diff --git a/v1/ast/check.go b/v1/ast/check.go
index 6e4d8ddd74..0aa7dfab1b 100644
--- a/v1/ast/check.go
+++ b/v1/ast/check.go
@@ -419,7 +419,18 @@ func (tc *typeChecker) checkExprBuiltin(env *TypeEnv, expr *Expr) *Error {
 		return newArgError(expr.Location, name, "too few arguments", getArgTypes(env, args), namedFargs)
 	}
 
+	pre := getArgTypes(env, args)
+
 	for i := range args {
+		// Check that pre-existing argument types are compatible with the expected types.
+		// Catching that case here avoids false negatives for builtins like sum([1, a]) where a is known to be a string.
+		if pre[i] != nil && !types.Nil(pre[i]) && !unifies(pre[i], fargs.Arg(i)) {
+			return newArgError(expr.Location, name, "invalid argument(s)", pre, namedFargs)
+		}
+
+		// unify1 infers types for untyped variables and checks resolved parts (constants, refs) inside partially-typed composites.
+		// The unifies pre-check above is skipped when the argument contains untyped variables,
+		// so unify1 is still needed to catch those errors.
 		if !unify1(env, args[i], fargs.Arg(i), false) {
 			post := make([]types.Type, len(args))
 			for i := range args {
@@ -549,6 +560,9 @@ func unify2Object(env *TypeEnv, a *Term, b *Term) bool {
 	return false
 }
 
+// unify1 walks into a term's structure (arrays, objects, sets, vars), checks
+// compatibility against the expected type, and infers types for variables by
+// assigning them in env. It uses unifies internally for leaf checks.
 func unify1(env *TypeEnv, term *Term, tpe types.Type, union bool) bool {
 	switch v := term.Value.(type) {
 	case *Array:
@@ -839,6 +853,7 @@ func (rc *refChecker) checkRefLeaf(tpe types.Type, ref Ref, idx int) *Error {
 	return rc.checkRefLeaf(types.Values(tpe), ref, idx+1)
 }
 
+// unifies checks whether two types are compatible with each other.
 func unifies(a, b types.Type) bool {
 
 	if a == nil || b == nil {
@@ -962,7 +977,14 @@ func unifiesObjects(a, b *types.Object) bool {
 
 func unifiesObjectsStatic(a, b *types.Object) bool {
 	for _, k := range a.Keys() {
-		if !unifies(a.Select(k), b.Select(k)) {
+		tpeB := b.Select(k)
+		if tpeB == nil {
+			if a.DynamicValue() != nil {
+				continue
+			}
+			return false
+		}
+		if !unifies(a.Select(k), tpeB) {
 			return false
 		}
 	}
@@ -1174,7 +1196,7 @@ func removeDuplicate(list []Value) []Value {
 func getArgTypes(env *TypeEnv, args []*Term) []types.Type {
 	pre := make([]types.Type, len(args))
 	for i := range args {
-		pre[i] = env.Get(args[i])
+		pre[i] = env.GetByValue(args[i].Value)
 	}
 	return pre
 }
diff --git a/v1/ast/check_test.go b/v1/ast/check_test.go
index a63fe7712f..deaaf60ba3 100644
--- a/v1/ast/check_test.go
+++ b/v1/ast/check_test.go
@@ -2643,6 +2643,90 @@ allow if {
 p if { [data.base.foo] }`,
 			expectedError: "policy.rego:3: rego_type_error: function data.base.foo used as reference, not called",
 		},
+		{
+			name: "wrong type used directly",
+			policy: `package p
+
+s := sum([1, "foo"])`,
+			expectedError: "sum: invalid argument(s)",
+		},
+		{
+			name: "wrong type as reference",
+			policy: `package p
+
+a := "foo"
+s := sum([1, a])`,
+			expectedError: "sum: invalid argument(s)",
+		},
+		{
+			name: "wrong type as reference within rule",
+			policy: `package p
+
+allow := s if {
+    a := "foo"
+    s := sum([1, a])
+}`,
+			expectedError: "sum: invalid argument(s)",
+		},
+		{
+			name: "compare partial object",
+			policy: `package p
+
+obj["a"] := input.a
+
+obj["b"] := input.b
+
+test_obj2 if {
+	{"a":"1"} == obj with input as {"a": "1"}
+}`,
+		},
+		// this policy verifies this issue: https://github.com/open-policy-agent/opa/issues/6751
+		{
+			name: "compare two partial objects",
+			policy: `package p
+
+obj1["a"] := input.a
+obj1["b"] := input.b
+
+obj2["x"] := input.x
+obj2["y"] := input.y
+
+test if {
+	obj1 == obj2
+}`,
+		},
+		// this policy verifies this issue: https://github.com/open-policy-agent/opa/issues/5594
+		{
+			name: "compare two partial objects 2",
+			policy: `package test
+
+obj["a"] := true
+
+obj["b"] := "foo" if {
+    input.foo == "bar"
+}
+
+test_obj if {
+    obj == {
+        "a": true
+    } with input as {"foo": "baz"}
+}`,
+		},
+		{
+			name: "sum with valid number args",
+			policy: `package p
+
+a := 1
+s := sum([1, a])`,
+		},
+		{
+			name: "sum with untyped var from input",
+			policy: `package p
+
+allow := s if {
+    s := sum([1, input.a])
+}`,
+		},
 	}
 
 	for _, tc := range tests {
@@ -2665,6 +2749,14 @@ p if { [data.base.foo] }`,
 			base := "package base\n" + body
 			compiler.Compile(map[string]*Module{"base": MustParseModuleWithOpts(base, pOpts), "policy.rego": module})
 
+			if tc.expectedError == "" {
+				if compiler.Failed() {
+					t.Fatalf("expected no error, but got %v", compiler.Errors.Error())
+				} else {
+					return
+				}
+			}
+
 			if !compiler.Failed() {
 				t.Fatal("expected error, got none")
 			}