diff --git a/tests/test_api_openai.py b/tests/test_api_openai.py
index 71bc3968..31915bc3 100644
--- a/tests/test_api_openai.py
+++ b/tests/test_api_openai.py
@@ -442,3 +442,230 @@ class TestAiExtractionEndpoint:
         assert data["status"] == "success"
         assert data["tags"] == ["contract"]
         assert data["parse_error"] is None
+
+    @patch("app.utils.ai_provider.get_ai_provider")
+    @patch("app.api.openai.settings")
+    def test_extraction_json_found_but_invalid_reports_parse_error(self, mock_settings, mock_get_provider, client):
+        """Test JSONDecodeError branch: response contains '{...}' but is not valid JSON."""
+        mock_settings.ai_provider = "openai"
+        mock_settings.ai_model = "gpt-4o-mini"
+        mock_settings.openai_model = "gpt-4o-mini"
+
+        # Looks like JSON (has { and }) but is NOT parseable
+        mock_provider = MagicMock()
+        mock_provider.chat_completion.return_value = "{this is not: valid json!!}"
+        mock_get_provider.return_value = mock_provider
+
+        response = client.post("/api/ai/test-extraction", json={"text": "Some document text"})
+        assert response.status_code == 200
+        data = response.json()
+
+        assert data["status"] == "success"
+        assert data["parsed_json"] is None
+        assert data["tags"] == []
+        assert data["parse_error"] is not None
+        assert "raw_response" in data
+
+    @patch("app.api.openai.settings")
+    def test_extraction_text_too_long_returns_422(self, mock_settings, client):
+        """Test that text exceeding max length returns 422 validation error."""
+        from app.api.openai import _MAX_EXTRACTION_TEXT_LEN
+
+        oversized_text = "x" * (_MAX_EXTRACTION_TEXT_LEN + 1)
+        response = client.post("/api/ai/test-extraction", json={"text": oversized_text})
+        assert response.status_code == 422
+
+    @patch("app.utils.ai_provider.get_ai_provider")
+    @patch("app.api.openai.settings")
+    def test_extraction_response_with_no_tags_key(self, mock_settings, mock_get_provider, client):
+        """Test extraction where parsed JSON has no 'tags' key returns empty tags list."""
+        import json as json_module
+
+        mock_settings.ai_provider = "openai"
+        mock_settings.ai_model = "gpt-4o-mini"
+        mock_settings.openai_model = "gpt-4o-mini"
+
+        # Valid JSON but no 'tags' key
+        payload = {"title": "Report", "document_type": "Report"}
+        mock_provider = MagicMock()
+        mock_provider.chat_completion.return_value = json_module.dumps(payload)
+        mock_get_provider.return_value = mock_provider
+
+        response = client.post("/api/ai/test-extraction", json={"text": "Report content"})
+        assert response.status_code == 200
+        data = response.json()
+
+        assert data["status"] == "success"
+        assert data["tags"] == []
+        assert data["parsed_json"]["title"] == "Report"
+        assert data["parse_error"] is None
+
+
+@pytest.mark.unit
+class TestAiProviderTestEndpoint:
+    """Tests for GET /api/ai/test endpoint."""
+
+    @patch("app.utils.ai_provider.get_ai_provider")
+    @patch("app.api.openai.settings")
+    def test_ai_test_success(self, mock_settings, mock_get_provider, client):
+        """Test successful AI provider connection."""
+        mock_settings.ai_provider = "openai"
+        mock_settings.ai_model = "gpt-4o-mini"
+        mock_settings.openai_model = "gpt-4o-mini"
+
+        mock_provider = MagicMock()
+        mock_provider.chat_completion.return_value = "ok"
+        mock_get_provider.return_value = mock_provider
+
+        response = client.get("/api/ai/test")
+        assert response.status_code == 200
+        data = response.json()
+
+        assert data["status"] == "success"
+        assert "reachable" in data["message"].lower()
+        assert data["provider"] == "openai"
+        assert data["model"] == "gpt-4o-mini"
+        assert "response_preview" in data
+
+    @patch("app.utils.ai_provider.get_ai_provider")
+    @patch("app.api.openai.settings")
+    def test_ai_test_value_error(self, mock_settings, mock_get_provider, client):
+        """Test GET /api/ai/test returns error on provider configuration ValueError."""
+        mock_settings.ai_provider = "anthropic"
+        mock_settings.ai_model = None
+        mock_settings.openai_model = "gpt-4o-mini"
+
+        mock_get_provider.side_effect = ValueError("ANTHROPIC_API_KEY must be set")
+
+        response = client.get("/api/ai/test")
+        assert response.status_code == 200
+        data = response.json()
+
+        assert data["status"] == "error"
+        assert "ANTHROPIC_API_KEY" in data["message"]
+        assert data["provider"] == "anthropic"
+
+    @patch("app.utils.ai_provider.get_ai_provider")
+    @patch("app.api.openai.settings")
+    def test_ai_test_connection_exception(self, mock_settings, mock_get_provider, client):
+        """Test GET /api/ai/test returns error on provider runtime exception."""
+        mock_settings.ai_provider = "openai"
+        mock_settings.ai_model = "gpt-4o-mini"
+        mock_settings.openai_model = "gpt-4o-mini"
+
+        mock_provider = MagicMock()
+        mock_provider.chat_completion.side_effect = Exception("Connection refused")
+        mock_get_provider.return_value = mock_provider
+
+        response = client.get("/api/ai/test")
+        assert response.status_code == 200
+        data = response.json()
+
+        assert data["status"] == "error"
+        assert "Connection refused" in data["message"]
+        assert data["provider"] == "openai"
+
+    @patch("app.utils.ai_provider.get_ai_provider")
+    @patch("app.api.openai.settings")
+    def test_ai_test_uses_openai_model_fallback(self, mock_settings, mock_get_provider, client):
+        """Test that ai_model=None falls back to openai_model."""
+        mock_settings.ai_provider = "openai"
+        mock_settings.ai_model = None
+        mock_settings.openai_model = "gpt-3.5-turbo"
+
+        mock_provider = MagicMock()
+        mock_provider.chat_completion.return_value = "ok"
+        mock_get_provider.return_value = mock_provider
+
+        response = client.get("/api/ai/test")
+        assert response.status_code == 200
+        data = response.json()
+
+        assert data["status"] == "success"
+        assert data["model"] == "gpt-3.5-turbo"
+
+
+@pytest.mark.unit
+class TestExceptionChainDetail:
+    """Unit tests for the _get_exception_chain_detail helper."""
+
+    def test_single_exception_no_chain(self):
+        """Test with a simple exception that has no cause."""
+        from app.api.openai import _get_exception_chain_detail
+
+        exc = ValueError("root cause")
+        result = _get_exception_chain_detail(exc)
+        assert result == "root cause"
+
+    def test_exception_with_cause(self):
+        """Test that chained exceptions are surfaced in the message."""
+        from app.api.openai import _get_exception_chain_detail
+
+        inner = OSError("DNS resolution failed")
+        outer = ConnectionError("Connection failed")
+        outer.__cause__ = inner
+
+        result = _get_exception_chain_detail(outer)
+        assert "Connection failed" in result
+        assert "DNS resolution failed" in result
+        assert "caused by" in result
+
+    def test_empty_cause_string_skipped(self):
+        """Test that a cause with empty string representation is not appended."""
+        from app.api.openai import _get_exception_chain_detail
+
+        inner = Exception("")  # str() returns ""
+        outer = RuntimeError("outer error")
+        outer.__cause__ = inner
+
+        result = _get_exception_chain_detail(outer)
+        # The empty-string cause should be skipped (branch 47->49)
+        assert result == "outer error"
+
+    def test_duplicate_cause_string_skipped(self):
+        """Test that a cause whose str() is already in parts is not duplicated."""
+        from app.api.openai import _get_exception_chain_detail
+
+        outer = RuntimeError("same message")
+        inner = RuntimeError("same message")  # same text as outer
+        outer.__cause__ = inner
+
+        result = _get_exception_chain_detail(outer)
+        # "same message" should appear only once (branch: cause_str already in parts)
+        assert result.count("same message") == 1
+
+
+@pytest.mark.unit
+class TestExtractJsonFromText:
+    """Unit tests for the _extract_json_from_text helper."""
+
+    def test_json_in_code_fence(self):
+        """Test extraction from markdown code fence."""
+        from app.api.openai import _extract_json_from_text
+
+        text = '```json\n{"key": "value"}\n```'
+        result = _extract_json_from_text(text)
+        assert result == '{"key": "value"}'
+
+    def test_json_in_plain_code_fence(self):
+        """Test extraction from plain (non-json) code fence."""
+        from app.api.openai import _extract_json_from_text
+
+        text = '```\n{"key": "value"}\n```'
+        result = _extract_json_from_text(text)
+        assert result == '{"key": "value"}'
+
+    def test_bare_json_object(self):
+        """Test extraction of a bare JSON object without code fence."""
+        from app.api.openai import _extract_json_from_text
+
+        text = 'Here is the result: {"title": "Invoice"} done.'
+        result = _extract_json_from_text(text)
+        assert result == '{"title": "Invoice"}'
+
+    def test_no_json_returns_none(self):
+        """Test that text with no JSON object returns None."""
+        from app.api.openai import _extract_json_from_text
+
+        result = _extract_json_from_text("No JSON here at all.")
+        assert result is None