diff --git a/.conformance-catalog-ref b/.conformance-catalog-ref
new file mode 100644
index 0000000..efa9db0
--- /dev/null
+++ b/.conformance-catalog-ref
@@ -0,0 +1 @@
+b4c758a7dac698d7fcacd32dafcd4bb2f5dbddaf
diff --git a/.github/scripts/fetch-conformance-catalog.sh b/.github/scripts/fetch-conformance-catalog.sh
new file mode 100755
index 0000000..5652a5b
--- /dev/null
+++ b/.github/scripts/fetch-conformance-catalog.sh
@@ -0,0 +1,53 @@
+#!/usr/bin/env bash
+#
+# Fetch the conformance catalog at the revision this repo pins.
+#
+# The catalog lives in github.com/AuthPlane/conformance and is updated
+# independently of this repo, so cloning its default branch would let a catalog
+# change turn an unrelated PR red here. The ref is pinned instead, single-sourced
+# from the tracked .conformance-catalog-ref at the repo root — bump it there when
+# adopting new catalog cases, together with the coverage for them, so a catalog
+# change can never break CI on its own.
+#
+# This script exists because the read/guard/fetch sequence is needed by more than
+# one workflow (ci.yml and release.yml). Keeping it inline in both meant the
+# guard could be tightened in one and not the other; the pin was single-sourced
+# but the logic reading it was not.
+#
+# Clones into $RUNNER_TEMP — outside $GITHUB_WORKSPACE — so the catalog stays out
+# of the working tree: it must never trip `go list ./...` or a coverage glob, and
+# `git add -A` in the release commit must never stage it as a gitlink.
+#
+# Requires: GITHUB_WORKSPACE, RUNNER_TEMP.
+
+set -euo pipefail
+
+: "${GITHUB_WORKSPACE:?GITHUB_WORKSPACE must be set}"
+: "${RUNNER_TEMP:?RUNNER_TEMP must be set}"
+
+REF_FILE="$GITHUB_WORKSPACE/.conformance-catalog-ref"
+DEST="$RUNNER_TEMP/conformance"
+CATALOG_REPO="https://github.com/AuthPlane/conformance.git"
+
+if [[ ! -f "$REF_FILE" ]]; then
+  echo "::error::$REF_FILE is missing; the conformance catalog revision is unpinned"
+  exit 1
+fi
+
+CONFORMANCE_CATALOG_REF="$(tr -d '[:space:]' < "$REF_FILE")"
+
+# Guard against un-pinning: the ref must be a full commit SHA, not a branch or
+# tag name, either of which would silently track a moving target.
+if ! grep -Eq '^[0-9a-f]{40}$' <<< "$CONFORMANCE_CATALOG_REF"; then
+  echo "::error::.conformance-catalog-ref must be a 40-hex commit SHA, got '$CONFORMANCE_CATALOG_REF'"
+  exit 1
+fi
+
+git init -q "$DEST"
+if ! git -C "$DEST" fetch --depth=1 "$CATALOG_REPO" "$CONFORMANCE_CATALOG_REF"; then
+  echo "::error::Pinned conformance catalog ref $CONFORMANCE_CATALOG_REF is unreachable"
+  exit 1
+fi
+git -C "$DEST" checkout -q FETCH_HEAD
+
+echo "Conformance catalog checked out at $CONFORMANCE_CATALOG_REF"
diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index e5d1adf..42dd3dd 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -38,10 +38,8 @@ jobs:
       # conformance suite).
       - name: Clone shared conformance catalog (out of tree)
         if: matrix.module == 'core'
-        run: |
-          git -c advice.detachedHead=false clone --depth=1 \
-            https://github.com/AuthPlane/conformance.git \
-            "$RUNNER_TEMP/conformance"
+        shell: bash
+        run: .github/scripts/fetch-conformance-catalog.sh
 
       - name: Setup Go
         uses: actions/setup-go@4a3601121dd01d1626a1e23e37211e3254c1c06c # v6.4.0
diff --git a/.github/workflows/conformance-catalog-drift.yml b/.github/workflows/conformance-catalog-drift.yml
new file mode 100644
index 0000000..700135a
--- /dev/null
+++ b/.github/workflows/conformance-catalog-drift.yml
@@ -0,0 +1,103 @@
+name: Conformance Catalog Drift
+
+# The main CI (ci.yml) and release (release.yml) workflows pin the shared
+# conformance catalog to a fixed SHA (.conformance-catalog-ref) so a catalog
+# change can never break PR CI on its own. The trade-off is that new catalog
+# cases stay invisible until someone bumps the pin. This job closes that gap:
+# on a weekly schedule it runs the SDK's catalog-alignment check against the
+# LATEST (unpinned) default branch of the catalog and FAILS the job on any
+# drift, so the scheduled run goes red and GitHub notifies maintainers (the
+# same convention as security.yml). This workflow has no pull_request trigger,
+# so a failure here can never block a PR.
+#
+# When this job fails on drift, adopt the new cases in core/conformancetests/
+# and bump .conformance-catalog-ref to the new catalog SHA in the same change.
+
+on:
+  schedule:
+    # Mondays at 06:00 UTC.
+    - cron: "0 6 * * 1"
+  workflow_dispatch:
+
+# Least-privilege default: this workflow only reads the repo.
+permissions:
+  contents: read
+
+jobs:
+  drift:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+
+      # Clone the catalog's DEFAULT branch (latest, unpinned) — deliberately
+      # NOT the pinned .conformance-catalog-ref — so newly added cases show up.
+      # Cloned to $RUNNER_TEMP, outside $GITHUB_WORKSPACE, so it stays out of
+      # the working tree. Source: github.com/AuthPlane/conformance.
+      - name: Clone latest conformance catalog (out of tree)
+        run: |
+          git clone --depth=1 https://github.com/AuthPlane/conformance.git \
+            "$RUNNER_TEMP/conformance"
+
+      - name: Setup Go
+        uses: actions/setup-go@4a3601121dd01d1626a1e23e37211e3254c1c06c # v6.4.0
+        with:
+          go-version-file: "core/go.mod"
+          check-latest: true
+          cache-dependency-path: "core/go.sum"
+
+      # The alignment check runs in TestMain, AFTER m.Run(), so the full suite
+      # must execute for every Case() registration to fire — the same command
+      # ci.yml runs, just pointed at the latest catalog. TestMain then fails the
+      # suite if the latest catalog holds a case ID with no matching Case()
+      # registration, which fails this step and the job — a red scheduled run is
+      # the signal GitHub notifies on. This workflow has no pull_request
+      # trigger, so the failure never blocks a PR.
+      - name: Run catalog-alignment check against latest catalog
+        id: align
+        working-directory: core
+        env:
+          CONFORMANCE_CATALOG_PATH: ${{ runner.temp }}/conformance/oauth-sdk-conformance-catalog.yaml
+        # `shell: bash` is load-bearing, not decoration. The default shell for a
+        # `run:` step on Linux is `bash -e {0}`, which does NOT set pipefail, so
+        # the pipeline below would exit with tee's status — always 0 — and this
+        # step would report success no matter what `go test` did. `shell: bash`
+        # is what adds `-o pipefail`.
+        shell: bash
+        run: go test ./conformancetests/ -v 2>&1 | tee "$RUNNER_TEMP/align.log"
+
+      - name: Report drift
+        if: always()
+        run: |
+          if [ "${{ steps.align.outcome }}" = "success" ]; then
+            echo "Conformance catalog alignment: no drift against the latest catalog." >> "$GITHUB_STEP_SUMMARY"
+          elif ! grep -qE "has no conformance test|registers unknown case" "$RUNNER_TEMP/align.log" 2>/dev/null; then
+            # Match the two messages that actually mean drift, not the bare
+            # "CATALOG ALIGNMENT:" prefix. verifyCatalogAlignment emits that
+            # prefix for three distinct outcomes (catalog_alignment_test.go):
+            #
+            #   catalog case %q has no conformance test          -> drift
+            #   conformance test registers unknown case %q       -> drift
+            #   load catalog: read catalog: ...                  -> harness problem
+            #
+            # The third fires when the catalog clone is missing or the path is
+            # wrong. Grepping the prefix would classify that as drift, which is
+            # exactly the case this branch exists to separate out.
+            echo "::warning::The catalog-alignment step failed without a drift message. This is a build or harness problem — a compile error, a failed module download, an unreadable catalog clone, or an unrelated conformance assertion — not catalog drift. Read the step log before touching .conformance-catalog-ref."
+            {
+              echo "## Conformance alignment check failed for another reason"
+              echo ""
+              echo "The step failed, but its output carries neither drift message"
+              echo "(\`has no conformance test\` / \`registers unknown case\`)."
+              echo "That points at a build or harness problem rather than a catalog change."
+            } >> "$GITHUB_STEP_SUMMARY"
+          else
+            echo "::warning::Conformance catalog drift detected — the latest catalog has cases not yet covered by the SDK. Adopt them in core/conformancetests/ and bump .conformance-catalog-ref."
+            {
+              echo "## Conformance catalog drift detected"
+              echo ""
+              echo "The latest (unpinned) conformance catalog contains cases the SDK does not yet cover, or the alignment check otherwise failed."
+              echo ""
+              echo "**Next steps:** adopt the new cases in \`core/conformancetests/\` and bump \`.conformance-catalog-ref\` to the new catalog SHA in the same change."
+            } >> "$GITHUB_STEP_SUMMARY"
+          fi
diff --git a/.github/workflows/release.yml b/.github/workflows/release.yml
index c9721bf..482af25 100644
--- a/.github/workflows/release.yml
+++ b/.github/workflows/release.yml
@@ -102,11 +102,8 @@ jobs:
           token: ${{ steps.app_token.outputs.token }}
 
       - name: Check out shared conformance catalog
-        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
-        with:
-          repository: AuthPlane/conformance
-          path: conformance
-          fetch-depth: 1
+        shell: bash
+        run: .github/scripts/fetch-conformance-catalog.sh
 
       - name: Set up Go 1.25
         uses: actions/setup-go@4a3601121dd01d1626a1e23e37211e3254c1c06c # v6.4.0
@@ -226,7 +223,7 @@ jobs:
 
       - name: Run tests in all four modules
         env:
-          CONFORMANCE_CATALOG_PATH: ${{ github.workspace }}/conformance/oauth-sdk-conformance-catalog.yaml
+          CONFORMANCE_CATALOG_PATH: ${{ runner.temp }}/conformance/oauth-sdk-conformance-catalog.yaml
         run: |
           (cd core      && go test ./...)
           (cd mcp       && go test ./...)
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 348c85a..70095c1 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -76,6 +76,19 @@ go install golang.org/x/vuln/cmd/govulncheck@latest
 (cd mcp  && govulncheck ./...)
 ```
 
+**Conformance catalog:**
+
+The `core` conformance suite maps to the shared [conformance catalog](https://github.com/AuthPlane/conformance). CI pins the catalog to the SHA tracked in [`.conformance-catalog-ref`](.conformance-catalog-ref) at the repo root, so a catalog change can never break CI on its own. To reproduce CI locally, check out that same ref:
+
+```bash
+git clone https://github.com/AuthPlane/conformance.git /path/to/catalog
+git -C /path/to/catalog checkout "$(cat .conformance-catalog-ref)"
+export CONFORMANCE_CATALOG_PATH=/path/to/catalog/oauth-sdk-conformance-catalog.yaml
+(cd core && go test ./conformancetests/ -v)
+```
+
+A weekly `conformance-catalog-drift` workflow runs the alignment check against the latest catalog and fails when new cases need adopting. When adopting them, update `core/conformancetests/` and bump `.conformance-catalog-ref` in the same change. See [`core/conformancetests/README.md`](core/conformancetests/README.md) for details.
+
 ## Pull Request Guidelines
 
 - Branch off `main`. Release branches (`release/v*`, `hotfix/v*`) are managed by the release flow — see [RELEASE_POLICY.md](RELEASE_POLICY.md).
diff --git a/RELEASE_GUIDE.md b/RELEASE_GUIDE.md
index 58aabfa..5fed83f 100644
--- a/RELEASE_GUIDE.md
+++ b/RELEASE_GUIDE.md
@@ -5,7 +5,7 @@ How to ship a new version of the Go SDK (`core`, `http`, `mcp`). All three modul
 ## Prerequisites
 
 - You are a maintainer on `AuthPlane/go-sdk`.
-- **`RELEASE_BOT_APP_ID`** and **`RELEASE_BOT_PRIVATE_KEY`** are set as organization secrets scoped to this repo. The Release Bot GitHub App mints a short-lived token used to push the four annotated tags and check out the conformance catalog. (`ci.yml` does not need these — the conformance repo is public.) `release.yml` fails fast with a clear error if either secret is missing — the workflow will not silently proceed.
+- **`RELEASE_BOT_APP_ID`** and **`RELEASE_BOT_PRIVATE_KEY`** are set as organization secrets scoped to this repo. The Release Bot GitHub App mints a short-lived token used to push the four annotated tags. (`ci.yml` does not need these — the conformance repo is public.) `release.yml` fails fast with a clear error if either secret is missing — the workflow will not silently proceed.
 - `CHANGELOG.md` on `main` has a populated `## [Unreleased]` section.
 
 There is no registry to configure. `proxy.golang.org` polls public tags and begins serving the new module versions within seconds of the atomic push — **the tag push is the publish**.
diff --git a/core/conformancetests/README.md b/core/conformancetests/README.md
index 61fc42e..fbef6b9 100644
--- a/core/conformancetests/README.md
+++ b/core/conformancetests/README.md
@@ -85,12 +85,17 @@ Set `CONFORMANCE_CATALOG_PATH` (or `AUTHPLANE_CONFORMANCE_CATALOG`) to the absol
 # Clone the catalog repo anywhere
 git clone git@github.com:AuthPlane/conformance.git /path/to/catalog
 
+# Check out the same pinned ref CI uses (single-sourced at the repo root)
+git -C /path/to/catalog checkout "$(cat /path/to/go-sdk/.conformance-catalog-ref)"
+
 # Point the harness at it
 export CONFORMANCE_CATALOG_PATH=/path/to/catalog/oauth-sdk-conformance-catalog.yaml
 ```
 
 This is useful in CI or when the catalog is not a sibling directory.
 
+CI pins the catalog to the SHA tracked in [`.conformance-catalog-ref`](../../.conformance-catalog-ref) at the repo root, so a catalog change can never break CI on its own. Check out that same ref locally (as above) to match CI exactly. A weekly `conformance-catalog-drift` workflow runs the alignment check against the latest catalog and fails when new cases need adopting; adopt them here and bump `.conformance-catalog-ref` in the same change.
+
 ## Running
 
 ```bash