feat: implement breaking change input

.tarpaulin.ci.toml

@@ -3,4 +3,4 @@ out = ["Xml"]
 target-dir = "coverage"
 output-dir = "coverage"
 fail-under = 60
-exclude-files = ["target/*", "private/*"]
+exclude-files = ["target/*", "private/*", "tests/*"]

.tarpaulin.local.toml

@@ -4,4 +4,4 @@ skip-clean = true
 target-dir = "coverage"
 output-dir = "coverage"
 fail-under = 60
-exclude-files = ["target/*", "private/*"]
+exclude-files = ["target/*", "private/*", "tests/*"]

Cargo.lock

@@ -28,9 +28,9 @@ dependencies = [
 
 [[package]]
 name = "anstream"
-version = "0.6.21"
+version = "1.0.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "43d5b281e737544384e969a5ccad3f1cdd24b48086a0fc1b2a5262a26b8f4f4a"
+checksum = "824a212faf96e9acacdbd09febd34438f8f711fb84e09a8916013cd7815ca28d"
 dependencies = [
  "anstyle",
  "anstyle-parse",
@@ -43,15 +43,15 @@ dependencies = [
 
 [[package]]
 name = "anstyle"
-version = "1.0.13"
+version = "1.0.14"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "5192cca8006f1fd4f7237516f40fa183bb07f8fbdfedaa0036de5ea9b0b45e78"
+checksum = "940b3a0ca603d1eade50a4846a2afffd5ef57a9feac2c0e2ec2e14f9ead76000"
 
 [[package]]
 name = "anstyle-parse"
-version = "0.2.7"
+version = "1.0.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "4e7644824f0aa2c7b9384579234ef10eb7efb6a0deb83f9630a49594dd9c15c2"
+checksum = "52ce7f38b242319f7cabaa6813055467063ecdc9d355bbb4ce0c68908cd8130e"
 dependencies = [
  "utf8parse",
 ]
@@ -99,9 +99,9 @@ checksum = "7c02d123df017efcdfbd739ef81735b36c5ba83ec3c59c80a9d7ecc718f92e50"
 
 [[package]]
 name = "assert_cmd"
-version = "2.1.2"
+version = "2.2.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "9c5bcfa8749ac45dd12cb11055aeeb6b27a3895560d60d71e3c23bf979e60514"
+checksum = "9a686bbee5efb88a82df0621b236e74d925f470e5445d3220a5648b892ec99c9"
 dependencies = [
  "anstyle",
  "bstr",
@@ -152,9 +152,9 @@ checksum = "3a8241f3ebb85c056b509d4327ad0358fbbba6ffb340bf388f26350aeda225b1"
 
 [[package]]
 name = "bitflags"
-version = "2.10.0"
+version = "2.11.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "812e12b5285cc515a9c72a5c1d3b6d46a19dac5acfef5265968c166106e31dd3"
+checksum = "843867be96c8daad0d758b57df9392b6d8d271134fce549de6ce169ff98a92af"
 
 [[package]]
 name = "blake2"
@@ -244,9 +244,9 @@ dependencies = [
 
 [[package]]
 name = "clap"
-version = "4.5.57"
+version = "4.6.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "6899ea499e3fb9305a65d5ebf6e3d2248c5fab291f300ad0a704fbe142eae31a"
+checksum = "b193af5b67834b676abd72466a96c1024e6a6ad978a1f484bd90b85c94041351"
 dependencies = [
  "clap_builder",
  "clap_derive",
@@ -254,9 +254,9 @@ dependencies = [
 
 [[package]]
 name = "clap_builder"
-version = "4.5.57"
+version = "4.6.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "7b12c8b680195a62a8364d16b8447b01b6c2c8f9aaf68bee653be34d4245e238"
+checksum = "714a53001bf66416adb0e2ef5ac857140e7dc3a0c48fb28b2f10762fc4b5069f"
 dependencies = [
  "anstream",
  "anstyle",
@@ -266,9 +266,9 @@ dependencies = [
 
 [[package]]
 name = "clap_derive"
-version = "4.5.55"
+version = "4.6.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "a92793da1a46a5f2a02a6f4c46c6496b28c43638adea8306fcb0caa1634f24e5"
+checksum = "1110bd8a634a1ab8cb04345d8d878267d57c3cf1b38d91b71af6686408bbca6a"
 dependencies = [
  "heck",
  "proc-macro2",
@@ -278,9 +278,9 @@ dependencies = [
 
 [[package]]
 name = "clap_lex"
-version = "0.7.7"
+version = "1.1.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "c3e64b0cc0439b12df2fa678eae89a1c56a529fd067a9115f7827f1fffd22b32"
+checksum = "c8d4a3bb8b1e0c1050499d1815f5ab16d04f0959b233085fb31653fbfc9d98f9"
 
 [[package]]
 name = "clru"
@@ -293,9 +293,9 @@ dependencies = [
 
 [[package]]
 name = "colorchoice"
-version = "1.0.4"
+version = "1.0.5"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "b05b61dc5112cbb17e4b6cd61790d9845d13888356391624cbe7e41efeac1e75"
+checksum = "1d07550c9036bf2ae0c684c4297d503f838287c83c53686d05370d0e139ae570"
 
 [[package]]
 name = "convert_case"
@@ -698,18 +698,6 @@ dependencies = [
  "version_check",
 ]
 
-[[package]]
-name = "getrandom"
-version = "0.3.4"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "899def5c37c4fd7b2664648c28120ecec138e4d395b459e5ca34f9cce2dd77fd"
-dependencies = [
- "cfg-if",
- "libc",
- "r-efi 5.3.0",
- "wasip2",
-]
-
 [[package]]
 name = "getrandom"
 version = "0.4.2"
@@ -718,7 +706,7 @@ checksum = "0de51e6874e94e7bf76d726fc5d13ba782deca734ff60d5bb2fb2607c7406555"
 dependencies = [
  "cfg-if",
  "libc",
- "r-efi 6.0.0",
+ "r-efi",
  "rand_core",
  "wasip2",
  "wasip3",
@@ -1575,9 +1563,9 @@ dependencies = [
 
 [[package]]
 name = "inquire"
-version = "0.9.2"
+version = "0.9.4"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "ae51d5da01ce7039024fbdec477767c102c454dbdb09d4e2a432ece705b1b25d"
+checksum = "6654738b8024300cf062d04a1c13c10c8e2cea598ec1c47dc9b6641159429756"
 dependencies = [
  "bitflags",
  "crossterm",
@@ -1673,6 +1661,7 @@ dependencies = [
  "jj-lib",
  "lazy-regex",
  "predicates",
+ "textwrap",
  "thiserror",
  "tokio",
 ]
@@ -1757,9 +1746,9 @@ dependencies = [
 
 [[package]]
 name = "lazy-regex"
-version = "3.5.1"
+version = "3.6.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "c5c13b6857ade4c8ee05c3c3dc97d2ab5415d691213825b90d3211c425c1f907"
+checksum = "6bae91019476d3ec7147de9aa291cadb6d870abf2f3015d2da73a90325ac1496"
 dependencies = [
  "lazy-regex-proc_macros",
  "once_cell",
@@ -1769,9 +1758,9 @@ dependencies = [
 
 [[package]]
 name = "lazy-regex-proc_macros"
-version = "3.5.1"
+version = "3.6.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "32a95c68db5d41694cea563c86a4ba4dc02141c16ef64814108cb23def4d5438"
+checksum = "4de9c1e1439d8b7b3061b2d209809f447ca33241733d9a3c01eabf2dc8d94358"
 dependencies = [
  "proc-macro2",
  "quote",
@@ -1793,9 +1782,9 @@ checksum = "09edd9e8b54e49e587e4f6295a7d29c3ea94d469cb40ab8ca70b288248a81db2"
 
 [[package]]
 name = "libc"
-version = "0.2.182"
+version = "0.2.183"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "6800badb6cb2082ffd7b6a67e6125bb39f18782f793520caee8cb8846be06112"
+checksum = "b5b646652bf6661599e1da8901b3b9522896f01e736bad5f723fe7a3a27f899d"
 
 [[package]]
 name = "libredox"
@@ -1889,9 +1878,9 @@ dependencies = [
 
 [[package]]
 name = "memchr"
-version = "2.7.6"
+version = "2.8.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "f52b00d39961fc5b2736ea853c9cc86238e165017a493d1d5c8eac6bdc4cc273"
+checksum = "f8ca58f447f06ed17d5fc4043ce1b10dd205e060fb3ce5b979b8ed8e59ff3f79"
 
 [[package]]
 name = "memmap2"
@@ -1937,9 +1926,9 @@ dependencies = [
 
 [[package]]
 name = "once_cell"
-version = "1.21.3"
+version = "1.21.4"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "42f5e15c9953c5e4ccceeb2e7382a716482c34515315f7b03532b8b4e8393d2d"
+checksum = "9f7c3e4beb33f85d45ae3e3a1792185706c8e16d043238c593331cc7cd313b50"
 
 [[package]]
 name = "once_cell_polyfill"
@@ -2021,9 +2010,9 @@ dependencies = [
 
 [[package]]
 name = "pin-project-lite"
-version = "0.2.16"
+version = "0.2.17"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "3b3cff922bd51709b605d9ead9aa71031d81447142d828eb4a6eba76fe619f9b"
+checksum = "a89322df9ebe1c1578d689c92318e070967d1042b512afbe49518723f4e6d5cd"
 
 [[package]]
 name = "plain"
@@ -2063,9 +2052,9 @@ dependencies = [
 
 [[package]]
 name = "predicates"
-version = "3.1.3"
+version = "3.1.4"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "a5d19ee57562043d37e82899fade9a22ebab7be9cef5026b07fda9cdd4293573"
+checksum = "ada8f2932f28a27ee7b70dd6c1c39ea0675c55a36879ab92f3a715eaa1e63cfe"
 dependencies = [
  "anstyle",
  "difflib",
@@ -2077,15 +2066,15 @@ dependencies = [
 
 [[package]]
 name = "predicates-core"
-version = "1.0.9"
+version = "1.0.10"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "727e462b119fe9c93fd0eb1429a5f7647394014cf3c04ab2c0350eeb09095ffa"
+checksum = "cad38746f3166b4031b1a0d39ad9f954dd291e7854fcc0eed52ee41a0b50d144"
 
 [[package]]
 name = "predicates-tree"
-version = "1.0.12"
+version = "1.0.13"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "72dd2d6d381dfb73a193c7fca536518d7caee39fc8503f74e7dc0be0531b425c"
+checksum = "d0de1b847b39c8131db0467e9df1ff60e6d0562ab8e9a16e568ad0fdb372e2f2"
 dependencies = [
  "predicates-core",
  "termtree",
@@ -2144,19 +2133,13 @@ dependencies = [
 
 [[package]]
 name = "quote"
-version = "1.0.44"
+version = "1.0.45"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "21b2ebcf727b7760c461f091f9f0f539b77b8e87f2fd88131e7f1b433b3cece4"
+checksum = "41f2619966050689382d2b44f664f4bc593e129785a36d6ee376ddf37259b924"
 dependencies = [
  "proc-macro2",
 ]
 
-[[package]]
-name = "r-efi"
-version = "5.3.0"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "69cdb34c158ceb288df11e18b4bd39de994f6657d83847bdffdbd7f346754b0f"
-
 [[package]]
 name = "r-efi"
 version = "6.0.0"
@@ -2170,7 +2153,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "bc266eb313df6c5c09c1c7b1fbe2510961e5bcd3add930c1e31f7ed9da0feff8"
 dependencies = [
  "chacha20",
- "getrandom 0.4.2",
+ "getrandom",
  "rand_core",
 ]
 
@@ -2279,9 +2262,9 @@ checksum = "cab834c73d247e67f4fae452806d17d3c7501756d98c8808d7c9c7aa7d18f973"
 
 [[package]]
 name = "regex-syntax"
-version = "0.8.9"
+version = "0.8.10"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "a96887878f22d7bad8a3b6dc5b7440e0ada9a245242924394987b21cf2210a4c"
+checksum = "dc897dd8d9e8bd1ed8cdad82b5966c3e0ecae09fb1907d58efaa013543185d0a"
 
 [[package]]
 name = "rustc_version"
@@ -2474,6 +2457,12 @@ dependencies = [
  "serde",
 ]
 
+[[package]]
+name = "smawk"
+version = "0.3.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "b7c388c1b5e93756d0c740965c41e8822f866621d41acbdf6336a6a168f8840c"
+
 [[package]]
 name = "stable_deref_trait"
 version = "1.2.1"
@@ -2500,9 +2489,9 @@ checksum = "13c2bddecc57b384dee18652358fb23172facb8a2c51ccc10d74c157bdea3292"
 
 [[package]]
 name = "syn"
-version = "2.0.114"
+version = "2.0.117"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "d4d107df263a3013ef9b1879b0df87d706ff80f65a86ea879bd9c31f9b307c2a"
+checksum = "e665b8803e7b1d2a727f4023456bbbbe74da67099c585258af0ad9c5013b9b99"
 dependencies = [
  "proc-macro2",
  "quote",
@@ -2511,12 +2500,12 @@ dependencies = [
 
 [[package]]
 name = "tempfile"
-version = "3.26.0"
+version = "3.27.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "82a72c767771b47409d2345987fda8628641887d5466101319899796367354a0"
+checksum = "32497e9a4c7b38532efcdebeef879707aa9f794296a4f0244f6f69e9bc8574bd"
 dependencies = [
  "fastrand",
- "getrandom 0.3.4",
+ "getrandom",
  "once_cell",
  "rustix",
  "windows-sys 0.61.2",
@@ -2528,6 +2517,17 @@ version = "0.5.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "8f50febec83f5ee1df3015341d8bd429f2d1cc62bcba7ea2076759d315084683"
 
+[[package]]
+name = "textwrap"
+version = "0.16.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "c13547615a44dc9c452a8a534638acdf07120d4b6847c8178705da06306a3057"
+dependencies = [
+ "smawk",
+ "unicode-linebreak",
+ "unicode-width",
+]
+
 [[package]]
 name = "thiserror"
 version = "2.0.18"
@@ -2574,9 +2574,9 @@ checksum = "1f3ccbac311fea05f86f61904b462b55fb3df8837a366dfc601a0161d0532f20"
 
 [[package]]
 name = "tokio"
-version = "1.49.0"
+version = "1.50.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "72a2903cd7736441aac9df9d7688bd0ce48edccaadf181c3b90be801e81d3d86"
+checksum = "27ad5e34374e03cfffefc301becb44e9dc3c17584f414349ebe29ed26661822d"
 dependencies = [
  "bytes",
  "pin-project-lite",
@@ -2585,9 +2585,9 @@ dependencies = [
 
 [[package]]
 name = "tokio-macros"
-version = "2.6.0"
+version = "2.6.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "af407857209536a95c8e56f8231ef2c2e2aff839b22e07a1ffcbc617e9db9fa5"
+checksum = "5c55a2eff8b69ce66c84f85e1da1c233edc36ceb85a2058d11b0d6a3c7e7569c"
 dependencies = [
  "proc-macro2",
  "quote",
@@ -2699,9 +2699,15 @@ checksum = "7eec5d1121208364f6793f7d2e222bf75a915c19557537745b195b253dd64217"
 
 [[package]]
 name = "unicode-ident"
-version = "1.0.22"
+version = "1.0.24"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "9312f7c4f6ff9069b165498234ce8be658059c6728633667c526e27dc2cf1df5"
+checksum = "e6e4313cd5fcd3dad5cafa179702e2b244f760991f45397d14d4ebf38247da75"
+
+[[package]]
+name = "unicode-linebreak"
+version = "0.1.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "3b09c83c3c29d37506a3e260c08c03743a6bb66a9cd432c6934ab501a190571f"
 
 [[package]]
 name = "unicode-normalization"
@@ -3038,9 +3044,9 @@ checksum = "589f6da84c646204747d1270a2a5661ea66ed1cced2631d546fdfb155959f9ec"
 
 [[package]]
 name = "winnow"
-version = "0.7.14"
+version = "0.7.15"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "5a5364e9d77fcdeeaa6062ced926ee3381faa2ee02d3eb83a5c27a8825540829"
+checksum = "df79d97927682d2fd8adb29682d1140b343be4ac0f08fd68b7765d9c059d3945"
 dependencies = [
  "memchr",
 ]
@@ -3145,18 +3151,18 @@ dependencies = [
 
 [[package]]
 name = "zerocopy"
-version = "0.8.40"
+version = "0.8.42"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "a789c6e490b576db9f7e6b6d661bcc9799f7c0ac8352f56ea20193b2681532e5"
+checksum = "f2578b716f8a7a858b7f02d5bd870c14bf4ddbbcf3a4c05414ba6503640505e3"
 dependencies = [
  "zerocopy-derive",
 ]
 
 [[package]]
 name = "zerocopy-derive"
-version = "0.8.40"
+version = "0.8.42"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "f65c489a7071a749c849713807783f70672b28094011623e200cb86dcb835953"
+checksum = "7e6cc098ea4d3bd6246687de65af3f920c430e236bee1e3bf2e441463f08a02f"
 dependencies = [
  "proc-macro2",
  "quote",

Cargo.toml

@@ -30,6 +30,7 @@ jj-lib = "0.39.0"
 lazy-regex = { version = "3.5.1", features = ["lite"] }
 thiserror = "2.0.18"
 tokio = { version = "1.49.0", features = ["macros", "rt-multi-thread"] }
+textwrap = "0.16.2"
 
 [dev-dependencies]
 assert_cmd = "2.1.2"

src/commit/types/breaking_change.rs

@@ -0,0 +1,125 @@
+use super::Footer;
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+#[repr(transparent)]
+pub struct BreakingChangeNote(String);
+
+impl Footer for BreakingChangeNote {
+    fn note(&self) -> &str {
+        &self.0
+    }
+
+    fn prefix(&self) -> &str {
+        "BREAKING CHANGE"
+    }
+}
+
+impl<T> From<T> for BreakingChangeNote
+where
+    T: ToString,
+{
+    fn from(value: T) -> Self {
+        Self(value.to_string())
+    }
+}
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub enum BreakingChange {
+    No,
+    Yes,
+    WithNote(BreakingChangeNote),
+}
+
+impl BreakingChange {
+    pub fn ignore(&self) -> bool {
+        matches!(self, BreakingChange::No)
+    }
+
+    pub fn header_segment(&self) -> &str {
+        match self {
+            Self::No => "",
+            _ => "!",
+        }
+    }
+
+    pub fn as_footer(&self) -> String {
+        match self {
+            BreakingChange::WithNote(footer) => footer.as_footer(),
+            _ => "".into(),
+        }
+    }
+}
+
+impl<T> From<T> for BreakingChange
+where
+    T: ToString,
+{
+    fn from(value: T) -> Self {
+        match value.to_string().trim() {
+            "" => Self::Yes,
+            value => Self::WithNote(value.into()),
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    /// Empty string produces Yes(None) — no footer, only '!' in the header
+    #[test]
+    fn from_empty_string_yields_yes_none() {
+        assert_eq!(BreakingChange::from(String::new()), BreakingChange::Yes);
+    }
+
+    /// Whitespace-only string produces Yes(None)
+    #[test]
+    fn from_whitespace_string_yields_yes_none() {
+        assert_eq!(BreakingChange::from("   ".to_string()), BreakingChange::Yes);
+    }
+
+    /// Mixed whitespace (tabs, newlines) produces Yes(None)
+    #[test]
+    fn from_tab_newline_string_yields_yes_none() {
+        assert_eq!(
+            BreakingChange::from("\t\n  ".to_string()),
+            BreakingChange::Yes
+        );
+    }
+
+    /// Non-empty string produces Yes(Some(...)) with the note preserved
+    #[test]
+    fn from_non_empty_string_yields_yes_some() {
+        assert_eq!(
+            BreakingChange::from("removes old API"),
+            BreakingChange::WithNote("removes old API".into()),
+        );
+    }
+
+    /// Surrounding whitespace is trimmed from the note
+    #[test]
+    fn from_string_trims_surrounding_whitespace() {
+        assert_eq!(
+            BreakingChange::from("  removes old API  "),
+            BreakingChange::WithNote("removes old API".into()),
+        );
+    }
+
+    /// Leading whitespace only is trimmed, leaving the non-empty part
+    #[test]
+    fn from_string_trims_leading_whitespace() {
+        assert_eq!(
+            BreakingChange::from("   removes old API"),
+            BreakingChange::WithNote("removes old API".into()),
+        );
+    }
+
+    /// Trailing whitespace only is trimmed, leaving the non-empty part
+    #[test]
+    fn from_string_trims_trailing_whitespace() {
+        assert_eq!(
+            BreakingChange::from("removes old API   "),
+            BreakingChange::WithNote("removes old API".into()),
+        );
+    }
+}

src/commit/types/footer.rs

@@ -0,0 +1,13 @@
+pub trait Footer {
+    fn prefix(&self) -> &str;
+    fn note(&self) -> &str;
+
+    fn as_footer(&self) -> String {
+        let default = format!("{}: {}", self.prefix(), self.note());
+        if default.chars().count() > 72 {
+            textwrap::wrap(&default, 71).join("\n ")
+        } else {
+            default
+        }
+    }
+}

src/commit/types/message.rs

@@ -1,4 +1,4 @@
-use super::{CommitType, Description, Scope};
+use super::{BreakingChange, CommitType, Description, Scope};
 use thiserror::Error;
 
 /// Errors that can occur when creating a ConventionalCommit
@@ -21,6 +21,7 @@ pub struct ConventionalCommit {
     commit_type: CommitType,
     scope: Scope,
     description: Description,
+    breaking_change: BreakingChange,
 }
 
 impl ConventionalCommit {
@@ -40,11 +41,13 @@ impl ConventionalCommit {
         commit_type: CommitType,
         scope: Scope,
         description: Description,
+        breaking_change: BreakingChange,
     ) -> Result<Self, CommitMessageError> {
         let commit = Self {
             commit_type,
             scope,
             description,
+            breaking_change,
         };
         let len = commit.first_line_len();
         if len > Self::FIRST_LINE_MAX_LENGTH {
@@ -65,13 +68,12 @@ impl ConventionalCommit {
     /// Calculate the length of the formatted first line
     ///
     /// Formula:
-    /// - With scope: `len(type) + len(scope) + 4 + len(description)`
+    /// - `len(type)` + `len(scope)` + `len(breaking_change)` + 2 + `len(description)`
-    ///   (the 4 accounts for parentheses, colon, and space: "()  ")
-    /// - Without scope: `len(type) + 2 + len(description)`
     ///   (the 2 accounts for colon and space: ": ")
     pub fn first_line_len(&self) -> usize {
         self.commit_type.len()
             + self.scope.header_segment_len()
+            + if self.breaking_change.ignore() { 0 } else { 1 }
             + 2 // ": "
             + self.description.len()
     }
@@ -81,7 +83,12 @@ impl ConventionalCommit {
     /// Returns `type(scope): description` if scope is non-empty, or
     /// `type: description` if scope is empty
     pub fn format(&self) -> String {
-        Self::format_preview(self.commit_type, &self.scope, &self.description)
+        Self::format_preview(
+            self.commit_type,
+            &self.scope,
+            &self.description,
+            &self.breaking_change,
+        )
     }
 
     /// Format a preview of the commit message without creating a validated instance
@@ -93,12 +100,18 @@ impl ConventionalCommit {
         commit_type: CommitType,
         scope: &Scope,
         description: &Description,
+        breaking_change: &BreakingChange,
     ) -> String {
-        if scope.is_empty() {
+        let scope = scope.header_segment();
-            format!("{}: {}", commit_type, description)
+        let breaking_change_header = breaking_change.header_segment();
-        } else {
+        let breaking_change_footer = breaking_change.as_footer();
-            format!("{}({}): {}", commit_type, scope, description)
+        format!(
-        }
+            r#"{commit_type}{scope}{breaking_change_header}: {description}
+
+{breaking_change_footer}"#,
+        )
+        .trim()
+        .to_string()
     }
 
     /// Returns the commit type
@@ -142,8 +155,9 @@ mod tests {
         commit_type: CommitType,
         scope: Scope,
         description: Description,
+        breaking_change: BreakingChange,
     ) -> ConventionalCommit {
-        ConventionalCommit::new(commit_type, scope, description)
+        ConventionalCommit::new(commit_type, scope, description, breaking_change)
             .expect("test commit should have valid line length")
     }
 
@@ -154,6 +168,7 @@ mod tests {
             CommitType::Feat,
             test_scope("cli"),
             test_description("add new feature"),
+            BreakingChange::No,
         );
         assert_eq!(commit.commit_type(), CommitType::Feat);
         assert_eq!(commit.scope().as_str(), "cli");
@@ -167,6 +182,7 @@ mod tests {
             CommitType::Fix,
             Scope::empty(),
             test_description("fix critical bug"),
+            BreakingChange::No,
         );
         assert_eq!(commit.commit_type(), CommitType::Fix);
         assert!(commit.scope().is_empty());
@@ -180,6 +196,7 @@ mod tests {
             CommitType::Feat,
             test_scope("auth"),
             test_description("add login"),
+            BreakingChange::No,
         );
         assert_eq!(commit.format(), "feat(auth): add login");
     }
@@ -192,6 +209,7 @@ mod tests {
             CommitType::Fix,
             test_scope("user-auth"),
             test_description("fix token refresh"),
+            BreakingChange::No,
         );
         assert_eq!(commit1.format(), "fix(user-auth): fix token refresh");
 
@@ -200,6 +218,7 @@ mod tests {
             CommitType::Docs,
             test_scope("api_docs"),
             test_description("update README"),
+            BreakingChange::No,
         );
         assert_eq!(commit2.format(), "docs(api_docs): update README");
 
@@ -208,6 +227,7 @@ mod tests {
             CommitType::Chore,
             test_scope("PROJ-123/cleanup"),
             test_description("remove unused code"),
+            BreakingChange::No,
         );
         assert_eq!(
             commit3.format(),
@@ -222,6 +242,7 @@ mod tests {
             CommitType::Feat,
             Scope::empty(),
             test_description("add login"),
+            BreakingChange::No,
         );
         assert_eq!(commit.format(), "feat: add login");
     }
@@ -233,6 +254,7 @@ mod tests {
             CommitType::Fix,
             Scope::empty(),
             test_description("fix critical bug"),
+            BreakingChange::No,
         );
         assert_eq!(commit1.format(), "fix: fix critical bug");
 
@@ -240,6 +262,7 @@ mod tests {
             CommitType::Docs,
             Scope::empty(),
             test_description("update installation guide"),
+            BreakingChange::No,
         );
         assert_eq!(commit2.format(), "docs: update installation guide");
     }
@@ -265,7 +288,7 @@ mod tests {
         ];
 
         for (commit_type, expected) in expected_formats {
-            let commit = test_commit(commit_type, scope.clone(), desc.clone());
+            let commit = test_commit(commit_type, scope.clone(), desc.clone(), BreakingChange::No);
             assert_eq!(
                 commit.format(),
                 expected,
@@ -295,7 +318,12 @@ mod tests {
         ];
 
         for (commit_type, expected) in expected_formats {
-            let commit = test_commit(commit_type, Scope::empty(), desc.clone());
+            let commit = test_commit(
+                commit_type,
+                Scope::empty(),
+                desc.clone(),
+                BreakingChange::No,
+            );
             assert_eq!(
                 commit.format(),
                 expected,
@@ -312,6 +340,7 @@ mod tests {
             CommitType::Feat,
             test_scope("auth"),
             test_description("add login"),
+            BreakingChange::No,
         );
         let display_output = format!("{}", commit);
         let format_output = commit.format();
@@ -325,6 +354,7 @@ mod tests {
             CommitType::Fix,
             test_scope("api"),
             test_description("handle null response"),
+            BreakingChange::No,
         );
         assert_eq!(format!("{}", commit), "fix(api): handle null response");
     }
@@ -336,6 +366,7 @@ mod tests {
             CommitType::Docs,
             Scope::empty(),
             test_description("improve README"),
+            BreakingChange::No,
         );
         assert_eq!(format!("{}", commit), "docs: improve README");
     }
@@ -345,8 +376,12 @@ mod tests {
     fn display_equals_format_for_all_types() {
         for commit_type in CommitType::all() {
             // With scope
-            let commit_with_scope =
+            let commit_with_scope = test_commit(
-                test_commit(*commit_type, test_scope("test"), test_description("change"));
+                *commit_type,
+                test_scope("test"),
+                test_description("change"),
+                BreakingChange::No,
+            );
             assert_eq!(
                 format!("{}", commit_with_scope),
                 commit_with_scope.format(),
@@ -355,8 +390,12 @@ mod tests {
             );
 
             // Without scope
-            let commit_without_scope =
+            let commit_without_scope = test_commit(
-                test_commit(*commit_type, Scope::empty(), test_description("change"));
+                *commit_type,
+                Scope::empty(),
+                test_description("change"),
+                BreakingChange::No,
+            );
             assert_eq!(
                 format!("{}", commit_without_scope),
                 commit_without_scope.format(),
@@ -370,7 +409,12 @@ mod tests {
     #[test]
     fn commit_type_accessor_returns_correct_type() {
         for commit_type in CommitType::all() {
-            let commit = test_commit(*commit_type, Scope::empty(), test_description("test"));
+            let commit = test_commit(
+                *commit_type,
+                Scope::empty(),
+                test_description("test"),
+                BreakingChange::No,
+            );
             assert_eq!(commit.commit_type(), *commit_type);
         }
     }
@@ -382,6 +426,7 @@ mod tests {
             CommitType::Feat,
             test_scope("auth"),
             test_description("add feature"),
+            BreakingChange::No,
         );
         assert_eq!(commit.scope().as_str(), "auth");
     }
@@ -393,6 +438,7 @@ mod tests {
             CommitType::Feat,
             Scope::empty(),
             test_description("add feature"),
+            BreakingChange::No,
         );
         assert!(commit.scope().is_empty());
     }
@@ -404,6 +450,7 @@ mod tests {
             CommitType::Feat,
             Scope::empty(),
             test_description("add new authentication flow"),
+            BreakingChange::No,
         );
         assert_eq!(commit.description().as_str(), "add new authentication flow");
     }
@@ -415,6 +462,7 @@ mod tests {
             CommitType::Feat,
             test_scope("cli"),
             test_description("add feature"),
+            BreakingChange::No,
         );
         let cloned = original.clone();
         assert_eq!(original, cloned);
@@ -427,11 +475,13 @@ mod tests {
             CommitType::Feat,
             test_scope("cli"),
             test_description("add feature"),
+            BreakingChange::No,
         );
         let commit2 = test_commit(
             CommitType::Feat,
             test_scope("cli"),
             test_description("add feature"),
+            BreakingChange::No,
         );
         assert_eq!(commit1, commit2);
     }
@@ -443,11 +493,13 @@ mod tests {
             CommitType::Feat,
             test_scope("cli"),
             test_description("change"),
+            BreakingChange::No,
         );
         let commit2 = test_commit(
             CommitType::Fix,
             test_scope("cli"),
             test_description("change"),
+            BreakingChange::No,
         );
         assert_ne!(commit1, commit2);
     }
@@ -459,11 +511,13 @@ mod tests {
             CommitType::Feat,
             test_scope("cli"),
             test_description("change"),
+            BreakingChange::No,
         );
         let commit2 = test_commit(
             CommitType::Feat,
             test_scope("api"),
             test_description("change"),
+            BreakingChange::No,
         );
         assert_ne!(commit1, commit2);
     }
@@ -475,11 +529,13 @@ mod tests {
             CommitType::Feat,
             test_scope("cli"),
             test_description("add feature"),
+            BreakingChange::No,
         );
         let commit2 = test_commit(
             CommitType::Feat,
             test_scope("cli"),
             test_description("fix bug"),
+            BreakingChange::No,
         );
         assert_ne!(commit1, commit2);
     }
@@ -491,6 +547,7 @@ mod tests {
             CommitType::Feat,
             test_scope("cli"),
             test_description("add feature"),
+            BreakingChange::No,
         );
         let debug_output = format!("{:?}", commit);
         assert!(debug_output.contains("ConventionalCommit"));
@@ -504,6 +561,7 @@ mod tests {
             CommitType::Feat,
             test_scope("auth"),
             test_description("implement OAuth2 login flow"),
+            BreakingChange::No,
         );
         assert_eq!(commit.format(), "feat(auth): implement OAuth2 login flow");
     }
@@ -515,6 +573,7 @@ mod tests {
             CommitType::Fix,
             Scope::empty(),
             test_description("prevent crash on empty input"),
+            BreakingChange::No,
         );
         assert_eq!(commit.format(), "fix: prevent crash on empty input");
     }
@@ -526,6 +585,7 @@ mod tests {
             CommitType::Docs,
             test_scope("README"),
             test_description("add installation instructions"),
+            BreakingChange::No,
         );
         assert_eq!(
             commit.format(),
@@ -540,6 +600,7 @@ mod tests {
             CommitType::Refactor,
             test_scope("core"),
             test_description("extract validation logic"),
+            BreakingChange::No,
         );
         assert_eq!(commit.format(), "refactor(core): extract validation logic");
     }
@@ -551,6 +612,7 @@ mod tests {
             CommitType::Ci,
             test_scope("github"),
             test_description("add release workflow"),
+            BreakingChange::No,
         );
         assert_eq!(commit.format(), "ci(github): add release workflow");
     }
@@ -563,6 +625,7 @@ mod tests {
             CommitType::Feat,
             Scope::empty(),
             Description::parse(&long_desc).unwrap(),
+            BreakingChange::No,
         );
         // Format should be "feat: " + 50 chars = 56 total chars
         let formatted = commit.format();
@@ -577,6 +640,7 @@ mod tests {
             CommitType::Feat,
             test_scope("my-scope_v2/feature"),
             test_description("add support"),
+            BreakingChange::No,
         );
         assert_eq!(commit.format(), "feat(my-scope_v2/feature): add support");
     }
@@ -594,6 +658,7 @@ mod tests {
             CommitType::Feat,
             Scope::empty(),
             test_description("add login"),
+            BreakingChange::No,
         );
         // "feat: add login" = 4 + 2 + 9 = 15
         assert_eq!(commit.first_line_len(), 15);
@@ -606,6 +671,7 @@ mod tests {
             CommitType::Feat,
             test_scope("auth"),
             test_description("add login"),
+            BreakingChange::No,
         );
         // "feat(auth): add login" = 4 + 4 + 4 + 9 = 21
         assert_eq!(commit.first_line_len(), 21);
@@ -622,6 +688,7 @@ mod tests {
             CommitType::Feat,
             Scope::parse(&scope_20).unwrap(),
             Description::parse(&desc_44).unwrap(),
+            BreakingChange::No,
         );
         assert!(result.is_ok());
         let commit = result.unwrap();
@@ -650,6 +717,7 @@ mod tests {
             CommitType::Refactor,
             Scope::parse(&scope_30).unwrap(),
             Description::parse(&desc_31).unwrap(),
+            BreakingChange::No,
         );
         assert!(result.is_err());
         assert_eq!(
@@ -672,6 +740,7 @@ mod tests {
             CommitType::Refactor,
             Scope::parse(&scope_30).unwrap(),
             Description::parse(&desc_40).unwrap(),
+            BreakingChange::No,
         );
         assert!(result.is_err());
         assert_eq!(
@@ -690,6 +759,7 @@ mod tests {
             CommitType::Fix,
             Scope::empty(),
             test_description("quick fix"),
+            BreakingChange::No,
         );
         assert!(result.is_ok());
     }
@@ -701,6 +771,7 @@ mod tests {
             CommitType::Feat,
             test_scope("cli"),
             test_description("add feature"),
+            BreakingChange::No,
         );
         assert!(result.is_ok());
     }
@@ -721,8 +792,12 @@ mod tests {
     /// Test new() returns Result type
     #[test]
     fn new_returns_result() {
-        let result =
+        let result = ConventionalCommit::new(
-            ConventionalCommit::new(CommitType::Feat, Scope::empty(), test_description("test"));
+            CommitType::Feat,
+            Scope::empty(),
+            test_description("test"),
+            BreakingChange::No,
+        );
         // Just verify it's a Result by using is_ok()
         assert!(result.is_ok());
     }
@@ -747,7 +822,7 @@ mod tests {
                     None => Scope::empty(),
                 };
                 let desc = Description::parse(*desc_str).unwrap();
-                let commit = ConventionalCommit::new(*commit_type, scope, desc);
+                let commit = ConventionalCommit::new(*commit_type, scope, desc, BreakingChange::No);
                 // new() itself calls git_conventional::Commit::parse internally, so
                 // if this is Ok, SC-002 is satisfied for this case.
                 assert!(
@@ -771,4 +846,273 @@ mod tests {
         assert!(msg.contains("git-conventional"));
         assert!(msg.contains("missing type"));
     }
+
+    /// Breaking change without note and without scope: header gets '!', no footer
+    #[test]
+    fn format_breaking_change_no_note_no_scope() {
+        let commit = test_commit(
+            CommitType::Feat,
+            Scope::empty(),
+            test_description("add login"),
+            BreakingChange::Yes,
+        );
+        assert_eq!(commit.format(), "feat!: add login");
+    }
+
+    /// Breaking change without note and with scope: '!' goes after closing paren
+    #[test]
+    fn format_breaking_change_no_note_with_scope() {
+        let commit = test_commit(
+            CommitType::Feat,
+            test_scope("auth"),
+            test_description("add login"),
+            BreakingChange::Yes,
+        );
+        assert_eq!(commit.format(), "feat(auth)!: add login");
+    }
+
+    /// Breaking change with note and without scope: footer is appended after a blank line
+    #[test]
+    fn format_breaking_change_with_note_no_scope() {
+        let commit = test_commit(
+            CommitType::Feat,
+            Scope::empty(),
+            test_description("drop Node 6"),
+            "Node 6 is no longer supported".into(),
+        );
+        assert_eq!(
+            commit.format(),
+            "feat!: drop Node 6\n\nBREAKING CHANGE: Node 6 is no longer supported",
+        );
+    }
+
+    /// Breaking change with note and with scope: both '!' and footer are present
+    #[test]
+    fn format_breaking_change_with_note_and_scope() {
+        let commit = test_commit(
+            CommitType::Fix,
+            test_scope("api"),
+            test_description("drop Node 6"),
+            "Node 6 is no longer supported".into(),
+        );
+        assert_eq!(
+            commit.format(),
+            "fix(api)!: drop Node 6\n\nBREAKING CHANGE: Node 6 is no longer supported",
+        );
+    }
+
+    /// Display with breaking change delegates to format() (no scope, with note)
+    #[test]
+    fn display_breaking_change_with_note() {
+        let commit = test_commit(
+            CommitType::Feat,
+            Scope::empty(),
+            test_description("drop Node 6"),
+            "Node 6 is no longer supported".into(),
+        );
+        assert_eq!(
+            format!("{}", commit),
+            "feat!: drop Node 6\n\nBREAKING CHANGE: Node 6 is no longer supported",
+        );
+    }
+
+    /// first_line_len() counts the '!' for a breaking change without scope
+    ///
+    /// "feat!: add login" = 4 + 1 + 2 + 9 = 16
+    #[test]
+    fn first_line_len_breaking_change_no_scope() {
+        let commit = test_commit(
+            CommitType::Feat,
+            Scope::empty(),
+            test_description("add login"),
+            BreakingChange::Yes,
+        );
+        assert_eq!(commit.first_line_len(), 16);
+    }
+
+    /// first_line_len() counts the '!' for a breaking change with scope
+    ///
+    /// "feat(auth)!: add login" = 4 + 6 + 1 + 2 + 9 = 22
+    #[test]
+    fn first_line_len_breaking_change_with_scope() {
+        let commit = test_commit(
+            CommitType::Feat,
+            test_scope("auth"),
+            test_description("add login"),
+            BreakingChange::Yes,
+        );
+        assert_eq!(commit.first_line_len(), 22);
+    }
+
+    /// The `!` counts toward the 72-character first-line limit
+    ///
+    /// The inputs below produce exactly 72 chars without a breaking change
+    /// (covered by `exactly_72_characters_accepted`). With `!` they reach
+    /// 73 and must be rejected.
+    #[test]
+    fn breaking_change_exclamation_counts_toward_line_limit() {
+        let scope_20 = "a".repeat(20);
+        let desc_44 = "b".repeat(44);
+        let result = ConventionalCommit::new(
+            CommitType::Feat,
+            Scope::parse(&scope_20).unwrap(),
+            Description::parse(&desc_44).unwrap(),
+            BreakingChange::Yes,
+        );
+        assert!(result.is_err());
+        assert_eq!(
+            result.unwrap_err(),
+            CommitMessageError::FirstLineTooLong {
+                actual: 73,
+                max: 72
+            },
+        );
+    }
+
+    /// Breaking change footer does not count toward the 72-character first-line limit
+    #[test]
+    fn breaking_change_footer_does_not_count_toward_line_limit() {
+        // First line is short; the note itself is long — should still be accepted.
+        let long_note = "x".repeat(200);
+        let result = ConventionalCommit::new(
+            CommitType::Fix,
+            Scope::empty(),
+            test_description("quick fix"),
+            long_note.into(),
+        );
+        assert!(result.is_ok());
+    }
+
+    /// format_preview() static method produces the same result as format() for identical inputs
+    #[test]
+    fn format_preview_matches_format() {
+        let commit = test_commit(
+            CommitType::Feat,
+            test_scope("auth"),
+            test_description("add login"),
+            BreakingChange::No,
+        );
+        let preview = ConventionalCommit::format_preview(
+            commit.commit_type(),
+            commit.scope(),
+            commit.description(),
+            &BreakingChange::No,
+        );
+        assert_eq!(preview, commit.format());
+    }
+
+    /// format_preview() with a breaking-change note produces the full multi-line message
+    #[test]
+    fn format_preview_breaking_change_with_note() {
+        let preview = ConventionalCommit::format_preview(
+            CommitType::Feat,
+            &Scope::empty(),
+            &test_description("drop legacy API"),
+            &"removes legacy endpoint".into(),
+        );
+        assert_eq!(
+            preview,
+            "feat!: drop legacy API\n\nBREAKING CHANGE: removes legacy endpoint"
+        );
+    }
+
+    /// format_preview() with scope and breaking-change note
+    #[test]
+    fn format_preview_breaking_change_with_scope_and_note() {
+        let preview = ConventionalCommit::format_preview(
+            CommitType::Fix,
+            &test_scope("api"),
+            &test_description("drop Node 6"),
+            &"Node 6 is no longer supported".into(),
+        );
+        assert_eq!(
+            preview,
+            "fix(api)!: drop Node 6\n\nBREAKING CHANGE: Node 6 is no longer supported"
+        );
+    }
+
+    /// Breaking-change footer is separated from the header by exactly one blank line
+    #[test]
+    fn format_breaking_change_footer_separator() {
+        let commit = test_commit(
+            CommitType::Fix,
+            Scope::empty(),
+            test_description("drop old API"),
+            "old API removed".into(),
+        );
+        let formatted = commit.format();
+        let parts: Vec<&str> = formatted.splitn(2, "\n\n").collect();
+        assert_eq!(
+            parts.len(),
+            2,
+            "expected header and footer separated by \\n\\n"
+        );
+        assert_eq!(parts[0], "fix!: drop old API");
+        assert_eq!(parts[1], "BREAKING CHANGE: old API removed");
+    }
+
+    /// format() output has no leading or trailing whitespace for any variant
+    #[test]
+    fn format_has_no_surrounding_whitespace() {
+        let no_bc = test_commit(
+            CommitType::Feat,
+            Scope::empty(),
+            test_description("add feature"),
+            BreakingChange::No,
+        );
+        let f = no_bc.format();
+        assert_eq!(
+            f,
+            f.trim(),
+            "format() must not have surrounding whitespace (no breaking change)"
+        );
+
+        let with_note = test_commit(
+            CommitType::Fix,
+            Scope::empty(),
+            test_description("fix bug"),
+            "important migration required".into(),
+        );
+        let f2 = with_note.format();
+        assert_eq!(
+            f2,
+            f2.trim(),
+            "format() must not have surrounding whitespace (with note)"
+        );
+    }
+
+    /// All commit types format correctly with breaking change and no note
+    #[test]
+    fn all_commit_types_format_with_breaking_change_no_note() {
+        let desc = test_description("test change");
+
+        let expected_formats = [
+            (CommitType::Feat, "feat!: test change"),
+            (CommitType::Fix, "fix!: test change"),
+            (CommitType::Docs, "docs!: test change"),
+            (CommitType::Style, "style!: test change"),
+            (CommitType::Refactor, "refactor!: test change"),
+            (CommitType::Perf, "perf!: test change"),
+            (CommitType::Test, "test!: test change"),
+            (CommitType::Build, "build!: test change"),
+            (CommitType::Ci, "ci!: test change"),
+            (CommitType::Chore, "chore!: test change"),
+            (CommitType::Revert, "revert!: test change"),
+        ];
+
+        for (commit_type, expected) in expected_formats {
+            let commit = test_commit(
+                commit_type,
+                Scope::empty(),
+                desc.clone(),
+                BreakingChange::Yes,
+            );
+            assert_eq!(
+                commit.format(),
+                expected,
+                "Format should be correct for {:?} with breaking change",
+                commit_type
+            );
+        }
+    }
 }

src/commit/types/mod.rs

@@ -1,3 +1,9 @@
+mod footer;
+pub use footer::Footer;
+
+mod breaking_change;
+pub use breaking_change::BreakingChange;
+
 mod commit_type;
 pub use commit_type::CommitType;
 

src/commit/types/scope.rs

@@ -18,9 +18,9 @@ impl Scope {
         if value.is_empty() {
             return Ok(Self::empty());
         }
-        if value.len() > Self::MAX_LENGTH {
+        if value.chars().count() > Self::MAX_LENGTH {
             return Err(ScopeError::TooLong {
-                actual: value.len(),
+                actual: value.chars().count(),
                 max: Self::MAX_LENGTH,
             });
         }
@@ -458,10 +458,6 @@ mod tests {
         assert!(msg.contains("30"));
     }
 
-    // =========================================================================
-    // header_segment() / header_segment_len() tests
-    // =========================================================================
-
     /// Test header_segment() returns empty string for empty scope
     #[test]
     fn header_segment_empty_scope_returns_empty_string() {
@@ -517,4 +513,50 @@ mod tests {
             );
         }
     }
+
+    /// A scope whose byte count exceeds MAX_LENGTH but whose char
+    /// count does not must be rejected with InvalidCharacter, not
+    /// TooLong.
+    ///
+    /// Before the fix the byte-based `.len()` check fired first,
+    /// producing a misleading "too long" error for a string that is
+    /// actually within the limit.
+    #[test]
+    fn length_limit_uses_char_count_not_byte_count() {
+        // "ñ" is 2 bytes in UTF-8; 16 × "ñ" = 16 chars, 32 bytes.
+        // char count 16 ≤ 30  →  length check passes
+        // regex rejects "ñ"   →  should return InvalidCharacter, not TooLong
+        let input = "ñ".repeat(16);
+        assert_eq!(input.chars().count(), 16, "sanity: 16 chars");
+        assert_eq!(input.len(), 32, "sanity: 32 bytes");
+
+        let result = Scope::parse(&input);
+        assert!(result.is_err());
+        assert_eq!(
+            result.unwrap_err(),
+            ScopeError::InvalidCharacter('ñ'),
+            "expected InvalidCharacter('ñ') for a 16-char / 32-byte input, not TooLong",
+        );
+    }
+
+    /// The actual length reported in TooLong must be the char count,
+    /// not the byte count.
+    ///
+    /// "a".repeat(30) + "é" is 31 chars and 32 bytes. The length
+    /// check should fire on char count (31 > 30) and report actual =
+    /// 31.
+    #[test]
+    fn too_long_error_actual_reports_char_count_not_byte_count() {
+        // 30 ASCII 'a' + 1 two-byte 'é' = 31 chars, 32 bytes
+        let input = "a".repeat(30) + "é";
+        assert_eq!(input.chars().count(), 31, "sanity: 31 chars");
+        assert_eq!(input.len(), 32, "sanity: 32 bytes");
+
+        let result = Scope::parse(&input);
+        assert_eq!(
+            result.unwrap_err(),
+            ScopeError::TooLong { actual: 31, max: 30 },
+            "actual should be the char count (31), not the byte count (32)",
+        );
+    }
 }

src/lib.rs

@@ -6,8 +6,8 @@ mod prompts;
 
 pub use crate::{
     commit::types::{
-        CommitMessageError, CommitType, ConventionalCommit, Description, DescriptionError, Scope,
+        BreakingChange, CommitMessageError, CommitType, ConventionalCommit, Description,
-        ScopeError,
+        DescriptionError, Scope, ScopeError,
     },
     error::Error,
     jj::{JjExecutor, lib_executor::JjLib},

src/prompts/mock.rs

@@ -8,7 +8,7 @@
 use std::sync::{Arc, Mutex};
 
 use crate::{
-    commit::types::{CommitType, Description, Scope},
+    commit::types::{BreakingChange, CommitType, Description, Scope},
     error::Error,
     prompts::prompter::Prompter,
 };
@@ -19,6 +19,7 @@ enum MockResponse {
     CommitType(CommitType),
     Scope(Scope),
     Description(Description),
+    BreakingChange(BreakingChange),
     Confirm(bool),
     Error(Error),
 }
@@ -70,6 +71,15 @@ impl MockPrompts {
         self
     }
 
+    /// Configure the mock to return a specific breaking change response
+    pub fn with_breaking_change(self, breaking_change: BreakingChange) -> Self {
+        self.responses
+            .lock()
+            .unwrap()
+            .push(MockResponse::BreakingChange(breaking_change));
+        self
+    }
+
     /// Configure the mock to return a specific confirmation response
     pub fn with_confirm(self, confirm: bool) -> Self {
         self.responses
@@ -112,6 +122,14 @@ impl MockPrompts {
             .contains(&"input_description".to_string())
     }
 
+    /// Check if input_breaking_change was called
+    pub fn was_breaking_change_called(&self) -> bool {
+        self.prompts_called
+            .lock()
+            .unwrap()
+            .contains(&"input_breaking_change".to_string())
+    }
+
     /// Check if confirm_apply was called
     pub fn was_confirm_called(&self) -> bool {
         self.prompts_called
@@ -166,6 +184,19 @@ impl Prompter for MockPrompts {
         }
     }
 
+    fn input_breaking_change(&self) -> Result<BreakingChange, Error> {
+        self.prompts_called
+            .lock()
+            .unwrap()
+            .push("input_breaking_change".to_string());
+
+        match self.responses.lock().unwrap().remove(0) {
+            MockResponse::BreakingChange(bc) => Ok(bc),
+            MockResponse::Error(e) => Err(e),
+            _ => panic!("MockPrompts: Expected BreakingChange response, got different type"),
+        }
+    }
+
     fn confirm_apply(&self, _message: &str) -> Result<bool, Error> {
         self.prompts_called
             .lock()
@@ -281,4 +312,64 @@ mod tests {
         let mock = MockPrompts::new();
         assert!(mock.emitted_messages().is_empty());
     }
+
+    #[test]
+    fn mock_input_breaking_change_no() {
+        let mock = MockPrompts::new().with_breaking_change(BreakingChange::No);
+        let result = mock.input_breaking_change();
+        assert!(result.is_ok());
+        assert_eq!(result.unwrap(), BreakingChange::No);
+        assert!(mock.was_breaking_change_called());
+    }
+
+    #[test]
+    fn mock_input_breaking_change_yes_no_note() {
+        let mock = MockPrompts::new().with_breaking_change(BreakingChange::Yes);
+        let result = mock.input_breaking_change();
+        assert!(result.is_ok());
+        assert_eq!(result.unwrap(), BreakingChange::Yes);
+        assert!(mock.was_breaking_change_called());
+    }
+
+    #[test]
+    fn mock_input_breaking_change_yes_with_note() {
+        let mock = MockPrompts::new().with_breaking_change("removes old API".into());
+        let result = mock.input_breaking_change();
+        assert!(result.is_ok());
+        assert_eq!(
+            result.unwrap(),
+            BreakingChange::WithNote("removes old API".into())
+        );
+        assert!(mock.was_breaking_change_called());
+    }
+
+    #[test]
+    fn mock_input_breaking_change_error() {
+        let mock = MockPrompts::new().with_error(Error::Cancelled);
+        let result = mock.input_breaking_change();
+        assert!(result.is_err());
+        assert!(matches!(result.unwrap_err(), Error::Cancelled));
+    }
+
+    #[test]
+    fn mock_tracks_breaking_change_call() {
+        let mock = MockPrompts::new()
+            .with_commit_type(CommitType::Fix)
+            .with_scope(Scope::empty())
+            .with_description(Description::parse("test").unwrap())
+            .with_breaking_change(BreakingChange::No)
+            .with_confirm(true);
+
+        mock.select_commit_type().unwrap();
+        mock.input_scope().unwrap();
+        mock.input_description().unwrap();
+        mock.input_breaking_change().unwrap();
+        mock.confirm_apply("test").unwrap();
+
+        assert!(mock.was_commit_type_called());
+        assert!(mock.was_scope_called());
+        assert!(mock.was_description_called());
+        assert!(mock.was_breaking_change_called());
+        assert!(mock.was_confirm_called());
+    }
 }

src/prompts/prompter.rs

@@ -5,8 +5,10 @@
 //! [`CommitWorkflow`](super::CommitWorkflow) to use real interactive prompts
 //! in production while accepting mock implementations in tests.
 
+use inquire::{Confirm, Text};
+
 use crate::{
-    commit::types::{CommitType, Description, Scope},
+    commit::types::{BreakingChange, CommitType, Description, Scope},
     error::Error,
 };
 
@@ -24,6 +26,9 @@ pub trait Prompter: Send + Sync {
     /// Prompt the user to input a required description
     fn input_description(&self) -> Result<Description, Error>;
 
+    /// Prompt the user for breaking change
+    fn input_breaking_change(&self) -> Result<BreakingChange, Error>;
+
     /// Prompt the user to confirm applying the commit message
     fn confirm_apply(&self, message: &str) -> Result<bool, Error>;
 
@@ -34,6 +39,18 @@ pub trait Prompter: Send + Sync {
     fn emit_message(&self, msg: &str);
 }
 
+fn format_message_box(message: &str) -> String {
+    let preview_width = 72 + 2; // max width + space padding
+    let mut lines: Vec<String> = Vec::new();
+    lines.push(format!("┌{}┐", "─".repeat(preview_width)));
+    for line in message.split("\n") {
+        let padding = 72_usize.saturating_sub(line.chars().count());
+        lines.push(format!("│ {line}{:padding$} │", ""));
+    }
+    lines.push(format!("└{}┘", "─".repeat(preview_width)));
+    lines.join("\n")
+}
+
 /// Production implementation of [`Prompter`] using the `inquire` crate
 #[derive(Debug)]
 pub struct RealPrompts;
@@ -137,18 +154,30 @@ impl Prompter for RealPrompts {
         }
     }
 
+    fn input_breaking_change(&self) -> Result<BreakingChange, Error> {
+        if !Confirm::new("Does this revision include a breaking change?")
+            .with_default(false)
+            .prompt()
+            .map_err(|_| Error::Cancelled)?
+        {
+            return Ok(BreakingChange::No);
+        }
+        let answer = Text::new("Enter the description of the breaking change:")
+            .with_help_message("Enter an empty message to skip creating a message footer")
+            .prompt()
+            .map_err(|_| Error::Cancelled)?;
+        let trimmed = answer.trim();
+        Ok(trimmed.into())
+    }
+
     fn confirm_apply(&self, message: &str) -> Result<bool, Error> {
         use inquire::Confirm;
 
         // Show preview
-        println!();
+        println!(
-        println!("📝 Commit Message Preview:");
+            "\n📝 Commit Message Preview:\n{}\n",
-        println!("┌────────────────────────────────────────────────────────────────────────┐");
+            format_message_box(message)
-        // Pad with spaces to fill the box
+        );
-        let padding = 72_usize.saturating_sub(message.chars().count()) - 1;
-        println!("│ {message}{:padding$}│", "");
-        println!("└────────────────────────────────────────────────────────────────────────┘");
-        println!();
 
         // Get confirmation
         Confirm::new("Apply this commit message?")
@@ -174,4 +203,71 @@ mod tests {
         fn _accepts_prompter(_p: impl Prompter) {}
         _accepts_prompter(real);
     }
+
+    /// Top border uses exactly preview_width (74) dashes; bottom likewise
+    #[test]
+    fn format_message_box_borders() {
+        let result = format_message_box("hello");
+        let lines: Vec<&str> = result.split('\n').collect();
+        let dashes = "─".repeat(74);
+        assert_eq!(lines[0], format!("┌{dashes}┐"));
+        assert_eq!(lines[lines.len() - 1], format!("└{dashes}┘"));
+    }
+
+    /// A single-line message produces exactly 3 rows: top, content, bottom
+    #[test]
+    fn format_message_box_single_line_row_count() {
+        let result = format_message_box("feat: add login");
+        assert_eq!(result.split('\n').count(), 3);
+    }
+
+    /// A message with one `\n` produces 4 rows: top, two content, bottom
+    #[test]
+    fn format_message_box_multi_line_row_count() {
+        let result = format_message_box("feat: add login\nsecond line");
+        assert_eq!(result.split('\n').count(), 4);
+    }
+
+    /// A breaking-change message (`\n\n`) produces an empty content row for the blank line
+    #[test]
+    fn format_message_box_blank_separator_line() {
+        let msg = "feat!: drop old API\n\nBREAKING CHANGE: removed";
+        let result = format_message_box(msg);
+        assert_eq!(result.split('\n').count(), 5); // top + 3 content + bottom
+    }
+
+    /// All output rows have identical char counts (the box is rectangular)
+    #[test]
+    fn format_message_box_all_rows_same_width() {
+        let msg = "feat(auth): add login\n\nBREAKING CHANGE: old API removed";
+        let result = format_message_box(msg);
+        let widths: Vec<usize> = result.split('\n').map(|l| l.chars().count()).collect();
+        let expected = widths[0];
+        assert!(
+            widths.iter().all(|&w| w == expected),
+            "rows have differing widths: {:?}",
+            widths
+        );
+    }
+
+    /// An empty message produces a single fully-padded content row
+    #[test]
+    fn format_message_box_empty_message() {
+        let result = format_message_box("");
+        let lines: Vec<&str> = result.split('\n').collect();
+        assert_eq!(lines.len(), 3);
+        // "│ " + 72 spaces + " │" = 76 chars
+        let expected = format!("│ {:72} │", "");
+        assert_eq!(lines[1], expected);
+    }
+
+    /// A line of exactly 72 characters leaves no right-hand padding
+    #[test]
+    fn format_message_box_line_exactly_72_chars() {
+        let line_72 = "a".repeat(72);
+        let result = format_message_box(&line_72);
+        let lines: Vec<&str> = result.split('\n').collect();
+        let expected = format!("│ {line_72} │");
+        assert_eq!(lines[1], expected);
+    }
 }

src/prompts/workflow.rs

@@ -4,7 +4,9 @@
 //! creating a conventional commit message using interactive prompts.
 
 use crate::{
-    commit::types::{CommitMessageError, CommitType, ConventionalCommit, Description, Scope},
+    commit::types::{
+        BreakingChange, CommitMessageError, CommitType, ConventionalCommit, Description, Scope,
+    },
     error::Error,
     jj::JjExecutor,
     prompts::prompter::{Prompter, RealPrompts},
@@ -52,30 +54,19 @@ impl<J: JjExecutor, P: Prompter> CommitWorkflow<J, P> {
     /// - Repository operation fails
     /// - Message validation fails
     pub async fn run(&self) -> Result<(), Error> {
-        // Verify we're in a jj repository
         if !self.executor.is_repository().await? {
             return Err(Error::NotARepository);
         }
-
-        // Step 1: Select commit type (kept across retries)
         let commit_type = self.type_selection().await?;
-
-        // Steps 2–4 loop: re-prompt scope and description when the combined
-        // first line would exceed 72 characters (issue 3.4).
         loop {
-            // Step 2: Input scope (optional)
             let scope = self.scope_input().await?;
-
-            // Step 3: Input description (required)
             let description = self.description_input().await?;
-
+            let breaking_change = self.breaking_change_input().await?;
-            // Step 4: Preview and confirm
             match self
-                .preview_and_confirm(commit_type, scope, description)
+                .preview_and_confirm(commit_type, scope, description, breaking_change)
                 .await
             {
                 Ok(conventional_commit) => {
-                    // Step 5: Apply the message
                     self.executor
                         .describe(&conventional_commit.to_string())
                         .await?;
@@ -99,35 +90,49 @@ impl<J: JjExecutor, P: Prompter> CommitWorkflow<J, P> {
 
     /// Prompt user to input an optional scope
     ///
-    /// Returns Ok(Scope) with the validated scope, or Error::Cancelled if user cancels
+    /// Returns Ok(Scope) with the validated scope, or
+    /// Error::Cancelled if user cancels
     async fn scope_input(&self) -> Result<Scope, Error> {
         self.prompts.input_scope()
     }
 
     /// Prompt user to input a required description
     ///
-    /// Returns Ok(Description) with the validated description, or Error::Cancelled if user cancels
+    /// Returns Ok(Description) with the validated description, or
+    /// Error::Cancelled if user cancels
     async fn description_input(&self) -> Result<Description, Error> {
         self.prompts.input_description()
     }
 
+    /// Prompt user for breaking change
+    ///
+    /// Returns Ok(BreakingChange) with the validated breaking change,
+    /// or Error::Cancel if user cancels
+    async fn breaking_change_input(&self) -> Result<BreakingChange, Error> {
+        self.prompts.input_breaking_change()
+    }
+
     /// Preview the formatted conventional commit message and get user confirmation
     ///
-    /// This method also validates that the complete first line doesn't exceed 72 characters
+    /// This method also validates that the complete first line
+    /// doesn't exceed 72 characters
     async fn preview_and_confirm(
         &self,
         commit_type: CommitType,
         scope: Scope,
         description: Description,
+        breaking_change: BreakingChange,
     ) -> Result<ConventionalCommit, Error> {
         // Format the message for preview
-        let message = ConventionalCommit::format_preview(commit_type, &scope, &description);
+        let message =
+            ConventionalCommit::format_preview(commit_type, &scope, &description, &breaking_change);
 
         // Try to build the conventional commit (this validates the 72-char limit)
         let conventional_commit: ConventionalCommit = match ConventionalCommit::new(
             commit_type,
             scope.clone(),
             description.clone(),
+            breaking_change,
         ) {
             Ok(cc) => cc,
             Err(CommitMessageError::FirstLineTooLong { actual, max }) => {
@@ -252,9 +257,9 @@ mod tests {
         let commit_type = CommitType::Feat;
         let scope = Scope::empty();
         let description = Description::parse("test description").unwrap();
-
+        let breaking_change = BreakingChange::No;
         let result = workflow
-            .preview_and_confirm(commit_type, scope, description)
+            .preview_and_confirm(commit_type, scope, description, breaking_change)
             .await;
         assert!(result.is_ok());
     }
@@ -299,6 +304,7 @@ mod tests {
             .with_commit_type(CommitType::Feat)
             .with_scope(Scope::empty())
             .with_description(Description::parse("add new feature").unwrap())
+            .with_breaking_change(BreakingChange::Yes)
             .with_confirm(true);
 
         // Create workflow with both mocks
@@ -330,6 +336,7 @@ mod tests {
             .with_commit_type(CommitType::Fix)
             .with_scope(Scope::parse("api").unwrap())
             .with_description(Description::parse("fix bug").unwrap())
+            .with_breaking_change(BreakingChange::No)
             .with_confirm(false); // User cancels at confirmation
 
         let workflow = CommitWorkflow::with_prompts(mock_executor, mock_prompts);
@@ -352,9 +359,11 @@ mod tests {
             // First iteration: scope + description exceed 72 chars combined
             .with_scope(Scope::parse("very-long-scope-name").unwrap())
             .with_description(Description::parse("a".repeat(45)).unwrap())
+            .with_breaking_change(BreakingChange::No)
             // Second iteration: short enough to succeed
             .with_scope(Scope::empty())
             .with_description(Description::parse("short description").unwrap())
+            .with_breaking_change(BreakingChange::No)
             .with_confirm(true);
 
         // Clone before moving into workflow so we can inspect emitted messages after
@@ -447,6 +456,7 @@ mod tests {
                 .with_commit_type(*commit_type)
                 .with_scope(Scope::empty())
                 .with_description(Description::parse("test").unwrap())
+                .with_breaking_change(BreakingChange::Yes)
                 .with_confirm(true);
 
             let workflow = CommitWorkflow::with_prompts(
@@ -468,6 +478,7 @@ mod tests {
             .with_commit_type(CommitType::Feat)
             .with_scope(Scope::empty())
             .with_description(Description::parse("test").unwrap())
+            .with_breaking_change(BreakingChange::Yes)
             .with_confirm(true);
 
         let workflow = CommitWorkflow::with_prompts(
@@ -484,6 +495,7 @@ mod tests {
             .with_commit_type(CommitType::Feat)
             .with_scope(Scope::parse("api").unwrap())
             .with_description(Description::parse("test").unwrap())
+            .with_breaking_change(BreakingChange::No)
             .with_confirm(true);
 
         let workflow = CommitWorkflow::with_prompts(
@@ -509,4 +521,99 @@ mod tests {
         let workflow = CommitWorkflow::with_prompts(mock_executor, mock_prompts);
         assert!(matches!(workflow, CommitWorkflow { .. }));
     }
+
+    /// Preview_and_confirm must forward BreakingChange::Yes to
+    /// ConventionalCommit::new(), producing a commit whose string
+    /// contains '!'.
+    ///
+    /// Before the fix the parameter was ignored and
+    /// BreakingChange::No was hard-coded, so a confirmed
+    /// breaking-change commit was silently applied without the '!'
+    /// marker.
+    #[tokio::test]
+    async fn preview_and_confirm_forwards_breaking_change_yes() {
+        let mock_executor = MockJjExecutor::new();
+        let mock_prompts = MockPrompts::new().with_confirm(true);
+        let workflow = CommitWorkflow::with_prompts(mock_executor, mock_prompts);
+
+        let result = workflow
+            .preview_and_confirm(
+                CommitType::Feat,
+                Scope::empty(),
+                Description::parse("remove old API").unwrap(),
+                BreakingChange::Yes,
+            )
+            .await;
+
+        assert!(result.is_ok(), "expected Ok, got: {:?}", result);
+        let message = result.unwrap().to_string();
+        assert!(
+            message.contains("feat!:"),
+            "expected '!' marker in described message, got: {:?}",
+            message,
+        );
+    }
+
+    /// Preview_and_confirm must forward BreakingChange::WithNote,
+    /// producing a commit with both the '!' header marker and the
+    /// BREAKING CHANGE footer.
+    #[tokio::test]
+    async fn preview_and_confirm_forwards_breaking_change_with_note() {
+        let mock_executor = MockJjExecutor::new();
+        let mock_prompts = MockPrompts::new().with_confirm(true);
+        let workflow = CommitWorkflow::with_prompts(mock_executor, mock_prompts);
+
+        let breaking_change: BreakingChange = "removes legacy endpoint".into();
+        let result = workflow
+            .preview_and_confirm(
+                CommitType::Feat,
+                Scope::empty(),
+                Description::parse("drop legacy API").unwrap(),
+                breaking_change,
+            )
+            .await;
+
+        assert!(result.is_ok(), "expected Ok, got: {:?}", result);
+        let message = result.unwrap().to_string();
+        assert!(
+            message.contains("feat!:"),
+            "expected '!' header marker in message, got: {:?}",
+            message,
+        );
+        assert!(
+            message.contains("BREAKING CHANGE:"),
+            "expected BREAKING CHANGE footer in message, got: {:?}",
+            message,
+        );
+    }
+
+    /// The message passed to executor.describe() must include the '!'
+    /// marker when the user selects a breaking change.
+    ///
+    /// This test exercises the full run() path and inspects what was
+    /// actually handed to the jj executor, which is the authoritative
+    /// check that the described commit is correct.
+    #[tokio::test]
+    async fn full_workflow_describes_commit_with_breaking_change_marker() {
+        let mock_executor = MockJjExecutor::new().with_is_repo_response(Ok(true));
+        let mock_prompts = MockPrompts::new()
+            .with_commit_type(CommitType::Feat)
+            .with_scope(Scope::empty())
+            .with_description(Description::parse("remove old API").unwrap())
+            .with_breaking_change(BreakingChange::Yes)
+            .with_confirm(true);
+
+        let workflow = CommitWorkflow::with_prompts(mock_executor, mock_prompts);
+        let result: Result<(), Error> = workflow.run().await;
+
+        assert!(result.is_ok(), "expected workflow to succeed, got: {:?}", result);
+
+        let messages = workflow.executor.describe_messages();
+        assert_eq!(messages.len(), 1, "expected exactly one describe() call");
+        assert!(
+            messages[0].contains("feat!:"),
+            "expected '!' marker in the described message, got: {:?}",
+            messages[0],
+        );
+    }
 }

tests/cli.rs

@@ -1,6 +1,6 @@
 use assert_fs::TempDir;
 #[cfg(feature = "test-utils")]
-use jj_cz::{CommitType, Description, MockPrompts, Scope};
+use jj_cz::{BreakingChange, CommitType, Description, MockPrompts, Scope};
 use jj_cz::{CommitWorkflow, Error, JjLib};
 #[cfg(feature = "test-utils")]
 use jj_lib::{config::StackedConfig, settings::UserSettings, workspace::Workspace};
@@ -27,6 +27,7 @@ async fn test_happy_path_integration() {
         .with_commit_type(CommitType::Feat)
         .with_scope(Scope::empty())
         .with_description(Description::parse("add new feature").unwrap())
+        .with_breaking_change(BreakingChange::No)
         .with_confirm(true);
 
     // Create a mock executor that tracks calls
@@ -85,6 +86,7 @@ async fn test_cancellation() {
         .with_commit_type(CommitType::Feat)
         .with_scope(Scope::empty())
         .with_description(Description::parse("test").unwrap())
+        .with_breaking_change(BreakingChange::No)
         .with_confirm(true);
     let workflow = CommitWorkflow::with_prompts(executor, mock_prompts);