fuj-management/docs/plans/2026-05-07-1033-fuj-sync-dry-run.md

Plan: add --dry-run to fuj sync

Context

fuj infer already supports --dry-run (it builds the planned BatchUpdateValues operations, prints them, and skips the actual write — see go/internal/services/banksync/infer.go:136-156 and the Dry run: would update N row(s). line in go/cmd/fuj/main.go:209-213).

fuj sync had no equivalent. It always committed three potential writes to the payments sheet: WriteHeader (if the header row is missing/wrong), AppendValues (for each new Fio transaction), and SortByDateColumn (if --sort, default true). For inspecting what a sync would do — useful when debugging dedupe, sanity-checking a date window, or wiring up the command for the first time on a new account — the only options were pointing at a throwaway spreadsheet or reading the diff after the fact.

This change mirrors infer's read-only mode for sync: same flag name, same output style, same "build the data structures, print instead of writing" shape.

Files modified

1. go/internal/services/banksync/sync.go — DryRun bool field added to SyncOpts; three write points gated on opts.DryRun
2. go/cmd/fuj/main.go — --dry-run flag added to syncCmd; final println split on *dryRun
3. go/internal/services/banksync/sync_test.go — TestSyncToSheets_DryRun added
4. CHANGELOG.md — entry added

Behaviour

When --dry-run is set:

• If the sheet header is missing/wrong → prints Dry run: would write header row; skips WriteHeader
• For each non-deduped Fio transaction → prints Dry run: would append date=… amount=… sender=… vs=… message=…; skips AppendValues
• If --sort is true → prints Dry run: would sort by date; skips SortByDateColumn
• Returns len(newRows) so the caller can print Dry run: would sync N new transaction(s).

The existing ID-dedup logic runs in full even during dry-run (reads the sheet, builds existingIDs), so the output reflects exactly what the next real sync would do.