diff --git a/.gitignore b/.gitignore
index ad08cff..cfbcbcc 100644
--- a/.gitignore
+++ b/.gitignore
@@ -227,6 +227,7 @@ tmp/
 tmp.txt
 wget-log
 Downloads/
+logs/
 *.gz
 *.rar
 *.zip
diff --git a/.pre-commit-config.yaml b/.pre-commit-config.yaml
index f7a2c97..fea0dab 100644
--- a/.pre-commit-config.yaml
+++ b/.pre-commit-config.yaml
@@ -105,7 +105,7 @@ repos:
         name: "🔒 security · Detect private keys"
 
   - repo: https://github.com/commitizen-tools/commitizen
-    rev: v4.17.0
+    rev: v4.18.1
     hooks:
       - id: commitizen
         name: "🌳 git · Validate commit message"
@@ -134,7 +134,7 @@ repos:
         additional_dependencies: [".[toml]"]
 
   - repo: https://github.com/semgrep/pre-commit
-    rev: 'v1.171.0'
+    rev: 'v1.177.0'
     hooks:
       - id: semgrep
         name: "🔒 security · Static analysis (semgrep)"
@@ -143,14 +143,14 @@ repos:
 
   # Spelling and typos
   - repo: https://github.com/crate-ci/typos
-    rev: v1.48.0
+    rev: v1.50.2
     hooks:
       - id: typos
         name: "📝 spelling · Check typos"
 
   # CI/CD validation
   - repo: https://github.com/python-jsonschema/check-jsonschema
-    rev: 0.37.4
+    rev: 0.38.0
     hooks:
       - id: check-dependabot
         name: "🔧 ci/cd · Validate Dependabot config"
@@ -159,7 +159,7 @@ repos:
         files: ^\.github/workflows/.*\.ya?ml$
 
   - repo: https://github.com/ariebovenberg/slotscheck
-    rev: v0.20.1
+    rev: v0.21.0
     hooks:
       - id: slotscheck
         name: "🔍 check · slotscheck"
@@ -172,7 +172,7 @@ repos:
           - responses
 
   - repo: https://github.com/astral-sh/ruff-pre-commit
-    rev: v0.16.1
+    rev: v0.16.8
     hooks:
       - id: ruff-check
         name: "🐍 lint · Check with Ruff"
@@ -191,7 +191,7 @@ repos:
 
   # Python type checking
   - repo: https://github.com/pre-commit/mirrors-mypy
-    rev: v2.3.0
+    rev: v2.3.1
     hooks:
       - id: mypy
         name: "🐍 types · Check with mypy"
@@ -202,14 +202,14 @@ repos:
         exclude: ^mailgun/examples/
 
   - repo: https://github.com/RobertCraigie/pyright-python
-    rev: v1.1.411
+    rev: v1.1.414
     hooks:
       - id: pyright
         name: "🐍 types · Check with pyright"
 
   # Python project configuration
   - repo: https://github.com/abravalheri/validate-pyproject
-    rev: v0.25
+    rev: '0.26'
     hooks:
       - id: validate-pyproject
         name: "🐍 config · Validate pyproject.toml"
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 9113bf1..12e6804 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,7 +2,9 @@
 
 We [keep a changelog.](http://keepachangelog.com/)
 
-## [Unreleased] (1.9.1)
+## [Unreleased]
+
+## [1.9.1] - 2026-09-21
 
 ### Security
 
@@ -35,9 +37,14 @@ We [keep a changelog.](http://keepachangelog.com/)
 
 ### Pull Requests Merged
 
+- PR #61: build(deps): Bump the minor-and-patch group across 1 directory with 3 updates.
+- PR #63: build(deps): Bump the minor-and-patch group across 1 directory with 2 updates.
+- PR #64: build(deps): Bump github/codeql-action/upload-sarif from 4.37.8 to 4.37.9 in the minor-and-patch group.
 - PR #65: Hardening security and stability.
+- PR #66: Release 1.9.1.
+- PR #67: build(deps): Bump github/codeql-action/upload-sarif from 4.37.9 to 4.38.0 in the minor-and-patch group.
 
-## v1.9.0 - 2026-08-04
+## [1.9.0] - 2026-08-04
 
 ### Added
 
@@ -76,7 +83,7 @@ We [keep a changelog.](http://keepachangelog.com/)
 - [PR_58](https://github.com/mailgun/mailgun-python/pull/58) - build(deps): Bump the minor-and-patch group with 2 updates.
 - [PR_59](https://github.com/mailgun/mailgun-python/pull/59) - build(deps): Bump actions/setup-python from 6.3.0 to 7.0.0.
 
-## v1.8.0 - 2026-07-20
+## [1.8.0] - 2026-07-20
 
 ### 🌟 Top Highlights (The "Big Wins")
 
@@ -505,4 +512,7 @@ We [keep a changelog.](http://keepachangelog.com/)
 [1.6.0]: https://github.com/mailgun/mailgun-python/releases/tag/v1.6.0
 [1.7.0]: https://github.com/mailgun/mailgun-python/releases/tag/v1.7.0
 [1.7.1]: https://github.com/mailgun/mailgun-python/releases/tag/v1.7.1
-[unreleased]: https://github.com/mailgun/mailgun-python/compare/v1.8.0...HEAD
+[1.8.0]: https://github.com/mailgun/mailgun-python/releases/tag/v1.8.0
+[1.9.0]: https://github.com/mailgun/mailgun-python/releases/tag/v1.9.0
+[1.9.1]: https://github.com/mailgun/mailgun-python/releases/tag/v1.9.1
+[unreleased]: https://github.com/mailgun/mailgun-python/compare/v1.9.1...HEAD
diff --git a/mailgun/_version.py b/mailgun/_version.py
index 0a0a43a..38cf6db 100644
--- a/mailgun/_version.py
+++ b/mailgun/_version.py
@@ -1 +1 @@
-__version__ = "1.9.0"
+__version__ = "1.9.1"
diff --git a/tests/fuzz/fuzz.dict b/tests/fuzz/fuzz.dict
index 807fcbf..f458c58 100644
--- a/tests/fuzz/fuzz.dict
+++ b/tests/fuzz/fuzz.dict
@@ -2209,3 +2209,36 @@
 "}{\xef\xbf\xbd"
 "}}}}\xef\xbf\xbd"
 "~\x13"
+"\xff\xff\xff\xff\xff\xff\xff\xc8"
+"qq;qCF        %2}\x7f\xef\xbf\xbd{\x7f\x7f"
+"_\x00\x00\x00\x00\x00\x00\x00"
+"<!>E"
+"im1"
+"<!><!I\x04I\x14"
+"vii"
+"\x00\x00\x00\x00\x00\x00\x00$"
+"<!-q"
+"<!do\x1ct}py"
+"<!/>ccccc"
+"<!doc3%\x00\x00"
+"7.\x02\x02{\"%80"
+"!%D5G%D\x7f\x7f\x7f\x7f\x01\x00"
+"\x01\x00\x00\x00\x00\x00\x00\x17"
+"\xcf\xff\xff\xff\xff\xff\xff\xff"
+"\xf9\xff\xff\xff\xff\xff\xff\xff"
+"<!$-->---"
+"\xff\xff\xff\xff\xff\xff\xff\xf0"
+"<![CDATa&"
+"ieg"
+"\x89\x0f\x00\x00\x00\x00\x00\x00"
+"<!><!</p>"
+"<!</p></b"
+"<!doc\x0b<f/"
+"\xff\xff\xff\xff\xff\xff\xff,"
+"<!0000uoo"
+"<!YCD/</p"
+"<!><"
+"<!d\x104}xe&"
+"\xf7\xff\x0f\x00\x00\x00\x00\x00"
+"\x00\x00\x00\x00\x00\x00\x08l"
+"\xec\x00\x00\x00\x00\x00\x00\x00"
diff --git a/tests/fuzz/fuzz_async_evil_server.py b/tests/fuzz/fuzz_async_evil_server.py
index 96235e4..fd53783 100755
--- a/tests/fuzz/fuzz_async_evil_server.py
+++ b/tests/fuzz/fuzz_async_evil_server.py
@@ -42,6 +42,10 @@
     "Wed, 21 Oct 2026 07:28:00 GMT",
     "invalid-date-string",
     "9999999999999999999999999999999",
+    "1e400",
+    "-1e400",
+    "1e999",
+    "9" * 150,
 ]
 
 
diff --git a/tests/fuzz/fuzz_audit_events.py b/tests/fuzz/fuzz_audit_events.py
index 17910e6..fa3733a 100644
--- a/tests/fuzz/fuzz_audit_events.py
+++ b/tests/fuzz/fuzz_audit_events.py
@@ -65,6 +65,27 @@ def TestOneInput(data: bytes) -> None:
             action = getattr(client.domains, fuzzed_method)
             action(domain=fuzzed_domain)
 
+
+        # Probe header injection & control character audit hooks
+        if fdp.ConsumeBool():
+            hostile_headers = {
+                fdp.ConsumeUnicodeNoSurrogates(16): fdp.PickValueInList(
+                    ["valid", "bad\r\nHeader: 1", "control\x01char", "null\x00byte"]
+                )
+            }
+            client.messages.create(
+                domain=fuzzed_domain,
+                data={"from": "test@example.com", "to": "user@example.com"},
+                headers=hostile_headers,
+            )
+
+        # Probe SSRF URL audit hooks
+        if fdp.ConsumeBool():
+            hostile_url = fdp.PickValueInList(
+                ["ftp://api.mailgun.net", "gopher://127.0.0.1", "http://attacker.com/v3"]
+            )
+            client.messages.api_call(method="get", url=hostile_url)
+
         # Verify invariant: if audit hook fired, arguments must be safe
         for event, args in _AUDIT_LOG:
             for arg in args:
diff --git a/tests/fuzz/fuzz_builders_advanced.py b/tests/fuzz/fuzz_builders_advanced.py
index d36cd2b..f77bbbb 100644
--- a/tests/fuzz/fuzz_builders_advanced.py
+++ b/tests/fuzz/fuzz_builders_advanced.py
@@ -11,12 +11,28 @@
 
 import atheris
 
+from mailgun.security import IdempotencyGuard
+
+
 with atheris.instrument_imports():
     from mailgun.builders import ChunkedStreamer, MailgunMessageBuilder
 
 logging.disable(logging.CRITICAL)
 
 
+def _test_cyclic_and_stream_pointer_invariants(
+    fdp: atheris.FuzzedDataProvider, builder: MailgunMessageBuilder
+) -> None:
+    # Explicitly annotate cyclic_dict as dict[str, Any] to allow self-referential structures
+    cyclic_dict: dict[str, Any] = {"domain": "test.com", "to": ["a@b.com"]}
+    cyclic_dict["self"] = cyclic_dict
+    cyclic_dict["nested"] = [cyclic_dict]
+
+    key = IdempotencyGuard.generate_key("test.com", cyclic_dict)
+    assert isinstance(key, str)
+    assert len(key) == 64
+
+
 def _generate_nested_ast(fdp: atheris.FuzzedDataProvider, depth: int = 0) -> Any:
     """Generate arbitrary nested JSON structures to stress idempotency hashing."""
     if depth > 4 or fdp.ConsumeBool():
@@ -57,7 +73,7 @@ def TestOneInput(data: bytes) -> None:
     try:
         num_operations = fdp.ConsumeIntInRange(1, 10)
         for _ in range(num_operations):
-            op_code = fdp.ConsumeIntInRange(0, 5)
+            op_code = fdp.ConsumeIntInRange(0, 6)
 
             if op_code == 0:
                 builder.set_idempotency_safe(safe=fdp.ConsumeBool())
@@ -101,6 +117,16 @@ def TestOneInput(data: bytes) -> None:
                     # Expected during fuzzing: optional API may be missing or reject malformed input.
                     pass
 
+            elif op_code == 6:
+                # Direct cyclic graph insertion into builder payload
+                cyclic: dict[str, Any] = {"nested": []}
+                cyclic["self"] = cyclic
+                cyclic["nested"].append(cyclic)
+                builder.add_custom_variable("cyclic_prop", cyclic)
+
+        # Exercise the standalone idempotency cyclic invariant
+        _test_cyclic_and_stream_pointer_invariants(fdp, builder)
+
         # Build and trigger hash serialization
         final_payload, files = builder.build()
 
diff --git a/tests/fuzz/fuzz_headers.py b/tests/fuzz/fuzz_headers.py
index edd3fed..57fc0aa 100755
--- a/tests/fuzz/fuzz_headers.py
+++ b/tests/fuzz/fuzz_headers.py
@@ -46,6 +46,9 @@
     "value\x00_null_byte",
     "value\x09tab_separated",
     "https://api.mailgun.net/v3\r\n\r\n<script>alert(1)</script>",
+    "просто-заголовок",
+    "header-with-emoji-🚀",
+    "\ud800\udc00",
 ]
 
 
@@ -118,7 +121,7 @@ def TestOneInput(data: bytes) -> None:
             if "\r" in v or "\n" in v or "\x00" in v:
                 raise RuntimeError(f"INJECTION LEAK in header value: {repr(v)}")
 
-    except (TypeError, ValueError):
+    except (TypeError, ValueError, UnicodeEncodeError):
         # Expected security rejection for malformed headers or control characters
         pass
     except Exception as e:
diff --git a/tests/fuzz/fuzz_pagination_stream.py b/tests/fuzz/fuzz_pagination_stream.py
index 86e4cd9..5782b43 100644
--- a/tests/fuzz/fuzz_pagination_stream.py
+++ b/tests/fuzz/fuzz_pagination_stream.py
@@ -26,6 +26,8 @@
     "https://api.mailgun.net/v3/events?limit=10&ascending=true&ascending=false",
     "https://api.mailgun.net/v3/events?tags=tag1&tags=tag2&tags=tag3",
     "https://api.mailgun.net/v3/events?\x00=corrupted",
+    "https://api.mailgun.net/v3/events?ascending=yes&limit=NaN&score=Infinity",
+    "https://api.mailgun.net/v3/events?tags=promo&tags=newsletter&threshold=1e300",
 ]
 
 
@@ -48,6 +50,11 @@ def TestOneInput(data: bytes) -> None:
             ["delivered", "failed", "opened", "clicked", None]
         )
 
+    if fdp.ConsumeBool():
+        initial_filters["tags"] = [
+            fdp.ConsumeUnicodeNoSurrogates(10) for _ in range(fdp.ConsumeIntInRange(1, 3))
+        ]
+
     # Next URL selection: either from hostile seeds or dynamically fuzzed
     if fdp.ConsumeBool():
         next_url = fdp.PickValueInList(_HOSTILE_PAGING_URLS)
diff --git a/tests/fuzz/fuzz_spamguard.py b/tests/fuzz/fuzz_spamguard.py
index b0c9d0e..797b685 100644
--- a/tests/fuzz/fuzz_spamguard.py
+++ b/tests/fuzz/fuzz_spamguard.py
@@ -18,12 +18,10 @@
 
 logging.disable(logging.CRITICAL)
 
-_MALFORMED_HTML_SNIPPETS = [
+_BENIGN_MALFORMED_SNIPPETS = [
     '<a href="http://evil.com">Click here</a>',
     '<img src="cid:missing.png" alt="No image">',
     '<div style="display:none;font-size:0px;color:#ffffff;background-color:#ffffff">Hidden Spam</div>',
-    '<script>alert("xss")</script>',
-    '<iframe src="javascript:alert(1)"></iframe>',
     '<!-- ' * 50 + 'Unclosed Comment',
     '<table' + ' border=1' * 200 + '><tr><td>Deep attr</td></tr></table>',
     '<a href="javascript:void(0)">Spam</a>' * 50,
@@ -31,6 +29,18 @@
     '<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"' + '>' * 100,
 ]
 
+_HOSTILE_EXECUTABLE_SNIPPETS = [
+    '<script>alert("xss")</script>',
+    '<iframe src="javascript:alert(1)"></iframe>',
+    '<object data="exploit.swf"></object>',
+    '<embed src="exploit.pdf">',
+    '<applet code="Exploit.class"></applet>',
+    '<img src="x" onerror="alert(1)">',
+    '<div onmouseover="stealCookies()">Hover me</div>',
+    '<body onload="init()">',
+    '<button onclick="leak()">Click</button>',
+]
+
 
 def TestOneInput(data: bytes) -> None:
     if len(data) < 8:
@@ -38,18 +48,26 @@ def TestOneInput(data: bytes) -> None:
 
     fdp = atheris.FuzzedDataProvider(data)
 
-    mode = fdp.ConsumeIntInRange(0, 2)
+    mode = fdp.ConsumeIntInRange(0, 3)
+
     if mode == 0:
-        # Mode 0: Structured HTML with known deliverability traps
+        # Mode 0: Mixed HTML snippets
         num_snippets = fdp.ConsumeIntInRange(1, 6)
-        parts = [
-            fdp.PickValueInList(_MALFORMED_HTML_SNIPPETS)
-            for _ in range(num_snippets)
-        ]
+        parts = []
+        for _ in range(num_snippets):
+            if fdp.ConsumeBool():
+                parts.append(fdp.PickValueInList(_BENIGN_MALFORMED_SNIPPETS))
+            else:
+                parts.append(fdp.PickValueInList(_HOSTILE_EXECUTABLE_SNIPPETS))
         html_content = f"<html><body>{''.join(parts)}</body></html>"
 
     elif mode == 1:
-        # Mode 1: Boundary stress test around MAX_HTML_SIZE_BYTES (100,000 bytes)
+        # Mode 1: Guaranteed un-commented exploit tag to verify detection invariant
+        exploit = fdp.PickValueInList(_HOSTILE_EXECUTABLE_SNIPPETS)
+        html_content = f"<html><body><div>{exploit}</div></body></html>"
+
+    elif mode == 2:
+        # Mode 2: Boundary stress test around MAX_HTML_SIZE_BYTES (100,000 bytes)
         size_choice = fdp.ConsumeIntInRange(0, 2)
         if size_choice == 0:
             target_size = 99_950
@@ -62,7 +80,7 @@ def TestOneInput(data: bytes) -> None:
         html_content = f"<html><body><p>{base_str}</p></body></html>"
 
     else:
-        # Mode 2: Unconstrained chaotic Unicode noise
+        # Mode 3: Unconstrained chaotic Unicode noise
         html_content = fdp.ConsumeUnicodeNoSurrogates(fdp.ConsumeIntInRange(10, 40000))
 
     try:
@@ -85,6 +103,13 @@ def TestOneInput(data: bytes) -> None:
         if not isinstance(report["issues"], list):
             raise RuntimeError(f"TYPE DRIFT: Issues must be a list, got {type(report['issues'])}")
 
+        # Invariant 4: Standalone unmasked hostile executable snippets must be flagged unsafe
+        if mode == 1:
+            if report["is_safe"]:
+                raise RuntimeError(
+                    f"SECURITY BYPASS: Active executable snippet marked safe: {html_content!r}"
+                )
+
     except (TypeError, ValueError):
         # Expected rejection for oversized payloads exceeding MAX_HTML_SIZE_BYTES
         pass