diff --git a/clients/rust/README.md b/clients/rust/README.md
index cf2f0e1..bc72195 100644
--- a/clients/rust/README.md
+++ b/clients/rust/README.md
@@ -28,11 +28,13 @@ tarball, where the rest of the mxaccessgw repo does not exist. The vendored
 copies are shipped in the published `.crate` via `Cargo.toml`'s `include`
 list, which is what makes the crate buildable standalone; they are build
 inputs only, never a second source of truth. **Refresh rule:** any commit
-that edits a Contracts proto (`mxaccess_gateway.proto`, `mxaccess_worker.proto`,
-`galaxy_repository.proto`) must copy the changed file(s) into
-`clients/rust/protos/` in that same commit — `scripts/check-codegen.ps1`
-Check 3 fails the build on byte drift between the vendored copies and the
-canonical Contracts protos. `tonic`/`prost` bindings are generated into
+that edits *or adds* a Contracts proto (today `mxaccess_gateway.proto`,
+`mxaccess_worker.proto`, `galaxy_repository.proto`) must copy the changed or
+new file(s) into `clients/rust/protos/` in that same commit —
+`scripts/check-codegen.ps1` Check 3 fails the build on byte drift between the
+vendored copies and the canonical Contracts protos, and equally on a canonical
+proto that has no vendored copy (which an in-repo build cannot catch, since
+`build.rs` reads the canonical directory here). `tonic`/`prost` bindings are generated into
 Cargo build output. `src/generated.rs` declares the Rust modules that
 include those generated files. `src/generated` remains reserved for checked-in
 generator output if the crate later changes to source-tree generation.
diff --git a/docs/ClientPackaging.md b/docs/ClientPackaging.md
index 8833e72..419b5f9 100644
--- a/docs/ClientPackaging.md
+++ b/docs/ClientPackaging.md
@@ -206,7 +206,8 @@ publish` ship them inside the `.crate`, making the crate buildable standalone
 with no access to the rest of the mxaccessgw repo. Any Contracts proto change
 must refresh `clients/rust/protos/` in the same commit; `scripts/check-codegen.ps1`
 Check 3 byte-compares the vendored copies against the canonical protos and
-fails on drift. Because the vendored protos make a standalone build possible,
+fails on drift — in both directions, so a newly added canonical proto that
+was never vendored fails there rather than at a consumer's standalone build. Because the vendored protos make a standalone build possible,
 `cargo package`/`cargo publish` run **with** verification (no `--no-verify`) —
 a `cargo package` that cannot build from the vendored tree alone would mean
 the vendored copies are stale, and verification is what catches that before
diff --git a/docs/GatewayTesting.md b/docs/GatewayTesting.md
index 8679b12..d87cd63 100644
--- a/docs/GatewayTesting.md
+++ b/docs/GatewayTesting.md
@@ -689,6 +689,11 @@ committed client descriptor set (Check 1), the C# `Generated/` (Check 2), the Ru
 protos (Check 3), or the Go/Python client bindings (Check 4, IPC-25) no longer match the current
 `.proto` sources — the codegen drift class this repo has hit repeatedly (stale client
 descriptors, net48 `CS0246` on unregenerated protos, silently stale Go/Python worker bindings).
+Check 3 sweeps both directions: a vendored copy that drifted from (or has no) canonical proto
+fails, and so does a canonical proto with no vendored copy at all. The second direction matters
+because `clients/rust/build.rs` prefers the canonical directory whenever it exists, so an
+unvendored proto builds fine in-repo and only breaks the standalone crate build a consumer runs
+from the published tarball — drift no in-repo build can surface.
 Check 4 regenerates the Go and Python bindings with their pinned generators (`protoc-gen-go`
 v1.36.11 / `protoc-gen-go-grpc` 1.6.2, `grpcio-tools` 1.80.0) and fails on any diff; a missing
 generator fails the check rather than skipping it. The **primary** guard for the
diff --git a/scripts/check-codegen.ps1 b/scripts/check-codegen.ps1
index ab19901..a7b7038 100644
--- a/scripts/check-codegen.ps1
+++ b/scripts/check-codegen.ps1
@@ -11,8 +11,11 @@
 #      (which breaks the net48 worker build with CS0246 — see docs/Contracts.md).
 #   3. The Rust crate's vendored protos (clients/rust/protos/*.proto — build inputs that make the
 #      crate buildable outside the repo, CLI-02) are byte-identical to the canonical Contracts
-#      protos. A drift means a .proto was edited without refreshing the vendored copies, which would
-#      publish a stale wire contract to crate consumers while the in-repo build stays correct.
+#      protos, and the two directories hold the same set of files. A drift means a .proto was edited
+#      without refreshing the vendored copies, which would publish a stale wire contract to crate
+#      consumers while the in-repo build stays correct. The sweep runs both directions: a canonical
+#      proto with no vendored copy is just as broken (the published crate cannot build standalone),
+#      and it is invisible in-repo because build.rs prefers the canonical directory when it exists.
 #   4. The committed Go and Python client bindings match a fresh regeneration (IPC-25). The two
 #      per-client generate-proto.ps1 scripts pin their generators (protoc-gen-go v1.36.11 /
 #      protoc-gen-go-grpc v1.6.2 for Go; grpcio-tools 1.80.0 for Python), so a clean checkout
@@ -88,6 +91,16 @@ try {
             $failures.Add("Rust vendored proto drifted from canonical: clients/rust/protos/$($vendored.Name). Refresh it from src/ZB.MOM.WW.MxGateway.Contracts/Protos/$($vendored.Name).")
         }
     }
+
+    # Reverse direction: a canonical proto that was never vendored passes the loop above (it only
+    # walks the vendored dir) but breaks a standalone crate build, because build.rs falls back to
+    # clients/rust/protos/ only outside the repo — in-repo it reads the canonical dir and stays green.
+    foreach ($canonicalProto in Get-ChildItem -Path $canonicalProtoDir -Filter '*.proto' -File) {
+        $vendoredCounterpart = Join-Path $vendoredProtoDir $canonicalProto.Name
+        if (-not (Test-Path $vendoredCounterpart)) {
+            $failures.Add("Canonical proto is not vendored for the Rust crate: $($canonicalProto.Name). Copy src/ZB.MOM.WW.MxGateway.Contracts/Protos/$($canonicalProto.Name) to clients/rust/protos/$($canonicalProto.Name) (and add it to build.rs's input list).")
+        }
+    }
 }
 catch {
     $failures.Add("Rust vendored proto check failed: $($_.Exception.Message)")