diff --git a/NEWS b/NEWS
index ead5684b7..f5af7bad2 100644
--- a/NEWS
+++ b/NEWS
@@ -27,6 +27,8 @@ New in spot 1.0a (not released):
       transition counts, just as the ltlcross tool.
     - ltlcross will display the number of timeouts at the end
       of its execution.
+    - ltlcross will diagnose tools with missing input or
+      output %-sequence before attempting to run any of them.
     - The parser for LBT's prefix-style LTL formulas will now
       read atomic propositions that are not of the form p1, p2...
       This makes it possible to process formulas written in
diff --git a/src/bin/ltlcross.cc b/src/bin/ltlcross.cc
index 58d5aba1d..c667729c4 100644
--- a/src/bin/ltlcross.cc
+++ b/src/bin/ltlcross.cc
@@ -494,10 +494,28 @@ namespace
       declare('N', &output);
       declare('T', &output);
 
+      std::vector<bool> has(256);
       size_t s = translators.size();
       assert(s);
       for (size_t n = 0; n < s; ++n)
-	prime(translators[n]);
+	{
+	  // Check that each translator uses at least one input and
+	  // one output.
+	  has.clear();
+	  scan(translators[n], has);
+	  if (!(has['f'] || has['s'] || has['l'] || has['w']
+		|| has['F'] || has['S'] || has['L'] || has['W']))
+	    error(2, 0, "no input %%-sequence in '%s'.\n       Use "
+		  "one of %%f,%%s,%%l,%%w,%%F,%%S,%%L,%%W to indicate how "
+		  "to pass the formula.", translators[n]);
+	  if (!(has['N'] || has['T']))
+	    error(2, 0, "no output %%-sequence in '%s'.\n      Use "
+		  "one of %%N,%%T to indicate where the automaton is saved.",
+		  translators[n]);
+
+	  // Remember the %-sequences used by all translators.
+	  prime(translators[n]);
+	}
 
     }
 
@@ -526,7 +544,7 @@ namespace
 	return string_ltl_wring;
       if (!string_ltl_lbt.val().empty())
 	return string_ltl_lbt;
-      error(2, 0, "None of the translators need the input formula?");
+      assert(!"None of the translators need the input formula?");
       return string_ltl_spot;
     }
 
@@ -560,9 +578,7 @@ namespace
       format(command, translators[translator_num]);
       toclean.push_back(output.val());
 
-      if (output.format == printable_result_filename::None)
-	error(2, 0, "no output sequence used in %s",
-	      translators[translator_num]);
+      assert(output.format != printable_result_filename::None);
 
       std::string cmd = command.str();
       std::cerr << "Running [" << l << translator_num << "]: "
diff --git a/src/misc/formater.cc b/src/misc/formater.cc
index 68acf2fb3..9851098c5 100644
--- a/src/misc/formater.cc
+++ b/src/misc/formater.cc
@@ -1,6 +1,6 @@
 // -*- coding: utf-8 -*-
-// Copyright (C) 2012 Laboratoire de Recherche et Développement de
-// l'Epita (LRDE).
+// Copyright (C) 2012, 2013 Laboratoire de Recherche et Développement
+// de l'Epita (LRDE).
 //
 // This file is part of Spot, a model checking library.
 //
@@ -23,18 +23,24 @@
 namespace spot
 {
   void
-  formater::prime(const char* fmt)
+  formater::scan(const char* fmt, std::vector<bool>& has) const
   {
     for (const char* pos = fmt; *pos; ++pos)
       if (*pos == '%')
 	{
 	  char c = *++pos;
-	  has_[c] = true;
+	  has[c] = true;
 	  if (!c)
 	    break;
 	}
   }
 
+  void
+  formater::prime(const char* fmt)
+  {
+    scan(fmt, has_);
+  }
+
   std::ostream&
   formater::format(const char* fmt)
     {
diff --git a/src/misc/formater.hh b/src/misc/formater.hh
index 0f3d83fd7..ff56cb08f 100644
--- a/src/misc/formater.hh
+++ b/src/misc/formater.hh
@@ -1,6 +1,6 @@
 // -*- coding: utf-8 -*-
-// Copyright (C) 2012 Laboratoire de Recherche et Développement de
-// l'Epita (LRDE).
+// Copyright (C) 2012, 2013 Laboratoire de Recherche et Développement
+// de l'Epita (LRDE).
 //
 // This file is part of Spot, a model checking library.
 //
@@ -117,16 +117,32 @@ namespace spot
     {
     }
 
+    /// \brief Scan the %-sequences occuring in \a fmt.
+    ///
+    /// Set has['c'] for each %c in \a fmt.   \a has must
+    /// be 256 wide.
+    /// @{
+    void
+    scan(const char* fmt, std::vector<bool>& has) const;
+
+    void
+    scan(const std::string& fmt, std::vector<bool>& has) const
+    {
+      scan(fmt.c_str(), has);
+    }
+    /// @}
+
     /// Collect the %-sequences occurring in \a fmt.
+    /// @{
     void
     prime(const char* fmt);
 
-    /// Collect the %-sequences occurring in \a fmt.
     void
     prime(const std::string& fmt)
     {
       prime(fmt.c_str());
     }
+    /// @}
 
     /// Whether %c occurred in the primed formats.
     bool