diff --git a/.github/workflows/single-account-guard.yml b/.github/workflows/single-account-guard.yml
new file mode 100644
index 0000000..4dfe1c8
--- /dev/null
+++ b/.github/workflows/single-account-guard.yml
@@ -0,0 +1,25 @@
+name: single-account guard
+
+# Anti-forgetting guard for the single-account invariant (PRD §7, FR44,
+# story 5.6 / EVO-1228). Fails the build when account-routing keywords land in
+# src/runners outside the documented allowlist. See
+# src/runners/single-account.spec.ts for the keyword list and exceptions.
+
+on:
+  pull_request:
+    branches: [main, develop]
+  push:
+    branches: [main, develop]
+
+jobs:
+  single-account:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-node@v4
+        with:
+          node-version: 20
+      - name: Install dependencies
+        run: npm ci
+      - name: Enforce single-account invariant
+        run: npx jest --runInBand src/runners/single-account.spec.ts
diff --git a/src/runners/single-account.spec.ts b/src/runners/single-account.spec.ts
new file mode 100644
index 0000000..cf79771
--- /dev/null
+++ b/src/runners/single-account.spec.ts
@@ -0,0 +1,308 @@
+import * as fs from 'fs';
+import * as path from 'path';
+
+/**
+ * Single-account invariant guard (story 5.6 / EVO-1228, FR44).
+ *
+ * evo-flow is single-account by PRD §7 and Architecture §Architectural
+ * Decisions. This spec greps every runner source file for account-routing
+ * keywords so a stray `if (account.tier === 'premium')` in a hot path fails
+ * CI instead of silently reintroducing multi-tenancy.
+ *
+ * To clear a failure: remove the account routing, or use one of the two
+ * escape hatches below — ALLOWED_LINE_PATTERNS for a genuinely neutral line
+ * (e.g. a log-only field; the pattern must match the entire line), or
+ * ALLOWED_TOKENS for a sanctioned identifier stripped before the scan.
+ * Exceptions live HERE, as literals, so every addition is visible in code
+ * review.
+ */
+
+const RUNNERS_DIR = __dirname;
+
+const RUNNER_MODES = [
+  'campaign-packer',
+  'campaign-sender',
+  'event-receiver',
+  'event-process',
+];
+
+interface KeywordPattern {
+  keyword: string;
+  pattern: RegExp;
+}
+
+interface SanctionedPattern {
+  pattern: RegExp;
+  reason: string;
+}
+
+// Substring matches on purpose: \b is camelCase-blind (`getAccountById` has
+// no word boundary before "Account"), and in a guard a loud false positive
+// beats a silent miss.
+const FORBIDDEN_PATTERNS: KeywordPattern[] = [
+  { keyword: 'accountId', pattern: /accountId/i },
+  { keyword: 'account_id', pattern: /account_id/i },
+  { keyword: 'Account.', pattern: /account\./i },
+  { keyword: 'tenant', pattern: /tenant/i },
+  { keyword: 'accountById', pattern: /accountById/i },
+  { keyword: 'byAccount', pattern: /byAccount/i },
+];
+
+/**
+ * Sanctioned tokens, STRIPPED from each line before the scan. Unlike
+ * ALLOWED_LINE_PATTERNS below (an anchored match of one entire literal
+ * line), a token exempts a single identifier — never the rest of its line,
+ * so `routeByTenant(); // TenantDbContext` still fails. Keep each entry
+ * justified — this list is the audit trail of every sanctioned mention.
+ */
+const ALLOWED_TOKENS: SanctionedPattern[] = [
+  {
+    // The DB seam (ADR14, story 10.1b): single-account in community, the RLS
+    // extension point in enterprise. Injecting it is the sanctioned way to
+    // reach Postgres — it is not account routing. Lookarounds keep the strip
+    // to the standalone identifier: glued forms (`tenantDbContextRouter`)
+    // stay on the line and trip the scan.
+    pattern: /(?<![A-Za-z0-9_])[Tt]enantDbContext(?![A-Za-z0-9_])/g,
+    reason: 'ADR14 tenant DB-context seam',
+  },
+  {
+    // The NestJS module that provides the seam — referenced by name in the
+    // runner modules' wiring docs. Sanctioned as its own full identifier so
+    // the bare-token strip can stay glue-proof.
+    pattern: /(?<![A-Za-z0-9_])[Tt]enantDbContextModule(?![A-Za-z0-9_])/g,
+    reason: 'ADR14 seam NestJS module',
+  },
+];
+
+/**
+ * Neutral-line exemptions (AC3): each pattern MUST match the ENTIRE trimmed
+ * line (`^...$`, no `g` flag — enforced by a self-test below). A line that
+ * fully matches is skipped; one extra character (e.g. routing glued onto the
+ * same line) breaks the anchor and the guard fires. Empty today — no runner
+ * has a neutral mention. Example entry for a log-only field:
+ *   { pattern: /^logger\.log\(\{ accountId: ingestion\.accountId \}\);$/,
+ *     reason: 'EVO-XXXX log-only field, no flow impact' },
+ */
+const ALLOWED_LINE_PATTERNS: SanctionedPattern[] = [];
+
+interface Violation {
+  file: string;
+  line: number;
+  keyword: string;
+  text: string;
+}
+
+function collectSourceFiles(dir: string): string[] {
+  return fs.readdirSync(dir, { withFileTypes: true }).flatMap((entry) => {
+    const fullPath = path.join(dir, entry.name);
+    if (entry.isDirectory()) return collectSourceFiles(fullPath);
+    if (!entry.name.endsWith('.ts') || entry.name.endsWith('.spec.ts')) {
+      return [];
+    }
+    return [fullPath];
+  });
+}
+
+function scanContent(
+  content: string,
+  fileLabel: string,
+  allowedLines: SanctionedPattern[] = ALLOWED_LINE_PATTERNS,
+): Violation[] {
+  const violations: Violation[] = [];
+  content.split('\n').forEach((text, index) => {
+    if (allowedLines.some(({ pattern }) => pattern.test(text.trim()))) {
+      return;
+    }
+    const scannable = ALLOWED_TOKENS.reduce(
+      (line, { pattern }) => line.replace(pattern, ''),
+      text,
+    );
+    for (const { keyword, pattern } of FORBIDDEN_PATTERNS) {
+      if (pattern.test(scannable)) {
+        violations.push({ file: fileLabel, line: index + 1, keyword, text });
+        break;
+      }
+    }
+  });
+  return violations;
+}
+
+function scanDirectory(dir: string): Violation[] {
+  return collectSourceFiles(dir).flatMap((file) =>
+    scanContent(
+      fs.readFileSync(file, 'utf8'),
+      path.relative(RUNNERS_DIR, file),
+    ),
+  );
+}
+
+function assertNoViolations(violations: Violation[], scope: string): void {
+  if (violations.length === 0) return;
+
+  const report = violations
+    .map(
+      (v) =>
+        `  ${v.file}:${v.line} — keyword "${v.keyword}" — ${v.text.trim()}`,
+    )
+    .join('\n');
+  throw new Error(
+    `Account routing detected in ${scope} (single-account invariant, FR44).\n` +
+      'Remove the routing/scoping. If the line is genuinely neutral (e.g. a ' +
+      'log-only field), add an anchored entry to ALLOWED_LINE_PATTERNS in ' +
+      'single-account.spec.ts; sanctioned identifiers belong in ' +
+      `ALLOWED_TOKENS.\n${report}`,
+  );
+}
+
+describe('single-account invariant (FR44 / EVO-1228)', () => {
+  RUNNER_MODES.forEach((mode) => {
+    it(`keeps src/runners/${mode} free of account routing`, () => {
+      const dir = path.join(RUNNERS_DIR, mode);
+      if (!fs.existsSync(dir)) {
+        throw new Error(
+          `Runner directory missing: src/runners/${mode} — update RUNNER_MODES if it was renamed.`,
+        );
+      }
+      assertNoViolations(scanDirectory(dir), `src/runners/${mode}`);
+    });
+  });
+
+  it('covers every runner directory, including future modes', () => {
+    assertNoViolations(scanDirectory(RUNNERS_DIR), 'src/runners');
+  });
+
+  it('detects account routing when introduced (guard self-test)', () => {
+    const snippet = [
+      'export function dispatch(account: { tier: string }) {',
+      "  if (account.tier === 'premium') {",
+      '    return fastLane();',
+      '  }',
+      '}',
+    ].join('\n');
+
+    const violations = scanContent(snippet, 'campaign-sender/example.ts');
+
+    expect(violations).toEqual([
+      expect.objectContaining({
+        file: 'campaign-sender/example.ts',
+        line: 2,
+        keyword: 'Account.',
+      }),
+    ]);
+  });
+
+  it('keeps allowed tokens exempt (documented exceptions)', () => {
+    const violations = scanContent(
+      'constructor(private readonly db: TenantDbContext) {}',
+      'campaign-packer/example.ts',
+    );
+
+    expect(violations).toEqual([]);
+  });
+
+  it('does not let an allowed token launder real routing on the same line', () => {
+    const violations = scanContent(
+      'routeByTenant(tenantId); // wrapped by TenantDbContext',
+      'campaign-sender/example.ts',
+    );
+
+    expect(violations).toEqual([
+      expect.objectContaining({ line: 1, keyword: 'tenant' }),
+    ]);
+  });
+
+  it('detects routing hidden by camelCase boundaries (getAccountById)', () => {
+    const violations = scanContent(
+      'return getAccountById(contact.ownerId);',
+      'campaign-sender/example.ts',
+    );
+
+    expect(violations).toEqual([
+      expect.objectContaining({ line: 1, keyword: 'accountById' }),
+    ]);
+  });
+
+  it('keeps the seam NestJS module identifier exempt', () => {
+    const violations = scanContent(
+      'imports: [TenantDbContextModule],',
+      'campaign-packer/example.ts',
+    );
+
+    expect(violations).toEqual([]);
+  });
+
+  it('does not strip the sanctioned token glued into a larger identifier', () => {
+    const violations = scanContent(
+      'tenantDbContextRouter().route(msg);',
+      'event-process/example.ts',
+    );
+
+    expect(violations).toEqual([
+      expect.objectContaining({ line: 1, keyword: 'tenant' }),
+    ]);
+  });
+
+  it('requires every sanctioned token to strip globally with glue-proof anchors', () => {
+    for (const { pattern } of ALLOWED_TOKENS) {
+      expect(pattern.flags).toContain('g');
+      expect(pattern.source.startsWith('(?<![A-Za-z0-9_])')).toBe(true);
+      expect(pattern.source.endsWith('(?![A-Za-z0-9_])')).toBe(true);
+    }
+  });
+
+  describe('neutral-line exemptions (AC3)', () => {
+    const NEUTRAL_LOG_ALLOWLIST = [
+      {
+        pattern: /^logger\.log\(\{ accountId: ingestion\.accountId \}\);$/,
+        reason: 'test fixture: log-only field, no flow impact',
+      },
+    ];
+
+    it('exempts a documented neutral log mention', () => {
+      const violations = scanContent(
+        '  logger.log({ accountId: ingestion.accountId });',
+        'event-process/example.ts',
+        NEUTRAL_LOG_ALLOWLIST,
+      );
+
+      expect(violations).toEqual([]);
+    });
+
+    it('still fails the same mention when it is not allowlisted', () => {
+      const violations = scanContent(
+        '  logger.log({ accountId: ingestion.accountId });',
+        'event-process/example.ts',
+      );
+
+      expect(violations).toEqual([
+        expect.objectContaining({ line: 1, keyword: 'accountId' }),
+      ]);
+    });
+
+    it('does not let an allowlisted line launder routing appended to it', () => {
+      const violations = scanContent(
+        'logger.log({ accountId: ingestion.accountId }); routeByAccount(accountId);',
+        'event-process/example.ts',
+        NEUTRAL_LOG_ALLOWLIST,
+      );
+
+      expect(violations).toEqual([
+        expect.objectContaining({ line: 1, keyword: 'accountId' }),
+      ]);
+    });
+
+    it('requires every entry to be anchored, stateless and wildcard-free', () => {
+      for (const { pattern } of [
+        ...ALLOWED_LINE_PATTERNS,
+        ...NEUTRAL_LOG_ALLOWLIST,
+      ]) {
+        expect(pattern.source.startsWith('^')).toBe(true);
+        expect(pattern.source.endsWith('$')).toBe(true);
+        expect(pattern.flags).not.toContain('g');
+        // `.*` / `.+` / negated classes would exempt arbitrary content on a
+        // matching line — the gutting vector the anchors exist to prevent.
+        expect(pattern.source).not.toMatch(/\.[*+]|\[\^/);
+      }
+    });
+  });
+});