[Bug]: session.revert fails with 400 'Expected object, got undefined' when rolling back all assistant turns #2789

Before submitting

• I searched existing issues and did not find a duplicate.
• I included enough detail to reproduce or investigate the problem.

Area

apps/server — provider adapter (OpenCodeAdapter.rollbackThread)

Summary

When reverting a number of turns equal to or greater than the total number of assistant turns in a conversation, rollbackThread calls session.revert() without a messageID in the body. The OpenCode server rejects this with 400 BadRequest: "Expected object, got undefined".

Root Cause

In apps/server/src/provider/Layers/OpenCodeAdapter.ts:1386-1392:

const targetIndex = assistantMessages.length - numTurns - 1;
const target = targetIndex >= 0 ? assistantMessages[targetIndex] : null;
yield* runOpenCodeSdk("session.revert", () =>
  context.client.session.revert({
    sessionID: context.openCodeSessionId,
    ...(target ? { messageID: target.info.id } : {}),  // {} when target is null
  }),
);

When numTurns >= assistantMessages.length, targetIndex is < 0, so target is null. The spread produces { sessionID: "…" } with no messageID. The SDK strips the empty body and sends a POST with no body. The OpenCode server requires messageID in the body and returns a 400.

Example: 2 assistant turns, reverting 2 turns → targetIndex = 2 - 2 - 1 = -1 → target = null → no messageID.

Reproduction Steps

1. Start a conversation with exactly 1 assistant turn (1 user prompt + 1 assistant response).
2. Right-click the assistant message and choose "Revert to this point" (or otherwise trigger a revert that removes all assistant turns).
3. Observe the error in the server logs:

OpenCodeRuntimeError:
    at opencode.session.revert
    …
[cause]: Error: Expected object, got undefined
[cause]: Error: {"body":{"name":"BadRequest","data":{"message":"Expected object, got undefined","kind":"Payload"}},"status":400}

Expected behavior

Reverting all assistant turns should succeed (reverting to before the first assistant message), or should be handled gracefully with a user-facing error.

Actual behavior

session.revert call fails with a 400, causing an unhandled error in the rollback flow.

Stack trace (from nightly)

OpenCodeRuntimeError: 
    at opencode.session.revert (apps/server/dist/bin.mjs:26670:16)
    at rollbackThread (apps/server/dist/bin.mjs:26402:26)
    at rollbackConversation
    at handleRevertRequested
    at processDomainEvent
  [cause]: Error: Expected object, got undefined
      at wrapClientError (@opencode-ai/sdk/dist/error-interceptor.js:26:16)
      at request (@opencode-ai/sdk/dist/v2/gen/client/client.gen.js:169:37)
    [cause]: Error: {"body":{"name":"BadRequest","data":{"message":"Expected object, got undefined","kind":"Payload"}},"status":400}

Fix direction

In OpenCodeAdapter.rollbackThread, when target is null (reverting before the first assistant message), the code must still provide a valid messageID — potentially the user message that initiated the conversation, or handle this edge case with an alternative approach.

Version

0.0.25-nightly.20260515.295

Environment

macOS