OpenAI 4xx errors render the raw response body instead of the inner error.message

[anandgupta42]

Bug

In packages/opencode/src/provider/error.ts, the OR-chain that extracts a human-readable error message from a JSON response body short-circuits on OpenAI's standard 4xx response shape, causing the raw response body to be dumped to the user instead of the inner error.message.

Reproduction

Trigger any OpenAI 4xx error — for example by selecting a deprecated/restricted model — so the API returns:

{
  "error": {
    "message": "The model `gpt-5-codex` does not exist or you do not have access to it.",
    "type": "invalid_request_error",
    "code": "model_not_found"
  }
}

The user sees:

APIError: Bad Request: {"error":{"message":"The model `gpt-5-codex` does not exist or you do not have access to it.","type":"invalid_request_error","code":"model_not_found"}}

…and on systems that pass the message through a redaction layer (telemetry, log scrubbers), even worse:

APIError: Bad Request: {?:?}

In production telemetry from a downstream fork, this caused users to retry the same broken model selection 3+ times in the same session because the surfaced error gave no actionable hint about what was wrong.

Root cause

packages/opencode/src/provider/error.ts:67:

const errMsg = body.message || body.error || body.error?.message
if (errMsg && typeof errMsg === "string") {
  return `${msg}: ${errMsg}`
}

For OpenAI's {error: {message, type, code}} shape:

1. body.message → undefined (falsy)
2. body.error → the object {message, type, code} → truthy → assigned to errMsg
3. body.error?.message → never evaluated (short-circuit)
4. typeof errMsg === "string" → false (it's an object) → block skipped
5. Falls through to line 89: return \${msg}: ${e.responseBody}`` → raw structured body dumped

The sibling function parseStreamError in the same file already uses the correct defensive pattern at line 153 (typeof body?.error?.message === "string" ? ... : default); only parseAPICallError has this bug.

Fix

PR #25763 replaces the OR-chain with explicit-typeof ternaries so a truthy non-string at any level cannot block a valid string further down the chain. After the fix the user sees:

APIError: Bad Request: The model `gpt-5-codex` does not exist or you do not have access to it.

Tests added cover OpenAI nested shape, top-level message, plain-string error, missing-message fallback, and non-string short-circuit guard.