feat: Expose missing App attributes in Express via app_opts() and/or an app accessor

Problem

Shiny Express apps configure app-level options via app_opts(), but it only covers a subset of what can be set on a shiny.App object in Core mode. Several important App attributes are completely inaccessible from Express:

Attribute	Core mode	Express app_opts()
static_assets	App(...) constructor	✅ supported
bookmark_store	App(...) constructor	✅ supported
debug	App(...) constructor	✅ supported
sanitize_errors	app.sanitize_errors = True	❌ missing
sanitize_error_msg	app.sanitize_error_msg = "..."	❌ missing
sanitize_otel_errors	app.sanitize_otel_errors = False	❌ missing
lib_prefix	app.lib_prefix = "assets/"	❌ missing

In Core mode, an app sets these like:

app = App(app_ui, server)
app.sanitize_errors = True
app.sanitize_otel_errors = False
app.sanitize_error_msg = "Something went wrong."

In Express, there is no equivalent — app_opts() doesn't support any of them and the underlying App object is never exposed to the user.

Proposed Solutions

Option A: Extend app_opts() with all missing attributes

Add the missing parameters to the existing app_opts() function:

app_opts(
    sanitize_errors=True,
    sanitize_otel_errors=False,
    sanitize_error_msg="Something went wrong.",
    lib_prefix="assets/",
)

Pros: Consistent with the existing pattern; all options in one place.
Cons: app_opts() grows with every new App attribute; may lag behind future additions.

Option B: Provide an accessor to the underlying App object

Expose a function (e.g., express.get_app()) that returns the live App instance, allowing direct attribute mutation:

from shiny.express import get_app

get_app().sanitize_errors = True
get_app().sanitize_otel_errors = False

Pros: No need to update app_opts() every time App gains a new attribute; users have full control; mirrors the Core mode experience.
Cons: Requires the App to exist at call time (it's created during create_express_app()); needs careful handling of the execution lifecycle; slightly less discoverable.

Option C: Both — extend app_opts() and add an accessor

Keep app_opts() for the most common options, and add get_app() as an escape hatch for advanced use.

Recommendation

Implement Option C:

1. Add sanitize_errors, sanitize_otel_errors, and sanitize_error_msg to app_opts() since they're immediately needed and commonly configured.
2. Add a get_app() (or similar name — current_app(), express_app()?) accessor as an escape hatch so future App attributes don't require app_opts() updates.

The naming of the accessor is worth discussion — options include:

• express.get_app()
• express.current_app()
• express.app() (but could conflict with the app module)

Related

• shiny/express/_run.py — app_opts() and AppOpts TypedDict
• shiny/_app.py — App class with all mutable attributes
• examples/req/app.py — Core mode example using app.sanitize_errors = True

Start with shiny/express/_run.py, especially app_opts() and the AppOpts TypedDict, then inspect shiny/_app.py for the mutable attributes listed in the issue. Review examples/req/app.py for the Core-mode behavior. Done means selecting and implementing the agreed accessor and/or option design so the missing Express configuration is available, with the lifecycle and API naming resolved.