[giladsamuels-cognition]

Summary

Adds tests/agents/test_tool_use_agent_discussion.py, the first coverage for StructuredDiscussion.flatten in the ToolUseAgent, which decides the exact message list sent to the model each step. Tests are pure objects (real AnthropicAPIMessageBuilder, no LLM, no env).

Pinned behaviors:

• keep_last_n_obs=None → every group verbatim, breakpoint on flattened index 0.
• non-tail groups collapse to group.summary, falling back to group.messages when no summary exists.
• the cache breakpoint lands on messages[len(groups) - keep_last_n_obs] — an index into the flattened list, not the group list — and all breakpoints appended before it (including tail messages already extended in that iteration) are cleared:

groups = [g0(sum), g1(sum), g2(m0,m1)], keep_last_n_obs=1
flatten() -> [sum0, sum1, m2_0, m2_1]
breakpoints -> [False, False, True, False]

No production code changed.

Devin-Org: engineering

Link to Devin session: https://app.devin.ai/sessions/70a85ee5d4b844e2ac5eb0a3a9c09c73
Open in Devin Desktop: https://app.devin.ai/desktop/session/70a85ee5d4b844e2ac5eb0a3a9c09c73?variant=devin
Requested by: @giladsamuels-cognition

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✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.