diff --git a/.github/workflows/check-coverage-overlap.yml b/.github/workflows/check-coverage-overlap.yml
new file mode 100644
index 00000000..eb4402bc
--- /dev/null
+++ b/.github/workflows/check-coverage-overlap.yml
@@ -0,0 +1,49 @@
+name: Check Coverage Overlap
+
+on:
+  push:
+    branches: ['main', 'release-prep', 'hotfix-*']
+  pull_request:
+    branches: ['main', 'release-prep', 'hotfix-*']
+
+permissions:
+  contents: read
+
+jobs:
+  check-coverage-overlap:
+    runs-on: ${{ matrix.os }}
+    strategy:
+      matrix:
+        os: [ubuntu-latest]
+    steps:
+      - name: Checkout git repo
+        uses: actions/checkout@v4
+        with:
+          path: extension-repo
+
+      - name: Checkout paranext-core repo to use its sub-packages
+        uses: actions/checkout@v4
+        with:
+          path: paranext-core
+          repository: paranext/paranext-core
+
+      - name: Setup Node.js
+        uses: actions/setup-node@v4
+        with:
+          cache: 'npm'
+          cache-dependency-path: |
+            extension-repo/package-lock.json
+            paranext-core/package-lock.json
+          node-version-file: extension-repo/package.json
+
+      - name: Install extension dependencies
+        working-directory: extension-repo
+        run: npm ci
+
+      - name: Install core dependencies
+        working-directory: paranext-core
+        run: npm ci --ignore-scripts
+
+      - name: Check coverage overlap
+        working-directory: extension-repo
+        run: npm run check:coverage-overlap
diff --git a/lib/check-coverage-overlap.ts b/lib/check-coverage-overlap.ts
new file mode 100644
index 00000000..9fa33259
--- /dev/null
+++ b/lib/check-coverage-overlap.ts
@@ -0,0 +1,331 @@
+/**
+ * Detects coverage overlap redundancies by running each test suite in isolation and reporting which
+ * suites cover functions or branches in source files outside their primary scope.
+ *
+ * Exit code 1 if any suite covers functions or branches in an out-of-scope source file.
+ */
+
+import { spawn } from 'node:child_process';
+import fs from 'node:fs';
+import os from 'node:os';
+import path from 'node:path';
+import { globSync } from 'glob';
+
+const rootDir = path.resolve(__dirname, '..');
+
+/** V8 coverage data for a single source file as written by Jest's coverage-final.json. */
+interface FileCoverage {
+  path: string;
+  /** Istanbul statement hit counts, keyed by statement id. */
+  s: Record<string, number>;
+  /** Istanbul branch hit counts, keyed by branch id; each value is an array of per-path counts. */
+  b: Record<string, number[]>;
+  /** Istanbul function hit counts, keyed by function id. */
+  f: Record<string, number>;
+  statementMap: Record<string, unknown>;
+  branchMap: Record<string, unknown>;
+  fnMap: Record<string, unknown>;
+}
+
+/** Coverage hits for a single out-of-scope file. */
+interface OutOfScopeHit {
+  file: string;
+  fnHit: number;
+  fnTotal: number;
+  branchHit: number;
+  branchTotal: number;
+}
+
+/** Result for one test suite run. */
+interface SuiteResult {
+  testFile: string;
+  primarySourceFile: string;
+  outOfScopeHits: OutOfScopeHit[];
+  elapsedMs: number;
+  error?: string;
+}
+
+/**
+ * Derives the primary source file path from a test file path by stripping the `__tests__/` segment
+ * and the `.test.` infix.
+ *
+ * Assumes a strict 1:1 test-to-source mapping with a mirrored directory structure
+ * (`src/__tests__/foo.test.ts` → `src/foo.ts`). Test files that cover multiple source files, use a
+ * non-standard naming convention, or don't mirror `src/` will derive a nonexistent primary source
+ * file, causing all their coverage to be flagged as out-of-scope.
+ *
+ * @param testFile - Absolute path to a test file.
+ * @returns Absolute path to the expected primary source file.
+ */
+function deriveSourcePath(testFile: string): string {
+  const relative = path.relative(rootDir, testFile);
+  const withoutTests = relative.replace(/src\/__tests__\//, 'src/');
+  const withoutInfix = withoutTests.replace(/\.test\.(tsx?)$/, '.$1');
+  return path.resolve(rootDir, withoutInfix);
+}
+
+/**
+ * Counts the number of entries in a record whose value is greater than zero.
+ *
+ * @param counts - Map of id to hit count.
+ * @returns Number of entries with a non-zero count.
+ */
+function countNonZero(counts: Record<string, number>): number {
+  return Object.values(counts).filter((n) => n > 0).length;
+}
+
+/**
+ * Counts the number of branch slots across all branch entries that were hit at least once.
+ *
+ * @param branches - Map of branch id to array of per-path hit counts.
+ * @returns Number of individual branch slots with a non-zero count.
+ */
+function countBranchHits(branches: Record<string, number[]>): number {
+  return Object.values(branches)
+    .flat()
+    .filter((n) => n > 0).length;
+}
+
+/**
+ * Returns the total number of branch slots across all branch entries.
+ *
+ * @param branches - Map of branch id to array of per-path hit counts.
+ * @returns Total number of individual branch slots.
+ */
+function countBranchTotal(branches: Record<string, number[]>): number {
+  return Object.values(branches).flat().length;
+}
+
+/**
+ * Formats a hit/total pair as a percentage string.
+ *
+ * @param hit - Number of covered items.
+ * @param total - Total number of items.
+ * @returns Percentage string, e.g. "50%", or "n/a" when total is zero.
+ */
+function pct(hit: number, total: number): string {
+  if (total === 0) return 'n/a';
+  return `${Math.round((hit / total) * 100)}%`;
+}
+
+/**
+ * Escapes special regex characters in a string for use in a Jest --testPathPattern argument.
+ *
+ * @param s - Raw string to escape.
+ * @returns Regex-safe escaped string.
+ */
+function escapeRegExp(s: string): string {
+  return s.replace(/[.*+?^${}()|[\]\\]/g, String.raw`\$&`);
+}
+
+/**
+ * Runs a single Jest test file with coverage enabled, writing results to `coverageDir`.
+ *
+ * @param testFile - Absolute path to the test file to run.
+ * @param coverageDir - Directory where Jest should write coverage output.
+ * @returns A promise that resolves when Jest exits successfully.
+ * @throws If Jest exits with a non-zero status.
+ */
+function runJest(testFile: string, coverageDir: string): Promise<void> {
+  return new Promise((resolve, reject) => {
+    const relativeTestFile = path.relative(rootDir, testFile);
+    const proc = spawn(
+      'node',
+      [
+        'node_modules/.bin/jest',
+        '--config',
+        'jest.config.ts',
+        '--coverage',
+        '--coverageDirectory',
+        coverageDir,
+        '--testPathPatterns',
+        `${escapeRegExp(relativeTestFile)}$`,
+        '--coverageThreshold',
+        '{}',
+        '--forceExit',
+        '--silent',
+      ],
+      { cwd: rootDir },
+    );
+
+    const chunks: Buffer[] = [];
+    proc.stdout.on('data', (chunk: Buffer) => chunks.push(chunk));
+    proc.stderr.on('data', (chunk: Buffer) => chunks.push(chunk));
+
+    proc.on('error', (err) => {
+      reject(new Error(`Failed to spawn Jest: ${err.message}`));
+    });
+
+    proc.on('close', (code) => {
+      if (code === 0) {
+        resolve();
+      } else {
+        reject(new Error(`Jest failed:\n${Buffer.concat(chunks).toString('utf8').trim()}`));
+      }
+    });
+  });
+}
+
+/**
+ * Analyses out-of-scope coverage for a single test file by running it in isolation and comparing
+ * covered source files against the expected primary source file.
+ *
+ * @param testFile - Absolute path to the test file.
+ * @returns A promise resolving to a SuiteResult describing any out-of-scope coverage hits.
+ */
+async function analyzeSuite(testFile: string): Promise<SuiteResult> {
+  const primarySourceFile = deriveSourcePath(testFile);
+  const tmpDir = fs.mkdtempSync(path.join(os.tmpdir(), 'coverage-overlap-'));
+  const start = Date.now();
+
+  try {
+    await runJest(testFile, tmpDir);
+
+    const coverageJson = path.join(tmpDir, 'coverage-final.json');
+    if (fs.existsSync(coverageJson)) {
+      const coverageData: Record<string, FileCoverage> = JSON.parse(
+        fs.readFileSync(coverageJson, 'utf8'),
+      );
+
+      const outOfScopeHits: OutOfScopeHit[] = Object.entries(coverageData)
+        .filter(([filePath]) => filePath !== primarySourceFile)
+        .flatMap(([filePath, data]) => {
+          const fnHit = countNonZero(data.f);
+          const fnTotal = Object.keys(data.fnMap).length;
+          const branchHit = countBranchHits(data.b);
+          const branchTotal = countBranchTotal(data.b);
+          if (fnHit === 0 && branchHit === 0) return [];
+          return [{ file: filePath, fnHit, fnTotal, branchHit, branchTotal }];
+        });
+
+      return { testFile, primarySourceFile, outOfScopeHits, elapsedMs: Date.now() - start };
+    }
+
+    return {
+      testFile,
+      primarySourceFile,
+      outOfScopeHits: [],
+      elapsedMs: Date.now() - start,
+      error: 'coverage-final.json not found after jest run',
+    };
+  } catch (err) {
+    return {
+      testFile,
+      primarySourceFile,
+      outOfScopeHits: [],
+      elapsedMs: Date.now() - start,
+      error: err instanceof Error ? err.message : String(err),
+    };
+  } finally {
+    fs.rmSync(tmpDir, { recursive: true, force: true });
+  }
+}
+
+/**
+ * Prints a human-readable report of coverage overlap results and returns whether any ERROR-level
+ * violations were found.
+ *
+ * @param results - Array of suite results to report.
+ * @returns True if any suite has functions or branches covered in an out-of-scope file.
+ */
+function printReport(results: SuiteResult[]): boolean {
+  console.log('\n=== Coverage Overlap Report ===\n');
+
+  const hasViolations = results.reduce(
+    (acc, { testFile, primarySourceFile, outOfScopeHits, elapsedMs, error }) => {
+      const label = path.relative(rootDir, testFile);
+      const primary = path.relative(rootDir, primarySourceFile);
+      const time = `${(elapsedMs / 1000).toFixed(1)}s`;
+
+      if (error) {
+        console.log(`  [ERR]  ${label}  (${time})`);
+        console.log(`         Error: ${error}`);
+        return true;
+      }
+
+      if (outOfScopeHits.length === 0) {
+        console.log(`  [OK]   ${label}  (${time})`);
+        return acc;
+      }
+
+      console.log(`  [!!]   ${label}  →  primary: ${primary}  (${time})`);
+      outOfScopeHits.forEach((hit) => {
+        const file = path.relative(rootDir, hit.file);
+        console.log(`         ERROR  ${file}`);
+        console.log(
+          `                functions ${pct(hit.fnHit, hit.fnTotal)}  ` +
+            `branches ${pct(hit.branchHit, hit.branchTotal)}`,
+        );
+      });
+
+      return true;
+    },
+    false,
+  );
+
+  console.log('');
+  if (hasViolations) {
+    console.log('FAIL: some test suites cover functions or branches in out-of-scope files.\n');
+  } else {
+    console.log('PASS: no out-of-scope function or branch coverage detected.\n');
+  }
+
+  return hasViolations;
+}
+
+/**
+ * Runs `analyzeSuite` over `testFiles` with at most `concurrency` suites active at a time.
+ *
+ * @param testFiles - Absolute paths to test files.
+ * @param concurrency - Maximum number of Jest processes to run simultaneously.
+ * @returns Array of SuiteResults in the same order as `testFiles`.
+ */
+async function runWithConcurrencyLimit(
+  testFiles: string[],
+  concurrency: number,
+): Promise<SuiteResult[]> {
+  const results: SuiteResult[] = new Array(testFiles.length);
+  let next = 0;
+
+  /** Pulls test files from the shared queue and runs them sequentially until the queue is exhausted. */
+  async function worker(): Promise<void> {
+    while (next < testFiles.length) {
+      const index = next;
+      next += 1;
+      const testFile = testFiles[index];
+      // eslint-disable-next-line no-await-in-loop
+      const result = await analyzeSuite(testFile);
+      results[index] = result;
+      console.log(
+        `  done (${(result.elapsedMs / 1000).toFixed(1)}s)  ${path.relative(rootDir, testFile)}`,
+      );
+    }
+  }
+
+  await Promise.all(Array.from({ length: Math.min(concurrency, testFiles.length) }, worker));
+  return results;
+}
+
+/**
+ * Discovers all test files, runs each in isolation with bounded concurrency, and reports coverage
+ * overlap.
+ */
+(async function main() {
+  const testFiles = globSync('src/**/__tests__/**/*.test.{ts,tsx}', {
+    cwd: rootDir,
+    absolute: true,
+  });
+
+  if (testFiles.length === 0) {
+    console.error('No test files found.');
+    process.exit(1);
+  }
+
+  console.log(`Analyzing coverage overlap for ${testFiles.length} test suite(s)...`);
+
+  const concurrency = Math.max(1, os.cpus().length);
+  const results = await runWithConcurrencyLimit(testFiles, concurrency);
+
+  const hasViolations = printReport(results);
+  process.exit(hasViolations ? 1 : 0);
+})();
diff --git a/package.json b/package.json
index 48153936..54035533 100644
--- a/package.json
+++ b/package.json
@@ -27,6 +27,7 @@
     "lint-fix": "npm run lint-fix:scripts && npm run lint:styles -- --fix",
     "lint-fix:scripts": "npm run format && npm run lint:scripts",
     "bump-versions": "ts-node ./lib/bump-versions.ts",
+    "check:coverage-overlap": "ts-node ./lib/check-coverage-overlap.ts",
     "test": "jest",
     "test:coverage": "jest --coverage",
     "core:start": "npm --prefix ../paranext-core start",