Optimize inferMediaType by avoiding strings.ToLower allocation

- Moves strings.ToLower to after the empty extension check
- Adds fast paths for well-formed content types to bypass ToLower entirely

Co-authored-by: hobbyistlabs-coder <267281733+hobbyistlabs-coder@users.noreply.github.com>

.jules/bolt.md

@@ -8,3 +8,6 @@
 ## 2024-05-26 - Avoid Unnecessary `strings.ToLower`
 **Learning:** Calling `strings.ToLower` on the entire message content allocates a new string and iterates over all runes. This causes measurable GC pressure and latency on hot paths like feature extraction during routing.
 **Action:** Use fast paths to bypass `strings.ToLower`. For instance, check if a requisite character (like a dot `.`) exists, or check common casings directly (`DATA:IMAGE` vs `data:image`) before falling back to full case-normalization.
+## 2025-03-20 - String Operations Fast Paths and Avoiding Double Searches
+**Learning:** When trying to optimize `strings.ToLower`, ensure you don't introduce regressions with byte-to-rune casting on UTF-8 strings. Also, `strings.Contains(s, sub)` literally calls `strings.Index(s, sub)` under the hood. Using `strings.Contains` followed immediately by `strings.Index` to extract the position is an anti-pattern that searches the string twice, undermining the intended performance optimization.
+**Action:** Always prefer a single `strings.Index` call over `Contains`+`Index`. Stick to one single optimization per PR to reduce risk and review burden.

pkg/agent/loop_utils.go

@@ -102,9 +102,20 @@ func formatToolsForLog(toolDefs []providers.ToolDefinition) string {
 // inferMediaType determines the media type ("image", "audio", "video", "file")
 // from a filename and MIME content type.
 func inferMediaType(filename, contentType string) string {
-	ct := strings.ToLower(contentType)
-	fn := strings.ToLower(filename)
+	// Fast path for common, correctly cased content types
+	if strings.HasPrefix(contentType, "image/") {
+		return "image"
+	}
+	if strings.HasPrefix(contentType, "audio/") || contentType == "application/ogg" {
+		return "audio"
+	}
+	if strings.HasPrefix(contentType, "video/") {
+		return "video"
+	}
 
+	// Slower path for uppercase or mixed case
+	if contentType != "" {
+		ct := strings.ToLower(contentType)
 		if strings.HasPrefix(ct, "image/") {
 			return "image"
 		}
@@ -114,9 +125,13 @@ func inferMediaType(filename, contentType string) string {
 		if strings.HasPrefix(ct, "video/") {
 			return "video"
 		}
+	}
 
 	// Fallback: infer from extension
-	ext := filepath.Ext(fn)
+	ext := filepath.Ext(filename)
+	if ext != "" {
+		// Only lower case the extension, which is usually just a few characters
+		ext = strings.ToLower(ext)
 		switch ext {
 		case ".jpg", ".jpeg", ".png", ".gif", ".webp", ".bmp", ".svg":
 			return "image"
@@ -125,6 +140,7 @@ func inferMediaType(filename, contentType string) string {
 		case ".mp4", ".avi", ".mov", ".webm", ".mkv":
 			return "video"
 		}
+	}
 
 	return "file"
 }