diff --git a/__snapshots__/packages/language-server/test-out/util/toLS.spec.js b/__snapshots__/packages/language-server/test-out/util/toLS.spec.js
index 856d1bc5a..e5cd1e502 100644
--- a/__snapshots__/packages/language-server/test-out/util/toLS.spec.js
+++ b/__snapshots__/packages/language-server/test-out/util/toLS.spec.js
@@ -1,4 +1,150 @@
-exports['semanticTokens Tokenize "foo" with multiline token support 1'] = [
+exports['toLS completionItem Should map correctly when cursor is in first line 1'] = {
+  "label": "advancement",
+  "textEdit": {
+    "newText": "advancement",
+    "insert": {
+      "start": {
+        "line": 0,
+        "character": 0
+      },
+      "end": {
+        "line": 0,
+        "character": 1
+      }
+    },
+    "replace": {
+      "start": {
+        "line": 0,
+        "character": 0
+      },
+      "end": {
+        "line": 0,
+        "character": 4
+      }
+    }
+  },
+  "insertTextFormat": 2,
+  "insertTextMode": 2,
+  "additionalTextEdits": [
+    {
+      "range": {
+        "start": {
+          "line": 0,
+          "character": 4
+        },
+        "end": {
+          "line": 2,
+          "character": 2
+        }
+      },
+      "newText": ""
+    }
+  ]
+}
+
+exports['toLS completionItem Should map correctly when cursor is in second line 1'] = {
+  "label": "advancement",
+  "filterText": "an\\",
+  "textEdit": {
+    "newText": "advancement",
+    "insert": {
+      "start": {
+        "line": 1,
+        "character": 0
+      },
+      "end": {
+        "line": 1,
+        "character": 1
+      }
+    },
+    "replace": {
+      "start": {
+        "line": 1,
+        "character": 0
+      },
+      "end": {
+        "line": 1,
+        "character": 3
+      }
+    }
+  },
+  "insertTextFormat": 2,
+  "insertTextMode": 2,
+  "additionalTextEdits": [
+    {
+      "range": {
+        "start": {
+          "line": 0,
+          "character": 0
+        },
+        "end": {
+          "line": 1,
+          "character": 0
+        }
+      },
+      "newText": ""
+    },
+    {
+      "range": {
+        "start": {
+          "line": 1,
+          "character": 3
+        },
+        "end": {
+          "line": 2,
+          "character": 2
+        }
+      },
+      "newText": ""
+    }
+  ]
+}
+
+exports['toLS completionItem Should map correctly when cursor is in third line 1'] = {
+  "label": "advancement",
+  "filterText": "ce",
+  "textEdit": {
+    "newText": "advancement",
+    "insert": {
+      "start": {
+        "line": 2,
+        "character": 0
+      },
+      "end": {
+        "line": 2,
+        "character": 1
+      }
+    },
+    "replace": {
+      "start": {
+        "line": 2,
+        "character": 0
+      },
+      "end": {
+        "line": 2,
+        "character": 2
+      }
+    }
+  },
+  "insertTextFormat": 2,
+  "insertTextMode": 2,
+  "additionalTextEdits": [
+    {
+      "range": {
+        "start": {
+          "line": 0,
+          "character": 0
+        },
+        "end": {
+          "line": 2,
+          "character": 0
+        }
+      },
+      "newText": ""
+    }
+  ]
+}
+exports['toLS semanticTokens Tokenize "foo" with multiline token support 1'] = [
   {
     "deltaLine": 0,
     "deltaStartChar": 0,
@@ -8,7 +154,7 @@ exports['semanticTokens Tokenize "foo" with multiline token support 1'] = [
   }
 ]
 
-exports['semanticTokens Tokenize "foo" without multiline token support 1'] = [
+exports['toLS semanticTokens Tokenize "foo" without multiline token support 1'] = [
   {
     "deltaLine": 0,
     "deltaStartChar": 0,
@@ -18,7 +164,7 @@ exports['semanticTokens Tokenize "foo" without multiline token support 1'] = [
   }
 ]
 
-exports['semanticTokens Tokenize "foo↓bar" with multiline token support 1'] = [
+exports['toLS semanticTokens Tokenize "foo↓bar" with multiline token support 1'] = [
   {
     "deltaLine": 0,
     "deltaStartChar": 0,
@@ -28,7 +174,7 @@ exports['semanticTokens Tokenize "foo↓bar" with multiline token support 1'] =
   }
 ]
 
-exports['semanticTokens Tokenize "foo↓bar" without multiline token support 1'] = [
+exports['toLS semanticTokens Tokenize "foo↓bar" without multiline token support 1'] = [
   {
     "deltaLine": 0,
     "deltaStartChar": 0,
@@ -45,7 +191,7 @@ exports['semanticTokens Tokenize "foo↓bar" without multiline token support 1']
   }
 ]
 
-exports['semanticTokens Tokenize "foo↓bar↓qux" with multiline token support 1'] = [
+exports['toLS semanticTokens Tokenize "foo↓bar↓qux" with multiline token support 1'] = [
   {
     "deltaLine": 0,
     "deltaStartChar": 0,
@@ -55,7 +201,7 @@ exports['semanticTokens Tokenize "foo↓bar↓qux" with multiline token support
   }
 ]
 
-exports['semanticTokens Tokenize "foo↓bar↓qux" without multiline token support 1'] = [
+exports['toLS semanticTokens Tokenize "foo↓bar↓qux" without multiline token support 1'] = [
   {
     "deltaLine": 0,
     "deltaStartChar": 0,
diff --git a/packages/language-server/src/server.ts b/packages/language-server/src/server.ts
index 23b78516d..1cd194a79 100644
--- a/packages/language-server/src/server.ts
+++ b/packages/language-server/src/server.ts
@@ -31,6 +31,7 @@ let capabilities!: ls.ClientCapabilities
 let workspaceFolders!: ls.WorkspaceFolder[]
 let hasShutdown = false
 let progressReporter: ls.WorkDoneProgressReporter | undefined
+let initializationOptions: CustomInitializationOptions | undefined
 
 const externals = NodeJsExternals
 const logger: core.Logger = {
@@ -46,7 +47,7 @@ const logger: core.Logger = {
 let service!: core.Service
 
 connection.onInitialize(async (params) => {
-	const initializationOptions = params.initializationOptions as
+	initializationOptions = params.initializationOptions as
 		| CustomInitializationOptions
 		| undefined
 
@@ -261,6 +262,7 @@ connection.onCompletion(
 				offset,
 				capabilities.textDocument?.completion?.completionItem
 					?.insertReplaceSupport,
+				initializationOptions?.hasFlawedMultiLineCompletionItemFiltering,
 			)
 		)
 	},
diff --git a/packages/language-server/src/util/toLS.ts b/packages/language-server/src/util/toLS.ts
index f083d4a99..7b4d879b4 100644
--- a/packages/language-server/src/util/toLS.ts
+++ b/packages/language-server/src/util/toLS.ts
@@ -216,30 +216,133 @@ export function completionItem(
 	doc: TextDocument,
 	requestedOffset: number,
 	insertReplaceSupport: boolean | undefined,
+	hasFlawedMultiLineSupport: boolean | undefined,
 ): ls.CompletionItem {
 	const insertText = completion.insertText ?? completion.label
-	const canInsertReplace = insertReplaceSupport &&
-		![core.CR, core.LF, core.CRLF].includes(insertText)
-	const textEdit: ls.TextEdit | ls.InsertReplaceEdit = canInsertReplace
+
+	/**
+	 * When VS Code receives a list of `CompletionItem`s, it filters the items
+	 * by checking if they start with a prefix. The prefix is the content between
+	 * the start of the item's ranges (which is required to be the same position
+	 * for insert range and replace range) and the cursor. It additionally
+	 * imposes the restrictions that (1) the prefix must be within one line and
+	 * (2) the prefix range must contain the cursor.
+	 * (See also VS Code's documentation for [vscode.CompletionItem#range](https://github.com/microsoft/vscode/blob/47b1183dfda519c32d78b591cfa721d3b0b874ae/src/vs/vscode.d.ts#L3803))
+	 *
+	 * Spyglass can provide `CompletionItem`s that have `textEdit` ranges
+	 * spanning multiple lines in cases such as a mcfunction literal with a
+	 * backslash and newline in itself. Such ranges cannot be used as the prefix
+	 * by VS Code and will cause no completions getting displayed to the user.
+	 *
+	 * To fix the problem, this function breaks down the actual range of the
+	 * `CompletionItem` into a `leadingRange` (that may be `undefined` or
+	 * multi-line), a `filterRange` (that is for VS Code's prefix matching
+	 * filtering, is single-line and contains the cursor), and a `trailingRange`
+	 * (that may be `undefined` or multi-line). The `filterRange` will be used as
+	 * the ranges in the main `textEdit` property of the `CompletionItem`, where
+	 * the `insertText` will be edited into, and the `leadingRange` and
+	 * `trailingRange` will be used in `additionalTextEdits` to delete their
+	 * contents. This will result in the same net edit as a single `TextEdit`
+	 * that replaces the content of the actual range with `insertText`.
+	 */
+	function breakDownRange(): {
+		leadingRange?: ls.Range | undefined
+		filterRange: ls.Range
+		trailingRange?: ls.Range | undefined
+	} {
+		const fullRange = range(completion.range, doc)
+
+		if (!hasFlawedMultiLineSupport) {
+			// No need to break down the range.
+			return { filterRange: fullRange }
+		}
+
+		// Find the `ls.Position` of the last and the next newline characters
+		// before the cursor within the replace range, if any.
+		const originalText = doc.getText(fullRange)
+		// Indexing (instead of iterating) here is appropriate, since LSP offset
+		// is calculated by UTF-16 code units, not by Unicode code points.
+		const lastLFRelativeOffset = originalText.slice(0, requestedOffset)
+			.lastIndexOf(core.LF)
+		const filterLineStartPosition = lastLFRelativeOffset === -1
+			? undefined
+			: doc.positionAt(completion.range.start + lastLFRelativeOffset + 1)
+		const nextCRLFRelativeOffset = originalText.indexOf(
+			core.CRLF,
+			requestedOffset,
+		)
+		const nextLFRelativeOffset = originalText.indexOf(
+			core.LF,
+			requestedOffset,
+		)
+		const nextNewlineRelativeOffset = nextCRLFRelativeOffset !== -1 &&
+				nextCRLFRelativeOffset < nextLFRelativeOffset
+			? nextCRLFRelativeOffset
+			: nextLFRelativeOffset
+		const filterLineEndPosition = nextNewlineRelativeOffset === -1
+			? undefined
+			: doc.positionAt(completion.range.start + nextNewlineRelativeOffset)
+
+		// Break down the full range according to the two newline positions.
+		return {
+			leadingRange: filterLineStartPosition
+				? ls.Range.create(fullRange.start, filterLineStartPosition)
+				: undefined,
+			// Clamp `filterRange` between the two newlines to make sure it is
+			// single-line.
+			filterRange: ls.Range.create(
+				filterLineStartPosition ?? fullRange.start,
+				filterLineEndPosition ?? fullRange.end,
+			),
+			trailingRange: filterLineEndPosition
+				? ls.Range.create(filterLineEndPosition, fullRange.end)
+				: undefined,
+		}
+	}
+
+	const { leadingRange, filterRange, trailingRange } = breakDownRange()
+	const textEdit: ls.TextEdit | ls.InsertReplaceEdit = insertReplaceSupport
 		? ls.InsertReplaceEdit.create(
 			insertText,
-			/* insert  */ range(
-				core.Range.create(completion.range.start, requestedOffset),
-				doc,
-			),
-			/* replace */ range(completion.range, doc),
+			ls.Range.create(filterRange.start, doc.positionAt(requestedOffset)),
+			filterRange,
 		)
-		: ls.TextEdit.replace(range(completion.range, doc), insertText)
+		: ls.TextEdit.replace(filterRange, insertText)
 	const ans: ls.CompletionItem = {
 		label: completion.label,
 		kind: completion.kind,
 		detail: completion.detail,
 		documentation: completion.documentation,
-		filterText: completion.filterText,
+		/*
+		 * When the `filterRange` starts at the start of the actual range, the
+		 * text content in `filterRange` is the actual prefix of the completed
+		 * value and VS Code filtering will work properly.
+		 *
+		 * However, when there is a `leadingRange`, the text content in the
+		 * `filterRange` will be in the middle of the completed value. VS Code's
+		 * prefix matching will fail, resulting in this `CompletionItem` not
+		 * getting displayed to the user. This is fixed by overwriting the
+		 * `filterText` to be the exact value as the content of `filterRange` so
+		 * that the `CompletionItem`s will be shown.
+		 *
+		 * TODO: Improve mcfunction completers so that they keep the user's insane
+		 * backslashes and we can implement our own proper prefix checking here
+		 * to make the filtering and sorting more correct for editors with flawed
+		 * multi-line support.
+		 */
+		filterText: hasFlawedMultiLineSupport && leadingRange
+			? doc.getText(filterRange)
+			: completion.filterText,
 		sortText: completion.sortText,
 		textEdit,
 		insertTextFormat: InsertTextFormat.Snippet,
 		insertTextMode: ls.InsertTextMode.adjustIndentation,
+		additionalTextEdits: (leadingRange || trailingRange)
+			? [
+				...leadingRange ? [ls.TextEdit.del(leadingRange)] : [],
+				...trailingRange ? [ls.TextEdit.del(trailingRange)] : [],
+			]
+			: undefined,
 		...(completion.deprecated
 			? { tags: [ls.CompletionItemTag.Deprecated] }
 			: {}),
diff --git a/packages/language-server/src/util/types.ts b/packages/language-server/src/util/types.ts
index cb51bf696..1f96790e6 100644
--- a/packages/language-server/src/util/types.ts
+++ b/packages/language-server/src/util/types.ts
@@ -1,4 +1,9 @@
 export interface CustomInitializationOptions {
+	/**
+	 * Set to `true` if the text editor cannot filter `CompletionItem`s properly
+	 * if the range of the item spans multiple lines.
+	 */
+	hasFlawedMultiLineCompletionItemFiltering?: boolean
 	inDevelopmentMode?: boolean
 }
 
diff --git a/packages/language-server/test/util/toLS.spec.ts b/packages/language-server/test/util/toLS.spec.ts
index 090783ca3..816d220c2 100644
--- a/packages/language-server/test/util/toLS.spec.ts
+++ b/packages/language-server/test/util/toLS.spec.ts
@@ -1,9 +1,9 @@
-import type * as core from '@spyglassmc/core'
+import * as core from '@spyglassmc/core'
 import { showWhitespaceGlyph } from '@spyglassmc/core/test-out/utils.js'
 import snapshot from 'snap-shot-it'
 import { TextDocument } from 'vscode-languageserver-textdocument'
 import type * as ls from 'vscode-languageserver/node.js'
-import { semanticTokens } from '../../lib/util/toLS.js'
+import * as toLS from '../../lib/util/toLS.js'
 
 /**
  * The result of decoding a semantic token from an integer list.
@@ -37,7 +37,7 @@ const decodeSemanticTokens = (
 	return decodedTokens
 }
 
-describe('semanticTokens', () => {
+describe('toLS semanticTokens', () => {
 	const tokens: core.ColorToken[] = [
 		{
 			range: {
@@ -62,7 +62,7 @@ describe('semanticTokens', () => {
 				showWhitespaceGlyph(content)
 			}" ${multilineStr}`
 			it(itTitle, () => {
-				const { data } = semanticTokens(
+				const { data } = toLS.semanticTokens(
 					tokens,
 					doc,
 					hasMultilineTokenSupport,
@@ -72,3 +72,25 @@ describe('semanticTokens', () => {
 		}
 	}
 })
+
+describe('toLS completionItem', () => {
+	const doc = TextDocument.create(
+		'spyglassmc:///test.mcfunction',
+		'mcfunction',
+		0,
+		'adv\\\nan\\\nce',
+	)
+	const item = core.CompletionItem.create(
+		'advancement',
+		core.Range.create(0, 11),
+	)
+	it('Should map correctly when cursor is in first line', () => {
+		snapshot(toLS.completionItem(item, doc, 1, true, true))
+	})
+	it('Should map correctly when cursor is in second line', () => {
+		snapshot(toLS.completionItem(item, doc, 6, true, true))
+	})
+	it('Should map correctly when cursor is in third line', () => {
+		snapshot(toLS.completionItem(item, doc, 10, true, true))
+	})
+})
diff --git a/packages/vscode-extension/package.json b/packages/vscode-extension/package.json
index a139fe131..cdb7dcb5a 100644
--- a/packages/vscode-extension/package.json
+++ b/packages/vscode-extension/package.json
@@ -191,10 +191,12 @@
         "editor.semanticHighlighting.enabled": true
       },
       "[mcdoc]": {
-        "editor.semanticHighlighting.enabled": true
+        "editor.semanticHighlighting.enabled": true,
+        "editor.suggest.insertMode": "replace"
       },
       "[mcfunction]": {
-        "editor.semanticHighlighting.enabled": true
+        "editor.semanticHighlighting.enabled": true,
+        "editor.suggest.insertMode": "replace"
       },
       "editor.semanticTokenColorCustomizations": {
         "rules": {
diff --git a/packages/vscode-extension/src/extension.mts b/packages/vscode-extension/src/extension.mts
index ecf50a305..31599a55c 100644
--- a/packages/vscode-extension/src/extension.mts
+++ b/packages/vscode-extension/src/extension.mts
@@ -53,6 +53,7 @@ export function activate(context: vsc.ExtensionContext) {
 	]
 
 	const initializationOptions: server.CustomInitializationOptions = {
+		hasFlawedMultiLineCompletionItemFiltering: true,
 		inDevelopmentMode:
 			context.extensionMode === vsc.ExtensionMode.Development,
 	}