Bookmark restore drops render-hook environments: tool-result displays with live bslib/htmltools tags fail to re-render

Summary

shinychat serializes chat turns for bookmarking with jsonlite::serializeJSON() and restores them with jsonlite::unserializeJSON() (see client_get_state() / client_set_state() for the Chat method). That round-trip rebuilds any embedded function from deparsed source text, discarding its original environment.

If a tool result's extra$display contains a live htmltools/bslib tag — i.e. one carrying a render hook (tag$.renderHooks) — restoring the bookmark and re-rendering the tool card throws, because the hook can no longer resolve the package internals it closed over:

Error in tag_require_client_side: could not find function "tag_require_client_side"

The transcript then fails to re-render, so the restored session is broken.

Minimal reproducible example

This reproduces the failure with no LLM and no Shiny app — it mirrors exactly what client_get_state() / client_set_state() do to a tool result whose display embeds a bslib::input_code_editor() (a Bootstrap web component, which attaches a render hook):

library(ellmer)

req <- ContentToolRequest(id = "call_1", name = "demo", arguments = list())
res <- ContentToolResult(
  value   = "done",
  request = req,
  extra   = list(display = list(
    html = bslib::input_code_editor(id = "x", value = "select 1", language = "sql")
  ))
)

# Mirror shinychat's bookmark round-trip
recorded   <- contents_record(res)
state_json <- jsonlite::serializeJSON(recorded)
restored   <- contents_replay(jsonlite::unserializeJSON(state_json))

htmltools::findDependencies(restored@extra$display$html)
#> Error in tag_require_client_side: could not find function "tag_require_client_side"

The same thing happens in a real app: enable bookmarking, have a tool produce a result whose display$html/display$footer contains such a tag, reload to restore — the error fires from client_set_ui() → chat_append_message() → as.tags.shinychat_tool_card() → htmltools::findDependencies().

Even more minimal, the core fragility is just a JSON round-trip of any bslib web component:

ce  <- bslib::input_code_editor(id = "x", value = "select 1", language = "sql")
ce2 <- jsonlite::unserializeJSON(jsonlite::serializeJSON(ce))
htmltools::findDependencies(ce2)
#> Error: could not find function "tag_require_client_side"

Root cause

1. bslib::input_code_editor() (and other Bootstrap components) returns a tag with a render hook in tag$.renderHooks. htmltools runs render hooks lazily at render time, i.e. when as.tags() / findDependencies() / renderTags() walk the tag. The hook's body calls bslib internals:

   function(x) {
     current_version <- theme_version(bs_current_theme())
     if (isTRUE(current_version < version)) stop(...)
     return(tag_require_client_side(x, version, caller))   # unexported bslib internal
   }
   

   Normally this resolves fine because the hook is a closure whose environment chains to bslib's namespace.

2. client_get_state() records turns with ellmer::contents_record() and serializes with jsonlite::serializeJSON(), which deparses the closure to source text.

3. client_set_state() restores with jsonlite::unserializeJSON(), which parse()s the text back into a function whose environment is now R_GlobalEnv — the captured variables (version, caller) and the link to bslib's namespace are gone.

4. On re-render, as.tags.shinychat_tool_card() calls htmltools::findDependencies() on display$html / display$footer, which runs the hook. Exported bslib functions (theme_version, bs_current_theme) still resolve via the attached search path, but the unexported tag_require_client_side does not → "could not find function".

This is not specific to input_code_editor — any tag carrying a namespace-bound render hook (many bslib components, htmlwidgets, etc.) embedded in a tool-result display will hit it. It surfaces only on restore, because the hook fires when the stored display is re-rendered, not when the result is first produced. (Discovered via querychat's visualization tool, which embeds a bslib::input_code_editor() in its tool-result display footer.)

Options to consider

A few directions for discussion — not a recommendation:

1. Freeze tool-result display tags before serialization. In client_get_state() (or when recording the display), resolve render hooks to static HTML + dependencies (e.g. htmltools::renderTags() → HTML() + html_dependency list) so only inert, serialization-safe values enter the bookmark. Keeps web-component markup and JS deps intact for client-side hydration. Trade-off: hooks run at bookmark time, outside the live session/theme, so the resolved dependency/version context may differ from render time.

2. Avoid serializeJSON/unserializeJSON for the parts that may hold tags. For server-side bookmark stores, a serializer that preserves environments (e.g. saveRDS/serialize) round-trips closures correctly. Doesn't help URL bookmarks (size + must be text), and changes the storage format.

3. Re-home rebuilt closures on restore. After unserializeJSON(), deserialized render-hook functions could have their environment reset to a context where package internals resolve. Fragile in general (can't know the right namespace), but might be scoped to known tag shapes.

4. Document/validate the constraint. Treat live tags with render hooks in extra$display as unsupported for bookmarking and warn (or freeze) at record time, pushing producers toward markdown/text displays or pre-rendered HTML.

Stack trace (from the original app-level report)

Warning: Error in tag_require_client_side: could not find function "tag_require_client_side"
...
141: hook [<text>#8]
140: as.tags.shiny.tag
...
127: htmltools::findDependencies
126: as.tags.shinychat_tool_card
125: htmltools::as.tags
124: split_html_islands
...
112: chat_append_message
...
90: method(client_set_ui, new_S3_class(c("Chat", "R6")))
88: client_set_ui
...
51: observe

Environment

• shinychat 0.4.0.9000
• ellmer 0.4.1
• bslib (current), htmltools (current)

Start with the Chat methods client_get_state() and client_set_state(), then follow the reported client_set_ui() → chat_append_message() → as.tags.shinychat_tool_card() path. Reproduce the jsonlite round-trip with the bslib::input_code_editor() example and determine the supported handling for render-hook tags. Done means restored tool-result displays re-render without the tag_require_client_side error, with coverage for bookmark restoration.