diff --git a/documentation/docs/98-reference/.generated/server-warnings.md b/documentation/docs/98-reference/.generated/server-warnings.md
index 70896e1694..c01f1bfd6e 100644
--- a/documentation/docs/98-reference/.generated/server-warnings.md
+++ b/documentation/docs/98-reference/.generated/server-warnings.md
@@ -1,12 +1,15 @@
 <!-- This file is generated by scripts/process-messages/index.js. Do not edit! -->
 
-### unused_hydratable
+### unresolved_hydratable
 
 ```
-A `hydratable` value with key `%key%` was created, but not used during the render.
+A `hydratable` value with key `%key%` was created, but at least part of it was not used during the render.
 
-Stack:
+The `hydratable` was initialized in:
 %stack%
+
+The unresolved data is:
+%unresolved_data%
 ```
 
 The most likely cause of this is creating a `hydratable` in the `script` block of your component and then `await`ing
@@ -30,3 +33,5 @@ the result inside a `svelte:boundary` with a `pending` snippet:
 ```
 
 Consider inlining the `hydratable` call inside the boundary so that it's not called on the server.
+
+Note that this can also happen when a `hydratable` contains multiple promises and some but not all of them have been used.
diff --git a/packages/svelte/messages/server-warnings/warnings.md b/packages/svelte/messages/server-warnings/warnings.md
index 461595ebe9..ec4e61fe46 100644
--- a/packages/svelte/messages/server-warnings/warnings.md
+++ b/packages/svelte/messages/server-warnings/warnings.md
@@ -1,9 +1,12 @@
-## unused_hydratable
+## unresolved_hydratable
 
-> A `hydratable` value with key `%key%` was created, but not used during the render.
+> A `hydratable` value with key `%key%` was created, but at least part of it was not used during the render.
 >
-> Stack:
+> The `hydratable` was initialized in:
 > %stack%
+>
+> The unresolved data is:
+> %unresolved_data%
 
 The most likely cause of this is creating a `hydratable` in the `script` block of your component and then `await`ing
 the result inside a `svelte:boundary` with a `pending` snippet:
@@ -26,3 +29,5 @@ the result inside a `svelte:boundary` with a `pending` snippet:
 ```
 
 Consider inlining the `hydratable` call inside the boundary so that it's not called on the server.
+
+Note that this can also happen when a `hydratable` contains multiple promises and some but not all of them have been used.
diff --git a/packages/svelte/src/internal/client/dev/tracing.js b/packages/svelte/src/internal/client/dev/tracing.js
index 4688637f5d..bd11138f66 100644
--- a/packages/svelte/src/internal/client/dev/tracing.js
+++ b/packages/svelte/src/internal/client/dev/tracing.js
@@ -5,6 +5,7 @@ import { define_property } from '../../shared/utils.js';
 import { DERIVED, ASYNC, PROXY_PATH_SYMBOL, STATE_SYMBOL } from '#client/constants';
 import { effect_tracking } from '../reactivity/effects.js';
 import { active_reaction, untrack } from '../runtime.js';
+import { get_infinite_stack } from '../../shared/dev.js';
 
 /**
  * @typedef {{
@@ -134,55 +135,37 @@ export function trace(label, fn) {
  * @returns {Error & { stack: string } | null}
  */
 export function get_stack(label) {
-	// @ts-ignore stackTraceLimit doesn't exist everywhere
-	const limit = Error.stackTraceLimit;
+	return get_infinite_stack(label, (stack) => {
+		if (!stack) return;
 
-	// @ts-ignore
-	Error.stackTraceLimit = Infinity;
-	let error = Error();
+		const lines = stack.split('\n');
+		const new_lines = ['\n'];
 
-	// @ts-ignore
-	Error.stackTraceLimit = limit;
+		for (let i = 0; i < lines.length; i++) {
+			const line = lines[i];
+			const posixified = line.replaceAll('\\', '/');
 
-	const stack = error.stack;
+			if (line === 'Error') {
+				continue;
+			}
 
-	if (!stack) return null;
+			if (line.includes('validate_each_keys')) {
+				return undefined;
+			}
 
-	const lines = stack.split('\n');
-	const new_lines = ['\n'];
+			if (posixified.includes('svelte/src/internal') || posixified.includes('node_modules/.vite')) {
+				continue;
+			}
 
-	for (let i = 0; i < lines.length; i++) {
-		const line = lines[i];
-		const posixified = line.replaceAll('\\', '/');
-
-		if (line === 'Error') {
-			continue;
-		}
-
-		if (line.includes('validate_each_keys')) {
-			return null;
+			new_lines.push(line);
 		}
 
-		if (posixified.includes('svelte/src/internal') || posixified.includes('node_modules/.vite')) {
-			continue;
+		if (new_lines.length === 1) {
+			return undefined;
 		}
 
-		new_lines.push(line);
-	}
-
-	if (new_lines.length === 1) {
-		return null;
-	}
-
-	define_property(error, 'stack', {
-		value: new_lines.join('\n')
+		return new_lines.join('\n');
 	});
-
-	define_property(error, 'name', {
-		value: label
-	});
-
-	return /** @type {Error & { stack: string }} */ (error);
 }
 
 /**
diff --git a/packages/svelte/src/internal/client/hydratable.js b/packages/svelte/src/internal/client/hydratable.js
index f5e1ed689b..2c88f453e2 100644
--- a/packages/svelte/src/internal/client/hydratable.js
+++ b/packages/svelte/src/internal/client/hydratable.js
@@ -29,8 +29,7 @@ export function hydratable(key, fn) {
 		return fn();
 	}
 
-	const val = /** @type {() => T} */ (store.get(key));
-	return val;
+	return /** @type {T} */ (store.get(key));
 }
 
 /** @param {string} key */
diff --git a/packages/svelte/src/internal/client/types.d.ts b/packages/svelte/src/internal/client/types.d.ts
index 42db18fd1a..33b45cf98a 100644
--- a/packages/svelte/src/internal/client/types.d.ts
+++ b/packages/svelte/src/internal/client/types.d.ts
@@ -6,7 +6,7 @@ declare global {
 	interface Window {
 		__svelte?: {
 			/** hydratables */
-			h?: Map<string, () => unknown>;
+			h?: Map<string, unknown>;
 			/** unused hydratable keys */
 			uh?: Set<string>;
 		};
diff --git a/packages/svelte/src/internal/server/dev.js b/packages/svelte/src/internal/server/dev.js
index 1211670f94..60c7bd9f46 100644
--- a/packages/svelte/src/internal/server/dev.js
+++ b/packages/svelte/src/internal/server/dev.js
@@ -4,6 +4,7 @@ import {
 	is_tag_valid_with_ancestor,
 	is_tag_valid_with_parent
 } from '../../html-tree-validation.js';
+import { get_infinite_stack } from '../shared/dev.js';
 import { set_ssr_context, ssr_context } from './context.js';
 import * as e from './errors.js';
 import { Renderer } from './renderer.js';
@@ -98,3 +99,36 @@ export function validate_snippet_args(renderer) {
 		e.invalid_snippet_arguments();
 	}
 }
+
+/**
+ * @param {string} label
+ * @returns {Error & { stack: string } | null}
+ */
+export function get_stack(label) {
+	return get_infinite_stack(label, (stack) => {
+		if (!stack) return;
+
+		const lines = stack.split('\n');
+		const new_lines = [];
+
+		for (const line of lines) {
+			const posixified = line.replaceAll('\\', '/');
+
+			if (line.startsWith('Error:')) {
+				continue;
+			}
+
+			if (posixified.includes('svelte/src/internal') || posixified.includes('node_modules/.vite')) {
+				continue;
+			}
+
+			new_lines.push(line);
+		}
+
+		if (new_lines.length === 1) {
+			return undefined;
+		}
+
+		return new_lines.join('\n');
+	});
+}
diff --git a/packages/svelte/src/internal/server/hydratable.js b/packages/svelte/src/internal/server/hydratable.js
index a40f551619..1d5aa73deb 100644
--- a/packages/svelte/src/internal/server/hydratable.js
+++ b/packages/svelte/src/internal/server/hydratable.js
@@ -1,8 +1,10 @@
-/** @import { HydratableContext } from '#server' */
+/** @import { HydratableContext, HydratableLookupEntry } from '#server' */
 import { async_mode_flag } from '../flags/index.js';
 import { get_render_context } from './render-context.js';
 import * as e from './errors.js';
 import { uneval } from 'devalue';
+import { get_stack } from './dev.js';
+import { DEV } from 'esm-env';
 
 /**
  * @template T
@@ -17,16 +19,23 @@ export function hydratable(key, fn) {
 
 	const store = get_render_context();
 
-	const entry = store.hydratable.lookup.get(key);
-	if (entry !== undefined) {
-		return /** @type {T} */ (entry.value);
+	const existing_entry = store.hydratable.lookup.get(key);
+	if (existing_entry !== undefined) {
+		return /** @type {T} */ (existing_entry.value);
 	}
 
 	const result = fn();
-	store.hydratable.lookup.set(key, {
+	/** @type {HydratableLookupEntry} */
+	const entry = {
 		value: result,
 		root_index: encode(result, key, store.hydratable)
-	});
+	};
+
+	if (DEV) {
+		entry.stack = get_stack(`hydratable"`)?.stack;
+	}
+
+	store.hydratable.lookup.set(key, entry);
 
 	return result;
 }
@@ -50,15 +59,17 @@ function encode(value, key, hydratable_context) {
 function create_replacer(key, hydratable_context) {
 	/**
 	 * @param {unknown} value
-	 * @param {(value: any) => string} inner_uneval
 	 */
-	const replacer = (value, inner_uneval) => {
+	const replacer = (value) => {
 		if (value instanceof Promise) {
-			hydratable_context.unresolved_promises.set(value, key);
-			value.finally(() => hydratable_context.unresolved_promises.delete(value));
 			// use the root-level uneval because we need a separate, top-level entry for each promise
-			const index =
-				hydratable_context.values.push(value.then((v) => `r(${uneval(v, replacer)})`)) - 1;
+			/** @type {Promise<string>} */
+			const serialize_promise = value.then((v) => `r(${uneval(v, replacer)})`);
+			hydratable_context.unresolved_promises.set(serialize_promise, key);
+			serialize_promise.finally(() =>
+				hydratable_context.unresolved_promises.delete(serialize_promise)
+			);
+			const index = hydratable_context.values.push(serialize_promise) - 1;
 			return `d(${index})`;
 		}
 	};
diff --git a/packages/svelte/src/internal/server/renderer.js b/packages/svelte/src/internal/server/renderer.js
index e701c069e8..1b5c5da9cc 100644
--- a/packages/svelte/src/internal/server/renderer.js
+++ b/packages/svelte/src/internal/server/renderer.js
@@ -8,9 +8,9 @@ import * as e from './errors.js';
 import * as w from './warnings.js';
 import { BLOCK_CLOSE, BLOCK_OPEN } from './hydration.js';
 import { attributes } from './index.js';
-import { uneval } from 'devalue';
 import { get_render_context, with_render_context, init_render_context } from './render-context.js';
 import { DEV } from 'esm-env';
+import { get_stack } from './dev.js';
 
 /** @typedef {'head' | 'body'} RendererType */
 /** @typedef {{ [key in RendererType]: string }} AccumulatedContent */
@@ -578,32 +578,16 @@ export class Renderer {
 	async #collect_hydratables() {
 		const ctx = get_render_context().hydratable;
 
-		// for (const [k, v] of ctx.lookup) {
-		// 	// TODO - root-level
-		// 	// if (ctx.unresolved_promises.has(/** @type {Promise<unknown>} */ (v.value))) {
-		// 	// 	// this is a problem -- it means we've finished the render but somehow not consumed a hydratable, which means we've done
-		// 	// 	// extra work that will get serialized and sent but then not used on the client
-		// 	// 	w.unused_hydratable(k, v.stack ?? 'unavailable');
-		// 	// 	unused_keys.push(k);
-		// 	// 	continue;
-		// 	// }
-
-		// 	// TODO - nested
-
-		// 	// const encoded = encode(await v.value);
-		// 	// if (DEV && v.dev_competing_entries?.length) {
-		// 	// 	for (const competing_entry of v.dev_competing_entries) {
-		// 	// 		const competing_encoded = (competing_entry.encode ?? uneval)(await competing_entry.value);
-		// 	// 		if (encoded !== competing_encoded) {
-		// 	// 			e.hydratable_clobbering(
-		// 	// 				k,
-		// 	// 				v.stack ?? 'unavailable',
-		// 	// 				competing_entry.stack ?? 'unavailable'
-		// 	// 			);
-		// 	// 		}
-		// 	// 	}
-		// 	// }
-		// }
+		for (const [promise, key] of ctx.unresolved_promises) {
+			// this is a problem -- it means we've finished the render but we're still waiting on a promise to resolve so we can
+			// serialize it, so we're blocking the response on useless content.
+			w.unresolved_hydratable(
+				key,
+				DEV ? ctx.lookup.get(key)?.stack ?? 'unavailable' : 'unavailable in production builds',
+				await promise
+			);
+		}
+
 		return await Renderer.#hydratable_block(ctx, []);
 	}
 
diff --git a/packages/svelte/src/internal/server/types.d.ts b/packages/svelte/src/internal/server/types.d.ts
index 73b2c45182..975de28beb 100644
--- a/packages/svelte/src/internal/server/types.d.ts
+++ b/packages/svelte/src/internal/server/types.d.ts
@@ -18,12 +18,13 @@ export interface SSRContext {
 export interface HydratableLookupEntry {
 	value: unknown;
 	root_index: number;
+	stack?: string;
 }
 
 export interface HydratableContext {
 	lookup: Map<string, HydratableLookupEntry>;
 	values: MaybePromise<string>[];
-	unresolved_promises: Map<Promise<unknown>, string>;
+	unresolved_promises: Map<Promise<string>, string>;
 }
 
 export interface RenderContext {
diff --git a/packages/svelte/src/internal/server/warnings.js b/packages/svelte/src/internal/server/warnings.js
index 5dd1bbcf14..2f9ec3da49 100644
--- a/packages/svelte/src/internal/server/warnings.js
+++ b/packages/svelte/src/internal/server/warnings.js
@@ -6,24 +6,31 @@ var bold = 'font-weight: bold';
 var normal = 'font-weight: normal';
 
 /**
- * A `hydratable` value with key `%key%` was created, but not used during the render.
+ * A `hydratable` value with key `%key%` was created, but at least part of it was not used during the render.
  * 
- * Stack:
+ * The `hydratable` was initialized in:
  * %stack%
+ * 
+ * The unresolved data is:
+ * %unresolved_data%
  * @param {string} key
  * @param {string} stack
+ * @param {string} unresolved_data
  */
-export function unused_hydratable(key, stack) {
+export function unresolved_hydratable(key, stack, unresolved_data) {
 	if (DEV) {
 		console.warn(
-			`%c[svelte] unused_hydratable\n%cA \`hydratable\` value with key \`${key}\` was created, but not used during the render.
+			`%c[svelte] unresolved_hydratable\n%cA \`hydratable\` value with key \`${key}\` was created, but at least part of it was not used during the render.
+
+The \`hydratable\` was initialized in:
+${stack}
 
-Stack:
-${stack}\nhttps://svelte.dev/e/unused_hydratable`,
+The unresolved data is:
+${unresolved_data}\nhttps://svelte.dev/e/unresolved_hydratable`,
 			bold,
 			normal
 		);
 	} else {
-		console.warn(`https://svelte.dev/e/unused_hydratable`);
+		console.warn(`https://svelte.dev/e/unresolved_hydratable`);
 	}
 }
\ No newline at end of file
diff --git a/packages/svelte/src/internal/shared/dev.js b/packages/svelte/src/internal/shared/dev.js
new file mode 100644
index 0000000000..3c7a3165b2
--- /dev/null
+++ b/packages/svelte/src/internal/shared/dev.js
@@ -0,0 +1,26 @@
+import { define_property } from './utils';
+
+/**
+ * @param {string} label
+ * @param {(stack: string | undefined) => string | undefined} fn
+ * @returns {Error & { stack: string } | null}
+ */
+export function get_infinite_stack(label, fn) {
+	const limit = Error.stackTraceLimit;
+	Error.stackTraceLimit = Infinity;
+	let error = Error();
+	Error.stackTraceLimit = limit;
+	const stack = fn(error.stack);
+
+	if (!stack) return null;
+
+	define_property(error, 'stack', {
+		value: stack
+	});
+
+	define_property(error, 'name', {
+		value: label
+	});
+
+	return /** @type {Error & { stack: string }} */ (error);
+}
diff --git a/packages/svelte/tests/runtime-legacy/shared.ts b/packages/svelte/tests/runtime-legacy/shared.ts
index 19483f2e1d..c5529747e4 100644
--- a/packages/svelte/tests/runtime-legacy/shared.ts
+++ b/packages/svelte/tests/runtime-legacy/shared.ts
@@ -86,6 +86,7 @@ export interface RuntimeTest<Props extends Record<string, any> = Record<string,
 	}) => void | Promise<void>;
 	test_ssr?: (args: {
 		logs: any[];
+		warnings: any[];
 		assert: Assert;
 		variant: 'ssr' | 'async-ssr';
 	}) => void | Promise<void>;
@@ -105,7 +106,7 @@ export interface RuntimeTest<Props extends Record<string, any> = Record<string,
 declare global {
 	var __svelte:
 		| {
-				h?: Map<string, () => unknown>;
+				h?: Map<string, unknown>;
 				uh?: Set<string>;
 		  }
 		| undefined;
@@ -266,7 +267,16 @@ async function run_test_variant(
 			i++;
 		}
 
-		if (str.slice(0, i).includes('logs')) {
+		let ssr_str = config.test_ssr?.toString() ?? '';
+		let sn = 0;
+		let si = 0;
+		while (si < ssr_str.length) {
+			if (ssr_str[si] === '(') sn++;
+			if (ssr_str[si] === ')' && --sn === 0) break;
+			si++;
+		}
+
+		if (str.slice(0, i).includes('logs') || ssr_str.slice(0, si).includes('logs')) {
 			// eslint-disable-next-line no-console
 			console.log = (...args) => {
 				logs.push(...args);
@@ -277,7 +287,11 @@ async function run_test_variant(
 			manual_hydrate = true;
 		}
 
-		if (str.slice(0, i).includes('warnings') || config.warnings) {
+		if (
+			str.slice(0, i).includes('warnings') ||
+			config.warnings ||
+			ssr_str.slice(0, si).includes('warnings')
+		) {
 			// eslint-disable-next-line no-console
 			console.warn = (...args) => {
 				if (typeof args[0] === 'string' && args[0].startsWith('%c[svelte]')) {
@@ -397,6 +411,7 @@ async function run_test_variant(
 			if (config.test_ssr) {
 				await config.test_ssr({
 					logs,
+					warnings,
 					// @ts-expect-error
 					assert: {
 						...assert,
diff --git a/packages/svelte/tests/runtime-runes/samples/hydratable-custom-transport/_config.js b/packages/svelte/tests/runtime-runes/samples/hydratable-custom-transport/_config.js
deleted file mode 100644
index 57904ef576..0000000000
--- a/packages/svelte/tests/runtime-runes/samples/hydratable-custom-transport/_config.js
+++ /dev/null
@@ -1,21 +0,0 @@
-import { ok, test } from '../../test';
-
-export default test({
-	skip_no_async: true,
-	skip_mode: ['server'],
-
-	server_props: { environment: 'server' },
-	ssrHtml: '<p>The current environment is: server</p>',
-
-	props: { environment: 'browser' },
-
-	async test({ assert, target, variant }) {
-		const p = target.querySelector('p');
-		ok(p);
-		if (variant === 'hydrate') {
-			assert.htmlEqual(p.outerHTML, '<p>The current environment is: server</p>');
-		} else {
-			assert.htmlEqual(p.outerHTML, '<p>The current environment is: browser</p>');
-		}
-	}
-});
diff --git a/packages/svelte/tests/runtime-runes/samples/hydratable-custom-transport/main.svelte b/packages/svelte/tests/runtime-runes/samples/hydratable-custom-transport/main.svelte
deleted file mode 100644
index 18b7f83467..0000000000
--- a/packages/svelte/tests/runtime-runes/samples/hydratable-custom-transport/main.svelte
+++ /dev/null
@@ -1,15 +0,0 @@
-<script lang="ts">
-	import { hydratable } from "svelte";
-
-	let { environment }: { environment: 'server' | 'browser' } = $props();
-
-	const value = hydratable("environment", () => environment, {
-		transport: environment === 'server' ? {
-			encode: (val: string) => JSON.stringify([val])
-		} : {
-			decode: (val: [string]) => val[0]
-		}
-	})
-</script>
-
-<p>The current environment is: {value}</p>
diff --git a/packages/svelte/tests/runtime-runes/samples/hydratable-error-on-missing/_config.js b/packages/svelte/tests/runtime-runes/samples/hydratable-error-on-missing/_config.js
index b04d81d639..3349cbcb66 100644
--- a/packages/svelte/tests/runtime-runes/samples/hydratable-error-on-missing/_config.js
+++ b/packages/svelte/tests/runtime-runes/samples/hydratable-error-on-missing/_config.js
@@ -9,5 +9,5 @@ export default test({
 
 	props: { environment: 'browser' },
 
-	runtime_error: 'hydratable_missing_but_expected_e'
+	runtime_error: 'hydratable_missing_but_required'
 });
diff --git a/packages/svelte/tests/runtime-runes/samples/hydratable-unused-keys/_config.js b/packages/svelte/tests/runtime-runes/samples/hydratable-unused-keys/_config.js
index 2b0239893c..0f4acc600b 100644
--- a/packages/svelte/tests/runtime-runes/samples/hydratable-unused-keys/_config.js
+++ b/packages/svelte/tests/runtime-runes/samples/hydratable-unused-keys/_config.js
@@ -1,5 +1,5 @@
 import { tick } from 'svelte';
-import { ok, test } from '../../test';
+import { test } from '../../test';
 
 export default test({
 	skip_no_async: true,
@@ -8,6 +8,12 @@ export default test({
 	server_props: { environment: 'server' },
 	ssrHtml: '<div>Loading...</div>',
 
+	test_ssr({ assert, warnings }) {
+		assert.strictEqual(warnings.length, 1);
+		// for some strange reason we trim the error code off the beginning of warnings so I can't actually assert it
+		assert.include(warnings[0], 'A `hydratable` value with key `unused_key`');
+	},
+
 	async test({ assert, target }) {
 		// let it hydrate and resolve the promise on the client
 		await tick();
diff --git a/packages/svelte/tests/runtime-runes/samples/hydratable-unused-keys/main.svelte b/packages/svelte/tests/runtime-runes/samples/hydratable-unused-keys/main.svelte
index 4ab4801ddf..67848e7f6f 100644
--- a/packages/svelte/tests/runtime-runes/samples/hydratable-unused-keys/main.svelte
+++ b/packages/svelte/tests/runtime-runes/samples/hydratable-unused-keys/main.svelte
@@ -6,7 +6,7 @@
 	const unresolved_hydratable = hydratable(
 		"unused_key",
 		() => new Promise(
-			(res) => environment === 'server' ? undefined : res('did you ever hear the tragedy of darth plagueis the wise?')
+			(res, rej) => environment === 'server' ? setTimeout(() => res('did you ever hear the tragedy of darth plagueis the wise?'), 0) : rej('should not run')
 		)
 	);
 </script>