diff --git a/CHANGELOG_PENDING.md b/CHANGELOG_PENDING.md
index bf5a76e2..3c65af26 100644
--- a/CHANGELOG_PENDING.md
+++ b/CHANGELOG_PENDING.md
@@ -17,3 +17,6 @@
 ### Bug Fixes
 
 ### Breaking changes
+
+- It is now a syntax error to call a builtin function incorrectly.
+  [449](https://github.com/pulumi/esc/pull/449)
\ No newline at end of file
diff --git a/ast/environment.go b/ast/environment.go
index 03e7b4d1..9ee598ca 100644
--- a/ast/environment.go
+++ b/ast/environment.go
@@ -128,6 +128,9 @@ func (d *MapDecl[T]) parse(name string, node syntax.Node) syntax.Diagnostics {
 
 		var v T
 		vname := name + "." + kvp.Key.Value()
+		if strings.HasPrefix(kvp.Key.Value(), "fn::") {
+			diags.Extend(syntax.NodeError(kvp.Key, fmt.Sprintf("builtin function call %q not allowed at the top level", kvp.Key.Value())))
+		}
 		vdiags := parseNode(vname, &v, kvp.Value)
 		diags.Extend(vdiags...)
 
diff --git a/ast/expr.go b/ast/expr.go
index 4dbe23c1..7eacf4c2 100644
--- a/ast/expr.go
+++ b/ast/expr.go
@@ -625,14 +625,20 @@ func FromBase64(value Expr) *FromBase64Expr {
 }
 
 func tryParseFunction(node *syntax.ObjectNode) (Expr, syntax.Diagnostics, bool) {
+	var diags syntax.Diagnostics
 	if node.Len() != 1 {
-		return nil, nil, false
+		for i := 0; i < node.Len(); i++ {
+			if k := node.Index(i).Key.Value(); strings.HasPrefix(k, "fn::") {
+				diags = append(diags, syntax.NodeError(node, fmt.Sprintf("illegal call to builtin function: %q must be the only key within its containing object", k)))
+
+			}
+		}
+		return nil, diags, false
 	}
 
 	kvp := node.Index(0)
 
 	var parse func(node *syntax.ObjectNode, name *StringExpr, args Expr) (Expr, syntax.Diagnostics)
-	var diags syntax.Diagnostics
 	switch kvp.Key.Value() {
 	case "fn::fromJSON":
 		parse = parseFromJSON
diff --git a/ast/testdata/parse/invalid-invocation/env.yaml b/ast/testdata/parse/invalid-invocation/env.yaml
new file mode 100644
index 00000000..a57d41da
--- /dev/null
+++ b/ast/testdata/parse/invalid-invocation/env.yaml
@@ -0,0 +1,6 @@
+values:
+  multiple-keys:
+    fn::rotate::provider:
+      inputs: {}
+    state:
+      ohno: true
diff --git a/ast/testdata/parse/invalid-invocation/expected.json b/ast/testdata/parse/invalid-invocation/expected.json
new file mode 100644
index 00000000..a46a3f07
--- /dev/null
+++ b/ast/testdata/parse/invalid-invocation/expected.json
@@ -0,0 +1,78 @@
+{
+    "decl": {
+        "Description": null,
+        "Imports": null,
+        "Values": {
+            "Entries": [
+                {
+                    "Key": {
+                        "Value": "multiple-keys"
+                    },
+                    "Value": {
+                        "Entries": [
+                            {
+                                "Key": {
+                                    "Value": "fn::rotate::provider"
+                                },
+                                "Value": {
+                                    "Entries": [
+                                        {
+                                            "Key": {
+                                                "Value": "inputs"
+                                            },
+                                            "Value": {
+                                                "Entries": []
+                                            }
+                                        }
+                                    ]
+                                }
+                            },
+                            {
+                                "Key": {
+                                    "Value": "state"
+                                },
+                                "Value": {
+                                    "Entries": [
+                                        {
+                                            "Key": {
+                                                "Value": "ohno"
+                                            },
+                                            "Value": {
+                                                "Value": true
+                                            }
+                                        }
+                                    ]
+                                }
+                            }
+                        ]
+                    }
+                }
+            ]
+        }
+    },
+    "diags": [
+        {
+            "Severity": 1,
+            "Summary": "illegal call to builtin function: \"fn::rotate::provider\" must be the only key within its containing object",
+            "Detail": "",
+            "Subject": {
+                "Filename": "invalid-invocation",
+                "Start": {
+                    "Line": 3,
+                    "Column": 5,
+                    "Byte": 29
+                },
+                "End": {
+                    "Line": 6,
+                    "Column": 17,
+                    "Byte": 95
+                }
+            },
+            "Context": null,
+            "Expression": null,
+            "EvalContext": null,
+            "Extra": null,
+            "Path": "values[\"multiple-keys\"]"
+        }
+    ]
+}
diff --git a/ast/testdata/parse/invalid-invocation2/env.yaml b/ast/testdata/parse/invalid-invocation2/env.yaml
new file mode 100644
index 00000000..dcaeaf33
--- /dev/null
+++ b/ast/testdata/parse/invalid-invocation2/env.yaml
@@ -0,0 +1,2 @@
+values:
+  fn::secret: "hello"
diff --git a/ast/testdata/parse/invalid-invocation2/expected.json b/ast/testdata/parse/invalid-invocation2/expected.json
new file mode 100644
index 00000000..fe24a84c
--- /dev/null
+++ b/ast/testdata/parse/invalid-invocation2/expected.json
@@ -0,0 +1,43 @@
+{
+    "decl": {
+        "Description": null,
+        "Imports": null,
+        "Values": {
+            "Entries": [
+                {
+                    "Key": {
+                        "Value": "fn::secret"
+                    },
+                    "Value": {
+                        "Value": "hello"
+                    }
+                }
+            ]
+        }
+    },
+    "diags": [
+        {
+            "Severity": 1,
+            "Summary": "builtin function call \"fn::secret\" not allowed at the top level",
+            "Detail": "",
+            "Subject": {
+                "Filename": "invalid-invocation2",
+                "Start": {
+                    "Line": 2,
+                    "Column": 3,
+                    "Byte": 10
+                },
+                "End": {
+                    "Line": 2,
+                    "Column": 13,
+                    "Byte": 20
+                }
+            },
+            "Context": null,
+            "Expression": null,
+            "EvalContext": null,
+            "Extra": null,
+            "Path": "values[\"fn::secret\"]"
+        }
+    ]
+}