diff --git a/.changeset/steady-jays-sing.md b/.changeset/steady-jays-sing.md
new file mode 100644
index 000000000..e6796101a
--- /dev/null
+++ b/.changeset/steady-jays-sing.md
@@ -0,0 +1,5 @@
+---
+type: Security
+pr: 572
+---
+Hardened resolveModelPolicy against prototype pollution: Object.hasOwn guards now block \_\_proto\_\_ / constructor keys in user-supplied provider/budget/runtime_tiers from reaching inherited prototype slots. Also fixed resolveModelForTier to check model_policy before dynamic_routing so provider presets are not silently bypassed when dynamic routing is enabled.
diff --git a/get-shit-done/bin/lib/core.cjs b/get-shit-done/bin/lib/core.cjs
index 49bc5781c..a60af82ac 100644
--- a/get-shit-done/bin/lib/core.cjs
+++ b/get-shit-done/bin/lib/core.cjs
@@ -1569,13 +1569,15 @@ function resolveModelPolicy(policy, tier) {
   // merging config.runtime into the policy object (see step 2.5 in resolveModelInternal).
   const runtime = policy.runtime;
   const rtOverrides = policy.runtime_tiers;
-  if (runtime && rtOverrides && typeof rtOverrides === 'object') {
-    const runtimeEntry = rtOverrides[runtime];
-    if (runtimeEntry && typeof runtimeEntry === 'object') {
-      const raw = runtimeEntry[tier];
-      if (raw != null) {
-        const entry = typeof raw === 'string' ? { model: raw } : raw;
-        if (entry && entry.model) return entry.model;
+  if (runtime && typeof runtime === 'string' && rtOverrides && typeof rtOverrides === 'object') {
+    if (Object.hasOwn(rtOverrides, runtime)) {
+      const runtimeEntry = rtOverrides[runtime];
+      if (runtimeEntry && typeof runtimeEntry === 'object' && Object.hasOwn(runtimeEntry, tier)) {
+        const raw = runtimeEntry[tier];
+        if (raw != null) {
+          const entry = typeof raw === 'string' ? { model: raw } : raw;
+          if (entry && entry.model) return entry.model;
+        }
       }
     }
   }
@@ -1595,14 +1597,19 @@ function resolveModelPolicy(policy, tier) {
     return (v && typeof v === 'string') ? v : null;
   }
 
+  // Object.hasOwn guards prevent __proto__ / constructor key escalation from
+  // user-controlled policy.provider / policy.budget reaching inherited slots.
+  if (!Object.hasOwn(PROVIDER_PRESETS, provider)) return null;
   const presetForProvider = PROVIDER_PRESETS[provider];
-  if (!presetForProvider) return null; // Unknown provider — fall through silently.
+  if (!presetForProvider || typeof presetForProvider !== 'object') return null;
 
+  if (!Object.hasOwn(presetForProvider, tier)) return null;
   const tierPresets = presetForProvider[tier];
-  if (!tierPresets) return null; // Tier not in preset (partial preset catalog entry).
+  if (!tierPresets || typeof tierPresets !== 'object') return null;
 
   // Budget defaults to 'medium' — mirrors the existing 'balanced' default bias.
   const budget = (policy.budget && typeof policy.budget === 'string') ? policy.budget : 'medium';
+  if (!Object.hasOwn(tierPresets, budget)) return null;
   const budgetEntry = tierPresets[budget];
   if (!budgetEntry || !budgetEntry.model) return null; // Missing or null budget slot.
 
@@ -1748,6 +1755,14 @@ function resolveModelForTier(cwd, agentType, attempt) {
   const override = config.model_overrides?.[agentType];
   if (override) return override;
 
+  // model_policy beats dynamic_routing (#49 Codex adversarial HIGH finding).
+  // Delegate to resolveModelInternal which handles step 2.5 (policy) correctly
+  // and falls through to runtimeTierDefaults on a miss. Gate on non-Claude
+  // runtime to match the same condition in resolveModelInternal step 2.5.
+  if (config.model_policy && config.runtime && config.runtime !== 'claude') {
+    return resolveModelInternal(cwd, agentType);
+  }
+
   const dr = config.dynamic_routing;
   // Disabled / missing / non-object → fall back to the existing resolver.
   if (!dr || typeof dr !== 'object' || dr.enabled !== true) {
diff --git a/tests/feat-49-model-policy-presets.test.cjs b/tests/feat-49-model-policy-presets.test.cjs
index 3d13efe8a..2d44cd211 100644
--- a/tests/feat-49-model-policy-presets.test.cjs
+++ b/tests/feat-49-model-policy-presets.test.cjs
@@ -63,6 +63,7 @@ const os = require('node:os');
 const {
   resolveModelInternal,
   resolveModelPolicy,
+  resolveModelForTier,
   KNOWN_PROVIDERS,
   _resetRuntimeWarningCacheForTests,
 } = require('../get-shit-done/bin/lib/core.cjs');
@@ -678,3 +679,108 @@ describe('#49 KNOWN_PROVIDERS exports from model-catalog.cjs and core.cjs', () =
       'KNOWN_PROVIDERS from core.cjs (re-export) must match model-catalog.cjs canonical export');
   });
 });
+
+// ─── resolveModelPolicy: Object.hasOwn prototype-pollution guards ────────────
+
+describe('#49 resolveModelPolicy: prototype-pollution guards', () => {
+  test('__proto__ as provider returns null without throwing', () => {
+    assert.strictEqual(resolveModelPolicy({ provider: '__proto__', budget: 'medium' }, 'sonnet'), null);
+  });
+
+  test('constructor as provider returns null without throwing', () => {
+    assert.strictEqual(resolveModelPolicy({ provider: 'constructor', budget: 'medium' }, 'sonnet'), null);
+  });
+
+  test('__proto__ as budget returns null without throwing', () => {
+    assert.strictEqual(resolveModelPolicy({ provider: 'openai', budget: '__proto__' }, 'haiku'), null);
+  });
+
+  test('toString as budget returns null without throwing', () => {
+    assert.strictEqual(resolveModelPolicy({ provider: 'openai', budget: 'toString' }, 'haiku'), null);
+  });
+
+  test('__proto__ as runtime_tiers key returns null without throwing', () => {
+    const policy = {
+      runtime: '__proto__',
+      runtime_tiers: { '__proto__': { haiku: { model: 'evil' } } },
+    };
+    assert.strictEqual(resolveModelPolicy(policy, 'haiku'), null);
+  });
+
+  test('__proto__ as tier inside runtime_tiers returns null without throwing', () => {
+    const policy = {
+      runtime: 'codex',
+      runtime_tiers: { codex: { '__proto__': { model: 'evil' } } },
+    };
+    assert.strictEqual(resolveModelPolicy(policy, '__proto__'), null);
+  });
+
+  test('valid provider+tier+budget still resolves correctly after guards', () => {
+    const result = resolveModelPolicy({ provider: 'openai', budget: 'low' }, 'haiku');
+    assert.ok(typeof result === 'string' && result.length > 0,
+      'valid openai/haiku/low lookup must still resolve after adding hasOwn guards');
+  });
+});
+
+// ─── resolveModelForTier: model_policy beats dynamic_routing ─────────────────
+
+describe('#49 resolveModelForTier: model_policy beats dynamic_routing', () => {
+  let tmpDir;
+  beforeEach(() => { tmpDir = makeTmp('for-tier-'); });
+  afterEach(() => { rmr(tmpDir); });
+
+  test('model_policy wins over dynamic_routing.tier_models when both are set', () => {
+    writeConfig(tmpDir, {
+      runtime: 'codex',
+      model_policy: { provider: 'openai', budget: 'low' },
+      dynamic_routing: {
+        enabled: true,
+        tier_models: { light: 'haiku', standard: 'sonnet', heavy: 'opus' },
+      },
+    });
+    // model_policy fires before dynamic_routing in resolveModelForTier
+    const result = resolveModelForTier(tmpDir, 'gsd-executor', 0);
+    // gsd-executor is standard/sonnet tier; openai+low+sonnet preset model
+    assert.ok(typeof result === 'string' && result.length > 0,
+      'model_policy must return a model string');
+    assert.notStrictEqual(result, 'sonnet',
+      'dynamic_routing tier alias must not win over model_policy');
+  });
+
+  test('model_overrides still beats model_policy in resolveModelForTier', () => {
+    writeConfig(tmpDir, {
+      runtime: 'codex',
+      model_policy: { provider: 'openai', budget: 'high' },
+      dynamic_routing: {
+        enabled: true,
+        tier_models: { light: 'haiku', standard: 'sonnet', heavy: 'opus' },
+      },
+      model_overrides: { 'gsd-planner': 'custom-model-id' },
+    });
+    assert.strictEqual(resolveModelForTier(tmpDir, 'gsd-planner', 0), 'custom-model-id');
+  });
+
+  test('dynamic_routing.tier_models used normally when model_policy absent', () => {
+    writeConfig(tmpDir, {
+      runtime: 'codex',
+      dynamic_routing: {
+        enabled: true,
+        tier_models: { light: 'haiku', standard: 'my-custom-sonnet', heavy: 'opus' },
+      },
+    });
+    assert.strictEqual(resolveModelForTier(tmpDir, 'gsd-executor', 0), 'my-custom-sonnet');
+  });
+
+  test('model_policy with Claude runtime does not interrupt dynamic_routing', () => {
+    // model_policy only gates on non-Claude runtimes; with runtime absent/claude,
+    // dynamic_routing must still work normally.
+    writeConfig(tmpDir, {
+      model_policy: { provider: 'openai', budget: 'low' },
+      dynamic_routing: {
+        enabled: true,
+        tier_models: { light: 'haiku', standard: 'my-sonnet', heavy: 'opus' },
+      },
+    });
+    assert.strictEqual(resolveModelForTier(tmpDir, 'gsd-executor', 0), 'my-sonnet');
+  });
+});