diff --git a/src/interface/ffi/src/main.zig b/src/interface/ffi/src/main.zig
index 2c6ef0a..649c7b4 100644
--- a/src/interface/ffi/src/main.zig
+++ b/src/interface/ffi/src/main.zig
@@ -246,10 +246,11 @@ export fn verisimiser_enable_dimension(
         return .invalid_param;
     };
 
-    // TODO: actually enable the dimension in the overlay index
-
-    clearError();
-    return .ok;
+    // The overlay index that persists per-entity dimension state is not yet
+    // wired into the FFI. Fail loudly rather than report a dimension as enabled
+    // when it is not (soundness: no silent success).
+    setError("octad dimension overlay not yet wired into the FFI");
+    return .sidecar_unavailable;
 }
 
 /// Get the active dimension bitmask for an entity.
@@ -296,10 +297,11 @@ export fn verisimiser_record_provenance(
         return .invalid_param;
     };
 
-    // TODO: compute SHA-256 hash, chain from previous, write to sidecar
-
-    clearError();
-    return .ok;
+    // The SHA-256 hash-chain sidecar append is not yet wired into the FFI.
+    // Fail loudly rather than report a provenance event as recorded when no
+    // entry was written (soundness: no phantom audit trail).
+    setError("provenance sidecar not yet wired into the FFI");
+    return .sidecar_unavailable;
 }
 
 /// Verify the integrity of an entity's provenance hash chain.
@@ -319,11 +321,12 @@ export fn verisimiser_verify_provenance(
 
     _ = entity_id;
 
-    // TODO: walk the hash chain, verify each link
-    // Return .chain_corrupted if any link fails verification
-
-    clearError();
-    return .ok;
+    // The hash-chain store is not yet wired into the FFI, so integrity cannot
+    // be confirmed. Return an error rather than .ok — reporting "verified" for
+    // an unchecked (possibly tampered) chain would be the worst kind of
+    // soundness hole.
+    setError("provenance sidecar not yet wired into the FFI; cannot verify integrity");
+    return .sidecar_unavailable;
 }
 
 /// Get the length of an entity's provenance chain.
@@ -364,10 +367,11 @@ export fn verisimiser_record_version(
     _ = snapshot_ptr;
     _ = snapshot_len;
 
-    // TODO: store snapshot in temporal sidecar with current timestamp
-
-    clearError();
-    return .ok;
+    // The temporal sidecar that stores version snapshots is not yet wired into
+    // the FFI. Fail loudly rather than report a version as recorded when no
+    // snapshot was stored (soundness: no phantom history).
+    setError("temporal sidecar not yet wired into the FFI");
+    return .sidecar_unavailable;
 }
 
 /// Query entity state at a specific point in time.
@@ -614,3 +618,46 @@ test "enable dimension with invalid dimension" {
     const result = verisimiser_enable_dimension(handle, 42, 99);
     try std.testing.expectEqual(Result.invalid_param, result);
 }
+
+// Soundness: the persistence-backed octad operations are not yet wired into
+// the FFI, so they must fail loudly rather than report a false success.
+
+test "verify provenance does not falsely report verified" {
+    const handle = verisimiser_init() orelse return error.InitFailed;
+    defer verisimiser_free(handle);
+
+    // A valid handle + entity must NOT return .ok while verification is unwired:
+    // claiming a chain is verified without checking it would be unsound.
+    const result = verisimiser_verify_provenance(handle, 42);
+    try std.testing.expect(result != Result.ok);
+    try std.testing.expectEqual(Result.sidecar_unavailable, result);
+}
+
+test "record provenance does not falsely report recorded" {
+    const handle = verisimiser_init() orelse return error.InitFailed;
+    defer verisimiser_free(handle);
+
+    const result = verisimiser_record_provenance(handle, 42, 0, 0);
+    try std.testing.expect(result != Result.ok);
+    try std.testing.expectEqual(Result.sidecar_unavailable, result);
+}
+
+test "record version does not falsely report stored" {
+    const handle = verisimiser_init() orelse return error.InitFailed;
+    defer verisimiser_free(handle);
+
+    const result = verisimiser_record_version(handle, 42, 0, 0);
+    try std.testing.expect(result != Result.ok);
+    try std.testing.expectEqual(Result.sidecar_unavailable, result);
+}
+
+test "enable dimension does not falsely report enabled" {
+    const handle = verisimiser_init() orelse return error.InitFailed;
+    defer verisimiser_free(handle);
+
+    // Dimension 2 (provenance) is a valid enum value; the call must still fail
+    // loudly because the overlay index is not wired in.
+    const result = verisimiser_enable_dimension(handle, 42, 2);
+    try std.testing.expect(result != Result.ok);
+    try std.testing.expectEqual(Result.sidecar_unavailable, result);
+}
diff --git a/src/manifest/mod.rs b/src/manifest/mod.rs
index 750d4f9..8ba2408 100644
--- a/src/manifest/mod.rs
+++ b/src/manifest/mod.rs
@@ -372,6 +372,43 @@ mod validate_manifest_tests {
         assert!(report.failed_count() == 0);
     }
 
+    /// A manifest that sets both `[database].backend` and the legacy
+    /// `target-db` to conflicting values must fail validation up front, not
+    /// silently pass and blow up later at generate time (V-L2-E1).
+    #[test]
+    fn conflicting_backend_fails_validation() {
+        let dir = tempfile::tempdir().expect("tempdir");
+        let path = dir.path().join("verisimiser.toml");
+        let sidecar_path = dir.path().join("sidecar.db");
+        let body = format!(
+            "[project]\n\
+             name = \"test\"\n\
+             [database]\n\
+             backend = \"sqlite\"\n\
+             target-db = \"postgresql\"\n\
+             [sidecar]\n\
+             storage = \"sqlite\"\n\
+             path = \"{}\"\n",
+            sidecar_path.display().to_string().replace('\\', "/")
+        );
+        std::fs::write(&path, body).expect("write");
+
+        let report = validate_manifest(path.to_str().unwrap());
+        assert!(
+            !report.passed,
+            "conflicting backend/target-db must fail validation; checks: {:?}",
+            report.checks
+        );
+        assert!(
+            report
+                .checks
+                .iter()
+                .any(|c| c.name == "backend-unambiguous" && !c.passed),
+            "expected a failed 'backend-unambiguous' check; checks: {:?}",
+            report.checks
+        );
+    }
+
     /// A schema-source pointing at a missing file must fail
     /// `schema-source-exists`.
     #[test]
@@ -960,6 +997,26 @@ pub fn validate_manifest(path: &str) -> ValidationReport {
                 },
             },
         );
+
+        // 5. Backend selection is unambiguous. `effective_backend()` rejects a
+        // manifest that sets both [database].backend and the legacy
+        // [database].target-db to conflicting values (V-L2-E1). Validation must
+        // exercise it, otherwise a latent conflict passes `validate` only to
+        // fail later at generate time.
+        let backend_check = ValidationCheck {
+            name: "backend-unambiguous".to_string(),
+            description: "[database].backend and legacy target-db do not conflict".to_string(),
+            passed: true,
+            detail: None,
+        };
+        checks.push(match m.database.effective_backend() {
+            Ok(_) => backend_check,
+            Err(e) => ValidationCheck {
+                passed: false,
+                detail: Some(e.to_string()),
+                ..backend_check
+            },
+        });
     }
 
     let passed = checks.iter().all(|c| c.passed);