diff --git a/lib/api/mock.ts b/lib/api/mock.ts
index 42c4848..f459ed6 100644
--- a/lib/api/mock.ts
+++ b/lib/api/mock.ts
@@ -6,6 +6,7 @@
  * The implementation is organised into focused modules under `lib/api/mock/`
  * so a structural mistake in one domain cannot break the whole API layer:
  *
+ *   - domains.ts     — canonical domain registry used by structure tests
  *   - fixtures.ts    — fixture/seeded data (communities, members, events…)
  *   - state.ts       — the in-memory per-community store + persistence
  *   - session.ts     — SIWE endpoints + cookie-session simulation
diff --git a/lib/api/mock/domains.ts b/lib/api/mock/domains.ts
new file mode 100644
index 0000000..17543c2
--- /dev/null
+++ b/lib/api/mock/domains.ts
@@ -0,0 +1,201 @@
+/**
+ * Canonical mock API domain registry.
+ *
+ * Each entry is an independently parseable module under `lib/api/mock/`.
+ * The aggregator (`lib/api/mock.ts`) must stay a thin composition layer so a
+ * syntax error in one domain does not require editing a monolithic file.
+ *
+ * `requiredExports` is the stable function/value surface that other mock
+ * modules and the aggregator rely on. Keep this list in lockstep with the
+ * domain file's public exports.
+ */
+export const MOCK_API_DOMAINS = [
+  {
+    file: 'fixtures.ts',
+    requiredExports: [
+      'DEFAULT_COMMUNITY',
+      'mockConnections',
+      'mockPrivacySettings',
+      'mockReports',
+      'setMockConnections',
+      'setMockPrivacySettings',
+      'setMockReports',
+    ],
+  },
+  {
+    file: 'state.ts',
+    requiredExports: ['communityStates', 'getCommunityState', 'initPromise'],
+  },
+  {
+    file: 'session.ts',
+    requiredExports: [
+      'mockGetNonce',
+      'mockGetSession',
+      'mockGetSessionStatus',
+      'mockSiweLogout',
+      'mockSiweRefresh',
+      'mockSiweVerify',
+    ],
+  },
+  {
+    file: 'core.ts',
+    requiredExports: [
+      'mockGetCommunity',
+      'mockGetMeta',
+      'mockGetPolicy',
+      'mockGetResource',
+      'mockListPolicies',
+      'mockListResources',
+      'mockVerifyWallet',
+    ],
+  },
+  {
+    file: 'members.ts',
+    requiredExports: ['mockGetMembership', 'mockGetProfile', 'mockListMembers', 'mockUpdateProfile'],
+  },
+  {
+    file: 'analytics.ts',
+    requiredExports: ['buildAnalyticsDataSource', 'mockGetAnalyticsSummary'],
+  },
+  {
+    file: 'webhooks.ts',
+    requiredExports: [
+      'mockListAdminEvents',
+      'mockListWebhookEvents',
+      'mockReplayEvent',
+      'mockSubscribeWebhookEvents',
+      'replayMockEvent',
+    ],
+  },
+  {
+    file: 'approvals.ts',
+    requiredExports: [
+      'mockApproveAction',
+      'mockAssignRole',
+      'mockGetPendingActions',
+      'mockRejectAction',
+      'mockRemoveRole',
+      'mockUpdateApprovalConfig',
+      'mockUpdatePolicy',
+    ],
+  },
+  {
+    file: 'social.ts',
+    requiredExports: [
+      'mockAcceptConnectionRequest',
+      'mockBlockMember',
+      'mockCreateConnectionRequest',
+      'mockGetConnections',
+      'mockGetPrivacySettings',
+      'mockRejectConnectionRequest',
+      'mockUnblockMember',
+      'mockUpdatePrivacySettings',
+    ],
+  },
+  {
+    file: 'moderation.ts',
+    requiredExports: ['mockGetReport', 'mockListReports', 'mockUpdateReportState'],
+  },
+  {
+    file: 'governance.ts',
+    requiredExports: [
+      'mockCastVote',
+      'mockCloseProposalVoting',
+      'mockCreateProposal',
+      'mockDeleteProposal',
+      'mockGetMemberVote',
+      'mockGetProposal',
+      'mockListProposalVotes',
+      'mockListProposals',
+      'mockPublishProposal',
+      'mockResolveProposal',
+      'mockUpdateProposal',
+    ],
+  },
+  {
+    file: 'controls.ts',
+    requiredExports: [
+      'MOCK_META_VERSION_OVERRIDE',
+      'setMockMetaVersion',
+      'setMockResourceFetchDelay',
+      'setMockResourceFetchFailure',
+      'setMockRoleMutationFailure',
+    ],
+  },
+  {
+    file: 'scenarios.ts',
+    requiredExports: ['applyMockScenario', 'resetMockData'],
+  },
+] as const
+
+export type MockApiDomainFile = (typeof MOCK_API_DOMAINS)[number]['file']
+
+/** Public aggregator re-exports that existing `lib/api/mock` consumers rely on. */
+export const MOCK_API_PUBLIC_REEXPORTS = [
+  'applyMockScenario',
+  'communityStates',
+  'getCommunityState',
+  'MOCK_META_VERSION_OVERRIDE',
+  'mockConnections',
+  'mockPrivacySettings',
+  'mockReports',
+  'replayMockEvent',
+  'resetMockData',
+  'setMockMetaVersion',
+  'setMockResourceFetchDelay',
+  'setMockResourceFetchFailure',
+  'setMockRoleMutationFailure',
+] as const
+
+/** AccessApi methods that MockAccessApi must keep implementing. */
+export const MOCK_ACCESS_API_METHODS = [
+  'getSession',
+  'getCommunity',
+  'getMembership',
+  'verifyWallet',
+  'getProfile',
+  'listMembers',
+  'listResources',
+  'listPolicies',
+  'getResource',
+  'getPolicy',
+  'updateProfile',
+  'getMeta',
+  'getConnections',
+  'getPrivacySettings',
+  'updatePrivacySettings',
+  'blockMember',
+  'unblockMember',
+  'createConnectionRequest',
+  'acceptConnectionRequest',
+  'rejectConnectionRequest',
+  'listWebhookEvents',
+  'listAdminEvents',
+  'subscribeWebhookEvents',
+  'getPendingActions',
+  'approveAction',
+  'rejectAction',
+  'updateApprovalConfig',
+  'assignRole',
+  'removeRole',
+  'updatePolicy',
+  'listReports',
+  'getReport',
+  'updateReportState',
+  'listProposals',
+  'getProposal',
+  'getMemberVote',
+  'listProposalVotes',
+  'castVote',
+  'createProposal',
+  'updateProposal',
+  'publishProposal',
+  'closeProposalVoting',
+  'resolveProposal',
+  'deleteProposal',
+  'getNonce',
+  'siweVerify',
+  'siweRefresh',
+  'siweLogout',
+  'getSessionStatus',
+] as const
diff --git a/test/mock-api-structure.test.ts b/test/mock-api-structure.test.ts
new file mode 100644
index 0000000..3ad6f8f
--- /dev/null
+++ b/test/mock-api-structure.test.ts
@@ -0,0 +1,170 @@
+import './setup-env'
+import { describe, test } from 'node:test'
+import * as assert from 'node:assert/strict'
+import { existsSync, readFileSync } from 'node:fs'
+import path from 'node:path'
+import ts from 'typescript'
+
+import { MockAccessApi } from '../lib/api/mock'
+import {
+  MOCK_ACCESS_API_METHODS,
+  MOCK_API_DOMAINS,
+  MOCK_API_PUBLIC_REEXPORTS,
+} from '../lib/api/mock/domains'
+
+const REPO_ROOT = path.resolve(__dirname, '..', '..')
+const MOCK_DIR = path.join(REPO_ROOT, 'lib', 'api', 'mock')
+const AGGREGATOR = path.join(REPO_ROOT, 'lib', 'api', 'mock.ts')
+
+function parseSyntactics(fileName: string, sourceText: string): readonly ts.Diagnostic[] {
+  const options: ts.CompilerOptions = {
+    noResolve: true,
+    noLib: true,
+    skipLibCheck: true,
+    isolatedModules: true,
+    target: ts.ScriptTarget.ES2020,
+    module: ts.ModuleKind.ESNext,
+  }
+
+  const host: ts.CompilerHost = {
+    getSourceFile: (requested, languageVersion) =>
+      requested === fileName ? ts.createSourceFile(fileName, sourceText, languageVersion, true) : undefined,
+    getDefaultLibFileName: () => '',
+    writeFile: () => undefined,
+    getCurrentDirectory: () => '',
+    getCanonicalFileName: (file) => file,
+    useCaseSensitiveFileNames: () => true,
+    getNewLine: () => '\n',
+    fileExists: (file) => file === fileName,
+    readFile: (file) => (file === fileName ? sourceText : undefined),
+  }
+
+  return ts.createProgram([fileName], options, host).getSyntacticDiagnostics()
+}
+
+function exportedNames(fileName: string, sourceText: string): Set<string> {
+  const sourceFile = ts.createSourceFile(fileName, sourceText, ts.ScriptTarget.ES2020, true, ts.ScriptKind.TS)
+  const names = new Set<string>()
+
+  for (const statement of sourceFile.statements) {
+    if (ts.isExportDeclaration(statement) && statement.exportClause && ts.isNamedExports(statement.exportClause)) {
+      for (const element of statement.exportClause.elements) {
+        names.add(element.name.text)
+      }
+      continue
+    }
+
+    const modifiers = ts.canHaveModifiers(statement) ? ts.getModifiers(statement) : undefined
+    const isExport = Boolean(modifiers?.some((modifier) => modifier.kind === ts.SyntaxKind.ExportKeyword))
+    if (!isExport) continue
+
+    if (ts.isFunctionDeclaration(statement) && statement.name) names.add(statement.name.text)
+    else if (ts.isClassDeclaration(statement) && statement.name) names.add(statement.name.text)
+    else if (ts.isVariableStatement(statement)) {
+      for (const declaration of statement.declarationList.declarations) {
+        if (ts.isIdentifier(declaration.name)) names.add(declaration.name.text)
+      }
+    }
+  }
+
+  return names
+}
+
+function formatDiagnostics(diagnostics: readonly ts.Diagnostic[]): string {
+  return ts.formatDiagnostics(diagnostics, {
+    getCanonicalFileName: (file) => file,
+    getCurrentDirectory: () => '',
+    getNewLine: () => '\n',
+  })
+}
+
+describe('mock API domain module structure', () => {
+  test('each domain is an independently parseable module with clear exports', () => {
+    for (const domain of MOCK_API_DOMAINS) {
+      const filePath = path.join(MOCK_DIR, domain.file)
+      assert.equal(existsSync(filePath), true, `missing domain module ${domain.file}`)
+
+      const sourceText = readFileSync(filePath, 'utf8')
+      const diagnostics = parseSyntactics(filePath, sourceText)
+      assert.equal(
+        diagnostics.length,
+        0,
+        `${domain.file} has syntax errors:\n${formatDiagnostics(diagnostics)}`,
+      )
+
+      const names = exportedNames(filePath, sourceText)
+      for (const required of domain.requiredExports) {
+        assert.equal(
+          names.has(required),
+          true,
+          `${domain.file} must export ${required} (found: ${[...names].sort().join(', ')})`,
+        )
+      }
+    }
+  })
+
+  test('a syntax error in one domain does not affect parsing of an unrelated domain', () => {
+    const brokenDomain = MOCK_API_DOMAINS.find((domain) => domain.file === 'governance.ts')
+    const healthyDomain = MOCK_API_DOMAINS.find((domain) => domain.file === 'members.ts')
+    assert.ok(brokenDomain && healthyDomain)
+
+    const brokenPath = path.join(MOCK_DIR, brokenDomain.file)
+    const healthyPath = path.join(MOCK_DIR, healthyDomain.file)
+    const brokenSource = `${readFileSync(brokenPath, 'utf8')}\n}}}\n`
+    const healthySource = readFileSync(healthyPath, 'utf8')
+
+    const brokenDiagnostics = parseSyntactics(brokenPath, brokenSource)
+    const healthyDiagnostics = parseSyntactics(healthyPath, healthySource)
+
+    assert.ok(brokenDiagnostics.length > 0, 'expected the malformed governance module to fail syntactic parse')
+    assert.ok(
+      brokenDiagnostics.every((diagnostic) => diagnostic.file?.fileName === brokenPath),
+      'syntax diagnostics must stay scoped to the malformed domain file',
+    )
+    assert.equal(
+      healthyDiagnostics.length,
+      0,
+      `unrelated domain ${healthyDomain.file} must still parse:\n${formatDiagnostics(healthyDiagnostics)}`,
+    )
+  })
+
+  test('aggregator stays a thin composition layer and keeps public exports stable', () => {
+    const aggregatorSource = readFileSync(AGGREGATOR, 'utf8')
+    const aggregatorLines = aggregatorSource.split('\n').length
+    assert.ok(
+      aggregatorLines < 500,
+      `mock.ts should remain a thin aggregator, not a 2000+ line monolith (was ${aggregatorLines} lines)`,
+    )
+    assert.match(aggregatorSource, /from '\.\/mock\//)
+    assert.doesNotMatch(
+      aggregatorSource,
+      /export async function mockGetMeta/,
+      'domain implementations must live in lib/api/mock/*.ts, not in the aggregator',
+    )
+
+    const names = exportedNames(AGGREGATOR, aggregatorSource)
+    assert.equal(names.has('MockAccessApi'), true)
+    for (const required of MOCK_API_PUBLIC_REEXPORTS) {
+      assert.equal(names.has(required), true, `mock.ts must re-export ${required}`)
+    }
+
+    const api = new MockAccessApi('0xabc')
+    for (const method of MOCK_ACCESS_API_METHODS) {
+      assert.equal(
+        typeof (api as unknown as Record<string, unknown>)[method],
+        'function',
+        `MockAccessApi must keep ${method} for existing consumers`,
+      )
+    }
+  })
+
+  test('application consumers depend on the API boundary, not mock domain files', () => {
+    const indexSource = readFileSync(path.join(REPO_ROOT, 'lib', 'api', 'index.ts'), 'utf8')
+    const navSource = readFileSync(path.join(REPO_ROOT, 'components', 'nav.tsx'), 'utf8')
+
+    assert.match(indexSource, /from '\.\/mock-boundary'/)
+    assert.doesNotMatch(indexSource, /from '\.\/mock\//)
+    assert.match(navSource, /from ["']@\/lib\/api["']/)
+    assert.doesNotMatch(navSource, /from ["']@\/lib\/api\/mock/)
+  })
+})
diff --git a/test/tsconfig.json b/test/tsconfig.json
index 2fda9e9..710b656 100644
--- a/test/tsconfig.json
+++ b/test/tsconfig.json
@@ -28,6 +28,8 @@
     "../lib/api/access-decision.ts",
     "../lib/rate-limit.ts",
     "../lib/api/mock.ts",
+    "../lib/api/mock/**/*.ts",
+    "../lib/api/mock-boundary.ts",
     "../lib/wallet/address.ts",
     "../lib/wallet/chains.ts",
     "../lib/wallet/config.ts",