diff --git a/README.md b/README.md
index c3c2d2f..2efb2a9 100644
--- a/README.md
+++ b/README.md
@@ -90,7 +90,7 @@ Common unsupported-pattern buckets include `sshd_connection_closed_preauth`,
 `pam_faillock_account_locked`, and `pam_unix_session_closed`. These buckets keep
 non-finding evidence reviewable without counting it as detector evidence.
 
-For the parser behavior contract, supported modes, and fixture map, see [`docs/parser-contract.md`](./docs/parser-contract.md).
+For the parser behavior contract, supported modes, and fixture map, see [`docs/parser-contract.md`](./docs/parser-contract.md). For the deliberately noisy parser-coverage sample, see [`docs/parser-coverage-notes.md`](./docs/parser-coverage-notes.md).
 
 LogLens does not currently detect:
 
diff --git a/assets/noisy_auth_sample.log b/assets/noisy_auth_sample.log
new file mode 100644
index 0000000..2ab2091
--- /dev/null
+++ b/assets/noisy_auth_sample.log
@@ -0,0 +1,27 @@
+Mar 10 25:61:00 alpha-host sshd[9001]: Failed password for invalid user bad-clock from 203.0.113.1 port 50000 ssh2
+Feb 31 08:00:02 alpha-host sshd[9002]: Failed password for invalid user bad-date from 203.0.113.2 port 50001 ssh2
+
+
+Mar 10 08:00:10 alpha-host sshd[1001]: Failed password for invalid user svc+deploy from 203.0.113.10 port 50100 ssh2
+Mar 10 08:00:20 beta-host sshd[1002]: Accepted password for ops.robot from 203.0.113.11 port 50101 ssh2
+Mar 10 08:00:30 gamma-host sudo:    svc-admin : TTY=pts/1 ; PWD=/home/user/project ; USER=root ; COMMAND=/usr/bin/systemctl status ssh
+Mar 10 08:00:40 alpha-host sudo[1004]: limited.user : user NOT in sudoers ; TTY=pts/2 ; PWD=/home/user/project ; USER=root ; COMMAND=/usr/bin/id
+Mar 10 08:00:50 beta-host sudo[1005]: blocked-user : command not allowed ; TTY=pts/3 ; PWD=/home/user/project ; USER=root ; COMMAND=/usr/bin/less /etc/hosts
+Mar 10 08:01:00 beta-host pam_unix(sshd:auth): authentication failure; logname= uid=0 euid=0 tty=ssh ruser= rhost=
+Mar 10 08:01:10 gamma-host pam_faillock(sshd:auth): Authentication failure for user svc.locked from 203.0.113.13
+Mar 10 08:01:20 delta-host sshd[1008]: input_userauth_request: invalid user weird/user [preauth]
+Mar 10 08:01:30 alpha-host sshd[1009]: Connection closed by authenticating user legacy-user 203.0.113.20 port 50200 [preauth]
+Mar 10 08:01:40 beta-host sshd[1010]: Connection reset by invalid user trial.user 203.0.113.21 port 50201 [preauth]
+Mar 10 08:01:50 gamma-host sshd[1011]: Received disconnect from 203.0.113.22 port 50202:11: disconnected by user
+Mar 10 08:02:00 delta-host sshd[1012]: Timeout, client not responding from 203.0.113.23 port 50203
+Mar 10 08:02:10 alpha-host sshd[1013]: Unable to negotiate with 203.0.113.24 port 50204: no matching key exchange method found
+Mar 10 08:02:20 beta-host sshd[1014]: Unable to negotiate with 203.0.113.25 port 50205: no matching host key type found
+Mar 10 08:02:30 gamma-host pam_unix(sshd:session): session closed for user svc+deploy
+Mar 10 08:02:40 delta-host pam_unix(sudo:session): session closed for user svc-admin
+Mar 10 08:02:50 alpha-host pam_faillock(sshd:auth): Account temporarily locked for user locked.user
+Mar 10 08:03:00 beta-host pam_faillock(sshd:auth): Account temporarily locked for user svc-temp
+Mar 10 08:03:10 gamma-host pam_sss(sshd:auth): received for user unknown.shadow: 10 (User not known to the underlying authentication module)
+Mar 10 08:03:20 beta-host sudo: pam_unix(sudo:auth): authentication failure; user=limited.user
+
+rotated
+Mar 10 08:03:30 alpha-host CRON[2001]: (root) CMD (/usr/bin/true)
diff --git a/docs/parser-contract.md b/docs/parser-contract.md
index b97dc2d..2069a8a 100644
--- a/docs/parser-contract.md
+++ b/docs/parser-contract.md
@@ -79,6 +79,7 @@ Parsed successes and audit-only events remain reportable but do not count as bru
 | [`assets/parser_fixture_matrix_journalctl_short_full.log`](../assets/parser_fixture_matrix_journalctl_short_full.log) | Journalctl short-full known/unknown parser matrix |
 | [`assets/parser_auth_families_syslog.log`](../assets/parser_auth_families_syslog.log) | Syslog PAM/auth-family parser coverage |
 | [`assets/parser_auth_families_journalctl_short_full.log`](../assets/parser_auth_families_journalctl_short_full.log) | Journalctl PAM/auth-family parser coverage |
+| [`assets/noisy_auth_sample.log`](../assets/noisy_auth_sample.log) and [`tests/fixtures/parser_matrix/noisy_auth_expected.json`](../tests/fixtures/parser_matrix/noisy_auth_expected.json) | Noisy syslog parser-coverage matrix for malformed, unsupported, blank, irrelevant, multi-host, and unusual-username input |
 | [`tests/test_report_contracts.cpp`](../tests/test_report_contracts.cpp) | Stable report-shape expectations for generated artifacts |
 
 ## Non-goals
diff --git a/docs/parser-coverage-notes.md b/docs/parser-coverage-notes.md
new file mode 100644
index 0000000..677ef29
--- /dev/null
+++ b/docs/parser-coverage-notes.md
@@ -0,0 +1,29 @@
+# Parser coverage notes
+
+LogLens parser coverage is intentionally visible. Noisy logs should produce a useful coverage shape instead of a quiet success claim.
+
+## Noisy auth matrix
+
+[`assets/noisy_auth_sample.log`](../assets/noisy_auth_sample.log) is a sanitized `syslog_legacy` sample for reviewer inspection. It mixes recognized authentication evidence with common log noise:
+
+- malformed timestamp evidence
+- unsupported but bucketed `sshd` preauth, disconnect, and negotiation lines
+- partial PAM evidence that is either lower-confidence parsed evidence or telemetry-only warning evidence
+- sudo denial variants that still become typed audit events
+- empty, blank, rotated, and irrelevant service lines
+- multiple hosts and intentionally unusual synthetic usernames
+
+The locked expected coverage summary lives in [`tests/fixtures/parser_matrix/noisy_auth_expected.json`](../tests/fixtures/parser_matrix/noisy_auth_expected.json). It focuses on parser quality fields rather than detector findings:
+
+- `total_input_lines`: 27
+- `skipped_blank_lines`: 3
+- `parsed_lines`: 8
+- `unparsed_lines`: 16
+- `parse_success_rate`: 0.3333333333
+- `top_unknown_patterns`: the five most common unsupported-pattern buckets
+
+## Reading the numbers
+
+A low parse success rate is not automatically a bug for this fixture. The sample is deliberately noisy, and the useful property is that unsupported evidence remains explainable through `warnings` and `top_unknown_patterns`.
+
+The matrix should stay defensive and public-safe: use documentation IP ranges, synthetic hostnames, and synthetic usernames only.
diff --git a/tests/fixtures/parser_matrix/noisy_auth_expected.json b/tests/fixtures/parser_matrix/noisy_auth_expected.json
new file mode 100644
index 0000000..b08344c
--- /dev/null
+++ b/tests/fixtures/parser_matrix/noisy_auth_expected.json
@@ -0,0 +1,38 @@
+{
+  "fixture": "assets/noisy_auth_sample.log",
+  "input_mode": "syslog_legacy",
+  "assume_year": 2026,
+  "total_input_lines": 27,
+  "total_lines": 24,
+  "skipped_blank_lines": 3,
+  "parsed_lines": 8,
+  "unparsed_lines": 16,
+  "parse_success_rate": 0.3333333333,
+  "parsed_event_count": 8,
+  "warning_count": 16,
+  "top_unknown_patterns": [
+    {"pattern": "pam_faillock_account_locked", "count": 2},
+    {"pattern": "pam_unix_session_closed", "count": 2},
+    {"pattern": "sshd_connection_closed_preauth", "count": 2},
+    {"pattern": "sshd_negotiation_failure", "count": 2},
+    {"pattern": "sshd_timeout_or_disconnection", "count": 2}
+  ],
+  "warnings": [
+    {"line_number": 1, "reason": "invalid time token"},
+    {"line_number": 2, "reason": "invalid calendar date"},
+    {"line_number": 13, "reason": "unrecognized auth pattern: sshd_connection_closed_preauth"},
+    {"line_number": 14, "reason": "unrecognized auth pattern: sshd_connection_closed_preauth"},
+    {"line_number": 15, "reason": "unrecognized auth pattern: sshd_timeout_or_disconnection"},
+    {"line_number": 16, "reason": "unrecognized auth pattern: sshd_timeout_or_disconnection"},
+    {"line_number": 17, "reason": "unrecognized auth pattern: sshd_negotiation_failure"},
+    {"line_number": 18, "reason": "unrecognized auth pattern: sshd_negotiation_failure"},
+    {"line_number": 19, "reason": "unrecognized auth pattern: pam_unix_session_closed"},
+    {"line_number": 20, "reason": "unrecognized auth pattern: pam_unix_session_closed"},
+    {"line_number": 21, "reason": "unrecognized auth pattern: pam_faillock_account_locked"},
+    {"line_number": 22, "reason": "unrecognized auth pattern: pam_faillock_account_locked"},
+    {"line_number": 23, "reason": "unrecognized auth pattern: pam_sss_unknown_user"},
+    {"line_number": 24, "reason": "unrecognized auth pattern: sudo_other"},
+    {"line_number": 26, "reason": "missing syslog header fields"},
+    {"line_number": 27, "reason": "unrecognized auth pattern: program_cron"}
+  ]
+}
diff --git a/tests/test_parser.cpp b/tests/test_parser.cpp
index c874d3d..dc111a2 100644
--- a/tests/test_parser.cpp
+++ b/tests/test_parser.cpp
@@ -2,6 +2,8 @@
 
 #include <cmath>
 #include <filesystem>
+#include <fstream>
+#include <iomanip>
 #include <sstream>
 #include <stdexcept>
 #include <string>
@@ -52,12 +54,67 @@ std::filesystem::path asset_path(std::string_view filename) {
     return repo_root() / "assets" / std::string(filename);
 }
 
+std::filesystem::path parser_matrix_fixture_path(std::string_view filename) {
+    return repo_root() / "tests" / "fixtures" / "parser_matrix" / std::string(filename);
+}
+
+std::string read_text_file(const std::filesystem::path& path) {
+    std::ifstream input(path);
+    if (!input) {
+        throw std::runtime_error("unable to read file: " + path.string());
+    }
+
+    return std::string((std::istreambuf_iterator<char>(input)), std::istreambuf_iterator<char>());
+}
+
 void expect_close(double actual, double expected, double tolerance, const std::string& message) {
     if (std::fabs(actual - expected) > tolerance) {
         throw std::runtime_error(message);
     }
 }
 
+std::size_t total_input_lines(const loglens::ParseReport& result) {
+    return result.quality.total_lines + result.quality.skipped_blank_lines;
+}
+
+std::string noisy_auth_coverage_json(const loglens::ParseReport& result) {
+    std::ostringstream output;
+    output << "{\n"
+           << "  \"fixture\": \"assets/noisy_auth_sample.log\",\n"
+           << "  \"input_mode\": \"" << loglens::to_string(result.metadata.input_mode) << "\",\n"
+           << "  \"assume_year\": " << *result.metadata.assume_year << ",\n"
+           << "  \"total_input_lines\": " << total_input_lines(result) << ",\n"
+           << "  \"total_lines\": " << result.quality.total_lines << ",\n"
+           << "  \"skipped_blank_lines\": " << result.quality.skipped_blank_lines << ",\n"
+           << "  \"parsed_lines\": " << result.quality.parsed_lines << ",\n"
+           << "  \"unparsed_lines\": " << result.quality.unparsed_lines << ",\n"
+           << "  \"parse_success_rate\": " << std::fixed << std::setprecision(10)
+           << result.quality.parse_success_rate << ",\n"
+           << "  \"parsed_event_count\": " << result.events.size() << ",\n"
+           << "  \"warning_count\": " << result.warnings.size() << ",\n"
+           << "  \"top_unknown_patterns\": [\n";
+
+    for (std::size_t index = 0; index < result.quality.top_unknown_patterns.size(); ++index) {
+        const auto& entry = result.quality.top_unknown_patterns[index];
+        output << "    {\"pattern\": \"" << entry.pattern << "\", \"count\": " << entry.count << "}";
+        output << (index + 1 == result.quality.top_unknown_patterns.size() ? "\n" : ",\n");
+    }
+
+    output << "  ],\n"
+           << "  \"warnings\": [\n";
+
+    for (std::size_t index = 0; index < result.warnings.size(); ++index) {
+        const auto& warning = result.warnings[index];
+        output << "    {\"line_number\": " << warning.line_number
+               << ", \"reason\": \"" << warning.reason << "\"}";
+        output << (index + 1 == result.warnings.size() ? "\n" : ",\n");
+    }
+
+    output << "  ]\n"
+           << "}\n";
+    return output.str();
+}
+
 void test_invalid_user_failure() {
     const auto parser = make_syslog_parser();
     std::string error;
@@ -967,6 +1024,42 @@ void test_journalctl_fixture_matrix_file() {
     expect(result.quality.top_unknown_patterns[3].count == 1, "expected one sshd negotiation-failure journalctl line");
 }
 
+void test_noisy_auth_fixture_matrix_file() {
+    const auto parser = make_syslog_parser();
+    const auto result = parser.parse_file(asset_path("noisy_auth_sample.log"));
+
+    expect(result.events.size() == 8, "expected eight parsed noisy-auth events");
+    expect(result.warnings.size() == 16, "expected sixteen noisy-auth warnings");
+    expect(total_input_lines(result) == 27, "expected noisy-auth total input line count");
+    expect(result.quality.total_lines == 24, "expected noisy-auth nonblank line count");
+    expect(result.quality.skipped_blank_lines == 3, "expected noisy-auth skipped blank line count");
+    expect(result.quality.parsed_lines == 8, "expected noisy-auth parsed line count");
+    expect(result.quality.unparsed_lines == 16, "expected noisy-auth unparsed line count");
+    expect_close(result.quality.parse_success_rate, 8.0 / 24.0, 1e-9,
+                 "expected noisy-auth parse success rate");
+
+    expect(result.events[0].hostname == "alpha-host", "expected first noisy-auth host");
+    expect(result.events[0].username == "svc+deploy", "expected unusual invalid-user username");
+    expect(result.events[1].hostname == "beta-host", "expected second noisy-auth host");
+    expect(result.events[1].username == "ops.robot", "expected dotted accepted-password username");
+    expect(result.events[2].event_type == loglens::EventType::SudoCommand,
+           "expected noisy-auth sudo command event");
+    expect(result.events[3].event_type == loglens::EventType::SudoPolicyDenied,
+           "expected noisy-auth sudoers denial event");
+    expect(result.events[4].event_type == loglens::EventType::SudoPolicyDenied,
+           "expected noisy-auth command-not-allowed denial event");
+    expect(result.events[5].event_type == loglens::EventType::PamAuthFailure,
+           "expected partial pam_unix failure to remain parsed lower-confidence evidence");
+    expect(result.events[5].username.empty(), "expected partial pam_unix failure to stay username-less");
+    expect(result.events[5].source_ip.empty(), "expected partial pam_unix failure to stay source-less");
+    expect(result.events[7].hostname == "delta-host", "expected noisy-auth multi-host coverage");
+    expect(result.events[7].username == "weird/user", "expected slash username in input_userauth_request");
+
+    const auto actual = noisy_auth_coverage_json(result);
+    const auto expected = read_text_file(parser_matrix_fixture_path("noisy_auth_expected.json"));
+    expect(actual == expected, "expected noisy auth coverage summary to match fixture");
+}
+
 }  // namespace
 
 int main() {
@@ -1016,5 +1109,6 @@ int main() {
     test_journalctl_rejects_empty_fractional_seconds();
     test_syslog_fixture_matrix_file();
     test_journalctl_fixture_matrix_file();
+    test_noisy_auth_fixture_matrix_file();
     return 0;
 }
diff --git a/tests/test_report.cpp b/tests/test_report.cpp
index 8dabc2e..858645b 100644
--- a/tests/test_report.cpp
+++ b/tests/test_report.cpp
@@ -29,6 +29,31 @@ std::string read_file(const std::filesystem::path& path) {
     return std::string((std::istreambuf_iterator<char>(input)), std::istreambuf_iterator<char>());
 }
 
+std::filesystem::path repo_root() {
+    const std::filesystem::path source_path{__FILE__};
+    std::vector<std::filesystem::path> candidates;
+
+    if (source_path.is_absolute()) {
+        candidates.push_back(source_path);
+    } else {
+        const auto cwd = std::filesystem::current_path();
+        candidates.push_back(cwd / source_path);
+        candidates.push_back(cwd.parent_path() / source_path);
+    }
+
+    for (const auto& candidate : candidates) {
+        if (std::filesystem::exists(candidate)) {
+            return candidate.parent_path().parent_path();
+        }
+    }
+
+    throw std::runtime_error("unable to resolve repository root from test source path");
+}
+
+std::filesystem::path asset_path(std::string_view filename) {
+    return repo_root() / "assets" / std::string(filename);
+}
+
 loglens::ReportData make_report_data() {
     loglens::ReportData data;
     data.input_path = std::filesystem::path{"assets/sample_auth.log"};
@@ -41,6 +66,48 @@ loglens::ReportData make_report_data() {
     return data;
 }
 
+void test_noisy_auth_report_json_keeps_unsupported_lines_visible() {
+    const auto input_path = asset_path("noisy_auth_sample.log");
+    const loglens::AuthLogParser parser(loglens::ParserConfig{
+        loglens::InputMode::SyslogLegacy,
+        2026});
+    const auto parsed = parser.parse_file(input_path);
+
+    const loglens::Detector detector;
+    const auto findings = detector.analyze(parsed.events);
+
+    loglens::ReportData data;
+    data.input_path = std::filesystem::path{"assets/noisy_auth_sample.log"};
+    data.parse_metadata = parsed.metadata;
+    data.parser_quality = parsed.quality;
+    data.events = parsed.events;
+    data.findings = findings;
+    data.warnings = parsed.warnings;
+    data.auth_signal_mappings = detector.config().auth_signal_mappings;
+
+    const auto json = loglens::render_json_report(data);
+
+    expect(findings.empty(), "expected noisy unsupported lines not to create findings");
+    expect(json.find("\"parse_success_rate\": 0.3333") != std::string::npos,
+           "expected noisy report json parse success rate");
+    expect(json.find("\"parsed_event_count\": 8") != std::string::npos,
+           "expected noisy report json parsed event count");
+    expect(json.find("\"warning_count\": 16") != std::string::npos,
+           "expected noisy report json warning count");
+    expect(json.find("\"finding_count\": 0") != std::string::npos,
+           "expected noisy report json finding count");
+    expect(json.find("\"pattern\": \"sshd_connection_closed_preauth\", \"count\": 2") != std::string::npos,
+           "expected noisy report json stable sshd preauth bucket");
+    expect(json.find("\"pattern\": \"pam_faillock_account_locked\", \"count\": 2") != std::string::npos,
+           "expected noisy report json stable pam_faillock account-lock bucket");
+    expect(json.find("\"line_number\": 13, \"reason\": \"unrecognized auth pattern: sshd_connection_closed_preauth\"")
+               != std::string::npos,
+           "expected noisy report json to keep unsupported sshd warning visible");
+    expect(json.find("\"line_number\": 24, \"reason\": \"unrecognized auth pattern: sudo_other\"")
+               != std::string::npos,
+           "expected noisy report json to keep unsupported partial sudo warning visible");
+}
+
 void test_markdown_table_cells_escape_user_controlled_values() {
     auto data = make_report_data();
 
@@ -244,6 +311,7 @@ void test_write_reports_reports_csv_write_failure() {
 }  // namespace
 
 int main() {
+    test_noisy_auth_report_json_keeps_unsupported_lines_visible();
     test_markdown_table_cells_escape_user_controlled_values();
     test_json_escapes_generic_control_characters();
     test_reports_include_total_input_line_count();