[shubham2345]

Fixes #1137

Problem

bench eval run --agent claude-agent-acp --model claude-opus-5-5 fails on every turn with ACP error -32603 ... API Error: 400 Claude Code 2.1.257 does not support this model; version 2.1.280 or newer is required.

The _CLAUDE_AGENT_ACP_PACKAGE pin (0.73.0) bundles claude-agent-sdk 0.3.257, i.e. Claude Code 2.1.257, which is too old for claude-opus-5-5.

Change

• src/benchflow/agents/registry.py: raise the pin to @agentclientprotocol/claude-agent-acp@0.81.0 (sdk 0.3.280, Claude Code 2.1.280) and update the explanatory comment.
• tests/test_agent_registry.py: update the pin assertion.
• CHANGELOG.md: add a Fixed entry.

Verification

• tests/test_agent_registry.py: 31 passed.
• Direct ACP check on 0.81.0 with a valid key: selecting claude-opus-5-5 and claude-fable-5-1 succeeds and a one-word prompt returns end_turn. On 0.73.0, selecting claude-opus-5-5 fails with Invalid value for config option model.
• Not run: a full bench eval run through the runner and a sandbox.

Notes

• The gated live guard (tests/test_acp_pinned_protocol_guard.py, RUN_ACP_DEP_GUARD=1) is unchanged. On 0.81.0 its Fable check needs ANTHROPIC_API_KEY, because the adapter validates a concrete model against the API when it is selected.
• Not included: the optional env override (BENCHFLOW_CLAUDE_AGENT_ACP_VERSION) mentioned in the issue; this PR only raises the pin.

[Benjamin-eecs]

Non-binding review from the #1137 reporter: this matches what FrontierPhysics ran.

• FrontierPhysics has run its claude-opus-5-5 cells on claude-agent-acp 0.81.0 since 2026-09-23 through a local re-pin of this constant (FrontierPhysics-Infra 3e20674, FPINFRA_CLAUDE_AGENT_ACP_VERSION=0.81.0), through the full bench eval run path on Daytona and on docker. agent/runtime.json records sandbox.claude_code_version = 2.1.280, and the model and effort config options took effect. That covers the "Not run" item in the description.
• 0.81.0 does not change #1138: a large image read still produces multi-MB ACP lines, as on 0.73.0.
• release/0.8 solves #1137 differently (6bd05f2: adapter 0.81.2 plus @anthropic-ai/claude-code@2.1.280 through CLAUDE_CODE_EXECUTABLE). For main, this one-line bump is enough; PR #1139 is the same bump and could be closed in favour of this one.
• Optional follow-up, not needed here: an env override for the adapter (or CLI) version, so the next model that needs a newer Claude Code does not wait for a release.