fix(providers): improve context overflow detection and classification

pkg/agent/loop.go

@@ -1949,6 +1949,7 @@ turnLoop:
 
 			isContextError := !isTimeoutError && (strings.Contains(errMsg, "context_length_exceeded") ||
 				strings.Contains(errMsg, "context window") ||
+				strings.Contains(errMsg, "context_window") ||
 				strings.Contains(errMsg, "maximum context length") ||
 				strings.Contains(errMsg, "token limit") ||
 				strings.Contains(errMsg, "too many tokens") ||

pkg/providers/error_classifier.go

@@ -84,6 +84,15 @@ var (
 		substr("messages.1.content.1.tool_use.id"),
 		substr("invalid request format"),
 	}
+	contextOverflowPatterns = []errorPattern{
+		rxp(`context[_ ]?length[_ ]?exceeded`),
+		rxp(`context[_ ]?window[_ ]?exceeded`),
+		substr("maximum context length"),
+		substr("token limit"),
+		substr("too many tokens"),
+		substr("prompt is too long"),
+		substr("request too large"),
+	}
 
 	imageDimensionPatterns = []errorPattern{
 		rxp(`image dimensions exceed max`),
@@ -201,6 +210,9 @@ func classifyByMessage(msg string) FailoverReason {
 	if matchesAny(msg, formatPatterns) {
 		return FailoverFormat
 	}
+	if matchesAny(msg, contextOverflowPatterns) {
+		return FailoverContextOverflow
+	}
 	return ""
 }
 

pkg/providers/types.go

@@ -71,13 +71,14 @@ type NativeSearchCapable interface {
 type FailoverReason string
 
 const (
-	FailoverAuth       FailoverReason = "auth"
-	FailoverRateLimit  FailoverReason = "rate_limit"
-	FailoverBilling    FailoverReason = "billing"
-	FailoverTimeout    FailoverReason = "timeout"
-	FailoverFormat     FailoverReason = "format"
-	FailoverOverloaded FailoverReason = "overloaded"
-	FailoverUnknown    FailoverReason = "unknown"
+	FailoverAuth            FailoverReason = "auth"
+	FailoverRateLimit       FailoverReason = "rate_limit"
+	FailoverBilling         FailoverReason = "billing"
+	FailoverTimeout         FailoverReason = "timeout"
+	FailoverFormat          FailoverReason = "format"
+	FailoverContextOverflow FailoverReason = "context_overflow"
+	FailoverOverloaded      FailoverReason = "overloaded"
+	FailoverUnknown         FailoverReason = "unknown"
 )
 
 // FailoverError wraps an LLM provider error with classification metadata.
@@ -101,7 +102,7 @@ func (e *FailoverError) Unwrap() error {
 // IsRetriable returns true if this error should trigger fallback to next candidate.
 // Non-retriable: Format errors (bad request structure, image dimension/size).
 func (e *FailoverError) IsRetriable() bool {
-	return e.Reason != FailoverFormat
+	return e.Reason != FailoverFormat && e.Reason != FailoverContextOverflow
 }
 
 // ModelConfig holds primary model and fallback list.