diff --git a/http/httphi/bench_test.go b/http/httphi/bench_test.go
index c7f25b7..5ea8fca 100644
--- a/http/httphi/bench_test.go
+++ b/http/httphi/bench_test.go
@@ -41,8 +41,9 @@ var benchBody = []byte("hello world")
 func benchExchange(b *testing.B, conn conn) *Exchange {
 	b.Helper()
 	const bufferSize = 1024
+	const numHeaderCap = 2
 	exch := new(Exchange)
-	exch.Configure(make([]byte, 2*bufferSize), bufferSize, false)
+	exch.Configure(make([]byte, 2*bufferSize), bufferSize, numHeaderCap, false)
 	if !exch.Acquire(conn) {
 		b.Fatal("fresh exchange failed to acquire connection")
 	}
diff --git a/http/httphi/exchange.go b/http/httphi/exchange.go
index 0dd2fda..49063a5 100644
--- a/http/httphi/exchange.go
+++ b/http/httphi/exchange.go
@@ -48,6 +48,13 @@ type Exchange struct {
 	readErr       error
 }
 
+type ExchangeConfig struct {
+	RawBuf                []byte
+	RequestBufferLim      int
+	NumHeaderCap          int
+	NormalizeOutgoingKeys bool
+}
+
 // HijackRaw is a low-level implementation of http.Hijacker interface.
 // A Hijack method is not exposed due to heap allocation implications and correctness concerns.
 // Below is what an actual implementation may look like:
@@ -82,14 +89,15 @@ func (exch *Exchange) HijackRaw(dstBody []byte) (conn, []byte, error) {
 // of which the first requestLim bytes are reserved for the request header.
 // Panics if requestLim exceeds the buffer. Set normalizeKeys to normalize
 // outgoing header keys, i.e: "content-type" to "Content-Type".
-func (exch *Exchange) Configure(rawbuf []byte, requestLim int, normalizeKeys bool) {
-	respSize := len(rawbuf) - requestLim
+func (exch *Exchange) Configure(cfg ExchangeConfig) {
+	respSize := len(cfg.RawBuf) - cfg.RequestBufferLim
 	if respSize < 0 {
 		panic("request lim larger than buffer")
 	}
-	exch.rawbuf = rawbuf
-	exch.reqHdr.Reset(rawbuf[:0:requestLim])
-	exch.normalizeKeys = normalizeKeys
+	exch.rawbuf = cfg.RawBuf
+	exch.reqHdr.Reset(cfg.RawBuf[:0:cfg.RequestBufferLim], cfg.NumHeaderCap)
+	exch.reqHdr.ConfigBufferGrowth(false)
+	exch.normalizeKeys = cfg.NormalizeOutgoingKeys
 }
 
 // Acquire claims the exchange for conn and resets it to serve a new request,
@@ -111,7 +119,7 @@ func (exch *Exchange) Acquire(conn conn) bool {
 	exch.rw = conn
 	exch.headerWritten = false
 	exch.nextFree = nil
-	exch.reqHdr.Reset(nil)
+	exch.reqHdr.Reset(nil, 0)
 	return true
 }
 
@@ -350,13 +358,11 @@ func (exch *Exchange) ReadBody(dst []byte) (n int, _ error) {
 		}
 		n = copy(dst, toRead)
 		exch.respRemains -= n
-		if len(dst) == n {
-			return n, nil
-		}
-		dst = dst[n:]
+		// hand over what already arrived since conn might have
+		// exhausted data and could block indefinetely.
+		return n, nil
 	}
-	nr, err := exch.rw.Read(dst)
-	return nr + n, err
+	return exch.rw.Read(dst)
 }
 
 func (exch *Exchange) remainingSurplusBody() ([]byte, error) {
diff --git a/http/httphi/exchange_test.go b/http/httphi/exchange_test.go
index 8889fe3..6d1fa34 100644
--- a/http/httphi/exchange_test.go
+++ b/http/httphi/exchange_test.go
@@ -5,6 +5,7 @@ import (
 	"errors"
 	"io"
 	"net/http"
+	"strconv"
 	"strings"
 	"unsafe"
 
@@ -19,10 +20,11 @@ import (
 func nopBackoff(consecutiveBackoffs uint) time.Duration { return lneto.BackoffFlagNop }
 
 // newExchange returns an Exchange acquired on conn, ready to serve a request.
-func newExchange(t *testing.T, conn conn, bufferSize int, normalizeKeys bool) *Exchange {
+func newExchange(t *testing.T, conn conn, cfg ExchangeConfig) *Exchange {
 	t.Helper()
 	exch := new(Exchange)
-	exch.Configure(make([]byte, 2*bufferSize), bufferSize, normalizeKeys)
+	const numHeaderCap = 1
+	exch.Configure(cfg)
 	if !exch.Acquire(conn) {
 		t.Fatal("fresh exchange failed to acquire connection")
 	}
@@ -46,6 +48,7 @@ func serve(t *testing.T, request string, mux Mux) *rwconn {
 // WriteHeader must emit a complete status line terminated in CRLF followed by
 // the end-of-headers CRLF, for every status code including the longest text.
 func TestExchangeWriteHeader(t *testing.T) {
+	var buf [128]byte
 	for _, test := range []struct {
 		code int
 		want string
@@ -57,7 +60,7 @@ func TestExchangeWriteHeader(t *testing.T) {
 		{code: 511, want: "HTTP/1.1 511 Network Authentication Required\r\n\r\n"},
 	} {
 		conn := newConn("")
-		exch := newExchange(t, conn, 128, false)
+		exch := newExchange(t, conn, ExchangeConfig{RawBuf: buf[:], RequestBufferLim: 64})
 		exch.WriteHeader(test.code)
 		if got := conn.ViewWritten(); got != test.want {
 			t.Errorf("code %d: want %q, got %q", test.code, test.want, got)
@@ -67,8 +70,9 @@ func TestExchangeWriteHeader(t *testing.T) {
 
 // Status line is written once: a second WriteHeader must not reach the wire.
 func TestExchangeWriteHeaderOnce(t *testing.T) {
+	var buf [128]byte
 	conn := newConn("")
-	exch := newExchange(t, conn, 128, false)
+	exch := newExchange(t, conn, ExchangeConfig{RawBuf: buf[:], RequestBufferLim: 64})
 	exch.WriteHeader(404)
 	exch.WriteHeader(500)
 	const want = "HTTP/1.1 404 Not Found\r\n\r\n"
@@ -79,9 +83,10 @@ func TestExchangeWriteHeaderOnce(t *testing.T) {
 
 // Write with no prior WriteHeader must flush a 200 header ahead of the body.
 func TestExchangeWriteFlushesHeader(t *testing.T) {
+	var buf [128]byte
 	const body = "hello"
 	conn := newConn("")
-	exch := newExchange(t, conn, 128, false)
+	exch := newExchange(t, conn, ExchangeConfig{RawBuf: buf[:], RequestBufferLim: 64})
 	n, err := exch.WriteBody([]byte(body))
 	if err != nil {
 		t.Fatal(err)
@@ -124,7 +129,7 @@ func TestExchangeSetHeader(t *testing.T) {
 	} {
 		t.Run(test.name, func(t *testing.T) {
 			conn := newConn("")
-			exch := newExchange(t, conn, 128, test.normalize)
+			exch := newExchange(t, conn, ExchangeConfig{RawBuf: make([]byte, 128), RequestBufferLim: 64, NormalizeOutgoingKeys: test.normalize})
 			for _, kv := range test.set {
 				if !exch.StageHeader(kv[0], kv[1]) {
 					t.Fatalf("SetHeader(%q,%q) reported insufficient memory", kv[0], kv[1])
@@ -147,7 +152,7 @@ func TestExchangeSetHeader(t *testing.T) {
 func TestExchangeSetHeaderOOM(t *testing.T) {
 	const bufferSize = 32
 	conn := newConn("")
-	exch := newExchange(t, conn, bufferSize, false)
+	exch := newExchange(t, conn, ExchangeConfig{RawBuf: make([]byte, bufferSize), RequestBufferLim: 64})
 	if exch.StageHeader("X-Big", strings.Repeat("v", 4*bufferSize)) {
 		t.Fatal("want insufficient memory reported for oversized header value")
 	}
@@ -346,10 +351,11 @@ func TestExchangeReadBody(t *testing.T) {
 func TestExchangeStageOKAndFail(t *testing.T) {
 	const key, value = "K", "V"
 	const field = len(key) + len(value) + len(":\r\n")
+	const numHeaderCap = 4
 	for _, bufLen := range []int{field + 2, field + 1, field} {
 		conn := newConn("")
 		exch := new(Exchange)
-		exch.Configure(make([]byte, bufLen), bufLen, false)
+		exch.Configure(make([]byte, bufLen), bufLen, numHeaderCap, false)
 		if !exch.Acquire(conn) {
 			t.Fatal("fresh exchange failed to acquire connection")
 		}
@@ -942,3 +948,144 @@ func TestExchangeRequestParseMultipartRejects(t *testing.T) {
 		}
 	}
 }
+
+// A request from a browser carries around twenty header fields. Serving one
+// must not depend on how many fields the parser happens to have room for: the
+// exchange's buffer is the memory the caller granted, and the field table comes
+// out of it.
+func TestHandleBrowserSizedRequest(t *testing.T) {
+	const wantMode = "navigate"
+	request := "GET /echo HTTP/1.1\r\nHost: lneto.test\r\n" +
+		"User-Agent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/120.0 Safari/537.36\r\n" +
+		"Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,image/webp,*/*;q=0.8\r\n" +
+		"Accept-Language: en-US,en;q=0.5\r\nAccept-Encoding: gzip, deflate, br\r\n" +
+		"Upgrade-Insecure-Requests: 1\r\nSec-Fetch-Dest: document\r\nSec-Fetch-Site: none\r\n" +
+		"Sec-Ch-Ua: \"Chromium\";v=\"120\"\r\nCache-Control: max-age=0\r\nDnt: 1\r\n" +
+		"Referer: https://lneto.test/index.html\r\nCookie: session=abcdef0123456789; theme=dark\r\n" +
+		"X-Trace: 0123456789abcdef\r\nX-Client: bench\r\nX-Seq: 42\r\nX-Tag: alpha\r\n" +
+		"X-Nonce: cafebabe\r\nX-Mode: " + wantMode + "\r\n\r\n"
+
+	var gotMode string
+	var fields int
+	var sm MuxSlice
+	sm.Reset(1)
+	sm.Handle("GET /echo", func(exch *Exchange) {
+		gotMode = string(exch.RequestHeader("X-Mode"))
+		exch.RequestHeaderRaw().ForEach(func(key, value []byte) error {
+			fields++
+			return nil
+		})
+	})
+	conn := newConn(request)
+	conn.Hangup()
+	exch := newExchange(t, conn, 8192, false)
+	if err := Handle(exch, &sm, nopBackoff); err != nil {
+		t.Fatalf("serving a browser sized request: %s", err)
+	}
+	if gotMode != wantMode {
+		t.Errorf("last header field read back as %q, want %q", gotMode, wantMode)
+	}
+	const sent = 19
+	if fields < sent {
+		t.Errorf("handler saw %d header fields, request carried %d", fields, sent)
+	}
+}
+
+// A request with more header fields than the exchange has room for must be
+// answered, not dropped: the peer learns its request was too large instead of
+// seeing the connection go away.
+func TestHandleTooManyHeaderFields(t *testing.T) {
+	request := "GET /echo HTTP/1.1\r\nHost: lneto.test\r\n"
+	for i := 0; i < 512; i++ {
+		request += "H" + strconv.Itoa(i) + ":v\r\n"
+	}
+	request += "\r\n"
+
+	var served bool
+	var sm MuxSlice
+	sm.Reset(1)
+	sm.Handle("GET /echo", func(exch *Exchange) { served = true })
+	conn := newConn(request)
+	conn.Hangup()
+	exch := newExchange(t, conn, 1024, false)
+	err := Handle(exch, &sm, nopBackoff)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if served {
+		t.Fatal("handler ran on a request the parser could not hold")
+	}
+	got := conn.ViewWritten()
+	if !strings.HasPrefix(got, "HTTP/1.1 431 ") {
+		t.Errorf("want a 431 answer, got %q", firstLine(got))
+	}
+}
+
+func firstLine(s string) string {
+	if i := strings.Index(s, "\r\n"); i >= 0 {
+		return s[:i]
+	}
+	return s
+}
+
+// A body that already arrived alongside the request header must be handed over
+// without touching the connection again. A peer that sent a whole request and
+// is waiting for its answer sends nothing more, so a read for bytes already in
+// hand blocks until the connection's deadline, or forever without one.
+func TestExchangeReadBodyDoesNotReadPastWhatArrived(t *testing.T) {
+	const body = "message body"
+	dst := make([]byte, 64) // Deliberately larger than the body.
+	var got string
+	var readErr error
+	var sm MuxSlice
+	sm.Reset(1)
+	sm.Handle("POST /", func(ex *Exchange) {
+		n, err := ex.ReadBody(dst)
+		got, readErr = string(dst[:n]), err
+		ex.WriteHeader(200)
+	})
+	// The peer is still there, waiting to be answered: a read for bytes it is
+	// not going to send blocks, exactly as it does on a socket.
+	conn := &blockingConn{request: "POST / HTTP/1.1\r\nHost: h\r\nContent-Length: 12\r\n\r\n" + body}
+	exch := newExchange(t, conn, 1024, false)
+	done := make(chan struct{})
+	go func() {
+		defer close(done)
+		Handle(exch, &sm, nopBackoff)
+	}()
+	select {
+	case <-done:
+	case <-time.After(2 * time.Second):
+		t.Fatal("ReadBody blocked waiting for a body that had already arrived")
+	}
+	if readErr != nil {
+		t.Fatal(readErr)
+	}
+	if got != body {
+		t.Errorf("want body %q, got %q", body, got)
+	}
+}
+
+// blockingConn delivers a request and then blocks on reads, the way a peer
+// awaiting its answer does. Writes are discarded.
+type blockingConn struct {
+	request string
+	read    int
+	blocked chan struct{}
+}
+
+func (c *blockingConn) Read(b []byte) (int, error) {
+	if c.read >= len(c.request) {
+		if c.blocked == nil {
+			c.blocked = make(chan struct{})
+		}
+		<-c.blocked // Nothing more is coming, and nothing unblocks this.
+		return 0, io.EOF
+	}
+	n := copy(b, c.request[c.read:])
+	c.read += n
+	return n, nil
+}
+
+func (c *blockingConn) Write(b []byte) (int, error) { return len(b), nil }
+func (c *blockingConn) Close() error                { return nil }
diff --git a/http/httphi/mux.go b/http/httphi/mux.go
index 0f1a09d..8d66a6e 100644
--- a/http/httphi/mux.go
+++ b/http/httphi/mux.go
@@ -5,6 +5,7 @@ import (
 	"unsafe"
 
 	"github.com/soypat/lneto"
+	"github.com/soypat/lneto/http/httpraw"
 	"github.com/soypat/lneto/internal"
 )
 
@@ -13,7 +14,7 @@ import (
 // Handle does not close the connection on any outcome: the caller owns it.
 func Handle(exch *Exchange, mux Mux, backoff lneto.BackoffStrategy) error {
 	reqhdr := &exch.reqHdr
-	reqhdr.Reset(nil)
+	reqhdr.Reset(nil, 0) // Assume exchange has been configured and reuse memory.
 	var consecutiveBackoffs uint
 	for {
 		n, err := reqhdr.ReadFromLimited(exch.rw, reqhdr.BufferFree())
@@ -31,6 +32,12 @@ func Handle(exch *Exchange, mux Mux, backoff lneto.BackoffStrategy) error {
 		if needMore {
 			continue // Request header split across reads, accumulate the rest.
 		} else if err != nil {
+			if err == httpraw.ErrHeaderTooMany || err == httpraw.ErrSmallHeaderBuffer {
+				// The peer is owed an answer: no larger buffer is coming, so
+				// say so instead of dropping the connection, RFC 6585 5.
+				exch.StageHeader("Content-Length", "0")
+				exch.WriteHeader(int(StatusRequestHeaderFieldsTooLarge))
+			}
 			return err
 		}
 		break // Done!
diff --git a/http/httphi/router.go b/http/httphi/router.go
index 0e25263..08a9f32 100644
--- a/http/httphi/router.go
+++ b/http/httphi/router.go
@@ -44,14 +44,15 @@ type conn = io.ReadWriteCloser
 // Methods are safe for concurrent use. The zero value is not usable: configure
 // it first.
 type Router struct {
-	mu            sync.Mutex
-	gen           atomic.Uint32
-	numGoro       int
-	reqBuf        int
-	respBuf       int
-	normalizeKeys bool
-	pendingConns  chan job
-	mux           Mux
+	mu              sync.Mutex
+	gen             atomic.Uint32
+	numGoro         int
+	reqBuf          int
+	respBuf         int
+	reqNumHeaderCap int
+	normalizeKeys   bool
+	pendingConns    chan job
+	mux             Mux
 
 	globbuf  []byte
 	exchs    []Exchange
@@ -79,6 +80,8 @@ type RouterConfig struct {
 	// "HTTP/1.1 200 OK\r\n" does not count towards this memory, only actual Headers key/value pairs use this memory.
 	// After memory is fully consumed [Exchange.StageHeader] will not append more headers.
 	ResponseHeaderMinBufferSize int
+	// Number of request header key/value pairs to parse before failing and returning [StatusRequestHeaderFieldsTooLarge].
+	RequestNumHeaderCap int
 
 	// NormalizeOutgoingKeys normalizes response header field keys as they are
 	// staged, i.e: "content-type" becomes "Content-Type".
@@ -154,6 +157,7 @@ func (r *Router) Configure(cfg RouterConfig) error {
 	gen := r.gen.Load()
 	numgoro := cfg.FixedNumGoroutines
 	workerMode := cfg.workerMode()
+	r.reqNumHeaderCap = cfg.RequestNumHeaderCap
 	r.reqBuf = cfg.RequestHeaderBufferSize
 	r.respBuf = cfg.ResponseHeaderMinBufferSize
 	r.mux = cfg.Mux
@@ -183,7 +187,13 @@ func (r *Router) Configure(cfg RouterConfig) error {
 		for i := range numgoro {
 			// TODO exchange buffer alloc
 			goff := i * rawBuflen
-			r.exchs[i].Configure(r.globbuf[goff:goff+rawBuflen], cfg.RequestHeaderBufferSize, cfg.NormalizeOutgoingKeys)
+			// r.globbuf[goff:goff+rawBuflen], cfg.RequestHeaderBufferSize, cfg.RequestNumHeaderCap, cfg.NormalizeOutgoingKeys
+			r.exchs[i].Configure(ExchangeConfig{
+				RawBuf:                r.globbuf[goff : goff+rawBuflen],
+				RequestBufferLim:      cfg.RequestHeaderBufferSize,
+				NumHeaderCap:          cfg.RequestNumHeaderCap,
+				NormalizeOutgoingKeys: cfg.NormalizeOutgoingKeys,
+			})
 			go r.goroWorker(gen, jobqueue, cfg.Backoff, cfg.Mux)
 		}
 		r.pendingConns = jobqueue
@@ -324,9 +334,14 @@ func (r *Router) getExchLocked(conn conn) (exch *Exchange) {
 		}
 	}
 
-	if r.numGoro == 0 {
+	if r.numGoro == -1 {
 		exch := new(Exchange)
-		exch.Configure(make([]byte, r.respBuf+r.reqBuf), r.reqBuf, r.normalizeKeys)
+		exch.Configure(ExchangeConfig{
+			RawBuf:                make([]byte, r.respBuf+r.reqBuf),
+			RequestBufferLim:      r.reqBuf,
+			NumHeaderCap:          r.reqNumHeaderCap,
+			NormalizeOutgoingKeys: r.normalizeKeys,
+		})
 		exch.Acquire(conn) // Fresh exchange, CAS cannot fail.
 		return exch
 	}
diff --git a/http/httpraw/header.go b/http/httpraw/header.go
index c85d1dc..868b449 100644
--- a/http/httpraw/header.go
+++ b/http/httpraw/header.go
@@ -65,10 +65,12 @@ type Header struct {
 // Flags returns [Flags] to signal status code has been set, Connection:Close or other useful signals provided by flags.
 func (h *Header) Flags() Flags { return h.flags }
 
-// EnableBufferGrowth disables buffer growth during parsing if b is false. Is enabled by default.
-// Disabling buffer growth prevents allocations but methods may throw errors on insufficient memory.
-func (h *Header) EnableBufferGrowth(b bool) {
-	if !b {
+// ConfigBufferGrowth configures the memory the header may use. Setting
+// outlives [Header.Reset]. Call before parsing/reading.
+//
+// enableBufferGrowth enables growing both the header buffer and the header key/value pair slice.
+func (h *Header) ConfigBufferGrowth(enableBufferGrowth bool) {
+	if !enableBufferGrowth {
 		h.flags |= flagNoBufferGrow
 	} else {
 		h.flags &^= flagNoBufferGrow
@@ -77,7 +79,7 @@ func (h *Header) EnableBufferGrowth(b bool) {
 
 // ParseBytes copies the bytes into buffer and parses the HTTP header. It fails if HTTP header data is incomplete.
 func (h *Header) ParseBytes(asResponse bool, b []byte) error {
-	h.Reset(nil)
+	h.Reset(nil, 0)
 	h.hbuf.readFromBytes(b)
 	return h.parse(asResponse)
 }
@@ -86,7 +88,7 @@ func (h *Header) ParseBytes(asResponse bool, b []byte) error {
 // It fails if HTTP data is incomplete.
 func (h *Header) Parse(asResponse bool) error {
 	debuglog("http:parse:reset")
-	h.Reset(h.hbuf.buf)
+	h.Reset(h.hbuf.buf, 0)
 	debuglog("http:parse:start")
 	return h.parse(asResponse)
 }
@@ -119,7 +121,7 @@ func (h *Header) TryParse(asResponse bool) (needMoreData bool, err error) {
 			return err == ErrNeedMoreData, err
 		}
 	}
-	err = h.parseNextHeaders()
+	err = h.parseNextHeaders(h.flags)
 	return err == ErrNeedMoreData, err
 }
 
@@ -133,7 +135,7 @@ func (h *Header) ParsingSuccess() bool {
 // If read is successful (read length>0) and reader returns [io.EOF] then ReadFromLimited will return a nil error.
 func (h *Header) ReadFromLimited(r io.Reader, maxBytesToRead int) (int, error) {
 	if maxBytesToRead <= 0 {
-		return 0, errSmallBuffer
+		return 0, ErrSmallHeaderBuffer
 	} else if h.flags.HasAny(flagMangledBuffer) {
 		return 0, errMangledBuffer
 	} else if h.flags.HasAny(flagReaderEOF) {
@@ -142,14 +144,14 @@ func (h *Header) ReadFromLimited(r io.Reader, maxBytesToRead int) (int, error) {
 	free := h.BufferFree()
 	if free < maxBytesToRead {
 		if h.flags.HasAny(flagNoBufferGrow) {
-			return 0, errSmallBuffer
+			return 0, ErrSmallHeaderBuffer
 		}
 		h.hbuf.buf = slices.Grow(h.hbuf.buf, maxBytesToRead)
 	}
 	blen := len(h.hbuf.buf)
 	b := h.hbuf.buf[blen:min(blen+maxBytesToRead, cap(h.hbuf.buf))]
 	if len(b) == 0 {
-		return 0, errSmallBuffer
+		return 0, ErrSmallHeaderBuffer
 	}
 	n, err := r.Read(b)
 	if err != nil && err == io.EOF {
@@ -166,12 +168,12 @@ func (h *Header) ReadFromLimited(r io.Reader, maxBytesToRead int) (int, error) {
 // Used to accumulate HTTP header for later parsing with [Header.TryParse].
 func (h *Header) ReadFromBytes(b []byte) (int, error) {
 	if len(b) == 0 {
-		return 0, errSmallBuffer
+		return 0, ErrSmallHeaderBuffer
 	}
 	free := h.BufferFree()
 	if free < len(b) {
 		if h.flags.HasAny(flagNoBufferGrow) {
-			return 0, errSmallBuffer
+			return 0, ErrSmallHeaderBuffer
 		}
 		h.hbuf.buf = slices.Grow(h.hbuf.buf, len(b))
 	}
@@ -253,13 +255,13 @@ func (hb *headerBuf) forEach(cb func(key, value []byte) error) error {
 //	h.Reset(prealloc[:0]); h.ParseBytes(httpHeader) // Tell header to use a pre-allocated buffer capacity.
 //	h.Reset(httpHeader); h.Parse() // Parse bytes in place with no copying.
 //	h.Reset(nil) // Reuse buffer previously set in a call to Reset.
-func (h *Header) Reset(buf []byte) {
-	if h.flags.HasAny(flagNoBufferGrow) && cap(buf) < 32 {
-		panic("small buffer and flagNoBufferGrow set")
-	}
+func (h *Header) Reset(buf []byte, numHeaderCapacity int) {
 	const persistentFlags = flagNoBufferGrow
 	debuglog("http:reset:hbuf")
-	h.hbuf.reset(buf)
+	h.hbuf.reset(buf, numHeaderCapacity)
+	if h.flags.HasAny(flagNoBufferGrow) && cap(h.hbuf.buf) < 32 {
+		panic("small buffer and flagNoBufferGrow set")
+	}
 	*h = Header{
 		hbuf:  h.hbuf,
 		flags: h.flags & persistentFlags,
diff --git a/http/httpraw/header_test.go b/http/httpraw/header_test.go
index edc8bd0..c7cf12f 100644
--- a/http/httpraw/header_test.go
+++ b/http/httpraw/header_test.go
@@ -2,6 +2,7 @@ package httpraw
 
 import (
 	"bytes"
+	"errors"
 	"fmt"
 	"net/http"
 	"strconv"
@@ -10,6 +11,8 @@ import (
 	"time"
 )
 
+const numHeaderCapacity = 16
+
 func TestHeaderParseRequest(t *testing.T) {
 	const (
 		wantMethod  = "GET"
@@ -387,7 +390,7 @@ func TestCopyDecodedPercentURLInPlace(t *testing.T) {
 
 func TestHeaderSetOverwrite(t *testing.T) {
 	var h Header
-	h.Reset(nil)
+	h.Reset(nil, numHeaderCapacity)
 	h.SetMethod("GET")
 	h.SetRequestTarget("/")
 	h.SetProtocol("HTTP/1.1")
@@ -410,7 +413,7 @@ func TestHeaderSetOverwrite(t *testing.T) {
 
 func TestHeaderSetBytesEmptyValue(t *testing.T) {
 	var h Header
-	h.Reset(nil)
+	h.Reset(nil, numHeaderCapacity)
 	h.SetBytes("X-Empty", nil)
 	if got := h.Get("X-Empty"); len(got) != 0 {
 		t.Errorf("want empty value, got %q", got)
@@ -465,7 +468,7 @@ func TestHeader_SplitBeforeColonStillParses(t *testing.T) {
 	const part2 = ": example.com\r\n\r\n"
 
 	var h Header
-	h.Reset(nil)
+	h.Reset(nil, numHeaderCapacity)
 	if _, err := h.ReadFromBytes([]byte(part1)); err != nil {
 		t.Fatal(err)
 	}
@@ -498,7 +501,7 @@ func TestHeader_AppendHeaderExactCapNoPanic(t *testing.T) {
 	const key, value = "K", "V"
 	buf := make([]byte, 0, len(key)+len(value)) // exact cap, no slack.
 	var h Header
-	h.Reset(buf)
+	h.Reset(buf, numHeaderCapacity)
 	defer func() {
 		if r := recover(); r != nil {
 			t.Fatalf("appendHeader panicked on exact-cap buffer: %v", r)
@@ -515,8 +518,8 @@ func TestHeader_AppendHeaderExactCapNoPanic(t *testing.T) {
 func TestHeader_AddFullBufferNoPanic(t *testing.T) {
 	buf := make([]byte, 0, 40) // Small cap; enough for Reset (len 0) but not the field below.
 	var h Header
-	h.Reset(buf)
-	h.EnableBufferGrowth(false)
+	h.Reset(buf, numHeaderCapacity)
+	h.ConfigBufferGrowth(false)
 	h.SetMethod("GET")
 	h.SetRequestTarget("/")
 	h.SetProtocol("HTTP/1.1")
@@ -550,7 +553,7 @@ func TestHeader_SetInt(t *testing.T) {
 	} {
 		t.Run(tc.name, func(t *testing.T) {
 			var h Header
-			h.Reset(nil)
+			h.Reset(nil, numHeaderCapacity)
 			h.SetInt("Content-Length", tc.value, tc.base)
 			if got := string(h.Get("Content-Length")); got != tc.want {
 				t.Fatalf("want %q, got %q", tc.want, got)
@@ -562,7 +565,7 @@ func TestHeader_SetInt(t *testing.T) {
 // SetInt on an existing key must reuse the slot in place (single field, latest value).
 func TestHeader_SetIntOverwrite(t *testing.T) {
 	var h Header
-	h.Reset(nil)
+	h.Reset(nil, numHeaderCapacity)
 	h.SetMethod("GET")
 	h.SetRequestTarget("/")
 	h.SetProtocol("HTTP/1.1")
@@ -586,8 +589,8 @@ func TestHeader_SetIntOverwrite(t *testing.T) {
 func TestHeader_SetIntNoAlloc(t *testing.T) {
 	buf := make([]byte, 0, 256)
 	var h Header
-	h.Reset(buf)
-	h.EnableBufferGrowth(false)
+	h.Reset(buf, numHeaderCapacity)
+	h.ConfigBufferGrowth(false)
 	h.Add("Content-Length", "0000000000000000000000") // pre-size a reusable slot.
 	allocs := testing.AllocsPerRun(100, func() {
 		h.SetInt("Content-Length", 1234567890, 10)
@@ -599,3 +602,43 @@ func TestHeader_SetIntNoAlloc(t *testing.T) {
 		t.Fatalf("want %q, got %q", "1234567890", got)
 	}
 }
+
+// A browser sends upwards of twenty header fields and an API client with a few
+// custom fields is not far behind. The field table must be sized from the
+// buffer the caller handed over, not fixed at a count that a real request
+// exceeds.
+func TestHeader_FieldTableSizedFromBuffer(t *testing.T) {
+	const wantVal = "the-canary-value"
+	raw := "GET / HTTP/1.1\r\nHost: lneto.test\r\n"
+	for i := 0; i < 40; i++ {
+		raw += "X-Field-" + strconv.Itoa(i) + ": value-of-a-realistic-length-here\r\n"
+	}
+	raw += "X-Canary: " + wantVal + "\r\n\r\n"
+
+	var h Header
+	h.Reset(make([]byte, 0, 8192), numHeaderCapacity) // Room for the block with plenty to spare.
+	err := h.ParseBytes(false, []byte(raw))
+	if err != nil {
+		t.Fatalf("parsing a 42 field request into an 8kB buffer: %s", err)
+	}
+	if got := string(h.Get("X-Canary")); got != wantVal {
+		t.Fatalf("want X-Canary %q, got %q", wantVal, got)
+	}
+}
+
+// A buffer too small for the fields it is handed must be refused with an error
+// the caller can act on, so a server answers 431 instead of dropping the peer.
+func TestHeader_FieldTableFullIsReported(t *testing.T) {
+	raw := "GET / HTTP/1.1\r\n"
+	for i := 0; i < 64; i++ {
+		raw += "H" + strconv.Itoa(i) + ":v\r\n" // As short as a field gets.
+	}
+	raw += "\r\n"
+	var h Header
+	h.Reset(make([]byte, 0, 512), numHeaderCapacity)
+	h.ConfigBufferGrowth(false)
+	err := h.ParseBytes(false, []byte(raw))
+	if !errors.Is(err, ErrHeaderTooMany) {
+		t.Fatalf("want ErrHeaderFieldsTooLarge, got %v", err)
+	}
+}
diff --git a/http/httpraw/parse.go b/http/httpraw/parse.go
index 8ae5cf3..5752c59 100644
--- a/http/httpraw/parse.go
+++ b/http/httpraw/parse.go
@@ -14,12 +14,17 @@ var (
 	errNoProto = errors.New("missing protocol, HTTP/0.9 unsupported")
 	// ErrNeedMoreData signals a parser was handed an incomplete buffer: append
 	// more data to it and call again.
-	ErrNeedMoreData = errors.New("need more data: cannot find trailing lf/delimiter")
-	errNoBoundary   = errors.New("httpraw: multipart boundary not set")
-	errUnparsed     = errors.New("need to finish parsing")
-	errInvalidName  = errors.New("invalid header name")
-	errSmallBuffer  = errors.New("small read buffer. Increase ReadBufferSize")
-	errOOM          = errors.New("httpraw: buffer out of memory")
+	ErrNeedMoreData      = errors.New("need more data: cannot find trailing lf/delimiter")
+	errNoBoundary        = errors.New("httpraw: multipart boundary not set")
+	errUnparsed          = errors.New("need to finish parsing")
+	errInvalidName       = errors.New("invalid header name")
+	ErrSmallHeaderBuffer = errors.New("httpraw: Header buffer exhausted, increase size")
+	errOOM               = errors.New("httpraw: Header incomplete due to OOM")
+	// ErrHeaderTooMany signals a header block carrying more fields than
+	// the buffer it is parsed into has room for, see [Header.Reset]. A server
+	// answers it with 431, RFC 6585 5: no larger buffer is coming, so reading
+	// the rest of the block would only spend memory on a request already lost.
+	ErrHeaderTooMany = errors.New("httpraw: more header fields than buffer holds")
 	// Header.Set and Header.Add mangles the buffer.
 	// Call them after retrieving the Body. Do not call them before parsing the header (why would you even do that?).
 	errMangledBuffer    = errors.New("httpraw: mangled buffer")
@@ -52,17 +57,21 @@ type headerBuf struct {
 	headers []argsKV
 }
 
-// reset sets the buffer data and discards all parsed data.
-func (h *headerBuf) reset(buf []byte) {
+// reset sets the buffer data and discards all parsed data. The field table is
+// grown to match the new buffer's capacity and never shrinks, so a header
+// reused across requests settles on its largest buffer and stops allocating.
+func (h *headerBuf) reset(buf []byte, numHeaderCapacity int) {
 	if buf == nil {
 		buf = h.buf[:0] // Reuse buffer but discard raw data on nil input.
 	}
-	if cap(h.headers) == 0 {
-		h.headers = make([]argsKV, 16)
+	if numHeaderCapacity != 0 {
+		internal.SliceReuse(&h.headers, numHeaderCapacity)
+	} else {
+		h.headers = h.headers[:0]
 	}
 	*h = headerBuf{
 		buf:     buf,
-		headers: h.headers[:0],
+		headers: h.headers,
 	}
 }
 
@@ -100,7 +109,7 @@ func (h *Header) parse(asResponse bool) (err error) {
 		return err
 	}
 	debuglog("http:firstline:done")
-	err = h.parseNextHeaders()
+	err = h.parseNextHeaders(h.flags)
 	debuglog("http:headers:done")
 	return err
 }
@@ -117,9 +126,9 @@ func (h *Header) parseFirstLine(asResponse bool) (err error) {
 	return err
 }
 
-func (h *Header) parseNextHeaders() error {
+func (h *Header) parseNextHeaders(flags Flags) error {
 	var ss scannerState
-	h.hbuf.parseNextHeaders(&ss)
+	h.hbuf.parseNextHeaders(&ss, flags)
 	if ss.err != nil {
 		h.flags |= flagConnClose
 		return ss.err
@@ -134,13 +143,13 @@ func (hb *headerBuf) readFromBytes(b []byte) {
 
 func (hb *headerBuf) free() int { return cap(hb.buf) - len(hb.buf) }
 
-func (hb *headerBuf) parseNextHeaders(ss *scannerState) {
+func (hb *headerBuf) parseNextHeaders(ss *scannerState, flags Flags) {
 	debuglog("http:nexthdr:loop")
 	for kv := hb.next(ss); kv.isValid(); kv = hb.next(ss) {
-		if len(hb.headers) == cap(hb.headers) {
-			// Refuse to grow the headers slice: caller must pre-allocate
-			// sufficient capacity via reset or use a larger initial size.
-			ss.err = errOOM
+		if len(hb.headers) == cap(hb.headers) && flags.HasAny(flagNoBufferGrow) {
+			// Refuse to grow the headers slice: the caller granted this much
+			// memory and no more, see [Header.Reset].
+			ss.err = ErrHeaderTooMany
 			return
 		}
 		hb.headers = append(hb.headers, kv)
diff --git a/http/httpraw/parse_test.go b/http/httpraw/parse_test.go
index f0ef65c..ae66cc7 100644
--- a/http/httpraw/parse_test.go
+++ b/http/httpraw/parse_test.go
@@ -10,7 +10,7 @@ func TestTryParse_IncrementalRequest(t *testing.T) {
 	// Full HTTP request split across multiple ReadFromBytes calls.
 	full := "GET /index.html HTTP/1.1\r\nHost: example.com\r\nContent-Type: text/html\r\n\r\nbody here"
 	var hdr Header
-	hdr.Reset(make([]byte, 0, 256))
+	hdr.Reset(make([]byte, 0, 256), numHeaderCapacity)
 
 	// Feed data in small chunks to exercise incremental parsing.
 	chunks := splitInto(full, 10)
@@ -85,7 +85,7 @@ func TestTryParse_IncrementalRequest(t *testing.T) {
 func TestTryParse_IncrementalResponse(t *testing.T) {
 	full := "HTTP/1.1 200 OK\r\nContent-Length: 5\r\nServer: lneto\r\n\r\nhello"
 	var hdr Header
-	hdr.Reset(make([]byte, 0, 256))
+	hdr.Reset(make([]byte, 0, 256), numHeaderCapacity)
 
 	chunks := splitInto(full, 8)
 	var done bool
@@ -137,7 +137,7 @@ func TestReadFromLimited(t *testing.T) {
 	r := strings.NewReader(data)
 
 	var hdr Header
-	hdr.Reset(make([]byte, 0, 256))
+	hdr.Reset(make([]byte, 0, 256), numHeaderCapacity)
 
 	// Read in one shot.
 	n, err := hdr.ReadFromLimited(r, 256)
@@ -163,7 +163,7 @@ func TestReadFromLimited(t *testing.T) {
 
 func TestReadFromLimited_MaxBytes(t *testing.T) {
 	var hdr Header
-	hdr.Reset(make([]byte, 0, 256))
+	hdr.Reset(make([]byte, 0, 256), numHeaderCapacity)
 
 	// Zero maxBytesToRead should error.
 	_, err := hdr.ReadFromLimited(strings.NewReader("data"), 0)
@@ -174,7 +174,7 @@ func TestReadFromLimited_MaxBytes(t *testing.T) {
 
 func TestReadFromBytes_Empty(t *testing.T) {
 	var hdr Header
-	hdr.Reset(make([]byte, 0, 256))
+	hdr.Reset(make([]byte, 0, 256), numHeaderCapacity)
 
 	_, err := hdr.ReadFromBytes(nil)
 	if err == nil {
@@ -184,7 +184,7 @@ func TestReadFromBytes_Empty(t *testing.T) {
 
 func TestBufferFreeAndCapacity(t *testing.T) {
 	var hdr Header
-	hdr.Reset(make([]byte, 0, 100))
+	hdr.Reset(make([]byte, 0, 100), numHeaderCapacity)
 
 	if hdr.BufferCapacity() != 100 {
 		t.Errorf("capacity = %d; want 100", hdr.BufferCapacity())
@@ -202,9 +202,8 @@ func TestBufferFreeAndCapacity(t *testing.T) {
 func TestEnableBufferGrowth(t *testing.T) {
 	var hdr Header
 	buf := make([]byte, 0, 64)
-	hdr.Reset(buf)
-	hdr.EnableBufferGrowth(false)
-
+	hdr.Reset(buf, numHeaderCapacity)
+	hdr.ConfigBufferGrowth(false)
 	// With growth disabled, reading more than capacity should fail.
 	big := make([]byte, 128)
 	for i := range big {
@@ -389,7 +388,7 @@ func TestHeader_MultilineValue(t *testing.T) {
 
 func TestHeader_ResponseRoundTrip(t *testing.T) {
 	var hdr Header
-	hdr.Reset(make([]byte, 0, 256))
+	hdr.Reset(make([]byte, 0, 256), numHeaderCapacity)
 	hdr.SetProtocol("HTTP/1.1")
 	hdr.SetStatus("404", "Not Found")
 	hdr.Add("Content-Type", "text/plain")
@@ -429,7 +428,7 @@ func TestHeader_ResponseRoundTrip(t *testing.T) {
 
 func TestHeader_RequestRoundTrip(t *testing.T) {
 	var hdr Header
-	hdr.Reset(make([]byte, 0, 256))
+	hdr.Reset(make([]byte, 0, 256), numHeaderCapacity)
 	hdr.SetProtocol("HTTP/1.1")
 	hdr.SetMethod("POST")
 	hdr.SetRequestTarget("/api/data")
diff --git a/examples/http-linux/conn-linux.go b/internal/rawsock/rawsock_linux.go
similarity index 100%
rename from examples/http-linux/conn-linux.go
rename to internal/rawsock/rawsock_linux.go