From 98849452b3eeaaede142188fa787cf8d7a300819 Mon Sep 17 00:00:00 2001
From: Elliott Johnson <hello@ell.iott.dev>
Date: Fri, 21 Nov 2025 16:09:19 -0700
Subject: [PATCH] make errors better

---
 .../98-reference/.generated/server-errors.md  | 11 +++++++++
 .../svelte/messages/server-errors/errors.md   |  9 ++++++++
 packages/svelte/src/internal/server/errors.js | 23 +++++++++++++++++++
 .../svelte/src/internal/server/hydratable.js  | 22 +++++++++++++++++-
 4 files changed, 64 insertions(+), 1 deletion(-)

diff --git a/documentation/docs/98-reference/.generated/server-errors.md b/documentation/docs/98-reference/.generated/server-errors.md
index 95bb5fa7ee..6d852a7524 100644
--- a/documentation/docs/98-reference/.generated/server-errors.md
+++ b/documentation/docs/98-reference/.generated/server-errors.md
@@ -41,6 +41,17 @@ This error occurs when using `hydratable` multiple times with the same key. To a
 </script>
 ```
 
+### hydratable_serialization_failed
+
+```
+Failed to serialize `hydratable` data.
+
+`hydratable` can serialize anything [`uneval` from `devalue`](https://npmjs.com/package/uneval) can, plus Promises. 
+
+Stack:
+%stack%
+```
+
 ### lifecycle_function_unavailable
 
 ```
diff --git a/packages/svelte/messages/server-errors/errors.md b/packages/svelte/messages/server-errors/errors.md
index be726b261a..30cf4cd2c2 100644
--- a/packages/svelte/messages/server-errors/errors.md
+++ b/packages/svelte/messages/server-errors/errors.md
@@ -33,6 +33,15 @@ This error occurs when using `hydratable` multiple times with the same key. To a
 </script>
 ```
 
+## hydratable_serialization_failed
+
+> Failed to serialize `hydratable` data.
+>
+> `hydratable` can serialize anything [`uneval` from `devalue`](https://npmjs.com/package/uneval) can, plus Promises. 
+>
+> Stack:
+> %stack%
+
 ## lifecycle_function_unavailable
 
 > `%name%(...)` is not available on the server
diff --git a/packages/svelte/src/internal/server/errors.js b/packages/svelte/src/internal/server/errors.js
index 480862b3c5..aab1659bf1 100644
--- a/packages/svelte/src/internal/server/errors.js
+++ b/packages/svelte/src/internal/server/errors.js
@@ -53,6 +53,29 @@ ${stack2}\nhttps://svelte.dev/e/hydratable_clobbering`);
 	throw error;
 }
 
+/**
+ * Failed to serialize `hydratable` data.
+ * 
+ * `hydratable` can serialize anything [`uneval` from `devalue`](https://npmjs.com/package/uneval) can, plus Promises. 
+ * 
+ * Stack:
+ * %stack%
+ * @param {string} stack
+ * @returns {never}
+ */
+export function hydratable_serialization_failed(stack) {
+	const error = new Error(`hydratable_serialization_failed\nFailed to serialize \`hydratable\` data.
+
+\`hydratable\` can serialize anything [\`uneval\` from \`devalue\`](https://npmjs.com/package/uneval) can, plus Promises. 
+
+Stack:
+${stack}\nhttps://svelte.dev/e/hydratable_serialization_failed`);
+
+	error.name = 'Svelte error';
+
+	throw error;
+}
+
 /**
  * `%name%(...)` is not available on the server
  * @param {string} name
diff --git a/packages/svelte/src/internal/server/hydratable.js b/packages/svelte/src/internal/server/hydratable.js
index 3beb5d50c2..6abc1e2735 100644
--- a/packages/svelte/src/internal/server/hydratable.js
+++ b/packages/svelte/src/internal/server/hydratable.js
@@ -66,7 +66,12 @@ function encode(key, value, values, unresolved) {
 				needs_thunk = false;
 				return result;
 			};
-			const serialize_promise = value.then(scoped_uneval);
+			const serialize_promise = value
+				.then(scoped_uneval)
+				.catch((devalue_error) =>
+					e.hydratable_serialization_failed(serialization_stack(entry.stack, devalue_error?.stack))
+				);
+			serialize_promise.catch(() => {});
 			unresolved?.set(serialize_promise, key);
 			serialize_promise.finally(() => unresolved?.delete(serialize_promise));
 
@@ -123,3 +128,18 @@ async function compare(key, a, b) {
 		);
 	}
 }
+
+/**
+ * @param {string | undefined} root_stack
+ * @param {string | undefined} uneval_stack
+ */
+function serialization_stack(root_stack, uneval_stack) {
+	let out = '';
+	if (root_stack) {
+		out += root_stack + '\n';
+	}
+	if (uneval_stack) {
+		out += 'Caused by:\n' + uneval_stack + '\n';
+	}
+	return out || '<missing stack trace>';
+}