diff --git a/beancount_format.py b/beancount_format.py
index 4cbc5c7..fbdfba9 100644
--- a/beancount_format.py
+++ b/beancount_format.py
@@ -115,13 +115,18 @@ def format_balance(
     account: str,
     amount: int,
     currency: str = "SATS"
-) -> str:
+) -> Dict[str, Any]:
     """
-    Format a balance assertion directive for Beancount.
+    Format a balance assertion directive for Fava's JSON API.
 
     Balance assertions verify that an account has an expected balance on a specific date.
     They are checked automatically by Beancount when the file is loaded.
 
+    Fava's `deserialise` (fava/serialisation.py) expects
+    `{"t": "Balance", "amount": {"number", "currency"}, ...}` — the
+    previous source-string return 500'd on every assertion create
+    (libra-#39).
+
     Args:
         date_val: Date of the balance assertion
         account: Account name (e.g., "Assets:Bitcoin:Lightning")
@@ -129,15 +134,15 @@ def format_balance(
         currency: Currency code (default: "SATS")
 
     Returns:
-        Beancount balance directive as a string
-
-    Example:
-        >>> format_balance(date(2025, 11, 10), "Assets:Bitcoin:Lightning", 1500000, "SATS")
-        '2025-11-10 balance Assets:Bitcoin:Lightning  1500000 SATS'
+        Fava API Balance entry dict ready for `fava.add_entry`.
     """
-    date_str = date_val.strftime('%Y-%m-%d')
-    # Two spaces between account and amount (Beancount convention)
-    return f"{date_str} balance {account}  {amount} {currency}"
+    return {
+        "t": "Balance",
+        "date": date_val.strftime('%Y-%m-%d'),
+        "account": account,
+        "amount": {"number": str(amount), "currency": currency},
+        "meta": {},
+    }
 
 
 def format_posting_with_cost(
diff --git a/tests/test_reconciliation_api.py b/tests/test_reconciliation_api.py
index 66757be..a168ff8 100644
--- a/tests/test_reconciliation_api.py
+++ b/tests/test_reconciliation_api.py
@@ -18,19 +18,6 @@ from uuid import uuid4
 import pytest
 
 
-# Tests that try to actually create + check an assertion all hit issue #39:
-# `format_balance` returns a Beancount source string but `fava.add_entry`
-# expects a dict, so Fava 500s on every assertion-create call. The contract
-# violation is on libra's side; mark these strict-xfail so they go green
-# automatically once #39 lands and the format_balance return shape is fixed.
-ASSERTION_CREATE_BROKEN = pytest.mark.xfail(
-    reason="libra/issues/39 — POST /assertions submits a Beancount source string "
-    "to Fava's JSON API and 500s. Drop this marker when the format_balance "
-    "return type is changed to a dict.",
-    strict=True,
-)
-
-
 # ---------------------------------------------------------------------------
 # helpers (local — assertion endpoints don't have wrapper helpers yet)
 # ---------------------------------------------------------------------------
@@ -58,7 +45,6 @@ async def _create_assertion(
 # ---------------------------------------------------------------------------
 
 
-@ASSERTION_CREATE_BROKEN
 @pytest.mark.anyio
 async def test_assertion_against_empty_account_passes(
     client, super_user_headers, standard_accounts,
@@ -79,7 +65,6 @@ async def test_assertion_against_empty_account_passes(
     assert body.get("difference_sats", 0) == 0
 
 
-@ASSERTION_CREATE_BROKEN
 @pytest.mark.anyio
 async def test_assertion_with_wrong_balance_returns_409(
     client, super_user_headers, standard_accounts,
@@ -102,7 +87,6 @@ async def test_assertion_with_wrong_balance_returns_409(
     assert detail.get("difference_sats") == 999_999 or detail.get("difference_sats") == -999_999
 
 
-@ASSERTION_CREATE_BROKEN
 @pytest.mark.anyio
 async def test_assertion_with_tolerance_accepts_small_diff(
     client, super_user_headers, standard_accounts,
@@ -119,7 +103,6 @@ async def test_assertion_with_tolerance_accepts_small_diff(
     assert r.json().get("status") == "passed"
 
 
-@ASSERTION_CREATE_BROKEN
 @pytest.mark.anyio
 async def test_list_assertions_returns_created(
     client, super_user_headers, standard_accounts,
@@ -145,7 +128,6 @@ async def test_list_assertions_returns_created(
     assert assertion_id in ids, f"created assertion {assertion_id} missing from list {ids}"
 
 
-@ASSERTION_CREATE_BROKEN
 @pytest.mark.anyio
 async def test_get_assertion_by_id(
     client, super_user_headers, standard_accounts,
@@ -167,7 +149,6 @@ async def test_get_assertion_by_id(
     assert r.json().get("id") == assertion_id
 
 
-@ASSERTION_CREATE_BROKEN
 @pytest.mark.anyio
 async def test_recheck_assertion_via_check_endpoint(
     client, super_user_headers, standard_accounts,
@@ -190,7 +171,6 @@ async def test_recheck_assertion_via_check_endpoint(
     assert r.json().get("status") == "passed"
 
 
-@ASSERTION_CREATE_BROKEN
 @pytest.mark.anyio
 async def test_delete_assertion_removes_it(
     client, super_user_headers, standard_accounts,