diff --git a/internal/thinking/provider/openai/apply.go b/internal/thinking/provider/openai/apply.go
index eaad30ee..e8a2562f 100644
--- a/internal/thinking/provider/openai/apply.go
+++ b/internal/thinking/provider/openai/apply.go
@@ -10,10 +10,53 @@ import (
 
 	"github.com/router-for-me/CLIProxyAPI/v6/internal/registry"
 	"github.com/router-for-me/CLIProxyAPI/v6/internal/thinking"
+	log "github.com/sirupsen/logrus"
 	"github.com/tidwall/gjson"
 	"github.com/tidwall/sjson"
 )
 
+// validReasoningEffortLevels contains the standard values accepted by the
+// OpenAI reasoning_effort field. Provider-specific extensions (xhigh, minimal,
+// auto) are NOT in this set and must be clamped before use.
+var validReasoningEffortLevels = map[string]struct{}{
+	"none":   {},
+	"low":    {},
+	"medium": {},
+	"high":   {},
+}
+
+// clampReasoningEffort maps any thinking level string to a value that is safe
+// to send as OpenAI reasoning_effort. Non-standard CPA-internal values are
+// mapped to the nearest standard equivalent.
+//
+// Mapping rules:
+//   - none / low / medium / high  → returned as-is (already valid)
+//   - xhigh                       → "high" (nearest lower standard level)
+//   - minimal                     → "low" (nearest higher standard level)
+//   - auto                        → "medium" (reasonable default)
+//   - anything else               → "medium" (safe default)
+func clampReasoningEffort(level string) string {
+	if _, ok := validReasoningEffortLevels[level]; ok {
+		return level
+	}
+	var clamped string
+	switch level {
+	case string(thinking.LevelXHigh):
+		clamped = string(thinking.LevelHigh)
+	case string(thinking.LevelMinimal):
+		clamped = string(thinking.LevelLow)
+	case string(thinking.LevelAuto):
+		clamped = string(thinking.LevelMedium)
+	default:
+		clamped = string(thinking.LevelMedium)
+	}
+	log.WithFields(log.Fields{
+		"original": level,
+		"clamped":  clamped,
+	}).Debug("openai: reasoning_effort clamped to nearest valid standard value")
+	return clamped
+}
+
 // Applier implements thinking.ProviderApplier for OpenAI models.
 //
 // OpenAI-specific behavior:
@@ -58,7 +101,7 @@ func (a *Applier) Apply(body []byte, config thinking.ThinkingConfig, modelInfo *
 	}
 
 	if config.Mode == thinking.ModeLevel {
-		result, _ := sjson.SetBytes(body, "reasoning_effort", string(config.Level))
+		result, _ := sjson.SetBytes(body, "reasoning_effort", clampReasoningEffort(string(config.Level)))
 		return result, nil
 	}
 
@@ -79,7 +122,7 @@ func (a *Applier) Apply(body []byte, config thinking.ThinkingConfig, modelInfo *
 		return body, nil
 	}
 
-	result, _ := sjson.SetBytes(body, "reasoning_effort", effort)
+	result, _ := sjson.SetBytes(body, "reasoning_effort", clampReasoningEffort(effort))
 	return result, nil
 }
 
@@ -114,7 +157,7 @@ func applyCompatibleOpenAI(body []byte, config thinking.ThinkingConfig) ([]byte,
 		return body, nil
 	}
 
-	result, _ := sjson.SetBytes(body, "reasoning_effort", effort)
+	result, _ := sjson.SetBytes(body, "reasoning_effort", clampReasoningEffort(effort))
 	return result, nil
 }