diff --git a/README.md b/README.md
index 3f3bfa2..91c1a7c 100644
--- a/README.md
+++ b/README.md
@@ -123,6 +123,11 @@ TypeGap — Type Coverage Report
 | `--min-coverage <n>` | Exit non-zero if coverage drops below `n`% |
 | `--pattern <pattern>` | Custom glob pattern for target files |
 
+Baselines are versioned JSON documents. TypeGap currently accepts version `1`, including the
+numeric project totals and an array of file entries with a non-empty `file` path plus numeric
+`total`, `annotated`, and `coverage` fields. Malformed JSON, unsupported versions, and invalid
+fields are rejected with a concise error; `--format json` returns that error as JSON.
+
 ## CI Integration
 
 CI should also run from a source checkout until the package is published:
diff --git a/src/cli.test.ts b/src/cli.test.ts
index d40b812..028e0ed 100644
--- a/src/cli.test.ts
+++ b/src/cli.test.ts
@@ -1,6 +1,6 @@
 import { describe, it, expect } from 'vitest';
-import { execSync } from 'node:child_process';
-import { readFileSync } from 'node:fs';
+import { execSync, spawnSync } from 'node:child_process';
+import { readFileSync, rmSync, writeFileSync } from 'node:fs';
 
 describe('CLI integration', () => {
   const cli = 'npx tsx src/cli.ts';
@@ -73,4 +73,24 @@ describe('CLI integration', () => {
       execSync(`${cli} fixtures/fully-typed --compare fixtures/no-baseline.json`, { encoding: 'utf-8' });
     }).toThrow();
   });
+
+  it.each(['text', 'json'])('reports invalid baselines without a stack trace in %s mode', (format) => {
+    const baseline = 'fixtures/invalid-baseline-test.json';
+    writeFileSync(baseline, '{not json\n');
+    try {
+      const result = spawnSync('npx', ['tsx', 'src/cli.ts', 'fixtures/fully-typed', '--compare', baseline, '--format', format], {
+        encoding: 'utf8',
+      });
+      expect(result.status).toBe(1);
+      const output = `${result.stdout}${result.stderr}`;
+      expect(output).toContain('Invalid baseline: malformed JSON');
+      expect(output).not.toContain('SyntaxError');
+      expect(output).not.toContain('at loadBaseline');
+      if (format === 'json') {
+        expect(JSON.parse(result.stdout)).toEqual({ error: 'Invalid baseline: malformed JSON' });
+      }
+    } finally {
+      rmSync(baseline, { force: true });
+    }
+  });
 });
diff --git a/src/cli.ts b/src/cli.ts
index fdfc9f4..114e110 100644
--- a/src/cli.ts
+++ b/src/cli.ts
@@ -63,14 +63,24 @@ program
       pattern: opts.pattern as string | undefined,
     });
 
-    const { output, exitCode } = generateReport(result, {
-      format,
-      detail: (opts.detail as boolean) ?? false,
-      saveBaseline: opts.baseline as string | undefined,
-      compareBaseline: opts.compare ? resolve(opts.compare as string) : undefined,
-      minCoverage,
-      cwd: process.cwd(),
-    });
+    let report;
+    try {
+      report = generateReport(result, {
+        format,
+        detail: (opts.detail as boolean) ?? false,
+        saveBaseline: opts.baseline as string | undefined,
+        compareBaseline: opts.compare ? resolve(opts.compare as string) : undefined,
+        minCoverage,
+        cwd: process.cwd(),
+      });
+    } catch (error) {
+      const message = error instanceof Error ? error.message : 'Unable to load baseline';
+      if (format === 'json') console.log(JSON.stringify({ error: message }));
+      else console.error(`Error: ${message}`);
+      process.exit(1);
+    }
+
+    const { output, exitCode } = report;
 
     console.log(output);
 
diff --git a/src/reporter.test.ts b/src/reporter.test.ts
index c54bd9b..98c826e 100644
--- a/src/reporter.test.ts
+++ b/src/reporter.test.ts
@@ -3,7 +3,7 @@ import { generateReport, saveBaseline, loadBaseline, compareWithBaseline, getBas
 import { buildProjectResult } from './analyzer.js';
 import { AnnotationStatus } from './types.js';
 import type { FileResult } from './types.js';
-import { rmSync, existsSync } from 'node:fs';
+import { rmSync, existsSync, readFileSync, writeFileSync } from 'node:fs';
 
 const BASELINE_FILE = 'fixtures/baseline-test.json';
 
@@ -164,6 +164,23 @@ describe('saveBaseline / loadBaseline', () => {
     expect(() => loadBaseline('nonexistent.json')).toThrow();
   });
 
+  it('rejects malformed and unsupported baselines with stable errors', () => {
+    writeFileSync(BASELINE_FILE, '{not json\n');
+    expect(() => loadBaseline(BASELINE_FILE)).toThrow('Invalid baseline: malformed JSON');
+
+    writeFileSync(BASELINE_FILE, JSON.stringify({ version: 2 }));
+    expect(() => loadBaseline(BASELINE_FILE)).toThrow('Invalid baseline: unsupported version 2; expected version 1');
+  });
+
+  it('rejects invalid version-1 baseline fields', () => {
+    saveBaseline(makeResult(), BASELINE_FILE);
+    const baseline = JSON.parse(readFileSync(BASELINE_FILE, 'utf8'));
+    baseline.files[0].coverage = '75';
+    writeFileSync(BASELINE_FILE, JSON.stringify(baseline));
+
+    expect(() => loadBaseline(BASELINE_FILE)).toThrow('Invalid baseline: files[0].coverage must be a finite number');
+  });
+
   it('includes timestamp in baseline', () => {
     const result = makeResult();
     saveBaseline(result, BASELINE_FILE);
diff --git a/src/reporter.ts b/src/reporter.ts
index 0a05cdd..f838e16 100644
--- a/src/reporter.ts
+++ b/src/reporter.ts
@@ -75,7 +75,52 @@ export function loadBaseline(filePath: string): Baseline {
   if (!existsSync(filePath)) {
     throw new Error(`Baseline file not found: ${filePath}`);
   }
-  return JSON.parse(readFileSync(filePath, 'utf-8')) as Baseline;
+  let value: unknown;
+  try {
+    value = JSON.parse(readFileSync(filePath, 'utf-8'));
+  } catch {
+    throw new Error('Invalid baseline: malformed JSON');
+  }
+  return validateBaseline(value);
+}
+
+function validateBaseline(value: unknown): Baseline {
+  if (!isRecord(value)) throw new Error('Invalid baseline: expected a JSON object');
+  if (value.version !== 1) {
+    const version = value.version === undefined ? 'missing' : JSON.stringify(value.version);
+    throw new Error(`Invalid baseline: unsupported version ${version}; expected version 1`);
+  }
+
+  for (const field of ['total', 'annotated', 'coverage', 'anyCount', 'unknownCount', 'implicitCount'] as const) {
+    requireFiniteNumber(value, field, field);
+  }
+  if (typeof value.timestamp !== 'string' || !Number.isFinite(Date.parse(value.timestamp))) {
+    throw new Error('Invalid baseline: timestamp must be a valid date string');
+  }
+  if (!Array.isArray(value.files)) throw new Error('Invalid baseline: files must be an array');
+
+  value.files.forEach((file, index) => {
+    const prefix = `files[${index}]`;
+    if (!isRecord(file)) throw new Error(`Invalid baseline: ${prefix} must be an object`);
+    if (typeof file.file !== 'string' || file.file.length === 0) {
+      throw new Error(`Invalid baseline: ${prefix}.file must be a non-empty string`);
+    }
+    for (const field of ['total', 'annotated', 'coverage'] as const) {
+      requireFiniteNumber(file, field, `${prefix}.${field}`);
+    }
+  });
+
+  return value as unknown as Baseline;
+}
+
+function isRecord(value: unknown): value is Record<string, unknown> {
+  return typeof value === 'object' && value !== null && !Array.isArray(value);
+}
+
+function requireFiniteNumber(value: Record<string, unknown>, field: string, label: string): void {
+  if (typeof value[field] !== 'number' || !Number.isFinite(value[field])) {
+    throw new Error(`Invalid baseline: ${label} must be a finite number`);
+  }
 }
 
 /** Compare current result against saved baseline */