diff --git a/fava_client.py b/fava_client.py
index 2c24cc9..c5f88bc 100644
--- a/fava_client.py
+++ b/fava_client.py
@@ -691,6 +691,9 @@ class FavaClient:
         """
         Add an account to the Beancount ledger via an Open directive.
 
+        NOTE: Fava's /api/add_entries endpoint does NOT support Open directives.
+        This method uses /api/source to directly edit the Beancount file.
+
         Args:
             account_name: Full account name (e.g., "Assets:Receivable:User-abc123")
             currencies: List of currencies for this account (e.g., ["EUR", "SATS"])
@@ -698,7 +701,7 @@ class FavaClient:
             metadata: Optional metadata for the account
 
         Returns:
-            Response from Fava ({"data": "Stored 1 entries.", "mtime": "..."})
+            Response from Fava ({"data": "new_sha256sum", "mtime": "..."})
 
         Example:
             # Add a user's receivable account
@@ -719,26 +722,67 @@ class FavaClient:
         if opening_date is None:
             opening_date = date_type.today()
 
-        # Format Open directive for Fava
-        open_directive = {
-            "t": "Open",
-            "date": opening_date.isoformat(),
-            "account": account_name,
-            "currencies": currencies,
-            "meta": metadata or {}
-        }
-
         try:
             async with httpx.AsyncClient(timeout=self.timeout) as client:
+                # Step 1: Get current source file
+                response = await client.get(f"{self.base_url}/source")
+                response.raise_for_status()
+                source_data = response.json()["data"]
+
+                file_path = source_data["file_path"]
+                sha256sum = source_data["sha256sum"]
+                source = source_data["source"]
+
+                # Step 2: Check if account already exists
+                if f"open {account_name}" in source:
+                    logger.info(f"Account {account_name} already exists in Beancount file")
+                    return {"data": sha256sum, "mtime": source_data.get("mtime", "")}
+
+                # Step 3: Find insertion point (after last Open directive)
+                lines = source.split('\n')
+                insert_index = 0
+                for i, line in enumerate(lines):
+                    if line.strip().startswith(('open ', f'{opening_date.year}-')) and 'open' in line:
+                        insert_index = i + 1
+
+                # Step 4: Format Open directive as Beancount text
+                currencies_str = ", ".join(currencies)
+                open_lines = [
+                    "",
+                    f"{opening_date.isoformat()} open {account_name} {currencies_str}"
+                ]
+
+                # Add metadata if provided
+                if metadata:
+                    for key, value in metadata.items():
+                        # Format metadata with proper indentation
+                        if isinstance(value, str):
+                            open_lines.append(f'  {key}: "{value}"')
+                        else:
+                            open_lines.append(f'  {key}: {value}')
+
+                # Step 5: Insert into source
+                for i, line in enumerate(open_lines):
+                    lines.insert(insert_index + i, line)
+
+                new_source = '\n'.join(lines)
+
+                # Step 6: Update source file via PUT /api/source
+                update_payload = {
+                    "file_path": file_path,
+                    "source": new_source,
+                    "sha256sum": sha256sum
+                }
+
                 response = await client.put(
-                    f"{self.base_url}/add_entries",
-                    json={"entries": [open_directive]},
+                    f"{self.base_url}/source",
+                    json=update_payload,
                     headers={"Content-Type": "application/json"}
                 )
                 response.raise_for_status()
                 result = response.json()
 
-                logger.info(f"Added account {account_name} to Fava with currencies {currencies}")
+                logger.info(f"Added account {account_name} to Beancount file with currencies {currencies}")
                 return result
 
         except httpx.HTTPStatusError as e: