diff --git a/internal/check-facet-changes-map.mjs b/internal/check-facet-changes-map.mjs
index c50bab114..eb3a35ed6 100644
--- a/internal/check-facet-changes-map.mjs
+++ b/internal/check-facet-changes-map.mjs
@@ -53,6 +53,26 @@ import { chromium } from 'playwright'
 // (`bun run serve` on 4173) to run it against a built leaf or the cap-5 shell.
 const BASE = process.env.BASE_URL || 'http://127.0.0.1:5173'
 const PATH_ = process.env.PROBE_PATH || '/'
+
+// ⚠ THE SERVER THAT ANSWERS IS NOT NECESSARILY THIS CHECKOUT. Several worktrees of this repo
+// run on one machine and a default port belongs to whichever started first — measured
+// 2026-09-24 on :4173. This gate reports on how a map's source data changes when a facet is
+// picked, so a run against another tree describes somebody else's data flow. The fingerprint
+// is public/doxa-maps/manifest.json; a dev server serves it straight off disk, so this stays
+// quiet in the normal `bun run dev` case and only speaks when the bytes really differ.
+{
+  const { servesThisTree } = await import('./_serves-this-tree.mjs')
+  const { fileURLToPath } = await import('node:url')
+  const { dirname, resolve } = await import('node:path')
+  const repo = resolve(dirname(fileURLToPath(import.meta.url)), '..')
+  const tree = await servesThisTree(BASE, repo)
+  if (/DIFFERENT build/.test(tree.why)) {
+    console.log('FACET-CHANGES-MAP: SKIP — the server is not serving THIS build.')
+    console.log(`  ${tree.why}`)
+    console.log('  Point BASE_URL at this checkout (bun run dev, or bun run serve).')
+    process.exit(0)
+  }
+}
 const TAB = process.env.PROBE_TAB || 'research-dashboard'
 const SELF_TEST = process.argv.includes('--self-test')
 // WHICH HALF OF THE FIX TO REVERT in --self-test. Default 'colour' reverts ONLY the paint, leaving
diff --git a/internal/check-filter-width-stable.mjs b/internal/check-filter-width-stable.mjs
index 4748f300c..fb518c521 100644
--- a/internal/check-filter-width-stable.mjs
+++ b/internal/check-filter-width-stable.mjs
@@ -36,9 +36,29 @@
  * NEGATIVE-TESTED: restoring `flex:1 1 160px; max-width:240px` on .filter-dropdown makes widths
  * diverge and this fails by name, naming the before/after arrays.
  */
+import { dirname, resolve } from 'node:path'
+import { fileURLToPath } from 'node:url'
 import { chromium } from 'playwright'
+import { servesThisTree } from './_serves-this-tree.mjs'
 
+const REPO = resolve(dirname(fileURLToPath(import.meta.url)), '..')
 const BASE = process.env.BASE_URL || 'http://127.0.0.1:4173/doxa-maps'
+
+// ⚠ "ANSWERS 200" IS NOT "IS THIS BUILD". This machine runs several worktrees of one repo and
+// the default port belongs to whichever server started first — measured 2026-09-24: :4173 was
+// another worktree's. This gate compares filter widths BEFORE and AFTER a tab change and fails
+// on divergence, so a run against someone else's tree can report a width regression in code
+// that was never touched here. Separate that from the unreachable case below, which is already
+// a SKIP: both are preconditions, neither is a product finding.
+{
+  const tree = await servesThisTree(BASE, REPO)
+  if (/DIFFERENT build/.test(tree.why)) {
+    console.log('FILTER-WIDTH: SKIP — the server is not serving THIS build.')
+    console.log(`  ${tree.why}`)
+    console.log('  Serve this checkout (`bun run serve`) and pass its port as BASE_URL.')
+    process.exit(0)
+  }
+}
 const WALK = `(sel)=>{const o=[],s=new Set();const w=(r)=>{if(!r||s.has(r))return;s.add(r);try{o.push(...r.querySelectorAll(sel))}catch(e){};for(const e of r.querySelectorAll('*'))if(e.shadowRoot)w(e.shadowRoot)};w(document);return o}`
 
 async function measure(page, label, activateTab) {
diff --git a/internal/check-maplibre-layer-drops.mjs b/internal/check-maplibre-layer-drops.mjs
index 9147b7e08..32455eafe 100644
--- a/internal/check-maplibre-layer-drops.mjs
+++ b/internal/check-maplibre-layer-drops.mjs
@@ -40,6 +40,7 @@ import { chromium } from 'playwright'
 import { readFileSync, existsSync } from 'node:fs'
 import { resolve, join, dirname } from 'node:path'
 import { fileURLToPath } from 'node:url'
+import { servesThisTree } from './_serves-this-tree.mjs'
 
 // Portable by construction — see check-no-absolute-paths.mjs. Never hardcode a worktree.
 const ROOT = resolve(dirname(fileURLToPath(import.meta.url)), '..')
@@ -52,6 +53,22 @@ if (!existsSync(MANIFEST)) {
   console.log('MAPLIBRE-LAYER-DROPS: SKIP — no built manifest (run `bun run tree` first).')
   process.exit(0)
 }
+// ⚠ REACHABLE AND MINE ARE DIFFERENT QUESTIONS. This machine runs several worktrees of one
+// repo and the default port belongs to whichever server started first — measured 2026-09-24:
+// :4173 was another worktree's, and this gate reported PASS 16/16 about a build that was not
+// on disk here. The layer-drop family is silent by nature (MapLibre omits a layer instead of
+// throwing), so a verdict taken from the wrong tree is exactly the kind nobody can catch by
+// looking. A mismatch is a precondition, not a product failure: SKIP, but name it.
+{
+  const tree = await servesThisTree(BASE, ROOT)
+  if (/DIFFERENT build/.test(tree.why)) {
+    console.log('MAPLIBRE-LAYER-DROPS: SKIP — the server is not serving THIS build.')
+    console.log('  ' + tree.why)
+    console.log('  Start `bun run serve` in this checkout and pass its port as BASE_URL.')
+    process.exit(0)
+  }
+}
+
 const manifest = JSON.parse(readFileSync(MANIFEST, 'utf8'))
 const leaves = []
 for (const b of manifest.bundles || []) {
diff --git a/internal/check-packet-browser.mjs b/internal/check-packet-browser.mjs
index 85042baf8..7d14f43fd 100644
--- a/internal/check-packet-browser.mjs
+++ b/internal/check-packet-browser.mjs
@@ -56,6 +56,24 @@ try {
   console.log('    python3 -m http.server 4898   (or set PACKET_ORIGIN)')
   process.exit(0)
 }
+
+// ⚠ AND IT MUST BE THIS CHECKOUT'S ROOT. Several worktrees of this repo run on one machine,
+// so a port that answers may belong to another one — measured 2026-09-24 on :4173, where a
+// browser gate reported on a build that was not on disk here. This gate imports the shared
+// renderer FROM THE SERVER, so the wrong origin means it verifies someone else's renderer
+// and says nothing about this change. Precondition, not a product failure: SKIP and name it.
+{
+  const { servesThisTree } = await import('./_serves-this-tree.mjs')
+  const repo = resolve(dirname(fileURLToPath(import.meta.url)), '..')
+  const tree = await servesThisTree(ORIGIN + '/public', repo)
+  if (/DIFFERENT build/.test(tree.why)) {
+    await b.close()
+    console.log('PACKET-BROWSER: SKIP — the server is not serving THIS checkout.')
+    console.log(`  ${tree.why}`)
+    console.log('  Serve THIS repo root and re-run (PACKET_ORIGIN=http://127.0.0.1:PORT).')
+    process.exit(0)
+  }
+}
 const html = await pg.evaluate(async ({RENDER, STRINGS, LANG, ORIGIN, API, ISO3, ZONE}) => {
   const [{ createPacketRenderer }, strings, norm] = await Promise.all([
     import(RENDER),
diff --git a/internal/check-provider-switch.mjs b/internal/check-provider-switch.mjs
index 4bd46a4fd..dc721f771 100644
--- a/internal/check-provider-switch.mjs
+++ b/internal/check-provider-switch.mjs
@@ -12,7 +12,12 @@
 // not calling its tile servers, and only the request log shows that.
 //
 // Usage: node internal/check-provider-switch.mjs <leafUrl>
+import { dirname, resolve } from 'node:path'
+import { fileURLToPath } from 'node:url'
 import { chromium } from 'playwright'
+import { servesThisTree } from './_serves-this-tree.mjs'
+
+const REPO = resolve(dirname(fileURLToPath(import.meta.url)), '..')
 
 // ⚠ NOT named `URL` — that shadows the global URL constructor, so every
 // `new URL(request.url())` below throws and the host tally stays EMPTY while the
@@ -39,6 +44,28 @@ const LEAF = process.argv[2] || DEFAULT_LEAF
     console.log('    python3 -m http.server 4173 --directory public   (or set BASE_URL)')
     process.exit(0)
   }
+
+  // ⚠ REACHABLE IS NOT THE SAME QUESTION AS MINE. Several worktrees of this repo run on one
+  // machine and the default port belongs to whichever started first — measured 2026-09-24:
+  // :4173 was another worktree's server, and an hour of probing reported on a build nobody
+  // here had changed. This gate's whole verdict is a request tally from a page; a tally taken
+  // from someone else's page is a confident statement about the wrong code.
+  // A mismatch is a PRECONDITION, not a product failure, so it SKIPs (exit 0) like the
+  // unreachable case above — but it says which, so nobody reads it as a pass.
+  // The helper wants the ORIGIN that serves public/, not the leaf page — passing the page URL
+  // makes it look for the manifest under the page's own path and report "nothing serving it",
+  // which would SKIP every run for the wrong reason. (Measured: it did, on both servers.)
+  const tree = await servesThisTree(new URL(LEAF).origin, REPO)
+  // Only a genuine MISMATCH skips. "No manifest to compare" (an unbuilt checkout) is a
+  // different precondition and the reachable check above already speaks for it — turning
+  // that into a skip here would silence the gate on exactly the builds it should run on.
+  if (/DIFFERENT build/.test(tree.why)) {
+    console.log('PROVIDER-SWITCH: SKIP — the server is not serving THIS build.')
+    console.log(`  ${tree.why}`)
+    console.log('  Serve your own tree and re-run, e.g.:')
+    console.log('    bun run serve   (then BASE_URL=http://127.0.0.1:PORT)')
+    process.exit(0)
+  }
 }
 
 async function run(provider) {