider returned error hides actual Kimi/Moonshot API error details"
• earendil-works/pi Feat/revolutionary ai enhancements #5169 — Previous kimi-k2.6 thinking-mode issue (fixed in pi v0.78.0)

Additional Context

• This is not the same as the previous thinking-mode issue. That was fixed by pi (client side). This requires server-side schema transformation.
• litellm's drop_params: true does not help because the issue is in the JSON Schema structure, not in parameters.
• The error is deterministic — it happens on every tool request with kimi-k2.6/k2.5.