fix(opencode): limit default GPT verbosity

[Lp-Francois]

Context: when using Bedrock with a Gateway such as LiteLLM, we see rejection errors from the Bedrock mantle.

A fix for now is to create a plugin such as:

// In plugins/gateway-compat.ts

import type { Plugin } from "@opencode-ai/plugin"

/**
 * gateway-compat: strip parameters the Contentful AI Gateway (LiteLLM) rejects.
 *
 * OpenCode auto-adds `textVerbosity: "low"` for GPT-5.x models. Via the
 * OpenAI-compatible provider this becomes `verbosity`, which Bedrock Mantle
 * behind the gateway rejects. Remove it for all contentful-gateway GPT-5.x
 * models.
 */
const GatewayCompatibility: Plugin = async () => ({
  "chat.params": async (input, output) => {
    if (input.model.providerID !== "contentful-gateway") return
    if (!input.model.api.id.includes("gpt-5")) return

    delete output.options.textVerbosity
  },
})

export default GatewayCompatibility

But this PR handles it at the root: opencode ✨

Everything is explained in the plugin comment, but TLDR: OpenCode auto-adds textVerbosity: "low" for GPT-5.x models. Bedrock rejects the option for some GTP models.

Note: reviewed by a human (me) and implemented by latest Luna max

---

Here starts the AI generated part:

Summary

• only add the GPT-5 textVerbosity default for provider packages known to support it
• avoid forwarding OpenAI-specific verbosity through generic OpenAI-compatible providers
• preserve the existing OpenAI and Bedrock Mantle behavior
• add regression coverage for OpenAI-compatible GPT-5 models

Testing

• bun test test/provider/transform.test.ts
• bun typecheck from packages/opencode
• bun turbo typecheck (pre-push hook)

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[Lp-Francois]

Let me create the issue now

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