diff --git a/README.md b/README.md
index 46408cc..0aa8b32 100644
--- a/README.md
+++ b/README.md
@@ -25,7 +25,6 @@ Writing a PR description after the coding is finished is easy to postpone. diffn
 
 ## Features
 
-### Choose your diff
 
 Summarize tracked working-tree changes, staged changes, a base ref, or a commit range.
 
diff --git a/src/diffnote/analyze.py b/src/diffnote/analyze.py
index 40e3d99..202df45 100644
--- a/src/diffnote/analyze.py
+++ b/src/diffnote/analyze.py
@@ -29,6 +29,8 @@
 
 CONV_RE = re.compile(r"^(feat|fix|refactor|perf|chore|docs|test|style|build|ci)"
                      r"(\([^)]+\))?!?:", re.I)
+BREAKING_COMMIT_RE = re.compile(r"^[a-z][a-z0-9-]*(?:\([^)]+\))?!:", re.I)
+PUBLIC_API_RE = re.compile(r"^(?:lib/|src/(?:[^/]+/)*api(?:/|$))", re.I)
 
 
 MINUS_RE = re.compile(r"^--- ")
@@ -120,9 +122,11 @@ def _join_text(file):
     return "\n".join(file["lines"])
 
 
-def assess_risks(files):
+def assess_risks(files, commit_messages=None):
     """Return list of risk dicts: {label, files:[...]}."""
     risks = {}
+    breaking_commit = any(BREAKING_COMMIT_RE.match(message) for message in (commit_messages or []))
+    breaking_files = []
     for f in files:
         path = f["new"]
         content = _join_text(f)
@@ -134,6 +138,10 @@ def assess_risks(files):
             risks.setdefault("dependency change", []).append(path)
         if f["added"] + f["deleted"] >= 500:
             risks.setdefault("large batch change (review carefully)", []).append(path)
+        if breaking_commit or PUBLIC_API_RE.match(path):
+            breaking_files.append(path)
+    if breaking_files:
+        risks["possible breaking change"] = breaking_files
     return [{"label": k, "files": v} for k, v in risks.items()]
 
 
@@ -167,7 +175,7 @@ def build_summary(files, commit_messages=None):
         for t in _categorize(f["new"]):
             areas[t] += 1
 
-    risks = assess_risks(files)
+    risks = assess_risks(files, commit_messages)
     title = suggest_title(files, commit_messages)
     section = suggest_section(files, commit_messages)
 
diff --git a/tests/test_analyze.py b/tests/test_analyze.py
index c9cd75b..d558cf5 100644
--- a/tests/test_analyze.py
+++ b/tests/test_analyze.py
@@ -1,5 +1,7 @@
+import json
 import textwrap
 from diffnote import analyze
+from diffnote import render
 
 SAMPLE = textwrap.dedent(
     """
@@ -36,6 +38,20 @@ def f():
 ).strip()
 
 
+def _files_for(*paths):
+    diff = []
+    for path in paths:
+        diff.extend([
+            f"diff --git a/{path} b/{path}",
+            f"--- a/{path}",
+            f"+++ b/{path}",
+            "@@ -1 +1 @@",
+            "-old value",
+            "+new value",
+        ])
+    return analyze.parse_diff("\n".join(diff))
+
+
 def test_parse_diff_counts():
     files = analyze.parse_diff(SAMPLE)
     assert len(files) == 4
@@ -67,3 +83,27 @@ def test_suggest_title_derives_migration():
     files = analyze.parse_diff(SAMPLE)
     s = analyze.build_summary(files, commit_messages=[])
     assert "schema" in s["title"] or s["title"].startswith("feat")
+
+
+def test_breaking_risk_covers_public_api_paths_only():
+    files = _files_for("lib/client.py", "src/app/api/routes.py", "src/app/apiary.py")
+    summary = analyze.build_summary(files)
+    risk = next(r for r in summary["risks"] if r["label"] == "possible breaking change")
+    assert risk["files"] == ["lib/client.py", "src/app/api/routes.py"]
+
+
+def test_breaking_commit_marker_covers_all_changed_files():
+    files = _files_for("src/app/core.py", "README.md")
+    summary = analyze.build_summary(files, ["feat(api)!: remove the old response field"])
+    risk = next(r for r in summary["risks"] if r["label"] == "possible breaking change")
+    assert risk["files"] == ["src/app/core.py", "README.md"]
+
+
+def test_breaking_risk_appears_in_terminal_markdown_and_json():
+    summary = analyze.build_summary(_files_for("src/api/client.py"), ["fix!: change public return type"])
+    terminal = render.render_terminal(summary, use_color=False)
+    markdown = render.render_markdown(summary)
+    machine = json.loads(render.render_json(summary))
+    assert "possible breaking change" in terminal
+    assert "possible breaking change" in markdown
+    assert any(risk["label"] == "possible breaking change" for risk in machine["risks"])