Cannot start a runtime from within a runtime (Tokio) Rust Origin: Application & API OSI layer 7 · Application Last updated 5 Nov 2026

Rust: thread 'main' panicked: Cannot start a runtime from within a runtime — a function like `block_on` tried to block a thread that drives async tasks

Blocking code (block_on, reqwest::blocking) was called inside an async context.

Exact message you may see 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 to drive asynchronous tasks.Cannot drop a runtime in a context where blocking is not allowedCannot start a runtime from within a runtimethread 'main' panicked: Cannot start a runtime from within a runtime
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Seen on: Rust

Meaning

Don't call block_on within async code. Make the caller async and .await, or use tokio::task::spawn_blocking for blocking work, or tokio::task::block_in_place (multi-thread runtime). reqwest::blocking inside #[tokio::main] triggers this: use the async client.

Common causes

  • Runtime::block_on inside async fn
  • reqwest::blocking::get in async code
  • Mixing sync libraries' blocking calls in tasks
  • Tests using #[tokio::test] with blocking clients
  • Nested runtimes

⚡ Quick fix

  1. Use async APIs and .await
  2. tokio::task::spawn_blocking(|| blocking_work()).await
  3. Replace reqwest::blocking with the async client

Detailed fix by platform

Rust

  1. `let r = tokio::task::spawn_blocking(move
  2. heavy()).await?;`

How to diagnose

  1. Call — block_on in async?
  2. Client — blocking?
  3. Fix — spawn_blocking?

🔧 Still not fixed?

Many errors look alike. If the steps above didn’t solve it, one of these is probably what you’re facing:

Where this error sits

Origin: Application & API. Errors that are often confused with or caused by this one (click to open):

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