diff --git a/packages/builtin-tools/src/tools/EnterWorktreeTool/UI.tsx b/packages/builtin-tools/src/tools/EnterWorktreeTool/UI.tsx
index e0d503b181..07b406d7a0 100644
--- a/packages/builtin-tools/src/tools/EnterWorktreeTool/UI.tsx
+++ b/packages/builtin-tools/src/tools/EnterWorktreeTool/UI.tsx
@@ -14,10 +14,17 @@ export function renderToolResultMessage(
   _progressMessagesForMessage: ProgressMessage<ToolProgressData>[],
   _options: { theme: ThemeName },
 ): React.ReactNode {
+  if (!output) return null;
   return (
     <Box flexDirection="column">
       <Text>
-        Switched to worktree on branch <Text bold>{output.worktreeBranch}</Text>
+        Switched to worktree
+        {output.worktreeBranch ? (
+          <>
+            {' '}
+            on branch <Text bold>{output.worktreeBranch}</Text>
+          </>
+        ) : null}
       </Text>
       <Text dimColor>{output.worktreePath}</Text>
     </Box>
diff --git a/packages/builtin-tools/src/tools/ExecuteTool/ExecuteTool.ts b/packages/builtin-tools/src/tools/ExecuteTool/ExecuteTool.ts
index bfe9fb32b4..da13524c14 100644
--- a/packages/builtin-tools/src/tools/ExecuteTool/ExecuteTool.ts
+++ b/packages/builtin-tools/src/tools/ExecuteTool/ExecuteTool.ts
@@ -250,6 +250,12 @@ export const ExecuteTool = buildTool({
   renderToolResultMessage(content, progressMessages, options) {
     const innerTool = options.tools.find(t => t.name === content.tool_name)
     if (!innerTool?.renderToolResultMessage) return null
+    // Guard against null/undefined result — several error branches in this
+    // tool (tool-not-found, validation-failed, permission-denied, etc.) set
+    // result: null, and delegating null to the inner tool's UI would crash
+    // on any property access (e.g. output.worktreeBranch).
+    if (content.result == null || typeof content.result !== 'object')
+      return null
     const innerInput = (options.input as { params?: unknown } | undefined)
       ?.params
     return innerTool.renderToolResultMessage(
diff --git a/packages/builtin-tools/src/tools/ExitWorktreeTool/UI.tsx b/packages/builtin-tools/src/tools/ExitWorktreeTool/UI.tsx
index 76e0de4dda..e5dd5c2477 100644
--- a/packages/builtin-tools/src/tools/ExitWorktreeTool/UI.tsx
+++ b/packages/builtin-tools/src/tools/ExitWorktreeTool/UI.tsx
@@ -14,6 +14,7 @@ export function renderToolResultMessage(
   _progressMessagesForMessage: ProgressMessage<ToolProgressData>[],
   _options: { theme: ThemeName },
 ): React.ReactNode {
+  if (!output) return null;
   const actionLabel = output.action === 'keep' ? 'Kept worktree' : 'Removed worktree';
   return (
     <Box flexDirection="column">
diff --git a/src/utils/debug.ts b/src/utils/debug.ts
index f70e9a68df..7e51efdee9 100644
--- a/src/utils/debug.ts
+++ b/src/utils/debug.ts
@@ -1,6 +1,6 @@
 import { appendFile, mkdir, symlink, unlink } from 'fs/promises'
 import memoize from 'lodash-es/memoize.js'
-import { dirname, join } from 'path'
+import { dirname, join, resolve } from 'path'
 import { getSessionId } from 'src/bootstrap/state.js'
 
 import { type BufferedWriter, createBufferedWriter } from './bufferedWriter.js'
@@ -90,10 +90,10 @@ export const getDebugFilePath = memoize((): string | null => {
   for (let i = 0; i < process.argv.length; i++) {
     const arg = process.argv[i]!
     if (arg.startsWith('--debug-file=')) {
-      return arg.substring('--debug-file='.length)
+      return resolve(arg.substring('--debug-file='.length))
     }
     if (arg === '--debug-file' && i + 1 < process.argv.length) {
-      return process.argv[i + 1]!
+      return resolve(process.argv[i + 1]!)
     }
   }
   return null
@@ -170,7 +170,20 @@ function getDebugWriter(): BufferedWriter {
               // Directory already exists
             }
           }
-          getFsImplementation().appendFileSync(path, content)
+          try {
+            getFsImplementation().appendFileSync(path, content)
+          } catch (e) {
+            // If the directory was removed between mkdirSync and
+            // appendFileSync (or the path was relative and CWD changed),
+            // retry once with a fresh mkdirSync. Swallow on second failure
+            // to avoid crashing the React render tree via the error boundary.
+            try {
+              getFsImplementation().mkdirSync(dir)
+              getFsImplementation().appendFileSync(path, content)
+            } catch {
+              // Best-effort: debug log write failed, don't crash the UI
+            }
+          }
           void updateLatestDebugLogSymlink()
           return
         }
@@ -228,7 +241,9 @@ export function logForDebugging(
 export function getDebugLogPath(): string {
   return (
     getDebugFilePath() ??
-    process.env.CLAUDE_CODE_DEBUG_LOGS_DIR ??
+    (process.env.CLAUDE_CODE_DEBUG_LOGS_DIR
+      ? resolve(process.env.CLAUDE_CODE_DEBUG_LOGS_DIR)
+      : null) ??
     join(getClaudeConfigHomeDir(), 'debug', `${getSessionId()}.txt`)
   )
 }