From 4f35c019fc110fe2f4c74cb293f26f270195345b Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Fri, 24 Oct 2025 11:22:27 -0400
Subject: [PATCH] add fork_discarded error

---
 .../98-reference/.generated/client-errors.md  |  6 +++
 .../svelte/messages/client-errors/errors.md   |  4 ++
 packages/svelte/src/internal/client/errors.js | 48 ++++++++++++-------
 .../src/internal/client/reactivity/batch.js   |  2 +-
 4 files changed, 43 insertions(+), 17 deletions(-)

diff --git a/documentation/docs/98-reference/.generated/client-errors.md b/documentation/docs/98-reference/.generated/client-errors.md
index d7a91fae01..d47d0eb5cd 100644
--- a/documentation/docs/98-reference/.generated/client-errors.md
+++ b/documentation/docs/98-reference/.generated/client-errors.md
@@ -146,6 +146,12 @@ The `flushSync()` function can be used to flush any pending effects synchronousl
 
 This restriction only applies when using the `experimental.async` option, which will be active by default in Svelte 6.
 
+### fork_discarded
+
+```
+Cannot commit a fork that was already committed or discarded (including as a result of a different fork being committed)
+```
+
 ### fork_timing
 
 ```
diff --git a/packages/svelte/messages/client-errors/errors.md b/packages/svelte/messages/client-errors/errors.md
index 40e6d82089..44afef58b4 100644
--- a/packages/svelte/messages/client-errors/errors.md
+++ b/packages/svelte/messages/client-errors/errors.md
@@ -112,6 +112,10 @@ The `flushSync()` function can be used to flush any pending effects synchronousl
 
 This restriction only applies when using the `experimental.async` option, which will be active by default in Svelte 6.
 
+## fork_discarded
+
+> Cannot commit a fork that was already committed or discarded (including as a result of a different fork being committed)
+
 ## fork_timing
 
 > Cannot create a fork inside an effect or when state changes are pending
diff --git a/packages/svelte/src/internal/client/errors.js b/packages/svelte/src/internal/client/errors.js
index 4916e630d7..8341688ea6 100644
--- a/packages/svelte/src/internal/client/errors.js
+++ b/packages/svelte/src/internal/client/errors.js
@@ -261,6 +261,38 @@ export function flush_sync_in_effect() {
 	}
 }
 
+/**
+ * Cannot create a fork inside an effect or when state changes are pending
+ * @returns {never}
+ */
+export function fork_timing() {
+	if (DEV) {
+		const error = new Error(`fork_timing\nCannot create a fork inside an effect or when state changes are pending\nhttps://svelte.dev/e/fork_timing`);
+
+		error.name = 'Svelte error';
+
+		throw error;
+	} else {
+		throw new Error(`https://svelte.dev/e/fork_timing`);
+	}
+}
+
+/**
+ * Cannot commit a fork that was already committed or discarded (including as a result of a different fork being committed)
+ * @returns {never}
+ */
+export function fork_discarded() {
+	if (DEV) {
+		const error = new Error(`fork_discarded\nCannot commit a fork that was already committed or discarded (including as a result of a different fork being committed)\nhttps://svelte.dev/e/fork_discarded`);
+
+		error.name = 'Svelte error';
+
+		throw error;
+	} else {
+		throw new Error(`https://svelte.dev/e/fork_discarded`);
+	}
+}
+
 /**
  * `getAbortSignal()` can only be called inside an effect or derived
  * @returns {never}
@@ -455,20 +487,4 @@ export function svelte_boundary_reset_onerror() {
 	} else {
 		throw new Error(`https://svelte.dev/e/svelte_boundary_reset_onerror`);
 	}
-}
-
-/**
- * Cannot create a fork inside an effect or when state changes are pending
- * @returns {never}
- */
-export function fork_timing() {
-	if (DEV) {
-		const error = new Error(`fork_timing\nCannot create a fork inside an effect or when state changes are pending\nhttps://svelte.dev/e/fork_timing`);
-
-		error.name = 'Svelte error';
-
-		throw error;
-	} else {
-		throw new Error(`https://svelte.dev/e/fork_timing`);
-	}
 }
\ No newline at end of file
diff --git a/packages/svelte/src/internal/client/reactivity/batch.js b/packages/svelte/src/internal/client/reactivity/batch.js
index 17c017728c..35c2d30918 100644
--- a/packages/svelte/src/internal/client/reactivity/batch.js
+++ b/packages/svelte/src/internal/client/reactivity/batch.js
@@ -907,7 +907,7 @@ export function fork(fn) {
 	return {
 		commit: async () => {
 			if (!batches.has(batch)) {
-				throw new Error('Cannot commit this batch'); // TODO better error
+				e.fork_discarded();
 			}
 
 			batch.is_fork = false;