diff --git a/.github/workflows/free-tests.yml b/.github/workflows/free-tests.yml
index 0851743df..e357d2f74 100644
--- a/.github/workflows/free-tests.yml
+++ b/.github/workflows/free-tests.yml
@@ -101,8 +101,12 @@ jobs:
       run_url: ${{ steps.find.outputs.run_url }}
       tree: ${{ steps.find.outputs.tree }}
     steps:
+      # On a pull request this checks out main (the base commit), never the
+      # merge commit, so the reuse script, its imports and the run profile are
+      # main's code; bun ignores any .env or bunfig.toml in the checkout.
       - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7
         with:
+          ref: ${{ github.event.pull_request.base.sha || github.sha }}
           persist-credentials: false
       - uses: oven-sh/setup-bun@0c5077e51419868618aeaa5fe8019c62421857d6 # v2
         with:
@@ -111,7 +115,7 @@ jobs:
         name: Find a green run of this exact tree that this run's pull request cannot control
         env:
           GH_TOKEN: ${{ github.token }}
-        run: bun run scripts/ci-tree-reuse.ts find --workflow free-tests.yml --profile ci-reuse/free-tests --pull-request
+        run: bun --no-env-file --config=/dev/null run scripts/ci-tree-reuse.ts find --workflow free-tests.yml --profile ci-reuse/free-tests --pull-request
 
   free-plan:
     needs: reuse
@@ -763,6 +767,7 @@ jobs:
         if: steps.route.outputs.reused != 'true'
         env:
           PR_HEAD_SHA: ${{ github.event.pull_request.head.sha }}
+          SIZING_OVERRIDE: ${{ inputs.free_runner }}${{ inputs.free_shards }}${{ inputs.free_jobs }}
         run: bun run scripts/ci-tree-reuse.ts record --plan "$RUNNER_TEMP/free-plan.json" --results "$RUNNER_TEMP/free-results" --os Linux,macOS,Windows --out "$RUNNER_TEMP/tested-tree/tree.json"
       - name: Upload the tested tree
         if: steps.route.outputs.reused != 'true'
diff --git a/docs/pregate.md b/docs/pregate.md
index 0984609f5..8f6e6a7da 100644
--- a/docs/pregate.md
+++ b/docs/pregate.md
@@ -1063,13 +1063,17 @@ runs nightly and by hand only (docs/support-tiers.md), never on a push, so its
 #### Pull requests reuse only a run they can't control
 
 Free Tests' `reuse` job also runs on `pull_request`, as
-`find … --pull-request` (`findPrReuse`). Main pushes stay the only consumers
+`find … --pull-request` (`findPrReuse`). On a pull request it checks out the
+base commit, never the merge commit, and runs bun with `--no-env-file
+--config=/dev/null`, so the reuse script, everything it imports and the run
+profile are main's code. Main pushes stay the only consumers
 of anything a pull request could certify for itself, so a `pull_request` run
 is never a source. A pull request run reuses a green run only when all of these
 hold:
 
-1. the pull request's head is on this repository (`fork_head`) and it targets
-   main (`not_main_pr`);
+1. the pull request's head is on this repository (`fork_head`), it targets
+   main (`not_main_pr`), and this is the run's first attempt (`rerun`: a re-run
+   never reuses, so a red earlier attempt stays visible);
 2. GitHub's merge commit has exactly two parents, main and the pull request's
    head (`merge_mismatch`);
 3. the merge commit leaves `.github/workflows/free-tests.yml`
@@ -1083,7 +1087,10 @@ hold:
 5. `verifyRun` accepts it under `ci-reuse/free-tests` with `push` and
    `workflow_dispatch` as the accepted events, its `tested-tree` record names
    the merge commit's tree, and no newer red run on its head vetoes it (the
-   pull request's own run in progress is not a veto).
+   pull request's own run in progress is not a veto). A dispatch that
+   overrode the runner, shards or sub-shards records `sizing_experiment` in
+   its `tested-tree`, and `verifyRun` never accepts it (`sizing_experiment`),
+   for any consumer.
 
 The aggregate's route step accepts a reuse on a `pull_request` run of
 `refs/pull/<n>/merge` as it does on a push to main. A reused pull request run
diff --git a/lib/run-evidence.ts b/lib/run-evidence.ts
index 19801123c..7266eeefa 100644
--- a/lib/run-evidence.ts
+++ b/lib/run-evidence.ts
@@ -50,7 +50,7 @@ export interface ApiRun {
   repository?: { full_name: string } | null; head_repository?: { full_name: string } | null;
 }
 export interface ApiJob { name: string; conclusion: string | null; completed_at?: string | null }
-export interface TreeRecord { run_id?: unknown; run_attempt?: unknown; event?: unknown; sha?: unknown; head_sha?: unknown; tree?: unknown; reused?: unknown; reuse?: unknown; os?: unknown; files?: unknown; tests?: unknown }
+export interface TreeRecord { run_id?: unknown; run_attempt?: unknown; event?: unknown; sha?: unknown; head_sha?: unknown; tree?: unknown; reused?: unknown; reuse?: unknown; sizing_experiment?: unknown; os?: unknown; files?: unknown; tests?: unknown }
 
 /** Everything verifyRun reads, injectable so every refusal has a fixture. */
 export interface RunDeps {
@@ -63,7 +63,7 @@ export interface RunDeps {
 
 export type RunRefusal = 'run_ref_invalid' | 'other_repository' | 'fork_run' | 'workflow_mismatch' | 'event_mismatch' | 'run_in_progress'
   | 'run_not_success' | 'job_missing' | 'job_ambiguous' | 'job_not_success' | 'artifact_missing' | 'artifact_mismatch'
-  | 'artifact_reused' | 'tree_mismatch' | 'newer_run_vetoes' | 'run_changed' | 'run_stale' | 'os_mismatch' | 'api_error';
+  | 'artifact_reused' | 'sizing_experiment' | 'tree_mismatch' | 'newer_run_vetoes' | 'run_changed' | 'run_stale' | 'os_mismatch' | 'api_error';
 export interface RunEvidence { run_id: number; run_attempt: number; head_sha: string; tree: string; profile: string; event: string; finished_at: string; files: string[]; tests: number | null }
 export type RunVerdict = { ok: true; evidence: RunEvidence } | { ok: false; reason: RunRefusal; detail: string };
 
@@ -169,6 +169,7 @@ export function verifyRun(deps: RunDeps, o: { repo: string; runRef: string; prof
     catch (e: any) { return fail('artifact_missing', `${at} has no readable ${p.artifact} tree.json: ${String(e?.message ?? e).slice(0, 200)}`); }
     if (!rec || typeof rec !== 'object') return fail('artifact_mismatch', `${at}'s ${p.artifact} tree.json is not a JSON object`);
     if (rec.reused === true || rec.reuse === true) return fail('artifact_reused', `${at} reused another run; a reuse is never evidence`);
+    if (rec.sizing_experiment !== undefined) return fail('sizing_experiment', `${at} was a runner-sizing experiment (a dispatch that overrode the runner, shards or sub-shards); it is never a Free Tests result`);
     const pr = run.event === 'pull_request';
     const recHeads = pr ? [['head_sha', rec.head_sha, run.head_sha] as const] : rec.head_sha === undefined && rec.sha === undefined ? [['head', undefined, run.head_sha] as const] : ([['sha', rec.sha, run.head_sha], ['head_sha', rec.head_sha, run.head_sha]] as const).filter(([, got]) => got !== undefined);
     const wrong = ([['run_id', rec.run_id, id], ['run_attempt', rec.run_attempt, run.run_attempt], ['event', rec.event, run.event], ...recHeads] as const).filter(([, got, want]) => got !== want);
diff --git a/scripts/ci-tree-reuse.ts b/scripts/ci-tree-reuse.ts
index d0558edbb..49256933f 100755
--- a/scripts/ci-tree-reuse.ts
+++ b/scripts/ci-tree-reuse.ts
@@ -6,7 +6,9 @@
  *       The producer. A Free Tests or Windows Free Tests aggregate runs it only after its strict verification
  *       passed, and uploads the file as the `tested-tree` artifact: run_id, run_attempt, event, sha (GITHUB_SHA),
  *       head_sha (the pull request head for pull_request, else GITHUB_SHA), tree (git's tree of GITHUB_SHA),
- *       workflow, os (the OSes the run's test jobs ran on), files (every test file in the plan) and tests.
+ *       workflow, os (the OSes the run's test jobs ran on), files (every test file in the plan) and tests, plus
+ *       sizing_experiment: true when a dispatch overrode the runner, shards or sub-shards (SIZING_OVERRIDE), which
+ *       verifyRun never accepts as evidence.
  *
  *   bun run scripts/ci-tree-reuse.ts find --workflow <file.yml> --profile <run_profiles name>
  *       The main-push consumer (the `reuse` job). A push to main whose tree a pull request already tested green
@@ -43,7 +45,7 @@ export interface PrReuseDeps extends RunDeps {
   commit(sha: string): { tree: string; parents: string[] } | null;
   blob(sha: string, file: string): string | null;
 }
-export interface PrContext { number: number; mergeSha: string; headSha: string; headRef: string; headRepo: string; baseRef: string; runId: number }
+export interface PrContext { number: number; mergeSha: string; headSha: string; headRef: string; headRepo: string; baseRef: string; runId: number; runAttempt: number }
 /** The files whose bytes decide what a reuse means: a pull request that edits one runs every job. */
 export const reuseGuardedFiles = (workflow: string) => [`.github/workflows/${workflow}`, 'scripts/ci-tree-reuse.ts', 'lib/run-evidence.ts'];
 export type ReuseVerdict =
@@ -95,6 +97,7 @@ export function findPrReuse(deps: PrReuseDeps, o: { repo: string; base: string;
   const { pr } = o;
   if (!SHA.test(pr.mergeSha) || !SHA.test(pr.headSha)) return refuse('usage', `merge ${JSON.stringify(pr.mergeSha)} or head ${JSON.stringify(pr.headSha)} is not a commit sha`);
   if (!Number.isSafeInteger(pr.number) || pr.number < 1 || !pr.headRef) return refuse('usage', 'the pull_request event names no pull request number or head branch');
+  if (pr.runAttempt !== 1) return refuse('rerun', `attempt ${pr.runAttempt} of run ${pr.runId}: a re-run never reuses, so an earlier red attempt can't be hidden`);
   if (pr.baseRef !== o.base) return refuse('not_main_pr', `#${pr.number} targets ${pr.baseRef}, not ${o.base}`);
   if (o.profile.workflow !== o.workflow) return refuse('profile_mismatch', `profile ${o.profileName} names ${o.profile.workflow}, not ${o.workflow}`);
   if (pr.headRepo.toLowerCase() !== o.repo.toLowerCase()) return refuse('fork_head', `#${pr.number}'s head is on ${pr.headRepo || 'a deleted repository'}, not ${o.repo}`);
@@ -146,7 +149,7 @@ export function treeRecord(env: NodeJS.ProcessEnv, o: { plan: { shards: Array<{
   if (!files.length) throw new Error('the plan lists no test files');
   const counts = o.results.map(r => r.outcome?.summary?.testsRan);
   const tests = o.results.length && counts.every(n => Number.isSafeInteger(n) && n! >= 0) ? counts.reduce((a, b) => a! + b!, 0)! : files.length;
-  return { run_id: int('GITHUB_RUN_ID'), run_attempt: int('GITHUB_RUN_ATTEMPT'), event, sha, head_sha: head, tree: o.tree, workflow, os: o.os, files, tests };
+  return { run_id: int('GITHUB_RUN_ID'), run_attempt: int('GITHUB_RUN_ATTEMPT'), event, sha, head_sha: head, tree: o.tree, workflow, os: o.os, files, tests, ...(env.SIZING_OVERRIDE ? { sizing_experiment: true } : {}) };
 }
 
 function flag(args: string[], name: string): string | undefined { const i = args.indexOf(name); return i >= 0 ? args[i + 1] : undefined; }
@@ -206,7 +209,7 @@ function find(args: string[]): number {
   let event: { pull_request?: { number?: number; head?: { sha?: string; ref?: string; repo?: { full_name?: string } | null }; base?: { ref?: string } } };
   try { event = JSON.parse(fs.readFileSync(env.GITHUB_EVENT_PATH ?? '', 'utf8')); } catch (e: any) { return settle(refuse('usage', `GITHUB_EVENT_PATH: ${String(e?.message ?? e).slice(0, 200)}`)); }
   const p = event.pull_request ?? {};
-  const pr: PrContext = { number: Number(p.number), mergeSha: env.GITHUB_SHA ?? '', headSha: p.head?.sha ?? '', headRef: p.head?.ref ?? '', headRepo: p.head?.repo?.full_name ?? '', baseRef: p.base?.ref ?? '', runId: Number(env.GITHUB_RUN_ID) };
+  const pr: PrContext = { number: Number(p.number), mergeSha: env.GITHUB_SHA ?? '', headSha: p.head?.sha ?? '', headRef: p.head?.ref ?? '', headRepo: p.head?.repo?.full_name ?? '', baseRef: p.base?.ref ?? '', runId: Number(env.GITHUB_RUN_ID), runAttempt: Number(env.GITHUB_RUN_ATTEMPT) };
   const prDeps: PrReuseDeps = {
     ...deps,
     runsForBranch: (wf, branch) => ghApiJson<{ workflow_runs?: ApiRun[] }>(`repos/${repo}/actions/workflows/${wf}/runs?branch=${encodeURIComponent(branch)}&per_page=${MAX_CANDIDATES}`).workflow_runs ?? [],
diff --git a/test/ci-reuse-workflow-wiring.test.ts b/test/ci-reuse-workflow-wiring.test.ts
index 10923d075..6cf72f2a1 100644
--- a/test/ci-reuse-workflow-wiring.test.ts
+++ b/test/ci-reuse-workflow-wiring.test.ts
@@ -1,8 +1,8 @@
 /**
  * CI reuse wiring (GSTA-156 P3b) for free-tests.yml and windows-free-tests.yml. Pins the properties that make reuse
  * safe: nothing a pull request controls certifies it (a push to main consumes; free-tests.yml's pull_request runs consume
- * only through find --pull-request, whose sources and guarded files test/ci-tree-reuse.test.ts pins; dispatch runs never
- * consume), the reuse job alone reads Actions with a job-level actions: read,
+ * only through find --pull-request run from main's checkout, whose sources and guarded files test/ci-tree-reuse.test.ts
+ * pins; a sizing-experiment dispatch records sizing_experiment and is never evidence; dispatch runs never consume), the reuse job alone reads Actions with a job-level actions: read,
  * a skipped or failed reuse job runs every job exactly as before (GitHub skips a job whose needed job was skipped
  * unless its condition uses a status function, so every dependent condition starts with !cancelled()), the
  * aggregate's reuse branch runs before any download and the strict path is otherwise unchanged, tested-tree is
@@ -52,8 +52,9 @@ for (const [file, spec] of Object.entries(FILES)) {
       expect(source).not.toContain('pull_request_target');
       const find = jobs.reuse.steps.find((s: any) => s.id === 'find');
       expect(find.env).toEqual({ GH_TOKEN: '${{ github.token }}' });
-      expect(find.run).toBe(`bun run scripts/ci-tree-reuse.ts find --workflow ${file} --profile ${spec.profile}${spec.pullRequest ? ' --pull-request' : ''}`);
+      expect(find.run).toBe(spec.pullRequest ? `bun --no-env-file --config=/dev/null run scripts/ci-tree-reuse.ts find --workflow ${file} --profile ${spec.profile} --pull-request` : `bun run scripts/ci-tree-reuse.ts find --workflow ${file} --profile ${spec.profile}`);
       expect(jobs.reuse.steps[0].with['persist-credentials']).toBe(false);
+      if (spec.pullRequest) expect(jobs.reuse.steps[0].with.ref).toBe('${{ github.event.pull_request.base.sha || github.sha }}');
       expect(Object.keys(jobs.reuse.outputs).sort()).toEqual(['reused', 'run_id', 'run_url', 'tree']);
     });
 
@@ -100,7 +101,7 @@ for (const [file, spec] of Object.entries(FILES)) {
       expect(steps.length).toBe(upload + 1);
       expect(steps[upload].uses).toStartWith('actions/upload-artifact@');
       expect(steps[upload].with['if-no-files-found']).toBe('error');
-      expect(steps[record].env).toEqual({ PR_HEAD_SHA: '${{ github.event.pull_request.head.sha }}' });
+      expect(steps[record].env).toEqual({ PR_HEAD_SHA: '${{ github.event.pull_request.head.sha }}', ...(spec.pullRequest ? { SIZING_OVERRIDE: '${{ inputs.free_runner }}${{ inputs.free_shards }}${{ inputs.free_jobs }}' } : {}) });
       expect(steps[1].run).toMatch(/set -eu\n(test "\$[A-Z_]+_RESULT" = success\n)+$/);
     });
 
diff --git a/test/ci-tree-reuse.test.ts b/test/ci-tree-reuse.test.ts
index 4987e0534..0c697dd3a 100644
--- a/test/ci-tree-reuse.test.ts
+++ b/test/ci-tree-reuse.test.ts
@@ -126,7 +126,7 @@ describe('findPrReuse: a pull request reuses only a run it cannot control', () =
   const SELF = 999;
   const GUARDED = ['.github/workflows/free-tests.yml', 'scripts/ci-tree-reuse.ts', 'lib/run-evidence.ts'];
   interface PrFake { id: number; event: string; branch: string; head: string; conclusion?: string | null; status?: string; started?: string; record?: Partial<TreeRecord> }
-  interface PrWorld { runs: PrFake[]; trees: Record<string, string>; blobs: Record<string, string>; parents: string[]; headRepo: string }
+  interface PrWorld { runs: PrFake[]; trees: Record<string, string>; blobs: Record<string, string>; parents: string[]; headRepo: string; attempt?: number }
   const blobs = (over: Record<string, string> = {}) => ({ ...Object.fromEntries(GUARDED.flatMap(p => [[`${MERGE}:${p}`, `blob-${p}`], [`${BASE}:${p}`, `blob-${p}`]])), ...over });
   const prWorld = (over: Partial<PrWorld> = {}): PrWorld => ({
     runs: [{ id: 500, event: 'push', branch: 'main', head: MAIN_HEAD }, { id: SELF, event: 'pull_request', branch: 'feat', head: HEAD, status: 'in_progress', conclusion: null, started: '2026-10-30T11:30:00Z' }],
@@ -148,7 +148,7 @@ describe('findPrReuse: a pull request reuses only a run it cannot control', () =
     };
   }
   const pr = (w: PrWorld) => findPrReuse(prDeps(w), { repo: REPO, base: 'main', workflow: 'free-tests.yml', profileName: 'ci-reuse/free-tests', profile: PROFILE, now: NOW,
-    pr: { number: 42, mergeSha: MERGE, headSha: HEAD, headRef: 'feat', headRepo: w.headRepo, baseRef: 'main', runId: SELF } });
+    pr: { number: 42, mergeSha: MERGE, headSha: HEAD, headRef: 'feat', headRepo: w.headRepo, baseRef: 'main', runId: SELF, runAttempt: w.attempt ?? 1 } });
   const prWhy = (w: PrWorld) => { const v = pr(w); return v.reused ? 'reused' : v.reason; };
 
   test('a green push run on main at the merge commit\'s exact tree is reused', () => {
@@ -186,6 +186,15 @@ describe('findPrReuse: a pull request reuses only a run it cannot control', () =
     expect(prWhy(prWorld({ headRepo: 'someone/gstack' }))).toBe('fork_head');
   });
 
+  test('a runner-sizing experiment dispatch is never a source (red team p1 #1)', () => {
+    const v = pr(prWorld({ runs: [{ id: 506, event: 'workflow_dispatch', branch: 'feat', head: HEAD, record: { sizing_experiment: true } }] }));
+    expect(v).toMatchObject({ reused: false, reason: 'no_verified_run', detail: expect.stringContaining('reason=sizing_experiment') });
+  });
+
+  test('a re-run attempt of the pull request run never reuses, so its own red earlier attempt stays visible (red team p1 #2)', () => {
+    expect(prWhy(prWorld({ attempt: 2, runs: [{ id: 501, event: 'workflow_dispatch', branch: 'feat', head: HEAD }] }))).toBe('rerun');
+  });
+
   test('a merge commit that is not base plus this head, or an unreadable one, is refused', () => {
     expect(prWhy(prWorld({ parents: [BASE, '6'.repeat(40)] }))).toBe('merge_mismatch');
     expect(prWhy(prWorld({ parents: [BASE] }))).toBe('merge_mismatch');
@@ -200,6 +209,8 @@ describe('treeRecord: the tested-tree producer', () => {
     expect(treeRecord(env(), { plan, results, os: ['Linux', 'macOS', 'Windows'], tree: TREE })).toEqual({ run_id: 300, run_attempt: 2, event: 'workflow_dispatch', sha: HEAD, head_sha: HEAD, tree: TREE, workflow: '.github/workflows/free-tests.yml', os: ['Linux', 'macOS', 'Windows'], files: ['test/a.test.ts', 'test/b.test.ts', 'test/c.test.ts'], tests: 12 });
     expect(treeRecord(env({ GITHUB_EVENT_NAME: 'pull_request', GITHUB_SHA: MERGE, PR_HEAD_SHA: HEAD }), { plan, results, os: ['Windows'], tree: TREE })).toMatchObject({ event: 'pull_request', sha: MERGE, head_sha: HEAD });
     expect(treeRecord(env(), { plan, results: [{ outcome: {} }], os: ['Linux'], tree: TREE }).tests).toBe(3);
+    expect(treeRecord(env({ SIZING_OVERRIDE: '1' }), { plan, results, os: ['Linux'], tree: TREE })).toMatchObject({ sizing_experiment: true });
+    expect(treeRecord(env({ SIZING_OVERRIDE: '' }), { plan, results, os: ['Linux'], tree: TREE })).not.toHaveProperty('sizing_experiment');
   });
   test('missing or malformed inputs throw, so the aggregate fails instead of uploading a record', () => {
     expect(() => treeRecord(env({ GITHUB_EVENT_NAME: 'pull_request', GITHUB_SHA: MERGE }), { plan, results, os: ['Linux'], tree: TREE })).toThrow(/PR_HEAD_SHA/);