diff --git a/README.md b/README.md
index 2ec05a9..a6e9997 100644
--- a/README.md
+++ b/README.md
@@ -28,7 +28,7 @@ Why `.github` and not a dedicated `github-actions` repo: `.github` is *the* GitH
 | `setup-python-uv` | Install uv + a pinned Python version + (default-on) `uv sync`. |
 | `setup-node-pnpm` | corepack + setup-node@v4 with pnpm cache + (default-on) `pnpm install --frozen-lockfile`. Accepts a `pnpm-filter` input for workspace filtering. |
 | `setup-dotnet` | setup-dotnet@v5 with NuGet cache keyed on `**/*.csproj` + (default-off) `dotnet tool restore`. |
-| `setup-go` | setup-go@v6 reading version from `go.mod`. Optional `private-modules: true` mints a short-lived read-only `pinpredict-argocd` App token and configures git + `GOPRIVATE` so `go`/`golangci-lint`/`goreleaser` fetch a private pinpredict module (e.g. `github.com/pinpredict/ppkit`) without vendoring — the non-Docker analogue of `docker-release.yml`'s `private-modules` secret. Default false. |
+| `setup-go` | setup-go@v6 reading version from `go.mod`, with the Go module + build caches keyed **per calling job** (`github.job`) instead of setup-go's built-in single-per-go.sum key — under the built-in scheme the first job to save wins the key and e.g. a `go test -race -cover…` job recompiles the whole dependency graph on every run (~3.5 min in k5s vs ~30 s of tests) because its race-instrumented artifacts are never saved. `cache-name` overrides the scope (fixed name to share between jobs; `"false"` disables). Optional `private-modules: true` mints a short-lived read-only `pinpredict-argocd` App token and configures git + `GOPRIVATE` so `go`/`golangci-lint`/`goreleaser` fetch a private pinpredict module (e.g. `github.com/pinpredict/ppkit`) without vendoring — the non-Docker analogue of `docker-release.yml`'s `private-modules` secret. Default false. |
 
 #### Language setup composites — usage
 
@@ -58,6 +58,9 @@ Why `.github` and not a dedicated `github-actions` repo: `.github` is *the* GitH
 - uses: pinpredict/.github/actions/setup-go@main
   with:
     go-version-file: "go.mod" # optional; default "go.mod"
+    # cache-name: "shared"    # optional; default "" = per-job cache scope
+                              # (github.job). Fixed name shares a cache across
+                              # jobs that compile identically; "false" disables.
 
 # Go, fetching a private pinpredict module without vendoring (e.g. k4a → ppkit)
 - uses: pinpredict/.github/actions/setup-go@main
diff --git a/actions/setup-go/action.yml b/actions/setup-go/action.yml
index 1d67c2a..d6191bc 100644
--- a/actions/setup-go/action.yml
+++ b/actions/setup-go/action.yml
@@ -4,10 +4,20 @@ description: |
   recipe for pinpredict Go repos (service-template and anything templated
   from it).
 
-  actions/setup-go@v6 caches the Go SDK + module cache by default; warm
-  runs skip both downloads. Reading the version from go.mod means the
-  caller doesn't pin Go in two places — bumping `go x.yy.z` in go.mod
-  drives CI.
+  Caches the Go module + build caches per CALLING JOB (the cache key
+  includes `github.job`) instead of using setup-go@v6's built-in cache.
+  The built-in cache keys once per go.sum for the whole repo, so the
+  first job to save wins the key and every sibling job restores
+  artifacts compiled with the wrong flags, recompiles from scratch, and
+  never saves its own ("Cache hit occurred on the primary key …, not
+  saving cache"). Concretely: a `go test -race -coverprofile` job kept
+  restoring the build job's plain artifacts and recompiled the entire
+  dependency graph on every run (~3.5 min in k5s against ~30 s of actual
+  test time). Per-job keys keep each flag set (plain build, race+cover
+  test, goreleaser) warm independently. `cache-name` overrides the
+  scope for jobs that should share; `cache-name: "false"` disables
+  caching. Reading the version from go.mod means the caller doesn't pin
+  Go in two places — bumping `go x.yy.z` in go.mod drives CI.
 
   Optional `private-modules: true` mints a short-lived, read-only
   pinpredict-argocd App token and configures git + GOPRIVATE so `go`,
@@ -21,6 +31,15 @@ inputs:
     description: "Path to go.mod (or any file with a `go` directive)."
     required: false
     default: "go.mod"
+  cache-name:
+    description: >-
+      Scope name for the Go module + build cache. Empty (the default)
+      scopes the cache to the calling job's id (`github.job`), so each
+      job's differently-flagged compile artifacts stay warm
+      independently. Set a fixed name to share one cache between jobs
+      that compile identically, or "false" to skip caching entirely.
+    required: false
+    default: ""
   private-modules:
     description: >-
       When true, mint a read-only pinpredict-argocd App token from
@@ -51,6 +70,29 @@ runs:
       uses: actions/setup-go@v6
       with:
         go-version-file: ${{ inputs.go-version-file }}
+        # Built-in caching stays off: it keys ONE cache per go.sum for every
+        # job in the repo, so only the first job to save ever populates it and
+        # differently-flagged sibling jobs (e.g. `go test -race -cover…`)
+        # recompile the world on every run. The per-job cache below replaces it.
+        cache: false
+
+    # Module + build caches, scoped per calling job (or per `cache-name`).
+    # restore-keys fall back first to the same scope with an older go.sum
+    # (near-warm after a dep bump), then to any scope (a cold test job still
+    # inherits the build job's module download cache). Paths are the Linux
+    # defaults — every pinpredict runner (GitHub-hosted and the EKS/ARC scale
+    # sets) is Linux; a macOS caller would just miss the cache, not break.
+    - name: Restore Go module + build caches
+      if: ${{ inputs.cache-name != 'false' }}
+      uses: actions/cache@v6
+      with:
+        path: |
+          ~/go/pkg/mod
+          ~/.cache/go-build
+        key: setup-go-job-${{ runner.os }}-${{ runner.arch }}-${{ inputs.cache-name || github.job }}-${{ hashFiles(inputs.go-version-file, '**/go.sum') }}
+        restore-keys: |
+          setup-go-job-${{ runner.os }}-${{ runner.arch }}-${{ inputs.cache-name || github.job }}-
+          setup-go-job-${{ runner.os }}-${{ runner.arch }}-
 
     # Opt-in (private-modules: true): mint a short-lived, read-only token for
     # the org-wide pinpredict-argocd App so `go mod download` / golangci /