diff --git a/sjsonnet/src/sjsonnet/Evaluator.scala b/sjsonnet/src/sjsonnet/Evaluator.scala
index f5631d69..8776d938 100644
--- a/sjsonnet/src/sjsonnet/Evaluator.scala
+++ b/sjsonnet/src/sjsonnet/Evaluator.scala
@@ -795,12 +795,19 @@ class Evaluator(
   }
 
   def visitObjExtend(e: ObjExtend)(implicit scope: ValScope): Val = {
-    val original = visitExpr(e.base).cast[Val.Obj]
-    e.ext match {
-      case ext: ObjBody.MemberList => visitMemberList(e.pos, ext, original)
-      case ext: ObjBody.ObjComp    => visitObjComp(ext, original)
-      case o: Val.Obj              => o.addSuper(e.pos, original)
-      case _                       => Error.fail("Should not have happened", e.pos)
+    val base = visitExpr(e.base)
+    base match {
+      case original: Val.Obj =>
+        e.ext match {
+          case ext: ObjBody.MemberList => visitMemberList(e.pos, ext, original)
+          case ext: ObjBody.ObjComp    => visitObjComp(ext, original)
+          case o: Val.Obj              => o.addSuper(e.pos, original)
+          case _                       => Error.fail("Should not have happened", e.pos)
+        }
+      case _ =>
+        // Desugar to + so stringification, RHS evaluation and errors share the same path.
+        // Reuse the evaluated base to avoid evaluating the original expression twice.
+        visitBinaryOp(BinaryOp(e.pos, base, BinaryOp.OP_+, e.ext))
     }
   }
 
diff --git a/sjsonnet/test/resources/new_test_suite/error.objextend_array_lhs.jsonnet b/sjsonnet/test/resources/new_test_suite/error.objextend_array_lhs.jsonnet
new file mode 100644
index 00000000..3bdcb6ef
--- /dev/null
+++ b/sjsonnet/test/resources/new_test_suite/error.objextend_array_lhs.jsonnet
@@ -0,0 +1 @@
+[1, 2] { x: 1 }
diff --git a/sjsonnet/test/resources/new_test_suite/error.objextend_array_lhs.jsonnet.golden b/sjsonnet/test/resources/new_test_suite/error.objextend_array_lhs.jsonnet.golden
new file mode 100644
index 00000000..90026184
--- /dev/null
+++ b/sjsonnet/test/resources/new_test_suite/error.objextend_array_lhs.jsonnet.golden
@@ -0,0 +1,2 @@
+sjsonnet.Error: Unknown binary operation: array + object
+    at [<root>].(error.objextend_array_lhs.jsonnet:1:8)
diff --git a/sjsonnet/test/resources/new_test_suite/error.objextend_number_lhs.jsonnet b/sjsonnet/test/resources/new_test_suite/error.objextend_number_lhs.jsonnet
new file mode 100644
index 00000000..b2b05a09
--- /dev/null
+++ b/sjsonnet/test/resources/new_test_suite/error.objextend_number_lhs.jsonnet
@@ -0,0 +1 @@
+42 { x: 1 }
diff --git a/sjsonnet/test/resources/new_test_suite/error.objextend_number_lhs.jsonnet.golden b/sjsonnet/test/resources/new_test_suite/error.objextend_number_lhs.jsonnet.golden
new file mode 100644
index 00000000..087dbc45
--- /dev/null
+++ b/sjsonnet/test/resources/new_test_suite/error.objextend_number_lhs.jsonnet.golden
@@ -0,0 +1,2 @@
+sjsonnet.Error: Unknown binary operation: number + object
+    at [<root>].(error.objextend_number_lhs.jsonnet:1:4)
diff --git a/sjsonnet/test/resources/new_test_suite/objextend_nonobject_lhs.jsonnet b/sjsonnet/test/resources/new_test_suite/objextend_nonobject_lhs.jsonnet
new file mode 100644
index 00000000..b2f88180
--- /dev/null
+++ b/sjsonnet/test/resources/new_test_suite/objextend_nonobject_lhs.jsonnet
@@ -0,0 +1,22 @@
+// Per the Jsonnet spec, lhs { body } is equivalent to lhs + { body }.
+local identity(x) = x;
+local base = { x: 1, y: self.x };
+
+std.assertEqual("hello" { x: 1 }, 'hello{"x": 1}') &&
+std.assertEqual(identity("hello") {}, "hello" + {}) &&
+std.assertEqual("" { local x = 2, x: x, y: self.x }, '{"x": 2, "y": 2}') &&
+std.assertEqual("" { x: 1, nested: { y: $.x } }, '{"nested": {"y": 1}, "x": 1}') &&
+std.assertEqual("" { hidden:: error "unused", x: 1 }, '{"x": 1}') &&
+std.assertEqual("" { assert self.x == 1, x: 1 }, '{"x": 1}') &&
+std.assertEqual("" { x+: 1, hasSuper: "x" in super }, '{"hasSuper": false, "x": 1}') &&
+std.assertEqual("" { [null]: error "unused" }, "" + {}) &&
+std.assertEqual("" { [k]: k for k in ["z", "a"] }, '{"a": "a", "z": "z"}') &&
+std.assertEqual("" { [k]: error "unused" for k in [] }, "" + {}) &&
+std.assertEqual("" { [k]: error "unused" for k in [null] }, "" + {}) &&
+std.assertEqual("hello" { x: 1 } { y: 2 }, 'hello{"x": 1}{"y": 2}') &&
+std.assertEqual(base { x: 2, z: super.x }, { x: 2, y: 2, z: 1 }) &&
+std.assertEqual(
+  std.foldl(function(acc, x) acc { x: x }, [1, 2], "hello"),
+  'hello{"x": 1}{"x": 2}'
+) &&
+true
diff --git a/sjsonnet/test/resources/new_test_suite/objextend_nonobject_lhs.jsonnet.golden b/sjsonnet/test/resources/new_test_suite/objextend_nonobject_lhs.jsonnet.golden
new file mode 100644
index 00000000..27ba77dd
--- /dev/null
+++ b/sjsonnet/test/resources/new_test_suite/objextend_nonobject_lhs.jsonnet.golden
@@ -0,0 +1 @@
+true
diff --git a/sjsonnet/test/src/sjsonnet/EvaluatorTests.scala b/sjsonnet/test/src/sjsonnet/EvaluatorTests.scala
index 5ba83efc..ff60c780 100644
--- a/sjsonnet/test/src/sjsonnet/EvaluatorTests.scala
+++ b/sjsonnet/test/src/sjsonnet/EvaluatorTests.scala
@@ -42,6 +42,130 @@ object EvaluatorTests extends TestSuite {
       eval("std.objectKeysValues({a: error 'unused'})[0].key") ==> ujson.Str("a")
       assert(evalErr("std.objectKeysValues({a: error 'boom'})[0].value").contains("boom"))
     }
+    test("objectExtension") {
+      test("matchesAddition") {
+        val bodies = Seq(
+          "{}",
+          "{ x: 1 }",
+          "{ z: 0, a: [true, null, { nested: 'value' }] }",
+          "{ local value = 2, x: value, y: self.x + 1 }",
+          "{ x: 1, nested: { y: $.x } }",
+          "{ hidden:: error 'unused', visible: 1 }",
+          "{ f(x):: x + 1, value: self.f(1) }",
+          "{ assert self.x == 1, x: 1 }",
+          "{ x+: 1, hasSuper: 'x' in super }",
+          "{ [null]: error 'unused', ['x' + 'y']: 1 }",
+          "{ [key]: key for key in ['z', 'a'] }",
+          "{ [key]: error 'unused' for key in [] }",
+          "{ [key]: error 'unused' for key in [null] }"
+        )
+        for {
+          body <- bodies
+          base <- Seq("'hello'", "(function(x) x)('prefix')", "('🙂' + '\\n\\\"')")
+          preserveOrder <- Seq(false, true)
+        } {
+          eval(s"($base) $body", preserveOrder = preserveOrder) ==>
+          eval(s"($base) + $body", preserveOrder = preserveOrder)
+        }
+      }
+      test("chainedExtensions") {
+        eval("'prefix' { x: 1 } { y: 2 }") ==>
+        eval("'prefix' + { x: 1 } + { y: 2 }")
+        eval("(function(x) x)('prefix') { x: 1 }", strict = true) ==>
+        ujson.Str("prefix{\"x\": 1}")
+        // Strict mode deliberately forbids consecutive object bodies as a syntax restriction.
+        assert(
+          evalErr("'prefix' {} {}", strict = true).contains("Adjacent object literals not allowed")
+        )
+      }
+      test("invalidBase") {
+        for {
+          base <- Seq("42", "[error 'unused']", "true", "false", "null", "function(x) x")
+          body <- Seq("{}", "{ x: 1 }", "{ local x = 1, x: x }", "{ [k]: 1 for k in ['x'] }")
+        } {
+          evalErr(s"($base) $body").takeWhile(_ != '\n') ==>
+          evalErr(s"($base) + $body").takeWhile(_ != '\n')
+        }
+      }
+      test("errorOrder") {
+        for (op <- Seq("", "+")) {
+          assert(
+            evalErr(s"(error 'base first') $op { [error 'key second']: 1 }")
+              .startsWith("sjsonnet.Error: base first")
+          )
+          for (base <- Seq("'prefix'", "42")) {
+            assert(
+              evalErr(s"$base $op { [error 'key first']: error 'value later' }")
+                .startsWith("sjsonnet.Error: key first")
+            )
+            assert(
+              evalErr(s"$base $op { [k]: 1 for k in error 'source first' }")
+                .startsWith("sjsonnet.Error: source first")
+            )
+            assert(
+              evalErr(s"$base $op { [k]: 1 for k in ['x', 'x'] }")
+                .contains("Duplicate key x")
+            )
+          }
+          assert(
+            evalErr(s"42 $op { assert false: 'unused assertion', x: error 'unused value' }")
+              .startsWith("sjsonnet.Error: Unknown binary operation: number + object")
+          )
+        }
+      }
+      test("materializationErrors") {
+        for (body <- Seq(
+               "{ assert false: 'assertion' }",
+               "{ x: error 'visible field' }",
+               "{ x: super.x }",
+               "{ f(x): x }"
+             )) {
+          evalErr(s"'prefix' $body").takeWhile(_ != '\n') ==>
+          evalErr(s"'prefix' + $body").takeWhile(_ != '\n')
+        }
+      }
+      test("evaluatedOnce") {
+        val body = "{ [std.trace('key', 'x')]: std.trace('value', 1) }"
+        val (value, traces) = evalWithTraces(s"std.trace('base', 'prefix') $body")
+        value ==> ujson.Str("prefix{\"x\": 1}")
+        traces.size ==> 3
+        assert(traces(0).endsWith("base"))
+        assert(traces(1).endsWith("key"))
+        assert(traces(2).endsWith("value"))
+      }
+      test("objectBase") {
+        for (op <- Seq("", "+")) {
+          eval(s"local base = { x: 1, y: self.x }; (base $op { x: 2, z: super.x })") ==>
+          ujson.Obj("x" -> 2, "y" -> 2, "z" -> 1)
+          eval(s"local base = { x: [1] }; base $op { x+: [2] }") ==>
+          ujson.Obj("x" -> ujson.Arr(1, 2))
+          eval(s"local base = { x: 1 }; base $op { [k]: super.x + 1 for k in ['y'] }") ==>
+          ujson.Obj("x" -> 1, "y" -> 2)
+          eval(s"local base = { assert self.x == 2, x: 1 }; base $op { x: 2 }") ==>
+          ujson.Obj("x" -> 2)
+          eval(s"local base = { x: error 'unused' }; (base $op { y: 2 }).y") ==>
+          ujson.Num(2)
+        }
+      }
+      test("foldlFallback") {
+        for (op <- Seq("", "+")) {
+          eval(s"std.foldl(function(acc, x) acc $op { x: x }, [1, 2], 'prefix')") ==>
+          ujson.Str("prefix{\"x\": 1}{\"x\": 2}")
+          eval(s"std.foldl(function(acc, x) acc $op { x: x }, [], 'prefix')") ==>
+          ujson.Str("prefix")
+        }
+      }
+      test("sharedBodyWithDifferentBaseTypes") {
+        def program(op: String): String =
+          s"""local extend(base) = base $op { z: 0, a: 1, hidden:: error 'unused' };
+             |[extend('prefix'), extend({ inherited: 2 }), extend('prefix')]
+             |""".stripMargin
+        for (preserveOrder <- Seq(false, true)) {
+          eval(program(""), preserveOrder = preserveOrder) ==>
+          eval(program("+"), preserveOrder = preserveOrder)
+        }
+      }
+    }
     test("arrays") {
       eval("[1, [2, 3], 4][1][0]") ==> ujson.Num(2)
       eval("([1, 2, 3] + [4, 5, 6])[3]") ==> ujson.Num(4)