diff --git a/DESCRIPTION b/DESCRIPTION
index 012b375..34ce3f0 100644
--- a/DESCRIPTION
+++ b/DESCRIPTION
@@ -15,6 +15,7 @@ Imports:
     S7,
     utils
 Suggests:
+    DBI,
     testthat (>= 3.0.0)
 URL: https://github.com/nbenn/sqlr
 BugReports: https://github.com/nbenn/sqlr/issues
@@ -38,3 +39,4 @@ Collate:
     'order.R'
     'render.R'
     'compare.R'
+    'conformance.R'
diff --git a/NAMESPACE b/NAMESPACE
index f6fb647..78a5ee1 100644
--- a/NAMESPACE
+++ b/NAMESPACE
@@ -10,6 +10,7 @@ export(sqlr_catalog)
 export(sqlr_char)
 export(sqlr_check)
 export(sqlr_column)
+export(sqlr_conformance_types)
 export(sqlr_constraint)
 export(sqlr_date)
 export(sqlr_decimal_type)
@@ -46,6 +47,7 @@ export(sqlr_smallint)
 export(sqlr_sql)
 export(sqlr_string_type)
 export(sqlr_table)
+export(sqlr_test_types)
 export(sqlr_text)
 export(sqlr_time)
 export(sqlr_time_type)
diff --git a/R/conformance.R b/R/conformance.R
new file mode 100644
index 0000000..ebbae4c
--- /dev/null
+++ b/R/conformance.R
@@ -0,0 +1,132 @@
+#' Dialect conformance tests
+#'
+#' A dialect package calls `sqlr_test_types()` from its test suite. It
+#' registers one test per entry of `sqlr_conformance_types()`. Each renders a
+#' table holding a single column of that type, creates it on a live database,
+#' reflects it and expects an empty [sqlr_diff()]. The list covers every type
+#' class sqlr models and each parameter it carries, and lives here rather than
+#' in the dialects, so a type added to sqlr is tested in every dialect without
+#' anyone adding it to a test.
+#'
+#' A dialect that cannot represent a type declares it in `skip`, with a reason,
+#' and the test for that type is skipped citing it.
+#'
+#' @param connect Function of no arguments returning a `DBIConnection`, or
+#'   skipping when no database is available. It is called once per type, and
+#'   the connection is closed afterwards. The dialect is resolved by
+#'   [sqlr_for()], and the table is created in the connection's default schema
+#'   and removed again.
+#' @param skip Named character vector: each name is an entry of
+#'   `sqlr_conformance_types()` the dialect cannot represent, each value the
+#'   reason.
+#'
+#' @return `sqlr_test_types()` returns `NULL` invisibly;
+#'   `sqlr_conformance_types()` returns a named list of [sqlr_type]s.
+#'
+#' @examples
+#' names(sqlr_conformance_types())
+#'
+#' # in tests/testthat/ of a dialect package, e.g.
+#' # sqlr_test_types(
+#' #   function() DBI::dbConnect(RSQLite::SQLite(), ":memory:"),
+#' #   skip = c(time_tz = "SQLite has no time zone-aware time type")
+#' # )
+#'
+#' @export
+sqlr_test_types <- function(connect, skip = character()) {
+  stopifnot(is.function(connect))
+
+  types <- sqlr_conformance_types()
+  check_skip(skip, names(types))
+
+  for (name in names(types)) {
+    testthat::test_that(paste0("sqlr type round trip: ", name), {
+      if (name %in% names(skip)) {
+        testthat::skip(paste0(name, ": ", skip[[name]]))
+      }
+
+      con <- connect()
+      on.exit(DBI::dbDisconnect(con))
+
+      differences <- round_trip_type(con, name, types[[name]])
+      testthat::expect(
+        length(differences) == 0L,
+        paste(c("Written vs reflected:", differences), collapse = "\n")
+      )
+    })
+  }
+
+  invisible(NULL)
+}
+
+#' @rdname sqlr_test_types
+#' @export
+sqlr_conformance_types <- function() {
+  list(
+    tinyint = sqlr_integer_type(bytes = 1L),
+    smallint = sqlr_smallint(),
+    integer = sqlr_int(),
+    bigint = sqlr_bigint(),
+    integer_unsigned = sqlr_int(unsigned = TRUE),
+    real = sqlr_real(),
+    double = sqlr_double(),
+    numeric = sqlr_numeric(),
+    numeric_precision = sqlr_numeric(10),
+    numeric_scale = sqlr_numeric(10, 2),
+    char = sqlr_char(3),
+    char_unsized = sqlr_char(),
+    varchar = sqlr_varchar(255),
+    text = sqlr_text(),
+    blob = sqlr_blob(),
+    varbinary = sqlr_blob(16),
+    binary = sqlr_binary_type(size = 16L, fixed = TRUE),
+    boolean = sqlr_boolean(),
+    date = sqlr_date(),
+    time = sqlr_time(),
+    time_precision = sqlr_time(precision = 3),
+    time_tz = sqlr_time(with_timezone = TRUE),
+    timestamp = sqlr_timestamp(),
+    timestamp_precision = sqlr_timestamp(precision = 3),
+    timestamp_tz = sqlr_timestamp(with_timezone = TRUE),
+    json = sqlr_json(),
+    json_binary = sqlr_json(binary = TRUE),
+    uuid = sqlr_uuid()
+  )
+}
+
+check_skip <- function(skip, types) {
+  named <- length(names(skip)) == length(skip) && all(nzchar(names(skip)))
+  reasoned <- is.character(skip) && !anyNA(skip) && all(nzchar(skip))
+  if (!named || !reasoned) {
+    stop("`skip` must name each type and give a reason for it", call. = FALSE)
+  }
+
+  unknown <- setdiff(names(skip), types)
+  if (length(unknown)) {
+    stop(
+      "`skip` names types sqlr_conformance_types() does not have: ",
+      paste0(unknown, collapse = ", "),
+      call. = FALSE
+    )
+  }
+}
+
+round_trip_type <- function(con, name, type) {
+  dialect <- sqlr_for(con)
+  table <- sqlr_table(
+    paste0("sqlr_", name, "_", Sys.getpid()),
+    sqlr_column("x", type)
+  )
+
+  on.exit(DBI::dbRemoveTable(con, table@name, fail_if_missing = FALSE))
+  for (stmt in sqlr_render(table, dialect)) {
+    DBI::dbExecute(con, stmt)
+  }
+
+  reflected <- schema_table(sqlr_reflect(con, dialect = dialect), table@name)
+  if (is.null(reflected)) {
+    return(paste0("table ", table@name, " was not reflected"))
+  }
+
+  sqlr_diff(table, reflected)
+}
diff --git a/README.Rmd b/README.Rmd
index 205f87d..84732b2 100644
--- a/README.Rmd
+++ b/README.Rmd
@@ -147,7 +147,10 @@ no dialect registered is an error rather than a guess.
 
 Writing one means implementing the protocol in `?sqlr_render` and
 `?sqlr_reflect`: how types spell in both directions, and whatever the engine
-does differently.
+does differently. Its test suite then calls `sqlr_test_types()`, which writes
+every type sqlr models to a live database and checks that each reads back
+unchanged. A type the engine cannot represent is declared with a reason and
+skipped.
 
 ## Scope
 
diff --git a/README.md b/README.md
index 731fd76..c16e196 100644
--- a/README.md
+++ b/README.md
@@ -160,7 +160,10 @@ than a guess.
 
 Writing one means implementing the protocol in `?sqlr_render` and
 `?sqlr_reflect`: how types spell in both directions, and whatever the
-engine does differently.
+engine does differently. Its test suite then calls `sqlr_test_types()`,
+which writes every type sqlr models to a live database and checks that
+each reads back unchanged. A type the engine cannot represent is
+declared with a reason and skipped.
 
 ## Scope
 
diff --git a/man/sqlr_test_types.Rd b/man/sqlr_test_types.Rd
new file mode 100644
index 0000000..da8c5c3
--- /dev/null
+++ b/man/sqlr_test_types.Rd
@@ -0,0 +1,49 @@
+% Generated by roxygen2: do not edit by hand
+% Please edit documentation in R/conformance.R
+\name{sqlr_test_types}
+\alias{sqlr_test_types}
+\alias{sqlr_conformance_types}
+\title{Dialect conformance tests}
+\usage{
+sqlr_test_types(connect, skip = character())
+
+sqlr_conformance_types()
+}
+\arguments{
+\item{connect}{Function of no arguments returning a `DBIConnection`, or
+skipping when no database is available. It is called once per type, and
+the connection is closed afterwards. The dialect is resolved by
+[sqlr_for()], and the table is created in the connection's default schema
+and removed again.}
+
+\item{skip}{Named character vector: each name is an entry of
+`sqlr_conformance_types()` the dialect cannot represent, each value the
+reason.}
+}
+\value{
+`sqlr_test_types()` returns `NULL` invisibly;
+  `sqlr_conformance_types()` returns a named list of [sqlr_type]s.
+}
+\description{
+A dialect package calls `sqlr_test_types()` from its test suite. It
+registers one test per entry of `sqlr_conformance_types()`. Each renders a
+table holding a single column of that type, creates it on a live database,
+reflects it and expects an empty [sqlr_diff()]. The list covers every type
+class sqlr models and each parameter it carries, and lives here rather than
+in the dialects, so a type added to sqlr is tested in every dialect without
+anyone adding it to a test.
+}
+\details{
+A dialect that cannot represent a type declares it in `skip`, with a reason,
+and the test for that type is skipped citing it.
+}
+\examples{
+names(sqlr_conformance_types())
+
+# in tests/testthat/ of a dialect package, e.g.
+# sqlr_test_types(
+#   function() DBI::dbConnect(RSQLite::SQLite(), ":memory:"),
+#   skip = c(time_tz = "SQLite has no time zone-aware time type")
+# )
+
+}
diff --git a/tests/testthat/test-conformance.R b/tests/testthat/test-conformance.R
new file mode 100644
index 0000000..779edf8
--- /dev/null
+++ b/tests/testthat/test-conformance.R
@@ -0,0 +1,59 @@
+extends_type <- function(cls) {
+  parent <- cls@parent
+  inherits(parent, "S7_class") &&
+    (identical(parent, sqlr_type) || extends_type(parent))
+}
+
+test_that("the conformance types cover every type class and parameter", {
+  ns <- asNamespace("sqlr")
+  classes <- Filter(
+    function(x) inherits(x, "S7_class") && extends_type(x),
+    mget(ls(ns), envir = ns)
+  )
+  classes$sqlr_other_type <- NULL
+
+  types <- sqlr_conformance_types()
+  uncovered <- character()
+
+  for (cls in classes) {
+    variants <- Filter(function(x) identical(S7_class(x), cls), types)
+    if (!length(variants)) {
+      uncovered <- c(uncovered, cls@name)
+      next
+    }
+
+    default <- cls()
+    for (p in setdiff(names(cls@properties), "raw")) {
+      varied <- vapply(
+        variants,
+        function(x) !identical(prop(x, p), prop(default, p)),
+        logical(1L)
+      )
+      if (!any(varied)) uncovered <- c(uncovered, paste0(cls@name, "@", p))
+    }
+  }
+
+  expect_equal(uncovered, character())
+})
+
+test_that("each conformance type has its own name", {
+  types <- sqlr_conformance_types()
+
+  expect_false(anyDuplicated(names(types)) > 0L)
+  expect_true(all(nzchar(names(types))))
+})
+
+test_that("a skipped type must be known and come with a reason", {
+  connect <- function() stop("not reached")
+
+  expect_error(
+    sqlr_test_types(connect, skip = c(no_such_type = "why")),
+    "no_such_type"
+  )
+  expect_error(sqlr_test_types(connect, skip = c(uuid = "")), "reason")
+  expect_error(sqlr_test_types(connect, skip = "why"), "reason")
+})
+
+test_that("a connection rather than a way to open one is rejected", {
+  expect_error(sqlr_test_types(list()), "is.function")
+})