diff --git a/companions/proto/Dockerfile b/companions/proto/Dockerfile
index baf8f559..1d757a56 100644
--- a/companions/proto/Dockerfile
+++ b/companions/proto/Dockerfile
@@ -76,11 +76,6 @@ COPY --from=codeflybase /bin/codefly /bin/codefly
 #   - Go-built plugins (buf, protoc-gen-*) ARE static (CGO_ENABLED=0
 #     explicitly in the builder), so we COPY just those binaries
 #     without bringing the Go runtime.
-# HOME=/tmp so the companion works when run as an arbitrary non-root UID
-# (codefly runs it as the host user on Linux CI). buf and npm/npx write
-# their caches under $HOME; the default /root isn't writable by that user.
-ENV HOME=/tmp
-
 # python3 + a dedicated /venv host the Python facade + strip plugins. The venv
 # is built here (not copied from the builder) so it is bound to this stage's
 # Python 3.13 — the builder may ship a different Python minor version, and a
@@ -146,11 +141,22 @@ RUN printf '#!/bin/sh\nexec /venv/bin/python /opt/codefly-facades/python/protoc_
 
 # Exercise target executables in both architecture builds. A successful native
 # compile alone would not catch copying the host Go distribution into runtime.
+# Keep installation caches under /root, then use the runtime home as a non-root
+# UID. The temporary mount prevents smoke checks from baking owned caches into
+# /tmp, where a different host UID must create its own Buf/Go/npm caches.
+ENV HOME=/tmp
+USER 65532:65532
 ARG TARGETARCH
-RUN test "$(go env GOARCH)" = "$TARGETARCH" && \
+RUN --mount=type=tmpfs,target=/tmp \
+    test "$(go env GOARCH)" = "$TARGETARCH" && \
     buf --version && \
+    mkdir /tmp/proto-check && \
+    printf 'syntax = "proto3";\npackage cachecheck;\nmessage Check {}\n' > /tmp/proto-check/check.proto && \
+    buf build /tmp/proto-check --output /tmp/proto-check.bin && \
     protoc-gen-go --version && \
     protoc-gen-go-grpc --version && \
-    swagger version
+    swagger version && \
+    npm cache verify
 
+USER 0:0
 WORKDIR /workspace
diff --git a/companions/proto/README.md b/companions/proto/README.md
index 09c83154..af198f6b 100644
--- a/companions/proto/README.md
+++ b/companions/proto/README.md
@@ -63,6 +63,10 @@ it never compiles under ARM emulation. A separate target-platform Go image
 supplies the runtime `go`/`gofmt`, so cross-building cannot accidentally ship
 the build host's compiler. Each image build executes Buf, the Go generators,
 Swagger and the runtime Go architecture check on its target platform.
+These checks run as a non-root user with temporary caches that never enter the
+image. Installation caches stay under `/root`; runtime tools use `/tmp` so
+Linux callers with arbitrary host UIDs can create their own Buf, Go, and npm
+caches. The tagged companion suite verifies this with a different non-root UID.
 Upstream downloads were checked: Buf 1.73.0's
 prebuilt binary carries Go 1.26.8 and x/net 0.58.0, including fixed findings,
 and several other plugin downloads carry still older Go toolchains. Retaining
@@ -111,8 +115,9 @@ may also contain handwritten Go, so only files carrying Go's standard
 
 `info.codefly.yaml` carries the image version. Version 0.0.21 removes Rust
 generation and its build dependencies; 0.0.22 removes emulated Go compilation.
+Version 0.0.23 fixes non-root runtime caches left owned by root in 0.0.22.
 Publish through `codefly companion publish` using the [publication procedure](../../docs/runbooks/publish-companions.md);
-changing the manifest alone does not publish an image. Core 0.19.14 selects
+changing the manifest alone does not publish an image. Core 0.19.15 selects
 this companion.
 
 ## Python generation
diff --git a/companions/proto/companion_plugins_test.go b/companions/proto/companion_plugins_test.go
index 2576d5e3..cbc006c7 100644
--- a/companions/proto/companion_plugins_test.go
+++ b/companions/proto/companion_plugins_test.go
@@ -313,7 +313,7 @@ func TestTsFacadeLockfileMatchesTheRuntime(t *testing.T) {
 // neighbour — the reader then checks the wrong line, finds something plausible,
 // and moves on.
 var runbookCitations = map[string]string{
-	"Dockerfile:94": "@bufbuild/protoc-gen-es@",
+	"Dockerfile:89": "@bufbuild/protoc-gen-es@",
 	"flake.nix:51":  "protocGenEsVersion =",
 	"flake.nix:59":  "hash =",
 	"flake.nix:61":  "npmDepsHash =",
diff --git a/companions/proto/info.codefly.yaml b/companions/proto/info.codefly.yaml
index 0a9d1e31..074a5d62 100644
--- a/companions/proto/info.codefly.yaml
+++ b/companions/proto/info.codefly.yaml
@@ -1 +1 @@
-version: 0.0.22
+version: 0.0.23
diff --git a/docs/runbooks/bump-protobuf-es.md b/docs/runbooks/bump-protobuf-es.md
index 7273b091..f46111b3 100644
--- a/docs/runbooks/bump-protobuf-es.md
+++ b/docs/runbooks/bump-protobuf-es.md
@@ -103,7 +103,7 @@ Six places, and they must agree:
 |---|---|---|
 | `companions/proto/facades/ts/package.json` | `@bufbuild/protoplugin` | yes |
 | `companions/proto/facades/ts/package-lock.json` | the resolved `protoplugin` + `protobuf` closure | yes |
-| `companions/proto/Dockerfile:94` | `@bufbuild/protoc-gen-es@` installed via npm | yes |
+| `companions/proto/Dockerfile:89` | `@bufbuild/protoc-gen-es@` installed via npm | yes |
 | `companions/proto/flake.nix:51` | `protocGenEsVersion`, built from source | yes |
 | `companions/proto/flake.nix:59` | `hash` — the protobuf-es source tarball | **no** |
 | `companions/proto/flake.nix:61` | `npmDepsHash` — its npm dependency closure | **no** |
diff --git a/internal/ciguard/proto_companion_test.go b/internal/ciguard/proto_companion_test.go
index cff222d5..f52984ae 100644
--- a/internal/ciguard/proto_companion_test.go
+++ b/internal/ciguard/proto_companion_test.go
@@ -47,6 +47,20 @@ func TestSelectedProtoCompanionShipsGoimports(t *testing.T) {
 	require.Contains(t, string(output), "goimports")
 }
 
+// Linux runs companions as the host UID. Build-time checks must not leave
+// root-owned caches that pass on macOS (root in Docker) and fail on Linux.
+func TestSelectedProtoCompanionCachesWorkAsNonRoot(t *testing.T) {
+	ctx := t.Context()
+	testutil.RequireProtoImage(t, ctx)
+	img, err := proto.CompanionImage(ctx)
+	require.NoError(t, err)
+	fixture := filepath.Join(repoRoot(t), "companions", "proto", "testdata", "facade", "streaming")
+	output, err := exec.CommandContext(ctx, "docker", "run", "--rm", "--network", "none",
+		"--user", "1001:1001", "-v", fixture+":/proto-check:ro", "--entrypoint", "sh", img.FullName(), "-ec",
+		"buf build /proto-check --output /tmp/proto-check.bin && swagger version && npm cache verify").CombinedOutput()
+	require.NoError(t, err, "the selected companion must create its caches as an arbitrary host UID: %s", output)
+}
+
 // goimports names a package through `go list` in the consumer's module and
 // refuses a module whose go.mod requires a newer Go than the image carries,
 // then formats syntax only. Ask the selected image which Go it runs and hold
diff --git a/version/info.codefly.yaml b/version/info.codefly.yaml
index 309a17ff..aee57416 100644
--- a/version/info.codefly.yaml
+++ b/version/info.codefly.yaml
@@ -1 +1 @@
-version: 0.19.14
+version: 0.19.15