diff --git a/corepkgs/fetchurl/fetchurl.fix b/corepkgs/fetchurl/fetchurl.fix
index f798c0becb..0221b612cf 100644
--- a/corepkgs/fetchurl/fetchurl.fix
+++ b/corepkgs/fetchurl/fetchurl.fix
@@ -4,6 +4,7 @@ Function(["url", "md5"],
     , ("url", Var("url"))
     , ("md5", Var("md5"))
     , ("name", BaseName(Var("url")))
+    , ("id", Var("md5"))
     ]
   )
 )
diff --git a/corepkgs/fetchurl/fetchurl.sh b/corepkgs/fetchurl/fetchurl.sh
index 1479e898b9..7b6243974d 100644
--- a/corepkgs/fetchurl/fetchurl.sh
+++ b/corepkgs/fetchurl/fetchurl.sh
@@ -4,9 +4,7 @@ echo "downloading $url into $out..."
 wget "$url" -O "$out" || exit 1
 
 actual=$(md5sum -b $out | cut -c1-32)
-if ! test "$md5" == "ignore"; then
-    if ! test "$actual" == "$md5"; then
-	echo "hash is $actual, expected $md5"
-	exit 1
-    fi
+if ! test "$actual" == "$md5"; then
+    echo "hash is $actual, expected $md5"
+    exit 1
 fi
diff --git a/src/fix.cc b/src/fix.cc
index 6e3809f828..93cc27cfca 100644
--- a/src/fix.cc
+++ b/src/fix.cc
@@ -153,8 +153,10 @@ static Expr evalExpr2(EvalState & state, Expr e)
         fs.slice.roots.push_back(id);
         fs.slice.elems.push_back(elem);
 
-        return ATmake("FSId(<str>)", 
-            ((string) writeTerm(unparseFState(fs), "")).c_str());
+        FSId termId = hashString("producer-" + (string) id
+            + "-" + dstPath);
+        writeTerm(unparseFState(fs), "", termId);
+        return ATmake("FSId(<str>)", ((string) termId).c_str());
     }
 
     /* Packages are transformed into Derive fstate expressions. */
@@ -176,6 +178,7 @@ static Expr evalExpr2(EvalState & state, Expr e)
         fs.type = FState::fsDerive;
         fs.derive.platform = SYSTEM;
         string name;
+        FSId outId;
         bnds = ATempty;
 
         for (map<string, ATerm>::iterator it = bndMap.begin();
@@ -195,6 +198,7 @@ static Expr evalExpr2(EvalState & state, Expr e)
             }
             else if (ATmatch(value, "<str>", &s1)) {
                 if (key == "name") name = s1;
+                if (key == "id") outId = parseHash(s1);
                 fs.derive.env.push_back(StringPair(key, s1));
             }
             else throw badTerm("invalid package argument", value);
@@ -211,7 +215,8 @@ static Expr evalExpr2(EvalState & state, Expr e)
         
         /* Hash the fstate-expression with no outputs to produce a
            unique but deterministic path name for this package. */
-        Hash outId = hashTerm(unparseFState(fs));
+        if (outId == FSId())
+            outId = hashTerm(unparseFState(fs));
         string outPath = 
             canonPath(nixStore + "/" + ((string) outId).c_str() + "-" + name);
         fs.derive.env.push_back(StringPair("out", outPath));
@@ -219,8 +224,10 @@ static Expr evalExpr2(EvalState & state, Expr e)
         debug(format("%1%: %2%") % (string) outId % name);
 
         /* Write the resulting term into the Nix store directory. */
-        return ATmake("FSId(<str>)", 
-            ((string) writeTerm(unparseFState(fs), "-d-" + name)).c_str());
+        FSId termId = hashString("producer-" + (string) outId
+            + "-" + outPath);
+        writeTerm(unparseFState(fs), "-d-" + name, termId);
+        return ATmake("FSId(<str>)", ((string) termId).c_str());
     }
 
     /* BaseName primitive function. */
diff --git a/src/fstate.cc b/src/fstate.cc
index 85f8c75cc5..1c8c6776f1 100644
--- a/src/fstate.cc
+++ b/src/fstate.cc
@@ -31,9 +31,10 @@ ATerm termFromId(const FSId & id)
 }
 
 
-FSId writeTerm(ATerm t, const string & suffix)
+FSId writeTerm(ATerm t, const string & suffix, FSId id)
 {
-    FSId id = hashTerm(t);
+    /* By default, the id of a term is its hash. */
+    if (id == FSId()) id = hashTerm(t);
 
     string path = canonPath(nixStore + "/" + 
         (string) id + suffix + ".nix");
diff --git a/src/fstate.hh b/src/fstate.hh
index 681a8d0941..969abe9e06 100644
--- a/src/fstate.hh
+++ b/src/fstate.hh
@@ -63,7 +63,7 @@ Hash hashTerm(ATerm t);
 ATerm termFromId(const FSId & id);
 
 /* Write an aterm to the Nix store directory, and return its hash. */
-FSId writeTerm(ATerm t, const string & suffix);
+FSId writeTerm(ATerm t, const string & suffix, FSId id = FSId());
 
 /* Parse an fstate-expression. */
 FState parseFState(ATerm t);
diff --git a/src/normalise.cc b/src/normalise.cc
index fcb9c4d0d3..d06c8bf360 100644
--- a/src/normalise.cc
+++ b/src/normalise.cc
@@ -88,19 +88,18 @@ Slice normaliseFState(FSId id)
 
     /* We can skip running the builder if we can expand all output
        paths from their ids. */
-    bool fastBuild = false;
-#if 0
+    bool fastBuild = true;
     for (OutPaths::iterator i = outPaths.begin(); 
          i != outPaths.end(); i++)
     {
         try {
             expandId(i->second, i->first);
-        } catch (...) {
+        } catch (Error & e) {
+            debug(format("fast build failed: %1%") % e.what());
             fastBuild = false;
             break;
         }
     }
-#endif
 
     if (!fastBuild) {