diff --git a/action.yml b/action.yml
new file mode 100644
index 0000000..7210e55
--- /dev/null
+++ b/action.yml
@@ -0,0 +1,57 @@
+name: 'cargo-semver-checks'
+description: 'Ensure your Rust crate's public API follows semantic versioning'
+inputs:
+  crate-name:
+    description: 'The crate whose API to check for semver'
+    required: false
+    default: ''
+  version-tag-prefix:
+    description: 'The prefix to use for the git tag for a version; the default "v" creates tags like "v1.0.0"'
+    required: false
+    default: 'v'
+runs:
+  using: "composite"
+  runs-on: ubuntu-latest
+  steps:
+    - name: Install rust
+      uses: actions-rs/toolchain@v1
+      with:
+        toolchain: nightly
+        profile: minimal
+    - run: |
+      # Colorize output, since GitHub Actions terminals support color.
+      export CARGO_TERM_COLOR=always
+
+      # Record the current git sha, so we can come back to it after generating the baseline.
+      export CURRENT_GIT_SHA="$(git rev-parse HEAD)"
+
+      # Ensure this action's scripts are available to run on the path.
+      echo "${{ github.action_path }}" >> $GITHUB_PATH
+
+      export PACKAGE_NAME="${{ inputs.crate-name }}"
+      if [[ "$PACKAGE_NAME" == '' ]]; then
+        export PACKAGE_NAME="$(find_workspace_crates.sh)"
+      fi
+      export PACKAGE_NAME_WITH_UNDERSCORES="$(echo $"PACKAGE_NAME" | tr '-' '_')"
+
+      # Switch to the tag for the correct baseline version,
+      # then build rustdoc JSON.
+      #
+      # We *do not* want to record and reuse the target directory path
+      # across different git commits, since it may be at a different location
+      # in different commits.
+      git checkout "${{ inputs.version-tag-prefix }}$(find_comparison_version.sh "$PACKAGE_NAME")"
+      cargo +nightly rustdoc -- -Zunstable-options --output-format json
+      mv "$(cargo metadata --format-version 1 | jq -r .target_directory)/doc/$PACKAGE_NAME_WITH_UNDERSCORES.json" /tmp/baseline.json
+
+      # Return to the original git sha.
+      git checkout "$CURRENT_GIT_SHA"
+
+      # Build rustdoc JSON for the current version, and move it to /tmp/
+      # so it doesn't get overwritten by the baseline build.
+      cargo +nightly rustdoc -- -Zunstable-options --output-format json
+      mv "$(cargo metadata --format-version 1 | jq -r .target_directory)/doc/$PACKAGE_NAME_WITH_UNDERSCORES.json" /tmp/current.json
+
+      # Check for semver violations.
+      cargo install cargo-semver-checks
+      cargo semver-checks check-release --current /tmp/current.json --baseline /tmp/baseline.json
diff --git a/find_comparison_version.sh b/find_comparison_version.sh
new file mode 100644
index 0000000..8c67569
--- /dev/null
+++ b/find_comparison_version.sh
@@ -0,0 +1,60 @@
+#!/usr/bin/env bash
+
+# Script requirements:
+# - curl
+# - jq
+# - sort with `-V` flag, available in `coreutils-7`
+#   On macOS this may require `brew install coreutils`.
+
+# Fail on first error, on undefined variables, and on failures in pipelines.
+set -euo pipefail
+
+# Go to the repo root directory.
+cd "$(git rev-parse --show-toplevel)"
+
+# The first argument should be the name of a crate.
+CRATE_NAME="$1"
+
+CURRENT_VERSION="$( \
+    cargo metadata --format-version 1 | \
+    jq --arg crate_name "$CRATE_NAME" --exit-status -r \
+        '.packages[] | select(.name == $crate_name) | .version' \
+)" || (echo >&2 "No crate named $CRATE_NAME found in workspace."; exit 1)
+echo >&2 "Crate $CRATE_NAME current version: $CURRENT_VERSION"
+
+# The leading whitespace is important! With it, we know that every version is both
+# preceded by and followed by whitespace. We use this fact to avoid matching
+# on substrings of versions.
+EXISTING_VERSIONS="
+$( \
+    curl 2>/dev/null "https://crates.io/api/v1/crates/$CRATE_NAME" | \
+    jq --exit-status -r .versions[].num \
+)"
+echo >&2 -e "Versions on crates.io:$EXISTING_VERSIONS\n"
+
+# Use version sort (sort -V) to get all versions in ascending order, then use grep to:
+# - grab the first line that matches the current version (--max-count=1)
+# - only match full lines (--line-regexp)
+# - get one line of leading context (-B 1) i.e. the immediately-smaller version, if one exists
+# - explicitly opt out of trailing context lines (-A 0)
+# Finally, use `head` to output only the first of the up-to-two lines output.
+# Now, either:
+# - two lines were output, and we grabbed the immediately-smaller version, or
+# - one line was output with only our version, because there was no immediately-smaller version,
+#   and we grabbed that one. We sort this out with the subsequent conditional.
+OUTPUT="$( \
+    echo -e "$CURRENT_VERSION$EXISTING_VERSIONS" | \
+    sort -V | \
+    grep -B 1 -A 0 --line-regexp --max-count=1 "$CURRENT_VERSION" | \
+    head -n 1 \
+)"
+
+if [[ "$OUTPUT" == "$CURRENT_VERSION" ]]; then
+    echo >&2 "There is no suitable comparison version."
+    echo >&2 \
+        "The current version $CURRENT_VERSION is smaller than any version published on crates.io"
+    exit 1
+fi
+
+echo "Comparison version: $OUTPUT" >&2
+echo "$OUTPUT"
diff --git a/find_workspace_crates.sh b/find_workspace_crates.sh
new file mode 100644
index 0000000..c4eff23
--- /dev/null
+++ b/find_workspace_crates.sh
@@ -0,0 +1,23 @@
+#!/usr/bin/env bash
+
+# Script requirements:
+# - jq
+
+# Fail on first error, on undefined variables, and on failures in pipelines.
+set -euo pipefail
+
+# Go to the repo root directory.
+cd "$(git rev-parse --show-toplevel)"
+
+crates="$(cargo metadata --format-version 1 | \
+    jq --exit-status -r \
+        '.workspace_members[] as $key | .packages[] | select(.id == $key) | .name')"
+crate_count="$(echo -e "${crates}" | wc -l)"
+
+if [[ "$crate_count" == "1" ]]; then
+    echo -e "${crates}"
+    exit 0
+else
+    echo >&2 "Multiple crates in workspace, please specify a crate in the 'crate-name' setting."
+    exit 1
+fi