# NousResearch/hermes-agent · Prompt caching cache_control on tool results causes 400 errors

## Open now

- Nothing open on GitHub.

These items concern a bug where cache_control fields on tool_result content arrays trigger non-retryable 400 errors from Anthropic-format APIs, breaking tool-using sessions. The members include a version report and two related fix attempts targeting LiteLLM Anthropic proxy configurations.

## TL;DR

- 3 member(s): 0 open issue(s), 1 closed issue(s), 0 open PR(s), 1 merged PR(s), 1 PR(s) closed without merging.
- A fix was merged to main in PR #97710. Verify it addresses your case.
- **Recommended:** read PR #97710 (and any competing PRs below) before filing. Confirm the merged change covers your symptom.

## Maintainer resolution verdict

- Internal-tracker verdict over this cluster's issue members: `resolved` · deciding PR(s): `pr-97710`. This is distinct from the fix status above. Weigh it yourself.

## How this cluster was grouped

- Link evidence (`STRUCTURAL_SPANNING`): every member is connected by structural evidence: shared changed files, or a closing/superseded reference.
- Pairwise link density `0.67`: the share of member pairs that carry a link, counting a structural link in full and a relevance-only link at 0.36. `1.00` means every pair is linked.

## Closed / duplicate issues

- #89886: v2026.8.18: cache_control on tool_result.content\[\] rejected by Anthropic-format API (non-retryable 400, kills any tool-using session)

## Prior & competing PRs

- **#97710: fix(prompt-caching): tool-using sessions no longer 400 behind LiteLLM Anthropic proxies (salvage of #89931)**. merged to main · fix tier `verified`
  - maintainer direction: category `merged`
  - files: `agent/agent_runtime_helpers.py`, `agent/conversation_loop.py`, `agent/moa_loop.py`, `agent/prompt_caching.py`, `tests/agent/test_litellm_tool_part_cache_markers.py`
- **#89931: fix(prompt-caching): tool-using sessions no longer 400 behind LiteLLM Anthropic proxies (#89886)**. closed without merge
  - maintainer direction: category `merged_via`; implication `superseded`; refs `1d8946b40b`, `#97710`, `#90971`

## Fix status

- Merged to main in PR #97710 (cited above). No release data is tracked. Verify downstream.
- Fix last verified (merged) `2026-08-29`.
- Subsystem: `agent`