diff --git a/TCT/trapi.py b/TCT/trapi.py
index 9bad9b9..467988d 100644
--- a/TCT/trapi.py
+++ b/TCT/trapi.py
@@ -12,7 +12,7 @@
 # TODO: incorporate object ids into the method.
 def build_query(subject_ids:list[str],
         object_categories:list[str], predicates:list[str],
-        return_json:bool=True,
+        return_json:bool=False,
         object_ids=None, subject_categories=None):
     """
     This constructs a query json for use with TRAPI. Queries are of the form [subject_ids]-[predicates]-[object_categories].
@@ -33,7 +33,7 @@ def build_query(subject_ids:list[str],
         A list of predicates that we are interested in. Example: ["biolink:positively_correlated_with", "biolink:physically_interacts_with"].
 
     return_json
-        If true, returns a json string; if false, returns a dict.
+        If true, returns a json string; if false, returns a dict (default).
 
     object_ids
         None by default
@@ -42,7 +42,7 @@ def build_query(subject_ids:list[str],
 
     Returns
     -------
-    A json string
+    A dict (default) or json string if return_json=True
 
     Examples
     --------
@@ -94,7 +94,7 @@ def process_result(result:dict):
     """
 
 
-def query(url:str, query:str):
+def query(url:str, query:dict):
     """
     Queries a single TRAPI endpoint.
 
@@ -102,8 +102,8 @@ def query(url:str, query:str):
     ------
     url : str
         The URL for the API endpoint.
-    query : str
-        A JSON string representing the query, as produced by build_query
+    query : dict
+        A dict representing the query, as produced by build_query
 
     Returns
     -------
@@ -111,11 +111,16 @@ def query(url:str, query:str):
 
     Examples
     --------
-    >>> query = build_query(['NCBIGene:3845'], ['biolink:Gene'], ['biolink:physically_interacts_with'])
-    >>> response = query(url, query)
+    >>> query_dict = build_query(['NCBIGene:3845'], ['biolink:Gene'], ['biolink:physically_interacts_with'])
+    >>> response = query(url, query_dict)
     >>> print(response)
     """
-    # example: 1. get APIs, 2. get APIs that have the target object and subject types, and the target predicates. 3. build the query and run the query.
+    if isinstance(query, str):
+        raise TypeError(
+            "query must be a dict, not a JSON string. "
+            "Use build_query(...) without return_json=True, "
+            "or pass json.loads(query) instead."
+        )
     response = requests.post(url, json=query)
     if response.status_code == 200:
         # TODO
diff --git a/tests/test_trapi.py b/tests/test_trapi.py
new file mode 100644
index 0000000..d3a131f
--- /dev/null
+++ b/tests/test_trapi.py
@@ -0,0 +1,84 @@
+"""Tests for TCT.trapi module, focused on the predicate_query branch changes:
+- build_query defaults to returning a dict (return_json=False)
+- query() accepts a dict instead of a JSON string
+"""
+
+import json
+import inspect
+
+import pytest
+
+from TCT.trapi import build_query, query
+
+
+# Test data
+EXAMPLE_QUERIES = [
+    {
+        'subject_ids': ['NCBIGene:3845'],
+        'object_categories': ['biolink:Gene'],
+        'predicates': ['biolink:physically_interacts_with'],
+    },
+    {
+        'subject_ids': ['NCBIGene:3845'],
+        'object_categories': ['biolink:Gene'],
+        'predicates': [
+            'biolink:positively_correlated_with',
+            'biolink:physically_interacts_with',
+        ],
+    },
+]
+
+
+def test_build_query_returns_dict_by_default():
+    q = EXAMPLE_QUERIES[0]
+    result = build_query(q['subject_ids'], q['object_categories'], q['predicates'])
+    assert isinstance(result, dict)
+
+
+def test_build_query_default_is_explicitly_false():
+    sig = inspect.signature(build_query)
+    assert sig.parameters['return_json'].default is False
+
+
+def test_build_query_returns_json_string_when_requested():
+    q = EXAMPLE_QUERIES[0]
+    result = build_query(q['subject_ids'], q['object_categories'], q['predicates'], return_json=True)
+    assert isinstance(result, str)
+    parsed = json.loads(result)
+    assert isinstance(parsed, dict)
+
+
+def test_build_query_dict_and_json_are_equivalent():
+    q = EXAMPLE_QUERIES[0]
+    dict_result = build_query(q['subject_ids'], q['object_categories'], q['predicates'], return_json=False)
+    json_result = build_query(q['subject_ids'], q['object_categories'], q['predicates'], return_json=True)
+    assert dict_result == json.loads(json_result)
+
+
+@pytest.mark.parametrize("example_query", EXAMPLE_QUERIES)
+def test_build_query_structure(example_query):
+    result = build_query(
+        example_query['subject_ids'],
+        example_query['object_categories'],
+        example_query['predicates'],
+    )
+    assert 'message' in result
+    qg = result['message']['query_graph']
+    assert qg['edges']['e00']['predicates'] == example_query['predicates']
+    assert qg['edges']['e00']['subject'] == 'n00'
+    assert qg['edges']['e00']['object'] == 'n01'
+    assert qg['nodes']['n00']['ids'] == example_query['subject_ids']
+    assert qg['nodes']['n01']['categories'] == example_query['object_categories']
+
+
+def test_query_signature_expects_dict():
+    sig = inspect.signature(query)
+    assert sig.parameters['query'].annotation is dict
+
+
+def test_build_query_output_matches_query_input_type():
+    """build_query's default output type should match what query() expects."""
+    q = EXAMPLE_QUERIES[0]
+    result = build_query(q['subject_ids'], q['object_categories'], q['predicates'])
+    sig = inspect.signature(query)
+    assert isinstance(result, sig.parameters['query'].annotation)