diff --git a/packages/svelte/src/internal/client/dev/tracing.js b/packages/svelte/src/internal/client/dev/tracing.js
index c6edfde933..7ad4ca40af 100644
--- a/packages/svelte/src/internal/client/dev/tracing.js
+++ b/packages/svelte/src/internal/client/dev/tracing.js
@@ -1,4 +1,4 @@
-/** @import { Derived, Reaction, Value } from '#client' */
+/** @import { Derived, Reaction, Source, Value } from '#client' */
 import { UNINITIALIZED } from '../../../constants.js';
 import { snapshot } from '../../shared/clone.js';
 import { DERIVED, ASYNC, PROXY_PATH_SYMBOL, STATE_SYMBOL } from '#client/constants';
@@ -131,8 +131,10 @@ export function trace(label, fn) {
 }
 
 /**
- * @param {Value} source
+ * @template {Value} T
+ * @param {T} source
  * @param {string} label
+ * @returns {T}
  */
 export function tag(source, label) {
 	source.label = label;
diff --git a/packages/svelte/src/internal/client/dom/blocks/boundary.js b/packages/svelte/src/internal/client/dom/blocks/boundary.js
index f5b9ce981d..5afe09adb4 100644
--- a/packages/svelte/src/internal/client/dom/blocks/boundary.js
+++ b/packages/svelte/src/internal/client/dom/blocks/boundary.js
@@ -110,9 +110,6 @@ export class Boundary {
 	/** @type {Set<Derived>} */
 	#dirty_deriveds = new Set();
 
-	/** @type {Set<Derived>} */
-	#maybe_dirty_deriveds = new Set();
-
 	/**
 	 * A source containing the number of pending async deriveds/expressions.
 	 * Only created if `$effect.pending()` is used inside the boundary,
@@ -341,12 +338,7 @@ export class Boundary {
 
 		// any effects that were previously deferred should be transferred
 		// to the batch, which will flush in the next microtask
-		batch.transfer_effects(
-			this.#dirty_effects,
-			this.#maybe_dirty_effects,
-			this.#dirty_deriveds,
-			this.#maybe_dirty_deriveds
-		);
+		batch.transfer_effects(this.#dirty_effects, this.#maybe_dirty_effects, this.#dirty_deriveds);
 	}
 
 	/**
@@ -354,13 +346,7 @@ export class Boundary {
 	 * @param {Effect} effect
 	 */
 	defer_effect(effect) {
-		defer_effect(
-			effect,
-			this.#dirty_effects,
-			this.#maybe_dirty_effects,
-			this.#dirty_deriveds,
-			this.#maybe_dirty_deriveds
-		);
+		defer_effect(effect, this.#dirty_effects, this.#maybe_dirty_effects, this.#dirty_deriveds);
 	}
 
 	/**
diff --git a/packages/svelte/src/internal/client/dom/blocks/each.js b/packages/svelte/src/internal/client/dom/blocks/each.js
index 65d3c9fe24..da2f1adebf 100644
--- a/packages/svelte/src/internal/client/dom/blocks/each.js
+++ b/packages/svelte/src/internal/client/dom/blocks/each.js
@@ -666,7 +666,7 @@ function reconcile(state, array, anchor, flags, get_key) {
  * @param {V} value
  * @param {unknown} key
  * @param {number} index
- * @param {(anchor: Node, item: V | Source<V>, index: number | Value<number>, collection: () => V[]) => void} render_fn
+ * @param {(anchor: Node, item: MaybeSource<V>, index: MaybeSource<number>, collection: () => V[]) => void} render_fn
  * @param {number} flags
  * @param {() => V[]} get_collection
  * @returns {EachItem}
diff --git a/packages/svelte/src/internal/client/reactivity/batch.js b/packages/svelte/src/internal/client/reactivity/batch.js
index cae469ce22..cc1545696e 100644
--- a/packages/svelte/src/internal/client/reactivity/batch.js
+++ b/packages/svelte/src/internal/client/reactivity/batch.js
@@ -827,7 +827,7 @@ export class Batch {
 	 * Tell a fork batch that a source has been updated. Will delete that source from the fork,
 	 * discarding it if it has no other sources left, and rerunning it else with the new value.
 	 * @param {Batch} batch A fork
-	 * @param {Source} source
+	 * @param {Value} source
 	 * @param {boolean} is_derived
 	 * @param {any} value
 	 */
@@ -844,7 +844,7 @@ export class Batch {
 				(!current || current[0] !== value) &&
 				((source.f & ASYNC) === 0 ||
 					!depends_on(
-						source.e,
+						/** @type {Effect} */ (/** @type {Source} */ (source).e),
 						[...batch.current.keys()].filter((s) => !this.current.has(s)),
 						new Map()
 					))
@@ -1283,7 +1283,7 @@ function mark_eager_effects(value, effects) {
 
 /**
  * @param {Reaction} reaction
- * @param {Source[]} sources
+ * @param {Value[]} sources
  * @param {Map<Reaction, boolean>} checked
  */
 function depends_on(reaction, sources, checked) {
diff --git a/packages/svelte/src/internal/client/reactivity/deriveds.js b/packages/svelte/src/internal/client/reactivity/deriveds.js
index 585151e02e..01bec8518c 100644
--- a/packages/svelte/src/internal/client/reactivity/deriveds.js
+++ b/packages/svelte/src/internal/client/reactivity/deriveds.js
@@ -139,7 +139,7 @@ export function async_derived(fn, label, location) {
 
 		if (DEV) {
 			reactivity_loss_tracker = { effect, effect_deps: new Set(), warned: false };
-			effect.label ??= label ?? fn.toString();
+			// effect.label ??= label ?? fn.toString(); // TODO add dev time labeling for effects in follow-up PR
 		}
 
 		/** @type {ReturnType<typeof deferred<V>>} */
diff --git a/packages/svelte/src/internal/client/reactivity/sources.js b/packages/svelte/src/internal/client/reactivity/sources.js
index 8230edc114..746aed25ae 100644
--- a/packages/svelte/src/internal/client/reactivity/sources.js
+++ b/packages/svelte/src/internal/client/reactivity/sources.js
@@ -47,7 +47,7 @@ import { set_signal_status, update_derived_status } from './status.js';
 /** @type {Set<Effect>} */
 export let eager_effects = new Set();
 
-/** @type {Map<Source, any>} */
+/** @type {Map<Value, any>} */
 export const old_values = new Map();
 
 /**
@@ -71,14 +71,15 @@ export function set_eager_effects_deferred() {
  */
 // TODO rename this to `state` throughout the codebase
 export function source(v, stack) {
-	/** @type {Value} */
+	/** @type {Source} */
 	var signal = {
-		f: 0, // TODO ideally we could skip this altogether, but it causes type errors
+		f: 0,
 		v,
 		reactions: null,
 		equals,
 		rv: 0,
-		wv: 0
+		wv: 0,
+		e: null
 	};
 
 	if (DEV && tracing_mode_flag) {
@@ -142,7 +143,7 @@ export function mutate(source, value) {
 
 /**
  * @template V
- * @param {Source<V>} source
+ * @param {Value<V>} source
  * @param {V} value
  * @param {boolean} [should_proxy]
  * @returns {V}
@@ -182,7 +183,7 @@ var count_deps = 0;
 
 /**
  * @template V
- * @param {Source<V>} source
+ * @param {Value<V>} source
  * @param {V} value
  * @param {Effect[] | null} [updated_during_traversal]
  * @returns {V}
diff --git a/packages/svelte/src/internal/client/reactivity/types.d.ts b/packages/svelte/src/internal/client/reactivity/types.d.ts
index 0ee8570c3d..8e9b4e7624 100644
--- a/packages/svelte/src/internal/client/reactivity/types.d.ts
+++ b/packages/svelte/src/internal/client/reactivity/types.d.ts
@@ -100,7 +100,10 @@ export interface Effect extends Reaction {
 	dev_stack?: DevStackEntry | null;
 }
 
-export type Source<V = unknown> = Value<V>;
+export interface Source<V = unknown> extends Value<V> {
+	/** Only set for ASYNC signals - the corresponding effect that writes to this source */
+	e: Effect | null;
+}
 
 export type MaybeSource<T = unknown> = T | Source<T>;
 
diff --git a/packages/svelte/src/internal/client/runtime.js b/packages/svelte/src/internal/client/runtime.js
index 563b0e692b..72a2621b05 100644
--- a/packages/svelte/src/internal/client/runtime.js
+++ b/packages/svelte/src/internal/client/runtime.js
@@ -96,9 +96,9 @@ export function set_active_effect(effect) {
 }
 
 /**
- * When sources are created within a reaction, reading and writing
+ * When sources/deriveds are created within a reaction, reading and writing
  * them within that reaction should not cause a re-run
- * @type {null | Set<Source>}
+ * @type {null | Set<Value>}
  */
 export let current_sources = null;
 
@@ -126,11 +126,11 @@ export let skipped_deps = 0;
 /**
  * Tracks writes that the effect it's executed in doesn't listen to yet,
  * so that the dependency can be added to the effect later on if it then reads it
- * @type {null | Source[]}
+ * @type {null | Value[]}
  */
 export let untracked_writes = null;
 
-/** @param {null | Source[]} value */
+/** @param {null | Value[]} value */
 export function set_untracked_writes(value) {
 	untracked_writes = value;
 }
diff --git a/packages/svelte/src/reactivity/date.js b/packages/svelte/src/reactivity/date.js
index f882c05d76..b70ba7969a 100644
--- a/packages/svelte/src/reactivity/date.js
+++ b/packages/svelte/src/reactivity/date.js
@@ -1,4 +1,4 @@
-/** @import { Source } from '#client' */
+/** @import { Derived } from '#client' */
 import { derived } from '../internal/client/index.js';
 import { set, state } from '../internal/client/reactivity/sources.js';
 import { tag } from '../internal/client/dev/tracing.js';
@@ -42,7 +42,7 @@ var inited = false;
 export class SvelteDate extends Date {
 	#time = state(super.getTime());
 
-	/** @type {Map<keyof Date, Source<unknown>>} */
+	/** @type {Map<keyof Date, Derived<unknown>>} */
 	#deriveds = new Map();
 
 	#reaction = active_reaction;