[Issacboy123]

Summary

Long tool loops could exceed the reserve threshold before the next compaction check. Checks before a prompt also missed the new input. This PR connects the request-preparation hook to automatic compaction. Each normal request checks its context after new input and tool results arrive, then continues with the rebuilt history.

Depends on fix/bound-proactive-compaction-attempts.

Reasoning

Checking at this point includes queued input and tool results. AgentSession still owns compaction and saves the result. Before rebuilding context, it verifies that live messages match the selected session history. If a hook changes messages so they no longer match, Diffex warns and continues with those messages. Changes made by later request-only transforms are outside the budget check.

Changes

• Checked the budget after any existing preparation hook, using the context window of the model selected for the pending request.
• Preserved pending user and custom messages, images, and the full latest group of tool calls and results.
• Adjusted where proactive compaction cuts history so large tool batches remain complete.
• Checked for changes to the selected history after asynchronous preparation and rejected extension cuts that would discard pending input or separate tool results from their calls.
• Returned rebuilt messages, system prompt, and tools to the running loop. Kept retained message objects without rerunning tools or restarting the run.
• Added tests that inspect messages sent to the model and when compaction occurs. Covered thresholds, input preservation, existing hooks, cancellation, repeated attempts, later context growth, and separate summarization requests.
• Updated compaction documentation and the Unreleased changelog.

Testing

• node ../../node_modules/vitest/dist/cli.js --run test/agent-loop.test.ts test/agent.test.ts
• node ../../node_modules/vitest/dist/cli.js --run test/suite/agent-session-compaction.test.ts test/agent-session-compaction.test.ts test/compaction.test.ts test/agent-session-auto-compaction-queue.test.ts test/suite/agent-session-prompt.test.ts test/suite/agent-session-queue.test.ts test/agent-session-stats.test.ts test/suite/agent-session-current-plan-restoration.test.ts test/session-manager/compaction-rollback.test.ts test/suite/regressions/pre-prompt-compaction-no-continue.test.ts test/suite/regressions/177-compaction-busy-state.test.ts test/suite/regressions/6647-compaction-retries-transient-stream-drop.test.ts test/suite/agent-session-context-usage.test.ts test/suite/agent-session-proactive-compaction.test.ts test/suite/subagent-coordination.test.ts test/suite/agent-session-request-compaction.test.ts
• node ../../node_modules/vitest/dist/cli.js --run test/interactive-mode-compaction.test.ts test/print-mode.test.ts test/rpc-jsonl.test.ts test/rpc-prompt-response-semantics.test.ts
• npm run check