fix: correct tool_result pairing after user rejection; add Anthropic repro; keep list serializer

[enyst]

Summary

• Ensures the assistant tool_use is immediately followed by a tool_result that references the same tool_call_id when the user rejects an action.
• Reverts forcing string serialization for tool results. Anthropic supports list-serialized content; the real requirement is strict adjacency between tool_use and tool_result.
• Adds a focused unit test verifying ordering and id-matching for the rejection path.
• Adds a minimal repro script to validate against Anthropic via the LiteLLM eval proxy.

Root cause

• Anthropic rejects requests when a tool_result is not the next message immediately after its tool_use (same call id). Any intervening message (even a user/system/meta message) causes a 400: "tool_use ids were found without tool_result blocks immediately after".

What changed in this PR

• sdk/event/llm_convertible/observation.py:
  • Keep tool_call_id threading in ObservationEvent and UserRejectObservation tool messages.
  • Do NOT force string serialization (reverted) — provider compatibility is fine with list serializer.
• tests/sdk/event/test_user_reject_tool_result_order.py:
  • Asserts that ActionEvent (tool_use) followed by UserRejectObservation produces messages where the tool_result immediately follows the tool_use with matching tool_call_id.
• examples/repro_reject_anthropic.py:
  • Small script to exercise the flow against Anthropic via the eval proxy.

Live validation against Anthropic

• Environment: eval proxy to Anthropic Claude Sonnet 4.5
  • model: litellm_proxy/anthropic/claude-sonnet-4-5-20250929
  • base_url: https://llm-proxy.eval.all-hands.dev
  • api key: LITELLM_API_KEY (LiteLLM proxy key)
• How to run:

  cd agent-sdk
  uv run python examples/repro_reject_anthropic.py

• Expected/observed log excerpt:
  • LiteLLM completion() model= anthropic/claude-sonnet-4-5-20250929; provider = litellm_proxy
  • UserRejectObservation emitted referencing original tool_call_id
  • Second run completed without Anthropic tool_result error.

Why this fixes the issue

• The user rejection now reliably manifests as a tool_result in the very next message after the corresponding tool_use, which satisfies Anthropic’s contract. The unit test enforces ordering and id-matching. The repro script demonstrates the same with a live Anthropic call via the eval proxy.

Tests

• Unit test added: tests/sdk/event/test_user_reject_tool_result_order.py
• Pre-commit: passing locally across the repo

Closes

• Fixes OpenHands/OpenHands-CLI issue [Bug]: tool problem OpenHands-CLI#163

Notes

• Title kept for continuity; description updated to reflect the revert and live validation.

[github-actions]

Coverage Report •

File	Stmts	Miss	Cover	Missing
openhands-sdk/openhands/sdk/event/llm_convertible
observation.py	62	18	70%	81–86, 89, 100–101, 106, 122–125, 130, 139–140, 145
TOTAL	12425	5653	54%

[openhands-ai]

Looks like there are a few issues preventing this PR from being merged!

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@OpenHands please fix the failing actions on PR #1344 at branch `openhands/fix-tool-result-on-reject`

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