model/parsers: suppress extra gemma4 closing tool tags (#15370)

We've observed Gemma 4 occasionally emitting extra <tool_call|> tags
after a valid tool call. We suppress leading close tags in this
immediate post-tool-call state so the extra close tags do not leak into
assistant content. The tradeoff is that if the model intentionally
begins its next content span with the literal string "<tool_call|>", we
will erroneously treat it as noise and drop it.

model/parsers/gemma4.go

@@ -17,6 +17,7 @@ const (
 	Gemma4CollectingContent Gemma4ParserState = iota
 	Gemma4CollectingThinking
 	Gemma4CollectingToolCall
+	Gemma4IgnoringPostToolCallNoise
 )
 
 const (
@@ -285,7 +286,7 @@ func (p *Gemma4Parser) eat(done bool) ([]gemma4Event, bool) {
 
 			p.buffer.Reset()
 			p.buffer.WriteString(remaining)
-			p.state = Gemma4CollectingContent
+			p.state = Gemma4IgnoringPostToolCallNoise
 
 			if toolCall, err := parseGemma4ToolCall(toolCallContent); err == nil {
 				events = append(events, gemma4EventToolCall{toolCall: toolCall})
@@ -310,6 +311,38 @@ func (p *Gemma4Parser) eat(done bool) ([]gemma4Event, bool) {
 
 		// Wait for closing tag
 		return events, false
+
+	case Gemma4IgnoringPostToolCallNoise:
+		// We've observed Gemma 4 occasionally emitting extra <tool_call|> tags
+		// after a valid tool call. We suppress leading close tags in this immediate
+		// post-tool-call state so the extra close tags do not leak into assistant
+		// content.  The tradeoff is that if the model intentionally begins its next
+		// content span with the literal string "<tool_call|>", we will erroneously
+		// treat it as noise and drop it.
+		bufStr = strings.TrimLeftFunc(bufStr, unicode.IsSpace)
+		p.buffer.Reset()
+		p.buffer.WriteString(bufStr)
+
+		for strings.HasPrefix(bufStr, gemma4ToolCallCloseTag) {
+			bufStr = strings.TrimLeftFunc(bufStr[len(gemma4ToolCallCloseTag):], unicode.IsSpace)
+			p.buffer.Reset()
+			p.buffer.WriteString(bufStr)
+		}
+
+		if bufStr == "" {
+			return events, false
+		}
+
+		if strings.HasPrefix(gemma4ToolCallCloseTag, bufStr) {
+			if done {
+				p.buffer.Reset()
+				p.state = Gemma4CollectingContent
+			}
+			return events, false
+		}
+
+		p.state = Gemma4CollectingContent
+		return events, true
 	}
 
 	return events, false

model/parsers/gemma4_test.go

@@ -457,6 +457,77 @@ func TestGemma4Parser_StreamingToolCall(t *testing.T) {
 	}
 }
 
+func TestGemma4Parser_IgnoresExtraToolCallCloseTags(t *testing.T) {
+	tests := []struct {
+		name            string
+		chunks          []string
+		expectedContent string
+	}{
+		{
+			name: "same_chunk_without_trailing_content",
+			chunks: []string{
+				`<|tool_call>call:get_weather{location:<|"|>Paris<|"|>}<tool_call|><tool_call|>`,
+			},
+			expectedContent: "",
+		},
+		{
+			name: "same_chunk_before_real_content",
+			chunks: []string{
+				`<|tool_call>call:get_weather{location:<|"|>Paris<|"|>}<tool_call|><tool_call|>Done.`,
+			},
+			expectedContent: "Done.",
+		},
+		{
+			name: "split_across_chunks_before_real_content",
+			chunks: []string{
+				`<|tool_call>call:get_weather{location:<|"|>Paris<|"|>}<tool_call|><tool_`,
+				`call|>Done.`,
+			},
+			expectedContent: "Done.",
+		},
+	}
+
+	expectedToolCalls := []api.ToolCall{
+		{
+			Function: api.ToolCallFunction{
+				Name: "get_weather",
+				Arguments: testArgs(map[string]any{
+					"location": "Paris",
+				}),
+			},
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			parser := &Gemma4Parser{hasThinkingSupport: false}
+			parser.Init(nil, nil, nil)
+
+			var finalContent strings.Builder
+			var finalToolCalls []api.ToolCall
+
+			for i, chunk := range tt.chunks {
+				done := i == len(tt.chunks)-1
+				content, _, toolCalls, err := parser.Add(chunk, done)
+				if err != nil {
+					t.Fatalf("Add() error on chunk %d: %v", i, err)
+				}
+
+				finalContent.WriteString(content)
+				finalToolCalls = append(finalToolCalls, toolCalls...)
+			}
+
+			if diff := cmp.Diff(tt.expectedContent, finalContent.String()); diff != "" {
+				t.Errorf("content mismatch (-want +got):\n%s", diff)
+			}
+
+			if diff := cmp.Diff(expectedToolCalls, finalToolCalls, argsComparer); diff != "" {
+				t.Errorf("tool calls mismatch (-want +got):\n%s", diff)
+			}
+		})
+	}
+}
+
 func TestGemma4Parser_StreamingSplitThinkingTag(t *testing.T) {
 	tests := []struct {
 		name             string