ResolveSystemPrompt: expand env, fall back to embedded by filename

prompts/prompts.go

@@ -5,6 +5,7 @@ import (
 	_ "embed"
 	"fmt"
 	"os"
+	"path/filepath"
 	"strings"
 	"time"
 )
@@ -21,6 +22,13 @@ var SessionDBus string
 //go:embed SYSTEM_PROMPT_QWEN.md
 var QwenSystemPrompt string
 
+// embeddedPromptByFile maps source filenames (without .md) to embedded prompt
+// content, so ResolveSystemPrompt can fall back to embedded prompts when the
+// filesystem path doesn't resolve.
+var embeddedPromptByFile = map[string]string{
+	"SYSTEM_PROMPT_QWEN": QwenSystemPrompt,
+}
+
 // Environment renders the environment context block with live values.
 func Environment(cwd, platform string, isGitRepo bool, sandbox string) string {
 	date := time.Now().Format("2006-01-02")
@@ -60,21 +68,27 @@ func OperationalModel(enable9P, enableDBus bool, envVars map[string]string) stri
 	return strings.Join(parts, "\n")
 }
 
-// ResolveSystemPrompt reads from overridePath if non-empty, otherwise returns
-// the embedded default. If overridePath is a known key ("qwen", "devstral"),
-// the corresponding embedded prompt is returned instead.
+// ResolveSystemPrompt expands env vars in overridePath, then tries to read the
+// file from the filesystem. If the file doesn't exist, it checks whether the
+// filename matches an embedded prompt (e.g. "SYSTEM_PROMPT_QWEN.md" -> Qwen
+// system prompt). If no match is found, the embedded default is returned.
 func ResolveSystemPrompt(overridePath string) string {
-	switch overridePath {
-	case "qwen":
-		return QwenSystemPrompt
+	if overridePath == "" {
+		return DefaultSystemPrompt
 	}
-	if overridePath != "" {
-		data, err := os.ReadFile(overridePath)
-		if err == nil {
-			return strings.TrimRight(string(data), "\n")
+	expanded := os.Expand(overridePath, os.Getenv)
+	data, err := os.ReadFile(expanded)
+	if err == nil {
+		return strings.TrimRight(string(data), "\n")
+	}
+	// File not found — check embedded prompts by basename.
+	base := filepath.Base(expanded)
+	if key := strings.TrimSuffix(base, filepath.Ext(base)); key != "" {
+		if content, ok := embeddedPromptByFile[key]; ok {
+			return content
 		}
-		// Fall through to embedded default if file can't be read.
-		fmt.Fprintf(os.Stderr, "system_prompt override %q: %v (using default)\n", overridePath, err)
 	}
+	// Fall through to embedded default.
+	fmt.Fprintf(os.Stderr, "system_prompt %q: %v (using default)\n", expanded, err)
 	return DefaultSystemPrompt
 }