todos update silently clears completion subscribers (merge misses the field the SDK can't see)

[jeremy]

🤖 Drafted with agent assistance; filed by @jeremy.

Summary

Every basecamp todos update invocation silently clears the to-do's completion subscribers ("When done, notify…"). Both code paths in newTodosUpdateCmd (internal/commands/todos.go:939) build their PUT body without completion_subscriber_ids, and the BC3 todos update endpoint has replace semantics — omitting a parameter clears it (bc3-api, todos: "Omitting a parameter will clear its value").

The CLI already knows about this server contract — #412/#413 added a read-modify-write merge precisely to stop partial PUTs from wiping untouched fields — but the merge preserves only content, description, due_on, starts_on, and assignee_ids. Completion subscribers were never included, because the SDK's Todo model doesn't map the completion_subscribers field at all (basecamp/basecamp-sdk#355), so the merge has nothing to preserve. git grep -i completion_subscriber across internal/ and cmd/ returns zero hits.

Reproduction

1. In Basecamp, create a to-do and add a person under "When done, notify…"
2. basecamp todos update <id> --title "New title" --in <project>
3. Reload the to-do in Basecamp: the completion subscriber is gone. No warning, exit 0.

Applies equally to the field-clearing path (--no-due, --no-description, --due ""), which hand-builds a raw PUT body (internal/commands/todos.go:1043) that likewise omits completion_subscriber_ids.

Root cause chain

Layer	Defect
BC3 API	Documented replace semantics: omitted field = cleared (long-established; not changing)
SDK	Todo model omits completion_subscribers → clients cannot read them to preserve them (basecamp/basecamp-sdk#355); Todos.Update itself is a partial PUT with the same wipe hazard on every field (basecamp/basecamp-sdk#372)
CLI	Read-modify-write merge (both branches of newTodosUpdateCmd) preserves five fields, not six — subscribers fall through

Context: this same field was among those wiped in two production support incidents (June, July 2026) caused by third-party scripts using the Go SDK's partial-PUT Update — see basecamp/basecamp-sdk#372 for the fleet-wide SDK fix (merge-safe Update, new Edit read-modify-write closure, explicit Replace). The CLI is the interim mitigation for its own users until that lands, which makes this gap in its merge worth closing now rather than waiting.

Fix plan

Phase 1 — close the gap now (this issue)

Preserve completion subscribers in both branches of newTodosUpdateCmd:

1. Fetch the current to-do's completion_subscribers ids. The SDK model doesn't carry them (Stamp plugin version at release time #355), so until it does, read them from the raw GET JSON — the CLI already has app.Account().Get/raw request plumbing (same style the needsClear branch uses for its raw PUT). The BC3 payload field is completion_subscribers, an array of person objects; collect .id.
2. Include completion_subscriber_ids in the PUT body in both branches, defaulting to the fetched ids (preserve). No new flag is required to fix the wipe.
3. Optional, natural companion (mirrors --assignee): --notify-on-completion <names-or-ids> and --no-notify-on-completion to set/clear explicitly, reusing resolveAssigneeIDs-style person resolution. If added, follow the existing conflict-detection pattern (--no-X vs --X usage error, internal/commands/todos.go:969).
4. Test: update a to-do that has completion subscribers via --title only → subscribers unchanged; via --no-due → subscribers unchanged; explicit clear (if flag added) → cleared. The e2e harness's existing todos update coverage is the place to extend.

Phase 2 — after basecamp/basecamp-sdk#372 lands (follow-up, tracked here for the map)

• Replace the hand-rolled merge (internal/commands/todos.go:1126 branch) and the needsClear raw-PUT workaround (internal/commands/todos.go:1029 branch, ~80 lines) with the SDK's Edit closure — both branches collapse into one: fetch-free flag mapping onto Edit, with --no-X flags becoming plain field-clears on the full representation.
• Delete the raw-JSON subscriber fetch from Phase 1 once the SDK model round-trips completion_subscribers (Todo model omits completion_subscribers, description_attachments, comments_count basecamp-sdk#355).

Phase 1 is deliberately shaped so Phase 2 deletes it cleanly: the ids ride the same merge that already exists, no new abstractions.

Acceptance criteria

• basecamp todos update <id> --title "x" leaves completion subscribers untouched (both merge and clear branches)
• All existing preserved fields (content, description, due_on, starts_on, assignee_ids) still preserved — regression-covered
• Raw GET fallback documented inline as temporary pending Todo model omits completion_subscribers, description_attachments, comments_count basecamp-sdk#355 (comment referencing both issues)
• e2e/unit coverage for subscriber preservation in both branches

Related

• Todos.Update silently wipes omitted fields (partial PUT against replace-semantics endpoint) — all 6 SDKs; add Edit/Replace, make Update merge-safe basecamp-sdk#372 — fleet-wide SDK fix (merge-safe Update / Edit / Replace); Phase 2 dependency
• Todo model omits completion_subscribers, description_attachments, comments_count basecamp-sdk#355 — Todo model missing completion_subscribers; blocks doing this via typed SDK calls
• Todo Update Due Date Not Working #412 / Fix todo update dropping untouched fields #413 — original partial-PUT wipe fix that established the CLI's read-modify-write merge
• todos update --description silently fails (wrong API endpoint) #422 / Todos.Update uses wrong endpoint - missing /buckets/{projectID}/ segment basecamp-sdk#259 — shorthand-endpoint description drop (same neighborhood, already tracked)

[jeremy]

🤖 Posted with agent assistance by @jeremy.

Heads-up from the SDK side: basecamp/basecamp-sdk#375 (v0.8.0) lands the merge-safe todos surface — Update is now GET → overlay → full PUT, Edit(ctx, id, fn) is the read-modify-write primary API, and Replace is the verbatim sparse PUT. The Go Todo model now maps CompletionSubscribers with the nil-vs-empty distinction preserved (server [] → non-nil empty slice, absent → nil), so this issue's raw-GET completionSubscriberIDs helper and both preservation call sites can be deleted on the SDK bump, with the CLI update path collapsing onto Edit.

Bump hazard to handle in the same change as the SDK bump: today's CLI clear paths rely on omission-clears through the sparse Update/raw PUT. After 0.8.0, Update never clears by omission — any caller clearing a field that way must move to Edit (set empty on the full representation) or Replace. One server-native detail worth mirroring: dates are a range, so clearing due_on alone collapses the range onto starts_on — a full date clear sets both empty.