diff --git a/.github/workflows/test.yaml b/.github/workflows/test.yaml
index 82f3a98..b5f2ea4 100644
--- a/.github/workflows/test.yaml
+++ b/.github/workflows/test.yaml
@@ -750,3 +750,114 @@ jobs:
             exit 1
           fi
           echo "PASS"
+
+  pr_comment_free_review_url_preserves_https_base:
+    runs-on: ubuntu-latest
+    name: Test pr-comment free review URL keeps the https:// scheme on URL-style base
+    # Regression test for a bug surfaced by oasdiff-test/test#59: when the
+    # workflow passes a raw.githubusercontent.com URL as the base input
+    # (as the integration-test repo does), the entrypoint's
+    #   base_path=$(echo "$base" | sed 's/.*://')
+    # stripped the "https:" prefix and left a broken "//raw.github..."
+    # URL. The free /review page then tried to GET that broken URL and
+    # rendered "access denied" with no useful diagnostic — because the
+    # generic access-denied screen masked the real cause (malformed URL).
+    # The fix passes URL-shaped inputs through unchanged.
+    steps:
+      - uses: actions/checkout@v6
+      - name: Stub oasdiff to a no-changes spec
+        run: |
+          set -euo pipefail
+          mkdir -p /tmp/stub
+          cat > /tmp/stub/oasdiff <<'STUB'
+          #!/bin/sh
+          echo '[]'
+          STUB
+          chmod +x /tmp/stub/oasdiff
+
+          mkdir -p /tmp/run
+          export GITHUB_REF=refs/pull/123/merge
+          export GITHUB_REPOSITORY=oasdiff-test/test
+          export GITHUB_SHA=deadbeef
+          export GITHUB_BASE_REF=main
+          export GITHUB_STEP_SUMMARY=/tmp/run/step-summary
+          cat > /tmp/run/event.json <<EVT
+          {"pull_request":{"head":{"sha":"deadbeef"},"base":{"sha":"baadcafe"}}}
+          EVT
+          export GITHUB_EVENT_PATH=/tmp/run/event.json
+
+          export PATH=/tmp/stub:$PATH
+          base_url='https://raw.githubusercontent.com/oasdiff-test/test/main/simple.yaml'
+          out=$(./pr-comment/entrypoint.sh \
+            "$base_url" 'simple.yaml' \
+            '' '' '' '' '' '' 2>&1)
+          echo "--- entrypoint output ---"
+          echo "$out"
+
+          if ! echo "$out" | grep -q "::notice::.*View API changes"; then
+            echo "FAIL: notice line missing" >&2
+            exit 1
+          fi
+          notice=$(echo "$out" | grep "::notice::.*View API changes")
+          # The encoded base_file= param must contain the full URL, not the
+          # stripped "//raw..." form. https:// URL-encodes to https%3A%2F%2F.
+          if echo "$notice" | grep -q 'base_file=https%3A%2F%2Fraw.githubusercontent.com'; then
+            echo "PASS: full https://raw... URL preserved in base_file"
+          elif echo "$notice" | grep -q 'base_file=%2F%2Fraw.githubusercontent.com'; then
+            echo "FAIL: 'https:' was stripped from base — the strip_ref_prefix URL guard is missing" >&2
+            echo "notice line: $notice" >&2
+            exit 1
+          else
+            echo "FAIL: base_file= param did not contain the raw.githubusercontent.com host as expected" >&2
+            echo "notice line: $notice" >&2
+            exit 1
+          fi
+
+  pr_comment_free_review_url_strips_git_ref_prefix:
+    runs-on: ubuntu-latest
+    name: "Test pr-comment free review URL strips origin/main: prefix from git-ref base"
+    # Companion to pr_comment_free_review_url_preserves_https_base: when
+    # the base input is git-ref form (origin/main:openapi.yaml), the
+    # prefix must still be stripped so the /review page receives just the
+    # path. The previous sed-based implementation handled this correctly;
+    # this test guards the case-branch refactor against breaking it.
+    steps:
+      - uses: actions/checkout@v6
+      - name: Stub oasdiff to a no-changes spec
+        run: |
+          set -euo pipefail
+          mkdir -p /tmp/stub
+          cat > /tmp/stub/oasdiff <<'STUB'
+          #!/bin/sh
+          echo '[]'
+          STUB
+          chmod +x /tmp/stub/oasdiff
+
+          mkdir -p /tmp/run
+          export GITHUB_REF=refs/pull/123/merge
+          export GITHUB_REPOSITORY=foo/bar
+          export GITHUB_SHA=deadbeef
+          export GITHUB_BASE_REF=main
+          export GITHUB_STEP_SUMMARY=/tmp/run/step-summary
+          cat > /tmp/run/event.json <<EVT
+          {"pull_request":{"head":{"sha":"deadbeef"},"base":{"sha":"baadcafe"}}}
+          EVT
+          export GITHUB_EVENT_PATH=/tmp/run/event.json
+
+          export PATH=/tmp/stub:$PATH
+          out=$(./pr-comment/entrypoint.sh \
+            'origin/main:multi-file/openapi.yaml' 'HEAD:multi-file/openapi.yaml' \
+            '' '' '' '' '' '' 2>&1)
+          echo "--- entrypoint output ---"
+          echo "$out"
+
+          notice=$(echo "$out" | grep "::notice::.*View API changes")
+          # base_file should be just "multi-file/openapi.yaml" (URL-encoded slash),
+          # not the full "origin/main:multi-file/openapi.yaml".
+          if echo "$notice" | grep -q 'base_file=multi-file%2Fopenapi.yaml'; then
+            echo "PASS: origin/main: prefix stripped from base"
+          else
+            echo "FAIL: base_file= did not have the git-ref prefix stripped" >&2
+            echo "notice line: $notice" >&2
+            exit 1
+          fi
diff --git a/pr-comment/entrypoint.sh b/pr-comment/entrypoint.sh
index 50f8893..8ab4340 100755
--- a/pr-comment/entrypoint.sh
+++ b/pr-comment/entrypoint.sh
@@ -77,9 +77,24 @@ repo="${GITHUB_REPOSITORY#*/}"
 
 # Emit free review annotation (public repos only — no auth required)
 urlencode() { printf '%s' "$1" | jq -sRr @uri; }
-# Strip git ref prefix (e.g. "origin/main:path.yaml" -> "path.yaml", "HEAD:path.yaml" -> "path.yaml")
-base_path=$(echo "$base" | sed 's/.*://')
-rev_path=$(echo "$revision" | sed 's/.*://')
+# Resolve the on-disk / on-repo path portion of `base` and `revision` for
+# the free /review URL. Two input shapes are supported:
+#
+#   1. Git-ref form  — "origin/main:openapi.yaml" or "HEAD:openapi.yaml".
+#      The sed strips everything up to the colon so we keep just the path.
+#   2. URL form      — "https://raw.githubusercontent.com/o/r/main/foo.yaml".
+#      The naive sed would also strip "https:" and leave a broken
+#      "//raw.githubusercontent.com/..." which the /review page then can't
+#      fetch (it renders as the access-denied screen with a misleading
+#      "owner doesn't have access" message). Pass URLs through unchanged.
+strip_ref_prefix() {
+    case "$1" in
+        http://*|https://*) printf '%s' "$1" ;;
+        *)                  printf '%s' "$1" | sed 's/.*://' ;;
+    esac
+}
+base_path=$(strip_ref_prefix "$base")
+rev_path=$(strip_ref_prefix "$revision")
 # Prefer the base SHA over the branch name so the link is commit-pinned.
 # Fall back through `git rev-parse origin/<branch>` before resorting to
 # the branch name itself, so push-event triggers (no pull_request payload)