diff --git a/CHANGELOG.md b/CHANGELOG.md
index 6ebc3a54..39c25f2f 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -28,6 +28,10 @@ called out in a **Breaking** section.
 
 - **An issue graduates into an intent without retyping.** `abcd capture promote <iss-N>` is the native verb for step 2 of the record walk: one invocation mints an intent draft — slug reused from the issue, body carrying a by-id pointer to the issue rather than a copy, `promoted_from: iss-N` in its frontmatter — and stamps the issue's `promoted_to` with the minted `itd-N`. Promotion works from any status folder and never moves the issue, a second promote is refused with the existing `itd-N`, and a stamp failure after the mint names the orphan draft and its repair: `capture promote <iss-N> --intent <itd-N>`, the stamp-only mode that links an existing draft instead of minting. (itd-119, spc-24; the `promoted_to` half of iss-245)
 
+### Fixed
+
+- **An uppercase YAML null no longer reads as a superseding handle.** `frontmatter.IsNull` — and its byte-identical lint-side copy `isNull` — now resolve all four YAML nulls (`null` | `Null` | `NULL` | `~`) plus the empty scalar, the same set the memory package's own scalar parser already held. The two uppercase spellings were previously missed, so `disembark pack` over a foreign target repo emitted a live (`status: accepted`) ADR carrying `superseded_by: NULL` as a `superseded-adr` finding into `graveyard/abandoned.json`, quoting the null literal as its evidence; the narrowing also sat behind record-lint's null gate. Both copies are widened together so the two-gate agreement `capture/validate.go` depends on cannot drift. (#290)
+
 ## [0.5.1] - 2026-08-16
 
 ### Added
diff --git a/internal/core/frontmatter/frontmatter.go b/internal/core/frontmatter/frontmatter.go
index 1b6b9a75..7f19ca45 100644
--- a/internal/core/frontmatter/frontmatter.go
+++ b/internal/core/frontmatter/frontmatter.go
@@ -63,7 +63,10 @@ func Fields(lines []string) map[string]Field {
 	return fields
 }
 
-// IsNull treats an empty value and the YAML nulls ""/"null"/"~" as null.
+// IsNull treats an empty value and the four YAML nulls ""/"null"/"Null"/"NULL"/"~"
+// as null. YAML 1.1 (!!null) and the YAML 1.2 core schema (§10.2.1.1) both resolve
+// the three cased spellings and the tilde; the memory package's own scalar parser
+// holds the same set (internal/core/memory/yaml.go).
 func IsNull(v string) bool {
-	return v == "" || v == "null" || v == "~"
+	return v == "" || v == "null" || v == "Null" || v == "NULL" || v == "~"
 }
diff --git a/internal/core/frontmatter/frontmatter_test.go b/internal/core/frontmatter/frontmatter_test.go
index f444cb95..d5c41fb1 100644
--- a/internal/core/frontmatter/frontmatter_test.go
+++ b/internal/core/frontmatter/frontmatter_test.go
@@ -59,12 +59,21 @@ func TestFieldsTrimsCarriageReturn(t *testing.T) {
 }
 
 func TestIsNull(t *testing.T) {
-	for _, v := range []string{"", "null", "~"} {
+	// The four YAML nulls (YAML 1.1 !!null / YAML 1.2 core schema §10.2.1.1:
+	// null | Null | NULL | ~) plus the empty scalar all read as null. The
+	// uppercase spellings were previously missed (iss #290) even though the
+	// repo's own YAML scalar parser already holds the full set.
+	for _, v := range []string{"", "null", "Null", "NULL", "~"} {
 		if !IsNull(v) {
 			t.Errorf("IsNull(%q) = false, want true", v)
 		}
 	}
-	for _, v := range []string{"itd-9", "spc-1", "standalone"} {
+	// Only the exact YAML null spellings count: near-misses and record handles
+	// must stay non-null, so a fix can never widen this to a case-fold or a
+	// substring match. Fields does not strip quotes, so a *quoted* null reaches
+	// IsNull with its quote characters intact and must stay a string — an
+	// explicit quoted scalar is not a YAML null.
+	for _, v := range []string{"itd-9", "spc-1", "standalone", "None", "nil", "NUL", "nullish", `"null"`, `"NULL"`, "'~'"} {
 		if IsNull(v) {
 			t.Errorf("IsNull(%q) = true, want false", v)
 		}
diff --git a/internal/core/lifeboat/graveyard_abandoned_test.go b/internal/core/lifeboat/graveyard_abandoned_test.go
index c83b77a5..73deff26 100644
--- a/internal/core/lifeboat/graveyard_abandoned_test.go
+++ b/internal/core/lifeboat/graveyard_abandoned_test.go
@@ -149,6 +149,48 @@ func TestAbandonedAcceptedADRIsNotReported(t *testing.T) {
 	}
 }
 
+func TestAbandonedAcceptedADRWithUppercaseNullIsNotReported(t *testing.T) {
+	// Regression (iss #290): superseded_by carrying an uppercase YAML null —
+	// NULL or Null — must read as "not superseded", exactly as lowercase null
+	// and ~ do. frontmatter.IsNull previously missed the uppercase spellings, so
+	// a live (status: accepted) ADR packed from a foreign repo via
+	// `disembark pack` was silently emitted as a superseded-adr finding, quoting
+	// `superseded_by: NULL` as its evidence. The status is not-superseded and
+	// case-folded, so only the null literal decides the finding.
+	//
+	// This walks the bare-spelling matrix on the lifeboat path: only the four
+	// UNQUOTED YAML nulls decide the finding here. A *quoted* null (`"NULL"`,
+	// `'Null'`) is deliberately NOT in this table: per YAML scalar semantics a
+	// quoted value is a string, and frontmatter.IsNull — which sees what Fields
+	// captured, quotes intact — must keep reading it as non-null (asserted by
+	// TestIsNull's negative controls). gvSupersededADRs currently calls
+	// gvUnquote BEFORE IsNull, so in the lifeboat path alone a quoted null
+	// happens to read as absent today; that is quote-insensitive sentinel
+	// behaviour of gvUnquote, not YAML null semantics, and it is an open
+	// heuristic decision for lifeboat supersession handling tracked separately —
+	// this regression pins only the unquoted spellings. A real record handle
+	// (`adr-9`) is the positive control: widening the null set must not
+	// suppress a genuine superseding pointer.
+	nulls := []string{"NULL", "Null", "null", "~"}
+	for _, nul := range nulls {
+		dir, write := abandonedWriter(t)
+		write(".abcd/development/decisions/adrs/0035-live.md",
+			"---\nid: adr-35\nstatus: accepted\nsuperseded_by: "+nul+"\n---\n\n# Live decision\n")
+		fs := gvSupersededADRs(abandonedCtx(t, dir))
+		if len(fs) != 0 {
+			t.Fatalf("superseded_by: %s is a YAML null and must not be reported, got %v", nul, fs)
+		}
+	}
+	// Positive control: a real handle still yields a superseded finding.
+	dir, write := abandonedWriter(t)
+	write(".abcd/development/decisions/adrs/0035-live.md",
+		"---\nid: adr-35\nstatus: accepted\nsuperseded_by: adr-9\n---\n\n# Live decision\n")
+	fs := gvSupersededADRs(abandonedCtx(t, dir))
+	if _, ok := gvFindingByID(fs, "adr-35"); !ok {
+		t.Fatalf("superseded_by: adr-9 is a real handle and must still be reported, got %v", fs)
+	}
+}
+
 func TestAbandonedSupersededADRAcrossBothHomesDedupes(t *testing.T) {
 	dir, write := abandonedWriter(t)
 	// Same ADR id present in the native home AND a conventional home.
diff --git a/internal/core/lint/isnull_test.go b/internal/core/lint/isnull_test.go
new file mode 100644
index 00000000..78051a4d
--- /dev/null
+++ b/internal/core/lint/isnull_test.go
@@ -0,0 +1,36 @@
+package lint
+
+import "testing"
+
+// TestIsNull covers lint's private null predicate, the second copy of
+// frontmatter.IsNull that record-lint gates on. It must recognise the same four
+// YAML nulls (null | Null | NULL | ~) plus the empty scalar as the frontmatter
+// copy, or the two-gate agreement capture/validate.go depends on breaks: an
+// `impact: NULL` record would pass one gate and be refused by the other.
+// Regression for iss #290 — the uppercase spellings were previously missed.
+func TestIsNull(t *testing.T) {
+	for _, v := range []string{"", "null", "Null", "NULL", "~"} {
+		if !isNull(v) {
+			t.Errorf("isNull(%q) = false, want true", v)
+		}
+	}
+	for _, v := range []string{"itd-9", "spc-1", "standalone", "None", "nil", "NUL", "nullish"} {
+		if isNull(v) {
+			t.Errorf("isNull(%q) = true, want false", v)
+		}
+	}
+}
+
+// TestIsAbsentValueUppercaseNull confirms isAbsentValue, which delegates to
+// isNull, inherits the widened set: an uppercase YAML null is an absence, not a
+// malformed value.
+func TestIsAbsentValueUppercaseNull(t *testing.T) {
+	for _, v := range []string{"NULL", "Null", "null", "~", "", "[]"} {
+		if !isAbsentValue(v) {
+			t.Errorf("isAbsentValue(%q) = false, want true", v)
+		}
+	}
+	if isAbsentValue("adr-9") {
+		t.Errorf("isAbsentValue(%q) = true, want false", "adr-9")
+	}
+}
diff --git a/internal/core/lint/lint.go b/internal/core/lint/lint.go
index 2a8211d6..e08b7c77 100644
--- a/internal/core/lint/lint.go
+++ b/internal/core/lint/lint.go
@@ -2394,8 +2394,11 @@ func contentExempt(rel string, fields map[string]fmField, cfg Config) bool {
 	return false
 }
 
+// isNull is lint's copy of frontmatter.IsNull; the two must recognise the same
+// four YAML nulls (null | Null | NULL | ~) plus the empty scalar so the gates
+// they back (record-lint and capture/validate.go) can never disagree on a value.
 func isNull(v string) bool {
-	return v == "" || v == "null" || v == "~"
+	return v == "" || v == "null" || v == "Null" || v == "NULL" || v == "~"
 }
 
 // fenceMask marks lines that are inside (or are a marker for) a triple-backtick
diff --git a/internal/surface/cli/guard_shim_test.go b/internal/surface/cli/guard_shim_test.go
index e3a33a47..26d1cbe3 100644
--- a/internal/surface/cli/guard_shim_test.go
+++ b/internal/surface/cli/guard_shim_test.go
@@ -63,7 +63,26 @@ func fakePluginRoot(t *testing.T, script string) string {
 func runShim(t *testing.T, command, pluginRoot string) (stderr string, code int) {
 	t.Helper()
 	cmd := exec.Command("/bin/sh", "-c", command)
-	cmd.Env = append(os.Environ(), "CLAUDE_PLUGIN_ROOT="+pluginRoot)
+	// Hermetic env: the shim's last resort is `command -v abcd`, so a
+	// developer machine with abcd installed on PATH (this one ships
+	// ~/.local/bin/abcd) would answer the "binary absent" case with the REAL
+	// binary — a silent allow, no UNGUARDED notice, and a failure that
+	// reproduces only on machines that use the tool. PATH keeps the system
+	// directories the shim's POSIX utilities live in (/bin/sh, find, printf);
+	// everything user-local is out of reach, and HOME moves off-machine for
+	// the same reason. Same lesson as iss-219's setupHermetic.
+	for _, e := range os.Environ() {
+		switch {
+		case strings.HasPrefix(e, "PATH="), strings.HasPrefix(e, "HOME="),
+			strings.HasPrefix(e, "CLAUDE_PLUGIN_ROOT="):
+		default:
+			cmd.Env = append(cmd.Env, e)
+		}
+	}
+	cmd.Env = append(cmd.Env,
+		"PATH=/usr/bin:/bin",
+		"HOME="+t.TempDir(),
+		"CLAUDE_PLUGIN_ROOT="+pluginRoot)
 	cmd.Stdin = strings.NewReader(`{"tool_name":"Bash","tool_input":{"command":"ls"}}`)
 	var se strings.Builder
 	cmd.Stderr = &se