diff --git a/coderd/workspaces.go b/coderd/workspaces.go
index 6da85c7608ca4..fdae93096a793 100644
--- a/coderd/workspaces.go
+++ b/coderd/workspaces.go
@@ -1072,6 +1072,17 @@ func (api *API) putWorkspaceAutostart(rw http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	// Autostart configuration is not supported for prebuilt workspaces.
+	// Prebuild lifecycle is managed by the reconciliation loop, with scheduling behavior
+	// defined per preset at the template level, not per workspace.
+	if workspace.IsPrebuild() {
+		httpapi.Write(ctx, rw, http.StatusConflict, codersdk.Response{
+			Message: "Autostart is not supported for prebuilt workspaces",
+			Detail:  "Prebuilt workspace scheduling is configured per preset at the template level. Workspace-level overrides are not supported.",
+		})
+		return
+	}
+
 	dbSched, err := validWorkspaceSchedule(req.Schedule)
 	if err != nil {
 		httpapi.Write(ctx, rw, http.StatusBadRequest, codersdk.Response{
@@ -1156,6 +1167,17 @@ func (api *API) putWorkspaceTTL(rw http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	// TTL updates are not supported for prebuilt workspaces.
+	// Prebuild lifecycle is managed by the reconciliation loop, with TTL behavior
+	// defined per preset at the template level, not per workspace.
+	if workspace.IsPrebuild() {
+		httpapi.Write(ctx, rw, http.StatusConflict, codersdk.Response{
+			Message: "TTL updates are not supported for prebuilt workspaces",
+			Detail:  "Prebuilt workspace TTL is configured per preset at the template level. Workspace-level overrides are not supported.",
+		})
+		return
+	}
+
 	var dbTTL sql.NullInt64
 
 	err := api.Database.InTx(func(s database.Store) error {
@@ -1272,6 +1294,16 @@ func (api *API) putWorkspaceDormant(rw http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	// Dormancy configuration is not supported for prebuilt workspaces.
+	// Prebuilds are managed by the reconciliation loop and are not subject to dormancy.
+	if oldWorkspace.IsPrebuild() {
+		httpapi.Write(ctx, rw, http.StatusConflict, codersdk.Response{
+			Message: "Dormancy configuration is not supported for prebuilt workspaces",
+			Detail:  "Prebuilt workspaces are not subject to dormancy. Dormancy behavior is only applicable to regular workspaces",
+		})
+		return
+	}
+
 	// If the workspace is already in the desired state do nothing!
 	if oldWorkspace.DormantAt.Valid == req.Dormant {
 		rw.WriteHeader(http.StatusNotModified)
@@ -1416,6 +1448,17 @@ func (api *API) putExtendWorkspace(rw http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	// Deadline extensions are not supported for prebuilt workspaces.
+	// Prebuilds are managed by the reconciliation loop and must always have
+	// Deadline and MaxDeadline unset.
+	if workspace.IsPrebuild() {
+		httpapi.Write(ctx, rw, http.StatusConflict, codersdk.Response{
+			Message: "Deadline extension is not supported for prebuilt workspaces",
+			Detail:  "Prebuilt workspaces do not support user deadline modifications. Deadline extension is only applicable to regular workspaces",
+		})
+		return
+	}
+
 	code := http.StatusOK
 	resp := codersdk.Response{}