JsonCodec2 deprecation warning when using `effort` (thinking budget) — byte body treated as string in `sdk-client.ts`

[gonzalezd13]

Describe the bug

First, thank you for creating this plugin. I'm a big fan of opencode so being able to use Kiro without needing to manage my own custom provider is so nice to have.

I'm using a -thinking model with a variant selected (via OMO, which sets the native OpenCode variant agent-config field — e.g. model: kiro/claude-sonnet-5-thinking, variant: "max"), which resolves a non-empty effort value in the plugin and triggers the buggy code path. Every such request logs:

@aws-sdk/core/protocols - WARN - JsonCodec2: you have called a string method on a Uint8Array request body. It has been automatically converted to string. In a future version this will throw an error.

Root cause: in src/plugin/sdk-client.ts, the addEffortConfig middleware does JSON.parse(args.request.body) assuming args.request.body is a plain string. With the currently-resolved @aws-sdk/core (3.977.6), the serializer produces a JsonBytesStringAdapter — a Uint8Array subclass whose toString()/valueOf() lazily decode to UTF-8 and log the deprecation warning on first use. JSON.parse() on a non-string argument implicitly calls .toString(), hitting that path.

It currently still works — decode succeeds, JSON.parse succeeds, request goes out fine — but AWS's own comment states a future SDK version will throw instead of auto-converting, so this will break outright once that lands.

To Reproduce

1. Install @zhafron/opencode-kiro-auth (v2.0.0, or any version once @aws-sdk/core resolves to a JsonCodec2-era release transitively via @aws/codewhisperer-streaming-client).
2. Configure a -thinking model with a variant, e.g.:

   { "model": "kiro/claude-sonnet-5-thinking", "variant": "max" }

   (any variant that makes getEffectiveEffort() resolve a defined effort works)
3. Send a message using that model/variant in OpenCode.
4. The JsonCodec2 warning appears in the chat area, as shown in the attached screenshot.

Expected behavior

No deprecation warning. The effort-injection middleware should decode the request body correctly regardless of whether the SDK hands it a string or a byte-backed body, without relying on implicit toString() coercion.

Screenshots

Desktop (please complete the following information):

• OS: NixOS (Linux)
• OpenCode version: 1.18.13
• Plugin: @zhafron/opencode-kiro-auth v2.0.0
• @aws-sdk/core: 3.977.6 (resolved via @aws/codewhisperer-streaming-client 1.0.45, declared range ^3.974.7)
• Install method: opencode plugin manager, @latest

Additional context

Verified fix locally by patching the installed dist/plugin/sdk-client.js:

if (args.request?.body) {
  try {
    const rawBody = args.request.body
    const bodyStr = typeof rawBody === 'string' ? rawBody : new TextDecoder().decode(rawBody)
    const body = JSON.parse(bodyStr)
    body.additionalModelRequestFields = {
      output_config: { effort }
    }
    args.request.body = JSON.stringify(body)
  } catch {
    // If body parsing fails, continue without modification
  }
}

Warning no longer appeared with the same model/variant config that reproduced it. This was just a quick fix I had my agent do to continue work. By no means am I suggesting that this is the exact fix. I would prefer your expertise on that matter.

This report and the suggested fix were drafted with the help of Kiro, then reviewed by me.