diff --git a/CHANGELOG.md b/CHANGELOG.md
index 6fa8545..a135806 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -14,6 +14,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Added `@pytest.mark.large` and `@pytest.mark.concurrent` test markers
 - Added `generate_content()` and `generate_content_with_hash()` test helpers
 - Added `sized_content` fixture for generating test content of specific sizes
+- Added upload API tests: upload without tag, artifact creation verification, S3 object creation
+- Added download API tests: tag: prefix resolution, 404 for nonexistent project/package/artifact
+- Added download header tests: Content-Type, Content-Length, Content-Disposition, ETag, X-Checksum-SHA256
+- Added error handling tests: timeout behavior, checksum validation, resource cleanup, graceful error responses
 - Added production deployment job triggered by semantic version tags (v1.0.0) with manual approval gate (#63)
 - Added production Helm values file with persistence enabled (20Gi PostgreSQL, 100Gi MinIO) (#63)
 - Added integration tests for production deployment (#63)
diff --git a/backend/tests/integration/test_error_handling.py b/backend/tests/integration/test_error_handling.py
new file mode 100644
index 0000000..ce1f767
--- /dev/null
+++ b/backend/tests/integration/test_error_handling.py
@@ -0,0 +1,322 @@
+"""
+Integration tests for error handling in upload and download operations.
+
+Tests cover:
+- Timeout handling
+- Invalid request handling
+- Resource cleanup on failures
+- Graceful error responses
+"""
+
+import pytest
+import io
+import time
+from tests.factories import (
+    compute_sha256,
+    upload_test_file,
+    generate_content_with_hash,
+)
+
+
+class TestUploadErrorHandling:
+    """Tests for upload error handling."""
+
+    @pytest.mark.integration
+    def test_upload_to_nonexistent_project_returns_404(
+        self, integration_client, unique_test_id
+    ):
+        """Test upload to nonexistent project returns 404."""
+        content = b"test content for nonexistent project"
+
+        files = {"file": ("test.bin", io.BytesIO(content), "application/octet-stream")}
+        response = integration_client.post(
+            f"/api/v1/project/nonexistent-project-{unique_test_id}/nonexistent-pkg/upload",
+            files=files,
+        )
+        assert response.status_code == 404
+
+    @pytest.mark.integration
+    def test_upload_to_nonexistent_package_returns_404(
+        self, integration_client, test_project, unique_test_id
+    ):
+        """Test upload to nonexistent package returns 404."""
+        content = b"test content for nonexistent package"
+
+        files = {"file": ("test.bin", io.BytesIO(content), "application/octet-stream")}
+        response = integration_client.post(
+            f"/api/v1/project/{test_project}/nonexistent-package-{unique_test_id}/upload",
+            files=files,
+        )
+        assert response.status_code == 404
+
+    @pytest.mark.integration
+    def test_upload_empty_file_rejected(self, integration_client, test_package):
+        """Test empty file upload is rejected."""
+        project, package = test_package
+
+        files = {"file": ("empty.bin", io.BytesIO(b""), "application/octet-stream")}
+        response = integration_client.post(
+            f"/api/v1/project/{project}/{package}/upload",
+            files=files,
+        )
+        assert response.status_code in [400, 422]
+
+    @pytest.mark.integration
+    def test_upload_missing_file_returns_422(self, integration_client, test_package):
+        """Test upload without file field returns 422."""
+        project, package = test_package
+
+        response = integration_client.post(
+            f"/api/v1/project/{project}/{package}/upload",
+            data={"tag": "no-file-provided"},
+        )
+        assert response.status_code == 422
+
+    @pytest.mark.integration
+    def test_upload_invalid_checksum_format_returns_400(
+        self, integration_client, test_package
+    ):
+        """Test upload with invalid checksum format returns 400."""
+        project, package = test_package
+        content = b"checksum format test"
+
+        files = {"file": ("test.bin", io.BytesIO(content), "application/octet-stream")}
+        response = integration_client.post(
+            f"/api/v1/project/{project}/{package}/upload",
+            files=files,
+            headers={"X-Checksum-SHA256": "invalid-hash-format"},
+        )
+        assert response.status_code == 400
+
+    @pytest.mark.integration
+    def test_upload_checksum_mismatch_returns_422(
+        self, integration_client, test_package
+    ):
+        """Test upload with mismatched checksum returns 422."""
+        project, package = test_package
+        content = b"checksum mismatch test"
+        wrong_hash = "0" * 64  # Valid format but wrong hash
+
+        files = {"file": ("test.bin", io.BytesIO(content), "application/octet-stream")}
+        response = integration_client.post(
+            f"/api/v1/project/{project}/{package}/upload",
+            files=files,
+            headers={"X-Checksum-SHA256": wrong_hash},
+        )
+        assert response.status_code == 422
+
+    @pytest.mark.integration
+    def test_upload_with_correct_checksum_succeeds(
+        self, integration_client, test_package
+    ):
+        """Test upload with correct checksum succeeds."""
+        project, package = test_package
+        content = b"correct checksum test"
+        correct_hash = compute_sha256(content)
+
+        files = {"file": ("test.bin", io.BytesIO(content), "application/octet-stream")}
+        response = integration_client.post(
+            f"/api/v1/project/{project}/{package}/upload",
+            files=files,
+            headers={"X-Checksum-SHA256": correct_hash},
+        )
+        assert response.status_code == 200
+        assert response.json()["artifact_id"] == correct_hash
+
+
+class TestDownloadErrorHandling:
+    """Tests for download error handling."""
+
+    @pytest.mark.integration
+    def test_download_nonexistent_tag_returns_404(
+        self, integration_client, test_package
+    ):
+        """Test download of nonexistent tag returns 404."""
+        project, package = test_package
+
+        response = integration_client.get(
+            f"/api/v1/project/{project}/{package}/+/nonexistent-tag-xyz"
+        )
+        assert response.status_code == 404
+
+    @pytest.mark.integration
+    def test_download_nonexistent_artifact_returns_404(
+        self, integration_client, test_package
+    ):
+        """Test download of nonexistent artifact ID returns 404."""
+        project, package = test_package
+        fake_hash = "a" * 64
+
+        response = integration_client.get(
+            f"/api/v1/project/{project}/{package}/+/artifact:{fake_hash}"
+        )
+        assert response.status_code == 404
+
+    @pytest.mark.integration
+    def test_download_invalid_artifact_id_format(
+        self, integration_client, test_package
+    ):
+        """Test download with invalid artifact ID format."""
+        project, package = test_package
+
+        # Too short
+        response = integration_client.get(
+            f"/api/v1/project/{project}/{package}/+/artifact:abc123"
+        )
+        assert response.status_code == 404
+
+    @pytest.mark.integration
+    def test_download_from_nonexistent_project_returns_404(
+        self, integration_client, unique_test_id
+    ):
+        """Test download from nonexistent project returns 404."""
+        response = integration_client.get(
+            f"/api/v1/project/nonexistent-{unique_test_id}/pkg/+/tag"
+        )
+        assert response.status_code == 404
+
+    @pytest.mark.integration
+    def test_download_from_nonexistent_package_returns_404(
+        self, integration_client, test_project, unique_test_id
+    ):
+        """Test download from nonexistent package returns 404."""
+        response = integration_client.get(
+            f"/api/v1/project/{test_project}/nonexistent-{unique_test_id}/+/tag"
+        )
+        assert response.status_code == 404
+
+
+class TestTimeoutBehavior:
+    """Tests for timeout behavior (integration level)."""
+
+    @pytest.mark.integration
+    @pytest.mark.slow
+    def test_large_upload_completes_within_reasonable_time(
+        self, integration_client, test_package, sized_content
+    ):
+        """Test that a 10MB upload completes within reasonable time."""
+        project, package = test_package
+        content, expected_hash = sized_content(10 * 1024 * 1024, seed=999)  # 10MB
+
+        start_time = time.time()
+        result = upload_test_file(
+            integration_client, project, package, content, tag="timeout-test"
+        )
+        elapsed = time.time() - start_time
+
+        assert result["artifact_id"] == expected_hash
+        # Should complete within 60 seconds for 10MB on local docker
+        assert elapsed < 60, f"Upload took too long: {elapsed:.2f}s"
+
+    @pytest.mark.integration
+    @pytest.mark.slow
+    def test_large_download_completes_within_reasonable_time(
+        self, integration_client, test_package, sized_content
+    ):
+        """Test that a 10MB download completes within reasonable time."""
+        project, package = test_package
+        content, expected_hash = sized_content(10 * 1024 * 1024, seed=998)  # 10MB
+
+        # First upload
+        upload_test_file(
+            integration_client, project, package, content, tag="download-timeout-test"
+        )
+
+        # Then download and time it
+        start_time = time.time()
+        response = integration_client.get(
+            f"/api/v1/project/{project}/{package}/+/download-timeout-test",
+            params={"mode": "proxy"},
+        )
+        elapsed = time.time() - start_time
+
+        assert response.status_code == 200
+        assert len(response.content) == len(content)
+        # Should complete within 60 seconds for 10MB on local docker
+        assert elapsed < 60, f"Download took too long: {elapsed:.2f}s"
+
+
+class TestResourceCleanup:
+    """Tests for proper resource cleanup on failures.
+
+    Note: More comprehensive cleanup tests are in test_upload_download_api.py
+    (TestUploadFailureCleanup class) including S3 object cleanup verification.
+    """
+
+    @pytest.mark.integration
+    def test_checksum_mismatch_no_orphaned_artifact(
+        self, integration_client, test_package, unique_test_id
+    ):
+        """Test checksum mismatch doesn't leave orphaned artifact."""
+        project, package = test_package
+        # Use unique content to ensure artifact doesn't exist from prior tests
+        content = f"checksum mismatch orphan test {unique_test_id}".encode()
+        wrong_hash = "0" * 64
+        actual_hash = compute_sha256(content)
+
+        # Verify artifact doesn't exist before test
+        pre_check = integration_client.get(f"/api/v1/artifact/{actual_hash}")
+        assert pre_check.status_code == 404, "Artifact should not exist before test"
+
+        files = {"file": ("test.bin", io.BytesIO(content), "application/octet-stream")}
+        response = integration_client.post(
+            f"/api/v1/project/{project}/{package}/upload",
+            files=files,
+            headers={"X-Checksum-SHA256": wrong_hash},
+        )
+        assert response.status_code == 422
+
+        # Verify no artifact was created with either hash
+        response1 = integration_client.get(f"/api/v1/artifact/{wrong_hash}")
+        response2 = integration_client.get(f"/api/v1/artifact/{actual_hash}")
+        assert response1.status_code == 404
+        assert response2.status_code == 404
+
+
+class TestGracefulErrorResponses:
+    """Tests for graceful and informative error responses."""
+
+    @pytest.mark.integration
+    def test_404_response_has_detail_message(
+        self, integration_client, test_package
+    ):
+        """Test 404 responses include a detail message."""
+        project, package = test_package
+
+        response = integration_client.get(
+            f"/api/v1/project/{project}/{package}/+/nonexistent-tag"
+        )
+        assert response.status_code == 404
+        data = response.json()
+        assert "detail" in data
+        assert len(data["detail"]) > 0
+
+    @pytest.mark.integration
+    def test_422_response_has_detail_message(self, integration_client, test_package):
+        """Test 422 responses include a detail message."""
+        project, package = test_package
+
+        # Upload with mismatched checksum
+        content = b"detail message test"
+        wrong_hash = "0" * 64
+
+        files = {"file": ("test.bin", io.BytesIO(content), "application/octet-stream")}
+        response = integration_client.post(
+            f"/api/v1/project/{project}/{package}/upload",
+            files=files,
+            headers={"X-Checksum-SHA256": wrong_hash},
+        )
+        assert response.status_code == 422
+        data = response.json()
+        assert "detail" in data
+
+    @pytest.mark.integration
+    def test_error_response_is_json(self, integration_client, unique_test_id):
+        """Test error responses are valid JSON."""
+        response = integration_client.get(
+            f"/api/v1/project/nonexistent-{unique_test_id}/pkg/+/tag"
+        )
+        assert response.status_code == 404
+        # Should not raise exception - valid JSON
+        data = response.json()
+        assert isinstance(data, dict)
diff --git a/backend/tests/integration/test_upload_download_api.py b/backend/tests/integration/test_upload_download_api.py
index 4d9b8b2..07e6130 100644
--- a/backend/tests/integration/test_upload_download_api.py
+++ b/backend/tests/integration/test_upload_download_api.py
@@ -25,6 +25,19 @@ from tests.factories import (
 class TestUploadBasics:
     """Tests for basic upload functionality."""
 
+    @pytest.mark.integration
+    def test_upload_returns_200(self, integration_client, test_package):
+        """Test upload with valid file returns 200."""
+        project, package = test_package
+        content = b"valid file upload test"
+
+        files = {"file": ("test.bin", io.BytesIO(content), "application/octet-stream")}
+        response = integration_client.post(
+            f"/api/v1/project/{project}/{package}/upload",
+            files=files,
+        )
+        assert response.status_code == 200
+
     @pytest.mark.integration
     def test_upload_returns_artifact_id(self, integration_client, test_package):
         """Test upload returns the artifact ID (SHA256 hash)."""
@@ -101,6 +114,82 @@ class TestUploadBasics:
         assert "created_at" in result
         assert result["created_at"] is not None
 
+    @pytest.mark.integration
+    def test_upload_without_tag_succeeds(self, integration_client, test_package):
+        """Test upload without tag succeeds (no tag created)."""
+        project, package = test_package
+        content = b"upload without tag test"
+        expected_hash = compute_sha256(content)
+
+        files = {"file": ("no_tag.bin", io.BytesIO(content), "application/octet-stream")}
+        response = integration_client.post(
+            f"/api/v1/project/{project}/{package}/upload",
+            files=files,
+            # No tag parameter
+        )
+        assert response.status_code == 200
+        result = response.json()
+        assert result["artifact_id"] == expected_hash
+
+        # Verify no tag was created - list tags and check
+        tags_response = integration_client.get(
+            f"/api/v1/project/{project}/{package}/tags"
+        )
+        assert tags_response.status_code == 200
+        tags = tags_response.json()
+        # Filter for tags pointing to this artifact
+        artifact_tags = [t for t in tags.get("items", tags) if t.get("artifact_id") == expected_hash]
+        assert len(artifact_tags) == 0, "Tag should not be created when not specified"
+
+    @pytest.mark.integration
+    def test_upload_creates_artifact_in_database(self, integration_client, test_package):
+        """Test upload creates artifact record in database."""
+        project, package = test_package
+        content = b"database artifact test"
+        expected_hash = compute_sha256(content)
+
+        upload_test_file(integration_client, project, package, content)
+
+        # Verify artifact exists via API
+        response = integration_client.get(f"/api/v1/artifact/{expected_hash}")
+        assert response.status_code == 200
+        artifact = response.json()
+        assert artifact["id"] == expected_hash
+        assert artifact["size"] == len(content)
+
+    @pytest.mark.integration
+    def test_upload_creates_object_in_s3(self, integration_client, test_package):
+        """Test upload creates object in S3 storage."""
+        project, package = test_package
+        content = b"s3 object creation test"
+        expected_hash = compute_sha256(content)
+
+        upload_test_file(integration_client, project, package, content)
+
+        # Verify S3 object exists
+        assert s3_object_exists(expected_hash), "S3 object should exist after upload"
+
+    @pytest.mark.integration
+    def test_upload_with_tag_creates_tag_record(self, integration_client, test_package):
+        """Test upload with tag creates tag record."""
+        project, package = test_package
+        content = b"tag creation test"
+        expected_hash = compute_sha256(content)
+        tag_name = "my-tag-v1"
+
+        upload_test_file(
+            integration_client, project, package, content, tag=tag_name
+        )
+
+        # Verify tag exists
+        tags_response = integration_client.get(
+            f"/api/v1/project/{project}/{package}/tags"
+        )
+        assert tags_response.status_code == 200
+        tags = tags_response.json()
+        tag_names = [t["name"] for t in tags.get("items", tags)]
+        assert tag_name in tag_names
+
 
 class TestDuplicateUploads:
     """Tests for duplicate upload deduplication behavior."""
@@ -248,6 +337,23 @@ class TestDownload:
         assert response.status_code == 200
         assert response.content == original_content
 
+    @pytest.mark.integration
+    def test_download_by_tag_prefix(self, integration_client, test_package):
+        """Test downloading artifact using tag: prefix."""
+        project, package = test_package
+        original_content = b"download by tag prefix test"
+
+        upload_test_file(
+            integration_client, project, package, original_content, tag="prefix-tag"
+        )
+
+        response = integration_client.get(
+            f"/api/v1/project/{project}/{package}/+/tag:prefix-tag",
+            params={"mode": "proxy"},
+        )
+        assert response.status_code == 200
+        assert response.content == original_content
+
     @pytest.mark.integration
     def test_download_nonexistent_tag(self, integration_client, test_package):
         """Test downloading nonexistent tag returns 404."""
@@ -258,6 +364,33 @@ class TestDownload:
         )
         assert response.status_code == 404
 
+    @pytest.mark.integration
+    def test_download_nonexistent_artifact(self, integration_client, test_package):
+        """Test downloading nonexistent artifact ID returns 404."""
+        project, package = test_package
+        fake_hash = "0" * 64
+
+        response = integration_client.get(
+            f"/api/v1/project/{project}/{package}/+/artifact:{fake_hash}"
+        )
+        assert response.status_code == 404
+
+    @pytest.mark.integration
+    def test_download_from_nonexistent_project(self, integration_client, unique_test_id):
+        """Test downloading from nonexistent project returns 404."""
+        response = integration_client.get(
+            f"/api/v1/project/nonexistent-project-{unique_test_id}/somepackage/+/sometag"
+        )
+        assert response.status_code == 404
+
+    @pytest.mark.integration
+    def test_download_from_nonexistent_package(self, integration_client, test_project, unique_test_id):
+        """Test downloading from nonexistent package returns 404."""
+        response = integration_client.get(
+            f"/api/v1/project/{test_project}/nonexistent-package-{unique_test_id}/+/sometag"
+        )
+        assert response.status_code == 404
+
     @pytest.mark.integration
     def test_content_matches_original(self, integration_client, test_package):
         """Test downloaded content matches original exactly."""
@@ -275,6 +408,111 @@ class TestDownload:
         assert response.content == original_content
 
 
+class TestDownloadHeaders:
+    """Tests for download response headers."""
+
+    @pytest.mark.integration
+    def test_download_content_type_header(self, integration_client, test_package):
+        """Test download returns correct Content-Type header."""
+        project, package = test_package
+        content = b"content type header test"
+
+        upload_test_file(
+            integration_client, project, package, content,
+            filename="test.txt", tag="content-type-test"
+        )
+
+        response = integration_client.get(
+            f"/api/v1/project/{project}/{package}/+/content-type-test",
+            params={"mode": "proxy"},
+        )
+        assert response.status_code == 200
+        # Content-Type should be set (either text/plain or application/octet-stream)
+        assert "content-type" in response.headers
+
+    @pytest.mark.integration
+    def test_download_content_length_header(self, integration_client, test_package):
+        """Test download returns correct Content-Length header."""
+        project, package = test_package
+        content = b"content length header test - exactly 41 bytes!"
+        expected_length = len(content)
+
+        upload_test_file(
+            integration_client, project, package, content, tag="content-length-test"
+        )
+
+        response = integration_client.get(
+            f"/api/v1/project/{project}/{package}/+/content-length-test",
+            params={"mode": "proxy"},
+        )
+        assert response.status_code == 200
+        assert "content-length" in response.headers
+        assert int(response.headers["content-length"]) == expected_length
+
+    @pytest.mark.integration
+    def test_download_content_disposition_header(self, integration_client, test_package):
+        """Test download returns correct Content-Disposition header."""
+        project, package = test_package
+        content = b"content disposition test"
+        filename = "my-test-file.bin"
+
+        upload_test_file(
+            integration_client, project, package, content,
+            filename=filename, tag="disposition-test"
+        )
+
+        response = integration_client.get(
+            f"/api/v1/project/{project}/{package}/+/disposition-test",
+            params={"mode": "proxy"},
+        )
+        assert response.status_code == 200
+        assert "content-disposition" in response.headers
+        disposition = response.headers["content-disposition"]
+        assert "attachment" in disposition
+        assert filename in disposition
+
+    @pytest.mark.integration
+    def test_download_checksum_headers(self, integration_client, test_package):
+        """Test download returns checksum headers."""
+        project, package = test_package
+        content = b"checksum header test content"
+        expected_hash = compute_sha256(content)
+
+        upload_test_file(
+            integration_client, project, package, content, tag="checksum-headers"
+        )
+
+        response = integration_client.get(
+            f"/api/v1/project/{project}/{package}/+/checksum-headers",
+            params={"mode": "proxy"},
+        )
+        assert response.status_code == 200
+        # Check for checksum headers
+        assert "x-checksum-sha256" in response.headers
+        assert response.headers["x-checksum-sha256"] == expected_hash
+
+    @pytest.mark.integration
+    def test_download_etag_header(self, integration_client, test_package):
+        """Test download returns ETag header (artifact ID)."""
+        project, package = test_package
+        content = b"etag header test"
+        expected_hash = compute_sha256(content)
+
+        upload_test_file(
+            integration_client, project, package, content, tag="etag-test"
+        )
+
+        response = integration_client.get(
+            f"/api/v1/project/{project}/{package}/+/etag-test",
+            params={"mode": "proxy"},
+        )
+        assert response.status_code == 200
+        assert "etag" in response.headers
+        # ETag should contain the artifact ID (hash)
+        etag = response.headers["etag"].strip('"')
+        assert etag == expected_hash
+
+
 class TestConcurrentUploads:
     """Tests for concurrent upload handling."""