Fix background callback context serialisation for non-dict request args

[i-murray]

Description

Fixes a serialisation failure that breaks all background=True callbacks on the FastAPI backend (and, for the same reason, Quart).

When a background callback is dispatched, Dash copies the callback context and hands it to the background worker (Celery via kombu, or Diskcache), where it must be serialised. The request args stored on the context is a backend-specific multi-dict starlette.datastructures.QueryParams on FastAPI, a werkzeug/quart MultiDict on Quart — which is not JSON serialisable, so dispatch fails with:

TypeError: Object of type QueryParams is not JSON serializable

Root cause

During request setup, Dash._initialize_context (dash/dash.py) stores the request data on the context. Unlike cookies and headers, which are coerced to plain dicts, args is stored as-is:

g.cookies = dict(adapter.cookies)
g.headers = dict(adapter.headers)
g.args = adapter.args            # backend-specific multi-dict, not coerced

This is intentional for the synchronous HTTP path, which relies on the multi-dict interface (e.g. adapter.args.getlist("oldJob") and callback_context.args.getlist("cancelJob")). The problem only appears when that object is carried into the background worker and serialised.

Fix

_setup_background_callback (dash/_callback.py) builds a copy of the context for the worker. We coerce args to a plain dict only on that copy, leaving the live request context (and its getlist interface) untouched:

ctx_value = AttributeDict(**context_value.get())   # copy handed to the worker
...
args_value = ctx_value.get("args")
if args_value is not None and not isinstance(args_value, dict):
    ctx_value["args"] = dict(args_value)

Because this mutates only the worker-bound copy:

• adapter.args.getlist("oldJob") (in _get_callback_manager) is unaffected, it reads the request adapter directly.
• callback_context.args.getlist("cancelJob") (the synchronous HTTP cancel path) is unaffected, it reads the live context_value, not the copy.
• The only consumer that sees the coerced dict is the background worker, which does not use getlist and otherwise could not receive the object at all.

Closes #3816

Contributor Checklist

• I have broken down my PR scope into the following TODO tasks
  • Coerce non-dict request args to a plain dict on the worker-bound context copy in _setup_background_callback
  • Add regression tests covering the serialisation behaviour
• I have run the tests locally and they passed. (refer to testing section in contributing)
• I have added tests, or extended existing tests, to cover any new features or bugs fixed in this PR

optionals

• I have added entry in the CHANGELOG.md
• If this PR needs a follow-up in dash docs, community thread, I have mentioned the relevant URLS as follows
  • this GitHub #PR number updates the dash docs
  • here is the show and tell thread in Plotly Dash community

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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