diff --git a/ext/gpgme/gpgme_n.c b/ext/gpgme/gpgme_n.c
index 8ebb198..c761936 100644
--- a/ext/gpgme/gpgme_n.c
+++ b/ext/gpgme/gpgme_n.c
@@ -104,7 +104,7 @@ static const rb_data_type_t gpgme_data_type = {
     .dsize = NULL,
   },
   .data = NULL,
-  .flags = RUBY_TYPED_FREE_IMMEDIATELY,
+  .flags = 0,
 };
 
 static void
@@ -122,7 +122,7 @@ static const rb_data_type_t gpgme_ctx_type = {
     .dsize = NULL,
   },
   .data = NULL,
-  .flags = RUBY_TYPED_FREE_IMMEDIATELY,
+  .flags = 0,
 };
 
 static void
@@ -140,7 +140,7 @@ static const rb_data_type_t gpgme_key_type = {
     .dsize = NULL,
   },
   .data = NULL,
-  .flags = RUBY_TYPED_FREE_IMMEDIATELY,
+  .flags = 0,
 };
 
 #if defined(GPGME_VERSION_NUMBER) && GPGME_VERSION_NUMBER < 0x020000
@@ -159,7 +159,7 @@ static const rb_data_type_t gpgme_trust_item_type = {
     .dsize = NULL,
   },
   .data = NULL,
-  .flags = RUBY_TYPED_FREE_IMMEDIATELY,
+  .flags = 0,
 };
 #endif
 
@@ -1210,6 +1210,9 @@ rb_s_gpgme_op_keylist_next (VALUE dummy, VALUE vctx, VALUE rkey)
       save_gpgme_key_attrs (vkey, key);
       rb_ary_store (rkey, 0, vkey);
     }
+
+  RB_GC_GUARD(vctx);
+
   return LONG2NUM(err);
 }
 
@@ -2137,6 +2140,11 @@ rb_s_gpgme_op_decrypt (VALUE dummy, VALUE vctx, VALUE vcipher, VALUE vplain)
   UNWRAP_GPGME_DATA(vplain, plain);
 
   err = gpgme_op_decrypt (ctx, cipher, plain);
+
+  RB_GC_GUARD(vctx);
+  RB_GC_GUARD(vcipher);
+  RB_GC_GUARD(vplain);
+
   return LONG2NUM(err);
 }
 
@@ -2216,6 +2224,12 @@ rb_s_gpgme_op_verify (VALUE dummy, VALUE vctx, VALUE vsig, VALUE vsigned_text,
     UNWRAP_GPGME_DATA(vplain, plain);
 
   err = gpgme_op_verify (ctx, sig, signed_text, plain);
+
+  RB_GC_GUARD(vctx);
+  RB_GC_GUARD(vsig);
+  RB_GC_GUARD(vsigned_text);
+  RB_GC_GUARD(vplain);
+
   return LONG2NUM(err);
 }
 
@@ -2320,6 +2334,11 @@ rb_s_gpgme_op_decrypt_verify (VALUE dummy, VALUE vctx, VALUE vcipher,
   UNWRAP_GPGME_DATA(vplain, plain);
 
   err = gpgme_op_decrypt_verify (ctx, cipher, plain);
+
+  RB_GC_GUARD(vctx);
+  RB_GC_GUARD(vcipher);
+  RB_GC_GUARD(vplain);
+
   return LONG2NUM(err);
 }
 
@@ -2404,6 +2423,11 @@ rb_s_gpgme_op_sign (VALUE dummy, VALUE vctx, VALUE vplain, VALUE vsig,
   UNWRAP_GPGME_DATA(vsig, sig);
 
   err = gpgme_op_sign (ctx, plain, sig, NUM2INT(vmode));
+
+  RB_GC_GUARD(vctx);
+  RB_GC_GUARD(vplain);
+  RB_GC_GUARD(vsig);
+
   return LONG2NUM(err);
 }
 
@@ -2513,6 +2537,12 @@ rb_s_gpgme_op_encrypt (VALUE dummy, VALUE vctx, VALUE vrecp, VALUE vflags,
   err = gpgme_op_encrypt (ctx, recp, NUM2INT(vflags), plain, cipher);
   if (recp)
     xfree (recp);
+
+  RB_GC_GUARD(vctx);
+  RB_GC_GUARD(vrecp);
+  RB_GC_GUARD(vplain);
+  RB_GC_GUARD(vcipher);
+
   return LONG2NUM(err);
 }
 
@@ -2612,6 +2642,12 @@ rb_s_gpgme_op_encrypt_sign (VALUE dummy, VALUE vctx, VALUE vrecp, VALUE vflags,
   err = gpgme_op_encrypt_sign (ctx, recp, NUM2INT(vflags), plain, cipher);
   if (recp)
     xfree (recp);
+
+  RB_GC_GUARD(vctx);
+  RB_GC_GUARD(vrecp);
+  RB_GC_GUARD(vplain);
+  RB_GC_GUARD(vcipher);
+
   return LONG2NUM(err);
 }
 
diff --git a/lib/gpgme.rb b/lib/gpgme.rb
index ad52cc3..73f81f9 100644
--- a/lib/gpgme.rb
+++ b/lib/gpgme.rb
@@ -1,6 +1,8 @@
 require 'gpgme_n'
+require 'monitor'
 
-# TODO without this call one can't GPGME::Ctx.new, find out why
+# This call initializes the GPGME library and must happen before
+# any GPGME operations (e.g. Ctx.new) can succeed.
 GPGME::gpgme_check_version(nil)
 
 require 'gpgme/constants'
@@ -19,8 +21,61 @@
 require 'gpgme/crypto'
 
 module GPGME
+
+  # Mutex for serializing GPGME operations when thread safety is enabled.
+  # While the underlying GPGME C library supports separate contexts in
+  # separate threads, the communication with gpg-agent over Unix domain
+  # sockets can produce "Bad file descriptor" errors under heavy concurrent
+  # load. Thread-safe mode is enabled by default and can be disabled
+  # if not needed (e.g. single-threaded applications).
+  #
+  # A Monitor is used instead of a Mutex because GPGME operations are
+  # reentrant — e.g. Crypto#sign calls Ctx.new, and within that block,
+  # Key.find calls Ctx.new again.
+  #
+  # @example Disable thread-safe mode for single-threaded apps
+  #   GPGME.thread_safe = false
+  #
+  @thread_safe_mutex = Monitor.new
+  @thread_safe = true
+
   class << self
 
+    # Enable or disable thread-safe mode. Enabled by default. When
+    # enabled, all high-level GPGME operations (encrypt, decrypt, sign,
+    # verify, key listing, etc.) are serialized through a global monitor
+    # to prevent concurrent access to gpg-agent from causing "Bad file
+    # descriptor" errors. Disable for single-threaded apps if the
+    # synchronization overhead is undesirable.
+    #
+    # @param [Boolean] value false to disable thread-safe mode
+    attr_writer :thread_safe
+
+    # Returns true if thread-safe mode is enabled.
+    def thread_safe?
+      @thread_safe
+    end
+
+    # The mutex used for thread-safe serialization. Can be used directly
+    # if you need finer-grained control over locking.
+    #
+    # @example manual locking
+    #   GPGME.synchronize do
+    #     # multiple GPGME operations atomically
+    #   end
+    attr_reader :thread_safe_mutex
+
+    # Execute a block with the GPGME mutex held if thread-safe mode is
+    # enabled. If thread-safe mode is disabled, the block is executed
+    # directly without locking.
+    def synchronize(&block)
+      if @thread_safe
+        @thread_safe_mutex.synchronize(&block)
+      else
+        yield
+      end
+    end
+
     # From the c extension
     alias pubkey_algo_name gpgme_pubkey_algo_name
     alias hash_algo_name gpgme_hash_algo_name
diff --git a/lib/gpgme/ctx.rb b/lib/gpgme/ctx.rb
index 54e9788..99a1169 100644
--- a/lib/gpgme/ctx.rb
+++ b/lib/gpgme/ctx.rb
@@ -76,10 +76,12 @@ def self.new(options = {})
       end
 
       if block_given?
-        begin
-          yield ctx
-        ensure
-          GPGME::gpgme_release(ctx)
+        GPGME.synchronize do
+          begin
+            yield ctx
+          ensure
+            GPGME::gpgme_release(ctx)
+          end
         end
       else
         ctx
@@ -618,10 +620,21 @@ def inspect
     private
 
     def self.pass_function(pass, uid_hint, passphrase_info, prev_was_bad, fd)
+      # Write the passphrase directly using IO.for_fd. We set autoclose=false
+      # to prevent Ruby from closing the fd (which belongs to GPGME/gpg-agent).
+      # The IO object is used only within this method scope and not stored,
+      # so we also ensure it isn't prematurely collected by keeping a strong
+      # reference until we're done.
       io = IO.for_fd(fd, 'w')
       io.autoclose = false
-      io.puts pass
-      io.flush
+      begin
+        io.write "#{pass}\n"
+        io.flush
+      rescue => e
+        # If the fd has become invalid (e.g. agent communication error),
+        # re-raise as a more descriptive error.
+        raise e
+      end
     end
 
   end