Source: https://frameworkrecipes.dev/ktor/cancellation-propagation/

# Preserve Kotlin coroutine cancellation in a Ktor service

A broad exception handler can turn request cancellation into normal success. That prevents structured parents from stopping work and may cause the caller to send stale data or continue a mutation. Handle ordinary failures separately from cancellation.

Save this requirement in Perch and check it after each agent edit. The rule gives your agent a named check to run, a finding to repair, and the same command to recheck.

## Implement the Ktor boundary

Save this illustrative integration module as `src/main/kotlin/example/OrderLookup.kt`.

```
package example
import kotlinx.coroutines.CancellationException

suspend fun <T> loadOrder(read: suspend () -> T, fallback: () -> T): T {
    return try {
        read()
    } catch (cancelled: CancellationException) {
        throw cancelled
    } catch (failure: Exception) {
        fallback()
    }
}
```

The order, inventory, or resource-lifetime scenario is adapted from Perch’s public [order-service source at commit 4af67af](https://github.com/lakeday-org/perch/blob/4af67af46d76ac99398d61c7ad0a45441760d4dc/test/fixtures/order-service/storage.py). This Ktor implementation is an illustrative adaptation.

## Integrate with your application

Use this boundary around a cancellable read operation in a Ktor handler. It is unsuitable for hiding write failures: mutations should report rollback and preserve the original error. Ensure the database/client adapter actually observes coroutine cancellation; wrapping a blocking driver does not make the operation cancellable. Run blocking work on the appropriate dispatcher and use driver timeouts.

## Exercise the boundary and failure paths

| Scenario | Expected outcome |
| --- | --- |
| Read succeeds | Original value |
| Ordinary read failure | Explicit fallback |
| Parent coroutine cancels | CancellationException propagates |
| Blocking driver ignores cancellation | Timeout and dispatcher test documents the limitation |

Add regression tests for these cases in your existing Ktor test project. Use separate connections or deliberately reordered completion where the case involves concurrency; a sequential happy-path test does not exercise that failure. Run the test command after creating the matching test file or suite:

```
./gradlew test --tests example.OrderLookupTest
```

A focused test to adapt to the paths and test dependencies in your project:

```
import kotlinx.coroutines.CancellationException
import kotlinx.coroutines.test.runTest
import kotlin.test.Test
import kotlin.test.assertFailsWith

class OrderLookupTest {
    @Test fun cancellationDoesNotBecomeFallback() = runTest {
        assertFailsWith<CancellationException> {
            loadOrder<Int>({ throw CancellationException("stop") }, { 0 })
        }
    }
}
```

## Introduce the specific agent regression

In a throwaway copy, replace this source span:

```
throw cancelled
```

with:

```
fallback()
```

Run the relevant negative test and the Perch command on that broken copy. Restore the corrected source before committing the application. The supplied mutation reproduces the exact regression for the rule check.

## Preserve the requirement as a Perch rule

Save this in `.perch/rules/perch-guide-ktor-cancellation-propagation.yaml` at the repository root, keeping your existing `perch.yaml`:

```
- name: coroutine-cancellation-rethrown
  where: "src/main/kotlin/example/OrderLookup.kt"
  ensure: >-
    A caught CancellationException must be rethrown, not turned into a successful fallback value.
```

Install and authenticate the CLI once, then check the current working file:

```
npm install -g @lakeday/perch
perch login
perch check src/main/kotlin/example/OrderLookup.kt --rules coroutine-cancellation-rethrown --json
```

Use Node 22 or newer. The framework can use another language; Perch’s CLI remains a Node command. A gated finding exits 3. Exits 1 or 2 mean the check did not complete and must be reported as a blocker.

## Inspect the captured rule check

On 2026-10-14, Perch using jev-latest checked the exact fixed and broken source provided here. The capture records SHA-256 hashes of both sources and the requirement.

| Source control | Exit | Outcome |
| --- | --- | --- |
| Broken | 3 | Rule finding |
| Corrected | 0 | No rule finding |

The rule distinguished these controls in this run. [Raw Perch capture](https://frameworkrecipes.dev/ktor/cancellation-propagation/capture.json) · [Exact sources and mutation](https://frameworkrecipes.dev/ktor/cancellation-propagation/source.json).

## Install the rule pack and configure your agent

[Download this rule pack ZIP](https://reporules.fyi/ktor/cancellation-propagation/rule-pack.zip) · [Download the YAML rules](https://reporules.fyi/ktor/cancellation-propagation/rules.yaml) · [Agent task](https://reporules.fyi/ktor/cancellation-propagation/agent-task.txt). The pack contains the rule, its example paths and the repair/recheck instructions. Review the `where` selector before using it in an existing project. Keep your root `perch.yaml`; install this rule as a separate file under `.perch/rules/`.

1. Install Perch using Node 22 or newer and sign in once:

```
npm install -g @lakeday/perch
perch login
```

1. Download the pack, inspect it, and extract it from your repository root. The extraction preserves any existing files:

```
curl -fL 'https://frameworkrecipes.dev/ktor/cancellation-propagation/rule-pack.zip' -o 'perch-guide-ktor-cancellation-propagation.zip'
unzip -n 'perch-guide-ktor-cancellation-propagation.zip'
```

The installed rule is `.perch/rules/perch-guide-ktor-cancellation-propagation.yaml`. Its selector matches `src/main/kotlin/example/OrderLookup.kt` in this example; adapt the selector to your application file before checking an existing project.

1. Configure the coding agent you use. Run one of these commands:

Claude Code:

```
perch setup claude-code
```

Codex:

```
perch setup codex
```

Cursor:

```
perch setup cursor
```

Pi:

```
perch setup pi
```

1. Ask the agent to edit the implementation and check the saved requirement:

> Use the installed Perch skill while editing src/main/kotlin/example/OrderLookup.kt. Preserve coroutine-cancellation-rethrown. Run perch check src/main/kotlin/example/OrderLookup.kt --rules coroutine-cancellation-rethrown --json after the edit. Read each finding and the affected code, fix the implementation, and run the identical check again after the fix. Keep the rule and regression tests intact. Run the application's relevant regression tests and report the commands and actual results. Exit 3 means a rule finding; exit 1 or 2 means the check did not complete. Report an unavailable or incomplete check as blocked. Do not claim the application tests ran if only the Perch check ran.

1. After repairing a finding, run the same check again and inspect the result:

```
perch check src/main/kotlin/example/OrderLookup.kt --rules coroutine-cancellation-rethrown --json
```

Commit the downloaded rule and the installed agent configuration with your application. The rule file adds no guarantee of correctness; keep the regression tests from this guide.

With the base reference fetched, use `perch scan --since origin/main --filter type=lint` in CI.

## Framework references and verification scope

- [kotlinlang.org reference](https://kotlinlang.org/docs/cancellation-and-timeouts.html)

The dated capture records the source check shown above. Implement the adapters, schema and regression tests described in this guide in your application. Unchanged requests reuse cached answers; fresh model judgments can vary. Model analysis sends relevant source to the configured service.

**Framework Recipes**

Implementation, failure path, check.

Published by Perch · © 2026 Verglas LLC. Framework Recipes is a Perch publication.

Perch’s MIT license covers the CLI; hosted model analysis is a paid service. Relevant source is sent to the configured model service.

[Perch CLI MIT license](https://github.com/lakeday-org/perch/blob/main/LICENSE) · [Article text](https://frameworkrecipes.dev/ktor/cancellation-propagation/index.md) · [Resource index](https://frameworkrecipes.dev/llms.txt)