Allow for prepending (instead of appending) context to the system mes…

…sage

... via new 'system-prepend'.

Certain use cases/LLMs may prefer to first see the context, then the system
message.

gptel-context.el

@@ -233,14 +233,14 @@ ADVANCE controls the overlay boundary behavior."
 
 MESSAGE is usually either the system message or the user prompt.
 The accumulated context from CONTEXTS is appended or prepended to
-it, respectively."
-  ;; Append context before/after system message.
+it, as requested."
   (let ((context-string (gptel-context--string contexts)))
     (if (> (length context-string) 0)
         (pcase-exhaustive gptel-use-context
-          ('system (concat message "\n\n" context-string))
-          ('user   (concat context-string "\n\n" message))
-          ('nil    message))
+          ('system         (concat message "\n\n" context-string))
+          ('system-prepend (concat context-string "\n\n" message))
+          ('user           (concat context-string "\n\n" message))
+          ('nil            message))
       message)))
 
 (defun gptel-context--collect-media (&optional contexts)

gptel-rewrite.el

@@ -326,8 +326,11 @@ the changed regions. BUF is the (current) buffer."
   (let* ((prompt (buffer-substring-no-properties
                   (region-beginning) (region-end)))
          (gptel--system-message (or rewrite-message gptel--rewrite-message))
-         ;; always send context with system message
-         (gptel-use-context (and gptel-use-context 'system)))
+         (gptel-use-context (or
+			     ;; Never send context with user prompt; instead,
+			     ;; send with with system message (append).
+			     (and (eq gptel-use-context 'user) 'system)
+			     gptel-use-context)))
     (deactivate-mark)
     (gptel-request prompt
       :context

gptel-transient.el

@@ -478,14 +478,16 @@ value of `gptel-use-context', set from here."
   :format " %k %d %v"
   :set-value #'gptel--set-with-scope
   :display-nil "No"
-  :display-map '((nil    . "No")
-                 (system . "with system message")
-                 (user   . "with user prompt"))
+  :display-map '((nil            . "No")
+                 (system         . "with system message (append)")
+                 (system-prepend . "with system message (prepend)")
+                 (user           . "with user prompt"))
   :key "-i"
   :reader (lambda (prompt &rest _)
-            (let* ((choices '(("No"                  . nil)
-                              ("with system message" . system)
-                              ("with user prompt"    . user)))
+            (let* ((choices '(("No"                            . nil)
+                              ("with system message (append)"  . system)
+                              ("with system message (prepend)" . system-prepend)
+                              ("with user prompt"              . user)))
                    (destination (completing-read prompt choices nil t)))
               (cdr (assoc destination choices)))))
 

gptel.el

@@ -598,13 +598,15 @@ included in the request.
 
 Currently supported options are:
 
-    nil     - Do not use the context.
-    system  - Include the context with the system message.
-    user    - Include the context with the user prompt."
+    nil            - Do not use the context.
+    system         - Include the context with the system message (append).
+    system-prepend - Include the context with the system message (prepend).
+    user           - Include the context with the user prompt."
   :group 'gptel
   :type '(choice
           (const :tag "Don't include context" nil)
-          (const :tag "With system message" system)
+          (const :tag "With system message (append)" system)
+          (const :tag "With system message (prepend)" system-prepend)
           (const :tag "With user prompt" user)))
 
 (defvar-local gptel--old-header-line nil)
@@ -1148,7 +1150,8 @@ Model parameters can be let-bound around calls to this function."
   (let* ((gptel--system-message
           ;Add context chunks to system message if required
           (if (and gptel-context--alist
-                   (eq gptel-use-context 'system))
+                   (or (eq gptel-use-context 'system)
+                       (eq gptel-use-context 'system-prepend)))
               (gptel-context--wrap system)
             system))
          (gptel-stream stream)