diff --git a/api/agent/agent_test.go b/api/agent/agent_test.go
index c6117eddb..d81e013ac 100644
--- a/api/agent/agent_test.go
+++ b/api/agent/agent_test.go
@@ -64,6 +64,7 @@ func TestCallConfigurationRequest(t *testing.T) {
 	req.Header.Add("MYREALHEADER", "FOOLORD")
 	req.Header.Add("MYREALHEADER", "FOOPEASANT")
 	req.Header.Add("Content-Length", contentLength)
+	req.Header.Add("FN_ROUTE", "thewrongroute") // ensures that this doesn't leak out, should be overwritten
 
 	call, err := a.GetCall(
 		WithWriter(w), // XXX (reed): order matters [for now]
diff --git a/api/agent/call.go b/api/agent/call.go
index d864d12ae..1d4179b31 100644
--- a/api/agent/call.go
+++ b/api/agent/call.go
@@ -113,13 +113,19 @@ func FromRequest(appName, path string, req *http.Request) CallOpt {
 		headerVars := make(map[string]string, len(req.Header))
 
 		for k, v := range req.Header {
-			headerVars[toEnvName("FN_HEADER", k)] = strings.Join(v, ", ")
+			if !noOverrideVars(k) { // NOTE if we don't do this, they'll leak in (don't want people relying on this behavior)
+				headerVars[toEnvName("FN_HEADER", k)] = strings.Join(v, ", ")
+			}
 		}
 
 		// add all the env vars we build to the request headers
-		// TODO should we save req.Headers and copy OVER app.Config / route.Config ?
 		for k, v := range envVars {
-			req.Header.Add(k, v)
+			if noOverrideVars(k) {
+				// overwrite the passed in request headers explicitly with the generated ones
+				req.Header.Set(k, v)
+			} else {
+				req.Header.Add(k, v)
+			}
 		}
 
 		for k, v := range headerVars {
@@ -171,6 +177,24 @@ func FromRequest(appName, path string, req *http.Request) CallOpt {
 	}
 }
 
+func noOverrideVars(key string) bool {
+	// descrepency in casing b/w req headers and env vars, force matches
+	return overrideVars[strings.ToUpper(key)]
+}
+
+// overrideVars means that the app config, route config or header vars
+// must not overwrite the generated values in call construction.
+var overrideVars = map[string]bool{
+	"FN_FORMAT":      true,
+	"FN_APP_NAME":    true,
+	"FN_ROUTE":       true,
+	"FN_MEMORY":      true,
+	"FN_TYPE":        true,
+	"FN_CALL_ID":     true,
+	"FN_METHOD":      true,
+	"FN_REQUEST_URL": true,
+}
+
 // TODO this currently relies on FromRequest having happened before to create the model
 // here, to be a fully qualified model. We probably should double check but having a way
 // to bypass will likely be what's used anyway unless forced.
diff --git a/api/agent/protocol/http.go b/api/agent/protocol/http.go
index 03d62991c..a273c85c6 100644
--- a/api/agent/protocol/http.go
+++ b/api/agent/protocol/http.go
@@ -42,7 +42,7 @@ func (h *HTTPProtocol) Dispatch(w io.Writer, req *http.Request) error {
 		}
 
 		for k, v := range res.Header {
-			rw.Header()[k] = v
+			rw.Header().Add(k, v) // on top of any specified on the route
 		}
 		rw.WriteHeader(res.StatusCode)
 		// TODO should we TCP_CORK ?