fix(gemini): support snake_case thinking config fields from Python SDK

Google official Gemini Python SDK sends thinking_level, thinking_budget,
and include_thoughts (snake_case) instead of thinkingLevel, thinkingBudget,
and includeThoughts (camelCase). This caused thinking configuration to be
ignored when using Python SDK.

Changes:
- Extract layer: extractGeminiConfig now reads snake_case as fallback
- Apply layer: Gemini/CLI/Antigravity appliers clean up snake_case fields
- Translator layer: Gemini->OpenAI/Claude/Codex translators support fallback
- Tests: Added 4 test cases for snake_case field coverage

Fixes #1426

internal/thinking/apply.go

@@ -388,7 +388,12 @@ func extractGeminiConfig(body []byte, provider string) ThinkingConfig {
 	}
 
 	// Check thinkingLevel first (Gemini 3 format takes precedence)
-	if level := gjson.GetBytes(body, prefix+".thinkingLevel"); level.Exists() {
+	level := gjson.GetBytes(body, prefix+".thinkingLevel")
+	if !level.Exists() {
+		// Google official Gemini Python SDK sends snake_case field names
+		level = gjson.GetBytes(body, prefix+".thinking_level")
+	}
+	if level.Exists() {
 		value := level.String()
 		switch value {
 		case "none":
@@ -401,7 +406,12 @@ func extractGeminiConfig(body []byte, provider string) ThinkingConfig {
 	}
 
 	// Check thinkingBudget (Gemini 2.5 format)
-	if budget := gjson.GetBytes(body, prefix+".thinkingBudget"); budget.Exists() {
+	budget := gjson.GetBytes(body, prefix+".thinkingBudget")
+	if !budget.Exists() {
+		// Google official Gemini Python SDK sends snake_case field names
+		budget = gjson.GetBytes(body, prefix+".thinking_budget")
+	}
+	if budget.Exists() {
 		value := int(budget.Int())
 		switch value {
 		case 0:

internal/thinking/provider/antigravity/apply.go

@@ -94,8 +94,10 @@ func (a *Applier) applyCompatible(body []byte, config thinking.ThinkingConfig, m
 }
 
 func (a *Applier) applyLevelFormat(body []byte, config thinking.ThinkingConfig) ([]byte, error) {
-	// Remove conflicting field to avoid both thinkingLevel and thinkingBudget in output
+	// Remove conflicting fields to avoid both thinkingLevel and thinkingBudget in output
 	result, _ := sjson.DeleteBytes(body, "request.generationConfig.thinkingConfig.thinkingBudget")
+	result, _ = sjson.DeleteBytes(result, "request.generationConfig.thinkingConfig.thinking_budget")
+	result, _ = sjson.DeleteBytes(result, "request.generationConfig.thinkingConfig.thinking_level")
 	// Normalize includeThoughts field name to avoid oneof conflicts in upstream JSON parsing.
 	result, _ = sjson.DeleteBytes(result, "request.generationConfig.thinkingConfig.include_thoughts")
 
@@ -119,8 +121,10 @@ func (a *Applier) applyLevelFormat(body []byte, config thinking.ThinkingConfig)
 }
 
 func (a *Applier) applyBudgetFormat(body []byte, config thinking.ThinkingConfig, modelInfo *registry.ModelInfo, isClaude bool) ([]byte, error) {
-	// Remove conflicting field to avoid both thinkingLevel and thinkingBudget in output
+	// Remove conflicting fields to avoid both thinkingLevel and thinkingBudget in output
 	result, _ := sjson.DeleteBytes(body, "request.generationConfig.thinkingConfig.thinkingLevel")
+	result, _ = sjson.DeleteBytes(result, "request.generationConfig.thinkingConfig.thinking_level")
+	result, _ = sjson.DeleteBytes(result, "request.generationConfig.thinkingConfig.thinking_budget")
 	// Normalize includeThoughts field name to avoid oneof conflicts in upstream JSON parsing.
 	result, _ = sjson.DeleteBytes(result, "request.generationConfig.thinkingConfig.include_thoughts")
 

internal/thinking/provider/gemini/apply.go

@@ -118,8 +118,10 @@ func (a *Applier) applyLevelFormat(body []byte, config thinking.ThinkingConfig)
 	//   - ModeNone + Budget>0: forced to think but hide output (includeThoughts=false)
 	// ValidateConfig sets config.Level to the lowest level when ModeNone + Budget > 0.
 
-	// Remove conflicting field to avoid both thinkingLevel and thinkingBudget in output
+	// Remove conflicting fields to avoid both thinkingLevel and thinkingBudget in output
 	result, _ := sjson.DeleteBytes(body, "generationConfig.thinkingConfig.thinkingBudget")
+	result, _ = sjson.DeleteBytes(result, "generationConfig.thinkingConfig.thinking_budget")
+	result, _ = sjson.DeleteBytes(result, "generationConfig.thinkingConfig.thinking_level")
 	// Normalize includeThoughts field name to avoid oneof conflicts in upstream JSON parsing.
 	result, _ = sjson.DeleteBytes(result, "generationConfig.thinkingConfig.include_thoughts")
 
@@ -143,8 +145,10 @@ func (a *Applier) applyLevelFormat(body []byte, config thinking.ThinkingConfig)
 }
 
 func (a *Applier) applyBudgetFormat(body []byte, config thinking.ThinkingConfig) ([]byte, error) {
-	// Remove conflicting field to avoid both thinkingLevel and thinkingBudget in output
+	// Remove conflicting fields to avoid both thinkingLevel and thinkingBudget in output
 	result, _ := sjson.DeleteBytes(body, "generationConfig.thinkingConfig.thinkingLevel")
+	result, _ = sjson.DeleteBytes(result, "generationConfig.thinkingConfig.thinking_level")
+	result, _ = sjson.DeleteBytes(result, "generationConfig.thinkingConfig.thinking_budget")
 	// Normalize includeThoughts field name to avoid oneof conflicts in upstream JSON parsing.
 	result, _ = sjson.DeleteBytes(result, "generationConfig.thinkingConfig.include_thoughts")
 

internal/thinking/provider/geminicli/apply.go

@@ -79,8 +79,10 @@ func (a *Applier) applyCompatible(body []byte, config thinking.ThinkingConfig) (
 }
 
 func (a *Applier) applyLevelFormat(body []byte, config thinking.ThinkingConfig) ([]byte, error) {
-	// Remove conflicting field to avoid both thinkingLevel and thinkingBudget in output
+	// Remove conflicting fields to avoid both thinkingLevel and thinkingBudget in output
 	result, _ := sjson.DeleteBytes(body, "request.generationConfig.thinkingConfig.thinkingBudget")
+	result, _ = sjson.DeleteBytes(result, "request.generationConfig.thinkingConfig.thinking_budget")
+	result, _ = sjson.DeleteBytes(result, "request.generationConfig.thinkingConfig.thinking_level")
 	// Normalize includeThoughts field name to avoid oneof conflicts in upstream JSON parsing.
 	result, _ = sjson.DeleteBytes(result, "request.generationConfig.thinkingConfig.include_thoughts")
 
@@ -104,8 +106,10 @@ func (a *Applier) applyLevelFormat(body []byte, config thinking.ThinkingConfig)
 }
 
 func (a *Applier) applyBudgetFormat(body []byte, config thinking.ThinkingConfig) ([]byte, error) {
-	// Remove conflicting field to avoid both thinkingLevel and thinkingBudget in output
+	// Remove conflicting fields to avoid both thinkingLevel and thinkingBudget in output
 	result, _ := sjson.DeleteBytes(body, "request.generationConfig.thinkingConfig.thinkingLevel")
+	result, _ = sjson.DeleteBytes(result, "request.generationConfig.thinkingConfig.thinking_level")
+	result, _ = sjson.DeleteBytes(result, "request.generationConfig.thinkingConfig.thinking_budget")
 	// Normalize includeThoughts field name to avoid oneof conflicts in upstream JSON parsing.
 	result, _ = sjson.DeleteBytes(result, "request.generationConfig.thinkingConfig.include_thoughts")
 

internal/translator/claude/gemini/claude_gemini_request.go

@@ -116,7 +116,11 @@ func ConvertGeminiRequestToClaude(modelName string, inputRawJSON []byte, stream
 		// Include thoughts configuration for reasoning process visibility
 		// Translator only does format conversion, ApplyThinking handles model capability validation.
 		if thinkingConfig := genConfig.Get("thinkingConfig"); thinkingConfig.Exists() && thinkingConfig.IsObject() {
-			if thinkingLevel := thinkingConfig.Get("thinkingLevel"); thinkingLevel.Exists() {
+			thinkingLevel := thinkingConfig.Get("thinkingLevel")
+			if !thinkingLevel.Exists() {
+				thinkingLevel = thinkingConfig.Get("thinking_level")
+			}
+			if thinkingLevel.Exists() {
 				level := strings.ToLower(strings.TrimSpace(thinkingLevel.String()))
 				switch level {
 				case "":
@@ -132,23 +136,29 @@ func ConvertGeminiRequestToClaude(modelName string, inputRawJSON []byte, stream
 						out, _ = sjson.Set(out, "thinking.budget_tokens", budget)
 					}
 				}
-			} else if thinkingBudget := thinkingConfig.Get("thinkingBudget"); thinkingBudget.Exists() {
-				budget := int(thinkingBudget.Int())
-				switch budget {
-				case 0:
-					out, _ = sjson.Set(out, "thinking.type", "disabled")
-					out, _ = sjson.Delete(out, "thinking.budget_tokens")
-				case -1:
-					out, _ = sjson.Set(out, "thinking.type", "enabled")
-					out, _ = sjson.Delete(out, "thinking.budget_tokens")
-				default:
-					out, _ = sjson.Set(out, "thinking.type", "enabled")
-					out, _ = sjson.Set(out, "thinking.budget_tokens", budget)
+			} else {
+				thinkingBudget := thinkingConfig.Get("thinkingBudget")
+				if !thinkingBudget.Exists() {
+					thinkingBudget = thinkingConfig.Get("thinking_budget")
+				}
+				if thinkingBudget.Exists() {
+					budget := int(thinkingBudget.Int())
+					switch budget {
+					case 0:
+						out, _ = sjson.Set(out, "thinking.type", "disabled")
+						out, _ = sjson.Delete(out, "thinking.budget_tokens")
+					case -1:
+						out, _ = sjson.Set(out, "thinking.type", "enabled")
+						out, _ = sjson.Delete(out, "thinking.budget_tokens")
+					default:
+						out, _ = sjson.Set(out, "thinking.type", "enabled")
+						out, _ = sjson.Set(out, "thinking.budget_tokens", budget)
+					}
+				} else if includeThoughts := thinkingConfig.Get("includeThoughts"); includeThoughts.Exists() && includeThoughts.Type == gjson.True {
+					out, _ = sjson.Set(out, "thinking.type", "enabled")
+				} else if includeThoughts := thinkingConfig.Get("include_thoughts"); includeThoughts.Exists() && includeThoughts.Type == gjson.True {
+					out, _ = sjson.Set(out, "thinking.type", "enabled")
 				}
-			} else if includeThoughts := thinkingConfig.Get("includeThoughts"); includeThoughts.Exists() && includeThoughts.Type == gjson.True {
-				out, _ = sjson.Set(out, "thinking.type", "enabled")
-			} else if includeThoughts := thinkingConfig.Get("include_thoughts"); includeThoughts.Exists() && includeThoughts.Type == gjson.True {
-				out, _ = sjson.Set(out, "thinking.type", "enabled")
 			}
 		}
 	}

internal/translator/codex/gemini/codex_gemini_request.go

@@ -243,19 +243,30 @@ func ConvertGeminiRequestToCodex(modelName string, inputRawJSON []byte, _ bool)
 	out, _ = sjson.Set(out, "parallel_tool_calls", true)
 
 	// Convert Gemini thinkingConfig to Codex reasoning.effort.
+	// Note: Google official Python SDK sends snake_case fields (thinking_level/thinking_budget).
 	effortSet := false
 	if genConfig := root.Get("generationConfig"); genConfig.Exists() {
 		if thinkingConfig := genConfig.Get("thinkingConfig"); thinkingConfig.Exists() && thinkingConfig.IsObject() {
-			if thinkingLevel := thinkingConfig.Get("thinkingLevel"); thinkingLevel.Exists() {
+			thinkingLevel := thinkingConfig.Get("thinkingLevel")
+			if !thinkingLevel.Exists() {
+				thinkingLevel = thinkingConfig.Get("thinking_level")
+			}
+			if thinkingLevel.Exists() {
 				effort := strings.ToLower(strings.TrimSpace(thinkingLevel.String()))
 				if effort != "" {
 					out, _ = sjson.Set(out, "reasoning.effort", effort)
 					effortSet = true
 				}
-			} else if thinkingBudget := thinkingConfig.Get("thinkingBudget"); thinkingBudget.Exists() {
-				if effort, ok := thinking.ConvertBudgetToLevel(int(thinkingBudget.Int())); ok {
-					out, _ = sjson.Set(out, "reasoning.effort", effort)
-					effortSet = true
+			} else {
+				thinkingBudget := thinkingConfig.Get("thinkingBudget")
+				if !thinkingBudget.Exists() {
+					thinkingBudget = thinkingConfig.Get("thinking_budget")
+				}
+				if thinkingBudget.Exists() {
+					if effort, ok := thinking.ConvertBudgetToLevel(int(thinkingBudget.Int())); ok {
+						out, _ = sjson.Set(out, "reasoning.effort", effort)
+						effortSet = true
+					}
 				}
 			}
 		}

internal/translator/openai/gemini/openai_gemini_request.go

@@ -83,16 +83,27 @@ func ConvertGeminiRequestToOpenAI(modelName string, inputRawJSON []byte, stream
 		}
 
 		// Map Gemini thinkingConfig to OpenAI reasoning_effort.
-		// Always perform conversion to support allowCompat models that may not be in registry
+		// Always perform conversion to support allowCompat models that may not be in registry.
+		// Note: Google official Python SDK sends snake_case fields (thinking_level/thinking_budget).
 		if thinkingConfig := genConfig.Get("thinkingConfig"); thinkingConfig.Exists() && thinkingConfig.IsObject() {
-			if thinkingLevel := thinkingConfig.Get("thinkingLevel"); thinkingLevel.Exists() {
+			thinkingLevel := thinkingConfig.Get("thinkingLevel")
+			if !thinkingLevel.Exists() {
+				thinkingLevel = thinkingConfig.Get("thinking_level")
+			}
+			if thinkingLevel.Exists() {
 				effort := strings.ToLower(strings.TrimSpace(thinkingLevel.String()))
 				if effort != "" {
 					out, _ = sjson.Set(out, "reasoning_effort", effort)
 				}
-			} else if thinkingBudget := thinkingConfig.Get("thinkingBudget"); thinkingBudget.Exists() {
-				if effort, ok := thinking.ConvertBudgetToLevel(int(thinkingBudget.Int())); ok {
-					out, _ = sjson.Set(out, "reasoning_effort", effort)
+			} else {
+				thinkingBudget := thinkingConfig.Get("thinkingBudget")
+				if !thinkingBudget.Exists() {
+					thinkingBudget = thinkingConfig.Get("thinking_budget")
+				}
+				if thinkingBudget.Exists() {
+					if effort, ok := thinking.ConvertBudgetToLevel(int(thinkingBudget.Int())); ok {
+						out, _ = sjson.Set(out, "reasoning_effort", effort)
+					}
 				}
 			}
 		}

test/thinking_conversion_test.go

@@ -1441,6 +1441,28 @@ func TestThinkingE2EMatrix_Body(t *testing.T) {
 			expectValue: "medium",
 			expectErr:   false,
 		},
+		// Case 9001: thinking_budget=64000 (snake_case) → high (Gemini -> Codex)
+		{
+			name:        "9001",
+			from:        "gemini",
+			to:          "codex",
+			model:       "level-model",
+			inputJSON:   `{"model":"level-model","contents":[{"role":"user","parts":[{"text":"hi"}]}],"generationConfig":{"thinkingConfig":{"thinking_budget":64000}}}`,
+			expectField: "reasoning.effort",
+			expectValue: "high",
+			expectErr:   false,
+		},
+		// Case 9002: thinking_level=high (snake_case) → reasoning_effort=high (Gemini -> OpenAI)
+		{
+			name:        "9002",
+			from:        "gemini",
+			to:          "openai",
+			model:       "level-model",
+			inputJSON:   `{"model":"level-model","contents":[{"role":"user","parts":[{"text":"hi"}]}],"generationConfig":{"thinkingConfig":{"thinking_level":"high"}}}`,
+			expectField: "reasoning_effort",
+			expectValue: "high",
+			expectErr:   false,
+		},
 		// Case 11: Claude no param → passthrough (no thinking)
 		{
 			name:        "11",
@@ -1451,6 +1473,17 @@ func TestThinkingE2EMatrix_Body(t *testing.T) {
 			expectField: "",
 			expectErr:   false,
 		},
+		// Case 9003: thinking_budget=8192 (snake_case) → thinking.budget_tokens=8192 (Gemini -> Claude)
+		{
+			name:        "9003",
+			from:        "gemini",
+			to:          "claude",
+			model:       "level-model",
+			inputJSON:   `{"model":"level-model","contents":[{"role":"user","parts":[{"text":"hi"}]}],"generationConfig":{"thinkingConfig":{"thinking_budget":8192}}}`,
+			expectField: "thinking.budget_tokens",
+			expectValue: "8192",
+			expectErr:   false,
+		},
 		// Case 12: thinking.budget_tokens=8192 → medium
 		{
 			name:        "12",
@@ -1524,6 +1557,19 @@ func TestThinkingE2EMatrix_Body(t *testing.T) {
 
 		// gemini-budget-model (Min=128, Max=20000, ZeroAllowed=false, DynamicAllowed=true)
 
+		// Case 9004: thinking_budget=8192 (snake_case) → passthrough+normalize to thinkingBudget (Gemini -> Gemini)
+		{
+			name:            "9004",
+			from:            "gemini",
+			to:              "gemini",
+			model:           "gemini-budget-model",
+			inputJSON:       `{"model":"gemini-budget-model","contents":[{"role":"user","parts":[{"text":"hi"}]}],"generationConfig":{"thinkingConfig":{"thinking_budget":8192}}}`,
+			expectField:     "generationConfig.thinkingConfig.thinkingBudget",
+			expectValue:     "8192",
+			includeThoughts: "true",
+			expectErr:       false,
+		},
+
 		// Case 18: No param → passthrough
 		{
 			name:        "18",