diff --git a/cmd/hyperguild/brain.go b/cmd/hyperguild/brain.go
index f224dc5..0dbf76d 100644
--- a/cmd/hyperguild/brain.go
+++ b/cmd/hyperguild/brain.go
@@ -52,8 +52,22 @@ func runBrainQuery(ctx context.Context, args []string, _ io.Reader, stdout, stde
 	return nil
 }
 
-// runBrainWrite is implemented in Task 5; stub now returns an explicit error
-// so the router compiles and tests for runBrainQuery can run.
-func runBrainWrite(_ context.Context, _ []string, _ io.Reader, _, _ io.Writer) error {
-	return errors.New("brain write: not implemented (Task 5)")
+func runBrainWrite(ctx context.Context, args []string, stdin io.Reader, stdout, stderr io.Writer) error {
+	fs := flag.NewFlagSet("brain write", flag.ContinueOnError)
+	fs.SetOutput(stderr)
+	if err := fs.Parse(args); err != nil {
+		return fmt.Errorf("parse flags: %w", err)
+	}
+	if fs.NArg() < 2 {
+		return errors.New("brain write: type and slug required (e.g. brain write knowledge my-slug)")
+	}
+	kind := fs.Arg(0)
+	slug := fs.Arg(1)
+
+	res, err := newBrainClient().Write(ctx, kind, slug, stdin)
+	if err != nil {
+		return err
+	}
+	fmt.Fprintln(stdout, res.Path) //nolint:errcheck
+	return nil
 }
diff --git a/cmd/hyperguild/brain_test.go b/cmd/hyperguild/brain_test.go
index 4c7f063..8cb3aa8 100644
--- a/cmd/hyperguild/brain_test.go
+++ b/cmd/hyperguild/brain_test.go
@@ -3,6 +3,8 @@ package main
 import (
 	"bytes"
 	"context"
+	"encoding/json"
+	"io"
 	"net/http"
 	"net/http/httptest"
 	"strings"
@@ -79,3 +81,70 @@ func TestRunBrain_UnknownSubsubcommand(t *testing.T) {
 	err := runBrain(context.Background(), []string{"bogus"}, strings.NewReader(""), &out, &errBuf)
 	assert.Error(t, err)
 }
+
+func TestRunBrainWrite_Success(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		assert.Equal(t, http.MethodPost, r.Method)
+		assert.Equal(t, "/write", r.URL.Path)
+		w.Header().Set("Content-Type", "application/json")
+		_, _ = w.Write([]byte(`{"path":"knowledge/test-slug.md"}`))
+	}))
+	defer srv.Close()
+	t.Setenv("BRAIN_URL", srv.URL)
+
+	var out, errBuf bytes.Buffer
+	err := runBrain(
+		context.Background(),
+		[]string{"write", "knowledge", "test-slug"},
+		strings.NewReader("# Test\n\nSome body content.\n"),
+		&out, &errBuf,
+	)
+	require.NoError(t, err)
+	assert.Contains(t, out.String(), "knowledge/test-slug.md")
+}
+
+func TestRunBrainWrite_MissingArgs(t *testing.T) {
+	var out, errBuf bytes.Buffer
+	err := runBrain(context.Background(), []string{"write", "knowledge"}, strings.NewReader("x"), &out, &errBuf)
+	assert.Error(t, err)
+	assert.Contains(t, err.Error(), "type and slug required")
+}
+
+func TestRunBrainWrite_BackendError(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusBadRequest)
+		_, _ = w.Write([]byte("invalid slug"))
+	}))
+	defer srv.Close()
+	t.Setenv("BRAIN_URL", srv.URL)
+
+	var out, errBuf bytes.Buffer
+	err := runBrain(
+		context.Background(),
+		[]string{"write", "knowledge", "bad slug"},
+		strings.NewReader("body"),
+		&out, &errBuf,
+	)
+	assert.Error(t, err)
+	assert.Contains(t, err.Error(), "400")
+}
+
+func TestRunBrainWrite_EmptyStdin(t *testing.T) {
+	gotLen := -1
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		body, _ := io.ReadAll(r.Body)
+		var p struct {
+			Content string `json:"content"`
+		}
+		_ = json.Unmarshal(body, &p)
+		gotLen = len(p.Content)
+		_, _ = w.Write([]byte(`{"path":"x.md"}`))
+	}))
+	defer srv.Close()
+	t.Setenv("BRAIN_URL", srv.URL)
+
+	var out, errBuf bytes.Buffer
+	err := runBrain(context.Background(), []string{"write", "knowledge", "empty"}, strings.NewReader(""), &out, &errBuf)
+	require.NoError(t, err)
+	assert.Equal(t, 0, gotLen, "empty stdin should produce empty content payload")
+}