diff --git a/.abcd/development/brief/04-surfaces/03-merge.md b/.abcd/development/brief/04-surfaces/03-merge.md
index c83af16..ad250ba 100644
--- a/.abcd/development/brief/04-surfaces/03-merge.md
+++ b/.abcd/development/brief/04-surfaces/03-merge.md
@@ -23,6 +23,11 @@ alongside the manifest.
   session-relative); interactions enter as `src: "event"` entries with epoch
   milliseconds converted via `t0_epoch_ms`, and are assigned sequential
   `ev-NNN` IDs at merge time.
+- When interactions are present, `t0_epoch_ms` is required: without it the
+  epoch-millisecond interaction times cannot be placed on the session clock, so
+  merge rejects the session rather than emit a silently corrupt timeline. A
+  transcript-only session (no interactions) is already session-relative and is
+  unaffected.
 - Entries are stably sorted by time and written one JSON object per line;
   schema in [`../05-internals/02-schemas.md`](../05-internals/02-schemas.md).
 - Prints the count: `merged N utterances + M events → <dir>/timeline.jsonl`.
diff --git a/.abcd/work/DECISIONS.md b/.abcd/work/DECISIONS.md
index 7792d47..3aae7dd 100644
--- a/.abcd/work/DECISIONS.md
+++ b/.abcd/work/DECISIONS.md
@@ -174,3 +174,15 @@ Architecture-shaping decisions graduate to an ADR under
   newline-less prefix) can no longer leave a partial JSONL line that corrupts one
   physical record and breaks merge's reader; corrected the false comment claiming
   `os.File.Write` gives newline atomicity.
+- 2026-07-18 — `analyze.indexTimeline` seeds `idx.end` on the first entry (`i == 0
+  || end > idx.end`), matching how `idx.start` is seeded, so a fully-negative
+  timeline (an external recording predating manifest t0) reports its true latest
+  (still-negative) entry end as `sessionEnd` instead of flooring it at the zero
+  value 0. Fixes an over-permissive finding-time upper bound that admitted a
+  finding anchored after the real session end.
+- 2026-07-18 — `timeline.Merge` rejects a session that has interactions but a
+  manifest lacking `t0_epoch_ms` (T0EpochMS == 0), since epoch-millisecond
+  interaction times cannot be placed on the session clock without it; previously
+  it used the zero-value anchor and wrote a silently corrupt timeline (~55-year
+  offsets, nonsense report duration) with exit 0. Transcript-only sessions are
+  unaffected.
diff --git a/docs/reference/cli.md b/docs/reference/cli.md
index 5494a64..07e977e 100644
--- a/docs/reference/cli.md
+++ b/docs/reference/cli.md
@@ -72,7 +72,7 @@ testimony merge -session DIR
 |---|---|---|
 | `-session` | *(required)* | session directory |
 
-Behaviour: reads `manifest.json` (required), `transcript.jsonl`, and `interactions.jsonl`; converts interaction epoch-millisecond times to session-relative seconds via `t0_epoch_ms`; writes the time-sorted `timeline.jsonl`; prints `merged N utterances + M events → <path>`. A missing `transcript.jsonl` or `interactions.jsonl` counts as zero records rather than an error, so a default audio-only `record` session (which never writes `interactions.jsonl`) still merges to a speech-only timeline.
+Behaviour: reads `manifest.json` (required), `transcript.jsonl`, and `interactions.jsonl`; converts interaction epoch-millisecond times to session-relative seconds via `t0_epoch_ms`; writes the time-sorted `timeline.jsonl`; prints `merged N utterances + M events → <path>`. A missing `transcript.jsonl` or `interactions.jsonl` counts as zero records rather than an error, so a default audio-only `record` session (which never writes `interactions.jsonl`) still merges to a speech-only timeline. When interactions are present, `t0_epoch_ms` is required: without it their epoch-millisecond times cannot be placed on the session clock, so merge fails rather than write a corrupt timeline.
 
 ## `testimony report`
 
diff --git a/internal/analyze/analyze_test.go b/internal/analyze/analyze_test.go
index 064adab..eec774a 100644
--- a/internal/analyze/analyze_test.go
+++ b/internal/analyze/analyze_test.go
@@ -185,6 +185,26 @@ func TestIngestAcceptsNegativeAnchoredFinding(t *testing.T) {
 	}
 }
 
+// TestIngestRejectsFindingAfterNegativeSessionEnd is the loose-upper-bound
+// regression: when every timeline entry sits at negative session-relative time
+// (an external recording predating manifest t0), the session end is the latest
+// still-negative entry, not 0. Pre-fix indexTimeline seeded idx.end from the zero
+// value 0 and only grew it on `end > idx.end`, so a fully-negative timeline
+// reported its end as 0 and admitted a finding anchored after the real session
+// end. negativeTimeline's latest end is utt-004's t1 = -1, so t = -0.5 is after
+// the session ended and must be rejected.
+func TestIngestRejectsFindingAfterNegativeSessionEnd(t *testing.T) {
+	dir := writeSession(t, negativeTimeline)
+	answer := `{"rubric":"testimony-analysis/v1","findings":[
+	 {"id":"F-001","t":-0.5,"type":"bug","severity":3,"quote":"I clicked save and nothing happened",
+	  "evidence":["utt-004","ev-003"],"status":"unverified"}
+	]}`
+	_, err := Ingest(dir, strings.NewReader(answer))
+	if err == nil || !strings.Contains(err.Error(), "outside the session") {
+		t.Fatalf("expected an out-of-range refusal for t after the negative session end, got %v", err)
+	}
+}
+
 // TestIngestRefusesEmptyFindings is the data-loss regression: an empty findings
 // array must not truncate a prior good findings.jsonl. Pre-fix Ingest wrote an
 // empty slice with O_TRUNC and reported success.
diff --git a/internal/analyze/validate.go b/internal/analyze/validate.go
index 7ef0270..1b855fe 100644
--- a/internal/analyze/validate.go
+++ b/internal/analyze/validate.go
@@ -53,7 +53,12 @@ func indexTimeline(entries []timeline.Entry) timelineIndex {
 				idx.routes[r] = true
 			}
 		}
-		if end > idx.end {
+		// Seed idx.end on the first entry, exactly as idx.start is seeded below, so
+		// the upper bound reflects the true maximum end even when every entry sits
+		// at negative session-relative time (a recording predating t0, audio-only).
+		// Growing from the zero value 0 would floor the end at 0 and admit a finding
+		// anchored after the real (negative) session end.
+		if i == 0 || end > idx.end {
 			idx.end = end
 		}
 		// Track the earliest entry start so the finding-time lower bound matches
diff --git a/internal/timeline/timeline.go b/internal/timeline/timeline.go
index 2ac43f1..c2e4335 100644
--- a/internal/timeline/timeline.go
+++ b/internal/timeline/timeline.go
@@ -156,6 +156,16 @@ func Merge(dir string) (speech, events int, err error) {
 		return 0, 0, fmt.Errorf("read interactions: %w", err)
 	}
 
+	// Interaction times are epoch milliseconds; without t0 they cannot be placed
+	// on the session clock. A manifest lacking t0_epoch_ms leaves T0EpochMS at the
+	// zero value, which would turn each epoch-ms timestamp into a ~55-year offset
+	// and write a silently corrupt timeline. Reject it rather than emit nonsense.
+	// A transcript-only session carries no interactions and is already
+	// session-relative, so it is unaffected.
+	if len(ints) > 0 && man.T0EpochMS == 0 {
+		return 0, 0, fmt.Errorf("manifest is missing t0_epoch_ms; cannot place interactions on the session clock")
+	}
+
 	entries := BuildEntries(man.T0EpochMS, utts, ints)
 	if err := session.WriteJSONL(filepath.Join(dir, session.TimelineFile), entries); err != nil {
 		return 0, 0, fmt.Errorf("write timeline: %w", err)
diff --git a/internal/timeline/timeline_test.go b/internal/timeline/timeline_test.go
index 80e2d65..5c7e80f 100644
--- a/internal/timeline/timeline_test.go
+++ b/internal/timeline/timeline_test.go
@@ -3,6 +3,7 @@ package timeline
 import (
 	"os"
 	"path/filepath"
+	"strings"
 	"testing"
 
 	"github.com/REPPL/Testimony/internal/session"
@@ -74,6 +75,30 @@ func TestMergeAudioOnlySession(t *testing.T) {
 	}
 }
 
+// TestMergeRejectsMissingT0WithInteractions is the missing-anchor regression: a
+// session with interactions.jsonl but a manifest lacking t0_epoch_ms cannot place
+// those epoch-ms interaction times on the session clock. Pre-fix Merge used the
+// zero-value t0 (0), turning each epoch-ms timestamp into a ~55-year offset and
+// writing a silently corrupt timeline while exiting 0. Merge must reject it.
+func TestMergeRejectsMissingT0WithInteractions(t *testing.T) {
+	dir := t.TempDir()
+	// Manifest without t0_epoch_ms (left at the zero value).
+	if err := session.SaveManifest(dir, session.Manifest{Session: "s"}); err != nil {
+		t.Fatalf("SaveManifest: %v", err)
+	}
+	ints := []Interaction{{T: t0 + 9_500, Kind: "click", Selector: "[data-testid=save-btn]"}}
+	if err := session.WriteJSONL(filepath.Join(dir, session.InteractionsFile), ints); err != nil {
+		t.Fatalf("write interactions: %v", err)
+	}
+	if _, _, err := Merge(dir); err == nil || !strings.Contains(err.Error(), "t0_epoch_ms") {
+		t.Fatalf("expected a missing-t0 error, got %v", err)
+	}
+	// The corrupt timeline must not have been written.
+	if _, statErr := os.Stat(filepath.Join(dir, session.TimelineFile)); statErr == nil {
+		t.Fatalf("timeline.jsonl was written despite the missing t0")
+	}
+}
+
 func TestEventsNearWindow(t *testing.T) {
 	entries := sample()
 	var speech []Entry