diff --git a/.gitignore b/.gitignore
index 61107d4..4de8b63 100644
--- a/.gitignore
+++ b/.gitignore
@@ -65,6 +65,7 @@ BACKLOG.md
 EXPERIMENTAL_MVCC.md
 HARDENING_DECOUPLED.md
 ICEBERG_LAZY_REFRESH.md
+CROSS_TIER_MOVE_DESIGN.md
 
 # Local secrets — NEVER commit. (One-off GHCR push token; revoke after use.)
 github_token_do_not_merge.txt
diff --git a/docs/architecture_tiered.md b/docs/architecture_tiered.md
index 1dc76cd..da481f3 100644
--- a/docs/architecture_tiered.md
+++ b/docs/architecture_tiered.md
@@ -226,6 +226,14 @@ AND of those, OR of those when all arms prove the same tier, BETWEEN
 Subqueries, UDF calls, and expressions on the partition column are
 AMBIGUOUS.
 
+The tier classification above applies to the WHERE clause. The SET
+clause carries a separate rule: an `UPDATE` that assigns the partition
+column itself is rejected, regardless of `coldfront.allow_mixed_writes`.
+Changing the partition column can move a row across the cutoff, and the
+in-place rewrite would leave the row in its old tier where the view's
+tier predicate then hides it. To change the partition column, delete the
+row and re-insert it with the new value.
+
 ## Write modes: strict vs permissive (`allow_mixed_writes`)
 
 When the predicate is AMBIGUOUS the hook picks one of two behaviours
diff --git a/extension/coldfront/Makefile b/extension/coldfront/Makefile
index 423842c..62585c3 100644
--- a/extension/coldfront/Makefile
+++ b/extension/coldfront/Makefile
@@ -13,6 +13,7 @@ EXTENSION   = coldfront
 DATA        = coldfront--1.0.sql coldfront--0.1--1.0.sql
 REGRESS     = load_order update_unregistered_view update_heap_table \
               update_hot_via_view update_cold_via_view update_ambiguous_rejected \
+              update_partition_key_blocked \
               returning_literal_in_where cast_literal_in_value \
               classify_between_in_or allow_mixed_writes cold_write_batch_size_guc \
               dollar_quote_in_value mixed_case_identifier \
diff --git a/extension/coldfront/src/coldfront.c b/extension/coldfront/src/coldfront.c
index a6cce09..07b0a26 100644
--- a/extension/coldfront/src/coldfront.c
+++ b/extension/coldfront/src/coldfront.c
@@ -1960,6 +1960,52 @@ cf_reject_multi_reference(Query *query, RangeTblEntry *rte)
                          "are not supported; reference it once.")));
 }
 
+/*
+ * Reject an UPDATE that assigns the partition column of a tiered view. Changing
+ * the partition column can move the row across the hot/cold cutoff; the in-place
+ * rewrite (hot, cold, or dual) updates the row where it already lives, so a moved
+ * row stays physically in its old tier while the view's tier predicate
+ * (ts >= cutoff / r[ts] < cutoff) filters it out — a silent disappearance
+ * (GitHub #20). Relocating the row across tiers is a separate, unimplemented
+ * feature; until then the partition column is read-only through the view. The
+ * targetList here is post-parse-analyze, so it holds exactly the SET-assigned
+ * columns (plus resjunk entries we skip) — not the full row. Blocks any assignment
+ * regardless of the new value's tier or coldfront.allow_mixed_writes: with no move
+ * to permit, allowing it would just reinstate the loss.
+ */
+static void
+cf_reject_partition_col_update(Query *query, RangeTblEntry *rte,
+                               TieredViewInfo *info)
+{
+    AttrNumber  partcol_attno;
+    ListCell   *lc;
+
+    if (query->commandType != CMD_UPDATE || info->partition_col == NULL)
+        return;
+
+    partcol_attno = get_attnum(rte->relid, info->partition_col);
+    if (partcol_attno == InvalidAttrNumber)
+        return;
+
+    foreach(lc, query->targetList)
+    {
+        TargetEntry *tle = (TargetEntry *) lfirst(lc);
+
+        if (tle->resjunk)
+            continue;
+        if (tle->resno == partcol_attno)
+            ereport(ERROR,
+                    (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+                     errmsg("UPDATE of partition column \"%s\" on tiered view \"%s\" is not supported",
+                            info->partition_col, get_rel_name(rte->relid)),
+                     errhint("Changing \"%s\" can move the row across the hot/cold "
+                             "boundary; that relocation is not yet supported. To "
+                             "change \"%s\", delete the row and re-insert it with "
+                             "the new value.",
+                             info->partition_col, info->partition_col)));
+    }
+}
+
 /*
  * Reject a multi-reference UPDATE/DELETE on a tiered view, then pick the emit
  * path (tiered-INSERT split, hot, cold, or dual) and return the rewritten SQL.
@@ -1972,6 +2018,7 @@ cf_dispatch_emit(Query *query, RangeTblEntry *rte, TieredViewInfo *info,
     TierClass tier;
 
     cf_reject_multi_reference(query, rte);
+    cf_reject_partition_col_update(query, rte, info);
 
     /* Tiered-view INSERT: bulk split-by-watermark via emit_tiered_insert.
      * Iceberg-only INSERT falls through to the unconditional cold path
diff --git a/extension/coldfront/test/expected/update_partition_key_blocked.out b/extension/coldfront/test/expected/update_partition_key_blocked.out
new file mode 100644
index 0000000..4bf400e
--- /dev/null
+++ b/extension/coldfront/test/expected/update_partition_key_blocked.out
@@ -0,0 +1,65 @@
+-- An UPDATE that assigns the partition column of a tiered view is blocked: such
+-- a SET can move the row across the hot/cold cutoff, which the in-place rewrite
+-- would lose silently (GitHub #20). The hook must ereport(ERROR) with SQLSTATE
+-- 0A000 (ERRCODE_FEATURE_NOT_SUPPORTED) regardless of coldfront.allow_mixed_writes
+-- and regardless of which tier the WHERE selects. Relocating the row is a separate
+-- feature; until then the partition column is read-only via the view.
+CREATE EXTENSION IF NOT EXISTS pg_duckdb;
+NOTICE:  extension "pg_duckdb" already exists, skipping
+CREATE EXTENSION IF NOT EXISTS coldfront;
+NOTICE:  extension "coldfront" already exists, skipping
+SET TIME ZONE 'UTC';
+-- White-box: checks the hooks' SQL/DDL, not Iceberg I/O. Real cold I/O is ci/journey.sh; see README.md.
+SET coldfront.warehouse = '';
+SET coldfront.lakekeeper_endpoint = '';
+CREATE TABLE public._events (id int, ts timestamptz, status text);
+INSERT INTO public._events VALUES (1, '2026-04-01 12:00:00+00', 'hot_orig');
+CREATE VIEW public.events AS SELECT * FROM public._events;
+INSERT INTO coldfront.tiered_views(schema_name, relname, hot_table, iceberg_table, partition_col)
+VALUES ('public', 'events', 'public._events', 'ice.default.events', 'ts');
+INSERT INTO coldfront.archive_watermark(table_name, cutoff_time)
+VALUES ('events', '2026-03-01'::timestamptz);
+-- Permissive mode (default): a partition-column SET is still blocked, because
+-- the move is not implemented and the dual-tier rewrite would lose the row.
+SET coldfront.allow_mixed_writes = on;
+-- Cold→hot crossing (the #20 repro): blocked.
+UPDATE public.events SET ts = '2026-06-18 10:00:00+00' WHERE status = 'hot_orig';
+ERROR:  UPDATE of partition column "ts" on tiered view "events" is not supported
+HINT:  Changing "ts" can move the row across the hot/cold boundary; that relocation is not yet supported. To change "ts", delete the row and re-insert it with the new value.
+-- Hot→cold crossing: blocked too (symmetric loss).
+UPDATE public.events SET ts = '2026-01-05 10:00:00+00' WHERE ts = '2026-04-01 12:00:00+00';
+ERROR:  UPDATE of partition column "ts" on tiered view "events" is not supported
+HINT:  Changing "ts" can move the row across the hot/cold boundary; that relocation is not yet supported. To change "ts", delete the row and re-insert it with the new value.
+-- Same-tier constant SET is also blocked (blunt block; no per-value proof).
+UPDATE public.events SET ts = '2026-04-02 12:00:00+00' WHERE ts = '2026-04-01 12:00:00+00';
+ERROR:  UPDATE of partition column "ts" on tiered view "events" is not supported
+HINT:  Changing "ts" can move the row across the hot/cold boundary; that relocation is not yet supported. To change "ts", delete the row and re-insert it with the new value.
+-- Non-constant partition-column SET (could cross per-row): blocked.
+UPDATE public.events SET ts = ts + interval '6 months' WHERE id = 1;
+ERROR:  UPDATE of partition column "ts" on tiered view "events" is not supported
+HINT:  Changing "ts" can move the row across the hot/cold boundary; that relocation is not yet supported. To change "ts", delete the row and re-insert it with the new value.
+-- A tier-deterministic WHERE does not rescue a partition-column SET: blocked.
+UPDATE public.events SET ts = '2026-06-18 10:00:00+00' WHERE ts = '2026-01-15 01:00:00+00';
+ERROR:  UPDATE of partition column "ts" on tiered view "events" is not supported
+HINT:  Changing "ts" can move the row across the hot/cold boundary; that relocation is not yet supported. To change "ts", delete the row and re-insert it with the new value.
+-- Strict mode: same block.
+SET coldfront.allow_mixed_writes = off;
+UPDATE public.events SET ts = '2026-06-18 10:00:00+00' WHERE status = 'hot_orig';
+ERROR:  UPDATE of partition column "ts" on tiered view "events" is not supported
+HINT:  Changing "ts" can move the row across the hot/cold boundary; that relocation is not yet supported. To change "ts", delete the row and re-insert it with the new value.
+-- A SET that does NOT touch the partition column is unaffected (still routes by
+-- WHERE tier — here hot, plain PG).
+UPDATE public.events SET status = 'ok' WHERE ts = '2026-04-01 12:00:00+00';
+-- _events: only the status update applied; every ts-changing statement errored.
+SELECT id, ts, status FROM public._events ORDER BY id;
+ id |              ts              | status 
+----+------------------------------+--------
+  1 | Wed Apr 01 12:00:00 2026 UTC | ok
+(1 row)
+
+-- Cleanup. Unregister before dropping: the DDL hook blocks DROP of a
+-- registered tiered table/view.
+DELETE FROM coldfront.tiered_views;
+DELETE FROM coldfront.archive_watermark;
+DROP VIEW public.events;
+DROP TABLE public._events;
diff --git a/extension/coldfront/test/sql/update_partition_key_blocked.sql b/extension/coldfront/test/sql/update_partition_key_blocked.sql
new file mode 100644
index 0000000..9b4144f
--- /dev/null
+++ b/extension/coldfront/test/sql/update_partition_key_blocked.sql
@@ -0,0 +1,60 @@
+-- An UPDATE that assigns the partition column of a tiered view is blocked: such
+-- a SET can move the row across the hot/cold cutoff, which the in-place rewrite
+-- would lose silently (GitHub #20). The hook must ereport(ERROR) with SQLSTATE
+-- 0A000 (ERRCODE_FEATURE_NOT_SUPPORTED) regardless of coldfront.allow_mixed_writes
+-- and regardless of which tier the WHERE selects. Relocating the row is a separate
+-- feature; until then the partition column is read-only via the view.
+
+CREATE EXTENSION IF NOT EXISTS pg_duckdb;
+CREATE EXTENSION IF NOT EXISTS coldfront;
+
+SET TIME ZONE 'UTC';
+-- White-box: checks the hooks' SQL/DDL, not Iceberg I/O. Real cold I/O is ci/journey.sh; see README.md.
+SET coldfront.warehouse = '';
+SET coldfront.lakekeeper_endpoint = '';
+
+CREATE TABLE public._events (id int, ts timestamptz, status text);
+INSERT INTO public._events VALUES (1, '2026-04-01 12:00:00+00', 'hot_orig');
+CREATE VIEW public.events AS SELECT * FROM public._events;
+
+INSERT INTO coldfront.tiered_views(schema_name, relname, hot_table, iceberg_table, partition_col)
+VALUES ('public', 'events', 'public._events', 'ice.default.events', 'ts');
+INSERT INTO coldfront.archive_watermark(table_name, cutoff_time)
+VALUES ('events', '2026-03-01'::timestamptz);
+
+-- Permissive mode (default): a partition-column SET is still blocked, because
+-- the move is not implemented and the dual-tier rewrite would lose the row.
+SET coldfront.allow_mixed_writes = on;
+
+-- Cold→hot crossing (the #20 repro): blocked.
+UPDATE public.events SET ts = '2026-06-18 10:00:00+00' WHERE status = 'hot_orig';
+
+-- Hot→cold crossing: blocked too (symmetric loss).
+UPDATE public.events SET ts = '2026-01-05 10:00:00+00' WHERE ts = '2026-04-01 12:00:00+00';
+
+-- Same-tier constant SET is also blocked (blunt block; no per-value proof).
+UPDATE public.events SET ts = '2026-04-02 12:00:00+00' WHERE ts = '2026-04-01 12:00:00+00';
+
+-- Non-constant partition-column SET (could cross per-row): blocked.
+UPDATE public.events SET ts = ts + interval '6 months' WHERE id = 1;
+
+-- A tier-deterministic WHERE does not rescue a partition-column SET: blocked.
+UPDATE public.events SET ts = '2026-06-18 10:00:00+00' WHERE ts = '2026-01-15 01:00:00+00';
+
+-- Strict mode: same block.
+SET coldfront.allow_mixed_writes = off;
+UPDATE public.events SET ts = '2026-06-18 10:00:00+00' WHERE status = 'hot_orig';
+
+-- A SET that does NOT touch the partition column is unaffected (still routes by
+-- WHERE tier — here hot, plain PG).
+UPDATE public.events SET status = 'ok' WHERE ts = '2026-04-01 12:00:00+00';
+
+-- _events: only the status update applied; every ts-changing statement errored.
+SELECT id, ts, status FROM public._events ORDER BY id;
+
+-- Cleanup. Unregister before dropping: the DDL hook blocks DROP of a
+-- registered tiered table/view.
+DELETE FROM coldfront.tiered_views;
+DELETE FROM coldfront.archive_watermark;
+DROP VIEW public.events;
+DROP TABLE public._events;