diff --git a/.changelog/next/fixed-issue-4175.md b/.changelog/next/fixed-issue-4175.md
new file mode 100644
index 0000000000..db0da3b767
--- /dev/null
+++ b/.changelog/next/fixed-issue-4175.md
@@ -0,0 +1 @@
+- Server and client mirror contracts now detect missing parity coverage before drift reaches users.
diff --git a/server/lib/appIdentity.mirror.test.js b/server/lib/appIdentity.mirror.test.js
new file mode 100644
index 0000000000..53af99262a
--- /dev/null
+++ b/server/lib/appIdentity.mirror.test.js
@@ -0,0 +1,9 @@
+import { describe, expect, it } from 'vitest';
+import { PORTOS_APP_ID as serverAppId } from './appIdentity.js';
+import { PORTOS_APP_ID as clientAppId } from '../../client/src/lib/appIdentity.js';
+
+describe('appIdentity — server/client mirror parity', () => {
+  it('keeps the baseline app id identical', () => {
+    expect(clientAppId).toBe(serverAppId);
+  });
+});
diff --git a/server/lib/issueLength.mirror.test.js b/server/lib/issueLength.mirror.test.js
new file mode 100644
index 0000000000..b49ae390b9
--- /dev/null
+++ b/server/lib/issueLength.mirror.test.js
@@ -0,0 +1,55 @@
+import { describe, expect, it } from 'vitest';
+import {
+  CUSTOM_MINUTE_MAX as serverMinuteMax,
+  CUSTOM_MINUTE_MIN as serverMinuteMin,
+  CUSTOM_PAGE_MAX as serverPageMax,
+  CUSTOM_PAGE_MIN as serverPageMin,
+  DEFAULT_LENGTH_PROFILE as serverDefaultProfile,
+  LENGTH_PROFILES as serverProfiles,
+} from './issueLength.js';
+import {
+  CUSTOM_MINUTE_MAX as clientMinuteMax,
+  CUSTOM_MINUTE_MIN as clientMinuteMin,
+  CUSTOM_PAGE_MAX as clientPageMax,
+  CUSTOM_PAGE_MIN as clientPageMin,
+  DEFAULT_LENGTH_PROFILE as clientDefaultProfile,
+  LENGTH_PROFILES as clientProfiles,
+} from '../../client/src/lib/issueLength.js';
+
+describe('issueLength — server/client picker parity', () => {
+  it('keeps the profiles the client displays aligned with server targets', () => {
+    const serverPickerProfiles = Object.fromEntries(Object.entries(serverProfiles).map(([id, profile]) => [
+      id,
+      {
+        label: profile.label,
+        pageTarget: profile.pageTarget,
+        minutesTarget: profile.minutesTarget,
+      },
+    ]));
+    const clientPickerProfiles = Object.fromEntries(Object.entries(clientProfiles).map(([id, profile]) => [
+      id,
+      {
+        label: profile.label,
+        pageTarget: profile.pageTarget,
+        minutesTarget: profile.minutesTarget,
+      },
+    ]));
+
+    expect(clientPickerProfiles).toEqual(serverPickerProfiles);
+    expect(clientDefaultProfile).toBe(serverDefaultProfile);
+  });
+
+  it('keeps every custom-override bound identical', () => {
+    expect({
+      pageMin: clientPageMin,
+      pageMax: clientPageMax,
+      minuteMin: clientMinuteMin,
+      minuteMax: clientMinuteMax,
+    }).toEqual({
+      pageMin: serverPageMin,
+      pageMax: serverPageMax,
+      minuteMin: serverMinuteMin,
+      minuteMax: serverMinuteMax,
+    });
+  });
+});
diff --git a/server/lib/mirrorCoverage.test.js b/server/lib/mirrorCoverage.test.js
new file mode 100644
index 0000000000..5f2891bbd8
--- /dev/null
+++ b/server/lib/mirrorCoverage.test.js
@@ -0,0 +1,187 @@
+import { describe, expect, it } from 'vitest';
+import { readFileSync, readdirSync } from 'fs';
+import { basename, dirname, join } from 'path';
+import { fileURLToPath } from 'url';
+
+const here = dirname(fileURLToPath(import.meta.url));
+const CLIENT_LIB = join(here, '../../client/src/lib');
+const CLIENT_README = join(CLIENT_LIB, 'README.md');
+const SERVER_README = join(here, 'README.md');
+
+function listTestFiles(dir) {
+  return readdirSync(dir, { withFileTypes: true }).flatMap((entry) => {
+    const path = join(dir, entry.name);
+    if (entry.isDirectory()) return listTestFiles(path);
+    return entry.name.endsWith('.test.js') ? [path] : [];
+  });
+}
+
+// Both catalogs declare mirrors the same way — a backtick-fenced filename in
+// column 1, a description in column 2 naming the counterpart path — differing
+// only in which side is "this file" vs. "the other file it mirrors". One
+// parameterized walker keeps that row-parsing logic from drifting between the
+// two catalogs the way the mirrored declarations it guards must not drift.
+function listedPairsFor(readme, otherPathRe) {
+  const rows = [...readme.matchAll(/^\|\s+`([^`]+\.js)`\s+\|\s+(.+)\|$/gm)];
+  return rows.flatMap(([, thisFile, description]) => {
+    if (!/\bmirror/i.test(description)) return [];
+    const otherMatch = description.match(otherPathRe);
+    if (!otherMatch || otherMatch[1].includes('/') || thisFile !== basename(otherMatch[1])) return [];
+    return [{ thisFile, otherFile: otherMatch[1] }];
+  });
+}
+
+function listedMirrorPairs(readme) {
+  return listedPairsFor(readme, /server\/lib\/([\w/-]+\.js)/)
+    .map(({ thisFile, otherFile }) => ({ clientFile: thisFile, serverFile: otherFile }));
+}
+
+function listedServerMirrorPairs(readme) {
+  return listedPairsFor(readme, /client\/src\/lib\/([\w/-]+\.js)/)
+    .map(({ thisFile, otherFile }) => ({ clientFile: otherFile, serverFile: thisFile }));
+}
+
+function uniquePairs(pairs) {
+  return [...new Map(pairs.map((pair) => [`${pair.serverFile}:${pair.clientFile}`, pair])).values()];
+}
+
+// Matches `ref` only when it appears as (part of) a quoted string — i.e. an
+// actual import/require specifier — not a bare substring. A prose comment
+// mentioning a filename, or an unrelated same-prefix fixture, must not count
+// as "this test imports that file".
+function importsRef(source, ref) {
+  const escaped = ref.replace(/[.*+?^${}()|[\]\\]/g, '\\$&');
+  // Quote characters only — no backtick. Backtick-fenced prose is this
+  // codebase's dominant style for referencing a file path in a comment or
+  // JSDoc header (see every existing parity-test docstring), so treating it
+  // as an import specifier would reopen the exact prose-mention bypass this
+  // helper exists to close.
+  return new RegExp(`['"][^'"]*${escaped}['"]`).test(source);
+}
+
+function missingParityPins(pairs, testSources) {
+  return pairs.filter(({ clientFile, serverFile }) => {
+    const serverName = basename(serverFile);
+    // For a direct mirror, clientFile and serverName are the identical
+    // string — so "reads the client copy" and "reads the server copy" can't
+    // be proven by a plain check on each independently, or the SAME
+    // occurrence satisfies both (a client-only test that only imports its
+    // own module via `'./example.js'` would otherwise pass as a valid
+    // parity pin, and symmetrically for a server-only test — see the
+    // bypass-probe tests below). Strip whichever string just proved "reads
+    // the client copy" before checking for server-copy evidence, so the two
+    // proofs must come from genuinely different occurrences.
+    const clientPathRef = `client/src/lib/${clientFile}`;
+    return !testSources.some(({ path, source }) => {
+      if (path === fileURLToPath(import.meta.url)) return false;
+      const inClientLib = path.startsWith(CLIENT_LIB);
+      const readsClient = importsRef(source, clientPathRef) || (inClientLib && importsRef(source, clientFile));
+      if (!readsClient) return false;
+      let remainder = source.split(clientPathRef).join('');
+      // Only strip the bare-import specifier when it was actually used as
+      // client-copy evidence above (inClientLib) — outside client/src/lib
+      // that same specifier is exactly what proves the SERVER copy was read
+      // (see catalogTypes.parity.test.js: `from './catalogTypes.js'`
+      // alongside the full client path), so stripping it unconditionally
+      // would erase legitimate server-copy evidence.
+      if (inClientLib) remainder = remainder.split(`'./${clientFile}'`).join('').split(`"./${clientFile}"`).join('');
+      return importsRef(remainder, serverName);
+    });
+  });
+}
+
+describe('declared server/client mirror coverage', () => {
+  const readme = readFileSync(CLIENT_README, 'utf8');
+  const pairs = uniquePairs([
+    ...listedMirrorPairs(readme),
+    ...listedServerMirrorPairs(readFileSync(SERVER_README, 'utf8')),
+  ]);
+  const testSources = [...listTestFiles(here), ...listTestFiles(CLIENT_LIB)].map((path) => ({
+    path,
+    source: readFileSync(path, 'utf8'),
+  }));
+
+  it('finds direct same-name mirror declarations in the client catalog', () => {
+    expect(pairs).toContainEqual({ clientFile: 'seasonStructure.js', serverFile: 'seasonStructure.js' });
+    expect(pairs).toContainEqual({ clientFile: 'shotGrammar.js', serverFile: 'shotGrammar.js' });
+    expect(pairs).toContainEqual({ clientFile: 'appIdentity.js', serverFile: 'appIdentity.js' });
+    expect(pairs).toContainEqual({ clientFile: 'issueLength.js', serverFile: 'issueLength.js' });
+  });
+
+  it('also includes direct same-name declarations from the server catalog', () => {
+    expect(pairs).toContainEqual({ clientFile: 'catalogTypes.js', serverFile: 'catalogTypes.js' });
+  });
+
+  it('requires every declared direct mirror to have a test that reads both copies', () => {
+    const missing = missingParityPins(pairs, testSources);
+    expect(missing, `missing parity pins: ${missing.map(({ clientFile }) => clientFile).join(', ')}`).toEqual([]);
+  });
+
+  it('reports a synthetic declared mirror when no test reads both copies', () => {
+    const synthetic = listedMirrorPairs('| `example.js` | Mirror of `server/lib/example.js`. |');
+    expect(missingParityPins(synthetic, [])).toEqual([
+      { clientFile: 'example.js', serverFile: 'example.js' },
+    ]);
+  });
+
+  it('does not accept a same-name server-only unit test as a parity pin (bypass probe)', () => {
+    // A direct mirror's clientFile and serverFile are the same string, so a
+    // plain server-side unit test importing its own module via a bare
+    // relative path (e.g. `bareUrl.test.js` doing `from './bareUrl.js'`)
+    // trivially contains both `'./example.js'` and the server filename
+    // without ever touching the client copy. Pin that this does NOT count.
+    const synthetic = listedMirrorPairs('| `example.js` | Mirror of `server/lib/example.js`. |');
+    const serverOnlyUnitTest = {
+      path: join(here, 'example.test.js'),
+      source: "import { thing } from './example.js';\n",
+    };
+    expect(missingParityPins(synthetic, [serverOnlyUnitTest])).toEqual([
+      { clientFile: 'example.js', serverFile: 'example.js' },
+    ]);
+  });
+
+  it('does not accept a same-name client-only unit test as a parity pin (bypass probe)', () => {
+    // The mirror image of the probe above: a plain client-side unit test
+    // importing its own module via a bare relative path (e.g.
+    // client/src/lib/catalogTypes.test.js doing `from './catalogTypes.js'`)
+    // never touches the server copy. Pin that this does NOT count either.
+    const synthetic = listedMirrorPairs('| `example.js` | Mirror of `server/lib/example.js`. |');
+    const clientOnlyUnitTest = {
+      path: join(CLIENT_LIB, 'example.test.js'),
+      source: "import { thing } from './example.js';\n",
+    };
+    expect(missingParityPins(synthetic, [clientOnlyUnitTest])).toEqual([
+      { clientFile: 'example.js', serverFile: 'example.js' },
+    ]);
+  });
+
+  it('does not accept a bare prose mention of the filename as a parity pin (bypass probe)', () => {
+    // A comment mentioning the server path in passing — without an actual
+    // import of it — must not satisfy the guard either, or a stray comment
+    // surviving the deletion of the real parity-pinning test would keep this
+    // suite silently green.
+    const synthetic = listedMirrorPairs('| `example.js` | Mirror of `server/lib/example.js`. |');
+    const commentOnlyMention = {
+      path: join(CLIENT_LIB, 'example.test.js'),
+      source: "import { thing } from './example.js';\n// keep this in sync with server/lib/example.js\n",
+    };
+    expect(missingParityPins(synthetic, [commentOnlyMention])).toEqual([
+      { clientFile: 'example.js', serverFile: 'example.js' },
+    ]);
+  });
+
+  it('does not accept a backtick-fenced prose mention as a parity pin (bypass probe)', () => {
+    // Backtick-fenced file references are this codebase's dominant docstring
+    // style (see every existing parity-test header) — a JSDoc comment that
+    // mentions both paths in backticks, with no real import backing it, must
+    // not count either.
+    const synthetic = listedMirrorPairs('| `example.js` | Mirror of `server/lib/example.js`. |');
+    const backtickOnlyMention = {
+      path: join(CLIENT_LIB, 'example.test.js'),
+      source: 'import { thing } from \'./example.js\';\n// mirrors `server/lib/example.js` in spirit only, no import here.\n',
+    };
+    expect(missingParityPins(synthetic, [backtickOnlyMention])).toEqual([
+      { clientFile: 'example.js', serverFile: 'example.js' },
+    ]);
+  });
+});
diff --git a/server/lib/scenePrompt.test.js b/server/lib/scenePrompt.test.js
index f05b4d1076..37c3f4c59e 100644
--- a/server/lib/scenePrompt.test.js
+++ b/server/lib/scenePrompt.test.js
@@ -1,4 +1,7 @@
 import { describe, it, expect } from 'vitest';
+import { readFileSync } from 'fs';
+import { dirname, join } from 'path';
+import { fileURLToPath } from 'url';
 import {
   normalizeSlugline,
   normCharKey,
@@ -9,6 +12,11 @@ import {
   buildScenePrompt,
   __testing,
 } from './scenePrompt.js';
+import { compareDeclaration } from './mirrorParity.js';
+
+const here = dirname(fileURLToPath(import.meta.url));
+const SERVER_COPY = join(here, 'scenePrompt.js');
+const CLIENT_COPY = join(here, '../../client/src/lib/scenePrompt.js');
 
 describe('scenePrompt — normalizeSlugline', () => {
   it('collapses em/en/hyphen + punctuation + spaces so equivalent sluglines match', () => {
@@ -307,3 +315,31 @@ describe('scenePrompt — buildScenePrompt wardrobe appearances', () => {
     expect(out).not.toContain('Wearing:');
   });
 });
+
+describe('scenePrompt — server/client mirror parity', () => {
+  const server = readFileSync(SERVER_COPY, 'utf8');
+  const client = readFileSync(CLIENT_COPY, 'utf8');
+  const mirroredDeclarations = [
+    'PROMPT_MAX',
+    'normalizeSlugline',
+    'normCharKey',
+    'buildCharByKey',
+    'matchSceneCharacters',
+    'matchCharactersInText',
+    'buildPlaceByKey',
+    'matchScenePlace',
+    'matchEntriesByCandidates',
+    'matchPlacesInText',
+    'matchObjectsInText',
+    'appendWardrobe',
+    'buildScenePrompt',
+  ];
+
+  for (const name of mirroredDeclarations) {
+    it(`keeps ${name} identical`, () => {
+      const { clientDecl, serverNorm, clientNorm } = compareDeclaration(server, client, name);
+      expect(clientDecl, `client/src/lib/scenePrompt.js is missing ${name}`).not.toBeNull();
+      expect(clientNorm).toBe(serverNorm);
+    });
+  }
+});
diff --git a/server/lib/seasonStructure.mirror.test.js b/server/lib/seasonStructure.mirror.test.js
new file mode 100644
index 0000000000..17f34d4612
--- /dev/null
+++ b/server/lib/seasonStructure.mirror.test.js
@@ -0,0 +1,23 @@
+import { describe, expect, it } from 'vitest';
+import { readFileSync } from 'fs';
+import { dirname, join } from 'path';
+import { fileURLToPath } from 'url';
+import { compareDeclaration } from './mirrorParity.js';
+
+const here = dirname(fileURLToPath(import.meta.url));
+const SERVER_COPY = join(here, 'seasonStructure.js');
+const CLIENT_COPY = join(here, '../../client/src/lib/seasonStructure.js');
+const MIRRORED_DECLARATIONS = ['pickSeasonCount', 'recommendStructure', 'describeStructure'];
+
+describe('seasonStructure — server/client mirror parity', () => {
+  const server = readFileSync(SERVER_COPY, 'utf8');
+  const client = readFileSync(CLIENT_COPY, 'utf8');
+
+  for (const name of MIRRORED_DECLARATIONS) {
+    it(`keeps ${name} identical`, () => {
+      const { clientDecl, serverNorm, clientNorm } = compareDeclaration(server, client, name);
+      expect(clientDecl, `client/src/lib/seasonStructure.js is missing ${name}`).not.toBeNull();
+      expect(clientNorm).toBe(serverNorm);
+    });
+  }
+});
diff --git a/server/lib/shotGrammar.mirror.test.js b/server/lib/shotGrammar.mirror.test.js
new file mode 100644
index 0000000000..11bf69b819
--- /dev/null
+++ b/server/lib/shotGrammar.mirror.test.js
@@ -0,0 +1,22 @@
+import { describe, expect, it } from 'vitest';
+import { readFileSync } from 'fs';
+import { dirname, join } from 'path';
+import { fileURLToPath } from 'url';
+import { compareDeclaration } from './mirrorParity.js';
+
+const here = dirname(fileURLToPath(import.meta.url));
+const SERVER_COPY = join(here, 'shotGrammar.js');
+const CLIENT_COPY = join(here, '../../client/src/lib/shotGrammar.js');
+
+describe('shotGrammar — server/client vocabulary parity', () => {
+  const server = readFileSync(SERVER_COPY, 'utf8');
+  const client = readFileSync(CLIENT_COPY, 'utf8');
+
+  for (const name of ['SHOT_TYPES', 'SCREEN_DIRECTIONS']) {
+    it(`keeps ${name} identical`, () => {
+      const { clientDecl, serverNorm, clientNorm } = compareDeclaration(server, client, name);
+      expect(clientDecl, `client/src/lib/shotGrammar.js is missing ${name}`).not.toBeNull();
+      expect(clientNorm).toBe(serverNorm);
+    });
+  }
+});