diff --git a/backend/TESTING.md b/backend/TESTING.md
index 266c10299..7db16901a 100644
--- a/backend/TESTING.md
+++ b/backend/TESTING.md
@@ -99,9 +99,13 @@ Frontend testing is documented separately at `frontend/TESTING.md`. Contracts te
 ./dev.sh test:integration       # Tier 1 against Docker Compose services (./dev.sh up required)
 ```
 
-## Node 26 kills any suite whose require graph reaches `buffer-equal-constant-time`
+## Node 25+ kills any suite whose require graph reaches `buffer-equal-constant-time`
 
-Node 26 removed `SlowBuffer`. `buffer-equal-constant-time/index.js:37` reads
+Node 25 removed `SlowBuffer` — 22 and 24 still have it, 25 and 26 do not
+(measured: `node -e "typeof require('buffer').SlowBuffer"` is `function` on 22
+and 24, `undefined` on 25 and 26; the removal landed at 25, so "Node 26" — how
+this was first described — understates the range).
+`buffer-equal-constant-time/index.js:37` reads
 `SlowBuffer.prototype.equal` at **module scope**, so it throws the moment it is
 required — before any test runs:
 
@@ -117,7 +121,7 @@ never runs. The unconditional **read** at `:37` is the one that fires.
 **`jsonwebtoken` is the common importer, not the failing package.** The chain is
 `jsonwebtoken` → `jws` → `jwa` → `buffer-equal-constant-time`, and both
 `require('jsonwebtoken')` and `require('buffer-equal-constant-time')` throw at
-the identical frame. So a "will this suite die on 26?" check must ask whether
+the identical frame. So a "will this suite die on 25+?" check must ask whether
 that leaf is in the require graph — grepping a suite for the string
 `jsonwebtoken` misses every suite that reaches it transitively, and blames the
 wrong package when it hits.
@@ -153,7 +157,7 @@ comparison function is a faithful replacement rather than a test-only shim.
 Note that is a stub of the *leaf*, not of `jsonwebtoken` — the caution below
 about stubbing `jsonwebtoken` doesn't apply, because real signing and verifying
 still run against a real comparison.
-That is the move **if Node 26 ever becomes mandatory** — not now, while 26 is
+That is the move **if Node 25+ ever becomes mandatory** — not now, while 25+ is
 optional and Node 22 is the right answer.
 
 Note the remedy has to intercept the `require`. Lines 36-37 are dead by
@@ -192,7 +196,7 @@ actually signs or verifies a token, which is most of the runtime-token service
 suites.
 
 Suites with no jwt in their graph are unaffected — `mongoose` and
-`mongodb-memory-server` both load clean on 26.
+`mongodb-memory-server` both load clean on 25+.
 
 ## CI
 
diff --git a/backend/__tests__/utils/globalSetup.js b/backend/__tests__/utils/globalSetup.js
index 8fc0fea1e..525f54610 100644
--- a/backend/__tests__/utils/globalSetup.js
+++ b/backend/__tests__/utils/globalSetup.js
@@ -9,6 +9,26 @@ const sleep = (ms) => new Promise((resolve) => { setTimeout(resolve, ms); });
 // cached binary and never race the download lock — the root cause of the flaky
 // "Cannot unlock file ... .lock" failures in CI.
 module.exports = async () => {
+  // Name the remedy before the failure, because the failure does not. On Node
+  // 25+ every suite whose require graph reaches jsonwebtoken dies with a bare
+  // `TypeError: Cannot read properties of undefined (reading 'prototype')`
+  // raised inside buffer-equal-constant-time — a package the test never
+  // imports, four frames under jws. Measured cost of not saying this: a seat
+  // read the stack, installed deps, wrote a SlowBuffer shim, and only then
+  // found TESTING.md. One line here is cheaper than that detour.
+  if (Number(process.versions.node.split('.')[0]) >= 25) {
+    // console is not stubbed in this process (that happens per-worker in
+    // setup.js), but stderr is the honest target either way.
+    process.stderr.write(
+      `[globalSetup] Node ${process.versions.node} removed buffer.SlowBuffer, which `
+      + 'buffer-equal-constant-time reads at module scope — any suite reaching '
+      + 'jsonwebtoken will fail to load. Run on Node 22 (what CI pins):\n'
+      + '  PATH=/opt/homebrew/opt/node@22/bin:$PATH npx jest <suite>\n'
+      + '  ...or without a local install: npx -y -p node@22 node node_modules/jest/bin/jest.js <suite>\n'
+      + 'See backend/TESTING.md for the full picture.\n',
+    );
+  }
+
   // Integration runs use a real Mongo (MONGO_URI); no in-memory binary needed.
   if (process.env.INTEGRATION_TEST === 'true') return;
 
diff --git a/docs/development/agent-experience-audit.md b/docs/development/agent-experience-audit.md
index cd2623904..cce5a4b91 100644
--- a/docs/development/agent-experience-audit.md
+++ b/docs/development/agent-experience-audit.md
@@ -3760,3 +3760,34 @@ orientation/tool-description tests; and align the CLI fork frame with the
 same AND semantics. The decision tool posts its own ask,
 remains advisory (not privileged-action consent), and does not require
 unrelated work to stop while a ruling is pending.
+
+## 57. A documented remedy that its own failure never points at (2026-09-18, sprint-impl)
+
+*Origin observation: sprint-impl running TASK-130's backend suites in this
+checkout; verification: `backend/TESTING.md` ("Node 25+ kills any suite whose
+require graph reaches `buffer-equal-constant-time`"), `backend/__tests__/utils/globalSetup.js`,
+measured with `npx -y -p node@{22,24,25,26} node -e "typeof require('buffer').SlowBuffer"`.*
+
+`backend/TESTING.md` already carried the whole story of this trap, including the
+exact Node-22 invocation. The seat still lost the detour, because the failure it
+describes points nowhere near the doc. Two dependency-shaped errors arrived in a
+row, neither naming a Node version: first
+`Module ts-jest in the transform option was not found` (this checkout simply had
+no `backend/node_modules`), then, after `npm ci`,
+`TypeError: Cannot read properties of undefined (reading 'prototype')` raised
+four frames under `jws` inside a package the test never imports. The remedy is a
+paragraph in a testing doc — findable only if the version is already suspected.
+
+Two of the doc's own facts were also off by one release boundary, so even a
+reader who found it could be misled: measured, `buffer.SlowBuffer` is a
+`function` on Node 22 and 24 and `undefined` on 25 and 26. The heading said
+"Node 26", which understates the range and sends a Node 25 user looking
+elsewhere.
+
+**Repair:** the doc now states the measured boundary rather than the version on
+which it was first noticed, and `globalSetup` writes the remedy to stderr when
+Node ≥ 25, before any worker fails. One line where the consumer is, instead of a
+paragraph where the consumer is not. Rule: when a doc exists to explain a
+failure that a bare dependency error will produce, put the pointer at the
+failure, not only in the doc — and state removals as measured boundaries, not as
+the version of the machine you happened to hit them on.