diff --git a/packages/core/src/loop/boringstack/gate-stages.ts b/packages/core/src/loop/boringstack/gate-stages.ts
index 4c7d7b3..f8153a2 100644
--- a/packages/core/src/loop/boringstack/gate-stages.ts
+++ b/packages/core/src/loop/boringstack/gate-stages.ts
@@ -143,7 +143,7 @@ export function signatureToError(sig: string): IErrorItem {
       message: isParse
         ? `${message}\n↳ SYNTAX/PARSE error — do NOT surgically patch it (a patch on a broken-parse file re-breaks its braces/generics/JSX). REWRITE THE WHOLE FILE \`${file}\` cleanly in one pass; once it parses, downstream errors clear.${parseSteer}`
         : isPathsWithMethod
-          ? `${message}\n↳ \`PathsWithMethod<…, "<verb>">\` means no generated key matches this path AND method. Three causes — check the call site FIRST (generate:api fixes NEITHER of the first two): (1) WRONG PATH string — the literal must be EXACTLY \`/api/v1/<resource>\` (every route is mounted under \`/api/v1/\` — \`/api/x\` or \`/x\` is wrong) with a literal \`{id}\` segment, value via \`{ params: { path: { id } } }\`, never interpolated; (2) WRONG VERB — the path exists but doesn't support this method (e.g. a GET-only path called with POST); call the method the route actually defines; (3) ONLY if path AND verb are already right is the route genuinely unregistered — ensure it exists AND is mounted (shows in /swagger/json), then the gate re-runs generate:api and the type appears. Do NOT re-run generate:api for a call-site (path/verb) bug.`
+          ? `${message}\n↳ \`PathsWithMethod<…, "<verb>">\` means no generated key matches this path AND method. Three causes — check the call site FIRST (generate:api fixes NEITHER of the first two): (1) WRONG PATH string — the literal must EXACTLY match a generated key: the COLLECTION root carries a TRAILING SLASH (list/create are \`/api/v1/<resource>/\`, e.g. \`POST "/api/v1/<resource>/"\` — a POST/GET to \`/api/v1/<resource>\` WITHOUT the slash is the usual cause here; ADD it), while by-id is \`/api/v1/<resource>/{id}\` (no trailing slash) with a literal \`{id}\` segment, value via \`{ params: { path: { id } } }\`, never interpolated; \`/api/x\` or \`/x\` (missing the \`/api/v1/\` prefix) is also wrong; (2) WRONG VERB — the path exists but doesn't support this method (e.g. a GET-only path called with POST); call the method the route actually defines; (3) ONLY if path AND verb are already right is the route genuinely unregistered — ensure it exists AND is mounted (shows in /swagger/json), then the gate re-runs generate:api and the type appears. Do NOT re-run generate:api for a call-site (path/verb) bug.`
           : message,
     };
   }
diff --git a/packages/core/src/loop/conventions.ts b/packages/core/src/loop/conventions.ts
index 29cbd0f..b44e21d 100644
--- a/packages/core/src/loop/conventions.ts
+++ b/packages/core/src/loop/conventions.ts
@@ -159,17 +159,22 @@ const GUIDES: Readonly<Record<ConventionTopic, string>> = {
     "`@/lib/api/client` — never `fetch`/`axios` (lint-banned). PATH STRINGS are the #1 thing " +
     "the model gets wrong: the path is a LITERAL that must exactly match a key in the generated " +
     "`paths` type, and every route is mounted under `/api/v1/` — so it is " +
-    '`apiClient.GET("/api/v1/<resource>")`, NOT `"/<resource>"` or `"/api/<resource>"` (a wrong ' +
+    '`apiClient.GET("/api/v1/<resource>/")`, NOT `"/<resource>"` or `"/api/<resource>"` (a wrong ' +
     "string is a `PathsWithMethod`/'not assignable' error that `generate:api` will NEVER fix — it " +
-    "is a usage bug, not a stale-spec bug). For a by-id path, use the LITERAL `{id}` segment and " +
-    "pass the value via params — never string-interpolate the id into the path. CALL SHAPE: the " +
+    "is a usage bug, not a stale-spec bug). TRAILING SLASH matters and is the #1 cause of a POST/GET " +
+    "`PathsWithMethod` on a path that otherwise looks right: the COLLECTION root carries a trailing " +
+    'slash — list is `GET "/api/v1/<resource>/"` and create is `POST "/api/v1/<resource>/"` (Elysia ' +
+    "mounts the group at `/api/v1/<resource>` and the handler at `/`, so the generated key is " +
+    '`/api/v1/<resource>/`). The by-id path has NO trailing slash: `"/api/v1/<resource>/{id}"` — use ' +
+    "the LITERAL `{id}` segment and pass the value via params, never string-interpolate the id. If a " +
+    "POST to `/api/v1/<resource>` (no slash) is rejected, ADD the trailing slash. CALL SHAPE: the " +
     "options object is OPTIONAL — a plain list GET is one arg (`GET(path)`); when you need params " +
     "and/or a body they ALL go together in ONE options object as the SECOND arg — there is NEVER a " +
     "third positional argument:\n" +
-    '• list:   `const { data } = await apiClient.GET("/api/v1/supplier")`\n' +
+    '• list:   `const { data } = await apiClient.GET("/api/v1/supplier/")`  (collection → TRAILING SLASH)\n' +
     '• by id:  `const { data } = await apiClient.GET("/api/v1/supplier/{id}", { params: { path: { id } } })`\n' +
-    '• query:  `await apiClient.GET("/api/v1/supplier", { params: { query: { status: "active" } } })`\n' +
-    '• create: `const { data } = await apiClient.POST("/api/v1/supplier", { body: input })`\n' +
+    '• query:  `await apiClient.GET("/api/v1/supplier/", { params: { query: { status: "active" } } })`\n' +
+    '• create: `const { data } = await apiClient.POST("/api/v1/supplier/", { body: input })`  (collection → TRAILING SLASH)\n' +
     '• update: `const { data } = await apiClient.PATCH("/api/v1/supplier/{id}", { params: { path: { id } }, body: input })`\n' +
     '• delete: `await apiClient.DELETE("/api/v1/supplier/{id}", { params: { path: { id } } })`\n' +
     "PATCH/PUT take path AND body in the SAME options object (one object as the second arg — " +
diff --git a/packages/core/tests/boringstack-conventions.test.ts b/packages/core/tests/boringstack-conventions.test.ts
index d12be41..5f6701a 100644
--- a/packages/core/tests/boringstack-conventions.test.ts
+++ b/packages/core/tests/boringstack-conventions.test.ts
@@ -133,13 +133,18 @@ describe("convention registry", () => {
     expect(g).toContain("/api/v1/");
     expect(g).toContain("PathsWithMethod");
     // Full method↔shape pairings (not loose fragments) — a fragment on the wrong method must fail.
-    expect(g).toContain('apiClient.POST("/api/v1/supplier", { body: input })');
+    // COLLECTION root carries a TRAILING SLASH (build14: the model sprayed on POST /api/v1/supplier
+    // for ~40 turns until it discovered the generated key is /api/v1/supplier/).
+    expect(g).toContain('apiClient.POST("/api/v1/supplier/", { body: input })');
     expect(g).toContain(
       'apiClient.PATCH("/api/v1/supplier/{id}", { params: { path: { id } }, body: input })'
     );
     expect(g).toContain(
       'apiClient.DELETE("/api/v1/supplier/{id}", { params: { path: { id } } })'
     );
+    // The trailing-slash rule is stated explicitly (collection root vs by-id).
+    expect(g).toContain("TRAILING SLASH");
+    expect(g).toContain('POST "/api/v1/<resource>/"');
     // Options are OPTIONAL and there is never a THIRD positional arg (the corrected contradiction).
     expect(g).toContain("options object is OPTIONAL");
     expect(g).toContain("NEVER a " + "third positional argument");
diff --git a/packages/core/tests/boringstack-gate-stages.test.ts b/packages/core/tests/boringstack-gate-stages.test.ts
index 2095950..ebb1b5f 100644
--- a/packages/core/tests/boringstack-gate-stages.test.ts
+++ b/packages/core/tests/boringstack-gate-stages.test.ts
@@ -157,6 +157,9 @@ describe("signatureToError", () => {
     expect(err.message).toContain("WRONG PATH");
     expect(err.message).toContain("WRONG VERB");
     expect(err.message).toContain("unregistered");
+    // The steer must name the collection TRAILING SLASH (build14 endgame) — else the per-error
+    // feedback contradicts the front-loaded guide when the model is stuck on a slashless POST.
+    expect(err.message).toContain("TRAILING SLASH");
     // …and the wrong-lever warning: don't re-run generate:api for a call-site string bug.
     expect(err.message).toContain("Do NOT re-run generate:api");
     // A plain type error (no PathsWithMethod) is left untouched.