diff --git a/server/src/at_record_server/shelf_index_postgres.gleam b/server/src/at_record_server/shelf_index_postgres.gleam
index e22cd28..867ae5d 100644
--- a/server/src/at_record_server/shelf_index_postgres.gleam
+++ b/server/src/at_record_server/shelf_index_postgres.gleam
@@ -13,6 +13,7 @@ import at_record_server/shelf_index.{
   SeenOps, ShelfEntryEvent, Store,
 }
 import gleam/dynamic/decode
+import gleam/int
 import gleam/option.{type Option, None, Some}
 import gleam/result
 import pog
@@ -45,93 +46,144 @@ pub fn table_store(conn: pog.Connection) -> Result(shelf_index.Store, String) {
 }
 
 fn migrate(conn: pog.Connection) -> Result(Nil, String) {
-  use _ <- result.try(
-    pog.query(
-      "create table if not exists shelf_entry_events (
-         event_uri text primary key,
-         entry_uri text not null,
-         did text not null,
-         rkey text not null,
-         action text not null,
-         created_at text not null,
-         record_json text not null,
-         indexed_at text not null
-       )",
-    )
-    |> pog.execute(conn)
-    |> result.replace_error("shelf_entry_events migration failed"),
-  )
-  use _ <- result.try(
-    pog.query(
-      "create index if not exists shelf_entry_events_entry_rkey_idx
-         on shelf_entry_events (entry_uri, rkey)",
-    )
-    |> pog.execute(conn)
-    |> result.replace_error("shelf_entry_events index migration failed"),
-  )
-  use _ <- result.try(
-    pog.query(
-      "create table if not exists shelf_entries (
-         entry_uri text primary key,
-         did text not null,
-         status text not null,
-         title text,
-         artist_display text,
-         year int,
-         format text,
-         thumb_url text,
-         cover_cid text,
-         media_grade text,
-         sleeve_grade text,
-         rating int,
-         folder text,
-         notes text,
-         release_uri text,
-         release_cid text,
-         price_amount int,
-         price_currency text,
-         counterparty text,
-         source_client_agent text,
-         source_origin text,
-         source_origin_url text,
-         source_external_provider text,
-         source_external_id text,
-         source_external_url text,
-         source_record_uri text,
-         source_record_cid text,
-         created_at text not null,
-         updated_at text not null,
-         indexed_at text not null
-       )",
-    )
-    |> pog.execute(conn)
-    |> result.replace_error("shelf_entries migration failed"),
-  )
-  use _ <- result.try(
-    pog.query(
-      "create index if not exists shelf_entries_did_status_idx
-         on shelf_entries (did, status)",
-    )
-    |> pog.execute(conn)
-    |> result.replace_error("shelf_entries did/status index migration failed"),
-  )
-  use _ <- result.try(
-    pog.query(
-      "create index if not exists shelf_entries_release_uri_idx
-         on shelf_entries (release_uri)",
-    )
-    |> pog.execute(conn)
-    |> result.replace_error("shelf_entries release_uri index migration failed"),
-  )
-  pog.query(
+  use _ <- result.try(migration_step(
+    conn,
+    "create table if not exists shelf_entry_events (
+       event_uri text primary key,
+       entry_uri text not null,
+       did text not null,
+       rkey text not null,
+       action text not null,
+       created_at text not null,
+       record_json text not null,
+       indexed_at text not null
+     )",
+    "shelf_entry_events migration failed",
+  ))
+  use _ <- result.try(migration_step(
+    conn,
+    "create index if not exists shelf_entry_events_entry_rkey_idx
+       on shelf_entry_events (entry_uri, rkey)",
+    "shelf_entry_events index migration failed",
+  ))
+  use _ <- result.try(migration_step(
+    conn,
+    "create table if not exists shelf_entries (
+       entry_uri text primary key,
+       did text not null,
+       status text not null,
+       title text,
+       artist_display text,
+       year int,
+       format text,
+       thumb_url text,
+       cover_cid text,
+       media_grade text,
+       sleeve_grade text,
+       rating int,
+       folder text,
+       notes text,
+       release_uri text,
+       release_cid text,
+       price_amount int,
+       price_currency text,
+       counterparty text,
+       source_client_agent text,
+       source_origin text,
+       source_origin_url text,
+       source_external_provider text,
+       source_external_id text,
+       source_external_url text,
+       source_record_uri text,
+       source_record_cid text,
+       created_at text not null,
+       updated_at text not null,
+       indexed_at text not null
+     )",
+    "shelf_entries migration failed",
+  ))
+  use _ <- result.try(migration_step(
+    conn,
+    "create index if not exists shelf_entries_did_status_idx
+       on shelf_entries (did, status)",
+    "shelf_entries did/status index migration failed",
+  ))
+  use _ <- result.try(migration_step(
+    conn,
+    "create index if not exists shelf_entries_release_uri_idx
+       on shelf_entries (release_uri)",
+    "shelf_entries release_uri index migration failed",
+  ))
+  migration_step(
+    conn,
     "create table if not exists shelf_index_seen (
        did text primary key,
        seen_at text not null
      )",
+    "shelf_index_seen migration failed",
   )
-  |> pog.execute(conn)
-  |> result.replace_error("shelf_index_seen migration failed")
-  |> result.map(fn(_) { Nil })
+}
+
+/// Runs one migration statement, treating a lost `if not exists` race (see
+/// `is_race_loser`) as success and anything else as a labeled failure.
+fn migration_step(
+  conn: pog.Connection,
+  sql: String,
+  label: String,
+) -> Result(Nil, String) {
+  case pog.query(sql) |> pog.execute(conn) {
+    Ok(_) -> Ok(Nil)
+    Error(e) ->
+      case is_race_loser(e) {
+        True -> Ok(Nil)
+        False -> Error(label <> ": " <> describe_query_error(e))
+      }
+  }
+}
+
+/// `create table/index if not exists` is not actually safe under concurrent
+/// callers (every `with_store`-backed test opens its own fresh pool and
+/// re-runs `migrate`): two sessions can both pass the "not exists" check
+/// before either commits, and the loser then fails inserting its own system
+/// catalog row -- a duplicate key on `pg_type_typname_nsp_index` (a table)
+/// or `pg_class_relname_nsp_index` (an index), not anything
+/// migration-specific. Confirmed locally: five concurrent `create table if
+/// not exists shelf_entry_events` statements against a real
+/// non-superuser-owned Postgres 18 database, four of five failed with
+/// exactly this. Losing this specific race means the object exists now,
+/// which was always the actual goal, so it is not a real failure.
+fn is_race_loser(e: pog.QueryError) -> Bool {
+  case e {
+    pog.ConstraintViolated(constraint:, ..) ->
+      constraint == "pg_type_typname_nsp_index"
+      || constraint == "pg_class_relname_nsp_index"
+    _ -> False
+  }
+}
+
+fn describe_query_error(e: pog.QueryError) -> String {
+  case e {
+    pog.ConstraintViolated(message:, constraint:, detail:) ->
+      "constraint "
+      <> constraint
+      <> " violated: "
+      <> message
+      <> " ("
+      <> detail
+      <> ")"
+    pog.PostgresqlError(code:, name:, message:) ->
+      "postgres error " <> code <> " (" <> name <> "): " <> message
+    pog.UnexpectedArgumentCount(expected:, got:) ->
+      "expected "
+      <> int.to_string(expected)
+      <> " argument(s), got "
+      <> int.to_string(got)
+    pog.UnexpectedArgumentType(expected:, got:) ->
+      "expected argument type " <> expected <> ", got " <> got
+    pog.UnexpectedResultType(_) -> "unexpected result shape"
+    pog.QueryTimeout -> "query timed out"
+    pog.ConnectionUnavailable -> "no connection available"
+  }
 }
 
 fn upsert_event(conn: pog.Connection, event: ShelfEntryEvent) -> Nil {