diff --git a/agents/base-chat.ts b/agents/base-chat.ts
index 0ce94245ac..e177738732 100644
--- a/agents/base-chat.ts
+++ b/agents/base-chat.ts
@@ -101,7 +101,8 @@ End every response by calling the suggest_followups tool with exactly 3 followup
 
     // `model` is absent only when the generator is driven directly (tests) or
     // by a runtime predating AgentStepContext.model.
-    const contextWindow = CONTEXT_WINDOWS[model ?? ''] ?? DEFAULT_CONTEXT_WINDOW
+    const contextWindow =
+      (model && CONTEXT_WINDOWS[model]) ?? DEFAULT_CONTEXT_WINDOW
     const maxContextLength = Math.floor(contextWindow * CONTEXT_BUDGET_FRACTION)
 
     while (true) {
diff --git a/common/src/util/__tests__/string.test.ts b/common/src/util/__tests__/string.test.ts
index 3a141ca6b6..965ddbeacc 100644
--- a/common/src/util/__tests__/string.test.ts
+++ b/common/src/util/__tests__/string.test.ts
@@ -1,6 +1,6 @@
 import { describe, expect, it } from 'bun:test'
 
-import { pluralize } from '../string'
+import { pluralize, truncateStringWithMessage } from '../string'
 
 describe('pluralize', () => {
   it('should handle singular and plural cases correctly', () => {
@@ -235,5 +235,127 @@ describe('pluralize', () => {
     expect(pluralize(2, 'query')).toBe('2 queries')
     expect(pluralize(2, 'dependency')).toBe('2 dependencies')
   })
+
+  describe('truncateStringWithMessage', () => {
+    it('should truncate from end by default', () => {
+      const result = truncateStringWithMessage({
+        str: 'Hello world, this is a test string',
+        maxLength: 20
+      })
+      expect(result).toContain('TRUNCATED')
+      expect(result.startsWith('Hello')).toBe(true)
+    })
+
+    it('should handle negative available length for END truncation', () => {
+      const result = truncateStringWithMessage({
+        str: 'Hello',
+        maxLength: 5,
+        remove: 'END'
+      })
+      expect(result).toBe('\n[TRUNCATED DUE TO LENGTH...]')
+    })
+
+    it('should handle zero available length for END truncation', () => {
+      const result = truncateStringWithMessage({
+        str: 'Hello world',
+        maxLength: 10,
+        remove: 'END'
+      })
+      expect(result).toBe('\n[TRUNCATED DUE TO LENGTH...]')
+    })
+
+    it('should truncate from start correctly', () => {
+      const result = truncateStringWithMessage({
+        str: 'Hello world, this is a test string',
+        maxLength: 50,
+        remove: 'START'
+      })
+      expect(result).toContain('TRUNCATED DUE TO LENGTH')
+      expect(result.endsWith('string')).toBe(true)
+    })
+
+    it('should handle negative available length for START truncation', () => {
+      const result = truncateStringWithMessage({
+        str: 'Hello world',
+        maxLength: 5,
+        remove: 'START'
+      })
+      expect(result).toBe('[...TRUNCATED DUE TO LENGTH]\n')
+    })
+
+    it('should handle zero available length for START truncation', () => {
+      const result = truncateStringWithMessage({
+        str: 'Hello world',
+        maxLength: 10,
+        remove: 'START'
+      })
+      expect(result).toBe('[...TRUNCATED DUE TO LENGTH]\n')
+    })
+
+    it('should truncate from middle correctly', () => {
+      const result = truncateStringWithMessage({
+        str: 'Hello world, this is a test string',
+        maxLength: 20,
+        remove: 'MIDDLE'
+      })
+      expect(result).toContain('TRUNCATED')
+      expect(result.startsWith('Hello')).toBe(true)
+      expect(result.endsWith('string')).toBe(true)
+    })
+
+    it('should truncate from start correctly', () => {
+      const result = truncateStringWithMessage({
+        str: 'Hello world, this is a test string',
+        maxLength: 50,
+        remove: 'START'
+      })
+      expect(result).toContain('TRUNCATED DUE TO LENGTH')
+      expect(result.endsWith('string')).toBe(true)
+    })
+
+    it('should handle negative available length for MIDDLE truncation', () => {
+      const result = truncateStringWithMessage({
+        str: 'Hello world',
+        maxLength: 5,
+        remove: 'MIDDLE'
+      })
+      expect(result).toBe('\n[...TRUNCATED DUE TO LENGTH...]\n')
+    })
+
+    it('should handle zero available length for MIDDLE truncation', () => {
+      const result = truncateStringWithMessage({
+        str: 'Hello world',
+        maxLength: 10,
+        remove: 'MIDDLE'
+      })
+      expect(result).toBe('\n[...TRUNCATED DUE TO LENGTH...]\n')
+    })
+
+    it('should return original string when within maxLength', () => {
+      const result = truncateStringWithMessage({
+        str: 'Short',
+        maxLength: 100
+      })
+      expect(result).toBe('Short')
+    })
+
+    it('should use custom message when provided', () => {
+      const result = truncateStringWithMessage({
+        str: 'Hello world, this is a test string',
+        maxLength: 20,
+        message: 'CUSTOM MSG'
+      })
+      expect(result).toContain('CUSTOM MSG')
+      expect(result.startsWith('Hello')).toBe(true)
+    })
+
+    it('should handle empty string', () => {
+      const result = truncateStringWithMessage({
+        str: '',
+        maxLength: 10
+      })
+      expect(result).toBe('')
+    })
+  })
 })
 
diff --git a/common/src/util/string.ts b/common/src/util/string.ts
index 506de962fd..96f31f73f4 100644
--- a/common/src/util/string.ts
+++ b/common/src/util/string.ts
@@ -24,15 +24,17 @@ export const truncateStringWithMessage = ({
 
   if (remove === 'END') {
     const suffix = `\n[${message}...]`
-    return str.slice(0, maxLength - suffix.length) + suffix
+    const availableLength = Math.max(0, maxLength - suffix.length)
+    return str.slice(0, availableLength) + suffix
   }
   if (remove === 'START') {
     const prefix = `[...${message}]\n`
-    return prefix + str.slice(str.length - maxLength + prefix.length)
+    const availableLength = Math.max(0, maxLength - prefix.length)
+    return prefix + str.slice(str.length - availableLength)
   }
 
   const middle = `\n[...${message}...]\n`
-  const length = Math.floor((maxLength - middle.length) / 2)
+  const length = Math.max(0, Math.floor((maxLength - middle.length) / 2))
   return str.slice(0, length) + middle + str.slice(-length)
 }