frontend/src/api/snippets.ts

@@ -20,9 +20,13 @@ export interface SnippetFields {
   path: string;
   symbol: string;
   locations: SnippetLocation[];
-  /** Ids of the snippets folded into this one by merge, oldest first. Read-only:
+  /** The snippets folded into this one by merge, oldest first. Read-only: merge
-   *  merge is the only thing that adds to it, and an edit carries it forward. */
+   *  is the only thing that adds to it, and an edit carries it forward.
-  merged_from: number[];
+   *
+   *  Each entry records what THAT source contributed — never what the survivor
+   *  already had — which is what lets un-merge subtract exactly. An entry with
+   *  no `locations`/`tags` predates that attribution and cannot be un-merged. */
+  merged_from: { id: number; locations?: SnippetLocation[]; tags?: string[] }[];
   code: string;
 }
 
@@ -183,6 +187,16 @@ export async function verifySnippet(
   return apiPost(`/api/snippets/${id}/verify`, verdict);
 }
 
+/** Pull one source back out of a merged survivor: restores it and strips exactly
+ *  what it contributed. Also repairs a half-undone merge — a source restored
+ *  from the trash by hand leaves the survivor still claiming its call sites. */
+export async function unmergeSnippet(
+  survivorId: number,
+  sourceId: number,
+): Promise<{ survivor: Snippet; restored: Snippet | null }> {
+  return apiPost(`/api/snippets/${survivorId}/unmerge`, { source_id: sourceId });
+}
+
 /** Unify `sourceIds` into the canonical snippet `targetId`. Returns the merged
  *  survivor plus `merged_ids` — the sources actually folded in and trashed. */
 export async function mergeSnippets(

frontend/src/views/SnippetDetailView.vue

@@ -1,7 +1,12 @@
 <script setup lang="ts">
 import { ref, computed, onMounted } from "vue";
 import { useRoute, useRouter } from "vue-router";
-import { getSnippet, deleteSnippet, type Snippet } from "@/api/snippets";
+import {
+  getSnippet,
+  deleteSnippet,
+  unmergeSnippet,
+  type Snippet,
+} from "@/api/snippets";
 import { useToastStore } from "@/stores/toast";
 import ConfirmDialog from "@/components/ConfirmDialog.vue";
 
@@ -31,6 +36,32 @@ function locParts(loc: { repo: string; path: string; symbol: string }): string[]
   return [loc.repo, loc.path, loc.symbol].filter((p) => p && p.trim());
 }
 
+// Un-merge (#2165)
+const unmerging = ref<number | null>(null);
+
+/** Only entries carrying what the source contributed can be reversed exactly.
+ *  Without that, subtraction would be a guess that could strip call sites the
+ *  survivor owns in its own right — so the control isn't offered. */
+function canUnmerge(entry: { locations?: unknown[]; tags?: unknown[] }): boolean {
+  return canWrite.value && (!!entry.locations?.length || !!entry.tags?.length);
+}
+
+async function doUnmerge(sourceId: number) {
+  unmerging.value = sourceId;
+  try {
+    await unmergeSnippet(id.value, sourceId);
+    toast.show(`#${sourceId} pulled back out and restored`);
+    await load();
+  } catch (e: unknown) {
+    // 409 carries the reason the record's state makes it impossible — show it
+    // rather than a generic failure, since it's the actionable part.
+    const detail = (e as { body?: { error?: string } }).body?.error;
+    toast.show(detail || `Couldn't un-merge #${sourceId}`, "error");
+  } finally {
+    unmerging.value = null;
+  }
+}
+
 async function load() {
   loading.value = true;
   error.value = null;
@@ -117,7 +148,27 @@ async function confirmDelete() {
         <template v-if="mergedFrom.length">
           <dt>Merged from</dt>
           <dd class="merged-from">
-            <span v-for="mid in mergedFrom" :key="mid">#{{ mid }}</span>
+            <span v-for="m in mergedFrom" :key="m.id" class="merged-entry">
+              <span>#{{ m.id }}</span>
+              <button
+                v-if="canUnmerge(m)"
+                class="unmerge-btn"
+                :disabled="unmerging === m.id"
+                :title="`Pull #${m.id} back out: restore it and remove the ${(m.locations ?? []).length} location(s) it contributed`"
+                @click="doUnmerge(m.id)"
+              >
+                {{ unmerging === m.id ? "…" : "un-merge" }}
+              </button>
+              <!-- Says why rather than hiding the control: a disabled thing with
+                   no explanation reads as a bug. -->
+              <span
+                v-else-if="canWrite"
+                class="unmerge-na"
+                :title="`This merge predates per-source provenance, so what #${m.id} contributed isn't recorded. Restore it from the trash and adjust both records by hand — subtracting a guess could strip call sites this record genuinely owns.`"
+              >
+                (not reversible)
+              </span>
+            </span>
             <span class="merged-hint">
               folded in here — the originals are in the trash
             </span>
@@ -297,6 +348,34 @@ async function confirmDelete() {
   color: var(--color-text-muted);
   font-size: 0.8rem;
 }
+.merged-entry {
+  display: inline-flex;
+  align-items: baseline;
+  gap: 0.3rem;
+}
+.unmerge-btn {
+  font-size: 0.72rem;
+  padding: 0.05rem 0.35rem;
+  border: 1px solid var(--color-border);
+  border-radius: 4px;
+  background: transparent;
+  color: var(--color-text-muted);
+  cursor: pointer;
+}
+.unmerge-btn:hover:not(:disabled) {
+  color: var(--color-text);
+  border-color: var(--color-text-muted);
+}
+.unmerge-btn:disabled {
+  opacity: 0.6;
+  cursor: default;
+}
+.unmerge-na {
+  font-size: 0.72rem;
+  color: var(--color-text-muted);
+  /* Cursor cues that the explanation is in the tooltip. */
+  cursor: help;
+}
 
 .code-block {
   border: 1px solid var(--color-border);

src/scribe/mcp/tools/snippets.py

@@ -190,6 +190,46 @@ async def get_snippet(snippet_id: int) -> dict:
     return data
 
 
+async def unmerge_snippet(survivor_id: int, source_id: int) -> dict:
+    """Reverse ONE source out of a merged snippet — the inverse of merge_snippets.
+
+    Restores the source record and strips exactly what it contributed from the
+    survivor: the locations and tags it ADDED at merge time, never the ones the
+    survivor already had. Reach for it when a merge turns out to have unified two
+    things that only looked alike.
+
+    Also the fix for a half-undone merge. Restoring a merged-in source from the
+    trash by hand brings the record back but leaves the survivor still claiming
+    its call sites, so both records claim the same places and the reverse lookup
+    reads the duplicate claims as real. Running this on an already-restored
+    source repairs that: it skips the restore and does the subtraction.
+
+    Args:
+        survivor_id: The snippet that absorbed the other.
+        source_id: The snippet to pull back out of it.
+
+    Returns {"survivor": {...}, "restored": {...}}.
+
+    Refuses, with the reason, when: the survivor has no record of absorbing that
+    id; the source was purged from the trash; or the merge predates per-source
+    provenance, in which case what it contributed isn't known and subtracting a
+    guess could strip call sites the survivor genuinely owns — restore it from
+    the trash and adjust both records by hand instead.
+    """
+    uid = current_user_id()
+    try:
+        result = await snippets_svc.unmerge_snippet(uid, survivor_id, source_id)
+    except snippets_svc.UnmergeError as exc:
+        raise ValueError(str(exc)) from exc
+    if result is None:
+        raise ValueError(f"snippet {survivor_id} not found")
+    survivor, restored = result
+    return {
+        "survivor": snippets_svc.snippet_to_dict(survivor),
+        "restored": snippets_svc.snippet_to_dict(restored) if restored else None,
+    }
+
+
 async def find_duplicate_snippets(threshold: float = 0.0) -> dict:
     """Find snippets already recorded that look like duplicates of each other.
 
@@ -376,6 +416,10 @@ async def merge_snippets(target_id: int, source_ids: list[int]) -> dict:
     fields and as a "Merged from: #ids" line in the body), so a variant that got
     folded in leaves a trace outside the trash. It accumulates across merges.
 
+    Reversible: each entry also records what that source contributed, so
+    `unmerge_snippet(target_id, source_id)` can restore it and strip exactly
+    those locations back off — never the ones the target already had.
+
     Args:
         target_id: The snippet to keep (the canonical record).
         source_ids: Snippet ids to fold into the target and retire. Ids that
@@ -404,5 +448,6 @@ def register(mcp) -> None:
     for fn in (
         list_snippets, create_snippet, get_snippet, update_snippet,
         delete_snippet, merge_snippets, verify_snippet, find_duplicate_snippets,
+        unmerge_snippet,
     ):
         mcp.tool(name=fn.__name__)(fn)

src/scribe/routes/snippets.py

@@ -205,6 +205,36 @@ async def update_snippet_route(snippet_id: int):
     return jsonify(out)
 
 
+@snippets_bp.route("/<int:snippet_id>/unmerge", methods=["POST"])
+@login_required
+async def unmerge_snippet_route(snippet_id: int):
+    """Pull one source back out of a merged survivor. Body: {"source_id": int}.
+
+    Also the repair for a half-undone merge: restoring a source from the trash by
+    hand leaves the survivor still claiming its call sites, and running this on an
+    already-restored source strips them."""
+    uid = get_current_user_id()
+    data = await request.get_json() or {}
+    source_id = data.get("source_id")
+    if not isinstance(source_id, int):
+        return jsonify({"error": "source_id (int) is required"}), 400
+    try:
+        result = await snippets_svc.unmerge_snippet(uid, snippet_id, source_id)
+    except PermissionError as exc:
+        return jsonify({"error": str(exc)}), 403
+    except snippets_svc.UnmergeError as exc:
+        # 409, not 400: the request is well-formed, the record's state is what
+        # makes it impossible — and the message says which.
+        return jsonify({"error": str(exc)}), 409
+    if result is None:
+        return not_found("Snippet")
+    survivor, restored = result
+    return jsonify({
+        "survivor": snippets_svc.snippet_to_dict(survivor),
+        "restored": snippets_svc.snippet_to_dict(restored) if restored else None,
+    })
+
+
 @snippets_bp.route("/duplicates", methods=["GET"])
 @login_required
 async def duplicate_snippets_route():

src/scribe/services/snippets.py

@@ -134,23 +134,58 @@ def _render_location_block(locations: list[dict]) -> str | None:
     return f"**Locations:**\n{lines}"
 
 
-def _normalize_merged_from(ids: list[int] | None) -> list[int]:
+def _normalize_merged_from(entries: list | None) -> list[dict]:
-    """Merge provenance as a clean id list: ints only, no dups, order kept.
+    """Merge provenance as `[{"id": int, "locations": [...], "tags": [...]}]`.
 
     Order is history, not sorting — earlier merges stay first, so the list reads
     as the sequence of things folded in.
+
+    Each entry records WHAT THAT SOURCE CONTRIBUTED, which is what makes un-merge
+    (#2165) exact. Two problems it solves at once:
+
+      - A location can arrive from a source AND genuinely be the survivor's own.
+        Recording only what the source ADDED means reversing it can never strip a
+        call site the survivor already had.
+      - Two sources can bring the same location. Only the first records it, so
+        un-merging the second leaves it in place — correctly, since the first
+        still claims it.
+
+    A bare int is accepted and normalized to `{"id": n}` with no attribution.
+    That is not legacy tolerance: `snippet_fields` falls back to PARSING THE BODY
+    when a row has no `data`, and the body's `**Merged from:** #ids` line can only
+    ever carry ids. Such an entry still shows provenance; un-merge refuses it
+    rather than guessing, because guessing is exactly the failure above.
     """
-    out: list[int] = []
+    out: list[dict] = []
-    for raw in ids or []:
+    seen: set[int] = set()
+    for raw in entries or []:
+        if isinstance(raw, dict):
+            ident, locs, tags = raw.get("id"), raw.get("locations"), raw.get("tags")
+        else:
+            ident, locs, tags = raw, None, None
         try:
-            i = int(raw)
+            i = int(ident)
         except (TypeError, ValueError):
             continue
-        if i > 0 and i not in out:
+        if i <= 0 or i in seen:
-            out.append(i)
+            continue
+        seen.add(i)
+        entry: dict = {"id": i}
+        norm_locs = _normalize_locations(locs) if locs else []
+        if norm_locs:
+            entry["locations"] = norm_locs
+        clean_tags = [t for t in (tags or []) if isinstance(t, str) and t.strip()]
+        if clean_tags:
+            entry["tags"] = clean_tags
+        out.append(entry)
     return out
 
 
+def merged_from_ids(entries: list | None) -> list[int]:
+    """Just the absorbed ids, in history order — for display and containment."""
+    return [e["id"] for e in _normalize_merged_from(entries)]
+
+
 def compose_body(
     *,
     code: str,
@@ -188,8 +223,11 @@ def compose_body(
         # Human-readable mirror of data["merged_from"]. Without it, a merge folds
         # variants in and the record of what was absorbed lives only in the trash
         # — recoverable only by someone who already knows to go looking.
+        # Ids only — the body is the human-readable mirror, and per-source
+        # attribution belongs in `data` where it can be queried rather than
+        # re-parsed out of prose.
         header.append(
-            "**Merged from:** " + ", ".join(f"#{i}" for i in merged)
+            "**Merged from:** " + ", ".join(f"#{e['id']}" for e in merged)
         )
     fence_lang = (language or "").strip().lower()
     code_block = f"```{fence_lang}\n{(code or '').rstrip()}\n```"
@@ -859,15 +897,32 @@ def merge_snippet_fields(
     """Pure merge: union locations (target's first, then each source in order)
     and union extra tags. The target's scalar fields (name/when_to_use/signature/
     language/code) win — only locations and tags accumulate. ``sources`` is a
-    list of (parsed_fields, tags). Returns (locations, extra_tags)."""
+    list of (parsed_fields, tags).
-    locations = list(target_fields.get("locations") or [])
+
+    Returns (locations, extra_tags, contributions) where `contributions` is one
+    `{"locations": [...], "tags": [...]}` per source, positionally aligned with
+    `sources`, holding ONLY what that source actually added — anything the
+    survivor (or an earlier source) already had is not attributed to it. That is
+    what lets un-merge subtract exactly, without stripping a call site the
+    survivor legitimately owns."""
+    locations = _normalize_locations(target_fields.get("locations") or [])
     extra = _extra_tags(target_tags, target_fields.get("language", ""))
+    contributions: list[dict] = []
     for sfields, stags in sources:
-        locations.extend(sfields.get("locations") or [])
+        before = {_location_str(loc) for loc in locations}
+        added_locs = []
+        for loc in _normalize_locations(sfields.get("locations") or []):
+            if _location_str(loc) not in before:
+                before.add(_location_str(loc))
+                locations.append(loc)
+                added_locs.append(loc)
+        added_tags = []
         for t in _extra_tags(stags, sfields.get("language", "")):
             if t not in extra:
                 extra.append(t)
-    return _normalize_locations(locations), extra
+                added_tags.append(t)
+        contributions.append({"locations": added_locs, "tags": added_tags})
+    return _normalize_locations(locations), extra, contributions
 
 
 async def merge_snippets(user_id: int, target_id: int, source_ids: list[int]):
@@ -917,15 +972,25 @@ async def merge_snippets(user_id: int, target_id: int, source_ids: list[int]):
 
     tgt_fields = snippet_fields(target)
     parsed_sources = [(snippet_fields(s), s.tags) for s in sources]
-    locations, extra_tags = merge_snippet_fields(tgt_fields, target.tags, parsed_sources)
+    locations, extra_tags, contributions = merge_snippet_fields(
+        tgt_fields, target.tags, parsed_sources
+    )
 
     # Provenance: what this record absorbed, and when it absorbed it, in order.
     # Merge keeps the target's scalar fields and trashes the sources, so without
     # this the fact that a variant ever existed survives only in the trash — and
     # only for someone who already knew to go looking. Accumulated, not replaced:
     # a target merged twice keeps both histories.
+    #
+    # Each entry also carries what THAT source contributed, which is what makes
+    # un-merge exact (#2165) rather than a blind subtraction that would strip
+    # call sites the survivor legitimately owns.
     merged_from = _normalize_merged_from(
-        list(tgt_fields.get("merged_from") or []) + [s.id for s in sources]
+        list(tgt_fields.get("merged_from") or [])
+        + [
+            {"id": s.id, **contrib}
+            for s, contrib in zip(sources, contributions)
+        ]
     )
 
     # Owner-scoped write, authorised above — same reason as update_snippet.
@@ -962,3 +1027,112 @@ async def merge_snippets(user_id: int, target_id: int, source_ids: list[int]):
 
     _embed_snippet(updated)
     return updated, merged_ids
+
+
+class UnmergeError(Exception):
+    """Un-merge refused — the reason is the message, meant for the operator."""
+
+
+async def unmerge_snippet(user_id: int, survivor_id: int, source_id: int):
+    """Reverse ONE source out of a merged survivor: restore it, and strip exactly
+    what it contributed.
+
+    WHY THIS OWNS THE RESTORE. The obvious alternative was to have trash-restore
+    notice that the record it's reviving was merged into something and offer to
+    reverse. That would make the generic trash path learn snippet semantics for
+    one record type. Instead un-merge performs the restore itself, so the inverse
+    is one operation with one authorization check and the trash path stays
+    ignorant. Restoring from the trash directly is still allowed and still leaves
+    both records claiming the same call sites — which is why this exists — but it
+    is no longer the only way back.
+
+    EXACTNESS. Subtraction uses the contribution recorded at merge time, not the
+    source's current locations. A source that was itself edited after being
+    merged would otherwise strip locations it never contributed, and a location
+    the survivor independently owned would be lost. When an entry carries no
+    attribution (provenance parsed back out of the body, which can only hold
+    ids), this REFUSES rather than guessing.
+
+    Returns (survivor_note, restored_source_note). Raises UnmergeError with a
+    reason the operator can act on; returns None if the survivor isn't a snippet
+    this caller can see.
+    """
+    from scribe.services.access import can_write_note
+
+    survivor = await get_snippet(user_id, survivor_id)
+    if survivor is None:
+        return None
+    if not await can_write_note(user_id, survivor_id):
+        raise PermissionError(
+            f"snippet {survivor_id} is shared with you read-only — you can't "
+            f"un-merge a record you can't edit"
+        )
+
+    fields = snippet_fields(survivor)
+    entries = _normalize_merged_from(fields.get("merged_from"))
+    entry = next((e for e in entries if e["id"] == int(source_id)), None)
+    if entry is None:
+        raise UnmergeError(
+            f"snippet {survivor_id} has no record of absorbing #{source_id}"
+        )
+    if "locations" not in entry and "tags" not in entry:
+        raise UnmergeError(
+            f"#{source_id} was folded into {survivor_id} before per-source "
+            f"provenance was recorded, so what it contributed isn't known. "
+            f"Restore it from the trash and adjust both records by hand — "
+            f"subtracting a guess could strip call sites {survivor_id} owns."
+        )
+
+    # Bring the source back FIRST: if it can't be revived there is nothing to
+    # un-merge into, and the survivor is better left whole than stripped of
+    # locations whose other claimant never returned.
+    #
+    # An ALREADY-ALIVE source is the common case, not an error — the operator
+    # restored it from the trash themselves, which is precisely the state that
+    # motivated this feature: both records then claim the same call sites, and
+    # nothing had ever stripped the survivor's copy. Skip the revive and go
+    # straight to the subtraction that fixes it.
+    from scribe.services.trash import restore_entity
+
+    restored = await get_snippet(user_id, int(source_id))
+    if restored is None:
+        if await restore_entity(survivor.user_id, "note", int(source_id)) is None:
+            raise UnmergeError(
+                f"#{source_id} could not be restored — it was most likely purged "
+                f"from the trash, and a purged source cannot be brought back"
+            )
+        restored = await get_snippet(user_id, int(source_id))
+
+    drop_locs = {_location_str(loc) for loc in (entry.get("locations") or [])}
+    drop_tags = set(entry.get("tags") or [])
+    kept_locations = [
+        loc for loc in (fields.get("locations") or [])
+        if _location_str(loc) not in drop_locs
+    ]
+    kept_extra = [
+        t for t in _extra_tags(survivor.tags, fields.get("language", ""))
+        if t not in drop_tags
+    ]
+    remaining = [e for e in entries if e["id"] != int(source_id)]
+
+    updated = await notes_svc.update_note(
+        survivor.user_id, survivor_id,
+        body=compose_body(
+            code=fields["code"], language=fields["language"],
+            signature=fields["signature"], when_to_use=fields["when_to_use"],
+            locations=kept_locations, merged_from=remaining,
+        ),
+        tags=compose_tags(fields["language"], kept_extra),
+        data=compose_data(
+            name=fields["name"], when_to_use=fields["when_to_use"],
+            signature=fields["signature"], language=fields["language"],
+            code=fields["code"], locations=kept_locations, merged_from=remaining,
+            # Same reasoning as merge: the survivor's location set just changed,
+            # so any prior drift verdict no longer describes it. Dropped rather
+            # than carried.
+        ),
+    )
+    if updated is None:
+        return None
+    _embed_snippet(updated)
+    return updated, restored

src/scribe/services/trash.py

@@ -181,6 +181,36 @@ async def restore(user_id: int, batch_id: str) -> int:
     return n
 
 
+async def restore_entity(user_id: int, entity_type: str, entity_id: int) -> int | None:
+    """Restore ONE trashed entity by id, by reviving the batch that took it.
+
+    The inverse of `delete(user_id, entity_type, entity_id)`, which returns a
+    batch id the caller usually doesn't keep. Callers that need to undo their own
+    soft-delete later — snippet un-merge (#2165) — know the entity id, not the
+    batch, and looking it up here keeps them from reaching into the column set.
+
+    Restores the whole batch on purpose, not just the row: the batch is the
+    entity plus its cascaded descendants, so reviving the parent alone would
+    leave them orphaned in the trash. That is the same thing the trash UI does.
+
+    Returns rows restored, or None if the entity isn't trashed / not owned.
+    """
+    model = next((m for m, label in _TYPE.items() if label == entity_type), None)
+    if model is None:
+        return None
+    async with async_session() as session:
+        batch = (await session.execute(
+            select(model.deleted_batch_id).where(
+                model.id == entity_id,
+                _owner_clause(model, user_id),
+                model.deleted_at.isnot(None),
+            )
+        )).scalars().first()
+    if not batch:
+        return None
+    return await restore(user_id, batch)
+
+
 async def purge(user_id: int, batch_id: str) -> int:
     """Hard-delete every row in the batch. Irreversible."""
     from sqlalchemy import delete as sql_delete

tests/test_mcp_tool_snippets.py

@@ -229,5 +229,5 @@ def test_register_attaches_all_tools():
     assert set(names) == {
         "list_snippets", "create_snippet", "get_snippet", "update_snippet",
         "delete_snippet", "merge_snippets", "verify_snippet",
-        "find_duplicate_snippets",
+        "find_duplicate_snippets", "unmerge_snippet",
     }

tests/test_routes_snippets.py

@@ -21,6 +21,7 @@ def test_snippet_handlers_callable():
         "list_snippets_route", "create_snippet_route", "get_snippet_route",
         "update_snippet_route", "delete_snippet_route", "merge_snippet_route",
         "verify_snippet_route", "duplicate_snippets_route",
+        "unmerge_snippet_route",
     ):
         assert callable(getattr(routes, name))
 
@@ -31,6 +32,7 @@ def test_service_functions_take_user_id():
     for fn_name in (
         "create_snippet", "list_snippets", "get_snippet", "update_snippet",
         "delete_snippet", "merge_snippets", "record_verification",
+        "unmerge_snippet",
     ):
         fn = getattr(svc, fn_name)
         assert callable(fn)
@@ -46,7 +48,8 @@ def test_agent_and_web_surfaces_stay_at_parity():
     from scribe.routes import snippets as routes
 
     # Every write verb the web surface offers, the agent surface offers too.
-    for verb in ("create", "get", "list", "update", "delete", "merge", "verify"):
+    for verb in ("create", "get", "list", "update", "delete", "merge", "verify",
+                 "unmerge"):
         assert callable(getattr(tools, f"{verb}_snippets", None) or
                         getattr(tools, f"{verb}_snippet", None)), f"MCP lacks {verb}"
 

tests/test_services_snippets.py

@@ -148,7 +148,9 @@ def test_merge_snippet_fields_unions_locations_and_tags():
             ["py", "snippet", "helper"])
     src2 = ({"language": "py", "locations": [{"repo": "a", "path": "a.py", "symbol": "f"}]},  # dup loc
             ["snippet"])
-    locs, extra = s.merge_snippet_fields(target_fields, ["py", "snippet", "core"], [src1, src2])
+    locs, extra, contributions = s.merge_snippet_fields(
+        target_fields, ["py", "snippet", "core"], [src1, src2]
+    )
     # target location first, then unique source locations; dup dropped.
     assert locs == [
         {"repo": "a", "path": "a.py", "symbol": "f"},
@@ -157,6 +159,14 @@ def test_merge_snippet_fields_unions_locations_and_tags():
     # extra tags unioned, language + "snippet" markers excluded.
     assert extra == ["core", "helper"]
 
+    # Attribution (#2165): only what each source ACTUALLY added. src2's location
+    # is one the target already had, so it contributed nothing — un-merging it
+    # must not strip a call site the survivor owns in its own right.
+    assert contributions[0]["locations"] == [{"repo": "b", "path": "b.py", "symbol": "g"}]
+    assert contributions[0]["tags"] == ["helper"]
+    assert contributions[1]["locations"] == []
+    assert contributions[1]["tags"] == []
+
 
 # --- the queryable mirror (notes.data, migration 0070) -----------------------
 
@@ -253,19 +263,44 @@ def test_data_and_body_round_trip_to_the_same_fields():
 
 def test_normalize_merged_from_keeps_history_order_and_drops_junk():
     """Order is history, not sorting — earlier merges stay first."""
-    assert s._normalize_merged_from([9, 3, 9, "4", None, 0, -2, "x"]) == [9, 3, 4]
+    assert s.merged_from_ids([9, 3, 9, "4", None, 0, -2, "x"]) == [9, 3, 4]
     assert s._normalize_merged_from(None) == []
 
 
+def test_normalize_merged_from_carries_per_source_attribution():
+    """The shape that makes un-merge exact (#2165): each entry holds what THAT
+    source contributed, so reversing one can't strip what the survivor owns."""
+    loc = {"repo": "a", "path": "a.py", "symbol": "f"}
+    out = s._normalize_merged_from([{"id": 7, "locations": [loc], "tags": ["x"]}])
+    assert out == [{"id": 7, "locations": [loc], "tags": ["x"]}]
+
+
+def test_a_bare_id_normalizes_to_an_entry_with_no_attribution():
+    """Not legacy tolerance: snippet_fields falls back to PARSING THE BODY when a
+    row has no `data`, and the body's provenance line can only carry ids. Such an
+    entry still shows history; un-merge refuses it rather than guessing."""
+    assert s._normalize_merged_from([5]) == [{"id": 5}]
+
+
 def test_compose_body_renders_merged_from_and_parse_reads_it_back():
     body = s.compose_body(code="x = 1", merged_from=[12, 13])
     assert "**Merged from:** #12, #13" in body
     got = s.parse_snippet_fields("n — u", body, None)
-    assert got["merged_from"] == [12, 13]
+    # Ids survive the body round-trip; attribution cannot, since the body only
+    # ever renders ids — which is exactly why `data` is the authority for it.
+    assert s.merged_from_ids(got["merged_from"]) == [12, 13]
     # The code fence still comes last — provenance is header metadata.
     assert body.rstrip().endswith("```")
 
 
+def test_compose_body_renders_ids_from_rich_entries():
+    body = s.compose_body(
+        code="x = 1",
+        merged_from=[{"id": 12, "locations": [{"repo": "", "path": "a.py", "symbol": ""}]}],
+    )
+    assert "**Merged from:** #12" in body
+
+
 def test_compose_body_omits_merged_from_when_there_is_none():
     """A snippet that was never merged says nothing about merging."""
     assert "Merged from" not in s.compose_body(code="x = 1")
@@ -273,7 +308,8 @@ def test_compose_body_omits_merged_from_when_there_is_none():
 
 
 def test_compose_data_mirrors_merged_from():
-    assert s.compose_data(name="f", merged_from=[3, 3, 2])["merged_from"] == [3, 2]
+    got = s.compose_data(name="f", merged_from=[3, 3, 2])["merged_from"]
+    assert s.merged_from_ids(got) == [3, 2]
     assert "merged_from" not in s.compose_data(name="f")
 
 
@@ -283,7 +319,7 @@ def test_snippet_fields_prefers_the_data_columns_merged_from():
     body = s.compose_body(code="x = 1", merged_from=[12])
     note = _Note(title="n — u", body=body, tags=["snippet"],
                  data=s.compose_data(name="n", merged_from=[12, 13]))
-    assert s.snippet_fields(note)["merged_from"] == [12, 13]
+    assert s.merged_from_ids(s.snippet_fields(note)["merged_from"]) == [12, 13]
 
 
 def test_snippet_to_dict_includes_parsed_fields():

tests/test_snippet_merge_provenance.py

@@ -51,7 +51,7 @@ async def test_merge_records_what_it_absorbed_in_body_and_mirror():
         _snippet(id=1), [_snippet(id=2), _snippet(id=3)], [2, 3],
     )
     assert "**Merged from:** #2, #3" in kwargs["body"]
-    assert kwargs["data"]["merged_from"] == [2, 3]
+    assert s.merged_from_ids(kwargs["data"]["merged_from"]) == [2, 3]
 
 
 @pytest.mark.asyncio
@@ -60,7 +60,7 @@ async def test_merge_accumulates_across_merges():
     replace the record of the first."""
     target = _snippet(id=1, data=s.compose_data(name="f", merged_from=[2]))
     kwargs = await _run_merge(target, [_snippet(id=3)], [3])
-    assert kwargs["data"]["merged_from"] == [2, 3]
+    assert s.merged_from_ids(kwargs["data"]["merged_from"]) == [2, 3]
     assert "**Merged from:** #2, #3" in kwargs["body"]
 
 
@@ -74,7 +74,7 @@ async def test_merge_does_not_record_a_skipped_cross_owner_source():
         [_snippet(id=2, owner=9), _snippet(id=3, owner=42)],
         [2, 3],
     )
-    assert kwargs["data"]["merged_from"] == [2]
+    assert s.merged_from_ids(kwargs["data"]["merged_from"]) == [2]
     assert "#3" not in kwargs["body"]
 
 
@@ -93,7 +93,7 @@ async def test_an_ordinary_edit_carries_provenance_forward():
         await s.update_snippet(7, 1, signature="f(ms) -> string")
 
     kwargs = mock_update.await_args.kwargs
-    assert kwargs["data"]["merged_from"] == [2, 3]
+    assert s.merged_from_ids(kwargs["data"]["merged_from"]) == [2, 3]
     assert "**Merged from:** #2, #3" in kwargs["body"]
 
 

tests/test_snippet_unmerge.py

@@ -0,0 +1,183 @@
+"""Tests for un-merge (#2165) — reversing one source out of a merged survivor.
+
+The design question this task said to settle first was how to subtract without
+stripping call sites the survivor legitimately owns. The answer is per-source
+attribution recorded AT MERGE TIME: each `merged_from` entry holds only what
+that source actually added. These tests pin that rule, and the refusal that
+guards the case where the attribution isn't there.
+"""
+from types import SimpleNamespace
+from unittest.mock import AsyncMock, MagicMock, patch
+
+import pytest
+
+from scribe.services import snippets as s
+
+
+def _loc(path, repo="r", symbol=""):
+    return {"repo": repo, "path": path, "symbol": symbol}
+
+
+def _survivor(locations, merged_from, tags=None, owner=7):
+    """A survivor note whose `data` carries locations + merge provenance."""
+    return SimpleNamespace(
+        id=1, user_id=owner, note_type="snippet", deleted_at=None,
+        title="f — does a thing", tags=tags or ["python", "snippet"],
+        body=s.compose_body(code="x = 1", language="python", locations=locations,
+                            merged_from=merged_from),
+        data=s.compose_data(name="f", when_to_use="does a thing", language="python",
+                            code="x = 1", locations=locations,
+                            merged_from=merged_from),
+    )
+
+
+async def _run_unmerge(survivor, *, source_alive=None, restore=1):
+    """Drive unmerge_snippet with the DB stubbed; return update_note's kwargs."""
+    async def fake_get(_uid, sid):
+        if sid == survivor.id:
+            return survivor
+        return source_alive
+
+    with (
+        patch.object(s, "get_snippet", AsyncMock(side_effect=fake_get)),
+        patch("scribe.services.access.can_write_note", AsyncMock(return_value=True)),
+        patch("scribe.services.trash.restore_entity", AsyncMock(return_value=restore)),
+        patch.object(s.notes_svc, "update_note",
+                     AsyncMock(return_value=survivor)) as upd,
+        patch.object(s, "_embed_snippet", MagicMock()),
+    ):
+        await s.unmerge_snippet(7, 1, 2)
+    return upd.await_args.kwargs
+
+
+# --- the subtraction ------------------------------------------------------
+
+
+async def test_unmerge_strips_only_what_the_source_contributed():
+    """The survivor's own location survives; the source's is removed."""
+    mine, theirs = _loc("mine.py"), _loc("theirs.py")
+    survivor = _survivor(
+        [mine, theirs],
+        [{"id": 2, "locations": [theirs], "tags": ["helper"]}],
+        tags=["python", "snippet", "helper", "core"],
+    )
+    kwargs = await _run_unmerge(survivor)
+    assert kwargs["data"]["locations"] == [mine]
+    assert "helper" not in kwargs["tags"]
+    assert "core" in kwargs["tags"]
+
+
+async def test_a_location_the_survivor_also_owned_is_never_stripped():
+    """The central hazard the task named. If a source brought a location the
+    survivor ALREADY had, merge attributes nothing to it — so reversing must
+    leave that call site in place."""
+    shared = _loc("shared.py")
+    survivor = _survivor([shared], [{"id": 2, "locations": [], "tags": ["t"]}])
+    kwargs = await _run_unmerge(survivor)
+    assert kwargs["data"]["locations"] == [shared]
+
+
+async def test_the_reversed_entry_leaves_the_provenance_list():
+    survivor = _survivor(
+        [_loc("a.py"), _loc("b.py")],
+        [{"id": 2, "locations": [_loc("b.py")]}, {"id": 3, "locations": []}],
+    )
+    kwargs = await _run_unmerge(survivor)
+    assert s.merged_from_ids(kwargs["data"]["merged_from"]) == [3]
+    assert "#2" not in kwargs["body"]
+    # The other source's history is untouched — un-merge reverses one thing.
+    assert "**Merged from:** #3" in kwargs["body"]
+
+
+async def test_unmerging_the_last_source_clears_the_provenance_line():
+    survivor = _survivor([_loc("a.py")], [{"id": 2, "locations": [], "tags": ["t"]}])
+    kwargs = await _run_unmerge(survivor)
+    assert "Merged from" not in kwargs["body"]
+
+
+# --- restoring the source -------------------------------------------------
+
+
+async def test_an_already_restored_source_is_repaired_not_refused():
+    """The scenario that motivated the feature: the operator restored the source
+    from the trash by hand, so both records claim its call sites and nothing ever
+    stripped the survivor's copy. Un-merge must fix that, not reject it."""
+    theirs = _loc("theirs.py")
+    survivor = _survivor([_loc("mine.py"), theirs],
+                         [{"id": 2, "locations": [theirs]}])
+    alive = SimpleNamespace(id=2, user_id=7, note_type="snippet", deleted_at=None,
+                            title="g — x", tags=["snippet"], body="", data=None)
+    with patch("scribe.services.trash.restore_entity", AsyncMock()) as revive:
+        kwargs = await _run_unmerge(survivor, source_alive=alive)
+    # Nothing to revive — it's already alive — but the subtraction still happens.
+    revive.assert_not_called()
+    assert kwargs["data"]["locations"] == [_loc("mine.py")]
+
+
+async def test_a_purged_source_is_refused_and_the_survivor_is_untouched():
+    """If the source can't come back, stripping the survivor would lose the
+    locations entirely — no record would claim them."""
+    survivor = _survivor([_loc("a.py")], [{"id": 2, "locations": [_loc("a.py")]}])
+
+    async def fake_get(_uid, sid):
+        return survivor if sid == 1 else None
+
+    with (
+        patch.object(s, "get_snippet", AsyncMock(side_effect=fake_get)),
+        patch("scribe.services.access.can_write_note", AsyncMock(return_value=True)),
+        patch("scribe.services.trash.restore_entity", AsyncMock(return_value=None)),
+        patch.object(s.notes_svc, "update_note", AsyncMock()) as upd,
+        patch.object(s, "_embed_snippet", MagicMock()),
+    ):
+        with pytest.raises(s.UnmergeError, match="purged"):
+            await s.unmerge_snippet(7, 1, 2)
+    upd.assert_not_called()
+
+
+# --- refusals -------------------------------------------------------------
+
+
+async def test_an_entry_without_attribution_is_refused_not_guessed():
+    """Bare-id provenance comes from parsing the body, which can only hold ids.
+    Subtracting a guess could strip call sites the survivor owns — so refuse and
+    say what to do instead."""
+    survivor = _survivor([_loc("a.py")], [2])
+
+    async def fake_get(_uid, sid):
+        return survivor if sid == 1 else None
+
+    with (
+        patch.object(s, "get_snippet", AsyncMock(side_effect=fake_get)),
+        patch("scribe.services.access.can_write_note", AsyncMock(return_value=True)),
+        patch.object(s.notes_svc, "update_note", AsyncMock()) as upd,
+        patch.object(s, "_embed_snippet", MagicMock()),
+    ):
+        with pytest.raises(s.UnmergeError, match="provenance"):
+            await s.unmerge_snippet(7, 1, 2)
+    upd.assert_not_called()
+
+
+async def test_unmerging_something_never_absorbed_is_refused():
+    survivor = _survivor([_loc("a.py")], [{"id": 99, "locations": []}])
+    with (
+        patch.object(s, "get_snippet", AsyncMock(return_value=survivor)),
+        patch("scribe.services.access.can_write_note", AsyncMock(return_value=True)),
+    ):
+        with pytest.raises(s.UnmergeError, match="no record of absorbing"):
+            await s.unmerge_snippet(7, 1, 2)
+
+
+async def test_unmerge_requires_write_access():
+    """Same rule as merge: a read-only share can see the record, not rearrange it."""
+    survivor = _survivor([_loc("a.py")], [{"id": 2, "locations": []}])
+    with (
+        patch.object(s, "get_snippet", AsyncMock(return_value=survivor)),
+        patch("scribe.services.access.can_write_note", AsyncMock(return_value=False)),
+    ):
+        with pytest.raises(PermissionError):
+            await s.unmerge_snippet(7, 1, 2)
+
+
+async def test_unmerge_on_a_missing_survivor_returns_none():
+    with patch.object(s, "get_snippet", AsyncMock(return_value=None)):
+        assert await s.unmerge_snippet(7, 1, 2) is None