diff --git a/CHANGELOG.md b/CHANGELOG.md
index 1decda0..77f69ea 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -6,6 +6,10 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/),
 
 ## [Unreleased]
 
+### Security
+
+- Redact secret-shaped tokens (at minimum HuggingFace `hf_...` tokens) from `service.log`, the raw child `service-stderr.log` tee, and `app.log` before they are written, so the shareable **Export Diagnostics…** bundle cannot leak a credential embedded in third-party output (e.g. a token surfaced in a dependency's traceback).
+
 ## [1.2.0] - 2026-05-08
 
 ### Added
diff --git a/backend/services/logging_setup.py b/backend/services/logging_setup.py
index 72ce481..aba0297 100644
--- a/backend/services/logging_setup.py
+++ b/backend/services/logging_setup.py
@@ -15,6 +15,7 @@
 from __future__ import annotations
 
 import logging
+import re
 from logging.handlers import RotatingFileHandler
 from pathlib import Path
 
@@ -28,6 +29,61 @@
 # transcription pipeline spawns workers that re-exec this binary).
 _HANDLER_MARKER = "_mt_service_log_handler"
 
+# Secret-shaped tokens to mask before anything reaches disk (story #139). Our own
+# code never logs the HuggingFace token, but a third-party dependency could embed
+# it in a traceback (e.g. a 401 body), which would then land in service.log and
+# the user-shareable diagnostics zip. Keep every pattern in this one list so more
+# credential shapes can be added later. The Swift side mirrors this list in
+# macos/Sources/MeetingTranscriberKit/Logging/SecretRedaction.swift — keep them
+# in sync (both are exercised with the shared `hf_TESTTOKEN...` test vector).
+_SECRET_PATTERNS: list[tuple[re.Pattern[str], str]] = [
+    (re.compile(r"hf_[A-Za-z0-9]+"), "hf_***"),
+]
+
+
+def redact_secrets(text: str) -> str:
+    """Mask every known secret shape in ``text``. Safe on arbitrary strings."""
+    for pattern, replacement in _SECRET_PATTERNS:
+        text = pattern.sub(replacement, text)
+    return text
+
+
+class SecretRedactingFilter(logging.Filter):
+    """Mask secrets in a record's message, traceback, and stack info.
+
+    Installed on the file handler (not the root logger): a filter added to a
+    logger only runs for records logged *directly* on it, so records propagated
+    up from child loggers — notably ``uvicorn`` — would bypass it. A handler
+    filter runs for every record the handler emits.
+
+    The traceback is pre-rendered here and stashed in ``record.exc_text`` so the
+    handler's formatter reuses the redacted text instead of re-rendering the raw
+    ``exc_info``. Fails open: any formatting error leaves the record as-is rather
+    than raising back at the log call site.
+    """
+
+    def filter(self, record: logging.LogRecord) -> bool:
+        try:
+            if record.args:
+                try:
+                    record.msg = record.getMessage()
+                except Exception:  # noqa: BLE001
+                    # Malformed format record (e.g. mismatched %-args): keep the
+                    # raw template but drop args so the formatter cannot raise.
+                    record.msg = str(record.msg)
+                record.args = None
+            if isinstance(record.msg, str):
+                record.msg = redact_secrets(record.msg)
+            if record.exc_info and not record.exc_text:
+                record.exc_text = logging.Formatter().formatException(record.exc_info)
+            if record.exc_text:
+                record.exc_text = redact_secrets(record.exc_text)
+            if record.stack_info:
+                record.stack_info = redact_secrets(record.stack_info)
+        except Exception:  # noqa: BLE001 — redaction must never break logging.
+            pass
+        return True
+
 
 def _already_installed(root: logging.Logger, log_file: Path) -> bool:
     for handler in root.handlers:
@@ -62,6 +118,7 @@ def configure_service_logging(log_dir: Path | None = None) -> Path:
     )
     handler.setLevel(logging.INFO)
     handler.setFormatter(logging.Formatter(_LOG_FORMAT))
+    handler.addFilter(SecretRedactingFilter())
     setattr(handler, _HANDLER_MARKER, True)
     root.addHandler(handler)
     return log_file
diff --git a/docs/plans/139-redact-secrets-from-logs.md b/docs/plans/139-redact-secrets-from-logs.md
new file mode 100644
index 0000000..bde7f60
--- /dev/null
+++ b/docs/plans/139-redact-secrets-from-logs.md
@@ -0,0 +1,70 @@
+# Plan: Redact secrets from logs and the diagnostics export
+
+**Story**: #139
+**Spec**: N/A (follow-up to #137 / PR #138)
+**Branch**: feature/139-redact-secrets-from-logs
+**Date**: 2026-09-03
+**Mode**: TDD — both redactors are pure functions with clear input/output.
+
+## Technical Decisions
+
+### TD-1: Attach the redacting filter to the handler, not the root logger
+- **Context**: AC1 asks for a filter "installed on the root logger". A `logging.Filter` added via `Logger.addFilter` only runs for records logged *directly* on that logger — records propagated up from child loggers (notably `uvicorn`/`uvicorn.access`) skip it.
+- **Decision**: Add the filter to the `RotatingFileHandler` (which lives on the root logger). Handler-level filters run for every record the handler emits, including propagated ones.
+- **Alternatives considered**: `root.addFilter(...)` — rejected, misses propagated records; a `RedactingFormatter` subclass — works but AC explicitly asks for a filter, and a filter keeps message/traceback handling in one place.
+
+### TD-2: Pre-render redacted traceback into `record.exc_text`
+- **Context**: A filter runs before the Formatter renders `record.exc_info`, so redacting `record.msg` alone leaves the traceback unmasked.
+- **Decision**: In the filter, format the exception via `logging.Formatter().formatException(record.exc_info)`, redact it, and assign to `record.exc_text`. `Formatter.format` reuses a non-empty `exc_text` instead of re-rendering, so the on-disk traceback is redacted.
+- **Alternatives considered**: clearing `exc_info` — rejected, loses structured info other handlers may want.
+
+### TD-3: Fail-open filter
+- **Context**: `record.getMessage()` / `formatException()` can raise (e.g. mismatched `%` args). Filters run before the handler's emit try/except, so a raise escapes to the log call site.
+- **Decision**: Wrap the redaction work in try/except and return `True` on failure — never drop or raise from the redaction filter.
+
+### TD-4: Per-chunk Swift tee redaction
+- **Context**: The raw child stderr tee (`service-stderr.log`) bypasses Python logging entirely. For stderr that never passes through Python (uncaught interpreter tracebacks, C-level/third-party direct prints), the Swift tee is the *sole* redaction sink, not a backstop.
+- **Decision**: Redact each `availableData` chunk at the single tee write site. Accepted limitation: a token split across two reads could evade masking (low risk — tracebacks arrive as a burst).
+- **Alternatives considered**: newline-buffered tee with EOF flush — more robust but adds a second buffer to a concurrency-sensitive drain loop; deferred for simplicity.
+
+## Files to Create or Modify
+
+- `backend/services/logging_setup.py` — add `_SECRET_PATTERNS`, `redact_secrets()`, `SecretRedactingFilter`; attach filter to the handler.
+- `tests/unit/test_logging_setup.py` — message/traceback on-disk redaction, fail-open, `redact_secrets` unit.
+- `macos/Sources/MeetingTranscriberKit/Logging/SecretRedaction.swift` (new) — `SecretRedaction.redact(_:)`.
+- `macos/Sources/MeetingTranscriberKit/Service/ServiceSupervisor.swift` — redact chunk before teeing.
+- `macos/Sources/MeetingTranscriberKit/Logging/AppLog.swift` — redact lines before writing (app.log is bundled too).
+- `macos/Sources/MeetingTranscriberKitTests/SecretRedactionTests.swift` (new) + `main.swift` registration.
+- `CHANGELOG.md` — Unreleased entry.
+
+## Approach per AC
+
+### AC1/AC2: Redacting filter on the root logger covering message + traceback
+`SecretRedactingFilter` attached to the `RotatingFileHandler`; redacts `record.msg` (args collapsed via `getMessage()`), `record.exc_text` (pre-rendered), and `record.stack_info`.
+
+### AC3: Raw stderr tee redacted Swift-side
+`SecretRedaction.redact` applied at the tee write site in `drainAndTee`; `AppLog` writes also routed through it.
+
+### AC4: Unit test on-disk
+Log a record/exception containing `hf_TESTTOKEN`; assert the on-disk `service.log` contains `hf_***`, not the token.
+
+### AC5: Diagnostics zip free of the token
+Python on-disk test as automated coverage; documented manual check in the PR (seed token, export, `unzip -p | grep`). Shared `hf_TESTTOKEN` vector in Python and Swift tests to surface pattern drift.
+
+## Commit Sequence
+
+1. Python redacting filter + tests
+2. Swift redactor + tee/AppLog redaction + tests
+3. CHANGELOG + plan doc
+
+## Risks and Trade-offs
+
+- Per-chunk Swift redaction: token split across reads could evade masking (documented, low risk).
+- Two pattern lists (Python + Swift) can drift; mitigated by cross-reference comments and a shared test token.
+- Redaction is forward-only — existing/rotated on-disk log content is not rewritten (our code never logged tokens historically).
+- app.log carries only Swift-authored lifecycle strings, but is routed through the redactor anyway as near-free defense-in-depth.
+
+## Deviations from Plan
+
+- The fail-open test (`test_filter_neutralizes_malformed_format_record`) exercises `SecretRedactingFilter.filter()` directly rather than through the logging pipeline: pytest's own capture handler re-raises a malformed-`%`-args record independent of our filter, so a full-pipeline test could not isolate our filter's behavior. The filter now also drops `record.args` when `getMessage()` raises, so no downstream formatter can raise either.
+- The Swift redactor also covers `app.log` (via `AppLog.line`), not only the `service-stderr.log` tee — near-free defense-in-depth for the third bundled sink.
diff --git a/macos/Sources/MeetingTranscriberIntegrationTests/main.swift b/macos/Sources/MeetingTranscriberIntegrationTests/main.swift
index d1d9469..450cdd7 100644
--- a/macos/Sources/MeetingTranscriberIntegrationTests/main.swift
+++ b/macos/Sources/MeetingTranscriberIntegrationTests/main.swift
@@ -181,11 +181,57 @@ func scenarioPostHandshakeFloodDoesNotBlock() {
     check(!supervisor.isRunning, "flood: child terminated")
 }
 
+// MARK: - Scenario 6: secrets in child stderr are redacted in the tee (story #139)
+
+func scenarioStderrTokenIsRedacted() {
+    // The child prints an hf_ token to stderr (as a third-party dep might in a
+    // traceback). The supervisor's tee must write the placeholder, not the token,
+    // to service-stderr.log — the file bundled into the diagnostics export. This
+    // drives the real drainAndTee → SecretRedaction seam end-to-end.
+    let tmp = FileManager.default.temporaryDirectory
+        .appendingPathComponent("supervisor-redact-\(UUID().uuidString)", isDirectory: true)
+    try? FileManager.default.createDirectory(at: tmp, withIntermediateDirectories: true)
+    defer { try? FileManager.default.removeItem(at: tmp) }
+
+    let token = "hf_TESTTOKEN0123456789abcdef"
+    let nonce = UUID().uuidString
+    let code = """
+    import json, os, sys, time
+    print(json.dumps({"event": "ready", "port": 45998, "nonce": os.environ.get("MT_SERVICE_NONCE", "")}), flush=True)
+    sys.stderr.write("401 Unauthorized for \(token)\\n"); sys.stderr.flush()
+    time.sleep(30)
+    """
+
+    let stderrLog = FileLog(fileName: "service-stderr.log", directory: tmp)
+    let supervisor = ServiceSupervisor(stderrLog: stderrLog)
+    do {
+        let port = try supervisor.start(python(code, nonce: nonce), timeout: 10)
+        check(port == 45998, "redact: handshake parsed (got \(port))")
+
+        // Give the tee a moment to drain the stderr line, then flush.
+        let deadline = Date().addingTimeInterval(5)
+        var body = ""
+        repeat {
+            stderrLog.flush()
+            body = (try? String(contentsOf: stderrLog.url, encoding: .utf8)) ?? ""
+            if body.contains("hf_***") { break }
+            Thread.sleep(forTimeInterval: 0.05)
+        } while Date() < deadline
+        check(!body.contains(token), "redact: token absent from service-stderr.log")
+        check(body.contains("hf_***"), "redact: placeholder written to service-stderr.log")
+    } catch {
+        check(false, "redact: start threw \(error)")
+    }
+    supervisor.terminate(gracePeriod: 2)
+    check(!supervisor.isRunning, "redact: child terminated")
+}
+
 await scenarioStubService()
 scenarioNoisyStdout()
 scenarioSigkillEscalation()
 scenarioNonceMismatch()
 scenarioPostHandshakeFloodDoesNotBlock()
+scenarioStderrTokenIsRedacted()
 
 let summary = "\n\(passed) passed, \(failed) failed\n"
 FileHandle.standardOutput.write(Data(summary.utf8))
diff --git a/macos/Sources/MeetingTranscriberKit/Logging/AppLog.swift b/macos/Sources/MeetingTranscriberKit/Logging/AppLog.swift
index 1b48b72..7bd3219 100644
--- a/macos/Sources/MeetingTranscriberKit/Logging/AppLog.swift
+++ b/macos/Sources/MeetingTranscriberKit/Logging/AppLog.swift
@@ -32,6 +32,10 @@ public enum AppLog {
     }
 
     private static func line(_ level: String, _ message: String) -> String {
-        "\(ISO8601DateFormatter().string(from: Date())) \(level) app: \(message)"
+        // app.log is bundled into the diagnostics export; redact as
+        // defense-in-depth even though these are Swift-authored breadcrumbs that
+        // never interpolate tokens today (story #139).
+        let safe = SecretRedaction.redact(message)
+        return "\(ISO8601DateFormatter().string(from: Date())) \(level) app: \(safe)"
     }
 }
diff --git a/macos/Sources/MeetingTranscriberKit/Logging/SecretRedaction.swift b/macos/Sources/MeetingTranscriberKit/Logging/SecretRedaction.swift
new file mode 100644
index 0000000..bc1465d
--- /dev/null
+++ b/macos/Sources/MeetingTranscriberKit/Logging/SecretRedaction.swift
@@ -0,0 +1,38 @@
+// Story #139 — redact secrets from logs and the diagnostics export.
+// Plan: docs/plans/139-redact-secrets-from-logs.md
+
+import Foundation
+
+/// Masks secret-shaped tokens before text is written to a shareable log file.
+///
+/// The raw child stderr tee (`service-stderr.log`) and the app's own `app.log`
+/// are both bundled into the user-shareable diagnostics zip, and the raw stderr
+/// bypasses Python's logging formatter entirely — so for third-party output that
+/// never passes through Python (e.g. an uncaught traceback printed by
+/// `huggingface_hub`/`pyannote`), this is the *sole* redaction sink, not a
+/// backstop. Keep the pattern list in sync with the Python side in
+/// `backend/services/logging_setup.py` (`_SECRET_PATTERNS`); both are exercised
+/// with the shared `hf_TESTTOKEN...` test vector so a divergence surfaces.
+public enum SecretRedaction {
+    /// (pattern, replacement) pairs. Add more credential shapes here.
+    private static let patterns: [(NSRegularExpression, String)] = {
+        let specs = [
+            (#"hf_[A-Za-z0-9]+"#, "hf_***"),
+        ]
+        return specs.compactMap { pattern, replacement in
+            (try? NSRegularExpression(pattern: pattern)).map { ($0, replacement) }
+        }
+    }()
+
+    /// Return `text` with every known secret shape masked. Safe on any string.
+    public static func redact(_ text: String) -> String {
+        var result = text
+        for (regex, replacement) in patterns {
+            let range = NSRange(result.startIndex..., in: result)
+            result = regex.stringByReplacingMatches(
+                in: result, range: range, withTemplate: replacement
+            )
+        }
+        return result
+    }
+}
diff --git a/macos/Sources/MeetingTranscriberKit/Service/ServiceSupervisor.swift b/macos/Sources/MeetingTranscriberKit/Service/ServiceSupervisor.swift
index f404e6c..53480b6 100644
--- a/macos/Sources/MeetingTranscriberKit/Service/ServiceSupervisor.swift
+++ b/macos/Sources/MeetingTranscriberKit/Service/ServiceSupervisor.swift
@@ -167,7 +167,18 @@ public final class ServiceSupervisor {
             while true {
                 let chunk = handle.availableData
                 if chunk.isEmpty { break } // EOF
-                if matched || teeBeforeMatch { self?.stderrLog?.write(chunk) }
+                // Redact secrets before teeing: this raw child output bypasses
+                // Python's logging redaction, and the file is bundled into the
+                // shareable diagnostics zip (story #139). This single write site
+                // serves both the stderr tee and the post-handshake stdout tee.
+                // Per-chunk redaction accepts one boundary limitation: a token
+                // split across two availableData reads could evade masking.
+                if matched || teeBeforeMatch {
+                    let redacted = SecretRedaction.redact(
+                        String(decoding: chunk, as: UTF8.self)
+                    )
+                    self?.stderrLog?.write(Data(redacted.utf8))
+                }
                 if matched { continue } // keep draining + teeing, stop parsing
                 buffer.append(chunk)
                 while let newline = buffer.firstIndex(of: 0x0A) {
diff --git a/macos/Sources/MeetingTranscriberKitTests/DiagnosticsExporterTests.swift b/macos/Sources/MeetingTranscriberKitTests/DiagnosticsExporterTests.swift
index 83881f6..6b1f638 100644
--- a/macos/Sources/MeetingTranscriberKitTests/DiagnosticsExporterTests.swift
+++ b/macos/Sources/MeetingTranscriberKitTests/DiagnosticsExporterTests.swift
@@ -35,5 +35,36 @@ func runDiagnosticsExporterTests() {
         // Summary carries the identifying header.
         expect(DiagnosticsExporter.summary().contains("MeetingTranscriber diagnostics"),
                "summary has header")
+
+        // AC5 (story #139): a token that passed through the redacting write path
+        // must be absent from the exported zip. Seed a log the way the stderr tee
+        // would write it (raw child output run through SecretRedaction.redact),
+        // export, extract, and grep the extracted tree for the raw token.
+        let seededToken = "hf_TESTTOKEN0123456789abcdef"
+        let teed = SecretRedaction.redact("Traceback: 401 Unauthorized for \(seededToken)\n")
+        try? Data(teed.utf8).write(to: logs.appendingPathComponent("service-stderr.log"))
+
+        let secretDest = tmp.appendingPathComponent("diag-secret.zip")
+        do {
+            try DiagnosticsExporter.exportDiagnostics(to: secretDest, logsDirectory: logs)
+        } catch {
+            expect(false, "secret export threw \(error)")
+        }
+
+        let extracted = tmp.appendingPathComponent("extracted-\(UUID().uuidString)", isDirectory: true)
+        let ditto = Process()
+        ditto.executableURL = URL(fileURLWithPath: "/usr/bin/ditto")
+        ditto.arguments = ["-x", "-k", secretDest.path, extracted.path]
+        try? ditto.run()
+        ditto.waitUntilExit()
+
+        var zipBody = ""
+        if let files = FileManager.default.enumerator(at: extracted, includingPropertiesForKeys: nil) {
+            for case let url as URL in files {
+                zipBody += (try? String(contentsOf: url, encoding: .utf8)) ?? ""
+            }
+        }
+        expect(!zipBody.contains(seededToken), "exported zip is free of the seeded token")
+        expect(zipBody.contains("hf_***"), "exported zip carries the redacted placeholder")
     }
 }
diff --git a/macos/Sources/MeetingTranscriberKitTests/SecretRedactionTests.swift b/macos/Sources/MeetingTranscriberKitTests/SecretRedactionTests.swift
new file mode 100644
index 0000000..5f82a84
--- /dev/null
+++ b/macos/Sources/MeetingTranscriberKitTests/SecretRedactionTests.swift
@@ -0,0 +1,43 @@
+// Story #139 — redact secrets from logs and the diagnostics export.
+// Plan: docs/plans/139-redact-secrets-from-logs.md
+
+import Foundation
+import MeetingTranscriberKit
+
+// Shared canonical vector — the Python test (`_SEEDED_TOKEN`) uses the same
+// string so a pattern divergence between the two runtimes surfaces.
+private let seededToken = "hf_TESTTOKEN0123456789abcdef"
+
+func runSecretRedactionTests() {
+    suite("SecretRedaction") {
+        // Masks an hf_ token embedded in surrounding text.
+        let redacted = SecretRedaction.redact("401 Unauthorized for \(seededToken) here")
+        expect(!redacted.contains(seededToken), "token removed, got \(redacted.debugDescription)")
+        expect(redacted.contains("hf_***"), "placeholder present, got \(redacted.debugDescription)")
+
+        // Leaves non-secret text untouched.
+        expectEqual(
+            SecretRedaction.redact("plain line, no secrets"),
+            "plain line, no secrets",
+            "non-secret text unchanged"
+        )
+
+        // Masks every occurrence, not just the first.
+        let multi = SecretRedaction.redact("\(seededToken) and \(seededToken)")
+        expect(!multi.contains(seededToken), "all occurrences masked, got \(multi.debugDescription)")
+
+        // The stderr tee redacts before writing to the bundled log file.
+        let dir = FileManager.default.temporaryDirectory
+            .appendingPathComponent("redaction-tests-\(UUID().uuidString)", isDirectory: true)
+        try? FileManager.default.createDirectory(at: dir, withIntermediateDirectories: true)
+        defer { try? FileManager.default.removeItem(at: dir) }
+
+        let stderrLog = FileLog(fileName: "service-stderr.log", directory: dir)
+        let chunk = "Traceback: RuntimeError 401 for \(seededToken)\n"
+        stderrLog.write(Data(SecretRedaction.redact(chunk).utf8))
+        stderrLog.flush()
+        let onDisk = (try? String(contentsOf: stderrLog.url, encoding: .utf8)) ?? ""
+        expect(!onDisk.contains(seededToken), "tee redacts token on disk, got \(onDisk.debugDescription)")
+        expect(onDisk.contains("hf_***"), "tee wrote placeholder, got \(onDisk.debugDescription)")
+    }
+}
diff --git a/macos/Sources/MeetingTranscriberKitTests/main.swift b/macos/Sources/MeetingTranscriberKitTests/main.swift
index b6e461d..480a863 100644
--- a/macos/Sources/MeetingTranscriberKitTests/main.swift
+++ b/macos/Sources/MeetingTranscriberKitTests/main.swift
@@ -17,6 +17,7 @@ runPlainTextRendererTests()
 runOverviewAggregationsTests()
 runFileLogTests()
 runDiagnosticsExporterTests()
+runSecretRedactionTests()
 
 // (unnamed-speaker suite runs inside runSpeakerColorTests)
 TestRunner.shared.finish()
diff --git a/tests/unit/test_logging_setup.py b/tests/unit/test_logging_setup.py
index f6415cd..e0b5d50 100644
--- a/tests/unit/test_logging_setup.py
+++ b/tests/unit/test_logging_setup.py
@@ -85,3 +85,63 @@ def test_defaults_to_app_paths_logs_dir(self, tmp_path: Path, monkeypatch: pytes
         log_file = logging_setup.configure_service_logging()
 
         assert log_file == (tmp_path / "Logs" / "service.log").resolve()
+
+
+# Shared canonical token; the Swift redaction test uses the same string so a
+# pattern divergence between the two runtimes surfaces (story #139).
+_SEEDED_TOKEN = "hf_TESTTOKEN0123456789abcdef"
+
+
+class TestRedactSecrets:
+    def test_masks_hf_token(self):
+        redacted = logging_setup.redact_secrets(f"auth failed for {_SEEDED_TOKEN} now")
+
+        assert _SEEDED_TOKEN not in redacted
+        assert "hf_***" in redacted
+
+    def test_leaves_non_secret_text_untouched(self):
+        assert logging_setup.redact_secrets("plain message, no secrets") == ("plain message, no secrets")
+
+
+class TestSecretRedactingFilterOnDisk:
+    def test_message_token_is_redacted_on_disk(self, tmp_path: Path):
+        log_file = logging_setup.configure_service_logging(log_dir=tmp_path)
+
+        logging.getLogger("backend.services.redaction_probe").info("received token %s from provider", _SEEDED_TOKEN)
+
+        contents = log_file.read_text()
+        assert _SEEDED_TOKEN not in contents
+        assert "hf_***" in contents
+
+    def test_exception_traceback_token_is_redacted_on_disk(self, tmp_path: Path):
+        log_file = logging_setup.configure_service_logging(log_dir=tmp_path)
+        logger = logging.getLogger("backend.services.redaction_probe")
+
+        try:
+            raise RuntimeError(f"401 Unauthorized: {_SEEDED_TOKEN}")
+        except RuntimeError:
+            logger.exception("diarization call failed")
+
+        contents = log_file.read_text()
+        assert _SEEDED_TOKEN not in contents
+        assert "hf_***" in contents
+        # The traceback itself must be present (redaction, not omission).
+        assert "Traceback" in contents
+
+    def test_filter_neutralizes_malformed_format_record(self):
+        # A mismatched-args record makes getMessage() raise. The filter must not
+        # raise and must drop args so a downstream formatter cannot raise either.
+        record = logging.LogRecord(
+            name="probe",
+            level=logging.INFO,
+            pathname=__file__,
+            lineno=1,
+            msg="value is %d and %d",  # too few args on purpose
+            args=(1,),
+            exc_info=None,
+        )
+
+        assert logging_setup.SecretRedactingFilter().filter(record) is True
+        assert record.args is None
+        # getMessage() must now be safe (no formatting) — proves no downstream raise.
+        assert record.getMessage() == "value is %d and %d"