Likely full transcript traversal/materialization on writes causes I/O and memory amplification in tool-heavy sessions

[baron]

Impact

If transcript updates traverse or materialize full session history on each write, tool-heavy sessions can incur cumulative O(n²) work: slow saves, memory spikes, and disk churn.

Actual behavior

A low-cost probe reported likely full-history traversal/materialization/write amplification around tool-heavy sessions. This is consistent with large local AgentSession JSON and rollout footprints, but needs measurement before storage redesign.

Expected behavior

• First deliverable: instrument and confirm or refute full-history write amplification.
• If confirmed, session persistence should be incremental, append-oriented, or dirty-segment based rather than full-document rewrite per update.
• Existing on-disk compatibility should be preserved or migrated deliberately.

Publishable evidence

• Probe finding: likely full-history traversal/materialization/write amplification.
• Local scan: about 2,075 AgentSession JSON files totaling about 64 MB.

Scope / non-goals

In scope:

• measurement/instrumentation,
• persistence strategy for AgentSession updates if confirmed.

Out of scope:

• rollout session_meta dedup, tracked in Rollout session_meta duplicated across many session records #403,
• compaction semantics, tracked in Transcript compaction is turn-based, not byte-based; current-turn tool storms bypass compaction entirely #398.

Likely affected areas

• Sources/RepoPrompt/Features/AgentMode/Runtime/Transcript/
• Sources/RepoPrompt/Infrastructure/Persistence/
• Tests/RepoPromptTests/AgentMode/Transcript/
• Tests/RepoPromptTests/Persistence/

Acceptance criteria

• Instrumentation reports write count/bytes per session update for tool-heavy sessions.
• Findings confirm or refute full-history write amplification with bounded numbers.
• If confirmed, updates to a session of N turns cost O(delta) or otherwise demonstrably bounded work.
• On-disk compatibility is preserved or migration is documented.

Interlinks

Part of #406. Related: #403.

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