fix(oauth): form-urlencode the token refresh (was JSON → /token 400s)

internal/api/oauth.go

@@ -7,6 +7,7 @@ import (
 	"fmt"
 	"io"
 	"net/http"
+	"net/url"
 	"strings"
 	"time"
 )
@@ -264,12 +265,17 @@ func (c *OAuthClient) PollToken(ctx context.Context, auth *DeviceAuth, sleep fun
 // Refresh exchanges a refresh token for a new access token. The server may
 // rotate the refresh token; callers must persist tr.RefreshToken if non-empty.
 func (c *OAuthClient) Refresh(ctx context.Context, refreshToken string) (*TokenResponse, error) {
-	body, _ := json.Marshal(map[string]string{
-		"grant_type":    grantTypeRefreshToken,
-		"refresh_token": refreshToken,
-		"client_id":     ClientID,
-	})
-	status, raw, err := c.post(ctx, pathToken, body)
+	// The /token endpoint is the @node-oauth/oauth2-server token handler, which
+	// REQUIRES application/x-www-form-urlencoded (it rejects JSON with "content
+	// must be application/x-www-form-urlencoded"). Unlike the custom device-flow
+	// endpoints (/device, /device-token) which accept JSON via post(), the refresh
+	// grant must be form-encoded.
+	form := url.Values{
+		"grant_type":    {grantTypeRefreshToken},
+		"refresh_token": {refreshToken},
+		"client_id":     {ClientID},
+	}
+	status, raw, err := c.postForm(ctx, pathToken, form)
 	if err != nil {
 		return nil, err
 	}
@@ -308,6 +314,25 @@ func (c *OAuthClient) post(ctx context.Context, path string, body []byte) (int,
 	return resp.StatusCode, raw, nil
 }
 
+// postForm sends an application/x-www-form-urlencoded POST. Used for the OAuth
+// /token endpoint (the refresh grant), whose @node-oauth/oauth2-server handler
+// requires form encoding and rejects JSON.
+func (c *OAuthClient) postForm(ctx context.Context, path string, form url.Values) (int, []byte, error) {
+	req, err := http.NewRequestWithContext(ctx, http.MethodPost, c.BaseURL+path, strings.NewReader(form.Encode()))
+	if err != nil {
+		return 0, nil, err
+	}
+	req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
+	req.Header.Set("Accept", "application/json")
+	resp, err := c.HTTP.Do(req)
+	if err != nil {
+		return 0, nil, err
+	}
+	defer resp.Body.Close()
+	raw, _ := io.ReadAll(io.LimitReader(resp.Body, 1<<20))
+	return resp.StatusCode, raw, nil
+}
+
 // oauthMsg extracts a human message from an OAuth error body.
 func oauthMsg(raw []byte) string {
 	var oe oauthErr

internal/api/oauth_test.go

@@ -152,10 +152,19 @@ func TestRefreshRotatesToken(t *testing.T) {
 		if r.URL.Path != pathToken {
 			t.Errorf("refresh hit wrong path %q", r.URL.Path)
 		}
-		var body map[string]string
-		_ = json.NewDecoder(r.Body).Decode(&body)
-		if body["grant_type"] != grantTypeRefreshToken || body["refresh_token"] != "old-rt" {
-			t.Errorf("refresh body = %v", body)
+		// The /token route (the @node-oauth/oauth2-server token handler) REQUIRES
+		// application/x-www-form-urlencoded and rejects JSON with "content must be
+		// application/x-www-form-urlencoded". Assert the CLI sends form encoding —
+		// this is the regression guard for the bug where Refresh() posted JSON and
+		// the real server rejected every refresh after the 1h access-token TTL.
+		if ct := r.Header.Get("Content-Type"); ct != "application/x-www-form-urlencoded" {
+			t.Errorf("refresh Content-Type = %q, want application/x-www-form-urlencoded", ct)
+		}
+		if err := r.ParseForm(); err != nil {
+			t.Fatalf("ParseForm: %v", err)
+		}
+		if r.PostFormValue("grant_type") != grantTypeRefreshToken || r.PostFormValue("refresh_token") != "old-rt" {
+			t.Errorf("refresh form = %v", r.PostForm)
 		}
 		_ = json.NewEncoder(w).Encode(TokenResponse{
 			AccessToken: "new-at", ExpiresIn: 3600, RefreshToken: "new-rt", Scope: DeviceScope,