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## What Enforce a function–field binding principle for function DDL (add / drop / alter function on an existing collection), so a function and its output field stay coupled and already-stored vectors can never be silently invalidated by DDL. BM25 and MinHash follow the strict binding: add/drop only via the unified `add_function_field` / `drop_function_field`, output field always coupled. **TextEmbedding is a temporary carve-out** — its legacy `AddCollectionFunction` / `DropCollectionFunction` paths are retained, so attach-over-existing-field and detach are still possible for it, because `add_function_field` embedding backfill is not yet implemented. It will be folded into the unified path in a follow-up. So the "always coupled" invariant currently holds for BM25/MinHash, not TextEmbedding. ## Changes - **AlterCollectionSchema invariant**: reject standalone add-function (a function must be added together with its new output field), reject detaching a function without dropping its output field, and reject function cascade (a new function's input being another function's output). - **Legacy RPCs**: `AddCollectionFunction` / `DropCollectionFunction` are rejected for BM25/MinHash (must use `add_function_field` / `drop_function_field`) and retained only for TextEmbedding (see carve-out). `AlterCollectionFunction` is kept and gains the whitelist below. In the alter and legacy-add paths, function field IDs are always re-derived from field names (never trusted from the request), so a request cannot inject an unrelated field ID that a later `drop_function_field` would delete. - **alter_function whitelist**: only connection/runtime params may change (TextEmbedding: `url`, `credential`, `timeout_ms`, `max_client_batch_size`, `region`, `location`, `projectid`, `user`); BM25/MinHash have no alterable params. Function identity (type, name, input/output fields) and output-shaping params (`dim`, `model_name`, `endpoint` — the TEI model identity, `normalize`, `truncate*`, prompts, ...) are immutable. Params are normalized (keys lowercased, duplicate keys rejected) so a crafted duplicate key cannot bypass the diff. - **drop_function_field**: allow any output-producing function (dropping needs no backfill), removing the previous BM25/MinHash-only restriction. ## Not in scope / follow-up - Extending `add_function_field` to TextEmbedding (post-creation add would require backfilling every existing row through the external embedding model) — deferred; folding the TextEmbedding legacy paths into the unified binding follows this. - MinHash input-field analyzer immutability lives on a different DDL path (`AlterCollectionField`); tracked as a separate follow-up. ## Breaking changes - `DropCollectionFunction` on a **MinHash** function (detach — remove the function but keep its output field) is no longer supported; use `drop_function_field` instead (drops the function together with its output field). If a released version supported MinHash detach, migrate any such call to `drop_function_field`. issue: #51348 🤖 Generated with [Claude Code](https://claude.com/claude-code) Signed-off-by: MrPresent-Han <chun.han@gmail.com> Co-authored-by: MrPresent-Han <chun.han@gmail.com> Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>