diff --git a/.github/workflows/diff_report.yaml b/.github/workflows/diff_report.yaml
new file mode 100644
index 000000000..2ddb85094
--- /dev/null
+++ b/.github/workflows/diff_report.yaml
@@ -0,0 +1,45 @@
+name: diff report
+
+on:
+  pull_request_target:
+    types: [opened, synchronize, reopened]
+
+jobs:
+  comment:
+    name: comment
+    runs-on: ubuntu-latest
+    timeout-minutes: 10
+    permissions:
+      contents: read
+      pull-requests: write
+      actions: read
+    steps:
+    - name: Wait for process replay
+      id: wait
+      continue-on-error: true
+      uses: lewagon/wait-on-check-action@v1.3.4
+      with:
+        ref: ${{ github.event.pull_request.head.sha }}
+        check-name: process replay
+        repo-token: ${{ secrets.GITHUB_TOKEN }}
+        allowed-conclusions: success,failure
+        wait-interval: 20
+    - name: Download diff
+      if: steps.wait.outcome == 'success'
+      uses: dawidd6/action-download-artifact@v6
+      with:
+        github_token: ${{ secrets.GITHUB_TOKEN }}
+        workflow: tests.yaml
+        workflow_conclusion: ''
+        pr: ${{ github.event.number }}
+        name: diff_report_${{ github.event.number }}
+        path: .
+        allow_forks: true
+    - name: Comment on PR
+      if: steps.wait.outcome == 'success'
+      uses: thollander/actions-comment-pull-request@v2
+      with:
+        filePath: diff_report.txt
+        comment_tag: diff_report
+        pr_number: ${{ github.event.number }}
+        GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
diff --git a/.github/workflows/tests.yaml b/.github/workflows/tests.yaml
index f4be0ad5a..91a8e5c32 100644
--- a/.github/workflows/tests.yaml
+++ b/.github/workflows/tests.yaml
@@ -139,12 +139,22 @@ jobs:
       id: print-diff
       if: always()
       run: cat selfdrive/test/process_replay/diff.txt
+    - name: Print diff report
+      if: always()
+      run: cat selfdrive/test/process_replay/diff_report.txt
     - uses: actions/upload-artifact@v6
       if: always()
       continue-on-error: true
       with:
         name: process_replay_diff.txt
         path: selfdrive/test/process_replay/diff.txt
+    - name: Upload diff report
+      uses: actions/upload-artifact@v6
+      if: always() && github.event_name == 'pull_request'
+      continue-on-error: true
+      with:
+        name: diff_report_${{ github.event.number }}
+        path: selfdrive/test/process_replay/diff_report.txt
     - name: Checkout ci-artifacts
       if: github.repository == 'commaai/openpilot' && github.ref == 'refs/heads/master'
       uses: actions/checkout@v4
diff --git a/selfdrive/test/process_replay/diff_report.py b/selfdrive/test/process_replay/diff_report.py
new file mode 100644
index 000000000..32f058f8e
--- /dev/null
+++ b/selfdrive/test/process_replay/diff_report.py
@@ -0,0 +1,92 @@
+import os
+from collections import defaultdict
+
+from opendbc.car.tests.car_diff import format_diff, format_numeric_diffs
+from openpilot.selfdrive.test.process_replay.compare_logs import compare_logs
+from openpilot.selfdrive.test.process_replay.process_replay import PROC_REPLAY_DIR
+
+
+class MsgWrap:
+  """Adapter so to_dict() includes defaults"""
+  def __init__(self, msg):
+    self._msg = msg
+  def to_dict(self) -> dict:
+    return self._msg.to_dict(verbose=True)
+
+
+def diff_process(cfg, ref_msgs, new_msgs) -> tuple | None:
+  ref = defaultdict(list)
+  new = defaultdict(list)
+  for m in ref_msgs:
+    if m.which() in cfg.subs:
+      ref[m.which()].append(m)
+  for m in new_msgs:
+    if m.which() in cfg.subs:
+      new[m.which()].append(m)
+
+  diffs = []
+  for sub in cfg.subs:
+    if len(ref[sub]) != len(new[sub]):
+      diffs.append((f"{sub} (message count)", 0, (len(ref[sub]), len(new[sub])), 0))
+    for i, (r, n) in enumerate(zip(ref[sub], new[sub], strict=False)):
+      for d in compare_logs([r], [n], cfg.ignore, tolerance=cfg.tolerance):
+        if d[0] == "change":
+          a, b = d[2]
+          if a != a and b != b:
+            continue
+          diffs.append((d[1], i, d[2], r.logMonoTime))
+        elif d[0] in ("add", "remove"):
+          for item in d[2]:
+            if item[1] != item[1]:
+              continue
+            diffs.append((f"{d[1]}.{item[0]}", i, (d[0], item[1]), r.logMonoTime))
+  return (diffs, ref, new) if diffs else None
+
+
+def diff_format(diffs, ref, new, field) -> list[str]:
+  if any(part.isdigit() for part in field.split(".")):
+    return format_numeric_diffs(diffs)
+  msg_type = field.split(".")[0]
+  ref_ts = [(m.logMonoTime, MsgWrap(m)) for m in ref.get(msg_type, [])]
+  new_wrapped = [MsgWrap(m) for m in new.get(msg_type, [])]
+  return format_diff(diffs, ref_ts, new_wrapped, field)
+
+
+def diff_report(replay_diffs, segments) -> None:
+  seg_to_plat = {seg: plat for plat, seg in segments}
+
+  with_diffs, errors, n_passed = [], [], 0
+  for seg, proc, data in replay_diffs:
+    plat = seg_to_plat.get(seg, "UNKNOWN")
+    if data is None:
+      n_passed += 1
+    elif isinstance(data, str):
+      errors.append((plat, seg, proc, data))
+    else:
+      with_diffs.append((plat, seg, proc, data))
+
+  icon = "⚠️" if with_diffs else "✅"
+  lines = [
+    "## Process replay diff report",
+    "Replays driving segments through this PR and compares the behavior to master.",
+    "Please review any changes carefully to ensure they are expected.\n",
+    f"{icon}  {len(with_diffs)} changed, {n_passed} passed, {len(errors)} errors",
+  ]
+
+  for plat, seg, proc, err in errors:
+    lines.append(f"\nERROR {plat} - {seg} [{proc}]: {err}")
+
+  if with_diffs:
+    lines.append("<details><summary><b>Show changes</b></summary>\n\n```")
+    for plat, seg, proc, (diffs, ref, new) in with_diffs:
+      lines.append(f"\n{plat} - {seg} [{proc}]")
+      by_field = defaultdict(list)
+      for d in diffs:
+        by_field[d[0]].append(d)
+      for field, fd in sorted(by_field.items()):
+        lines.append(f"\n  {field} ({len(fd)} diffs)")
+        lines.extend(diff_format(fd, ref, new, field))
+    lines.append("```\n</details>")
+
+  with open(os.path.join(PROC_REPLAY_DIR, "diff_report.txt"), "w") as f:
+    f.write("\n".join(lines))
diff --git a/selfdrive/test/process_replay/test_processes.py b/selfdrive/test/process_replay/test_processes.py
index fbe300a7c..bc0085534 100755
--- a/selfdrive/test/process_replay/test_processes.py
+++ b/selfdrive/test/process_replay/test_processes.py
@@ -3,6 +3,7 @@ import argparse
 import concurrent.futures
 import os
 import sys
+import traceback
 from collections import defaultdict
 from tqdm import tqdm
 from typing import Any
@@ -11,6 +12,7 @@ from opendbc.car.car_helpers import interface_names
 from openpilot.common.git import get_commit
 from openpilot.tools.lib.openpilotci import get_url
 from openpilot.selfdrive.test.process_replay.compare_logs import compare_logs, format_diff
+from openpilot.selfdrive.test.process_replay.diff_report import diff_process, diff_report
 from openpilot.selfdrive.test.process_replay.process_replay import CONFIGS, PROC_REPLAY_DIR, FAKEDATA, replay_process, \
                                                                    check_most_messages_valid
 from openpilot.tools.lib.filereader import FileReader
@@ -72,11 +74,16 @@ EXCLUDED_PROCS = {"modeld", "dmonitoringmodeld"}
 
 def run_test_process(data):
   segment, cfg, args, cur_log_fn, ref_log_path, lr_dat = data
+  ref_log_msgs = list(LogReader(ref_log_path))
   lr = LogReader.from_bytes(lr_dat)
-  res, log_msgs = test_process(cfg, lr, segment, ref_log_path, cur_log_fn, args.ignore_fields, args.ignore_msgs)
+  res, log_msgs = test_process(cfg, lr, segment, ref_log_msgs, cur_log_fn, args.ignore_fields, args.ignore_msgs)
   # save logs so we can update refs
   save_log(cur_log_fn, log_msgs)
-  return (segment, cfg.proc_name, res)
+  try:
+    diff_data = diff_process(cfg, ref_log_msgs, log_msgs)
+  except Exception:
+    diff_data = traceback.format_exc()
+  return (segment, cfg.proc_name, res, diff_data)
 
 
 def get_log_data(segment):
@@ -85,14 +92,12 @@ def get_log_data(segment):
     return (segment, f.read())
 
 
-def test_process(cfg, lr, segment, ref_log_path, new_log_path, ignore_fields=None, ignore_msgs=None):
+def test_process(cfg, lr, segment, ref_log_msgs, new_log_path, ignore_fields=None, ignore_msgs=None):
   if ignore_fields is None:
     ignore_fields = []
   if ignore_msgs is None:
     ignore_msgs = []
 
-  ref_log_msgs = list(LogReader(ref_log_path))
-
   try:
     log_msgs = replay_process(cfg, lr, disable_progress=True)
   except Exception as e:
@@ -201,9 +206,11 @@ if __name__ == "__main__":
         log_paths[segment][cfg.proc_name]['new'] = cur_log_fn
 
     results: Any = defaultdict(dict)
+    diffs: list = []
     p2 = pool.map(run_test_process, pool_args)
-    for (segment, proc, result) in tqdm(p2, desc="Running Tests", total=len(pool_args)):
+    for (segment, proc, result, diff_data) in tqdm(p2, desc="Running Tests", total=len(pool_args)):
       results[segment][proc] = result
+      diffs.append((segment, proc, diff_data))
 
   diff_short, diff_long, failed = format_diff(results, log_paths, ref_commit)
   if not args.update_refs:
@@ -211,6 +218,11 @@ if __name__ == "__main__":
       f.write(diff_long)
     print(diff_short)
 
+    try:
+      diff_report(diffs, segments)
+    except Exception:
+      print(f"failed to generate diff report:\n{traceback.format_exc()}")
+
     if failed:
       print("TEST FAILED")
     else: