diff --git a/go/ai/document.go b/go/ai/document.go
index 95ca54c176..f4c94f1be9 100644
--- a/go/ai/document.go
+++ b/go/ai/document.go
@@ -94,8 +94,15 @@ func NewCustomPart(customData map[string]any) *Part {
 }
 
 // NewReasoningPart returns a Part containing reasoning text
-func NewReasoningPart(text string) *Part {
-	return &Part{Kind: PartReasoning, ContentType: "plain/text", Text: text}
+func NewReasoningPart(text string, signature []byte) *Part {
+	return &Part{
+		Kind:        PartReasoning,
+		ContentType: "plain/text",
+		Text:        text,
+		Metadata: map[string]any{
+			"signature": signature,
+		},
+	}
 }
 
 // IsText reports whether the [Part] contains plain text.
diff --git a/go/go.mod b/go/go.mod
index 2a0fb5564d..370e946a5d 100644
--- a/go/go.mod
+++ b/go/go.mod
@@ -35,7 +35,7 @@ require (
 	golang.org/x/exp v0.0.0-20250408133849-7e4ce0ab07d0
 	golang.org/x/tools v0.32.0
 	google.golang.org/api v0.230.0
-	google.golang.org/genai v1.5.0
+	google.golang.org/genai v1.8.0
 )
 
 require (
diff --git a/go/go.sum b/go/go.sum
index 7a5cec1edd..d4605238e3 100644
--- a/go/go.sum
+++ b/go/go.sum
@@ -418,8 +418,8 @@ google.golang.org/api v0.230.0 h1:2u1hni3E+UXAXrONrrkfWpi/V6cyKVAbfGVeGtC3OxM=
 google.golang.org/api v0.230.0/go.mod h1:aqvtoMk7YkiXx+6U12arQFExiRV9D/ekvMCwCd/TksQ=
 google.golang.org/appengine/v2 v2.0.6 h1:LvPZLGuchSBslPBp+LAhihBeGSiRh1myRoYK4NtuBIw=
 google.golang.org/appengine/v2 v2.0.6/go.mod h1:WoEXGoXNfa0mLvaH5sV3ZSGXwVmy8yf7Z1JKf3J3wLI=
-google.golang.org/genai v1.5.0 h1:6wB3MCW4JpCMHURJH2gBNxCU/9iN1YjKYQj362mDTbY=
-google.golang.org/genai v1.5.0/go.mod h1:TyfOKRz/QyCaj6f/ZDt505x+YreXnY40l2I6k8TvgqY=
+google.golang.org/genai v1.8.0 h1:unX2CNWSiKDO2MSTKK3RstXg/vHp9hr42LIcL6f3Cik=
+google.golang.org/genai v1.8.0/go.mod h1:TyfOKRz/QyCaj6f/ZDt505x+YreXnY40l2I6k8TvgqY=
 google.golang.org/genproto v0.0.0-20250106144421-5f5ef82da422 h1:6GUHKGv2huWOHKmDXLMNE94q3fBDlEHI+oTRIZSebK0=
 google.golang.org/genproto v0.0.0-20250106144421-5f5ef82da422/go.mod h1:1NPAxoesyw/SgLPqaUp9u1f9PWCLAk/jVmhx7gJZStg=
 google.golang.org/genproto/googleapis/api v0.0.0-20250218202821-56aae31c358a h1:nwKuGPlUAt+aR+pcrkfFRrTU1BVrSmYyYMxYbUIVHr0=
diff --git a/go/plugins/googlegenai/gemini.go b/go/plugins/googlegenai/gemini.go
index 51890953c7..b50527d245 100644
--- a/go/plugins/googlegenai/gemini.go
+++ b/go/plugins/googlegenai/gemini.go
@@ -786,12 +786,13 @@ func translateCandidate(cand *genai.Candidate) *ai.ModelResponse {
 		var p *ai.Part
 		partFound := 0
 
-		if part.Text != "" {
+		if part.Thought {
+			p = ai.NewReasoningPart(part.Text, part.ThoughtSignature)
 			partFound++
+		}
+		if part.Text != "" {
 			p = ai.NewTextPart(part.Text)
-			if part.Thought {
-				p = ai.NewReasoningPart(part.Text)
-			}
+			partFound++
 		}
 		if part.InlineData != nil {
 			partFound++
@@ -871,9 +872,20 @@ func toGeminiParts(parts []*ai.Part) ([]*genai.Part, error) {
 // toGeminiPart converts a [ai.Part] to a [genai.Part].
 func toGeminiPart(p *ai.Part) (*genai.Part, error) {
 	switch {
-	case p.IsText():
-		return genai.NewPartFromText(p.Text), nil
 	case p.IsReasoning():
+		// TODO: go-genai does not support genai.NewPartFromThought()
+		signature := []byte{}
+		if p.Metadata != nil {
+			if sig, ok := p.Metadata["signature"].([]byte); ok {
+				signature = sig
+			}
+		}
+		return &genai.Part{
+			Thought:          true,
+			Text:             p.Text,
+			ThoughtSignature: signature,
+		}, nil
+	case p.IsText():
 		return genai.NewPartFromText(p.Text), nil
 	case p.IsMedia():
 		if strings.HasPrefix(p.Text, "data:") {
diff --git a/go/plugins/googlegenai/googleai_live_test.go b/go/plugins/googlegenai/googleai_live_test.go
index b7af41df5f..94dad0a32f 100644
--- a/go/plugins/googlegenai/googleai_live_test.go
+++ b/go/plugins/googlegenai/googleai_live_test.go
@@ -166,6 +166,28 @@ func TestGoogleAILive(t *testing.T) {
 			t.Errorf("got %q, expecting it to contain %q", out, want)
 		}
 	})
+	t.Run("tool with thinking", func(t *testing.T) {
+		m := googlegenai.GoogleAIModel(g, "gemini-2.5-flash-preview-04-17")
+		resp, err := genkit.Generate(ctx, g,
+			ai.WithConfig(&googlegenai.GeminiConfig{
+				ThinkingConfig: &googlegenai.ThinkingConfig{
+					ThinkingBudget: 1000,
+				},
+			}),
+			ai.WithModel(m),
+			ai.WithPrompt("what is a gablorken of 2 over 3.5?"),
+			ai.WithTools(gablorkenTool))
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		out := resp.Message.Content[0].Text
+		const want = "11.31"
+		if !strings.Contains(out, want) {
+			t.Errorf("got %q, expecting it to contain %q", out, want)
+		}
+	})
+
 	t.Run("tool with json output", func(t *testing.T) {
 		type weatherQuery struct {
 			Location string `json:"location"`
@@ -414,7 +436,7 @@ func TestGoogleAILive(t *testing.T) {
 				Temperature: 0.4,
 				ThinkingConfig: &googlegenai.ThinkingConfig{
 					IncludeThoughts: true,
-					ThinkingBudget:  100,
+					ThinkingBudget:  1000,
 				},
 			}),
 			ai.WithModel(m),
@@ -425,8 +447,8 @@ func TestGoogleAILive(t *testing.T) {
 		if resp == nil {
 			t.Fatal("nil response obtanied")
 		}
-		if resp.Usage.ThoughtsTokens == 0 || resp.Usage.ThoughtsTokens > 100 {
-			t.Fatal("thoughts tokens should not be zero or greater than 100")
+		if resp.Usage.ThoughtsTokens == 0 || resp.Usage.ThoughtsTokens > 1000 {
+			t.Fatalf("thoughts tokens should not be zero or greater than 100, got: %d", resp.Usage.ThoughtsTokens)
 		}
 	})
 	t.Run("thinking disabled", func(t *testing.T) {