diff --git a/internal/cloudinit/cloudinit.go b/internal/cloudinit/cloudinit.go
index 5fedbf8..b574fc1 100644
--- a/internal/cloudinit/cloudinit.go
+++ b/internal/cloudinit/cloudinit.go
@@ -4,11 +4,15 @@
 package cloudinit
 
 import (
+	"errors"
+	"fmt"
 	"maps"
 
 	"gopkg.in/yaml.v3"
 )
 
+var ErrReservedKey = errors.New("reserved key in Extra map")
+
 // WriteFile represents a file to be written by cloud-init.
 type WriteFile struct {
 	Path        string `yaml:"path"`
@@ -68,6 +72,18 @@ func BuildCloudConfig(opts CloudConfigOpts) (string, error) {
 		doc["users"] = []map[string]interface{}{user}
 	}
 
+	// Validate that Extra keys don't collide with reserved keys
+	reservedKeys := []string{"packages", "write_files", "runcmd", "users", "ssh_pwauth"}
+	for _, key := range reservedKeys {
+		if _, exists := opts.Extra[key]; exists {
+			return "", fmt.Errorf(
+				"extra[%q] collides with reserved cloud-config key: %w",
+				key,
+				ErrReservedKey,
+			)
+		}
+	}
+
 	// Merge extra keys at top level
 	maps.Copy(doc, opts.Extra)
 
diff --git a/internal/cloudinit/cloudinit_test.go b/internal/cloudinit/cloudinit_test.go
index 1ef6563..e79f3fc 100644
--- a/internal/cloudinit/cloudinit_test.go
+++ b/internal/cloudinit/cloudinit_test.go
@@ -333,4 +333,89 @@ var _ = Describe("BuildCloudConfig", func() {
 			Expect(user["name"]).To(Equal("testuser"))
 		})
 	})
+
+	Context("Extra key collision validation", func() {
+		It("should reject Extra with 'packages' key", func() {
+			opts := cloudinit.CloudConfigOpts{
+				Packages: []string{"vim"},
+				Extra: map[string]interface{}{
+					"packages": []string{"curl"},
+				},
+			}
+			_, err := cloudinit.BuildCloudConfig(opts)
+			Expect(err).To(HaveOccurred())
+			Expect(err).To(MatchError(ContainSubstring("packages")))
+			Expect(err).To(MatchError(ContainSubstring("reserved")))
+		})
+
+		It("should reject Extra with 'write_files' key", func() {
+			opts := cloudinit.CloudConfigOpts{
+				Extra: map[string]interface{}{
+					"write_files": []interface{}{},
+				},
+			}
+			_, err := cloudinit.BuildCloudConfig(opts)
+			Expect(err).To(HaveOccurred())
+			Expect(err).To(MatchError(ContainSubstring("write_files")))
+		})
+
+		It("should reject Extra with 'runcmd' key", func() {
+			opts := cloudinit.CloudConfigOpts{
+				Extra: map[string]interface{}{
+					"runcmd": []interface{}{},
+				},
+			}
+			_, err := cloudinit.BuildCloudConfig(opts)
+			Expect(err).To(HaveOccurred())
+			Expect(err).To(MatchError(ContainSubstring("runcmd")))
+		})
+
+		It("should reject Extra with 'users' key", func() {
+			opts := cloudinit.CloudConfigOpts{
+				Extra: map[string]interface{}{
+					"users": []interface{}{},
+				},
+			}
+			_, err := cloudinit.BuildCloudConfig(opts)
+			Expect(err).To(HaveOccurred())
+			Expect(err).To(MatchError(ContainSubstring("users")))
+		})
+
+		It("should reject Extra with 'ssh_pwauth' key", func() {
+			opts := cloudinit.CloudConfigOpts{
+				Extra: map[string]interface{}{
+					"ssh_pwauth": true,
+				},
+			}
+			_, err := cloudinit.BuildCloudConfig(opts)
+			Expect(err).To(HaveOccurred())
+			Expect(err).To(MatchError(ContainSubstring("ssh_pwauth")))
+		})
+
+		It("should allow Extra with non-reserved keys", func() {
+			opts := cloudinit.CloudConfigOpts{
+				Extra: map[string]interface{}{
+					"timezone": "UTC",
+					"hostname": "test-vm",
+					"locale":   "en_US.UTF-8",
+				},
+			}
+			result, err := cloudinit.BuildCloudConfig(opts)
+			Expect(err).NotTo(HaveOccurred())
+
+			var parsed map[string]interface{}
+			Expect(yaml.Unmarshal([]byte(result), &parsed)).To(Succeed())
+			Expect(parsed["timezone"]).To(Equal("UTC"))
+			Expect(parsed["hostname"]).To(Equal("test-vm"))
+			Expect(parsed["locale"]).To(Equal("en_US.UTF-8"))
+		})
+
+		It("should allow empty Extra map", func() {
+			opts := cloudinit.CloudConfigOpts{
+				Extra: map[string]interface{}{},
+			}
+			_, err := cloudinit.BuildCloudConfig(opts)
+			Expect(err).NotTo(HaveOccurred())
+		})
+	})
 })