Fixes #2871

Problem

After the first session compaction in a multi-agent run, the agent halts mid-task and replies as if it has no conversation history ("I understand you're looking for a session summary, but ... no previous conversation history visible").

Root cause

Two compounding bugs, surfaced by ead9745 / 8dba51f (2026-05-18) which expanded when compaction activates — four days before the issue was filed:

1. Phantom trigger in multi-agent runs: compactIfNeeded estimated newly-added tokens via sess.GetAllMessages(), which recurses into sub-sessions. The content produced by a transfer_task child was attributed to the parent session even though it never enters the parent's prompt (GetMessages skips sub-session items). The phantom tokens triggered compaction of a parent conversation that was actually tiny; with everything fitting the keep budget, the split resolved to the "compact everything, keep nothing" sentinel — wiping the user's task and the in-flight tool exchange. The agent's next prompt was literally just Session Summary: ..., which models read as the user asking for a summary. This also explains the "first compaction only" symptom: the first compaction fires while the parent history is still tiny; after re-prompting, later compactions keep a real tail.

2. Fixed budgets break small context windows: MaxSummaryTokens (16k) and maxKeepTokens (20k) are absolute constants. For models whose window resolves from provider_opts.context_size and is ≤ ~16k, the summarizer's input budget went to zero — it received only its own prompts, fabricated a "no history" non-summary, and that text replaced the entire session history.

Fix

• Session.OwnMessages() (no sub-session recursion) now drives the compaction trigger's token accounting, so sub-agent work no longer causes phantom parent compactions.
• Summary/keep budgets scale with the window (min(16k, limit/4) / min(20k, limit/5)); the scaled cap is also used for the summary call's max_tokens.
• Safety net: RunLLM no-ops when no conversation message fits the summarization budget, instead of running the summarizer on an empty conversation and wiping history with the result.
• ComputeFirstKeptEntry gains a contextLimit parameter so hook-supplied summaries share the same kept-tail policy.

Tests

• TestCompactIfNeeded_IgnoresSubSessionTokens — regression test, verified to fail against the old trigger code.
• TestCompactIfNeeded_TriggersOnOwnMessages — large own tool results still trigger.
• TestRunLLM_SmallContextWindow — summarizer receives real conversation on an 8k window and a tail is kept.
• TestRunLLM_NoConversationFits_NoOps — empty summarizer input no-ops instead of wiping history.

task build, task test, task lint all pass (only pre-existing, environment-dependent pkg/sandbox.TestExtraWorkspace failure, which also fails on clean main).

Note for reviewers

One residual hazard left untouched (documented contract defended in 1e9512e): a legitimately triggered compaction whose whole conversation fits the keep budget (possible with image-heavy histories — token estimates ignore images) still drops the tail via the "compact everything" sentinel. Happy to follow up with a "keep the last user turn on threshold/overflow compaction" change if desired.

Assisted-By: docker-agent