User-Agent in custom provider options.headers is overridden #24594

Description

For custom openai-compatible providers (any provider where providerID doesn't start with opencode), opencode hardcodes User-Agent: opencode/${InstallationVersion} in streamText({ headers }) at packages/opencode/src/session/llm.ts:381. The AI SDK merges call-level headers after provider.getHeaders(), so this silently overrides any User-Agent the user set via provider.<name>.options.headers.

This breaks corporate gateways that allowlist on User-Agent and surprises anyone relying on documented options.headers.

OpenCode version

1.14.28

Steps to reproduce

1. Configure a custom openai-compatible provider with a User-Agent in options.headers:

   {
     "provider": {
       "foo": {
         "npm": "@ai-sdk/openai-compatible",
         "options": {
           "baseURL": "https://example.com/v1",
           "headers": { "User-Agent": "myua/1.0" }
         }
       }
     }
   }

2. Send any chat request through this provider.
3. Inspect the outgoing request — UA is opencode/1.14.28 ai-sdk/provider-utils/X runtime/bun/Y. myua/1.0 is gone.

Operating System

macOS 15 (Darwin 24.6.0)