diff --git a/internal/plugin/releases.go b/internal/plugin/releases.go
index cf7d1f8..88f325a 100644
--- a/internal/plugin/releases.go
+++ b/internal/plugin/releases.go
@@ -241,6 +241,11 @@ func (c *client) uploadReleaseAsset(ctx context.Context, cfg Config, release *Re
 	}
 	defer func() { _ = file.Close() }()
 
+	stat, err := file.Stat()
+	if err != nil {
+		return fmt.Errorf("stat asset: %w", err)
+	}
+
 	contentType := mime.TypeByExtension(filepath.Ext(assetPath))
 	if contentType == "" {
 		contentType = "application/octet-stream"
@@ -250,6 +255,10 @@ func (c *client) uploadReleaseAsset(ctx context.Context, cfg Config, release *Re
 	if err != nil {
 		return fmt.Errorf("build upload request: %w", err)
 	}
+	// GitHub's asset-upload endpoint requires an explicit Content-Length and
+	// rejects chunked transfer encoding. http.NewRequest does not infer the
+	// length for an *os.File body, so it must be set manually.
+	req.ContentLength = stat.Size()
 	setGitHubHeaders(req, cfg.Token, contentType, baseURL)
 
 	resp, err := c.httpClient.Do(req)
diff --git a/internal/plugin/releases_test.go b/internal/plugin/releases_test.go
index 4f06a01..06bf941 100644
--- a/internal/plugin/releases_test.go
+++ b/internal/plugin/releases_test.go
@@ -327,10 +327,19 @@ func TestUploadReleaseAssetsExpandsGlobs(t *testing.T) {
 		if got := r.URL.Query().Get("name"); got == "" {
 			t.Fatal("missing asset name query")
 		}
+		// GitHub's real asset-upload endpoint requires an explicit
+		// Content-Length and rejects chunked transfer encoding, so guard
+		// against regressing to an *os.File body without a set length.
+		if r.ContentLength < 0 {
+			t.Fatalf("ContentLength = %d, want a non-negative explicit length (chunked transfer encoding not allowed)", r.ContentLength)
+		}
 		body, err := io.ReadAll(r.Body)
 		if err != nil {
 			t.Fatalf("ReadAll() error = %v", err)
 		}
+		if int64(len(body)) != r.ContentLength {
+			t.Fatalf("body length = %d, Content-Length header = %d", len(body), r.ContentLength)
+		}
 		uploaded <- r.URL.Query().Get("name") + ":" + string(body)
 		w.WriteHeader(http.StatusCreated)
 		_, _ = w.Write([]byte(`{"state":"uploaded"}`))