# Cannot start a runtime from within a runtime. This happens because a function (like `block_on`) attempted to block the current thread while the thread is being used by another runtime.

- **ID:** `rust/tokio-runtime-enter-error`
- **Domain:** rust
- **Category:** runtime_error
- **Verification:** ai_generated
- **Fix Rate:** 85%

## Root Cause

Attempting to create or enter a Tokio runtime (e.g., via `block_on`, `runtime.block_on`, or `#[tokio::main]`) while already inside an active Tokio runtime context on the same thread.

## Version Compatibility

| Version | Status | Introduced | Deprecated |
|---------|--------|------------|------------|
| tokio 1.35.0 | active | — | — |
| tokio 1.36.0 | active | — | — |
| tokio 1.37.0 | active | — | — |
| rustc 1.75.0 | active | — | — |

## Workarounds

1. **Use `tokio::task::spawn_blocking` to run blocking code on a separate thread pool without creating a new runtime: `tokio::task::spawn_blocking(move || { /* blocking work */ }).await`** (90% success)
   ```
   Use `tokio::task::spawn_blocking` to run blocking code on a separate thread pool without creating a new runtime: `tokio::task::spawn_blocking(move || { /* blocking work */ }).await`
   ```
2. **Pass the runtime handle around using `tokio::runtime::Handle::current()` and use `handle.block_on()` only from non-async contexts; avoid calling `block_on` inside async code.** (85% success)
   ```
   Pass the runtime handle around using `tokio::runtime::Handle::current()` and use `handle.block_on()` only from non-async contexts; avoid calling `block_on` inside async code.
   ```
3. **Restructure the code to use a single runtime: if you need to run blocking code, use `tokio::task::spawn_blocking`; if you need to call async code from sync code, use `tokio::runtime::Runtime::block_on` only at the top-level entry point.** (95% success)
   ```
   Restructure the code to use a single runtime: if you need to run blocking code, use `tokio::task::spawn_blocking`; if you need to call async code from sync code, use `tokio::runtime::Runtime::block_on` only at the top-level entry point.
   ```

## Dead Ends

- **** — If the spawned thread also tries to create a runtime, it will succeed on a new thread, but the original thread still has the nested runtime issue; also, sharing data across threads may require `Send + Sync`. (75% fail)
- **** — This still attempts to enter a runtime from within another runtime on the same thread; the error persists. (90% fail)
- **** — This can lead to multiple runtime instances and still cause the same error if any runtime is entered while another is active; it also complicates code structure. (80% fail)