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Cian JohnstonandCopilot Autofix powered by AI
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## Summary Adds test coverage for the three compaction-decision functions in chatd that had zero tests: `latestPromptUsage`, `shouldCompactPromptUsage`, and `contextTokensFromUsage`. AIGOV-585 hypothesized that chatd's token counting logic was incorrect — that it compared a cumulative sum of prompt tokens across all agentic-loop steps against the context window. The tests disprove this: `latestPromptUsage` returns the last persisted assistant message's usage, not a sum. The actual bug was in the aibridge streaming interceptor, which summed usage across SSE chunks and persisted inflated values (fixed in `ad100452d4`). ## What's tested - `TestLatestPromptUsage` — pins that the compaction path reads the last step's usage (5,400), not a cumulative sum across steps (15,600). If someone wires `TotalUsage` into the compaction path as the issue suggested, this fails. - `TestShouldCompactPromptUsage` — covers the threshold decision with the inflated value from the issue (417,012 → compacts), the correct value (6,000 → doesn't compact), cache token counting, and both disable guards (threshold=100, contextLimit=0). <details> <summary>Plan / investigation notes</summary> - Traced the full flow: `chatloop.go:993` sets `result.usage = part.Usage` from the per-step `StreamPartTypeFinish` event, not the accumulated `TotalUsage` from `agent.go:544`. chatd never calls fantasy's `Agent` interface. - The `TotalUsage` accumulation in `agent.go:544` is only used for cost attribution, not context occupancy. - Commit `ad100452d4` fixed the real bug in `aibridge/intercept/chatcompletions/streaming.go` (cross-chunk usage summation for vLLM-style backends). - Tests reuse existing `dbMessage` and `withUsage` helpers from `message_conversion_test.go` (same package). </details> Generated by [Coder Agents](https://coder.com) --------- Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>