diff --git a/src/components/panels/agent-detail-tabs.tsx b/src/components/panels/agent-detail-tabs.tsx
index 7e9b106..1ae2f4f 100644
--- a/src/components/panels/agent-detail-tabs.tsx
+++ b/src/components/panels/agent-detail-tabs.tsx
@@ -1443,7 +1443,7 @@ export function ConfigTab({
   const updateModelConfig = (updater: (current: { primary?: string; fallbacks?: string[] }) => { primary?: string; fallbacks?: string[] }) => {
     setConfig((prev: any) => {
       const nextModel = updater({ ...(prev?.model || {}) })
-      const dedupedFallbacks = [...new Set((nextModel.fallbacks || []).map((value) => value.trim()).filter(Boolean))]
+      const dedupedFallbacks = [...new Set((nextModel.fallbacks || []).map((value) => (value || '').trim()).filter(Boolean))]
       return {
         ...prev,
         model: {
@@ -2788,8 +2788,8 @@ export function ModelsTab({ agent }: { agent: Agent }) {
         body: JSON.stringify({
           gateway_config: {
             model: {
-              primary: primary.trim(),
-              fallbacks: fallbacks.filter(f => f.trim()),
+              primary: (primary || '').trim(),
+              fallbacks: fallbacks.filter(f => f && f.trim()),
             },
           },
           write_to_gateway: true,
diff --git a/src/lib/__tests__/agent-model-trim.test.ts b/src/lib/__tests__/agent-model-trim.test.ts
new file mode 100644
index 0000000..08b57b7
--- /dev/null
+++ b/src/lib/__tests__/agent-model-trim.test.ts
@@ -0,0 +1,86 @@
+import { describe, expect, it } from 'vitest'
+
+/**
+ * Tests for the null-safe trim patterns used in agent model config editing.
+ * These mirror the logic in agent-detail-tabs.tsx (updateModelConfig + handleSave).
+ */
+
+function dedupFallbacks(fallbacks: (string | undefined | null)[]): string[] {
+  return [...new Set((fallbacks || []).map((value) => (value || '').trim()).filter(Boolean))]
+}
+
+function safePrimary(primary: string | undefined | null): string {
+  return (primary || '').trim()
+}
+
+function filterFallbacks(fallbacks: (string | undefined | null)[]): string[] {
+  return fallbacks.filter(f => f && f.trim()) as string[]
+}
+
+describe('agent model config trim safety', () => {
+  describe('dedupFallbacks (updateModelConfig pattern)', () => {
+    it('handles normal string values', () => {
+      expect(dedupFallbacks(['gpt-4', 'claude-3'])).toEqual(['gpt-4', 'claude-3'])
+    })
+
+    it('handles undefined values without throwing', () => {
+      expect(dedupFallbacks([undefined, 'gpt-4', undefined])).toEqual(['gpt-4'])
+    })
+
+    it('handles null values without throwing', () => {
+      expect(dedupFallbacks([null, 'gpt-4'])).toEqual(['gpt-4'])
+    })
+
+    it('filters out empty strings', () => {
+      expect(dedupFallbacks(['', '  ', 'gpt-4'])).toEqual(['gpt-4'])
+    })
+
+    it('deduplicates models', () => {
+      expect(dedupFallbacks(['gpt-4', 'gpt-4', 'claude-3'])).toEqual(['gpt-4', 'claude-3'])
+    })
+
+    it('handles empty array', () => {
+      expect(dedupFallbacks([])).toEqual([])
+    })
+  })
+
+  describe('safePrimary (handleSave pattern)', () => {
+    it('trims normal string', () => {
+      expect(safePrimary('  gpt-4  ')).toBe('gpt-4')
+    })
+
+    it('handles undefined without throwing', () => {
+      expect(safePrimary(undefined)).toBe('')
+    })
+
+    it('handles null without throwing', () => {
+      expect(safePrimary(null)).toBe('')
+    })
+
+    it('handles empty string', () => {
+      expect(safePrimary('')).toBe('')
+    })
+  })
+
+  describe('filterFallbacks (handleSave pattern)', () => {
+    it('filters valid values', () => {
+      expect(filterFallbacks(['gpt-4', 'claude-3'])).toEqual(['gpt-4', 'claude-3'])
+    })
+
+    it('filters out undefined without throwing', () => {
+      expect(filterFallbacks([undefined, 'gpt-4'])).toEqual(['gpt-4'])
+    })
+
+    it('filters out null without throwing', () => {
+      expect(filterFallbacks([null, 'gpt-4'])).toEqual(['gpt-4'])
+    })
+
+    it('filters out empty strings', () => {
+      expect(filterFallbacks(['', 'gpt-4'])).toEqual(['gpt-4'])
+    })
+
+    it('filters out whitespace-only strings', () => {
+      expect(filterFallbacks(['   ', 'gpt-4'])).toEqual(['gpt-4'])
+    })
+  })
+})
diff --git a/tests/agent-model-config.spec.ts b/tests/agent-model-config.spec.ts
new file mode 100644
index 0000000..ccb25d3
--- /dev/null
+++ b/tests/agent-model-config.spec.ts
@@ -0,0 +1,95 @@
+import { test, expect } from '@playwright/test'
+import { API_KEY_HEADER, createTestAgent, deleteTestAgent } from './helpers'
+
+/**
+ * E2E tests for agent model configuration updates.
+ * Verifies the API handles edge cases like empty/null model values.
+ */
+
+test.describe('Agent Model Config', () => {
+  const cleanup: number[] = []
+
+  test.afterEach(async ({ request }) => {
+    for (const id of cleanup) {
+      await deleteTestAgent(request, id).catch(() => {})
+    }
+    cleanup.length = 0
+  })
+
+  test('PUT with valid model config succeeds', async ({ request }) => {
+    const { id } = await createTestAgent(request)
+    cleanup.push(id)
+
+    const res = await request.put(`/api/agents/${id}`, {
+      headers: API_KEY_HEADER,
+      data: {
+        gateway_config: {
+          model: {
+            primary: 'claude-3-5-sonnet-20241022',
+            fallbacks: ['gpt-4o'],
+          },
+        },
+      },
+    })
+
+    expect(res.status()).toBe(200)
+  })
+
+  test('PUT with empty fallbacks array succeeds', async ({ request }) => {
+    const { id } = await createTestAgent(request)
+    cleanup.push(id)
+
+    const res = await request.put(`/api/agents/${id}`, {
+      headers: API_KEY_HEADER,
+      data: {
+        gateway_config: {
+          model: {
+            primary: 'claude-3-5-sonnet-20241022',
+            fallbacks: [],
+          },
+        },
+      },
+    })
+
+    expect(res.status()).toBe(200)
+  })
+
+  test('PUT with empty string primary returns appropriate response', async ({ request }) => {
+    const { id } = await createTestAgent(request)
+    cleanup.push(id)
+
+    const res = await request.put(`/api/agents/${id}`, {
+      headers: API_KEY_HEADER,
+      data: {
+        gateway_config: {
+          model: {
+            primary: '',
+            fallbacks: [],
+          },
+        },
+      },
+    })
+
+    // Server should accept the update (model config is optional gateway config)
+    expect([200, 400]).toContain(res.status())
+  })
+
+  test('PUT with whitespace-only fallbacks filters them', async ({ request }) => {
+    const { id } = await createTestAgent(request)
+    cleanup.push(id)
+
+    const res = await request.put(`/api/agents/${id}`, {
+      headers: API_KEY_HEADER,
+      data: {
+        gateway_config: {
+          model: {
+            primary: 'claude-3-5-sonnet-20241022',
+            fallbacks: ['  ', '', 'gpt-4o'],
+          },
+        },
+      },
+    })
+
+    expect(res.status()).toBe(200)
+  })
+})