diff --git a/src/main/runtime/harness-runtime.ts b/src/main/runtime/harness-runtime.ts
index 7555bdbb..47067dca 100644
--- a/src/main/runtime/harness-runtime.ts
+++ b/src/main/runtime/harness-runtime.ts
@@ -160,7 +160,14 @@ export class HarnessRuntime {
       this.writeLog(`[node] Harness process exited (${detail})`)
       if (this.child !== child) return
       this.child = undefined
-      this.setState('failed', `Harness stopped unexpectedly (${detail}).`)
+      const cause = extractFailureCause(this.logLines)
+      this.setState(
+        'failed',
+        cause
+          ? `Harness stopped unexpectedly (${detail}).
+${cause}`
+          : `Harness stopped unexpectedly (${detail}).`
+      )
     })
 
     const startedAt = Date.now()
@@ -241,6 +248,51 @@ export class HarnessRuntime {
   }
 }
 
+export function extractFailureCause(logLines: readonly string[]): string | undefined {
+  const stderrLines: string[] = []
+  let dshEntryError: string | undefined
+  let uncaughtError: string | undefined
+
+  for (const line of logLines) {
+    if (!line.startsWith('[stderr] ')) continue
+    const text = line.slice(8)
+    stderrLines.push(text)
+
+    if (dshEntryError === undefined) {
+      const m = text.match(/DSH entry failed:\s*(.+)/)
+      if (m && m[1]) dshEntryError = m[1].trim()
+    }
+
+    if (uncaughtError === undefined) {
+      const m1 = text.match(/uncaught exception:\s*(.+)/)
+      if (m1 && m1[1]) {
+        uncaughtError = m1[1].trim()
+      } else {
+        const m2 = text.match(/unhandled rejection:\s*(.+)/)
+        if (m2 && m2[1]) uncaughtError = m2[1].trim()
+      }
+    }
+  }
+
+  if (dshEntryError) return dshEntryError
+  if (uncaughtError) return uncaughtError
+
+  for (let i = stderrLines.length - 1; i >= 0; i--) {
+    const line = stderrLines[i]?.trim()
+    if (!line) continue
+    if (line.length < 200 && /\b(error|Error|ERROR|failed|Failed|FAILED)\b/.test(line)) {
+      return line
+    }
+  }
+
+  if (stderrLines.length > 0) {
+    const last = stderrLines[stderrLines.length - 1]?.trim()
+    if (last && last.length < 200) return last
+  }
+
+  return undefined
+}
+
 export function formatExitCode(code: number): string {
   const unsigned = code >>> 0
   const hexadecimal = `0x${unsigned.toString(16).padStart(8, '0').toUpperCase()}`
diff --git a/test/runtime.test.ts b/test/runtime.test.ts
index 816548ab..563dcab0 100644
--- a/test/runtime.test.ts
+++ b/test/runtime.test.ts
@@ -3,6 +3,7 @@ import {
   buildHarnessArguments,
   buildHarnessSpawnOptions,
   buildNodeArguments,
+  extractFailureCause,
   formatExitCode
 } from '../src/main/runtime/harness-runtime'
 import { canGrantWindowPermission, isTrustedAppUrl } from '../src/main/security-policy'
@@ -88,6 +89,79 @@ describe('Harness launch contract', () => {
   })
 })
 
+
+
+describe('harness failure cause extraction', () => {
+  it('extracts the DSH entry failure message from stderr', () => {
+    const logs = [
+      '[stderr] [harness-node] DSH entry failed: Error: dsh: plugin tree failed to load',
+      '[stderr] AggregateError: loader entries failed to apply',
+    ]
+    expect(extractFailureCause(logs)).toBe('Error: dsh: plugin tree failed to load')
+  })
+
+  it('extracts uncaught exception messages from stderr', () => {
+    const logs = [
+      '[stderr] [harness-node] uncaught exception: ReferenceError: foo is not defined',
+    ]
+    expect(extractFailureCause(logs)).toBe('ReferenceError: foo is not defined')
+  })
+
+  it('extracts unhandled rejection messages from stderr', () => {
+    const logs = [
+      '[stderr] [harness-node] unhandled rejection: TypeError: cannot read property x of null',
+    ]
+    expect(extractFailureCause(logs)).toBe('TypeError: cannot read property x of null')
+  })
+
+  it('prefers DSH entry failure over uncaught error', () => {
+    const logs = [
+      '[stderr] [harness-node] uncaught exception: some error',
+      '[stderr] [harness-node] DSH entry failed: Error: plugin failed to load',
+    ]
+    expect(extractFailureCause(logs)).toBe('Error: plugin failed to load')
+  })
+
+  it('falls back to the last error-like stderr line', () => {
+    const logs = [
+      '[stderr] some random output',
+      '[stderr] another line',
+      '[stderr] FATAL: configuration error in settings.yaml',
+    ]
+    expect(extractFailureCause(logs)).toBe('FATAL: configuration error in settings.yaml')
+  })
+
+  it('falls back to the last stderr line when nothing matches', () => {
+    const logs = [
+      '[stderr] starting up',
+      '[stderr] something happened',
+      '[stderr] process exiting now',
+    ]
+    expect(extractFailureCause(logs)).toBe('process exiting now')
+  })
+
+  it('returns undefined when there are no stderr lines', () => {
+    const logs = [
+      '[stdout] normal output',
+      '[desktop] starting harness',
+    ]
+    expect(extractFailureCause(logs)).toBeUndefined()
+  })
+
+  it('returns undefined for empty log array', () => {
+    expect(extractFailureCause([])).toBeUndefined()
+  })
+
+  it('ignores long error lines (>200 chars) when falling back', () => {
+    const longLine = 'x'.repeat(250)
+    const logs = [
+      `[stderr] ${longLine}`,
+      '[stderr] short error message',
+    ]
+    expect(extractFailureCause(logs)).toBe('short error message')
+  })
+})
+
 describe('navigation trust boundary', () => {
   it('only trusts the launcher and loopback HTTP pages', () => {
     expect(isTrustedAppUrl('file:///app/index.html')).toBe(true)