diff --git a/bin/httui-lsp/httui_lsp.ml b/bin/httui-lsp/httui_lsp.ml
index c3c2172..039bbbd 100644
--- a/bin/httui-lsp/httui_lsp.ml
+++ b/bin/httui-lsp/httui_lsp.ml
@@ -84,7 +84,10 @@ let values_for_uri uri =
 let publish_diagnostics uri text =
   let blocks = Httui_lang.Fence_scanner.scan text in
   let diagnostics =
-    Httui_lang.Analyze.diagnostics ~shapes:(shapes_for_uri uri) blocks
+    Httui_lang.Analyze.diagnostics ~shapes:(shapes_for_uri uri)
+      ~sql_tables_for:(fun connection_id ->
+        Sql_schema_store.tables_for ~connection_id)
+      ~doc:text blocks
     |> List.map (fun (d : Httui_lang.Analyze.diagnostic) ->
         T.Diagnostic.create
           ~range:(range_of_offsets text ~start:d.start_ ~stop:d.stop_)
diff --git a/lib/analyze.ml b/lib/analyze.ml
index b439437..4b46502 100644
--- a/lib/analyze.ml
+++ b/lib/analyze.ml
@@ -139,45 +139,148 @@ let field_diagnostic ~resolve (r : Refs.occurrence) segs =
           severity = Warning;
         }
 
-let diagnostics ?(shapes = []) blocks =
+(* --- cross-language ref-vs-column type check (db-* blocks) --------------- *)
+
+let comparison_ops = [ "="; "<>"; "!="; "<="; ">="; "<"; ">" ]
+
+(* Text strictly between byte offsets [a] and [b] (doc-absolute), trimmed. *)
+let between text a b =
+  if b <= a || a < 0 || b > String.length text then ""
+  else String.trim (String.sub text a (b - a))
+
+(* The data type of [col] across [tables]: the unique type when the column
+   name appears with a single category, else [None] (ambiguous columns are
+   skipped — leniency). *)
+let column_category tables col =
+  let col = String.lowercase_ascii col in
+  let cats =
+    List.concat_map
+      (fun (t : Sql.table) ->
+        List.filter_map
+          (fun (c : Sql.column) ->
+            if String.lowercase_ascii c.name = col then
+              Option.map Sql_types.of_column c.data_type
+            else None)
+          t.columns)
+      tables
+    |> List.sort_uniq compare
+  in
+  match cats with [ c ] -> Some c | _ -> None
+
+(* For db-* block [b], pair each ref compared to a column (`col OP {{ref}}`
+   or `{{ref}} OP col`) and warn when their type categories are
+   incompatible. [resolve_type] turns a ref occurrence into its inferred
+   category (via the shape cache); [tables] is the connection's schema. *)
+let sql_ref_diagnostics ~tables ~resolve_type doc (b : Block.t) =
+  let fields =
+    Sql.walk ~base:b.content_offset b.content
+    |> List.filter_map (fun (n : Sql.node) ->
+        if n.kind = "field" then
+          Some (String.sub doc n.start_ (n.stop_ - n.start_), n.start_, n.stop_)
+        else None)
+  in
+  Refs.of_block b
+  |> List.filter_map (fun (r : Refs.occurrence) ->
+      (* a column whose comparison operator sits in the gap to the ref,
+         on either side *)
+      let paired =
+        List.find_map
+          (fun (name, fs, fe) ->
+            if
+              fe <= r.ref_start
+              && List.mem (between doc fe r.ref_start) comparison_ops
+            then Some name
+            else if
+              r.ref_stop <= fs
+              && List.mem (between doc r.ref_stop fs) comparison_ops
+            then Some name
+            else None)
+          fields
+      in
+      match paired with
+      | None -> None
+      | Some col -> (
+          match (column_category tables col, resolve_type r) with
+          | Some cat, Some ref_cat when not (Sql_types.compatible cat ref_cat)
+            ->
+              Some
+                {
+                  start_ = r.name_start;
+                  stop_ = r.ref_stop;
+                  message =
+                    Printf.sprintf
+                      "Type mismatch: column '%s' is %s but {{%s}} is %s" col
+                      (Sql_types.to_string cat) r.name
+                      (Sql_types.to_string ref_cat);
+                  severity = Warning;
+                }
+          | _ -> None))
+
+let diagnostics ?(shapes = []) ?(sql_tables_for = fun _ -> []) ?(doc = "")
+    blocks =
   List.concat
     (List.mapi
        (fun i (b : Block.t) ->
          if not (Block.is_executable b) then []
          else
            let scope = aliases_above blocks ~index:i in
-           Refs.of_block b
-           |> List.filter_map (fun (r : Refs.occurrence) ->
-               let typo_check () =
-                 Option.bind
-                   (typed_ctx ~shapes ~values:[] ~blocks ~index:i ~scope r.name)
-                   (fun ctx ->
-                     field_diagnostic ~resolve:ctx.resolve r (segments_of b r))
-               in
-               if r.name = prev_name then
-                 if prev_decl blocks ~index:i = None then
+           let ref_diags =
+             Refs.of_block b
+             |> List.filter_map (fun (r : Refs.occurrence) ->
+                 let typo_check () =
+                   Option.bind
+                     (typed_ctx ~shapes ~values:[] ~blocks ~index:i ~scope
+                        r.name) (fun ctx ->
+                       field_diagnostic ~resolve:ctx.resolve r (segments_of b r))
+                 in
+                 if r.name = prev_name then
+                   if prev_decl blocks ~index:i = None then
+                     Some
+                       {
+                         start_ = r.name_start;
+                         stop_ = r.name_stop;
+                         message =
+                           "No previous block to reference with {{$prev}}";
+                         severity = Error;
+                       }
+                   else typo_check ()
+                 else if not r.has_path then None
+                 else if not (List.mem_assoc r.name scope) then
                    Some
                      {
                        start_ = r.name_start;
                        stop_ = r.name_stop;
-                       message = "No previous block to reference with {{$prev}}";
+                       message =
+                         Printf.sprintf
+                           "Unknown block alias '%s' — no block above declares \
+                            alias=%s"
+                           r.name r.name;
                        severity = Error;
                      }
-                 else typo_check ()
-               else if not r.has_path then None
-               else if not (List.mem_assoc r.name scope) then
-                 Some
-                   {
-                     start_ = r.name_start;
-                     stop_ = r.name_stop;
-                     message =
-                       Printf.sprintf
-                         "Unknown block alias '%s' — no block above declares \
-                          alias=%s"
-                         r.name r.name;
-                     severity = Error;
-                   }
-               else typo_check ()))
+                 else typo_check ())
+           in
+           let sql_diags =
+             if not (is_db_lang b.lang) then []
+             else
+               let connection_id =
+                 if String.length b.lang > 3 then
+                   String.sub b.lang 3 (String.length b.lang - 3)
+                 else ""
+               in
+               let resolve_type (r : Refs.occurrence) =
+                 Option.bind
+                   (typed_ctx ~shapes ~values:[] ~blocks ~index:i ~scope r.name)
+                   (fun ctx ->
+                     match ctx.resolve (List.map fst (segments_of b r)) with
+                     | Found shape ->
+                         Some (Sql_types.of_shape_type (Shape.type_name shape))
+                     | _ -> None)
+               in
+               sql_ref_diagnostics
+                 ~tables:(sql_tables_for connection_id)
+                 ~resolve_type doc b
+           in
+           ref_diags @ sql_diags)
        blocks)
 
 let block_index_at blocks ~offset =
diff --git a/lib/sql_types.ml b/lib/sql_types.ml
new file mode 100644
index 0000000..1af24f2
--- /dev/null
+++ b/lib/sql_types.ml
@@ -0,0 +1,63 @@
+(* A small type lattice that normalises the many native SQL column types
+   (Postgres / MySQL / SQLite) and the inferred ref shape types down to a
+   handful of comparable categories. Used by the cross-language check:
+   `WHERE col = {{ref}}` warns when the column category and the ref category
+   are genuinely incompatible. Leniency is deliberate — a false squiggle
+   erodes trust faster than a missed one, so anything [Unknown] passes. *)
+
+type t = Number | String | Boolean | Json | Datetime | Unknown
+
+let to_string = function
+  | Number -> "number"
+  | String -> "string"
+  | Boolean -> "boolean"
+  | Json -> "json"
+  | Datetime -> "datetime"
+  | Unknown -> "unknown"
+
+let contains ~needle s =
+  let nl = String.length needle and sl = String.length s in
+  let rec go i =
+    if i + nl > sl then false
+    else if String.sub s i nl = needle then true
+    else go (i + 1)
+  in
+  nl = 0 || go 0
+
+(* Column data-type string (from information_schema / PRAGMA) -> category.
+   Substring matching mirrors SQLite's affinity algorithm and covers the
+   common PG/MySQL spellings (int4, bigint, varchar, timestamptz, jsonb…). *)
+let of_column raw =
+  let s = String.lowercase_ascii (String.trim raw) in
+  let has n = contains ~needle:n s in
+  if has "bool" then Boolean
+  else if has "json" then Json
+  else if
+    has "int" || has "serial" || has "numeric" || has "decimal" || has "real"
+    || has "double" || has "float" || has "money"
+  then Number
+  else if has "timestamp" || has "date" || has "time" then Datetime
+  else if
+    has "char" || has "text" || has "clob" || has "uuid" || has "string"
+    || has "enum" || has "name"
+  then String
+  else Unknown
+
+(* Inferred ref shape type name (Shape.type_name: "number" | "string" |
+   "boolean" | "null" | "object" | "array<…>") -> category. *)
+let of_shape_type name =
+  match String.lowercase_ascii name with
+  | "number" -> Number
+  | "string" -> String
+  | "boolean" -> Boolean
+  | "object" -> Json
+  | s when String.length s >= 5 && String.sub s 0 5 = "array" -> Json
+  | _ -> Unknown
+
+(* Two categories are compatible unless both are known and different.
+   `Datetime` and `String` are treated as compatible (dates flow as ISO
+   strings on the value path). *)
+let compatible a b =
+  a = b || a = Unknown || b = Unknown
+  || (a = Datetime && b = String)
+  || (a = String && b = Datetime)
diff --git a/test/test_httui_lang.ml b/test/test_httui_lang.ml
index bb50343..c6b975d 100644
--- a/test/test_httui_lang.ml
+++ b/test/test_httui_lang.ml
@@ -769,4 +769,45 @@ let () =
        (fun (c : Httui_lang.Sql.completion) -> c.is_table)
        (complete "SELECT * FROM "));
 
+  (* --- cross-language ref-vs-column type check --- *)
+  let tc_shape =
+    Httui_lang.Shape.(Object_ [ ("body", Object_ [ ("id", Scalar "number") ]) ])
+  in
+  let tc_tables col_type =
+    [
+      Httui_lang.Sql.
+        {
+          schema = None;
+          name = "t";
+          columns = [ { name = "email"; data_type = Some col_type } ];
+        };
+    ]
+  in
+  let tc_diags col_type =
+    let doc =
+      "```http alias=req1\n\
+       GET /u\n\
+       ```\n\n\
+       ```db-pg\n\
+       SELECT * FROM t WHERE email = {{req1.response.body.id}}\n\
+       ```\n"
+    in
+    Httui_lang.Analyze.diagnostics
+      ~shapes:[ ("req1", tc_shape) ]
+      ~sql_tables_for:(fun _ -> tc_tables col_type)
+      ~doc
+      (Httui_lang.Fence_scanner.scan doc)
+  in
+  check "type mismatch (text column vs number ref) is flagged"
+    (List.exists
+       (fun (d : Httui_lang.Analyze.diagnostic) ->
+         has_sub d.message "Type mismatch" && has_sub d.message "email")
+       (tc_diags "text"));
+  check "compatible types (numeric column vs number ref) are not flagged"
+    (not
+       (List.exists
+          (fun (d : Httui_lang.Analyze.diagnostic) ->
+            has_sub d.message "Type mismatch")
+          (tc_diags "integer")));
+
   if !failures > 0 then exit 1