diff --git a/test/behavior-checker.test.ts b/test/behavior-checker.test.ts
new file mode 100644
index 0000000..a1f961e
--- /dev/null
+++ b/test/behavior-checker.test.ts
@@ -0,0 +1,175 @@
+import { describe, test, expect } from 'bun:test';
+import { checkBehaviors, loadBehaviorSpec, listBehaviorSpecs } from './helpers/behavior-checker';
+import type { BehaviorSpec } from './helpers/behavior-checker';
+
+describe('behavior-checker', () => {
+  test('listBehaviorSpecs returns all spec files', () => {
+    const specs = listBehaviorSpecs();
+    expect(specs).toContain('review');
+    expect(specs).toContain('retro');
+    expect(specs).toContain('qa');
+    expect(specs).toContain('ship');
+    expect(specs).toContain('plan-ceo-review');
+    expect(specs).toContain('plan-eng-review');
+    expect(specs.length).toBe(6);
+  });
+
+  test('loadBehaviorSpec returns null for nonexistent skill', () => {
+    expect(loadBehaviorSpec('nonexistent-skill')).toBeNull();
+  });
+
+  test('loadBehaviorSpec loads valid spec', () => {
+    const spec = loadBehaviorSpec('review');
+    expect(spec).not.toBeNull();
+    expect(spec!.skill).toBe('review');
+    expect(spec!.assertions.length).toBeGreaterThan(0);
+  });
+
+  test('pattern_exists passes when pattern is found', () => {
+    const spec: BehaviorSpec = {
+      skill: 'test',
+      assertions: [
+        { type: 'pattern_exists', pattern: 'hello', description: 'find hello' },
+      ],
+    };
+    const result = checkBehaviors('hello world', spec);
+    expect(result.passed).toBe(true);
+    expect(result.results[0].passed).toBe(true);
+  });
+
+  test('pattern_exists fails when pattern is missing', () => {
+    const spec: BehaviorSpec = {
+      skill: 'test',
+      assertions: [
+        { type: 'pattern_exists', pattern: 'goodbye', description: 'find goodbye' },
+      ],
+    };
+    const result = checkBehaviors('hello world', spec);
+    expect(result.passed).toBe(false);
+    expect(result.results[0].detail).toContain('not found');
+  });
+
+  test('pattern_exists with regex', () => {
+    const spec: BehaviorSpec = {
+      skill: 'test',
+      assertions: [
+        { type: 'pattern_exists', pattern: '\\d+ commits', regex: true, description: 'metric' },
+      ],
+    };
+    expect(checkBehaviors('Found 42 commits this week', spec).passed).toBe(true);
+    expect(checkBehaviors('No metrics here', spec).passed).toBe(false);
+  });
+
+  test('pattern_exists with case_insensitive', () => {
+    const spec: BehaviorSpec = {
+      skill: 'test',
+      assertions: [
+        { type: 'pattern_exists', pattern: 'CRITICAL', regex: true, case_insensitive: true, description: 'crit' },
+      ],
+    };
+    expect(checkBehaviors('critical finding: SQL injection', spec).passed).toBe(true);
+    expect(checkBehaviors('CRITICAL: race condition', spec).passed).toBe(true);
+  });
+
+  test('pattern_absent passes when pattern is not found', () => {
+    const spec: BehaviorSpec = {
+      skill: 'test',
+      assertions: [
+        { type: 'pattern_absent', pattern: 'variable name', regex: true, case_insensitive: true, description: 'no nitpick' },
+      ],
+    };
+    expect(checkBehaviors('SQL injection in user controller', spec).passed).toBe(true);
+  });
+
+  test('pattern_absent fails when pattern is found', () => {
+    const spec: BehaviorSpec = {
+      skill: 'test',
+      assertions: [
+        { type: 'pattern_absent', pattern: 'variable name', regex: true, case_insensitive: true, description: 'no nitpick' },
+      ],
+    };
+    const result = checkBehaviors('Consider renaming this variable name to something clearer', spec);
+    expect(result.passed).toBe(false);
+    expect(result.results[0].detail).toContain('Unwanted');
+  });
+
+  test('min_sections counts headings correctly', () => {
+    const spec: BehaviorSpec = {
+      skill: 'test',
+      assertions: [
+        { type: 'min_sections', heading_level: 2, min_count: 3, description: 'enough sections' },
+      ],
+    };
+    const output = `# Title
+## Section One
+content
+## Section Two
+content
+## Section Three
+content`;
+    expect(checkBehaviors(output, spec).passed).toBe(true);
+  });
+
+  test('min_sections fails when not enough headings', () => {
+    const spec: BehaviorSpec = {
+      skill: 'test',
+      assertions: [
+        { type: 'min_sections', heading_level: 2, min_count: 3, description: 'enough sections' },
+      ],
+    };
+    const result = checkBehaviors('## Only One\ncontent', spec);
+    expect(result.passed).toBe(false);
+    expect(result.results[0].detail).toContain('Found 1');
+  });
+
+  test('all spec files parse correctly', () => {
+    const specs = listBehaviorSpecs();
+    for (const name of specs) {
+      const spec = loadBehaviorSpec(name);
+      expect(spec).not.toBeNull();
+      expect(spec!.skill).toBe(name);
+      expect(spec!.assertions.length).toBeGreaterThan(0);
+      for (const a of spec!.assertions) {
+        expect(a.type).toBeDefined();
+        expect(a.description).toBeDefined();
+      }
+    }
+  });
+
+  test('review spec passes on realistic output', () => {
+    const spec = loadBehaviorSpec('review')!;
+    const output = `# Pre-Landing Review
+
+## CRITICAL Findings
+
+1. SQL injection in user_controller.rb line 15
+
+## INFORMATIONAL Findings
+
+1. Missing index on users.email column
+2. Consider adding rate limiting to login endpoint`;
+    const result = checkBehaviors(output, spec);
+    expect(result.passed).toBe(true);
+  });
+
+  test('retro spec passes on realistic output', () => {
+    const spec = loadBehaviorSpec('retro')!;
+    const output = `# Weekly Retrospective
+
+## Summary
+
+15 commits by 3 contributors this week.
+
+## Team Contributions
+
+### Alice
+- 8 commits, 2 PRs merged
+- Strong work on the auth module
+
+### Bob
+- 5 commits focused on testing
+- 1 PR merged`;
+    const result = checkBehaviors(output, spec);
+    expect(result.passed).toBe(true);
+  });
+});
diff --git a/test/fixtures/behaviors/plan-ceo-review.json b/test/fixtures/behaviors/plan-ceo-review.json
new file mode 100644
index 0000000..b6ac56b
--- /dev/null
+++ b/test/fixtures/behaviors/plan-ceo-review.json
@@ -0,0 +1,30 @@
+{
+  "skill": "plan-ceo-review",
+  "assertions": [
+    {
+      "type": "section_exists",
+      "pattern": "NOT in scope|not in scope|Out of scope|out of scope",
+      "regex": true,
+      "description": "Output must define scope boundaries"
+    },
+    {
+      "type": "pattern_exists",
+      "pattern": "SCOPE EXPANSION|HOLD SCOPE|SCOPE REDUCTION",
+      "regex": true,
+      "description": "Output must reference the selected scope mode"
+    },
+    {
+      "type": "min_sections",
+      "heading_level": 2,
+      "min_count": 3,
+      "description": "Output must have at least 3 major sections"
+    },
+    {
+      "type": "pattern_exists",
+      "pattern": "edge case|failure mode|error|rollback",
+      "regex": true,
+      "case_insensitive": true,
+      "description": "Output must address failure modes or edge cases"
+    }
+  ]
+}
diff --git a/test/fixtures/behaviors/plan-eng-review.json b/test/fixtures/behaviors/plan-eng-review.json
new file mode 100644
index 0000000..aa41a56
--- /dev/null
+++ b/test/fixtures/behaviors/plan-eng-review.json
@@ -0,0 +1,31 @@
+{
+  "skill": "plan-eng-review",
+  "assertions": [
+    {
+      "type": "section_exists",
+      "pattern": "Step 0|Scope Challenge|scope",
+      "regex": true,
+      "case_insensitive": true,
+      "description": "Output must include Step 0 scope challenge"
+    },
+    {
+      "type": "pattern_exists",
+      "pattern": "test|Test|testing|coverage",
+      "regex": true,
+      "description": "Output must address test coverage"
+    },
+    {
+      "type": "pattern_exists",
+      "pattern": "diagram|ascii|flow|\\+--",
+      "regex": true,
+      "case_insensitive": true,
+      "description": "Output should include diagrams or diagram references"
+    },
+    {
+      "type": "min_sections",
+      "heading_level": 2,
+      "min_count": 2,
+      "description": "Output must have at least 2 major sections"
+    }
+  ]
+}
diff --git a/test/fixtures/behaviors/qa.json b/test/fixtures/behaviors/qa.json
new file mode 100644
index 0000000..c43d69d
--- /dev/null
+++ b/test/fixtures/behaviors/qa.json
@@ -0,0 +1,26 @@
+{
+  "skill": "qa",
+  "assertions": [
+    {
+      "type": "pattern_exists",
+      "pattern": "bug|issue|finding|error|defect",
+      "regex": true,
+      "case_insensitive": true,
+      "description": "Output must report bugs, issues, or findings"
+    },
+    {
+      "type": "pattern_exists",
+      "pattern": "severity|critical|high|medium|low",
+      "regex": true,
+      "case_insensitive": true,
+      "description": "Output must classify findings by severity"
+    },
+    {
+      "type": "pattern_exists",
+      "pattern": "screenshot|evidence|console|observed",
+      "regex": true,
+      "case_insensitive": true,
+      "description": "Output must include evidence for findings"
+    }
+  ]
+}
diff --git a/test/fixtures/behaviors/retro.json b/test/fixtures/behaviors/retro.json
new file mode 100644
index 0000000..a5bc621
--- /dev/null
+++ b/test/fixtures/behaviors/retro.json
@@ -0,0 +1,25 @@
+{
+  "skill": "retro",
+  "assertions": [
+    {
+      "type": "pattern_exists",
+      "pattern": "\\d+\\s*(commit|PR|merge|push)",
+      "regex": true,
+      "case_insensitive": true,
+      "description": "Output must contain quantitative commit or PR metrics"
+    },
+    {
+      "type": "pattern_exists",
+      "pattern": "contributor|author|team|person",
+      "regex": true,
+      "case_insensitive": true,
+      "description": "Output must reference contributors or team members"
+    },
+    {
+      "type": "min_sections",
+      "heading_level": 2,
+      "min_count": 2,
+      "description": "Output must have at least 2 major sections"
+    }
+  ]
+}
diff --git a/test/fixtures/behaviors/review.json b/test/fixtures/behaviors/review.json
new file mode 100644
index 0000000..834452b
--- /dev/null
+++ b/test/fixtures/behaviors/review.json
@@ -0,0 +1,24 @@
+{
+  "skill": "review",
+  "assertions": [
+    {
+      "type": "pattern_exists",
+      "pattern": "CRITICAL|Critical|critical",
+      "regex": true,
+      "description": "Output must contain critical findings or explicitly state none found"
+    },
+    {
+      "type": "pattern_exists",
+      "pattern": "INFORMATIONAL|Informational|informational|info",
+      "regex": true,
+      "description": "Output must contain informational findings section"
+    },
+    {
+      "type": "pattern_absent",
+      "pattern": "variable name|naming convention|import order|whitespace style",
+      "regex": true,
+      "case_insensitive": true,
+      "description": "Output should not nitpick style issues"
+    }
+  ]
+}
diff --git a/test/fixtures/behaviors/ship.json b/test/fixtures/behaviors/ship.json
new file mode 100644
index 0000000..8caca1f
--- /dev/null
+++ b/test/fixtures/behaviors/ship.json
@@ -0,0 +1,19 @@
+{
+  "skill": "ship",
+  "assertions": [
+    {
+      "type": "pattern_exists",
+      "pattern": "PR|pull request|push|branch",
+      "regex": true,
+      "case_insensitive": true,
+      "description": "Output must reference PR creation or branch push"
+    },
+    {
+      "type": "pattern_exists",
+      "pattern": "test|passing|passed|green|CI",
+      "regex": true,
+      "case_insensitive": true,
+      "description": "Output must reference test results"
+    }
+  ]
+}
diff --git a/test/helpers/behavior-checker.ts b/test/helpers/behavior-checker.ts
new file mode 100644
index 0000000..311dfd7
--- /dev/null
+++ b/test/helpers/behavior-checker.ts
@@ -0,0 +1,161 @@
+/**
+ * Behavior assertion runner for skill E2E tests.
+ *
+ * Loads JSON behavior specs from test/fixtures/behaviors/ and checks
+ * that skill output contains (or does not contain) expected patterns.
+ * Deterministic, free (no API cost), and fast.
+ *
+ * Used by test/skill-e2e.test.ts after E2E tests capture output.
+ */
+
+import * as fs from 'fs';
+import * as path from 'path';
+
+export interface Assertion {
+  type: 'section_exists' | 'pattern_exists' | 'pattern_absent' | 'min_sections';
+  pattern?: string;
+  regex?: boolean;
+  case_insensitive?: boolean;
+  heading_level?: number;
+  min_count?: number;
+  description: string;
+}
+
+export interface BehaviorSpec {
+  skill: string;
+  assertions: Assertion[];
+}
+
+export interface AssertionResult {
+  assertion: Assertion;
+  passed: boolean;
+  detail?: string;
+}
+
+export interface BehaviorCheckResult {
+  passed: boolean;
+  results: AssertionResult[];
+}
+
+const BEHAVIORS_DIR = path.join(__dirname, '..', 'fixtures', 'behaviors');
+
+/**
+ * Load a behavior spec for a skill by name.
+ * Returns null if no spec file exists.
+ */
+export function loadBehaviorSpec(skillName: string): BehaviorSpec | null {
+  const specPath = path.join(BEHAVIORS_DIR, `${skillName}.json`);
+  if (!fs.existsSync(specPath)) return null;
+  return JSON.parse(fs.readFileSync(specPath, 'utf-8'));
+}
+
+/**
+ * List all available behavior specs.
+ */
+export function listBehaviorSpecs(): string[] {
+  if (!fs.existsSync(BEHAVIORS_DIR)) return [];
+  return fs.readdirSync(BEHAVIORS_DIR)
+    .filter(f => f.endsWith('.json'))
+    .map(f => f.replace('.json', ''));
+}
+
+/**
+ * Run all assertions from a behavior spec against the given output text.
+ */
+export function checkBehaviors(output: string, spec: BehaviorSpec): BehaviorCheckResult {
+  const results: AssertionResult[] = [];
+
+  for (const assertion of spec.assertions) {
+    const result = runAssertion(output, assertion);
+    results.push(result);
+  }
+
+  return {
+    passed: results.every(r => r.passed),
+    results,
+  };
+}
+
+function runAssertion(output: string, assertion: Assertion): AssertionResult {
+  switch (assertion.type) {
+    case 'section_exists':
+    case 'pattern_exists':
+      return checkPatternExists(output, assertion);
+    case 'pattern_absent':
+      return checkPatternAbsent(output, assertion);
+    case 'min_sections':
+      return checkMinSections(output, assertion);
+    default:
+      return { assertion, passed: false, detail: `Unknown assertion type: ${assertion.type}` };
+  }
+}
+
+function checkPatternExists(output: string, assertion: Assertion): AssertionResult {
+  if (!assertion.pattern) {
+    return { assertion, passed: false, detail: 'No pattern specified' };
+  }
+
+  const flags = assertion.case_insensitive ? 'i' : '';
+
+  if (assertion.regex) {
+    const re = new RegExp(assertion.pattern, flags);
+    const match = re.test(output);
+    return {
+      assertion,
+      passed: match,
+      detail: match ? undefined : `Pattern /${assertion.pattern}/${flags} not found in output`,
+    };
+  }
+
+  const haystack = assertion.case_insensitive ? output.toLowerCase() : output;
+  const needle = assertion.case_insensitive ? assertion.pattern.toLowerCase() : assertion.pattern;
+  const found = haystack.includes(needle);
+  return {
+    assertion,
+    passed: found,
+    detail: found ? undefined : `"${assertion.pattern}" not found in output`,
+  };
+}
+
+function checkPatternAbsent(output: string, assertion: Assertion): AssertionResult {
+  if (!assertion.pattern) {
+    return { assertion, passed: false, detail: 'No pattern specified' };
+  }
+
+  const flags = assertion.case_insensitive ? 'i' : '';
+
+  if (assertion.regex) {
+    const re = new RegExp(assertion.pattern, flags);
+    const match = re.test(output);
+    return {
+      assertion,
+      passed: !match,
+      detail: match ? `Unwanted pattern /${assertion.pattern}/${flags} found in output` : undefined,
+    };
+  }
+
+  const haystack = assertion.case_insensitive ? output.toLowerCase() : output;
+  const needle = assertion.case_insensitive ? assertion.pattern.toLowerCase() : assertion.pattern;
+  const found = haystack.includes(needle);
+  return {
+    assertion,
+    passed: !found,
+    detail: found ? `Unwanted pattern "${assertion.pattern}" found in output` : undefined,
+  };
+}
+
+function checkMinSections(output: string, assertion: Assertion): AssertionResult {
+  const level = assertion.heading_level ?? 2;
+  const minCount = assertion.min_count ?? 1;
+  const prefix = '#'.repeat(level) + ' ';
+
+  const headingCount = output.split('\n').filter(line => line.startsWith(prefix)).length;
+
+  return {
+    assertion,
+    passed: headingCount >= minCount,
+    detail: headingCount >= minCount
+      ? undefined
+      : `Found ${headingCount} h${level} headings, expected at least ${minCount}`,
+  };
+}