diff --git a/.github/scripts/resolve-workflow-run-pr.mjs b/.github/scripts/resolve-workflow-run-pr.mjs
new file mode 100644
index 0000000..3a027db
--- /dev/null
+++ b/.github/scripts/resolve-workflow-run-pr.mjs
@@ -0,0 +1,43 @@
+export default async function resolveWorkflowRunPr({
+  github,
+  context,
+  expectedBaseBranch,
+}) {
+  const workflowRun = context.payload.workflow_run;
+  const headRepository = workflowRun?.head_repository?.full_name;
+  const headBranch = workflowRun?.head_branch;
+  const headSha = workflowRun?.head_sha;
+
+  if (!headRepository || !headBranch || !headSha) {
+    throw new Error("Workflow run is missing head repository, branch, or SHA");
+  }
+
+  const baseRepository = `${context.repo.owner}/${context.repo.repo}`;
+  const pullRequests = await github.paginate(github.rest.pulls.list, {
+    ...context.repo,
+    state: "open",
+    base: expectedBaseBranch,
+    per_page: 100,
+  });
+  const matches = pullRequests.filter(
+    (pullRequest) =>
+      pullRequest.head.repo?.full_name === headRepository &&
+      pullRequest.head.ref === headBranch &&
+      pullRequest.head.sha === headSha &&
+      pullRequest.base.repo?.full_name === baseRepository &&
+      pullRequest.base.ref === expectedBaseBranch,
+  );
+
+  if (matches.length !== 1) {
+    throw new Error(
+      `Expected exactly one open pull request for workflow run ${workflowRun.id}; found ${matches.length}`,
+    );
+  }
+
+  const pullRequestNumber = matches[0].number;
+  if (!Number.isSafeInteger(pullRequestNumber) || pullRequestNumber < 1) {
+    throw new Error("GitHub returned an invalid pull request number");
+  }
+
+  return pullRequestNumber;
+}
diff --git a/.github/scripts/resolve-workflow-run-pr.test.mjs b/.github/scripts/resolve-workflow-run-pr.test.mjs
new file mode 100644
index 0000000..a41cf13
--- /dev/null
+++ b/.github/scripts/resolve-workflow-run-pr.test.mjs
@@ -0,0 +1,109 @@
+import assert from "node:assert/strict";
+import test from "node:test";
+
+import resolveWorkflowRunPr from "./resolve-workflow-run-pr.mjs";
+
+const listPullRequests = () => {};
+
+function makeContext(overrides = {}) {
+  return {
+    repo: { owner: "graphql", repo: "gaps" },
+    payload: {
+      workflow_run: {
+        id: 123,
+        head_repository: { full_name: "contributor/gaps" },
+        head_branch: "preview",
+        head_sha: "abc123",
+        ...overrides,
+      },
+    },
+  };
+}
+
+function makePullRequest(overrides = {}) {
+  return {
+    number: 40,
+    head: {
+      repo: { full_name: "contributor/gaps" },
+      ref: "preview",
+      sha: "abc123",
+    },
+    base: {
+      repo: { full_name: "graphql/gaps" },
+      ref: "main",
+    },
+    ...overrides,
+  };
+}
+
+function makeGithub(pullRequests) {
+  return {
+    rest: { pulls: { list: listPullRequests } },
+    paginate: async (method, parameters) => {
+      assert.equal(method, listPullRequests);
+      assert.deepEqual(parameters, {
+        owner: "graphql",
+        repo: "gaps",
+        state: "open",
+        base: "main",
+        per_page: 100,
+      });
+      return pullRequests;
+    },
+  };
+}
+
+test("resolves the PR using the complete workflow identity", async () => {
+  const shellMetacharacterBranch = "preview$(touch${IFS}/tmp/pwned)";
+  const exactMatch = makePullRequest({
+    head: {
+      repo: { full_name: "contributor/gaps" },
+      ref: shellMetacharacterBranch,
+      sha: "abc123",
+    },
+  });
+  const sameBranchFromAnotherFork = makePullRequest({
+    number: 41,
+    head: {
+      repo: { full_name: "attacker/gaps" },
+      ref: shellMetacharacterBranch,
+      sha: "abc123",
+    },
+  });
+
+  const result = await resolveWorkflowRunPr({
+    github: makeGithub([sameBranchFromAnotherFork, exactMatch]),
+    context: makeContext({ head_branch: shellMetacharacterBranch }),
+    expectedBaseBranch: "main",
+  });
+
+  assert.equal(result, 40);
+});
+
+test("rejects stale and ambiguous workflow runs", async () => {
+  await assert.rejects(
+    resolveWorkflowRunPr({
+      github: makeGithub([
+        makePullRequest({
+          head: {
+            repo: { full_name: "contributor/gaps" },
+            ref: "preview",
+            sha: "newer-sha",
+          },
+        }),
+      ]),
+      context: makeContext(),
+      expectedBaseBranch: "main",
+    }),
+    /found 0/,
+  );
+
+  await assert.rejects(
+    resolveWorkflowRunPr({
+      github: makeGithub([makePullRequest(), makePullRequest({ number: 41 })]),
+      context: makeContext(),
+      expectedBaseBranch: "main",
+    }),
+    /found 2/,
+  );
+});
diff --git a/.github/workflows/preview-deploy.yml b/.github/workflows/preview-deploy.yml
index 5956201..ea541c5 100644
--- a/.github/workflows/preview-deploy.yml
+++ b/.github/workflows/preview-deploy.yml
@@ -15,19 +15,36 @@ jobs:
     name: Deploy preview to Cloudflare
     runs-on: ubuntu-latest
     if: ${{ github.event.workflow_run.conclusion == 'success' && github.event.workflow_run.event == 'pull_request' }}
+    concurrency:
+      group: preview-deploy-${{ github.event.workflow_run.head_repository.id }}-${{ github.event.workflow_run.head_branch }}
+      cancel-in-progress: true
 
     steps:
       - name: Checkout wrangler config
         uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7
         with:
           ref: main
+          persist-credentials: false
+          sparse-checkout-cone-mode: false
           sparse-checkout: |
             wrangler.jsonc
             .github/scripts
 
+      - name: Resolve current PR
+        id: pr
+        uses: actions/github-script@3a2844b7e9c422d3c10d287c895573f7108da1b3 # v9
+        with:
+          result-encoding: string
+          script: |
+            const { default: resolveWorkflowRunPr } = await import('${{ github.workspace }}/.github/scripts/resolve-workflow-run-pr.mjs');
+            return resolveWorkflowRunPr({
+              github,
+              context,
+              expectedBaseBranch: 'main',
+            });
+
       # Security: actions/download-artifact handles zip extraction safely
-      # (immune to zip-slip), and the artifact only contains static HTML/CSS/JS
-      # built by the unprivileged preview-build workflow.
+      # (immune to zip-slip). Treat every extracted file as attacker-controlled.
       - name: Download build artifact
         uses: actions/download-artifact@3e5f45b2cfb9172054b4087a40e8e0b5a5461e7c # v8
         with:
@@ -36,19 +53,15 @@ jobs:
           github-token: ${{ secrets.GITHUB_TOKEN }}
           path: _site
 
-      - name: Get PR number
-        id: pr
-        env:
-          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+      # These are valid Cloudflare configuration files. The PR-built artifact must
+      # not ever supply these. If the main branch ever adds these files for real,
+      # copy them over.
+      - name: Reject deployment config from artifact
         run: |
-          PR_NUM=$(gh pr list --repo "${{ github.repository }}" \
-            --head "${{ github.event.workflow_run.head_branch }}" \
-            --json number --jq '.[0].number')
-          if ! [[ "$PR_NUM" =~ ^[0-9]+$ ]]; then
-            echo "::error::Could not determine PR number"
+          if [[ -e _site/.assetsignore || -e _site/_headers || -e _site/_redirects ]]; then
+            echo "::error::Preview artifact contains Cloudflare configuration"
             exit 1
           fi
-          echo "number=$PR_NUM" >> "$GITHUB_OUTPUT"
 
       # Security: Wrangler is configured for static asset serving only (see
       # wrangler.jsonc). The uploaded content is pre-built HTML/CSS/JS with no
@@ -59,40 +72,51 @@ jobs:
         with:
           apiToken: ${{ secrets.CLOUDFLARE_API_TOKEN }}
           accountId: ${{ secrets.CLOUDFLARE_ACCOUNT_ID }}
-          command: versions upload --no-bundle --preview-alias "pr-${{ steps.pr.outputs.number }}" --message "PR #${{ steps.pr.outputs.number }} preview (${{ github.event.workflow_run.head_sha }})"
+          wranglerVersion: "4.124.0"
+          command: versions upload --no-bundle --preview-alias "pr-${{ steps.pr.outputs.result }}" --message "PR #${{ steps.pr.outputs.result }} preview (${{ github.event.workflow_run.head_sha }})"
 
       - name: Hide old deploy preview comments
         uses: actions/github-script@3a2844b7e9c422d3c10d287c895573f7108da1b3 # v9
+        env:
+          PR_NUMBER: ${{ steps.pr.outputs.result }}
         with:
           script: |
             const { default: run } = await import('${{ github.workspace }}/.github/scripts/hide-old-deploy-comments.mjs');
-            await run({ github, context, prNumber: ${{ steps.pr.outputs.number }} });
+            await run({ github, context, prNumber: Number(process.env.PR_NUMBER) });
 
       - name: Comment on PR (success)
         uses: actions/github-script@3a2844b7e9c422d3c10d287c895573f7108da1b3 # v9
+        env:
+          DEPLOYMENT_URL: ${{ steps.deploy.outputs.deployment-url }}
+          PR_NUMBER: ${{ steps.pr.outputs.result }}
         with:
           script: |
+            const prNumber = Number(process.env.PR_NUMBER);
             await github.rest.issues.createComment({
               owner: context.repo.owner,
               repo: context.repo.repo,
-              issue_number: ${{ steps.pr.outputs.number }},
+              issue_number: prNumber,
               body: [
                 '<!-- deploy-preview -->',
                 '### 🚀 Deploy Preview',
                 '',
-                '- **This commit:** ${{ steps.deploy.outputs.deployment-url }}',
-                '- **This PR** <sub>(kept up to date)</sub>: https://pr-${{ steps.pr.outputs.number }}-gaps.graphql-foundation.workers.dev'
+                `- **This commit:** ${process.env.DEPLOYMENT_URL}`,
+                `- **This PR** <sub>(kept up to date)</sub>: https://pr-${prNumber}-gaps.graphql-foundation.workers.dev`
               ].join('\n')
             });
 
+      # GitHub requires a PR number for this comment. If checkout or PR
+      # resolution fails, that number is unavailable.
       - name: Comment on PR (failure)
-        if: failure()
+        if: ${{ failure() && steps.pr.outcome == 'success' }}
         uses: actions/github-script@3a2844b7e9c422d3c10d287c895573f7108da1b3 # v9
+        env:
+          PR_NUMBER: ${{ steps.pr.outputs.result }}
         with:
           script: |
             await github.rest.issues.createComment({
               owner: context.repo.owner,
               repo: context.repo.repo,
-              issue_number: ${{ steps.pr.outputs.number }},
-              body: 'Deploy preview failed.\n\n```\ngh run view ${{ github.run_id }} --repo ${{ github.repository }} --log-failed\n```'
+              issue_number: Number(process.env.PR_NUMBER),
+              body: `Deploy preview failed.\n\n\`\`\`\ngh run view ${context.runId} --repo ${context.repo.owner}/${context.repo.repo} --log-failed\n\`\`\``
             });
diff --git a/.github/workflows/validate.yml b/.github/workflows/validate.yml
index f06af2c..d07135e 100644
--- a/.github/workflows/validate.yml
+++ b/.github/workflows/validate.yml
@@ -24,5 +24,8 @@ jobs:
       - name: Install dependencies
         run: npm ci
 
+      - name: Test workflow helpers
+        run: npm run test:workflow-helpers
+
       - name: Validate GAP structure
         run: npm run test:structure
diff --git a/package.json b/package.json
index a2621eb..b7abe40 100644
--- a/package.json
+++ b/package.json
@@ -13,6 +13,7 @@
     "test:format": "prettier --check . || npm run suggest:format",
     "test:spelling": "cspell \"spec/**/*.md\" README.md LICENSE.md",
     "test:structure": "find ./gaps -maxdepth 1 -type d -name 'GAP-*' | xargs -I{} ./scripts/validate-structure.js {}",
+    "test:workflow-helpers": "node --test .github/scripts/*.test.mjs",
     "sync:codeowners": "node scripts/sync-codeowners.js"
   },
   "devDependencies": {