[devin-ai-integration]

Summary

Closes a code-scan coverage gap: StructuredDiscussion.flatten builds the exact message list sent to the model on every ToolUseAgent step, and had no test anywhere. A regression there silently changes model context (dropped observations) or prompt caching (cost/correctness), with no failing test.

New file tests/agents/test_tool_use_agent.py, pure objects, no LLM and no network. It pins both halves of the index-sensitive logic:

• tail selection by keep_last_n_obs — non-tail groups collapse to group.summary, the last n groups stay verbatim, and a non-tail group without a summary falls back to its messages:

  groups = [(2 msgs, summary), (1 msg, summary), (1 msg, no summary)]
  keep_last_n_obs=1  -> [g0_summary, g1_summary, g2_m0]
  keep_last_n_obs=None -> every message verbatim
  

• cache-breakpoint rewrite at the boundary — earlier _cache_breakpoint flags (including a stale one pre-set on an earlier message) are cleared and exactly one breakpoint is set at the tail boundary; when keep_last_n_obs exceeds the group count the boundary index is negative and no breakpoint is set at all.

Tests use the real AnthropicAPIMessageBuilder (the only builder that actually implements mark_all_previous_msg_for_caching via _cache_breakpoint) rather than stubs, so the assertions track real message objects.

Note for reviewers: flatten marks messages[i] where i is the group index, which coincides with the first message of the boundary group only when every collapsed group contributes exactly one message. The tests document current behavior; changing that indexing is out of scope here.

Devin-Org: engineering

Link to Devin session: https://app.devin.ai/sessions/5a670129ef3840b1ad2fd78b2267b90c
Open in Devin Desktop: https://app.devin.ai/desktop/session/5a670129ef3840b1ad2fd78b2267b90c?variant=devin

[devin-ai-integration]

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