diff --git a/.abcd/work/DECISIONS.md b/.abcd/work/DECISIONS.md
index 3e6f8ed..4d2807e 100644
--- a/.abcd/work/DECISIONS.md
+++ b/.abcd/work/DECISIONS.md
@@ -283,3 +283,13 @@ Architecture-shaping decisions graduate to an ADR under
   not running in CI (tooling), doubled CI runs on PR branches (trigger
   set is tied to the merge queue), and the /dev/null TTY gate (per
   round 8).
+- 2026-07-30 — Bug-hunt round 11: hunts and refutations ran sequentially in
+  one session (the subagent model was persistently overloaded server-side;
+  the routine's sequential fallback). One substantive fix: an explicit
+  `transcribe -offset` is validated at the CLI (finite, within ±1e9 s) —
+  the flag was the one unbounded offset entry point, running the engine
+  before a bare JSON error, or writing a transcript merge refuses and a
+  sidecar transcribe's own reader refuses. Nitpicks: a demo bind failure
+  (port taken) no longer creates a stray session directory; the CHANGELOG's
+  Alice-persona claim corrected to Bob. Refuted: documenting the capture
+  endpoints' 500 (the enumerated codes are the request-refusal contract).
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 86bfb91..f0441b8 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -40,7 +40,8 @@ Capture and diagnostics:
   unopenable `http://localhost:0`; the same applies to `record -demo`.
 - The demo page falls back to `fetch` when `sendBeacon` refuses to queue a
   capture batch, which used to drop the batch silently; its seeded display
-  name now uses the Alice persona.
+  name now uses the Bob persona, so the tutorial's rename to Alice is a real
+  change.
 - Diagnostic stderr tails cut on rune boundaries rather than mid-character;
   a record refused by the JSONL writer is named as a 1-based line of the
   output file; `transcribe` defaults its log sink instead of panicking when
@@ -99,6 +100,16 @@ Invocation contract:
   `-finding`/`-verdict` pairing and verdict syntax, an unknown
   `transcribe -engine`, and a malformed capture `-addr` (which also no longer
   creates a session directory before refusing).
+- An explicit `transcribe -offset` is held to the same bound as a derived or
+  persisted one: a non-finite value used to fail only after the conversion or
+  engine work was already spent, with a bare JSON encoding error at the
+  runtime status, and a finite but absurd one wrote a transcript at exit 0
+  that `merge` refuses one command later — naming `transcript.jsonl` rather
+  than the flag — while an external run persisted a sidecar `transcribe`'s
+  own reader refuses.
+- A `demo` whose well-formed address fails to bind (the port is already taken)
+  no longer creates a session directory first, which left a stray session —
+  manifest plus two empty stream files — behind at every refused bind.
 
 Capture integrity:
 
diff --git a/docs/reference/cli.md b/docs/reference/cli.md
index 221d2fd..d602a77 100644
--- a/docs/reference/cli.md
+++ b/docs/reference/cli.md
@@ -58,7 +58,7 @@ testimony transcribe -session DIR [-audio FILE]
 | `-device` | `auto` | (whisperx) inference device: `auto`, `cpu`, or `cuda`. `auto` picks `cuda` only when an NVIDIA GPU is present, and never on macOS |
 | `-compute_type` | `auto` | (whisperx) compute type: `auto`, `int8`, `float16`, … . `auto` follows the device: `float16` on CUDA, `int8` on CPU |
 | `-vad` | `auto` | (whisperx) VAD method: `auto`, `silero`, or `pyannote`. `auto` picks `silero`; `pyannote` fails under newer torch versions |
-| `-offset` | derived | audio-to-session clock offset in seconds. When not given: with `-audio`, derived from the recording's creation time minus the manifest's `t0_epoch_ms`, or 0 when derivation is impossible; without it, read back from `audio.offset.json` when the session has one, else 0 |
+| `-offset` | derived | audio-to-session clock offset in seconds. When not given: with `-audio`, derived from the recording's creation time minus the manifest's `t0_epoch_ms`, or 0 when derivation is impossible; without it, read back from `audio.offset.json` when the session has one, else 0. A non-finite value, or one beyond ±10⁹ seconds (the bound every derived or persisted offset already meets), is a usage error |
 
 Behaviour: with `-audio`, requires ffmpeg on PATH and converts the recording to 16 kHz mono `audio.wav` in the session directory; without it (or when `-audio` points at the session's own `audio.wav`), it uses the existing `audio.wav` in place and skips the conversion. It then runs the engine, applies the offset, and writes `transcript.jsonl`. Always prints the offset it used and its provenance — one of:
 
diff --git a/internal/cli/cli.go b/internal/cli/cli.go
index 3563057..c8f4976 100644
--- a/internal/cli/cli.go
+++ b/internal/cli/cli.go
@@ -188,6 +188,17 @@ func Run(args []string) int {
 				offsetSet = true
 			}
 		})
+		// An unusable explicit offset is a wrong invocation (exit 2), refused
+		// before any conversion or engine work starts — the -window precedent
+		// above. Unchecked, a non-finite -offset failed only after that work
+		// was already spent, with a bare JSON encoding error at exit 1, and a
+		// finite but absurd one wrote a transcript at exit 0 that merge
+		// refuses one command later, naming transcript.jsonl rather than the flag.
+		if offsetSet {
+			if err := transcribe.CheckOffset(*offset); err != nil {
+				return usageErr(fmt.Errorf("transcribe: %w", err))
+			}
+		}
 		n, err := transcribe.Run(transcribe.Options{
 			SessionDir: *dir,
 			Audio:      *audio,
diff --git a/internal/cli/cli_test.go b/internal/cli/cli_test.go
index f5a9b78..35eb7ce 100644
--- a/internal/cli/cli_test.go
+++ b/internal/cli/cli_test.go
@@ -132,6 +132,31 @@ func TestReportRejectsNonFiniteWindow(t *testing.T) {
 	}
 }
 
+// TestTranscribeRejectsUnusableOffset pins -offset to the same invocation
+// contract as -window: a non-finite or over-magnitude value is a wrong
+// invocation (exit 2), refused before any conversion or engine work. Unchecked,
+// `-offset NaN` failed only after that work was already spent, with a bare
+// JSON encoding error at exit 1, and `-offset 1e300` wrote a transcript at
+// exit 0 that merge refuses one command later, naming transcript.jsonl
+// rather than the flag. The refusal must precede engine detection, so this test
+// needs no ASR engine on PATH — on the pre-fix path these invocations instead
+// failed with the engine-missing (or JSON encoding) runtime error at exit 1.
+func TestTranscribeRejectsUnusableOffset(t *testing.T) {
+	dir := miniSession(t)
+	for _, v := range []string{"NaN", "Inf", "-Inf", "1e300"} {
+		var code int
+		stderr := captureStderr(t, func() {
+			code = Run([]string{"transcribe", "-session", dir, "-offset", v})
+		})
+		if code != 2 {
+			t.Errorf("-offset %s: exit %d, want 2 (usage error, like -window)", v, code)
+		}
+		if !strings.Contains(stderr, "testimony: transcribe: -offset") {
+			t.Errorf("-offset %s: want the -offset refusal on stderr, got %q", v, stderr)
+		}
+	}
+}
+
 // TestStrayPositionalIsAUsageError pins the other half of the invocation
 // contract: no command takes positional arguments (docs/reference/cli.md), and
 // flag parsing stops at the first non-flag argument, so a stray positional
diff --git a/internal/demo/demo.go b/internal/demo/demo.go
index baf4dae..4b244e4 100644
--- a/internal/demo/demo.go
+++ b/internal/demo/demo.go
@@ -71,16 +71,33 @@ const maxBatchBody = 8 << 20
 // Run starts the demo capture server on addr, creating a new session
 // directory under outRoot. It blocks until the process is interrupted.
 func Run(addr, outRoot string) error {
+	// Bind before the session directory exists. The CLI's CheckAddr already
+	// refuses a malformed -addr up front, but a well-formed address can still
+	// fail to bind (the port is taken, or the host cannot be listened on), and
+	// creating the directory first left a stray session behind — a manifest
+	// plus two empty stream files — for a server that never served. Binding is
+	// the only way to learn the answer, so it goes first; a directory-creation
+	// failure after it just closes the listener, leaving nothing behind either
+	// way.
+	bind, err := listenAddr(addr)
+	if err != nil {
+		return err
+	}
+	ln, err := net.Listen("tcp", bind)
+	if err != nil {
+		return err
+	}
 	dir, err := session.Create(outRoot, time.Now(), session.Manifest{
 		App:         DefaultApp,
 		Participant: "P1",
 		Tasks:       []string{DefaultTask},
 	})
 	if err != nil {
+		ln.Close()
 		return err
 	}
 
-	srv, err := Serve(addr, dir)
+	srv, err := serveOn(ln, addr, dir)
 	if err != nil {
 		return err
 	}
@@ -127,24 +144,41 @@ func Shutdown(srv *http.Server) error {
 // streams into the existing session directory dir. It binds synchronously (so
 // a bind failure is returned) and then serves in the background, returning the
 // running *http.Server for the caller to Shutdown. record reuses this to run
-// the demo app into the same directory as the recorders.
+// the demo app into the same directory as the recorders; demo.Run binds first
+// itself (so a refused bind never creates a session directory) and enters at
+// serveOn.
 func Serve(addr, dir string) (*http.Server, error) {
-	// Resolve the bind address before touching the session directory, so an addr
-	// that will be refused never leaves empty stream files behind.
+	// Resolve the bind address and bind it before touching the session
+	// directory, so an addr that will be refused never leaves empty stream
+	// files behind.
 	bind, err := listenAddr(addr)
 	if err != nil {
 		return nil, err
 	}
+	ln, err := net.Listen("tcp", bind)
+	if err != nil {
+		return nil, err
+	}
+	return serveOn(ln, addr, dir)
+}
+
+// serveOn serves the demo capture app on an already-bound listener, taking
+// ownership of it: on any error the listener is closed. addr is the address
+// the operator requested, kept only to surface the requested host alongside
+// the bound port on the returned server.
+func serveOn(ln net.Listener, addr, dir string) (*http.Server, error) {
 	// The interaction shape check needs the manifest's t0 anchor, and every
 	// caller creates the session (and so the manifest) before serving into it.
 	// Loading it here also refuses a session whose anchor merge could never
 	// use, before any capture is accepted against it.
 	man, err := session.LoadManifest(dir)
 	if err != nil {
+		ln.Close()
 		return nil, err
 	}
 	t0, err := man.T0()
 	if err != nil {
+		ln.Close()
 		return nil, err
 	}
 	open := func(name string) (*os.File, error) {
@@ -152,11 +186,13 @@ func Serve(addr, dir string) (*http.Server, error) {
 	}
 	inter, err := open(session.InteractionsFile)
 	if err != nil {
+		ln.Close()
 		return nil, err
 	}
 	raw, err := open(session.RawEventsFile)
 	if err != nil {
 		inter.Close()
+		ln.Close()
 		return nil, err
 	}
 	s := &server{interactions: inter, rawEvents: raw, t0: t0}
@@ -170,12 +206,6 @@ func Serve(addr, dir string) (*http.Server, error) {
 	mux.HandleFunc("/api/interactions", s.handleInteraction)
 	mux.HandleFunc("/api/events", s.handleRawEvents)
 
-	ln, err := net.Listen("tcp", bind)
-	if err != nil {
-		inter.Close()
-		raw.Close()
-		return nil, err
-	}
 	// A deliberately wider bind serves the page to other devices, but allowWrite
 	// still pins capture posts to loopback clients — lifting that pin would
 	// reopen the CSRF/DNS-rebinding surface the guard exists for. The operator
@@ -183,8 +213,8 @@ func Serve(addr, dir string) (*http.Server, error) {
 	// which surfaces no status to the page, so without this line the first
 	// signal that a remote participant's session recorded nothing was merge
 	// counting 0 events.
-	if !loopbackHost(bind) {
-		fmt.Fprintf(os.Stderr, "testimony demo: warning: bound to %s, but capture posts are accepted from loopback clients only — a page opened from another device is served yet records nothing\n", bind)
+	if !loopbackHost(ln.Addr().String()) {
+		fmt.Fprintf(os.Stderr, "testimony demo: warning: bound to %s, but capture posts are accepted from loopback clients only — a page opened from another device is served yet records nothing\n", ln.Addr().String())
 	}
 	// The two stream files use direct O_APPEND writes (no buffering), so their
 	// data is durable without an explicit Close; the OS reclaims them on exit,
diff --git a/internal/demo/demo_test.go b/internal/demo/demo_test.go
index 25a3bab..7ada25b 100644
--- a/internal/demo/demo_test.go
+++ b/internal/demo/demo_test.go
@@ -364,6 +364,32 @@ func TestWriteEndpointGuard(t *testing.T) {
 	}
 }
 
+// TestRunBindFailureCreatesNoSessionDir pins Run's ordering: the bind comes
+// before the session directory is created. A well-formed -addr can still fail
+// to bind (most plainly, the port is already taken — a second `testimony demo`
+// beside a first), and creating the directory first left a stray session
+// behind — manifest plus two empty stream files — for a server that never
+// served, the same class the malformed-addr refusal already forecloses. Run
+// only blocks after a successful bind, so with the port held it returns.
+func TestRunBindFailureCreatesNoSessionDir(t *testing.T) {
+	ln, err := net.Listen("tcp", "127.0.0.1:0")
+	if err != nil {
+		t.Fatalf("hold a port: %v", err)
+	}
+	defer ln.Close()
+	out := t.TempDir()
+	if err := Run(ln.Addr().String(), out); err == nil {
+		t.Fatal("Run on a taken port: want a bind error, got nil")
+	}
+	entries, err := os.ReadDir(out)
+	if err != nil {
+		t.Fatalf("read out root: %v", err)
+	}
+	if len(entries) != 0 {
+		t.Errorf("a refused bind created a session directory anyway: %v", entries)
+	}
+}
+
 // TestWiderBindWarnsCaptureStaysLoopback pins the operator signal for the
 // advertised wider bind: an explicit non-loopback host serves the page to
 // other devices, but allowWrite still pins capture posts to loopback clients,
diff --git a/internal/transcribe/transcribe.go b/internal/transcribe/transcribe.go
index b644f13..0a7f2d5 100644
--- a/internal/transcribe/transcribe.go
+++ b/internal/transcribe/transcribe.go
@@ -319,6 +319,30 @@ const maxOffsetSidecarBytes = 64 << 10
 // so the operator learns which input is wrong.
 const maxOffsetSeconds = 1e9
 
+// CheckOffset validates an explicit -offset value without touching the
+// session, so the CLI can refuse an unusable offset as a usage error before
+// any work starts — the CheckEngine/CheckAddr pattern. The derived and
+// sidecar paths already refuse an offset beyond maxOffsetSeconds where the
+// bad value enters (resolveOffset, readOffsetSidecar); the explicit flag was
+// the one unbounded entry point. A non-finite flag failed only after the
+// conversion (external, at the sidecar persist) or the whole engine run
+// (in place, at the transcript write) was already spent, with a bare JSON
+// encoding error at the runtime status; a finite but absurd one wrote a
+// transcript at exit 0 that merge refuses one command later — naming
+// transcript.jsonl rather than the flag — while the external path persisted
+// a sidecar readOffsetSidecar itself refuses on the next bare run. No
+// genuine offset is refused: a value past ±1e9 seconds already fails every
+// downstream reader.
+func CheckOffset(v float64) error {
+	if math.IsNaN(v) || math.IsInf(v, 0) {
+		return fmt.Errorf("-offset must be a finite number of seconds, got %v", v)
+	}
+	if math.Abs(v) > maxOffsetSeconds {
+		return fmt.Errorf("-offset %g exceeds %g seconds in magnitude; no audio→session offset is that large", v, maxOffsetSeconds)
+	}
+	return nil
+}
+
 // writeOffsetSidecar persists offset (with its provenance, for the operator)
 // beside audio.wav. The write is atomic (temp + rename) as well as
 // no-follow: a plain truncating write destroyed the prior sidecar's bytes
diff --git a/internal/transcribe/transcribe_test.go b/internal/transcribe/transcribe_test.go
index 0d222da..2a63365 100644
--- a/internal/transcribe/transcribe_test.go
+++ b/internal/transcribe/transcribe_test.go
@@ -3,6 +3,7 @@ package transcribe
 import (
 	"errors"
 	"io"
+	"math"
 	"os"
 	"os/exec"
 	"path/filepath"
@@ -202,6 +203,23 @@ func TestMapSegmentsNegativeOffset(t *testing.T) {
 	}
 }
 
+// TestCheckOffset pins the explicit-flag bound: the derived and sidecar paths
+// refuse a non-finite or over-magnitude offset where the bad value enters, and
+// the flag path must apply the same rule. Every genuine offset (including the
+// exact ±1e9 boundary and negative values) stays accepted.
+func TestCheckOffset(t *testing.T) {
+	for _, v := range []float64{math.NaN(), math.Inf(1), math.Inf(-1), 1e300, -1e300, 1e9 + 1} {
+		if err := CheckOffset(v); err == nil {
+			t.Errorf("CheckOffset(%v): want a refusal, got nil", v)
+		}
+	}
+	for _, v := range []float64{0, 4.25, -12.4, 1e9, -1e9} {
+		if err := CheckOffset(v); err != nil {
+			t.Errorf("CheckOffset(%v): want acceptance, got %v", v, err)
+		}
+	}
+}
+
 func TestResolveOffsetFlagWins(t *testing.T) {
 	off, prov, err := resolveOffset(Options{Offset: 4.25, OffsetSet: true}, session.Manifest{T0EpochMS: 0}, true)
 	if err != nil {