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backendcritical PriorityID: fastapi-anyio-structured-taskgroup-boundary

FastAPI 0.115+ AnyIO TaskGroup Structured Concurrency Boundary

Enforces strict structured concurrency in FastAPI 0.115+ lifespans and background tasks using anyio.create_task_group. Eliminates dangling coroutines and unhandled ExceptionGroups across shutdown cycles by establishing rigid cancellation scopes and exception propagation boundaries.

Prohibited Anti-Pattern: Spawning Unmanaged Background Tasks via asyncio.create_task Inside Lifespans or Endpoints

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---
description: FastAPI 0.115+ AnyIO TaskGroup Structured Concurrency Boundary - Enforces strict structured concurrency in FastAPI 0.115+ lifespans and background tasks using anyio.create_task_group. Eliminates dangling coroutines and unhandled ExceptionGroups across shutdown cycles by establishing rigid cancellation scopes and exception propagation boundaries.
globs: **/*.py
alwaysApply: true
verified_by: "agentrules.org"
license: "CC-BY-4.0"
source: "https://agentrules.org/rules/fastapi-anyio-structured-taskgroup-boundary"
---

# FastAPI 0.115+ AnyIO TaskGroup Structured Concurrency Boundary

> **Severity**: CRITICAL | **Category**: backend
> **Anti-Pattern Warning**: Spawning Unmanaged Background Tasks via asyncio.create_task Inside Lifespans or Endpoints

## System Constraints
You are a Principal Software Architect enforcing structured concurrency invariants in FastAPI 0.115+ systems built over AnyIO. You must reject any usage of `asyncio.create_task` or unmanaged global thread/task dispatching that circumvents FastAPI's AnyIO runtime boundary. Mandate explicit `anyio.create_task_group()` context managers, robust `ExceptionGroup` handling (via Python 3.11+ `except*` or AnyIO fallback unpackers), and non-blocking lifespan resource disposal to ensure graceful shutdowns and eliminate orphaned tasks.

## Strict Guidelines
- Engineers MUST ALWAYS manage background concurrent processes within an explicit `anyio.create_task_group()` context block bound to the FastAPI lifespan or an explicit request-scoped cancellation manager.
- Engineers MUST NOT call `asyncio.create_task` or bare `threading.Thread` directly within FastAPI lifecycle handlers or request endpoints.
- Lifespan context managers MUST cancel task groups deterministically upon application shutdown and await active child task completion before releasing external connections (e.g., DB pools, Redis).
- Services MUST catch child task failures using Python 3.11+ `except*` syntax or handle unpacked exceptions from `anyio.ExceptionGroup` to prevent masked silent failures or unhandled teardown crashes.
- Tasks started via `task_group.start_soon()` MUST accept cancellation gracefully and MUST NOT swallow `anyio.get_cancelled_exc_class()` without re-raising.
- Components NEVER attach fire-and-forget background tasks to the global event loop without registering an active cancellation token accessible during the lifespan teardown phase.

## Bad vs Good Implementations
### Lifespan Task Management with AnyIO Structured Concurrency

#### Bad Antipattern (AI Common Hallucination)
> **Why it fails**: Using asyncio.create_task leaves the task unmanaged within FastAPI's AnyIO event harness. If the task raises an exception, it is unhandled until garbage collection. On application shutdown, the cancellation is non-deterministic and can cause teardown race conditions or masked ExceptionGroup errors.
```python
import asyncio
from contextlib import asynccontextmanager
from fastapi import FastAPI

async def worker_loop():
    while True:
        await asyncio.sleep(1)

@asynccontextmanager
async def lifespan(app: FastAPI):
    # ANTI-PATTERN: Fire-and-forget task detaches from runtime lifespan hierarchy.
    # On SIGTERM, worker_loop becomes orphaned and leaks DB connections or fails silently.
    task = asyncio.create_task(worker_loop())
    yield
    # Teardown does not safely await cancellation; app exits while task is in-flight.
    task.cancel()

app = FastAPI(lifespan=lifespan)
```

#### Production Standard (Correct)
> **Standard**: Wrapping lifespan execution in an anyio.create_task_group ensures that all spawned workers are part of the structured concurrency boundary. When the application receives a termination signal, tg.cancel_scope.cancel() gracefully signals the tasks, and the context exit guarantees all children exit before database/resource handles tear down.
```python
import anyio
from contextlib import asynccontextmanager
from fastapi import FastAPI

async def worker_loop():
    try:
        while True:
            await anyio.sleep(1)
    except anyio.get_cancelled_exc_class():
        # Optional clean cleanup hook, cancellation must re-raise or be re-propagated
        raise

@asynccontextmanager
async def lifespan(app: FastAPI):
    async with anyio.create_task_group() as tg:
        # Invariant: Background worker bound to the active AnyIO TaskGroup
        tg.start_soon(worker_loop)
        try:
            yield
        finally:
            # Invariant: Explicit cancellation scope triggers deterministic worker shutdown
            tg.cancel_scope.cancel()

app = FastAPI(lifespan=lifespan)
```
Recommended MCP Servers (mcp-serverlist.com)

Equip agent with system tools to verify code for this stack.

Python Concurrency Runtime Diagnostics
MCP server providing AST linting and runtime execution analysis for Python async concurrency safety.
Recommended Agent Skills (askills.dev)

Ready-to-use executable skill functions tailored to this stack.

Ruff Async and Concurrency Linter
High-performance Python linter enforcing ASYNC rules (ASYNC100, ASYNC101, ASYNC102) to ban asyncio leakage inside structured frameworks.
Architectural Specification & Invariant Rationale

Architectural Invariant Design: FastAPI 0.115+ AnyIO TaskGroup Structured Concurrency Boundary

FastAPI 0.115+ AnyIO TaskGroup Structured Concurrency Boundary defines an industrial-grade behavioral standard for backend engineering. Enforces strict structured concurrency in FastAPI 0.115+ lifespans and background tasks using anyio.create_task_group. Eliminates dangling coroutines and unhandled ExceptionGroups across shutdown cycles by establishing rigid cancellation scopes and exception propagation boundaries. Modern software architectures require strict adherence to deterministic patterns to guarantee high availability, maintainability, and zero security vulnerabilities.

Framework Paradigm Shift & Drift MechanicsLLMs are trained on millions of open-source repositories containing outdated code, legacy idioms, and unpatched antipatterns. When coding assistants generate implementations for this stack, they default to common statistical tokens: Spawning Unmanaged Background Tasks via asyncio.create_task Inside Lifespans or Endpoints Without an explicit system instruction file, developers spend hours debugging silent failures.
Failure Modes & Production Blast Radius

Hazard Analysis: What Breaks Without This Rule

The following failure modes have been cataloged from hundreds of hours of production code reviews where AI assistants produced non-compliant implementations:

Failure ModeAI Symptom (The Anti-Pattern)SeverityProduction Impact & Remedy
AI Hallucination & Deprecation DriftSpawning Unmanaged Background Tasks via asyncio.create_task Inside Lifespans or EndpointsCritical
Violating this industrial standard introduces regression bugs, security risks, and unexpected runtime failures in production.
Enforce explicit behavioral system directives across all AI coding assistants in your repository.
Silent Architecture InconsistencyAI assistants produce mixed paradigms across files, creating fragmented codebase conventions.Medium
Increases cognitive load during code reviews and leads to maintenance debt across engineering teams.
Pin project conventions in .cursor/rules, CLAUDE.md, and AGENTS.md so all team agents follow identical guardrails.
Architectural Directives in Detail

Step-by-Step Production Directives

Each rule constraint below is injected into the AI agent's system prompt to establish deterministic boundaries:

1

Engineers MUST ALWAYS manage background concurrent processes within an explicit `anyio.create_task_group()` context block bound to the FastAPI lifespan or an explicit request-scoped cancellation manager.

Engineers MUST ALWAYS manage background concurrent processes within an explicit `anyio.create_task_group()` context block bound to the FastAPI lifespan or an explicit request-scoped cancellation manager.

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.
Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.
2

Engineers MUST NOT call `asyncio.create_task` or bare `threading.Thread` directly within FastAPI lifecycle handlers or request endpoints.

Engineers MUST NOT call `asyncio.create_task` or bare `threading.Thread` directly within FastAPI lifecycle handlers or request endpoints.

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.
Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.
3

Lifespan context managers MUST cancel task groups deterministically upon application shutdown and await active child task completion before releasing external connections (e.g., DB pools, Redis).

Lifespan context managers MUST cancel task groups deterministically upon application shutdown and await active child task completion before releasing external connections (e.g., DB pools, Redis).

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.
Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.
4

Services MUST catch child task failures using Python 3.11+ `except*` syntax or handle unpacked exceptions from `anyio.ExceptionGroup` to prevent masked silent failures or unhandled teardown crashes.

Services MUST catch child task failures using Python 3.11+ `except*` syntax or handle unpacked exceptions from `anyio.ExceptionGroup` to prevent masked silent failures or unhandled teardown crashes.

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.
Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.
5

Tasks started via `task_group.start_soon()` MUST accept cancellation gracefully and MUST NOT swallow `anyio.get_cancelled_exc_class()` without re-raising.

Tasks started via `task_group.start_soon()` MUST accept cancellation gracefully and MUST NOT swallow `anyio.get_cancelled_exc_class()` without re-raising.

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.
Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.
6

Components NEVER attach fire-and-forget background tasks to the global event loop without registering an active cancellation token accessible during the lifespan teardown phase.

Components NEVER attach fire-and-forget background tasks to the global event loop without registering an active cancellation token accessible during the lifespan teardown phase.

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.
Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.
Technical Questions & Answers

Frequently Asked Technical Questions

Common questions developers face when configuring AI coding assistants for this stack:

Q:Why is FastAPI 0.115+ AnyIO TaskGroup Structured Concurrency Boundary critical for AI coding assistants?

Enforces strict structured concurrency in FastAPI 0.115+ lifespans and background tasks using anyio.create_task_group. Eliminates dangling coroutines and unhandled ExceptionGroups across shutdown cycles by establishing rigid cancellation scopes and exception propagation boundaries. Without explicit behavioral boundaries, AI coding assistants (Cursor, Claude Code, Windsurf, Codex) rely on training data averages, which frequently produce deprecated, inefficient, or vulnerable code patterns.

Q:How does adopting this rule prevent AI hallucinations?

This rule explicitly declares what the AI must avoid (Spawning Unmanaged Background Tasks via asyncio.create_task Inside Lifespans or Endpoints) and prescribes verified positive examples. Grounding the LLM with negative and positive pairs eliminates guesswork.

Q:Which AI coding environments are supported?

This specification is multi-platform compatible: Cursor (.cursor/rules/*.mdc & .cursorrules), Anthropic Claude Code (CLAUDE.md), Windsurf (.windsurfrules), Codex / Cline (AGENTS.md), and GitHub Copilot (.github/copilot-instructions.md).

Q:Can I combine this rule with other technology stacks?

Yes. Use the Interactive Stack Builder or download an Agent Pack to merge this rule with other frontend, backend, database, security, and DevOps rules without syntax collision.

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