diff --git a/internal/registry/model_definitions.go b/internal/registry/model_definitions.go
index daa95a4d..77b06b72 100644
--- a/internal/registry/model_definitions.go
+++ b/internal/registry/model_definitions.go
@@ -24,49 +24,54 @@ func GetClaudeModels() []*ModelInfo {
 			DisplayName: "Claude 4.5 Sonnet",
 		},
 		{
-			ID:          "claude-sonnet-4-5-thinking",
-			Object:      "model",
-			Created:     1759104000, // 2025-09-29
-			OwnedBy:     "anthropic",
-			Type:        "claude",
-			DisplayName: "Claude 4.5 Sonnet Thinking",
-			Thinking:    &ThinkingSupport{Min: 1024, Max: 100000, ZeroAllowed: false, DynamicAllowed: true},
+			ID:                  "claude-sonnet-4-5-thinking",
+			Object:              "model",
+			Created:             1759104000, // 2025-09-29
+			OwnedBy:             "anthropic",
+			Type:                "claude",
+			DisplayName:         "Claude 4.5 Sonnet Thinking",
+			MaxCompletionTokens: 128000,
+			Thinking:            &ThinkingSupport{Min: 1024, Max: 100000, ZeroAllowed: false, DynamicAllowed: true},
 		},
 		{
-			ID:          "claude-opus-4-5-thinking",
-			Object:      "model",
-			Created:     1761955200, // 2025-11-01
-			OwnedBy:     "anthropic",
-			Type:        "claude",
-			DisplayName: "Claude 4.5 Opus Thinking",
-			Thinking:    &ThinkingSupport{Min: 1024, Max: 100000, ZeroAllowed: false, DynamicAllowed: true},
+			ID:                  "claude-opus-4-5-thinking",
+			Object:              "model",
+			Created:             1761955200, // 2025-11-01
+			OwnedBy:             "anthropic",
+			Type:                "claude",
+			DisplayName:         "Claude 4.5 Opus Thinking",
+			MaxCompletionTokens: 128000,
+			Thinking:            &ThinkingSupport{Min: 1024, Max: 100000, ZeroAllowed: false, DynamicAllowed: true},
 		},
 		{
-			ID:          "claude-opus-4-5-thinking-low",
-			Object:      "model",
-			Created:     1761955200, // 2025-11-01
-			OwnedBy:     "anthropic",
-			Type:        "claude",
-			DisplayName: "Claude 4.5 Opus Thinking Low",
-			Thinking:    &ThinkingSupport{Min: 1024, Max: 100000, ZeroAllowed: false, DynamicAllowed: true},
+			ID:                  "claude-opus-4-5-thinking-low",
+			Object:              "model",
+			Created:             1761955200, // 2025-11-01
+			OwnedBy:             "anthropic",
+			Type:                "claude",
+			DisplayName:         "Claude 4.5 Opus Thinking Low",
+			MaxCompletionTokens: 128000,
+			Thinking:            &ThinkingSupport{Min: 1024, Max: 100000, ZeroAllowed: false, DynamicAllowed: true},
 		},
 		{
-			ID:          "claude-opus-4-5-thinking-medium",
-			Object:      "model",
-			Created:     1761955200, // 2025-11-01
-			OwnedBy:     "anthropic",
-			Type:        "claude",
-			DisplayName: "Claude 4.5 Opus Thinking Medium",
-			Thinking:    &ThinkingSupport{Min: 1024, Max: 100000, ZeroAllowed: false, DynamicAllowed: true},
+			ID:                  "claude-opus-4-5-thinking-medium",
+			Object:              "model",
+			Created:             1761955200, // 2025-11-01
+			OwnedBy:             "anthropic",
+			Type:                "claude",
+			DisplayName:         "Claude 4.5 Opus Thinking Medium",
+			MaxCompletionTokens: 128000,
+			Thinking:            &ThinkingSupport{Min: 1024, Max: 100000, ZeroAllowed: false, DynamicAllowed: true},
 		},
 		{
-			ID:          "claude-opus-4-5-thinking-high",
-			Object:      "model",
-			Created:     1761955200, // 2025-11-01
-			OwnedBy:     "anthropic",
-			Type:        "claude",
-			DisplayName: "Claude 4.5 Opus Thinking High",
-			Thinking:    &ThinkingSupport{Min: 1024, Max: 100000, ZeroAllowed: false, DynamicAllowed: true},
+			ID:                  "claude-opus-4-5-thinking-high",
+			Object:              "model",
+			Created:             1761955200, // 2025-11-01
+			OwnedBy:             "anthropic",
+			Type:                "claude",
+			DisplayName:         "Claude 4.5 Opus Thinking High",
+			MaxCompletionTokens: 128000,
+			Thinking:            &ThinkingSupport{Min: 1024, Max: 100000, ZeroAllowed: false, DynamicAllowed: true},
 		},
 		{
 			ID:                  "claude-opus-4-5-20251101",
diff --git a/internal/runtime/executor/claude_executor.go b/internal/runtime/executor/claude_executor.go
index 591b8f9e..1a18c46a 100644
--- a/internal/runtime/executor/claude_executor.go
+++ b/internal/runtime/executor/claude_executor.go
@@ -17,6 +17,7 @@ import (
 	claudeauth "github.com/router-for-me/CLIProxyAPI/v6/internal/auth/claude"
 	"github.com/router-for-me/CLIProxyAPI/v6/internal/config"
 	"github.com/router-for-me/CLIProxyAPI/v6/internal/misc"
+	"github.com/router-for-me/CLIProxyAPI/v6/internal/registry"
 	"github.com/router-for-me/CLIProxyAPI/v6/internal/util"
 	cliproxyauth "github.com/router-for-me/CLIProxyAPI/v6/sdk/cliproxy/auth"
 	cliproxyexecutor "github.com/router-for-me/CLIProxyAPI/v6/sdk/cliproxy/executor"
@@ -66,6 +67,9 @@ func (e *ClaudeExecutor) Execute(ctx context.Context, auth *cliproxyauth.Auth, r
 	}
 	body = applyPayloadConfig(e.cfg, req.Model, body)
 
+	// Ensure max_tokens > thinking.budget_tokens when thinking is enabled
+	body = ensureMaxTokensForThinking(req.Model, body)
+
 	// Extract betas from body and convert to header
 	var extraBetas []string
 	extraBetas, body = extractAndRemoveBetas(body)
@@ -165,6 +169,9 @@ func (e *ClaudeExecutor) ExecuteStream(ctx context.Context, auth *cliproxyauth.A
 	body = checkSystemInstructions(body)
 	body = applyPayloadConfig(e.cfg, req.Model, body)
 
+	// Ensure max_tokens > thinking.budget_tokens when thinking is enabled
+	body = ensureMaxTokensForThinking(req.Model, body)
+
 	// Extract betas from body and convert to header
 	var extraBetas []string
 	extraBetas, body = extractAndRemoveBetas(body)
@@ -447,6 +454,42 @@ func (e *ClaudeExecutor) injectThinkingConfig(modelName string, body []byte) []b
 	return body
 }
 
+// ensureMaxTokensForThinking ensures max_tokens > thinking.budget_tokens when thinking is enabled.
+// Anthropic API requires this constraint; violating it returns a 400 error.
+// This function should be called after all thinking configuration is finalized.
+// It looks up the model's MaxCompletionTokens from the registry to use as the cap.
+func ensureMaxTokensForThinking(modelName string, body []byte) []byte {
+	thinkingType := gjson.GetBytes(body, "thinking.type").String()
+	if thinkingType != "enabled" {
+		return body
+	}
+
+	budgetTokens := gjson.GetBytes(body, "thinking.budget_tokens").Int()
+	if budgetTokens <= 0 {
+		return body
+	}
+
+	maxTokens := gjson.GetBytes(body, "max_tokens").Int()
+
+	// Look up the model's max completion tokens from the registry
+	maxCompletionTokens := 0
+	if modelInfo := registry.GetGlobalRegistry().GetModelInfo(modelName); modelInfo != nil {
+		maxCompletionTokens = modelInfo.MaxCompletionTokens
+	}
+
+	// Fall back to budget + buffer if registry lookup fails or returns 0
+	const fallbackBuffer = 4000
+	requiredMaxTokens := budgetTokens + fallbackBuffer
+	if maxCompletionTokens > 0 {
+		requiredMaxTokens = int64(maxCompletionTokens)
+	}
+
+	if maxTokens < requiredMaxTokens {
+		body, _ = sjson.SetBytes(body, "max_tokens", requiredMaxTokens)
+	}
+	return body
+}
+
 func (e *ClaudeExecutor) resolveUpstreamModel(alias string, auth *cliproxyauth.Auth) string {
 	if alias == "" {
 		return ""