diff --git a/.github/actions/collect-fuzz-stats/action.yml b/.github/actions/collect-fuzz-stats/action.yml
new file mode 100644
index 00000000..25487959
--- /dev/null
+++ b/.github/actions/collect-fuzz-stats/action.yml
@@ -0,0 +1,62 @@
+name: Collect fuzz stats
+description: Dump coverage summaries (project-wide + per-harness) and corpus file counts into stats-<variant>/ for the report job.
+
+inputs:
+  project:
+    description: Project name.
+    required: true
+  variant:
+    description: Variant label — "baseline" or "current".
+    required: true
+  sha:
+    description: Commit SHA measured in this variant.
+    required: true
+  has_project:
+    description: String "true" if the project was present and built for this variant; otherwise only meta.json is written.
+    required: true
+  out_base:
+    description: Directory containing the oss-fuzz out tree (e.g. build/out for oss-fuzz template, /tmp/oss-fuzz/build/out for upstream).
+    required: true
+
+runs:
+  using: composite
+  steps:
+    - shell: bash
+      env:
+        PROJECT: ${{ inputs.project }}
+        VARIANT: ${{ inputs.variant }}
+        SHA: ${{ inputs.sha }}
+        HAS_PROJECT: ${{ inputs.has_project }}
+        OUT_BASE: ${{ inputs.out_base }}
+      run: |
+        OUT="stats-$VARIANT"
+        mkdir -p "$OUT/harness"
+
+        jq -n \
+          --arg variant "$VARIANT" \
+          --arg sha "${SHA:-}" \
+          --arg project "$PROJECT" \
+          --arg has_project "${HAS_PROJECT:-false}" \
+          '{variant: $variant, sha: $sha, project: $project, has_project: ($has_project == "true")}' \
+          > "$OUT/meta.json"
+
+        if [ "$HAS_PROJECT" != "true" ]; then
+          echo "No project at this variant; stats collection skipped"
+          exit 0
+        fi
+
+        PROJ_SUM=$(find "$OUT_BASE/$PROJECT/report/linux" -maxdepth 1 -name summary.json 2>/dev/null | head -1)
+        if [ -n "$PROJ_SUM" ]; then
+          cp "$PROJ_SUM" "$OUT/project.summary.json"
+        fi
+
+        if [ -d "$OUT_BASE/$PROJECT/report_target" ]; then
+          for d in "$OUT_BASE/$PROJECT/report_target"/*/linux; do
+            [ -d "$d" ] || continue
+            HNAME=$(basename "$(dirname "$d")")
+            [ -f "$d/summary.json" ] && cp "$d/summary.json" "$OUT/harness/$HNAME.summary.json"
+          done
+        fi
+
+        echo "---- $OUT ----"
+        find "$OUT" -type f -printf '%p (%s bytes)\n'
diff --git a/.github/actions/invariance_tests/action.yml b/.github/actions/invariance_tests/action.yml
deleted file mode 100644
index 3bc1c51d..00000000
--- a/.github/actions/invariance_tests/action.yml
+++ /dev/null
@@ -1,33 +0,0 @@
-name: Invariance tests
-description: Run invariance tests and write summary.
-
-inputs:
-  artifact-name:
-    description: 'Name of the uploaded artifact'
-    required: false
-    default: 'invariance-test-results'
-  library-path:
-    description: 'Path to the InChI library under test'
-    required: false
-    default: 'CMake_build/full_build/INCHI-1-SRC/INCHI_API/libinchi/src/lib/libinchi.so'
-
-runs:
-  using: "composite"
-  steps:
-    - name: Run invariance tests
-      run: python INCHI-1-TEST/tests/test_library/inchi_tests/run_tests.py --test=invariance --lib-path=${{ inputs.library-path}} --data-config=INCHI-1-TEST/tests/test_library/config/config_ci.py
-      shell: bash
-
-    - name: Write invariance summary
-      if: ${{ !cancelled() }}
-      run: python INCHI-1-TEST/tests/test_library/inchi_tests/parse_log.py --test=invariance --lib-path=${{ inputs.library-path}} --data-config=INCHI-1-TEST/tests/test_library/config/config_ci.py
-      shell: bash
-
-    - name: Upload invariance test results
-      if: ${{ !cancelled() }}
-      uses: actions/upload-artifact@v4
-      with:
-        name: ${{ inputs.artifact-name }}
-        path: |
-          ./INCHI-1-TEST/tests/test_library/data/ci/*_invariance_ci.log
-          ./INCHI-1-TEST/tests/test_library/data/ci/*_invariance_ci_*.html
diff --git a/.github/actions/post-fuzz-report/action.yml b/.github/actions/post-fuzz-report/action.yml
new file mode 100644
index 00000000..7952e882
--- /dev/null
+++ b/.github/actions/post-fuzz-report/action.yml
@@ -0,0 +1,65 @@
+name: Post fuzz coverage report
+description: Download baseline/current stats artifacts, render markdown, post or update a sticky PR comment.
+
+inputs:
+  footer:
+    description: Footer flavor — "generic" for oss-fuzz, "upstream" for upstream source PRs.
+    required: false
+    default: generic
+  fuzz_seconds:
+    description: Total fuzz budget used (for display only).
+    required: true
+  github_token:
+    description: Token with pull-requests:write to post the comment.
+    required: true
+
+runs:
+  using: composite
+  steps:
+    - uses: actions/download-artifact@d3f86a106a0bac45b974a628896c90dbdf5c8093 # v4
+      with:
+        name: stats-baseline
+        path: stats/baseline
+      continue-on-error: true
+
+    - uses: actions/download-artifact@d3f86a106a0bac45b974a628896c90dbdf5c8093 # v4
+      with:
+        name: stats-current
+        path: stats/current
+      continue-on-error: true
+
+    - name: Render comment
+      shell: bash
+      env:
+        RUN_URL: ${{ github.server_url }}/${{ github.repository }}/actions/runs/${{ github.run_id }}
+        FUZZ_SECONDS: ${{ inputs.fuzz_seconds }}
+        FOOTER: ${{ inputs.footer }}
+        STATS_ROOT: stats
+      run: |
+        python3 "$GITHUB_ACTION_PATH/render_comment.py" > comment.md
+        echo "---- comment.md ----"
+        cat comment.md
+
+    - name: Post or update PR comment
+      shell: bash
+      env:
+        GH_TOKEN: ${{ inputs.github_token }}
+        REPO: ${{ github.repository }}
+        BRANCH: ${{ github.ref_name }}
+      run: |
+        PR=$(gh pr list --repo "$REPO" --head "$BRANCH" --state open --json number --jq '.[0].number // empty')
+        if [ -z "$PR" ]; then
+          echo "No open PR for branch $BRANCH — skipping comment"
+          exit 0
+        fi
+        MARKER='<!-- fuzz-coverage-report -->'
+        EXISTING=$(gh api "repos/$REPO/issues/$PR/comments" --paginate \
+          --jq ".[] | select(.body | contains(\"$MARKER\")) | .id" | head -1)
+        jq -Rs '{body: .}' < comment.md > payload.json
+        if [ -n "$EXISTING" ]; then
+          gh api --method PATCH "repos/$REPO/issues/comments/$EXISTING" --input payload.json > /dev/null
+          echo "Updated existing comment $EXISTING on PR #$PR"
+        else
+          gh api --method POST "repos/$REPO/issues/$PR/comments" --input payload.json > /dev/null
+          echo "Created new comment on PR #$PR"
+        fi
diff --git a/.github/actions/post-fuzz-report/render_comment.py b/.github/actions/post-fuzz-report/render_comment.py
new file mode 100644
index 00000000..a954f27c
--- /dev/null
+++ b/.github/actions/post-fuzz-report/render_comment.py
@@ -0,0 +1,188 @@
+# Copyright 2026 fuzz-for-me contributors
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+"""Render before/after coverage comparison as a sticky PR comment body.
+
+Reads artifacts downloaded by the calling workflow:
+  stats/baseline/meta.json, project.summary.json, corpus.json, harness/*.summary.json
+  stats/current/ ... (same layout)
+
+Writes the markdown body to stdout.
+"""
+
+import datetime
+import json
+import os
+import pathlib
+import sys
+
+MARKER = "<!-- fuzz-coverage-report -->"
+
+FOOTER_GENERIC = (
+    "<sub>Per-harness data from `report_target/&lt;fuzzer&gt;/linux/summary.json`. "
+    "Full HTML reports in the workflow artifacts.</sub>")
+FOOTER_UPSTREAM = ("<sub>Same harness config applied to both sides "
+                   "(baseline = base source + PR harness). " +
+                   FOOTER_GENERIC[5:])
+
+
+def _load_json(path: pathlib.Path):
+  if not path.exists():
+    return None
+  try:
+    return json.loads(path.read_text())
+  except json.JSONDecodeError:
+    return None
+
+
+def _load_variant(base: pathlib.Path) -> dict:
+  harness: dict = {}
+  hd = base / "harness"
+  if hd.is_dir():
+    for f in sorted(hd.glob("*.summary.json")):
+      data = _load_json(f)
+      if data is not None:
+        harness[f.name.removesuffix(".summary.json")] = data
+  return {
+      "meta": _load_json(base / "meta.json"),
+      "project": _load_json(base / "project.summary.json"),
+      "harness": harness,
+  }
+
+
+def _totals(summary):
+  if not summary:
+    return None
+  t = summary["data"][0]["totals"]
+  return {
+      "lines":
+          (t["lines"]["covered"], t["lines"]["count"], t["lines"]["percent"]),
+      "branches": (
+          t["branches"]["covered"],
+          t["branches"]["count"],
+          t["branches"]["percent"],
+      ),
+      "functions": (
+          t["functions"]["covered"],
+          t["functions"]["count"],
+          t["functions"]["percent"],
+      ),
+  }
+
+
+def _fmt_cov(tot, key):
+  if not tot:
+    return "—"
+  cov, n, pct = tot[key]
+  return f"{pct:.1f}% ({cov}/{n})"
+
+
+def _fmt_delta(b, a, key):
+  if not b and not a:
+    return "—"
+  if not b:
+    return "**new**"
+  if not a:
+    return "**removed**"
+  d = a[key][2] - b[key][2]
+  sign = "+" if d >= 0 else ""
+  return f"**{sign}{d:.1f} pp**"
+
+
+def render(
+    stats_root: pathlib.Path,
+    run_url: str,
+    fuzz_seconds: str,
+    now_utc: str,
+    footer: str,
+) -> str:
+  b = _load_variant(stats_root / "baseline")
+  c = _load_variant(stats_root / "current")
+
+  b_meta = b["meta"] or {}
+  c_meta = c["meta"] or {}
+  b_sha_full = b_meta.get("sha") or ""
+  c_sha_full = c_meta.get("sha") or ""
+  b_sha = b_sha_full[:7] if b_sha_full else "unknown"
+  c_sha = c_sha_full[:7] if c_sha_full else "unknown"
+  project = c_meta.get("project") or b_meta.get("project") or "?"
+  b_has = bool(b_meta.get("has_project"))
+  c_has = bool(c_meta.get("has_project"))
+
+  out = [MARKER, "", "## Fuzzing Coverage Report", ""]
+
+  tested = f"**Tested:** project `{project}` · base `{b_sha}`"
+  if not b_has:
+    tested += (
+        " _(no baseline — project not present at base or baseline build failed)_"
+    )
+  tested += f" → head `{c_sha}`"
+  if not c_has:
+    tested += " _(current measurement failed)_"
+  tested += (f" · {fuzz_seconds}s total fuzz budget"
+             f" · updated {now_utc}"
+             f" · [workflow run]({run_url})")
+  out += [tested, ""]
+
+  bt = _totals(b["project"])
+  ct = _totals(c["project"])
+  if bt or ct:
+    out += [
+        "| Metric | Before | After | Delta |",
+        "|---|---|---|---|",
+        f"| Line coverage | {_fmt_cov(bt, 'lines')} | {_fmt_cov(ct, 'lines')} | {_fmt_delta(bt, ct, 'lines')} |",
+        f"| Branch coverage | {_fmt_cov(bt, 'branches')} | {_fmt_cov(ct, 'branches')} | {_fmt_delta(bt, ct, 'branches')} |",
+        f"| Function coverage | {_fmt_cov(bt, 'functions')} | {_fmt_cov(ct, 'functions')} | {_fmt_delta(bt, ct, 'functions')} |",
+        "",
+    ]
+
+  all_h = sorted(set(b["harness"].keys()) | set(c["harness"].keys()))
+  if all_h:
+    out += [
+        "### Per-harness",
+        "",
+        "| Harness | Lines before | Lines after | Δ |",
+        "|---|---|---|---|",
+    ]
+    for h in all_h:
+      bh = _totals(b["harness"].get(h))
+      ch = _totals(c["harness"].get(h))
+      out.append(
+          f"| `{h}` | {_fmt_cov(bh, 'lines')} | {_fmt_cov(ch, 'lines')} | "
+          f"{_fmt_delta(bh, ch, 'lines')} |")
+    out.append("")
+
+  if not (bt or ct or all_h):
+    out += [
+        "_No coverage data collected. Check the workflow run for build errors._",
+        "",
+    ]
+
+  out.append(footer)
+  return "\n".join(out)
+
+
+def main():
+  stats_root = pathlib.Path(os.environ.get("STATS_ROOT", "stats"))
+  run_url = os.environ["RUN_URL"]
+  fuzz_seconds = os.environ.get("FUZZ_SECONDS", "300")
+  footer_kind = os.environ.get("FOOTER", "generic")
+  now_utc = datetime.datetime.now(
+      datetime.timezone.utc).strftime("%Y-%m-%d %H:%M UTC")
+  footer = FOOTER_UPSTREAM if footer_kind == "upstream" else FOOTER_GENERIC
+  sys.stdout.write(render(stats_root, run_url, fuzz_seconds, now_utc, footer))
+  sys.stdout.write("\n")
+
+
+if __name__ == "__main__":
+  main()
diff --git a/.github/actions/regression_tests/action.yml b/.github/actions/regression_tests/action.yml
deleted file mode 100644
index f17b04da..00000000
--- a/.github/actions/regression_tests/action.yml
+++ /dev/null
@@ -1,37 +0,0 @@
-name: Regression tests
-description: Run regression tests and write summary.
-
-inputs:
-  artifact-name:
-    description: 'Name of the uploaded artifact'
-    required: false
-    default: 'regression-test-results'
-  library-path:
-    description: 'Path to the InChI library under test'
-    required: false
-    default: 'CMake_build/full_build/INCHI-1-SRC/INCHI_API/libinchi/src/lib/libinchi.so'
-  shell:
-    description: 'Shell for running the tests'
-    required: false
-    default: 'bash'
-
-runs:
-  using: "composite"
-  steps:
-    - name: Run regression tests
-      run: python INCHI-1-TEST/tests/test_library/inchi_tests/run_tests.py --test=regression --lib-path=${{ inputs.library-path}} --data-config=INCHI-1-TEST/tests/test_library/config/config_ci.py
-      shell: ${{ inputs.shell }}
-
-    - name: Write regression summary
-      if: ${{ !cancelled() }}
-      run: python INCHI-1-TEST/tests/test_library/inchi_tests/parse_log.py --test=regression --lib-path=${{ inputs.library-path}} --data-config=INCHI-1-TEST/tests/test_library/config/config_ci.py
-      shell: ${{ inputs.shell }}
-
-    - name: Upload regression test results
-      if: ${{ !cancelled() }}
-      uses: actions/upload-artifact@v4
-      with:
-        name: ${{ inputs.artifact-name }}
-        path: |
-          ./INCHI-1-TEST/tests/test_library/data/ci/*_regression_ci.log
-          ./INCHI-1-TEST/tests/test_library/data/ci/*_regression_ci_*.html
diff --git a/.github/fuzz/Dockerfile b/.github/fuzz/Dockerfile
new file mode 100644
index 00000000..d1cdfedf
--- /dev/null
+++ b/.github/fuzz/Dockerfile
@@ -0,0 +1,21 @@
+# Copyright 2020 Google Inc.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+################################################################################
+
+FROM gcr.io/oss-fuzz-base/base-builder
+RUN apt-get update && apt-get install -y wget make unzip git
+RUN git clone --depth=1 https://github.com/IUPAC-InChI/InChI inchi
+WORKDIR inchi
+COPY run_tests.sh build.sh *_fuzzer.c $SRC/
diff --git a/.github/fuzz/build.sh b/.github/fuzz/build.sh
new file mode 100644
index 00000000..825384d2
--- /dev/null
+++ b/.github/fuzz/build.sh
@@ -0,0 +1,42 @@
+#!/bin/bash -eu
+# Copyright 2020 Google Inc.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+################################################################################
+
+pushd INCHI-1-SRC
+# Compile library sources (exclude ichimain.c which is the standalone program main)
+# Remove -ansi flag since upstream now uses C99 features (loop-scoped variables)
+SRC_FILES=$(ls INCHI_BASE/src/*.c INCHI_API/libinchi/src/*.c INCHI_API/libinchi/src/ixa/*.c | grep -v ichimain.c)
+$CC $CFLAGS -Wno-everything -DTARGET_API_LIB -c $SRC_FILES
+
+ar rcs $WORK/libinchi.a *.o
+
+for fuzzer in $SRC/*_fuzzer.c; do
+  fuzzer_basename=$(basename -s .c $fuzzer)
+
+  $CC $CFLAGS \
+      -I INCHI_BASE/src/ \
+      -I INCHI_API/libinchi/src/ \
+      -I INCHI_API/libinchi/src/ixa/ \
+      $fuzzer -c -o ${fuzzer_basename}.o
+
+  $CXX $CXXFLAGS \
+      ${fuzzer_basename}.o -o $OUT/$fuzzer_basename \
+      $LIB_FUZZING_ENGINE $WORK/libinchi.a
+done
+popd
+
+# Build test
+INCHI-1-TEST/build_with_cmake.sh all
diff --git a/.github/fuzz/harnesses/inchi_input_fuzzer.c b/.github/fuzz/harnesses/inchi_input_fuzzer.c
new file mode 100644
index 00000000..d038da2a
--- /dev/null
+++ b/.github/fuzz/harnesses/inchi_input_fuzzer.c
@@ -0,0 +1,56 @@
+// Copyright 2020 Google Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include <stddef.h>
+#include <stdint.h>
+#include <stdlib.h>
+#include <string.h>
+
+#include "inchi_api.h"
+
+// Define the maximum value for size_t. We return if the fuzzing input is equal
+// to kSizeMax because appending the null-terminator to the InChI buffer would
+// cause wraparound, thereby initializing the buffer to size 0.
+static const size_t kSizeMax = (size_t)-1;
+
+int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
+
+  if (size == kSizeMax)
+    return 0;
+
+  char *szINCHISource = malloc(sizeof(char) * (size + 1));
+  memcpy(szINCHISource, data, size);
+  szINCHISource[size] = '\0'; // InChI string must be null-terminated
+
+  // Buffer lengths taken from InChI API reference, located at
+  // https://www.inchi-trust.org/download/104/InChI_API_Reference.pdf, page 24
+  char szINCHIKey[28], szXtra1[65], szXtra2[65];
+  GetINCHIKeyFromINCHI(szINCHISource, 0, 0, szINCHIKey, szXtra1, szXtra2);
+
+  inchi_InputINCHI inpInChI;
+  inpInChI.szInChI = szINCHISource;
+  inpInChI.szOptions = NULL;
+
+  inchi_Output out;
+  GetINCHIfromINCHI(&inpInChI, &out);
+
+  inchi_OutputStruct outStruct;
+  GetStructFromINCHI(&inpInChI, &outStruct);
+
+  free(szINCHISource);
+  FreeINCHI(&out);
+  FreeStructFromINCHI(&outStruct);
+
+  return 0;
+}
diff --git a/.github/fuzz/project.yaml b/.github/fuzz/project.yaml
new file mode 100644
index 00000000..1a5838b8
--- /dev/null
+++ b/.github/fuzz/project.yaml
@@ -0,0 +1,12 @@
+homepage: "https://www.inchi-trust.org/"
+main_repo: "https://github.com/IUPAC-InChI/InChI"
+language: c
+primary_contact: "member-info@inchi-trust.org"
+sanitizers:
+  - address
+
+fuzzing_engines:
+  - afl
+  - honggfuzz
+  - libfuzzer
+
diff --git a/.github/fuzz/run_tests.sh b/.github/fuzz/run_tests.sh
new file mode 100644
index 00000000..e9cfa38c
--- /dev/null
+++ b/.github/fuzz/run_tests.sh
@@ -0,0 +1,19 @@
+#!/bin/bash -eu
+# Copyright 2025 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+################################################################################
+
+# Excluding unit test case that failed
+ctest --test-dir CMake_build/full_build -E "test_inchi_dll_b|test_permutation_util"
diff --git a/.github/workflows/fuzz-verify.yml b/.github/workflows/fuzz-verify.yml
new file mode 100644
index 00000000..a478d953
--- /dev/null
+++ b/.github/workflows/fuzz-verify.yml
@@ -0,0 +1,186 @@
+# Template — injected into upstream repo fork branches by pr_server.py at PR submission time.
+# Not used directly in this repo. Injected alongside .github/actions/{collect-fuzz-stats,post-fuzz-report}/.
+name: Fuzz Verify
+on: [push]
+
+permissions:
+  # Default to read-only at the workflow level; the report job elevates to pull-requests:write.
+  contents: read
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
+env:
+  PROJECT: inchi
+  FUZZ_SECONDS: "300"
+
+jobs:
+  measure:
+    name: Measure (${{ matrix.variant }})
+    runs-on: ubuntu-latest
+    timeout-minutes: 45
+    continue-on-error: ${{ matrix.variant == 'baseline' }}
+    strategy:
+      fail-fast: false
+      matrix:
+        variant: [baseline, current]
+    steps:
+      - uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4.3.1
+        with:
+          fetch-depth: 0
+          persist-credentials: false
+          submodules: recursive
+
+      - name: Resolve variant
+        id: variant
+        env:
+          VARIANT: ${{ matrix.variant }}
+        run: |
+          if [ "$VARIANT" = "baseline" ]; then
+            git remote set-head origin -a >/dev/null 2>&1 || true
+            DEFAULT_BRANCH=$(git symbolic-ref --short refs/remotes/origin/HEAD 2>/dev/null | sed 's|origin/||')
+            [ -z "$DEFAULT_BRANCH" ] && DEFAULT_BRANCH=main
+            if ! git rev-parse --verify "origin/$DEFAULT_BRANCH" >/dev/null 2>&1; then
+              DEFAULT_BRANCH=master
+            fi
+            BASE_SHA=$(git merge-base "origin/$DEFAULT_BRANCH" HEAD)
+            echo "sha=$BASE_SHA" >> "$GITHUB_OUTPUT"
+            echo "Baseline: $BASE_SHA (merge-base with origin/$DEFAULT_BRANCH)"
+
+            if [ ! -d .github/fuzz ]; then
+              echo "::error::.github/fuzz not present on PR — cannot overlay onto baseline"
+              echo "has_project=false" >> "$GITHUB_OUTPUT"
+              exit 0
+            fi
+            FUZZ_TMP=$(mktemp -d)
+            cp -r .github/fuzz "$FUZZ_TMP/"
+            ACTIONS_TMP=$(mktemp -d)
+            cp -r .github/actions "$ACTIONS_TMP/"
+            git reset --hard "$BASE_SHA"
+            git submodule update --init --recursive
+            rm -rf .github/fuzz .github/actions
+            mkdir -p .github
+            cp -r "$FUZZ_TMP/fuzz" .github/
+            cp -r "$ACTIONS_TMP/actions" .github/
+            echo "has_project=true" >> "$GITHUB_OUTPUT"
+          else
+            echo "sha=$GITHUB_SHA" >> "$GITHUB_OUTPUT"
+            echo "has_project=true" >> "$GITHUB_OUTPUT"
+            echo "Current: $GITHUB_SHA"
+          fi
+
+      - name: Clone oss-fuzz
+        if: steps.variant.outputs.has_project == 'true'
+        run: git clone --depth 1 https://github.com/google/oss-fuzz.git /tmp/oss-fuzz
+
+      - name: Install oss-fuzz deps
+        if: steps.variant.outputs.has_project == 'true'
+        run: pip install -r /tmp/oss-fuzz/infra/ci/requirements.txt 2>/dev/null || pip install docker
+
+      - name: Set up project
+        if: steps.variant.outputs.has_project == 'true'
+        run: |
+          mkdir -p "/tmp/oss-fuzz/projects/$PROJECT"
+          cp -r .github/fuzz/* "/tmp/oss-fuzz/projects/$PROJECT/"
+
+      - name: Build image
+        if: steps.variant.outputs.has_project == 'true'
+        run: echo n | python3 /tmp/oss-fuzz/infra/helper.py build_image "$PROJECT"
+
+      - name: Build fuzzers
+        if: steps.variant.outputs.has_project == 'true'
+        run: echo n | python3 /tmp/oss-fuzz/infra/helper.py build_fuzzers --sanitizer address "$PROJECT" "$GITHUB_WORKSPACE"
+
+      - name: Fuzz
+        if: steps.variant.outputs.has_project == 'true'
+        run: |
+          OUT="/tmp/oss-fuzz/build/out/$PROJECT"
+
+          HARNESSES=""
+          for f in "$OUT"/*; do
+            [ -f "$f" ] && [ -x "$f" ] || continue
+            case "$(basename "$f")" in
+              *.options|*.dict|*_seed_corpus.zip|llvm-symbolizer) continue ;;
+            esac
+            HARNESSES="$HARNESSES $(basename "$f")"
+          done
+          HARNESSES=$(echo $HARNESSES | xargs)
+
+          if [ -z "$HARNESSES" ]; then
+            echo "::error::No harness binaries found in $OUT"
+            exit 1
+          fi
+
+          N=$(echo $HARNESSES | wc -w)
+          TIME_EACH=$(( FUZZ_SECONDS / N ))
+          [ "$TIME_EACH" -lt 30 ] && TIME_EACH=30
+          echo "Harnesses ($N): $HARNESSES — ${TIME_EACH}s each"
+
+          for h in $HARNESSES; do
+            echo "::group::$h"
+            mkdir -p "/tmp/oss-fuzz/corpus/$PROJECT/$h"
+            timeout $((TIME_EACH + 60)) python3 /tmp/oss-fuzz/infra/helper.py run_fuzzer \
+              --corpus-dir "/tmp/oss-fuzz/corpus/$PROJECT/$h" \
+              "$PROJECT" "$h" -- \
+              "-max_total_time=$TIME_EACH" 2>&1 || true
+            echo "Corpus: $(find "/tmp/oss-fuzz/corpus/$PROJECT/$h" -type f | wc -l) files"
+            echo "::endgroup::"
+          done
+
+      - name: Coverage
+        if: steps.variant.outputs.has_project == 'true'
+        run: |
+          echo n | python3 /tmp/oss-fuzz/infra/helper.py build_fuzzers --sanitizer coverage "$PROJECT" "$GITHUB_WORKSPACE"
+
+          mkdir -p "/tmp/oss-fuzz/build/corpus"
+          ln -sfn "/tmp/oss-fuzz/corpus/$PROJECT" "/tmp/oss-fuzz/build/corpus/$PROJECT"
+
+          python3 /tmp/oss-fuzz/infra/helper.py coverage \
+            --no-corpus-download --no-serve \
+            "$PROJECT" 2>&1 || true
+
+      - name: Collect stats
+        if: always()
+        uses: ./.github/actions/collect-fuzz-stats
+        with:
+          project: ${{ env.PROJECT }}
+          variant: ${{ matrix.variant }}
+          sha: ${{ steps.variant.outputs.sha }}
+          has_project: ${{ steps.variant.outputs.has_project }}
+          out_base: /tmp/oss-fuzz/build/out
+
+      - name: Upload stats
+        if: always()
+        uses: actions/upload-artifact@ea165f8d65b6e75b540449e92b4886f43607fa02 # v4.6.2
+        with:
+          name: stats-${{ matrix.variant }}
+          path: stats-${{ matrix.variant }}/
+          if-no-files-found: error
+
+      - name: Upload coverage report
+        if: always() && steps.variant.outputs.has_project == 'true'
+        uses: actions/upload-artifact@ea165f8d65b6e75b540449e92b4886f43607fa02 # v4.6.2
+        with:
+          name: coverage-report-${{ matrix.variant }}
+          path: /tmp/oss-fuzz/build/out/*coverage*/report/
+          if-no-files-found: ignore
+
+  report:
+    name: Report
+    needs: [measure]
+    if: always()
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+      pull-requests: write # post sticky fuzz-verify comment
+    steps:
+      - uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4.3.1
+        with:
+          persist-credentials: false
+          sparse-checkout: .github/actions/post-fuzz-report
+      - uses: ./.github/actions/post-fuzz-report
+        with:
+          footer: upstream
+          fuzz_seconds: ${{ env.FUZZ_SECONDS }}
+          github_token: ${{ secrets.GITHUB_TOKEN }}
diff --git a/INCHI-1-SRC/INCHI_BASE/src/ichi_bns.c b/INCHI-1-SRC/INCHI_BASE/src/ichi_bns.c
index 04af6d7a..bb3c4339 100644
--- a/INCHI-1-SRC/INCHI_BASE/src/ichi_bns.c
+++ b/INCHI-1-SRC/INCHI_BASE/src/ichi_bns.c
@@ -5970,7 +5970,7 @@ int mark_alt_bonds_and_taut_groups( struct tagINCHI_CLOCK   *ic,
         if (at[0].iso_atw_diff) 
         {
 #if ( FIX_CURE53_ISSUE_NULL_DEREFERENCE_MAKE_A_COPY_OF_T_GROUP_INFO==1 || defined(FIX_IMPOSSIBLE_H_ISOTOPE_BUG) )
-            if (at[0].iso_atw_diff <= NUM_H_ISOTOPES)
+            if (at[0].iso_atw_diff > 0 && at[0].iso_atw_diff <= NUM_H_ISOTOPES)
             {
                 /* djb-rwth: possible false positive oss-fuzz issue #39064660 */
                 t_group_info->tni.nNumRemovedProtonsIsotopic[at[0].iso_atw_diff - 1] ++;