Address review: harden analytics status codes, add failure-path tests

207 alone is indistinguishable from 200 to callers that only check
response.ok, so /api/analytics now raises 500 when every requested
metric fails and reserves 207 for genuine partial failure. Adds
regression tests for the temporal, analytics, and ontology-create
failure paths introduced in this PR, and logs the fix in the
changelog's Unreleased section.

CHANGELOG.md

@@ -9,6 +9,14 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Fixed
+
+- **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
+  - `GET /api/analytics` sets `response.status_code = 207` (Multi-Status) when some, but not all, of the requested metrics fail, and raises `HTTPException(500)` when every requested metric fails — a plain 2xx (including 207) reads as success to callers that only check `response.ok`, so an all-failed request now surfaces as a hard error rather than a body full of `{"error": ...}`
+  - Added regression tests covering all three failure paths (`test_patterns_failure_returns_500`, `test_analytics_partial_failure_returns_207`, `test_analytics_total_failure_returns_500`, and two `TestOntologyCreateFailures` cases)
+
 ## [0.6.0] - 2026-07-21
 
 ### Added

semantica/explorer/routes/analytics.py

@@ -5,7 +5,7 @@ Analytics routes for graph metrics and validation.
 import asyncio
 from typing import Optional
 
-from fastapi import APIRouter, Depends, Query, Response
+from fastapi import APIRouter, Depends, HTTPException, Query, Response
 
 from ..dependencies import get_session
 from ..schemas import AnalyticsResponse, ValidationIssue, ValidationReportResponse
@@ -26,9 +26,11 @@ async def get_analytics(
     requested = set((metrics or "centrality,community,connectivity").split(","))
     graph_dict = await asyncio.to_thread(session.build_graph_dict)
     result: dict = {}
-    any_failed = False
+    attempted = 0
+    failed = 0
 
     if "centrality" in requested and session.centrality is not None:
+        attempted += 1
         try:
             result["centrality"] = await asyncio.to_thread(
                 session.centrality.calculate_degree_centrality,
@@ -36,9 +38,10 @@ async def get_analytics(
             )
         except Exception as exc:
             result["centrality"] = {"error": str(exc)}
-            any_failed = True
+            failed += 1
 
     if "community" in requested and session.community is not None:
+        attempted += 1
         try:
             result["community"] = await asyncio.to_thread(
                 session.community.detect_communities,
@@ -46,9 +49,10 @@ async def get_analytics(
             )
         except Exception as exc:
             result["community"] = {"error": str(exc)}
-            any_failed = True
+            failed += 1
 
     if "connectivity" in requested and session.connectivity is not None:
+        attempted += 1
         try:
             result["connectivity"] = await asyncio.to_thread(
                 session.connectivity.analyze_connectivity,
@@ -56,9 +60,15 @@ async def get_analytics(
             )
         except Exception as exc:
             result["connectivity"] = {"error": str(exc)}
-            any_failed = True
+            failed += 1
 
-    if any_failed:
+    if attempted and failed == attempted:
+        # Every requested metric raised: a plain 2xx (even 207) reads as
+        # success to callers that only check `response.ok`, so surface this
+        # as a hard failure rather than a body full of {"error": ...}.
+        raise HTTPException(status_code=500, detail="All requested analytics metrics failed to compute")
+
+    if failed:
         response.status_code = 207
 
     return AnalyticsResponse(**result)

tests/explorer/test_explorer_api.py

@@ -474,6 +474,16 @@ class TestTemporal:
         assert response.status_code == 200
         assert "patterns" in response.json()
 
+    def test_patterns_failure_returns_500(self, client, monkeypatch):
+        session = client.app.state.session
+
+        def _boom():
+            raise RuntimeError("boom")
+
+        monkeypatch.setattr(session, "build_graph_dict", _boom)
+        response = client.get("/api/temporal/patterns")
+        assert response.status_code == 500
+
     def test_bounds(self, client):
         response = client.get("/api/temporal/bounds")
         assert response.status_code == 200
@@ -488,6 +498,29 @@ class TestAnalytics:
         assert response.status_code == 200
         assert "centrality" in response.json()
 
+    def test_analytics_partial_failure_returns_207(self, client, monkeypatch):
+        session = client.app.state.session
+
+        def _boom(*_args, **_kwargs):
+            raise RuntimeError("boom")
+
+        monkeypatch.setattr(session.centrality, "calculate_degree_centrality", _boom)
+        response = client.get("/api/analytics?metrics=centrality,community")
+        assert response.status_code == 207
+        payload = response.json()
+        assert payload["centrality"]["error"]
+        assert payload["community"] is not None and "error" not in payload["community"]
+
+    def test_analytics_total_failure_returns_500(self, client, monkeypatch):
+        session = client.app.state.session
+
+        def _boom(*_args, **_kwargs):
+            raise RuntimeError("boom")
+
+        monkeypatch.setattr(session.centrality, "calculate_degree_centrality", _boom)
+        response = client.get("/api/analytics?metrics=centrality")
+        assert response.status_code == 500
+
     def test_validation(self, client):
         response = client.get("/api/analytics/validation")
         assert response.status_code == 200
@@ -496,6 +529,44 @@ class TestAnalytics:
         assert "issues" in payload
 
 
+class TestOntologyCreateFailures:
+    def test_create_from_sample_data_failure_returns_500(self, client, monkeypatch):
+        from semantica.ontology import OntologyEngine
+
+        def _boom(self, *_args, **_kwargs):
+            raise RuntimeError("boom")
+
+        monkeypatch.setattr(OntologyEngine, "from_data", _boom)
+        response = client.post(
+            "/api/ontology/create",
+            json={
+                "mode": "data",
+                "namespace": "http://example.org/create-failure-data",
+                "name": "Create Failure (data)",
+                "sample_data": "id,name\n1,Alice\n2,Bob",
+            },
+        )
+        assert response.status_code == 500
+
+    def test_create_from_text_failure_returns_500(self, client, monkeypatch):
+        from semantica.ontology import OntologyEngine
+
+        def _boom(self, *_args, **_kwargs):
+            raise RuntimeError("boom")
+
+        monkeypatch.setattr(OntologyEngine, "from_text", _boom)
+        response = client.post(
+            "/api/ontology/create",
+            json={
+                "mode": "text",
+                "namespace": "http://example.org/create-failure-text",
+                "name": "Create Failure (text)",
+                "schema_text": "Class: Person\nProperty: knows",
+            },
+        )
+        assert response.status_code == 500
+
+
 class TestEnrichment:
     def test_reasoning(self, client):
         response = client.post(