feat(thinking): remove thinkingConfig for ModeNone with zero budget and no level

- Updated Gemini, Gemini CLI, and Antigravity logic to delete `thinkingConfig` when `ModeNone` is set, `Budget=0`, and `Level` is empty.
- Adjusted tests to validate this behavior across multiple scenarios and models with zero-allowed configurations.
- Extended test cases for additional coverage of mixed-model behavior.

Closes: #3138

internal/thinking/provider/antigravity/apply.go

@@ -102,6 +102,10 @@ func (a *Applier) applyLevelFormat(body []byte, config thinking.ThinkingConfig)
 	result, _ = sjson.DeleteBytes(result, "request.generationConfig.thinkingConfig.include_thoughts")
 
 	if config.Mode == thinking.ModeNone {
+		if config.Budget == 0 && config.Level == "" {
+			result, _ = sjson.DeleteBytes(result, "request.generationConfig.thinkingConfig")
+			return result, nil
+		}
 		result, _ = sjson.SetBytes(result, "request.generationConfig.thinkingConfig.includeThoughts", false)
 		if config.Level != "" {
 			result, _ = sjson.SetBytes(result, "request.generationConfig.thinkingConfig.thinkingLevel", string(config.Level))

internal/thinking/provider/gemini/apply.go

@@ -22,7 +22,7 @@ import (
 //
 // Gemini-specific behavior:
 //   - Gemini 2.5: thinkingBudget format, flash series supports ZeroAllowed
-//   - Gemini 3.x: thinkingLevel format, cannot be disabled
+//   - Gemini 3.x: thinkingLevel format, disable by removing thinkingConfig when zero is allowed
 //   - Use ThinkingSupport.Levels to decide output format
 type Applier struct{}
 
@@ -114,7 +114,7 @@ func (a *Applier) applyCompatible(body []byte, config thinking.ThinkingConfig) (
 
 func (a *Applier) applyLevelFormat(body []byte, config thinking.ThinkingConfig) ([]byte, error) {
 	// ModeNone semantics:
-	//   - ModeNone + Budget=0: completely disable thinking (not possible for Level-only models)
+	//   - ModeNone + Budget=0: remove thinkingConfig to disable thinking
 	//   - ModeNone + Budget>0: forced to think but hide output (includeThoughts=false)
 	// ValidateConfig sets config.Level to the lowest level when ModeNone + Budget > 0.
 
@@ -126,6 +126,10 @@ func (a *Applier) applyLevelFormat(body []byte, config thinking.ThinkingConfig)
 	result, _ = sjson.DeleteBytes(result, "generationConfig.thinkingConfig.include_thoughts")
 
 	if config.Mode == thinking.ModeNone {
+		if config.Budget == 0 && config.Level == "" {
+			result, _ = sjson.DeleteBytes(result, "generationConfig.thinkingConfig")
+			return result, nil
+		}
 		result, _ = sjson.SetBytes(result, "generationConfig.thinkingConfig.includeThoughts", false)
 		if config.Level != "" {
 			result, _ = sjson.SetBytes(result, "generationConfig.thinkingConfig.thinkingLevel", string(config.Level))

internal/thinking/provider/geminicli/apply.go

@@ -87,6 +87,10 @@ func (a *Applier) applyLevelFormat(body []byte, config thinking.ThinkingConfig)
 	result, _ = sjson.DeleteBytes(result, "request.generationConfig.thinkingConfig.include_thoughts")
 
 	if config.Mode == thinking.ModeNone {
+		if config.Budget == 0 && config.Level == "" {
+			result, _ = sjson.DeleteBytes(result, "request.generationConfig.thinkingConfig")
+			return result, nil
+		}
 		result, _ = sjson.SetBytes(result, "request.generationConfig.thinkingConfig.includeThoughts", false)
 		if config.Level != "" {
 			result, _ = sjson.SetBytes(result, "request.generationConfig.thinkingConfig.thinkingLevel", string(config.Level))

test/thinking_conversion_test.go

@@ -1515,6 +1515,66 @@ func TestThinkingE2EMatrix_Body(t *testing.T) {
 			includeThoughts: "false",
 			expectErr:       false,
 		},
+		// Case 31A: reasoning_effort=none with zero allowed → delete thinkingConfig
+		{
+			name:        "31A",
+			from:        "openai",
+			to:          "gemini",
+			model:       "gemini-zero-mixed-model",
+			inputJSON:   `{"model":"gemini-zero-mixed-model","messages":[{"role":"user","content":"hi"}],"reasoning_effort":"none"}`,
+			expectField: "",
+			expectErr:   false,
+		},
+		// Case 31B: reasoning_effort=none with zero allowed to Gemini CLI → delete thinkingConfig
+		{
+			name:        "31B",
+			from:        "openai",
+			to:          "gemini-cli",
+			model:       "gemini-zero-mixed-model",
+			inputJSON:   `{"model":"gemini-zero-mixed-model","messages":[{"role":"user","content":"hi"}],"reasoning_effort":"none"}`,
+			expectField: "",
+			expectErr:   false,
+		},
+		// Case 31C: reasoning_effort=none with zero allowed to Antigravity → delete thinkingConfig
+		{
+			name:        "31C",
+			from:        "openai",
+			to:          "antigravity",
+			model:       "gemini-zero-mixed-model",
+			inputJSON:   `{"model":"gemini-zero-mixed-model","messages":[{"role":"user","content":"hi"}],"reasoning_effort":"none"}`,
+			expectField: "",
+			expectErr:   false,
+		},
+		// Case 31D: reasoning.effort=none with zero allowed → delete thinkingConfig
+		{
+			name:        "31D",
+			from:        "openai-response",
+			to:          "gemini",
+			model:       "gemini-zero-mixed-model",
+			inputJSON:   `{"model":"gemini-zero-mixed-model","input":[{"role":"user","content":"hi"}],"reasoning":{"effort":"none"}}`,
+			expectField: "",
+			expectErr:   false,
+		},
+		// Case 31E: reasoning.effort=none with zero allowed to Gemini CLI → delete thinkingConfig
+		{
+			name:        "31E",
+			from:        "openai-response",
+			to:          "gemini-cli",
+			model:       "gemini-zero-mixed-model",
+			inputJSON:   `{"model":"gemini-zero-mixed-model","input":[{"role":"user","content":"hi"}],"reasoning":{"effort":"none"}}`,
+			expectField: "",
+			expectErr:   false,
+		},
+		// Case 31F: reasoning.effort=none with zero allowed to Antigravity → delete thinkingConfig
+		{
+			name:        "31F",
+			from:        "openai-response",
+			to:          "antigravity",
+			model:       "gemini-zero-mixed-model",
+			inputJSON:   `{"model":"gemini-zero-mixed-model","input":[{"role":"user","content":"hi"}],"reasoning":{"effort":"none"}}`,
+			expectField: "",
+			expectErr:   false,
+		},
 		// Case 32: reasoning_effort=auto → -1 (DynamicAllowed=true)
 		{
 			name:            "32",
@@ -2957,6 +3017,15 @@ func getTestModels() []*registry.ModelInfo {
 			DisplayName: "Gemini Mixed Model",
 			Thinking:    &registry.ThinkingSupport{Min: 128, Max: 32768, Levels: []string{"low", "high"}, ZeroAllowed: false, DynamicAllowed: true},
 		},
+		{
+			ID:          "gemini-zero-mixed-model",
+			Object:      "model",
+			Created:     1700000000,
+			OwnedBy:     "test",
+			Type:        "gemini",
+			DisplayName: "Gemini Zero Mixed Model",
+			Thinking:    &registry.ThinkingSupport{Min: 1, Max: 65535, Levels: []string{"minimal", "low", "medium", "high"}, ZeroAllowed: true, DynamicAllowed: true},
+		},
 		{
 			ID:          "claude-budget-model",
 			Object:      "model",