diff --git a/CHANGELOG.md b/CHANGELOG.md
index 3082ee6..50785ce 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -19,6 +19,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - 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 version API tests: version creation, auto-detection, listing, download by version prefix, deletion
+- Added integrity verification tests: round-trip hash verification, client-side verification workflow, size variants (1KB-10MB)
+- Added consistency check endpoint tests with response format validation
+- Added corruption detection tests: bit flip, truncation, appended content, size mismatch, missing S3 objects
+- Added Digest header tests (RFC 3230) and verification mode tests
 - Added `package_versions` table for immutable version tracking separate from mutable tags (#56)
   - Versions are set at upload time via explicit `version` parameter or auto-detected from filename/metadata
   - Version detection priority: explicit parameter > package metadata > filename pattern
diff --git a/backend/tests/integration/test_integrity_verification.py b/backend/tests/integration/test_integrity_verification.py
new file mode 100644
index 0000000..556fc2a
--- /dev/null
+++ b/backend/tests/integration/test_integrity_verification.py
@@ -0,0 +1,764 @@
+"""
+Integration tests for artifact integrity verification.
+
+Tests cover:
+- Round-trip verification (upload -> download -> verify hash)
+- Consistency check endpoint
+- Header-based verification
+- Integrity verification across file sizes
+- Client-side verification workflow
+"""
+
+import pytest
+import io
+import hashlib
+from tests.factories import (
+    compute_sha256,
+    upload_test_file,
+    generate_content_with_hash,
+    s3_object_exists,
+    get_s3_client,
+    get_s3_bucket,
+)
+from tests.conftest import (
+    SIZE_1KB,
+    SIZE_10KB,
+    SIZE_100KB,
+    SIZE_1MB,
+    SIZE_10MB,
+)
+
+
+class TestRoundTripVerification:
+    """Tests for complete round-trip integrity verification."""
+
+    @pytest.mark.integration
+    def test_upload_download_hash_matches(self, integration_client, test_package):
+        """Test that upload -> download round trip preserves content integrity."""
+        project, package = test_package
+        content = b"Round trip integrity test content"
+        expected_hash = compute_sha256(content)
+
+        # Upload and capture returned hash
+        result = upload_test_file(
+            integration_client, project, package, content, tag="roundtrip"
+        )
+        uploaded_hash = result["artifact_id"]
+
+        # Verify upload returned correct hash
+        assert uploaded_hash == expected_hash
+
+        # Download artifact
+        response = integration_client.get(
+            f"/api/v1/project/{project}/{package}/+/roundtrip",
+            params={"mode": "proxy"},
+        )
+        assert response.status_code == 200
+
+        # Compute hash of downloaded content
+        downloaded_hash = compute_sha256(response.content)
+
+        # All three hashes should match
+        assert downloaded_hash == expected_hash
+        assert downloaded_hash == uploaded_hash
+
+    @pytest.mark.integration
+    def test_upload_response_contains_hash(self, integration_client, test_package):
+        """Test upload response contains artifact_id which is the SHA256 hash."""
+        project, package = test_package
+        content = b"Upload response hash test"
+        expected_hash = compute_sha256(content)
+
+        result = upload_test_file(integration_client, project, package, content)
+
+        assert "artifact_id" in result
+        assert result["artifact_id"] == expected_hash
+        assert len(result["artifact_id"]) == 64
+        assert all(c in "0123456789abcdef" for c in result["artifact_id"])
+
+    @pytest.mark.integration
+    def test_download_header_matches_artifact_id(self, integration_client, test_package):
+        """Test X-Checksum-SHA256 header matches artifact ID."""
+        project, package = test_package
+        content = b"Header verification test"
+        expected_hash = compute_sha256(content)
+
+        upload_test_file(
+            integration_client, project, package, content, tag="header-check"
+        )
+
+        response = integration_client.get(
+            f"/api/v1/project/{project}/{package}/+/header-check",
+            params={"mode": "proxy"},
+        )
+        assert response.status_code == 200
+        assert response.headers.get("X-Checksum-SHA256") == expected_hash
+
+    @pytest.mark.integration
+    def test_etag_matches_artifact_id(self, integration_client, test_package):
+        """Test ETag header matches artifact ID."""
+        project, package = test_package
+        content = b"ETag verification test"
+        expected_hash = compute_sha256(content)
+
+        upload_test_file(
+            integration_client, project, package, content, tag="etag-check"
+        )
+
+        response = integration_client.get(
+            f"/api/v1/project/{project}/{package}/+/etag-check",
+            params={"mode": "proxy"},
+        )
+        assert response.status_code == 200
+        etag = response.headers.get("ETag", "").strip('"')
+        assert etag == expected_hash
+
+    @pytest.mark.integration
+    def test_artifact_endpoint_returns_correct_hash(self, integration_client, test_package):
+        """Test artifact endpoint returns correct hash/ID."""
+        project, package = test_package
+        content = b"Artifact endpoint hash test"
+        expected_hash = compute_sha256(content)
+
+        upload_test_file(integration_client, project, package, content)
+
+        # Query artifact directly
+        response = integration_client.get(f"/api/v1/artifact/{expected_hash}")
+        assert response.status_code == 200
+        data = response.json()
+        assert data["id"] == expected_hash
+        assert data.get("sha256") == expected_hash
+
+
+class TestClientSideVerificationWorkflow:
+    """Tests for client-side verification workflow."""
+
+    @pytest.mark.integration
+    def test_client_can_verify_before_upload(self, integration_client, test_package):
+        """Test client can compute hash before upload and verify response matches."""
+        project, package = test_package
+        content = b"Client pre-upload verification test"
+
+        # Client computes hash locally before upload
+        client_hash = compute_sha256(content)
+
+        # Upload
+        result = upload_test_file(integration_client, project, package, content)
+
+        # Client verifies server returned the same hash
+        assert result["artifact_id"] == client_hash
+
+    @pytest.mark.integration
+    def test_client_can_provide_checksum_header(self, integration_client, test_package):
+        """Test client can provide X-Checksum-SHA256 header for verification."""
+        project, package = test_package
+        content = b"Client checksum header test"
+        client_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": client_hash},
+        )
+        assert response.status_code == 200
+        assert response.json()["artifact_id"] == client_hash
+
+    @pytest.mark.integration
+    def test_checksum_mismatch_rejected(self, integration_client, test_package):
+        """Test upload with wrong client checksum is rejected."""
+        project, package = test_package
+        content = b"Checksum mismatch 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
+
+    @pytest.mark.integration
+    def test_client_can_verify_after_download(self, integration_client, test_package):
+        """Test client can verify downloaded content matches header hash."""
+        project, package = test_package
+        content = b"Client post-download verification"
+
+        upload_test_file(
+            integration_client, project, package, content, tag="verify-after"
+        )
+
+        response = integration_client.get(
+            f"/api/v1/project/{project}/{package}/+/verify-after",
+            params={"mode": "proxy"},
+        )
+        assert response.status_code == 200
+
+        # Client gets hash from header
+        header_hash = response.headers.get("X-Checksum-SHA256")
+
+        # Client computes hash of downloaded content
+        downloaded_hash = compute_sha256(response.content)
+
+        # Client verifies they match
+        assert downloaded_hash == header_hash
+
+
+class TestIntegritySizeVariants:
+    """Tests for integrity verification across different file sizes."""
+
+    @pytest.mark.integration
+    def test_integrity_1kb(self, integration_client, test_package, sized_content):
+        """Test integrity verification for 1KB file."""
+        project, package = test_package
+        content, expected_hash = sized_content(SIZE_1KB, seed=100)
+
+        result = upload_test_file(
+            integration_client, project, package, content, tag="int-1kb"
+        )
+        assert result["artifact_id"] == expected_hash
+
+        response = integration_client.get(
+            f"/api/v1/project/{project}/{package}/+/int-1kb",
+            params={"mode": "proxy"},
+        )
+        assert response.status_code == 200
+        assert compute_sha256(response.content) == expected_hash
+        assert response.headers.get("X-Checksum-SHA256") == expected_hash
+
+    @pytest.mark.integration
+    def test_integrity_100kb(self, integration_client, test_package, sized_content):
+        """Test integrity verification for 100KB file."""
+        project, package = test_package
+        content, expected_hash = sized_content(SIZE_100KB, seed=101)
+
+        result = upload_test_file(
+            integration_client, project, package, content, tag="int-100kb"
+        )
+        assert result["artifact_id"] == expected_hash
+
+        response = integration_client.get(
+            f"/api/v1/project/{project}/{package}/+/int-100kb",
+            params={"mode": "proxy"},
+        )
+        assert response.status_code == 200
+        assert compute_sha256(response.content) == expected_hash
+        assert response.headers.get("X-Checksum-SHA256") == expected_hash
+
+    @pytest.mark.integration
+    def test_integrity_1mb(self, integration_client, test_package, sized_content):
+        """Test integrity verification for 1MB file."""
+        project, package = test_package
+        content, expected_hash = sized_content(SIZE_1MB, seed=102)
+
+        result = upload_test_file(
+            integration_client, project, package, content, tag="int-1mb"
+        )
+        assert result["artifact_id"] == expected_hash
+
+        response = integration_client.get(
+            f"/api/v1/project/{project}/{package}/+/int-1mb",
+            params={"mode": "proxy"},
+        )
+        assert response.status_code == 200
+        assert compute_sha256(response.content) == expected_hash
+        assert response.headers.get("X-Checksum-SHA256") == expected_hash
+
+    @pytest.mark.integration
+    @pytest.mark.slow
+    def test_integrity_10mb(self, integration_client, test_package, sized_content):
+        """Test integrity verification for 10MB file."""
+        project, package = test_package
+        content, expected_hash = sized_content(SIZE_10MB, seed=103)
+
+        result = upload_test_file(
+            integration_client, project, package, content, tag="int-10mb"
+        )
+        assert result["artifact_id"] == expected_hash
+
+        response = integration_client.get(
+            f"/api/v1/project/{project}/{package}/+/int-10mb",
+            params={"mode": "proxy"},
+        )
+        assert response.status_code == 200
+        assert compute_sha256(response.content) == expected_hash
+        assert response.headers.get("X-Checksum-SHA256") == expected_hash
+
+
+class TestConsistencyCheck:
+    """Tests for the admin consistency check endpoint."""
+
+    @pytest.mark.integration
+    def test_consistency_check_returns_200(self, integration_client):
+        """Test consistency check endpoint returns 200."""
+        response = integration_client.get("/api/v1/admin/consistency-check")
+        assert response.status_code == 200
+
+    @pytest.mark.integration
+    def test_consistency_check_response_format(self, integration_client):
+        """Test consistency check returns expected response format."""
+        response = integration_client.get("/api/v1/admin/consistency-check")
+        assert response.status_code == 200
+        data = response.json()
+
+        # Check expected fields
+        assert "total_artifacts_checked" in data
+        assert "orphaned_s3_objects" in data
+        assert "missing_s3_objects" in data
+        assert "size_mismatches" in data
+        assert "healthy" in data
+        assert "orphaned_s3_keys" in data
+        assert "missing_s3_keys" in data
+        assert "size_mismatch_artifacts" in data
+        # Verify types
+        assert isinstance(data["total_artifacts_checked"], int)
+        assert isinstance(data["orphaned_s3_objects"], int)
+        assert isinstance(data["missing_s3_objects"], int)
+        assert isinstance(data["size_mismatches"], int)
+        assert isinstance(data["healthy"], bool)
+        assert isinstance(data["orphaned_s3_keys"], list)
+        assert isinstance(data["missing_s3_keys"], list)
+        assert isinstance(data["size_mismatch_artifacts"], list)
+
+    @pytest.mark.integration
+    def test_consistency_check_after_upload(self, integration_client, test_package):
+        """Test consistency check passes after valid upload."""
+        project, package = test_package
+        content = b"Consistency check test content"
+
+        # Upload artifact
+        upload_test_file(integration_client, project, package, content)
+
+        # Run consistency check
+        response = integration_client.get("/api/v1/admin/consistency-check")
+        assert response.status_code == 200
+        data = response.json()
+
+        # Verify check ran and no issues
+        assert data["total_artifacts_checked"] >= 1
+        assert data["healthy"] is True
+
+    @pytest.mark.integration
+    def test_consistency_check_limit_parameter(self, integration_client):
+        """Test consistency check respects limit parameter."""
+        response = integration_client.get(
+            "/api/v1/admin/consistency-check",
+            params={"limit": 10}
+        )
+        assert response.status_code == 200
+        data = response.json()
+
+        # Lists should not exceed limit
+        assert len(data["orphaned_s3_keys"]) <= 10
+        assert len(data["missing_s3_keys"]) <= 10
+        assert len(data["size_mismatch_artifacts"]) <= 10
+
+
+class TestDigestHeader:
+    """Tests for RFC 3230 Digest header."""
+
+    @pytest.mark.integration
+    def test_download_includes_digest_header(self, integration_client, test_package):
+        """Test download includes Digest header in RFC 3230 format."""
+        project, package = test_package
+        content = b"Digest header test"
+        expected_hash = compute_sha256(content)
+
+        upload_test_file(
+            integration_client, project, package, content, tag="digest-test"
+        )
+
+        response = integration_client.get(
+            f"/api/v1/project/{project}/{package}/+/digest-test",
+            params={"mode": "proxy"},
+        )
+        assert response.status_code == 200
+        assert "Digest" in response.headers
+
+        # Verify Digest format (sha-256=base64hash)
+        digest = response.headers["Digest"]
+        assert digest.startswith("sha-256=")
+
+    @pytest.mark.integration
+    def test_digest_header_base64_valid(self, integration_client, test_package):
+        """Test Digest header contains valid base64 encoding."""
+        import base64
+
+        project, package = test_package
+        content = b"Digest base64 test"
+        expected_hash = compute_sha256(content)
+
+        upload_test_file(
+            integration_client, project, package, content, tag="digest-b64"
+        )
+
+        response = integration_client.get(
+            f"/api/v1/project/{project}/{package}/+/digest-b64",
+            params={"mode": "proxy"},
+        )
+        assert response.status_code == 200
+
+        digest = response.headers["Digest"]
+        base64_part = digest.split("=", 1)[1]
+
+        # Should be valid base64
+        try:
+            decoded = base64.b64decode(base64_part)
+            assert len(decoded) == 32  # SHA256 is 32 bytes
+        except Exception as e:
+            pytest.fail(f"Invalid base64 in Digest header: {e}")
+
+
+class TestVerificationModes:
+    """Tests for download verification modes."""
+
+    @pytest.mark.integration
+    def test_pre_verification_mode(self, integration_client, test_package):
+        """Test pre-verification mode verifies before streaming."""
+        project, package = test_package
+        content = b"Pre-verification mode test"
+
+        upload_test_file(
+            integration_client, project, package, content, tag="pre-verify"
+        )
+
+        response = integration_client.get(
+            f"/api/v1/project/{project}/{package}/+/pre-verify",
+            params={"mode": "proxy", "verify": "true", "verify_mode": "pre"},
+        )
+        assert response.status_code == 200
+        assert response.content == content
+
+        # X-Verified header should be true
+        assert response.headers.get("X-Verified") == "true"
+
+    @pytest.mark.integration
+    def test_stream_verification_mode(self, integration_client, test_package):
+        """Test streaming verification mode."""
+        project, package = test_package
+        content = b"Stream verification mode test"
+
+        upload_test_file(
+            integration_client, project, package, content, tag="stream-verify"
+        )
+
+        response = integration_client.get(
+            f"/api/v1/project/{project}/{package}/+/stream-verify",
+            params={"mode": "proxy", "verify": "true", "verify_mode": "stream"},
+        )
+        assert response.status_code == 200
+        assert response.content == content
+
+
+class TestArtifactIntegrityEndpoint:
+    """Tests for artifact-specific integrity operations."""
+
+    @pytest.mark.integration
+    def test_artifact_size_matches(self, integration_client, test_package):
+        """Test artifact endpoint returns correct size."""
+        project, package = test_package
+        content = b"Artifact size test content"
+        expected_size = len(content)
+
+        result = upload_test_file(integration_client, project, package, content)
+        artifact_id = result["artifact_id"]
+
+        response = integration_client.get(f"/api/v1/artifact/{artifact_id}")
+        assert response.status_code == 200
+        data = response.json()
+        assert data["size"] == expected_size
+
+    @pytest.mark.integration
+    def test_content_length_header_matches_size(self, integration_client, test_package):
+        """Test Content-Length header matches artifact size."""
+        project, package = test_package
+        content = b"Content-Length header test"
+        expected_size = len(content)
+
+        upload_test_file(
+            integration_client, project, package, content, tag="content-len"
+        )
+
+        response = integration_client.get(
+            f"/api/v1/project/{project}/{package}/+/content-len",
+            params={"mode": "proxy"},
+        )
+        assert response.status_code == 200
+        assert int(response.headers.get("Content-Length", 0)) == expected_size
+        assert len(response.content) == expected_size
+
+
+class TestCorruptionDetection:
+    """Tests for detecting corrupted S3 objects.
+
+    These tests directly manipulate S3 objects to simulate corruption
+    and verify that the system can detect hash mismatches.
+    """
+
+    @pytest.mark.integration
+    def test_detection_of_corrupted_content(self, integration_client, test_package):
+        """Test that corrupted S3 content is detected via hash mismatch.
+
+        Uploads content, then directly modifies the S3 object, then
+        verifies that the downloaded content hash doesn't match.
+        """
+        project, package = test_package
+        content = b"Original content for corruption test"
+        expected_hash = compute_sha256(content)
+
+        # Upload original content
+        result = upload_test_file(
+            integration_client, project, package, content, tag="corrupt-test"
+        )
+        assert result["artifact_id"] == expected_hash
+
+        # Get the S3 object and corrupt it
+        s3_client = get_s3_client()
+        bucket = get_s3_bucket()
+        s3_key = f"fruits/{expected_hash[:2]}/{expected_hash[2:4]}/{expected_hash}"
+
+        # Replace with corrupted content
+        corrupted_content = b"Corrupted content - different from original!"
+        s3_client.put_object(Bucket=bucket, Key=s3_key, Body=corrupted_content)
+
+        # Download via proxy (bypasses hash verification)
+        response = integration_client.get(
+            f"/api/v1/project/{project}/{package}/+/corrupt-test",
+            params={"mode": "proxy"},
+        )
+        assert response.status_code == 200
+
+        # Verify the downloaded content doesn't match original hash
+        downloaded_hash = compute_sha256(response.content)
+        assert downloaded_hash != expected_hash, "Corruption was not detected - hashes match"
+        assert response.content == corrupted_content
+
+        # The X-Checksum-SHA256 header should still show the original hash (from DB)
+        # but the actual content hash is different
+        header_hash = response.headers.get("X-Checksum-SHA256")
+        assert header_hash == expected_hash  # Header shows expected hash
+        assert downloaded_hash != header_hash  # But content is corrupted
+
+        # Restore original content for cleanup
+        s3_client.put_object(Bucket=bucket, Key=s3_key, Body=content)
+
+    @pytest.mark.integration
+    def test_detection_of_single_bit_flip(self, integration_client, test_package):
+        """Test detection of a single bit flip in S3 object content."""
+        project, package = test_package
+        content = b"Content for single bit flip detection test"
+        expected_hash = compute_sha256(content)
+
+        result = upload_test_file(
+            integration_client, project, package, content, tag="bitflip-test"
+        )
+        assert result["artifact_id"] == expected_hash
+
+        # Get S3 object and flip a single bit
+        s3_client = get_s3_client()
+        bucket = get_s3_bucket()
+        s3_key = f"fruits/{expected_hash[:2]}/{expected_hash[2:4]}/{expected_hash}"
+
+        # Flip the first bit of the first byte
+        corrupted_content = bytearray(content)
+        corrupted_content[0] ^= 0x01
+        corrupted_content = bytes(corrupted_content)
+
+        s3_client.put_object(Bucket=bucket, Key=s3_key, Body=corrupted_content)
+
+        # Download and verify hash mismatch
+        response = integration_client.get(
+            f"/api/v1/project/{project}/{package}/+/bitflip-test",
+            params={"mode": "proxy"},
+        )
+        assert response.status_code == 200
+
+        downloaded_hash = compute_sha256(response.content)
+        assert downloaded_hash != expected_hash, "Single bit flip not detected"
+
+        # Restore original
+        s3_client.put_object(Bucket=bucket, Key=s3_key, Body=content)
+
+    @pytest.mark.integration
+    def test_detection_of_truncated_content(self, integration_client, test_package):
+        """Test detection of truncated S3 object."""
+        project, package = test_package
+        content = b"This is content that will be truncated for testing purposes"
+        expected_hash = compute_sha256(content)
+
+        result = upload_test_file(
+            integration_client, project, package, content, tag="truncate-test"
+        )
+        assert result["artifact_id"] == expected_hash
+
+        # Get S3 object and truncate it
+        s3_client = get_s3_client()
+        bucket = get_s3_bucket()
+        s3_key = f"fruits/{expected_hash[:2]}/{expected_hash[2:4]}/{expected_hash}"
+
+        # Truncate to half the original size
+        truncated_content = content[: len(content) // 2]
+        s3_client.put_object(Bucket=bucket, Key=s3_key, Body=truncated_content)
+
+        # Download and verify hash mismatch
+        response = integration_client.get(
+            f"/api/v1/project/{project}/{package}/+/truncate-test",
+            params={"mode": "proxy"},
+        )
+        assert response.status_code == 200
+
+        downloaded_hash = compute_sha256(response.content)
+        assert downloaded_hash != expected_hash, "Truncation not detected"
+        assert len(response.content) < len(content), "Content was not truncated"
+
+        # Restore original
+        s3_client.put_object(Bucket=bucket, Key=s3_key, Body=content)
+
+    @pytest.mark.integration
+    def test_detection_of_appended_content(self, integration_client, test_package):
+        """Test detection of content with extra bytes appended."""
+        project, package = test_package
+        content = b"Original content"
+        expected_hash = compute_sha256(content)
+
+        result = upload_test_file(
+            integration_client, project, package, content, tag="append-test"
+        )
+        assert result["artifact_id"] == expected_hash
+
+        # Get S3 object and append extra bytes
+        s3_client = get_s3_client()
+        bucket = get_s3_bucket()
+        s3_key = f"fruits/{expected_hash[:2]}/{expected_hash[2:4]}/{expected_hash}"
+
+        appended_content = content + b" - extra bytes appended"
+        s3_client.put_object(Bucket=bucket, Key=s3_key, Body=appended_content)
+
+        # Download and verify hash mismatch
+        response = integration_client.get(
+            f"/api/v1/project/{project}/{package}/+/append-test",
+            params={"mode": "proxy"},
+        )
+        assert response.status_code == 200
+
+        downloaded_hash = compute_sha256(response.content)
+        assert downloaded_hash != expected_hash, "Appended content not detected"
+        assert len(response.content) > len(content), "Content was not extended"
+
+        # Restore original
+        s3_client.put_object(Bucket=bucket, Key=s3_key, Body=content)
+
+    @pytest.mark.integration
+    def test_client_detects_hash_mismatch_post_download(
+        self, integration_client, test_package
+    ):
+        """Test that a client can detect hash mismatch after downloading corrupted content.
+
+        This simulates the full client verification workflow:
+        1. Download content
+        2. Get expected hash from header
+        3. Compute actual hash of content
+        4. Verify they match (or detect corruption)
+        """
+        project, package = test_package
+        content = b"Content for client-side corruption detection"
+        expected_hash = compute_sha256(content)
+
+        result = upload_test_file(
+            integration_client, project, package, content, tag="client-detect"
+        )
+
+        # Corrupt the S3 object
+        s3_client = get_s3_client()
+        bucket = get_s3_bucket()
+        s3_key = f"fruits/{expected_hash[:2]}/{expected_hash[2:4]}/{expected_hash}"
+        corrupted = b"This is completely different content"
+        s3_client.put_object(Bucket=bucket, Key=s3_key, Body=corrupted)
+
+        # Simulate client download and verification
+        response = integration_client.get(
+            f"/api/v1/project/{project}/{package}/+/client-detect",
+            params={"mode": "proxy"},
+        )
+        assert response.status_code == 200
+
+        # Client gets expected hash from header
+        header_hash = response.headers.get("X-Checksum-SHA256")
+
+        # Client computes hash of downloaded content
+        actual_hash = compute_sha256(response.content)
+
+        # Client detects the mismatch
+        corruption_detected = actual_hash != header_hash
+        assert corruption_detected, "Client should detect hash mismatch"
+
+        # Restore original
+        s3_client.put_object(Bucket=bucket, Key=s3_key, Body=content)
+
+    @pytest.mark.integration
+    def test_consistency_check_detects_size_mismatch(
+        self, integration_client, test_package, unique_test_id
+    ):
+        """Test that consistency check detects size mismatches.
+
+        Uploads content, modifies S3 object size, then runs consistency check.
+        """
+        project, package = test_package
+        content = b"Content for size mismatch consistency check test " + unique_test_id.encode()
+        expected_hash = compute_sha256(content)
+
+        result = upload_test_file(
+            integration_client, project, package, content, tag="size-mismatch"
+        )
+
+        # Modify S3 object to have different size
+        s3_client = get_s3_client()
+        bucket = get_s3_bucket()
+        s3_key = f"fruits/{expected_hash[:2]}/{expected_hash[2:4]}/{expected_hash}"
+        different_size_content = content + b"extra extra extra"
+        s3_client.put_object(Bucket=bucket, Key=s3_key, Body=different_size_content)
+
+        # Run consistency check
+        response = integration_client.get("/api/v1/admin/consistency-check")
+        assert response.status_code == 200
+        data = response.json()
+
+        # Should detect the size mismatch
+        assert data["size_mismatches"] >= 1 or len(data["size_mismatch_artifacts"]) >= 1
+
+        # Restore original
+        s3_client.put_object(Bucket=bucket, Key=s3_key, Body=content)
+
+    @pytest.mark.integration
+    def test_consistency_check_detects_missing_s3_object(
+        self, integration_client, test_package, unique_test_id
+    ):
+        """Test that consistency check detects missing S3 objects.
+
+        Uploads content, deletes S3 object, then runs consistency check.
+        """
+        project, package = test_package
+        content = b"Content for missing S3 object test " + unique_test_id.encode()
+        expected_hash = compute_sha256(content)
+
+        result = upload_test_file(
+            integration_client, project, package, content, tag="missing-s3"
+        )
+
+        # Delete the S3 object
+        s3_client = get_s3_client()
+        bucket = get_s3_bucket()
+        s3_key = f"fruits/{expected_hash[:2]}/{expected_hash[2:4]}/{expected_hash}"
+        s3_client.delete_object(Bucket=bucket, Key=s3_key)
+
+        # Run consistency check
+        response = integration_client.get("/api/v1/admin/consistency-check")
+        assert response.status_code == 200
+        data = response.json()
+
+        # Should detect the missing S3 object
+        assert data["missing_s3_objects"] >= 1 or len(data["missing_s3_keys"]) >= 1
+
+        # Restore the object for cleanup
+        s3_client.put_object(Bucket=bucket, Key=s3_key, Body=content)