diff --git a/internal/handlers/api.go b/internal/handlers/api.go
index 3ad7068..0843b31 100644
--- a/internal/handlers/api.go
+++ b/internal/handlers/api.go
@@ -188,7 +188,7 @@ func (hdlr *Handlers) handleCreateSession(
 
 	var payload createRequest
 
-	err := json.NewDecoder(request.Body).Decode(&payload)
+	err := decodeJSON(request, &payload)
 	if err != nil {
 		hdlr.respondError(
 			writer, request,
@@ -925,7 +925,7 @@ func (hdlr *Handlers) HandleSendCommand() http.HandlerFunc {
 
 		var payload commandRequest
 
-		err := json.NewDecoder(request.Body).Decode(&payload)
+		err := decodeJSON(request, &payload)
 		if err != nil {
 			hdlr.respondError(
 				writer, request,
diff --git a/internal/handlers/api_test.go b/internal/handlers/api_test.go
index 7ed1aac..305be3f 100644
--- a/internal/handlers/api_test.go
+++ b/internal/handlers/api_test.go
@@ -797,6 +797,107 @@ func TestCreateSessionMalformed(t *testing.T) {
 	}
 }
 
+func (tserver *testServer) postBody(
+	path, token, body string,
+) (int, map[string]any) {
+	tserver.t.Helper()
+
+	resp, err := doRequestAuth(
+		tserver.t, http.MethodPost, tserver.url(path), token,
+		strings.NewReader(body),
+	)
+	if err != nil {
+		tserver.t.Fatalf("post %s: %v", path, err)
+	}
+
+	defer func() { _ = resp.Body.Close() }()
+
+	var result map[string]any
+
+	_ = json.NewDecoder(resp.Body).Decode(&result)
+
+	return resp.StatusCode, result
+}
+
+// TestRequestBodyLimit sends each endpoint a small valid JSON value
+// followed by other bytes. Only whitespace may follow the value, and
+// the whole body counts towards MAX_MESSAGE_SIZE.
+func TestRequestBodyLimit(t *testing.T) {
+	t.Parallel()
+
+	const (
+		maxMessageSize = 4096 // the default MAX_MESSAGE_SIZE
+		nick           = "alice_bodylimit"
+	)
+
+	endpoints := []struct {
+		name, path, value string
+		acceptedStatus    int
+	}{
+		{
+			"create session", apiSession,
+			`{"nick":"bob_bodylimit"}`,
+			http.StatusCreated,
+		},
+		{
+			"login", "/api/v1/login",
+			`{"nick":"` + nick + `","password":"` + testPassword + `"}`,
+			http.StatusOK,
+		},
+		{
+			"post message", apiMessages,
+			`{"command":"PING"}`,
+			http.StatusOK,
+		},
+	}
+
+	for _, endpoint := range endpoints {
+		t.Run(endpoint.name, func(t *testing.T) {
+			t.Parallel()
+
+			tserver := newTestServer(t)
+			token := tserver.createSession(nick)
+
+			tserver.sendCommand(token, map[string]any{
+				commandKey: irc.CmdPass,
+				bodyKey:    []string{testPassword},
+			})
+
+			bytesToLimit := maxMessageSize - len(endpoint.value)
+
+			for _, trailing := range []string{
+				strings.Repeat(" ", bytesToLimit+1),
+				strings.Repeat("x", bytesToLimit+1),
+				"x",
+				"{}",
+			} {
+				status, result := tserver.postBody(
+					endpoint.path, token, endpoint.value+trailing,
+				)
+				if status != http.StatusBadRequest ||
+					result["error"] != "invalid request body" {
+					t.Errorf(
+						"%d bytes of %.10q after the value: expected "+
+							"400 invalid request body, got %d: %v",
+						len(trailing), trailing, status, result,
+					)
+				}
+			}
+
+			status, result := tserver.postBody(
+				endpoint.path, token,
+				endpoint.value+strings.Repeat(" ", bytesToLimit),
+			)
+			if status != endpoint.acceptedStatus {
+				t.Errorf(
+					"body of exactly the limit: expected %d, got %d: %v",
+					endpoint.acceptedStatus, status, result,
+				)
+			}
+		})
+	}
+}
+
 func TestAuthNoCookie(t *testing.T) {
 	t.Parallel()
 
diff --git a/internal/handlers/auth.go b/internal/handlers/auth.go
index 21d9bc6..172a790 100644
--- a/internal/handlers/auth.go
+++ b/internal/handlers/auth.go
@@ -1,7 +1,6 @@
 package handlers
 
 import (
-	"encoding/json"
 	"net/http"
 	"strings"
 
@@ -55,7 +54,7 @@ func (hdlr *Handlers) handleLogin(
 
 	var payload loginRequest
 
-	err := json.NewDecoder(request.Body).Decode(&payload)
+	err := decodeJSON(request, &payload)
 	if err != nil {
 		hdlr.respondError(
 			writer, request,
diff --git a/internal/handlers/handlers.go b/internal/handlers/handlers.go
index 2326fff..58ba9c5 100644
--- a/internal/handlers/handlers.go
+++ b/internal/handlers/handlers.go
@@ -5,6 +5,7 @@ import (
 	"context"
 	"encoding/json"
 	"errors"
+	"io"
 	"log/slog"
 	"net/http"
 	"time"
@@ -159,6 +160,18 @@ func (hdlr *Handlers) respondError(
 	)
 }
 
+// decodeJSON reads the whole request body and decodes it into dest.
+// It fails if the body is larger than the http.MaxBytesReader limit,
+// or holds anything other than one JSON value and whitespace.
+func decodeJSON(request *http.Request, dest any) error {
+	body, err := io.ReadAll(request.Body)
+	if err != nil {
+		return err
+	}
+
+	return json.Unmarshal(body, dest)
+}
+
 func (hdlr *Handlers) idleTimeout() time.Duration {
 	raw := hdlr.params.Config.SessionIdleTimeout
 	if raw == "" {