diff --git a/pkg-py/src/querychat/_viz_tools.py b/pkg-py/src/querychat/_viz_tools.py
index c57b297c..46bbb324 100644
--- a/pkg-py/src/querychat/_viz_tools.py
+++ b/pkg-py/src/querychat/_viz_tools.py
@@ -5,6 +5,7 @@
 import base64
 import copy
 import io
+import warnings
 from typing import TYPE_CHECKING, Any, TypedDict
 from uuid import uuid4
 
@@ -151,6 +152,35 @@ def __init__(
 # ---------------------------------------------------------------------------
 
 
+def warn_viz_url_bookmarking() -> bool:
+    """
+    Warn (once) when URL bookmarking is active.
+
+    With URL bookmarking each chart embeds a base64 PNG preview (for LLM
+    feedback) that can push the bookmarked state past URL length limits and
+    corrupt it on restore. Server-side bookmarking has no such limit. This
+    check is best-effort and must never interrupt chart creation.
+    """
+    try:
+        session = get_current_session()
+        is_url = session is not None and session.bookmark.store == "url"
+    except Exception:
+        return False
+    if not is_url:
+        return False
+    try:
+        warnings.warn(
+            "URL bookmarking may not reliably restore visualizations. Each "
+            "chart embeds a PNG preview that can exceed URL length limits and "
+            "corrupt the saved state on restore. Consider server-side "
+            'bookmarking instead: App(..., bookmark_store="server").',
+            stacklevel=2,
+        )
+    except Exception:
+        return False
+    return True
+
+
 def visualize_impl(
     executor: QueryExecutor,
     update_fn: Callable[[VisualizeData], None],
@@ -188,6 +218,8 @@ def visualize(
             except Exception:
                 png_bytes = None
 
+            warn_viz_url_bookmarking()
+
             update_fn(
                 {"ggsql": ggsql, "title": title, "widget_id": altair_widget.widget_id}
             )
diff --git a/pkg-r/R/querychat_module.R b/pkg-r/R/querychat_module.R
index efc9904b..cb1c7ff8 100644
--- a/pkg-r/R/querychat_module.R
+++ b/pkg-r/R/querychat_module.R
@@ -267,7 +267,7 @@ mod_server <- function(
         if (!is.null(state$values$querychat_viz_widgets)) {
           restored <- restore_viz_widgets(
             executor,
-            state$values$querychat_viz_widgets,
+            restore_record_list(state$values$querychat_viz_widgets),
             session
           )
           viz_widgets <<- restored
@@ -329,6 +329,27 @@ GREETING_PROMPT <- paste(
   "using the suggestion card format from your instructions."
 )
 
+# A list of records (named lists) bookmarked to the URL comes back from Shiny's
+# decoder as a data.frame, because jsonlite simplifies a JSON array of objects
+# (simplifyDataFrame = TRUE). Rebuild the list-of-lists shape row by row,
+# dropping absent (NA) optional fields. A value restored from a server-side
+# store (or already a list) is passed through unchanged.
+restore_record_list <- function(x) {
+  if (is.null(x)) {
+    return(NULL)
+  }
+  if (is.data.frame(x)) {
+    return(lapply(seq_len(nrow(x)), function(i) {
+      row <- as.list(x[i, , drop = FALSE])
+      row <- lapply(row, function(v) {
+        if (length(v) == 1 && is.na(v)) NULL else v
+      })
+      row[!vapply(row, is.null, logical(1))]
+    }))
+  }
+  as.list(x)
+}
+
 restore_viz_widgets <- function(executor, saved_widgets, session) {
   if (!rlang::is_installed("ggsql")) {
     warning(
diff --git a/pkg-r/R/querychat_viz.R b/pkg-r/R/querychat_viz.R
index a1b084fe..c84d15bd 100644
--- a/pkg-r/R/querychat_viz.R
+++ b/pkg-r/R/querychat_viz.R
@@ -78,6 +78,7 @@ visualize_result <- function(
 
   viz_container <- NULL
   if (!is.null(session)) {
+    warn_viz_url_bookmarking()
     session$output[[widget_id]] <- ggsql::renderGgsql(spec)
     viz_container <- htmltools::div(
       class = "querychat-viz-container",
@@ -146,19 +147,47 @@ visualize_result <- function(
   )
   extra <- list(
     display = list(
-      html = viz_container,
+      html = freeze_tags(viz_container),
       title = if (nzchar(title)) title else "Query Visualization",
       show_request = FALSE,
       open = querychat_tool_starts_open("visualize"),
       full_screen = TRUE,
       icon = viz_icon(),
-      footer = footer
+      footer = freeze_tags(footer)
     )
   )
 
   ellmer::ContentToolResult(value = value, extra = extra)
 }
 
+# Best-effort, additive-only check: with URL bookmarking each chart embeds a
+# base64 PNG preview (for LLM feedback) that can push the bookmarked state past
+# URL length limits and corrupt it on restore. Recommend server-side
+# bookmarking, which has no such limit. Detection and the warning are wrapped so
+# this never interrupts chart creation, and the warning fires only once.
+warn_viz_url_bookmarking <- function() {
+  is_url <- tryCatch(
+    identical(shiny::getShinyOption("bookmarkStore", ""), "url"),
+    error = function(e) FALSE
+  )
+  if (!isTRUE(is_url)) {
+    return(invisible(FALSE))
+  }
+  tryCatch(
+    cli::cli_warn(
+      c(
+        "URL bookmarking may not reliably restore visualizations.",
+        "i" = "Each chart embeds a PNG preview that can exceed URL length limits and corrupt the saved state on restore.",
+        "i" = "Consider server-side bookmarking instead: {.code shiny::enableBookmarking(\"server\")}."
+      ),
+      .frequency = "once",
+      .frequency_id = "querychat_viz_url_bookmarking"
+    ),
+    error = function(e) NULL
+  )
+  invisible(TRUE)
+}
+
 collapse_validation_errors <- function(validated) {
   errors <- validated$errors
   if (is.null(errors) || !nrow(errors)) {
diff --git a/pkg-r/R/utils-html.R b/pkg-r/R/utils-html.R
new file mode 100644
index 00000000..2ef05b29
--- /dev/null
+++ b/pkg-r/R/utils-html.R
@@ -0,0 +1,22 @@
+# Resolve a tag's render hooks now and return inert HTML plus its html
+# dependencies. shinychat bookmarks chat turns with jsonlite::serializeJSON()
+# and restores them with unserializeJSON(), which rebuilds any embedded closure
+# from deparsed text and drops its environment. A live bslib/htmltools tag (e.g.
+# input_code_editor()) carries render hooks that close over package internals,
+# so after the round-trip re-rendering fails with "could not find function".
+# Freezing here -- while the hooks can still reach those internals -- keeps only
+# inert HTML and html_dependency objects, which survive the round-trip.
+# See posit-dev/shinychat#261.
+freeze_tags <- function(tags) {
+  if (is.null(tags) || is.character(tags)) {
+    return(tags)
+  }
+  rendered <- htmltools::renderTags(tags)
+  htmltools::tagList(
+    htmltools::HTML(rendered$html),
+    if (nzchar(rendered$head)) {
+      htmltools::tags$head(htmltools::HTML(rendered$head))
+    },
+    rendered$dependencies
+  )
+}