Summary

Adds exception handling guidance to the BackgroundTasks tutorial to help users avoid silent failures in production.

Context

Several users have reported confusion around exception handling in BackgroundTasks:

• Exceptions with handlers are replaced in a background task #2505 – Exceptions in background tasks can be hard to debug
• BackgroundTasks do not run when request failed #2604 – BackgroundTasks do not run when the request fails
• Raising exceptions with registered exception handlers from BackgroundTasks causes starlette exception #3589 – Exceptions in background tasks can obscure original errors
• Discussion Is it possible to register a background task before raising an HTTP error? #7532 – Users asking for clearer documentation
• Discussion BackgroundTasks do not run when request failed #8987 – Confusion around background task execution

Maintainers have previously noted that this behavior should be more explicit in the documentation.

Motivation

While building production systems, I explored async exception visibility patterns and noticed the same confusion affects FastAPI's BackgroundTasks. Exceptions in background tasks don't propagate to the response handler, which can lead to silent failures that are difficult to debug.

Changes

Adds a focused section to the BackgroundTasks tutorial explaining:

• How exceptions behave in background tasks (not propagated to the request handler)
• An example showing a silent failure scenario
• An example showing proper error handling with logging
• A pointer to task queue libraries for more complex production use cases

Scope

• Single documentation section (~50 lines)
• No API changes
• Located immediately after the basic BackgroundTasks examples
• Minimal and focused on production visibility

Happy to adjust wording or move this to a more appropriate section if you think that fits better.