diff --git a/beancount_format.py b/beancount_format.py
index 06f0afb..2819e67 100644
--- a/beancount_format.py
+++ b/beancount_format.py
@@ -14,6 +14,31 @@ Key concepts:
 from datetime import date, datetime
 from decimal import Decimal
 from typing import Any, Dict, List, Optional
+import re
+
+
+def sanitize_link(text: str) -> str:
+    """
+    Sanitize a string to make it valid for Beancount links.
+
+    Beancount links can only contain: A-Z, a-z, 0-9, -, _, /, .
+    All other characters are replaced with hyphens.
+
+    Examples:
+        >>> sanitize_link("Test (pending)")
+        'Test-pending'
+        >>> sanitize_link("Invoice #123")
+        'Invoice-123'
+        >>> sanitize_link("castle-abc123")
+        'castle-abc123'
+    """
+    # Replace any character that's not alphanumeric, dash, underscore, slash, or period with a hyphen
+    sanitized = re.sub(r'[^A-Za-z0-9\-_/.]', '-', text)
+    # Remove consecutive hyphens
+    sanitized = re.sub(r'-+', '-', sanitized)
+    # Remove leading/trailing hyphens
+    sanitized = sanitized.strip('-')
+    return sanitized
 
 
 def format_transaction(
diff --git a/views_api.py b/views_api.py
index 783309c..b6e95dc 100644
--- a/views_api.py
+++ b/views_api.py
@@ -802,7 +802,7 @@ async def api_create_expense_entry(
 
     # Format as Beancount entry and submit to Fava
     from .fava_client import get_fava_client
-    from .beancount_format import format_expense_entry
+    from .beancount_format import format_expense_entry, sanitize_link
 
     fava = get_fava_client()
 
@@ -814,10 +814,10 @@ async def api_create_expense_entry(
     import uuid
     entry_id = str(uuid.uuid4()).replace("-", "")[:16]
 
-    # Add castle ID as reference/link
+    # Add castle ID as reference/link (sanitized for Beancount)
     castle_reference = f"castle-{entry_id}"
     if data.reference:
-        castle_reference = f"{data.reference}-{entry_id}"
+        castle_reference = f"{sanitize_link(data.reference)}-{entry_id}"
 
     # Format Beancount entry
     entry = format_expense_entry(
@@ -930,7 +930,7 @@ async def api_create_receivable_entry(
 
     # Format as Beancount entry and submit to Fava
     from .fava_client import get_fava_client
-    from .beancount_format import format_receivable_entry
+    from .beancount_format import format_receivable_entry, sanitize_link
 
     fava = get_fava_client()
 
@@ -942,10 +942,10 @@ async def api_create_receivable_entry(
     import uuid
     entry_id = str(uuid.uuid4()).replace("-", "")[:16]
 
-    # Add castle ID as reference/link
+    # Add castle ID as reference/link (sanitized for Beancount)
     castle_reference = f"castle-{entry_id}"
     if data.reference:
-        castle_reference = f"{data.reference}-{entry_id}"
+        castle_reference = f"{sanitize_link(data.reference)}-{entry_id}"
 
     # Format Beancount entry
     entry = format_receivable_entry(
@@ -1007,7 +1007,7 @@ async def api_create_revenue_entry(
     Submits entry to Fava/Beancount.
     """
     from .fava_client import get_fava_client
-    from .beancount_format import format_revenue_entry
+    from .beancount_format import format_revenue_entry, sanitize_link
 
     # Get revenue account
     revenue_account = await get_account_by_name(data.revenue_account)
@@ -1057,10 +1057,10 @@ async def api_create_revenue_entry(
     import uuid
     entry_id = str(uuid.uuid4()).replace("-", "")[:16]
 
-    # Add castle ID as reference/link
+    # Add castle ID as reference/link (sanitized for Beancount)
     castle_reference = f"castle-{entry_id}"
     if data.reference:
-        castle_reference = f"{data.reference}-{entry_id}"
+        castle_reference = f"{sanitize_link(data.reference)}-{entry_id}"
 
     entry = format_revenue_entry(
         payment_account=payment_account.name,