diff --git a/build/commands/lib/buildDiagnostics.test.ts b/build/commands/lib/buildDiagnostics.test.ts
new file mode 100644
index 00000000000..66cb8130d63
--- /dev/null
+++ b/build/commands/lib/buildDiagnostics.test.ts
@@ -0,0 +1,99 @@
+// Copyright (c) 2026 The Brave Authors. All rights reserved.
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this file,
+// You can obtain one at https://mozilla.org/MPL/2.0/.
+
+import fs from 'node:fs'
+import os from 'node:os'
+import path from 'node:path'
+import { dumpBuildHangDiagnostics } from './buildDiagnostics.ts'
+import Log from './logging.js'
+
+jest.mock('./logging.js', () => ({
+  error: jest.fn(),
+}))
+
+describe('dumpBuildHangDiagnostics', () => {
+  let tempDir: string
+  let consoleLogSpy: jest.SpyInstance
+
+  beforeEach(() => {
+    tempDir = fs.mkdtempSync(path.join(os.tmpdir(), 'ci-diagnostics-'))
+    consoleLogSpy = jest.spyOn(console, 'log').mockImplementation(() => {})
+    jest.clearAllMocks()
+  })
+
+  afterEach(() => {
+    consoleLogSpy.mockRestore()
+    jest.restoreAllMocks()
+    fs.rmSync(tempDir, { recursive: true, force: true })
+  })
+
+  it('logs the contents of an existing diagnostics file', () => {
+    const diagPath = path.join(tempDir, 'siso.INFO')
+    fs.writeFileSync(diagPath, 'first line\nsecond line\n', 'utf8')
+
+    dumpBuildHangDiagnostics(tempDir)
+
+    expect(consoleLogSpy).toHaveBeenCalledWith(`--- tail of ${diagPath} ---`)
+    expect(consoleLogSpy).toHaveBeenCalledWith('first line\nsecond line\n')
+    expect(Log.error).not.toHaveBeenCalled()
+  })
+
+  it('reads a redirected diagnostics file when the local file is missing', () => {
+    const redirectedTarget = path.join(tempDir, 'redirect-target.INFO')
+    const redirectedMarker = path.join(tempDir, 'siso.exe.INFO.redirected')
+    fs.writeFileSync(redirectedTarget, 'redirected output\n', 'utf8')
+    fs.writeFileSync(redirectedMarker, `${redirectedTarget}\n`, 'utf8')
+
+    dumpBuildHangDiagnostics(tempDir)
+
+    expect(consoleLogSpy).toHaveBeenCalledWith(
+      `--- tail of ${redirectedTarget} ---`,
+    )
+    expect(consoleLogSpy).toHaveBeenCalledWith('redirected output\n')
+    expect(Log.error).not.toHaveBeenCalled()
+  })
+
+  it('logs an error when a redirected target does not exist', () => {
+    const redirectedTarget = path.join(tempDir, 'missing.INFO')
+    const redirectedMarker = path.join(tempDir, 'siso_output.redirected')
+    fs.writeFileSync(redirectedMarker, redirectedTarget, 'utf8')
+
+    dumpBuildHangDiagnostics(tempDir)
+
+    expect(Log.error).toHaveBeenCalledWith(
+      `Redirected file ${redirectedTarget} does not exist`,
+    )
+    expect(consoleLogSpy).not.toHaveBeenCalled()
+  })
+
+  it('logs read errors without throwing', () => {
+    const diagPath = path.join(tempDir, 'siso.INFO')
+    fs.writeFileSync(diagPath, 'content that will not be read', 'utf8')
+
+    const realFs = jest.requireActual('node:fs') as typeof import('node:fs')
+    jest.spyOn(fs, 'openSync').mockImplementation((...args) => {
+      if (args[0] === diagPath) {
+        throw new Error('open failed')
+      }
+      return realFs.openSync(...args)
+    })
+
+    dumpBuildHangDiagnostics(tempDir)
+
+    expect(consoleLogSpy).toHaveBeenCalledWith(`--- tail of ${diagPath} ---`)
+    expect(Log.error).toHaveBeenCalledWith('open failed')
+  })
+
+  it('prints only the final complete lines for oversized files', () => {
+    const diagPath = path.join(tempDir, 'siso.INFO')
+    const content = `${'a'.repeat(130 * 1024)}\nkept line 1\nkept line 2\n`
+    fs.writeFileSync(diagPath, content, 'utf8')
+
+    dumpBuildHangDiagnostics(tempDir)
+
+    expect(consoleLogSpy).toHaveBeenCalledWith('kept line 1\nkept line 2\n')
+    expect(Log.error).not.toHaveBeenCalled()
+  })
+})
diff --git a/build/commands/lib/buildDiagnostics.ts b/build/commands/lib/buildDiagnostics.ts
new file mode 100644
index 00000000000..7989b38b3ac
--- /dev/null
+++ b/build/commands/lib/buildDiagnostics.ts
@@ -0,0 +1,71 @@
+// Copyright (c) 2026 The Brave Authors. All rights reserved.
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this file,
+// You can obtain one at https://mozilla.org/MPL/2.0/.
+
+import fs from 'node:fs'
+import path from 'node:path'
+import Log from './logging.js'
+
+const kReadFileTailLimitBytes = 128 * 1024
+
+/**
+ * Dumps the hanged build diagnostics for a build.
+ * @param {string} outputDir
+ */
+export function dumpBuildHangDiagnostics(outputDir: string) {
+  const relativePaths = ['siso.INFO', 'siso.exe.INFO', 'siso_output']
+  for (const rel of relativePaths) {
+    let filePath = path.join(outputDir, rel)
+    if (!fs.existsSync(filePath)) {
+      const filePathRedirected = path.join(outputDir, rel + '.redirected')
+      if (fs.existsSync(filePathRedirected)) {
+        // read the redirected file and get the path
+        const redirectedContent = fs.readFileSync(filePathRedirected, 'utf8')
+        const redirectedPath = redirectedContent.trim()
+        if (fs.existsSync(redirectedPath)) {
+          filePath = redirectedPath
+        } else {
+          Log.error(`Redirected file ${redirectedPath} does not exist`)
+          continue
+        }
+      } else {
+        continue
+      }
+    }
+    console.log(`--- tail of ${filePath} ---`)
+    try {
+      console.log(readFileTailUtf8Sync(filePath, kReadFileTailLimitBytes))
+    } catch (err) {
+      Log.error(err instanceof Error ? err.message : String(err))
+    }
+  }
+}
+
+/**
+ * Reads only the last `maxBytes` bytes from a file.
+ */
+function readFileTailUtf8Sync(filePath: string, maxBytes: number): string {
+  const fd = fs.openSync(filePath, 'r')
+  try {
+    const size = fs.fstatSync(fd).size
+    if (size === 0) {
+      return ''
+    }
+    const toRead = Math.min(maxBytes, size)
+    const buf = Buffer.alloc(toRead)
+    fs.readSync(fd, buf, 0, toRead, size - toRead)
+    // May start mid-sequence for UTF-8; Node replaces invalid bytes at slice
+    // edge.
+    let str = buf.toString('utf8')
+    if (toRead < size) {
+      const nl = str.indexOf('\n')
+      if (nl !== -1) {
+        str = str.slice(nl + 1)
+      }
+    }
+    return str
+  } finally {
+    fs.closeSync(fd)
+  }
+}
diff --git a/build/commands/lib/util.js b/build/commands/lib/util.js
index 4d4b1f82a72..cb4c39fca61 100644
--- a/build/commands/lib/util.js
+++ b/build/commands/lib/util.js
@@ -18,6 +18,7 @@ import ActionGuard from './actionGuard.js'
 import { GitPatcher } from './gitPatcher.js'
 import { getBuildArgs } from './buildArgs.ts'
 import { isCI, isTeamcity } from './ciDetect.ts'
+import { dumpBuildHangDiagnostics } from './buildDiagnostics.ts'
 
 // Do not limit the number of listeners to avoid warnings from EventEmitter.
 process.setMaxListeners(0)
@@ -228,13 +229,22 @@ const util = {
     args = [],
     options = /** @type {Record<string, any>} */ ({}),
   ) => {
-    let { continueOnFail, verbose, onStdErrLine, onStdOutLine, ...cmdOptions } =
-      options
+    let {
+      continueOnFail,
+      verbose,
+      onSpawn,
+      onStdErrLine,
+      onStdOutLine,
+      ...cmdOptions
+    } = options
     if (verbose !== false) {
       Log.command(cmdOptions.cwd, cmd, args)
     }
     return new Promise((resolve, reject) => {
       const prog = spawn(...normalizeCommand(cmd, args), cmdOptions)
+      if (onSpawn) {
+        onSpawn(prog)
+      }
       const signalsToForward = ['SIGINT', 'SIGTERM', 'SIGQUIT', 'SIGHUP']
       const signalHandler = (s) => {
         prog.kill(s)
@@ -708,11 +718,14 @@ const util = {
     // Collect build statistics into this variable to display in a separate TC
     // block.
     let buildStats = ''
+    // Updated on every autoninja log line when CI pipes output (idle watchdog).
+    let lastBuildLogTime = Date.now()
 
     // Parse output to display the build progress on Teamcity.
     if (isTeamcity) {
       let lastStatusTime = Date.now()
       options.onStdOutLine = (line) => {
+        lastBuildLogTime = Date.now()
         if (
           buildStats
           || /^(RBE Stats:|metric\s+count|build finished)\s+/.test(line)
@@ -732,6 +745,14 @@ const util = {
       }
       options.onStdErrLine = options.onStdOutLine
       options.stdio = 'pipe'
+    } else if (isCI) {
+      const onLine = (line) => {
+        lastBuildLogTime = Date.now()
+        console.log(line)
+      }
+      options.onStdOutLine = onLine
+      options.onStdErrLine = onLine
+      options.stdio = 'pipe'
     }
 
     // Enable to allow error post-processing after autoninja/siso failure.
@@ -743,6 +764,14 @@ const util = {
       fs.unlinkSync(sisoOutputFile)
     }
 
+    let buildIdleWatchdogInterval = null
+    const clearBuildIdleWatchdog = () => {
+      if (buildIdleWatchdogInterval) {
+        clearInterval(buildIdleWatchdogInterval)
+        buildIdleWatchdogInterval = null
+      }
+    }
+
     const buildGuard = new ActionGuard(path.join(outputDir, 'build.guard'))
     try {
       if (
@@ -755,9 +784,36 @@ const util = {
         await util.runAsync('gn', ['clean', outputDir], options)
       }
       buildGuard.markStarted()
-      await util.runAsync('autoninja', ninjaOpts, options)
+
+      let buildProcess = null
+      const autoninjaOptions = {
+        ...options,
+        onSpawn: (prog) => {
+          buildProcess = prog
+        },
+      }
+
+      if (isCI) {
+        const idleTimeoutMs = 90 * 60 * 1000 // 90 minutes
+        lastBuildLogTime = Date.now()
+        buildIdleWatchdogInterval = setInterval(() => {
+          if (Date.now() - lastBuildLogTime <= idleTimeoutMs) {
+            return
+          }
+          clearBuildIdleWatchdog()
+          Log.error(
+            `Build aborted: no autoninja output for ${idleTimeoutMs / 1000}s `,
+          )
+          dumpBuildHangDiagnostics(outputDir)
+          util.killProcessTree(buildProcess)
+        }, 10 * 1000)
+      }
+
+      await util.runAsync('autoninja', ninjaOpts, autoninjaOptions)
+      clearBuildIdleWatchdog()
       buildGuard.markFinished()
     } catch (e) {
+      clearBuildIdleWatchdog()
       // Display siso_output on CI after a build failure.
       if (isCI && fs.existsSync(sisoOutputFile)) {
         const sisoOutput = fs.readFileSync(sisoOutputFile, 'utf8')
@@ -1029,6 +1085,29 @@ const util = {
       config.defaultOptions,
     )
   },
+
+  /**
+   * Stop a process and its descendants. On Windows, `child.kill()` often only
+   * affects `cmd.exe`; `taskkill /T` tears down the full tree.
+   * @param {import('node:child_process').ChildProcess | null} child
+   */
+  killProcessTree: (child) => {
+    if (!child?.pid) {
+      return
+    }
+    if (process.platform === 'win32') {
+      spawnSync('taskkill', ['/PID', String(child.pid), '/T', '/F'], {
+        stdio: 'ignore',
+        windowsHide: true,
+      })
+      return
+    }
+    try {
+      child.kill('SIGKILL')
+    } catch {
+      // Process may already have exited.
+    }
+  },
 }
 
 export default util