diff --git a/lsp/extensions.go b/lsp/extensions.go
new file mode 100644
index 0000000..d72c3db
--- /dev/null
+++ b/lsp/extensions.go
@@ -0,0 +1,348 @@
+// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
+// SPDX-License-Identifier: BSD-3-Clause
+
+// The language-server side of the extended-instruction layer.  The registry
+// in asm carries the mnemonics the generated architecture tables do not know;
+// this file lets completion and hover document those mnemonics from the
+// layer's own metadata, and lets diagnostics leave the layer's verdicts to
+// it.  Nothing the server states about a registered mnemonic is invented:
+// the summaries, forms, features, encodings and references are read from the
+// metadata, and the operand spellings are measured against the layer's own
+// encoder, so the editor surface can never promise a spelling the assembler
+// would refuse.
+
+package lsp
+
+import (
+	"fmt"
+	"slices"
+	"strings"
+	"sync"
+
+	"sourcedock.dev/petrbalvin/gasm-sdk/arch"
+	"sourcedock.dev/petrbalvin/gasm-sdk/asm"
+)
+
+// extFormDoc is the rendering of one registered form: everything hover and
+// completion state about it.
+type extFormDoc struct {
+	summary string // the metadata's one-line description ("" when absent)
+	form    string // the operand shape's label, from ExtForm.String
+	shape   string // the operand spellings the layer's encoder accepts
+	feature string // the architecture feature, from ExtFeature
+	word    uint32 // the fixed encoding bits
+	ref     string // the manual entry the encoding is transcribed from
+}
+
+// extDocsKey addresses one mnemonic's renderings for one architecture.
+type extDocsKey struct {
+	a    arch.Arch
+	name string
+}
+
+// extDocsCache memoises the per-form renderings: the probe walks the layer's
+// encoder once per mnemonic and architecture, and every later completion or
+// hover reuses the answer.
+var extDocsCache sync.Map // map[extDocsKey][]extFormDoc
+
+// The probe candidates.  They cover every operand kind the layer defines
+// today; a kind arriving later joins through its case in extBuild, and an
+// unknown kind degrades to a bare label rather than a wrong spelling.
+var extProbeArrangements = []arch.ExtArrangement{
+	arch.ExtArrB, arch.ExtArrH, arch.ExtArrS, arch.ExtArrD, arch.ExtArrQ, arch.ExtArrNone,
+}
+
+var extProbeQualifiers = []arch.ExtQualifier{
+	arch.ExtQualMerging, arch.ExtQualZeroing, arch.ExtQualNone,
+}
+
+var extProbeImmediates = []int64{1, 0, 255, -1}
+
+// extForms returns the renderings of one mnemonic's registered forms on a,
+// computing and memoising them on first use.  It returns nil when the layer
+// registers nothing for the mnemonic.
+func extForms(a arch.Arch, mnemonic string) []extFormDoc {
+	key := extDocsKey{a: a, name: strings.ToUpper(mnemonic)}
+	if v, ok := extDocsCache.Load(key); ok {
+		docs, _ := v.([]extFormDoc)
+		return docs
+	}
+	var docs []extFormDoc
+	if cands, ok := asm.LookupExtension(a, mnemonic); ok {
+		docs = make([]extFormDoc, 0, len(cands))
+		for _, in := range cands {
+			docs = append(docs, extFormDoc{
+				summary: in.Summary,
+				form:    in.Form.String(),
+				shape:   extFormShape(in),
+				feature: string(in.Feature),
+				word:    in.Word,
+				ref:     in.Ref,
+			})
+		}
+	}
+	extDocsCache.Store(key, docs)
+	return docs
+}
+
+// extBuild builds one candidate operand list for the form: every vector
+// position at reg with the arrangement arr, every predicate position at reg
+// with the qualifier qual, every immediate at imm.
+func extBuild(in arch.ExtInstr, reg int, arr arch.ExtArrangement, qual arch.ExtQualifier, imm int64) []arch.ExtOperand {
+	kinds := in.Form.Kinds()
+	ops := make([]arch.ExtOperand, len(kinds))
+	for i, k := range kinds {
+		switch k {
+		case arch.ExtZReg:
+			ops[i] = arch.ExtVector(reg, arr)
+		case arch.ExtPReg:
+			ops[i] = arch.ExtPredicate(reg, qual)
+		default: // arch.ExtImm and anything the probe does not model
+			ops[i] = arch.ExtImmediate(imm)
+		}
+	}
+	return ops
+}
+
+// extEncodes reports whether the form's encoder accepts the operand list.
+func extEncodes(in arch.ExtInstr, ops []arch.ExtOperand) bool {
+	_, err := in.Encode(ops)
+	return err == nil
+}
+
+// extAnchor finds one operand list the form encodes, to anchor the probes on;
+// ok is false when no probe combination encodes, and the shape then renders
+// from the operand kinds alone.
+func extAnchor(in arch.ExtInstr) (base []arch.ExtOperand, arr arch.ExtArrangement, qual arch.ExtQualifier, imm int64, ok bool) {
+	for _, a := range extProbeArrangements {
+		for _, q := range extProbeQualifiers {
+			for _, v := range extProbeImmediates {
+				ops := extBuild(in, 0, a, q, v)
+				if extEncodes(in, ops) {
+					return ops, a, q, v, true
+				}
+			}
+		}
+	}
+	return nil, arch.ExtArrNone, arch.ExtQualNone, 0, false
+}
+
+// extAcceptedArrangements returns the arrangements the form's vector
+// positions accept, probed together: the encoder requires the vector
+// positions of a form to agree, so one position alone cannot carry the probe.
+func extAcceptedArrangements(in arch.ExtInstr, qual arch.ExtQualifier, imm int64) []arch.ExtArrangement {
+	var out []arch.ExtArrangement
+	for _, a := range extProbeArrangements {
+		if extEncodes(in, extBuild(in, 0, a, qual, imm)) {
+			out = append(out, a)
+		}
+	}
+	return out
+}
+
+// extAcceptedQualifiers returns the qualifiers the form's predicate
+// positions accept, probed together for the same reason.
+func extAcceptedQualifiers(in arch.ExtInstr, arr arch.ExtArrangement, imm int64) []arch.ExtQualifier {
+	var out []arch.ExtQualifier
+	for _, q := range extProbeQualifiers {
+		if extEncodes(in, extBuild(in, 0, arr, q, imm)) {
+			out = append(out, q)
+		}
+	}
+	return out
+}
+
+// extRegRange walks the register numbers one operand position accepts and
+// returns a "Z0-Z31" style span when the accepted set is one contiguous run
+// from its low end.  ok is false when no register encodes there or the set
+// is not contiguous, and the caller falls back to a bare label.
+func extRegRange(in arch.ExtInstr, base []arch.ExtOperand, pos int, letter string) (span string, ok bool) {
+	const limit = 32 // both register files the layer defines sit inside 32
+	lo, hi, n := -1, -1, 0
+	for r := range limit {
+		ops := slices.Clone(base)
+		ops[pos].Reg = r
+		if !extEncodes(in, ops) {
+			continue
+		}
+		n++
+		if lo < 0 {
+			lo = r
+		}
+		hi = r
+	}
+	if n == 0 || hi-lo+1 != n {
+		return "", false
+	}
+	return fmt.Sprintf("%s%d-%s%d", letter, lo, letter, hi), true
+}
+
+// extFormShape measures the operand spellings the form accepts: the register
+// spans, the arrangement suffixes and the predicate qualifiers its own
+// encoder takes, rendered the way the source writes them.  Where a probe
+// finds nothing to anchor on, the shape degrades to the operand kinds and
+// states nothing the encoder has not proven.
+func extFormShape(in arch.ExtInstr) string {
+	kinds := in.Form.Kinds()
+	if len(kinds) == 0 {
+		return ""
+	}
+	base, arr, qual, imm, anchored := extAnchor(in)
+
+	// The arrangement suffixes and the qualifiers, rendered in enum order.
+	suffix := ""
+	quals := []arch.ExtQualifier{qual}
+	if anchored {
+		var spelled []string
+		for _, a := range extAcceptedArrangements(in, qual, imm) {
+			if a != arch.ExtArrNone {
+				spelled = append(spelled, a.String())
+			}
+		}
+		if len(spelled) > 0 {
+			suffix = strings.Join(spelled, "/")
+		}
+		quals = extAcceptedQualifiers(in, arr, imm)
+	}
+
+	var parts []string
+	for i, k := range kinds {
+		switch k {
+		case arch.ExtZReg:
+			label := "Z<n>"
+			if anchored {
+				if span, ok := extRegRange(in, base, i, "Z"); ok {
+					label = span
+				}
+			}
+			parts = append(parts, label+suffix)
+		case arch.ExtPReg:
+			label := "P<n>"
+			if anchored {
+				if span, ok := extRegRange(in, base, i, "P"); ok {
+					label = span
+				}
+			}
+			var spelled []string
+			for _, q := range quals {
+				if q != arch.ExtQualNone {
+					spelled = append(spelled, q.String())
+				}
+			}
+			if len(spelled) > 0 {
+				label += strings.Join(spelled, " or ")
+			}
+			parts = append(parts, label)
+		default:
+			parts = append(parts, "$imm")
+		}
+	}
+	return strings.Join(parts, ", ")
+}
+
+// extFeatureList names the architecture features the forms belong to, in
+// first-occurrence order, "" when the metadata carries none.
+func extFeatureList(docs []extFormDoc) string {
+	var out []string
+	for _, d := range docs {
+		if d.feature != "" && !slices.Contains(out, d.feature) {
+			out = append(out, d.feature)
+		}
+	}
+	return strings.Join(out, ", ")
+}
+
+// renderExtensionHover renders the hover documentation of a registered
+// extended mnemonic: the fact that it is an extension above the toolchain,
+// then one entry per registered form.  A form with no summary in the
+// metadata states the encoding facts alone: the shape label, the feature,
+// the fixed encoding bits and the manual reference, and nothing more.
+func renderExtensionHover(mnemonic string, docs []extFormDoc) string {
+	if len(docs) == 0 {
+		return ""
+	}
+	var b strings.Builder
+	b.WriteString("**" + mnemonic + "**: extension above the Go toolchain")
+	if f := extFeatureList(docs); f != "" {
+		b.WriteString(" (" + f + ")")
+	}
+	for _, d := range docs {
+		b.WriteString("\n\n- ")
+		// The summary leads where the metadata carries one; without it the
+		// form's own label stands in, and the bullet states the encoding
+		// facts and nothing more.
+		lead := d.summary
+		if lead == "" {
+			lead = d.form
+		}
+		if lead != "" {
+			b.WriteString(lead + ": ")
+		}
+		b.WriteString(d.shape)
+		var clauses []string
+		if d.summary != "" && d.form != "" {
+			clauses = append(clauses, d.form)
+		}
+		if d.feature != "" {
+			clauses = append(clauses, d.feature)
+		}
+		if d.word != 0 {
+			clauses = append(clauses, fmt.Sprintf("encoding 0x%08x", d.word))
+		}
+		if len(clauses) > 0 {
+			b.WriteString(" (" + strings.Join(clauses, ", ") + ")")
+		}
+		if d.ref != "" {
+			b.WriteString("\n  " + d.ref)
+		}
+	}
+	return b.String()
+}
+
+// extensionHover returns the hover documentation of a registered extended
+// mnemonic, "" when the layer registers nothing for it.
+func extensionHover(a arch.Arch, word string) string {
+	return renderExtensionHover(strings.ToUpper(word), extForms(a, word))
+}
+
+// applyExtensionCompletion merges the registered extended mnemonics into the
+// completion list, so they are offered exactly where the toolchain
+// mnemonics are: a mnemonic the base table also knows enriches its existing
+// item, a registry-only one gains its own.  The detail carries the operand
+// shapes the layer's encoder accepts.
+func applyExtensionCompletion(items []CompletionItem, a arch.Arch) []CompletionItem {
+	byLabel := make(map[string]int, len(items))
+	for i, it := range items {
+		if _, ok := byLabel[it.Label]; !ok {
+			byLabel[it.Label] = i
+		}
+	}
+	for _, name := range asm.ExtensionNames(a) {
+		docs := extForms(a, name)
+		if len(docs) == 0 {
+			continue
+		}
+		shapes := make([]string, 0, len(docs))
+		for _, d := range docs {
+			shapes = append(shapes, d.shape)
+		}
+		hover := extensionHover(a, name)
+		if i, ok := byLabel[name]; ok {
+			items[i].Detail += "; extension: " + strings.Join(shapes, "; ")
+			if items[i].Documentation == "" {
+				items[i].Documentation = hover
+			} else {
+				items[i].Documentation += "\n\n" + hover
+			}
+			continue
+		}
+		byLabel[name] = len(items)
+		items = append(items, CompletionItem{
+			Label:         name,
+			Kind:          ciFunction,
+			Detail:        "extension: " + strings.Join(shapes, "; "),
+			Documentation: hover,
+		})
+	}
+	return items
+}
diff --git a/lsp/extensions_test.go b/lsp/extensions_test.go
new file mode 100644
index 0000000..cb1007a
--- /dev/null
+++ b/lsp/extensions_test.go
@@ -0,0 +1,285 @@
+// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
+// SPDX-License-Identifier: BSD-3-Clause
+
+package lsp
+
+import (
+	"encoding/json"
+	"strings"
+	"testing"
+
+	"sourcedock.dev/petrbalvin/gasm-sdk/arch"
+	"sourcedock.dev/petrbalvin/gasm-sdk/asm"
+)
+
+// sveDoc is an arm64 kernel over the registered extended-instruction layer:
+// every SVE form of ADD, a registry-only mnemonic, the signed-immediate MUL,
+// and one mnemonic no layer registers, which must stay reported wherever the
+// registry is silent.
+const sveDoc = "#include \"textflag.h\"\n" +
+	"TEXT \u00b7f(SB), NOSPLIT, $0\n" +
+	"\tADD Z0.S, Z1.S, Z2.S\n" +
+	"\tADD Z1.S, P0/M, Z0.S\n" +
+	"\tADD $255, Z0.S\n" +
+	"\tSQADD Z3.S, Z4.S, Z5.S\n" +
+	"\tMUL $-128, Z0.B\n" +
+	"\tNOSUCHTHING Z0.S, Z1.S\n" +
+	"\tRET\n"
+
+// completionItems runs one completion request and indexes the answers by
+// label.  The base surface predates this change and carries a few duplicate
+// labels (a directive or pseudo-register can also sit in the generated
+// tables); callers that care about a specific label count it in the slice.
+func completionItems(t *testing.T, uri, text string, id int) ([]CompletionItem, map[string]CompletionItem) {
+	t.Helper()
+	in := session(uri, text) +
+		frame(id, "textDocument/completion", map[string]any{
+			"textDocument": map[string]any{"uri": uri},
+			"position":     map[string]any{"line": 2, "character": 1},
+		}) + frame(nil, "exit", nil)
+	msgs := run(t, in)
+	resp := findByID(msgs, id)
+	if resp == nil {
+		t.Fatal("no completion response")
+	}
+	var items []CompletionItem
+	if err := json.Unmarshal(mustResult(t, resp), &items); err != nil {
+		t.Fatal(err)
+	}
+	out := make(map[string]CompletionItem, len(items))
+	for _, it := range items {
+		if _, dup := out[it.Label]; !dup {
+			out[it.Label] = it
+		}
+	}
+	return items, out
+}
+
+// countLabel counts the items carrying one label.
+func countLabel(items []CompletionItem, label string) int {
+	n := 0
+	for _, it := range items {
+		if it.Label == label {
+			n++
+		}
+	}
+	return n
+}
+
+// hoverAt runs one hover request and returns the rendered markdown.
+func hoverAt(t *testing.T, uri, text string, id, line, char int) *Hover {
+	t.Helper()
+	in := session(uri, text) +
+		frame(id, "textDocument/hover", map[string]any{
+			"textDocument": map[string]any{"uri": uri},
+			"position":     map[string]any{"line": line, "character": char},
+		}) + frame(nil, "exit", nil)
+	msgs := run(t, in)
+	resp := findByID(msgs, id)
+	if resp == nil {
+		t.Fatal("no hover response")
+	}
+	var h *Hover
+	if err := json.Unmarshal(mustResult(t, resp), &h); err != nil {
+		t.Fatal(err)
+	}
+	return h
+}
+
+// publishedDiagnostics runs one didOpen and returns the pushed diagnostics.
+func publishedDiagnostics(t *testing.T, uri, text string) []Diagnostic {
+	t.Helper()
+	msgs := run(t, session(uri, text)+frame(nil, "exit", nil))
+	pub := findMethod(msgs, "textDocument/publishDiagnostics")
+	if pub == nil {
+		t.Fatal("no publishDiagnostics notification")
+	}
+	var p publishDiagnosticsParams
+	json.Unmarshal(pub.Params, &p)
+	return p.Diagnostics
+}
+
+// TestCompletionExtensionMnemonicsARM64 checks that the registered extended
+// mnemonics are offered where the toolchain mnemonics are, with the operand
+// shapes the registry accepts, and that a mnemonic both layers know merges
+// into one item instead of two.
+func TestCompletionExtensionMnemonicsARM64(t *testing.T) {
+	items, byLabel := completionItems(t, "file:///v_arm64.s", sveDoc, 40)
+
+	sqadd, ok := byLabel["SQADD"]
+	if !ok {
+		t.Fatal("completion missing the registry mnemonic SQADD")
+	}
+	if sqadd.Kind != ciFunction {
+		t.Errorf("SQADD kind = %d, want the instruction kind", sqadd.Kind)
+	}
+	for _, want := range []string{"extension: ", "Z0-Z31.B/.H/.S/.D", "$imm"} {
+		if !strings.Contains(sqadd.Detail, want) {
+			t.Errorf("SQADD detail = %q, want it to carry %q", sqadd.Detail, want)
+		}
+	}
+	if !strings.Contains(sqadd.Documentation, "extension above the Go toolchain") {
+		t.Errorf("SQADD documentation = %q, want the extension notice", sqadd.Documentation)
+	}
+
+	subr, ok := byLabel["SUBR"]
+	if !ok {
+		t.Fatal("completion missing the registry mnemonic SUBR")
+	}
+	if !strings.Contains(subr.Detail, "P0-P7/M") {
+		t.Errorf("SUBR detail = %q, want the predicate spelling P0-P7/M", subr.Detail)
+	}
+
+	// ADD sits in both layers: one item, the table entry carrying the
+	// extension shapes beside its own summary.
+	if n := countLabel(items, "ADD"); n != 1 {
+		t.Errorf("ADD offered %d times, want one merged item", n)
+	}
+	add := byLabel["ADD"]
+	if !strings.Contains(add.Detail, "64-bit") || !strings.Contains(add.Detail, "extension: ") {
+		t.Errorf("ADD detail = %q, want the merged table and extension text", add.Detail)
+	}
+	if !strings.Contains(add.Documentation, "extension above the Go toolchain") {
+		t.Errorf("ADD documentation = %q, want the extension notice beside the table summary", add.Documentation)
+	}
+
+	// The toolchain surface is untouched beside the additions.
+	for _, want := range []string{"TEXT", "MOVD", "NOSPLIT"} {
+		if _, ok := byLabel[want]; !ok {
+			t.Errorf("completion lost the toolchain entry %q", want)
+		}
+	}
+	// No extended mnemonic arrives as a second item beside its merge.
+	for _, name := range asm.ExtensionNames(arch.ARM64) {
+		if n := countLabel(items, name); n != 1 {
+			t.Errorf("extended mnemonic %q offered %d times, want once", name, n)
+		}
+	}
+}
+
+// TestCompletionNoExtensionMnemonicsAMD64 is the per-architecture negative
+// case: the extension layer registers nothing on amd64, so no extended
+// mnemonic may be invented into the list and the toolchain surface stands.
+func TestCompletionNoExtensionMnemonicsAMD64(t *testing.T) {
+	_, byLabel := completionItems(t, "file:///v_amd64.s", sveDoc, 41)
+	for _, banned := range []string{"SQADD", "SUBR", "UQADD", "UQSUB", "SMULH"} {
+		if _, ok := byLabel[banned]; ok {
+			t.Errorf("completion invented %q on amd64, whose layer registers nothing", banned)
+		}
+	}
+	if _, ok := byLabel["MOVQ"]; !ok {
+		t.Error("completion lost MOVQ on amd64")
+	}
+	if add, ok := byLabel["ADD"]; ok && strings.Contains(add.Detail, "extension: ") {
+		t.Errorf("ADD detail = %q, want no extension text where the layer is absent", add.Detail)
+	}
+}
+
+// TestHoverExtensionMnemonicARM64 checks that a registered extended mnemonic
+// documents itself from the layer's metadata: the extension notice, the
+// family, the operand shapes, the fixed encoding and the manual reference.
+func TestHoverExtensionMnemonicARM64(t *testing.T) {
+	h := hoverAt(t, "file:///v_arm64.s", sveDoc, 42, 5, 3) // on SQADD
+	if h == nil {
+		t.Fatal("hover over SQADD returned nothing")
+	}
+	for _, want := range []string{
+		"**SQADD**: extension above the Go toolchain (sve)",
+		"Add signed saturating scalable vector elements, unpredicated: Z0-Z31.B/.H/.S/.D, Z0-Z31.B/.H/.S/.D, Z0-Z31.B/.H/.S/.D",
+		"unpredicated vectors, sve, encoding 0x04201000",
+		"$imm, Z0-Z31.B/.H/.S/.D",
+		"DDI 0487J",
+	} {
+		if !strings.Contains(h.Contents.Value, want) {
+			t.Errorf("hover = %q, want it to carry %q", h.Contents.Value, want)
+		}
+	}
+	if strings.Contains(h.Contents.Value, ".Q") {
+		t.Errorf("hover = %q, states an arrangement the layer's encoder refuses", h.Contents.Value)
+	}
+}
+
+// TestHoverSharedMnemonicARM64 checks the merge on a mnemonic both layers
+// know: the table entry keeps its summary and the registered forms attach
+// beside it.
+func TestHoverSharedMnemonicARM64(t *testing.T) {
+	h := hoverAt(t, "file:///v_arm64.s", sveDoc, 43, 2, 3) // on ADD
+	if h == nil {
+		t.Fatal("hover over ADD returned nothing")
+	}
+	if !strings.Contains(h.Contents.Value, "**ADD**: ADD (64-bit)") {
+		t.Errorf("hover = %q, want the generated table summary", h.Contents.Value)
+	}
+	if !strings.Contains(h.Contents.Value, "extension above the Go toolchain") {
+		t.Errorf("hover = %q, want the registered forms beside the summary", h.Contents.Value)
+	}
+	if !strings.Contains(h.Contents.Value, "P0-P7/M") {
+		t.Errorf("hover = %q, want the predicated form's spelling", h.Contents.Value)
+	}
+}
+
+// TestHoverNoExtensionMnemonicAMD64 is the per-architecture negative case:
+// with no registered layer on amd64, a hover over a registry-only mnemonic
+// answers null exactly as before.
+func TestHoverNoExtensionMnemonicAMD64(t *testing.T) {
+	if h := hoverAt(t, "file:///v_amd64.s", sveDoc, 44, 5, 3); h != nil {
+		t.Errorf("hover over SQADD on amd64 = %q, want null", h.Contents.Value)
+	}
+}
+
+// TestExtensionShapesMatchRegistry pins the measuring: the rendered shapes
+// are exactly the spellings the registry's encoder accepts, checked by
+// round-tripping the boundary spellings through asm.EncodeExtension.
+func TestExtensionShapesMatchRegistry(t *testing.T) {
+	docs := extForms(arch.ARM64, "ADD")
+	if len(docs) != 3 {
+		t.Fatalf("ADD renders %d forms, want 3", len(docs))
+	}
+	wantShapes := []string{
+		"Z0-Z31.B/.H/.S/.D, Z0-Z31.B/.H/.S/.D, Z0-Z31.B/.H/.S/.D",
+		"Z0-Z31.B/.H/.S/.D, P0-P7/M, Z0-Z31.B/.H/.S/.D",
+		"$imm, Z0-Z31.B/.H/.S/.D",
+	}
+	for i, want := range wantShapes {
+		if docs[i].shape != want {
+			t.Errorf("form %d shape = %q, want %q", i, docs[i].shape, want)
+		}
+	}
+	// The predicate span ends where the encoder's three-bit field ends.
+	if _, err := asm.EncodeExtension(arch.ARM64, "ADD",
+		arch.ExtVector(0, arch.ExtArrS), arch.ExtPredicate(7, arch.ExtQualMerging), arch.ExtVector(1, arch.ExtArrS)); err != nil {
+		t.Errorf("P7/M refused: %v", err)
+	}
+	if _, err := asm.EncodeExtension(arch.ARM64, "ADD",
+		arch.ExtVector(0, arch.ExtArrS), arch.ExtPredicate(8, arch.ExtQualMerging), arch.ExtVector(1, arch.ExtArrS)); err == nil {
+		t.Error("P8/M encoded, the span overstates the registry")
+	}
+	// The arrangement suffixes stop at .D: .Q refuses while the shape omits it.
+	if _, err := asm.EncodeExtension(arch.ARM64, "ADD",
+		arch.ExtVector(0, arch.ExtArrQ), arch.ExtVector(1, arch.ExtArrQ), arch.ExtVector(2, arch.ExtArrQ)); err == nil {
+		t.Error(".Q encoded, the suffixes overstate the registry")
+	}
+	// And .B, which the shape spells, encodes.
+	if _, err := asm.EncodeExtension(arch.ARM64, "ADD",
+		arch.ExtVector(0, arch.ExtArrB), arch.ExtVector(1, arch.ExtArrB), arch.ExtVector(2, arch.ExtArrB)); err != nil {
+		t.Errorf(".B refused: %v", err)
+	}
+}
+
+// TestExtensionHoverWithoutProse checks the no-prose contract: a form whose
+// metadata carries no summary states the encoding facts it has and nothing
+// invented in their place.
+func TestExtensionHoverWithoutProse(t *testing.T) {
+	docs := []extFormDoc{
+		{form: "unsigned immediate", shape: "$imm, Z0-Z31.B/.H/.S/.D", feature: "sve", word: 0x2520c000},
+	}
+	md := renderExtensionHover("TESTMNEM", docs)
+	for _, want := range []string{
+		"**TESTMNEM**: extension above the Go toolchain (sve)",
+		"- unsigned immediate: $imm, Z0-Z31.B/.H/.S/.D (sve, encoding 0x2520c000)",
+	} {
+		if !strings.Contains(md, want) {
+			t.Errorf("hover = %q, want it to carry %q", md, want)
+		}
+	}
+}
diff --git a/lsp/handlers.go b/lsp/handlers.go
index 9c5fcda..9017f88 100644
--- a/lsp/handlers.go
+++ b/lsp/handlers.go
@@ -114,6 +114,9 @@ func (s *Server) completion(p completionParams) []CompletionItem {
 		}
 		items = append(items, CompletionItem{Label: r.Name, Kind: kind, Detail: r.Desc})
 	}
+	// The extended-instruction layer, beside the toolchain entries: the
+	// registered mnemonics with the operand shapes the registry accepts.
+	items = applyExtensionCompletion(items, a.Arch)
 	// Local labels defined in the document.
 	if f, _ := parser.Parse("", s.docs[p.TextDocument.URI]); f != nil {
 		for _, name := range labelNames(f) {
@@ -134,6 +137,7 @@ func (s *Server) hover(p hoverParams) *Hover {
 	a := arch.ForArch(arch.FromFilename(uriPath(p.TextDocument.URI)))
 
 	var md string
+	ext := extensionHover(a.Arch, word)
 	// Directives first: several of their names (TEXT, PCALIGN, FUNCDATA,
 	// the BYTE family) also sit in the generated instruction tables with
 	// empty summaries, and the directive documentation is the answer a
@@ -142,6 +146,13 @@ func (s *Server) hover(p hoverParams) *Hover {
 		md = doc
 	} else if in, ok := a.Lookup(word); ok {
 		md = "**" + in.Name + "**: " + in.Summary
+		// A mnemonic the extension layer registers documents its registered
+		// forms beside the table entry.
+		if ext != "" {
+			md += "\n\n" + ext
+		}
+	} else if ext != "" {
+		md = ext
 	} else if r, ok := a.Register(word); ok {
 		md = "**" + r.Name + "**: " + r.Class.String() + " register. " + r.Desc
 	} else if desc, ok := arch.PseudoRegDesc(word); ok {