diff --git a/CHANGELOG.md b/CHANGELOG.md
index 263f02f0..8047c022 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -11,6 +11,11 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 
+- **`tests/explorer/test_explorer_api.py` failed with `TypeError: Client.__init__() got an unexpected keyword argument 'app'` on current httpx** (#788, #789) by @Sameer6305
+  - `httpx>=0.28.0` removed the `app=` kwarg that Starlette's `TestClient` relies on to wrap a FastAPI app for testing; `httpx` wasn't pinned anywhere in `pyproject.toml`, so different environments could independently resolve an incompatible transitive version and hit the same break
+  - Added an explicit `httpx<0.28.0` constraint to the main `[project.dependencies]` array (not just a dev extra), so it applies globally across production, dev, and CI installs
+  - Without the pin, the full test suite fails to even complete collection (fails immediately on `tests/explorer/test_vocabulary.py` with the same `TestClient` error); with it, `tests/explorer/test_explorer_api.py` goes from 7 failed/12 passed/58 errors to 77 passed, 0 errors
+
 - **Explorer backend routes returned HTTP 200 with error/empty bodies on failure, defeating frontend error handling** (#770, #787) by @Sameer6305 and @KaifAhmad1
   - `GET /api/temporal/patterns` now raises `HTTPException(500)` on a genuine computation failure instead of silently returning an empty-but-valid `TemporalPatternResponse`; the `ImportError` fallback (optional `kg` extra not installed) is unchanged and still degrades gracefully to an empty list
   - `POST /api/ontology/create` now raises `HTTPException(500)` when ontology generation fails in either the `sample_data` or `schema_text` mode, instead of silently falling back to a partial/minimal ontology with a misleading `nodes_added` count
diff --git a/pyproject.toml b/pyproject.toml
index 9e003ba4..a8f9aa71 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -85,7 +85,8 @@ dependencies = [
   "python-dotenv>=1.2.1",
   "loguru>=0.7.3",
   "structlog>=22.1.0",
-  "gensim>=4.4.0"
+  "gensim>=4.4.0",
+  "httpx<0.28.0"
 ]
 
 [project.urls]