disallow late setContext calls

documentation/docs/98-reference/.generated/client-errors.md

@@ -110,6 +110,12 @@ Rest element properties of `$props()` such as `%property%` are readonly
 The `%rune%` rune is only available inside `.svelte` and `.svelte.js/ts` files
 ```
 
+### set_context_after_init
+
+```
+`setContext` must be called when a component first initializes, not in a subsequent effect or after an `await` expression
+```
+
 ### state_descriptors_fixed
 
 ```

packages/svelte/messages/client-errors/errors.md

@@ -72,6 +72,10 @@ See the [migration guide](/docs/svelte/v5-migration-guide#Components-are-no-long
 
 > The `%rune%` rune is only available inside `.svelte` and `.svelte.js/ts` files
 
+## set_context_after_init
+
+> `setContext` must be called when a component first initializes, not in a subsequent effect or after an `await` expression
+
 ## state_descriptors_fixed
 
 > Property descriptors defined on `$state` objects must contain `value` and always be `enumerable`, `configurable` and `writable`.

packages/svelte/src/internal/client/context.js

@@ -2,6 +2,7 @@
 
 import { DEV } from 'esm-env';
 import { lifecycle_outside_component } from '../shared/errors.js';
+import * as e from './errors.js';
 import { source } from './reactivity/sources.js';
 import {
 	active_effect,
@@ -10,7 +11,7 @@ import {
 	set_active_reaction
 } from './runtime.js';
 import { effect, teardown } from './reactivity/effects.js';
-import { legacy_mode_flag } from '../flags/index.js';
+import { async_mode_flag, legacy_mode_flag } from '../flags/index.js';
 
 /** @type {ComponentContext | null} */
 export let component_context = null;
@@ -65,6 +66,13 @@ export function getContext(key) {
  */
 export function setContext(key, context) {
 	const context_map = get_or_init_context_map('setContext');
+
+	if (async_mode_flag) {
+		if (/** @type {ComponentContext} */ (component_context).m) {
+			e.set_context_after_init();
+		}
+	}
+
 	context_map.set(key, context);
 	return context;
 }

packages/svelte/src/internal/client/errors.js

@@ -197,6 +197,21 @@ export function hydration_failed() {
 	}
 }
 
+/**
+ * `setContext` must be called when a component first initializes, not in a subsequent effect or after an `await` expression
+ * @returns {never}
+ */
+export function set_context_after_init() {
+	if (DEV) {
+		const error = new Error(`set_context_after_init\n\`setContext\` must be called when a component first initializes, not in a subsequent effect or after an \`await\` expression\nhttps://svelte.dev/e/set_context_after_init`);
+
+		error.name = 'Svelte error';
+		throw error;
+	} else {
+		throw new Error(`https://svelte.dev/e/set_context_after_init`);
+	}
+}
+
 /**
  * Could not `{@render}` snippet due to the expression being `null` or `undefined`. Consider using optional chaining `{@render snippet?.()}`
  * @returns {never}

packages/svelte/tests/runtime-runes/samples/set-context-after-mount/_config.js

@@ -0,0 +1,11 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	async test({ target, assert, logs }) {
+		const button = target.querySelector('button');
+
+		flushSync(() => button?.click());
+		assert.ok(logs[0].startsWith('set_context_after_init'));
+	}
+});

packages/svelte/tests/runtime-runes/samples/set-context-after-mount/main.svelte

@@ -0,0 +1,19 @@
+<script>
+	import { setContext } from 'svelte';
+
+	let condition = $state(false);
+
+	$effect(() => {
+		if (condition) {
+			try {
+				setContext('potato', {});
+			} catch (e) {
+				console.log(e.message);
+			}
+		}
+	});
+</script>
+
+<button onclick={() => condition = !condition}>
+	{condition}
+</button>