diff --git a/.github/workflows/semantic-line-breaks.yml b/.github/workflows/semantic-line-breaks.yml
new file mode 100644
index 00000000..d6cc0a0c
--- /dev/null
+++ b/.github/workflows/semantic-line-breaks.yml
@@ -0,0 +1,28 @@
+---
+name: Semantic line breaks
+
+on:
+  pull_request:
+  push:
+    branches: [main]
+
+permissions: {}
+
+jobs:
+  semantic-line-breaks:
+    name: Snapper
+    runs-on: ubuntu-latest
+    steps:
+      - name: Check out repository
+        uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1  # v7.0.1
+        with:
+          persist-credentials: false
+      - uses: astral-sh/setup-uv@20cfd1bf945f4377ade1205e4dbc17946fc9a30d  # v10.0.1
+      - name: Check semantic line breaks
+        run: >-
+          git ls-files -z -- '*.md' '*.rst' ':(exclude)vendored/**'
+          | xargs -0 --no-run-if-empty
+          uvx
+          --from
+          'snapper-fmt==0.11.0'
+          snapper --max-width 0 --check
diff --git a/.vale.ini b/.vale.ini
index e502c137..68e95e9b 100644
--- a/.vale.ini
+++ b/.vale.ini
@@ -4,7 +4,8 @@ MinAlertLevel = error
 Packages = https://github.com/tbhb/vale-ai-tells/releases/download/v1.29.0/ai-tells.zip
 
 [*.{rst,md}]
-BasedOnStyles = ai-tells
+BasedOnStyles = ai-tells, snapper
+snapper.SemanticLineBreaks = error
 
 # These rules misclassify established technical, example, or release-note prose
 # in this repository. All other ai-tells rules remain enforced.
diff --git a/docs/source/unreleased.rst b/docs/source/unreleased.rst
index 22ac7472..d297d197 100644
--- a/docs/source/unreleased.rst
+++ b/docs/source/unreleased.rst
@@ -1,8 +1,8 @@
 Unreleased changes
 ==================
 
-Changes that have landed on the main branch but are not yet part of a
-tagged release.  These entries are assembled into the
+Changes that have landed on the main branch but are not yet part of a tagged release.
+These entries are assembled into the
 :doc:`changelog` when the next release is published.
 
 .. towncrier-draft-entries::
diff --git a/styles/snapper/LongProseLine.yml b/styles/snapper/LongProseLine.yml
new file mode 100644
index 00000000..1f5029f7
--- /dev/null
+++ b/styles/snapper/LongProseLine.yml
@@ -0,0 +1,11 @@
+---
+# Vale style rule: flag prose lines that are suspiciously long.
+# Likely indicates wrapped text that should use semantic line breaks.
+extends: existence
+message: Prose line exceeds 120 characters. Consider running 'snapper' to break at
+  sentence boundaries.
+level: suggestion
+scope: paragraph
+# This catches lines > 120 chars that contain a period followed by a space.
+# Pure code or URLs are less likely to match.
+tokens: ['.{120,}']
diff --git a/styles/snapper/SemanticLineBreaks.yml b/styles/snapper/SemanticLineBreaks.yml
new file mode 100644
index 00000000..3f4edaf5
--- /dev/null
+++ b/styles/snapper/SemanticLineBreaks.yml
@@ -0,0 +1,11 @@
+---
+# Vale style rule: flag lines containing multiple sentences.
+# These should be broken at sentence boundaries.
+extends: existence
+message: Line contains multiple sentences. Run 'snapper --check' or reformat with
+  'snapper --in-place'.
+level: warning
+scope: paragraph
+# Match a sentence-ending punctuation followed by a space and a capital letter
+# on the same line (indicating two sentences on one line).
+tokens: ['[.!?]\\s+[A-Z]']