diff --git a/.github/scripts/arch-lint.ts b/.github/scripts/arch-lint.ts
index 117c2d65b..2925a5cbe 100644
--- a/.github/scripts/arch-lint.ts
+++ b/.github/scripts/arch-lint.ts
@@ -15,7 +15,7 @@ import { err, ok, Result } from "neverthrow";
 import { type Node, parseSync } from "oxc-parser";
 import { parseDocument } from "yaml";
 import { z } from "zod";
-import { countNoun } from "../../src/text.js";
+import { countNoun, quote } from "../../src/text.js";
 
 export const ARCHITECTURE_PATH = "architecture.yml";
 
@@ -83,15 +83,33 @@ export function parseArchitecture(root: string): Result<Architecture, string[]>
     return err(doc.error);
   }
   const parsed = ARCHITECTURE.safeParse(doc.value);
-  return parsed.success
-    ? ok(parsed.data)
-    : err(located(parsed.error.issues.flatMap((issue) => describeIssue(doc.value, issue))));
+  if (!parsed.success) {
+    return err(located(parsed.error.issues.flatMap((issue) => describeIssue(doc.value, issue))));
+  }
+  const merged = mergedMermaidNodes(parsed.data);
+  return merged.length > 0 ? err(located(merged)) : ok(parsed.data);
 }
 
 function located(problems: readonly string[]): string[] {
   return problems.map((problem) => `${ARCHITECTURE_PATH}: ${problem}`);
 }
 
+/** A hyphen in a layer name is edge syntax to mermaid, so ids swap it for an underscore. */
+function mermaidId(layer: string): string {
+  return layer.replace(/-/g, "_");
+}
+
+/** The id mapping is not injective, so two layer names can share a node; the declaration refuses that pair. */
+function mergedMermaidNodes(arch: Architecture): string[] {
+  return [...Map.groupBy(Object.keys(arch.layers), mermaidId)].flatMap(([id, layers]) =>
+    layers.length > 1
+      ? [
+          `layers ${layers.map(quote).join(" and ")} share the mermaid id ${id}, which would draw them as one node of the module map; rename all but one`,
+        ]
+      : [],
+  );
+}
+
 /** For callers that render rather than lint (docs, tests): a malformed declaration is fatal to them. */
 export function readArchitecture(root: string): Architecture {
   return parseArchitecture(root).match(
@@ -279,14 +297,14 @@ export function lintArchitecture(root: string, arch = readArchitecture(root)): s
   return problems;
 }
 
-/** A hyphen in a layer name is edge syntax to mermaid, so ids swap it for an underscore. */
 export function renderArchitectureMermaid(arch: Architecture): string {
-  const id = (layer: string): string => layer.replace(/-/g, "_");
   return [
     "graph TD",
-    ...Object.entries(arch.layers).map(([name, paths]) => `  ${id(name)}["${paths.join("<br>")}"]`),
+    ...Object.entries(arch.layers).map(
+      ([name, paths]) => `  ${mermaidId(name)}["${paths.join("<br>")}"]`,
+    ),
     ...Object.entries(arch.edges).flatMap(([from, targets]) =>
-      targets.map((to) => `  ${id(from)} --> ${id(to)}`),
+      targets.map((to) => `  ${mermaidId(from)} --> ${mermaidId(to)}`),
     ),
   ].join("\n");
 }
diff --git a/.github/scripts/lib/count-word.ts b/.github/scripts/lib/count-word.ts
index 2238de350..e2abc94fb 100644
--- a/.github/scripts/lib/count-word.ts
+++ b/.github/scripts/lib/count-word.ts
@@ -1,7 +1,8 @@
 /**
  * The written-out counts docs prose uses ("Fifteen sections list ..."), shared by the generators
- * and the docs pins. The loud throw is the tripwire: a derived list outgrowing the words fails
- * the build instead of the prose silently falling back to a numeral.
+ * and the docs pins. Every caller counts a list the code derives (a registry, a constant), never a
+ * number read from a page, so a count past the words is a BUG: invariant: the derived list outgrew
+ * the table and the build fails instead of the prose silently falling back to a numeral.
  */
 
 const COUNT_WORDS = [
@@ -32,7 +33,7 @@ export function countWord(n: number): string {
   const word = COUNT_WORDS[n];
   if (word === undefined) {
     throw new Error(
-      `extend COUNT_WORDS (.github/scripts/lib/count-word.ts): no word for count ${n}`,
+      `BUG: extend COUNT_WORDS (.github/scripts/lib/count-word.ts): no word for count ${n}`,
     );
   }
   return word;
diff --git a/test/architecture/architecture.test.ts b/test/architecture/architecture.test.ts
index a1a2e9f97..a135079ed 100644
--- a/test/architecture/architecture.test.ts
+++ b/test/architecture/architecture.test.ts
@@ -152,6 +152,15 @@ describe("parseArchitecture", () => {
         "Flow sequence in block collection must be sufficiently indented and end with a ] at line 4, column 1",
       ],
     ],
+    [
+      // Mermaid reads a hyphen as edge syntax, so the renderer swaps it for an underscore; two names one id would
+      // merge into a single node of the module map, silently, unless the declaration refuses them first.
+      "two layers the mermaid map would draw as one node",
+      "layers: {plain-data: [src/plain-data.ts], plain_data: [src/plain_data.ts]}\nexclude: []\nedges: {}",
+      [
+        'layers "plain-data" and "plain_data" share the mermaid id plain_data, which would draw them as one node of the module map; rename all but one',
+      ],
+    ],
   ])("%s fails naming the key", (_case, document, problems) =>
     withTempDir("arch-lint-parse-", (dir) => {
       expect(parse(dir, document)).toEqual(err(problems.map((p) => `${ARCHITECTURE_PATH}: ${p}`)));
diff --git a/test/scripts/count-word.test.ts b/test/scripts/count-word.test.ts
index 51ea9a5e5..cf8a88860 100644
--- a/test/scripts/count-word.test.ts
+++ b/test/scripts/count-word.test.ts
@@ -1,12 +1,15 @@
-/** The written-out counts (.github/scripts/lib/count-word.ts): the tripwire past the range, never a numeral fallback. */
+/**
+ * The written-out counts (.github/scripts/lib/count-word.ts): the tripwire past the range is a BUG: invariant,
+ * never a numeral fallback, so a derived list outgrowing the words fails the build as a programming error.
+ */
 
 import { expect, test } from "bun:test";
 import { countWord } from "../../.github/scripts/lib/count-word.js";
 
-test("a count with no word throws naming the list to extend, whatever shape the count has", () => {
+test("a count with no word throws a BUG: invariant naming the list to extend, whatever shape the count has", () => {
   for (const count of [21, -1, 1.5, Number.NaN]) {
     expect(() => countWord(count)).toThrow(
-      `extend COUNT_WORDS (.github/scripts/lib/count-word.ts): no word for count ${count}`,
+      `BUG: extend COUNT_WORDS (.github/scripts/lib/count-word.ts): no word for count ${count}`,
     );
   }
 });