WIP thinking API support

- Allows specifying whether thinking mode should be on or not
- Templates get passed a new option so, e.g., qwen3's template can put
  `/think` or `/no_think` in the system prompt depending on the value of
  the setting
- Add parsing for thinking blocks in both streaming/non-streaming mode
- Update the CLI to make use of these changes

api/types.go

@@ -83,6 +83,12 @@ type GenerateRequest struct {
 	// Options lists model-specific options. For example, temperature can be
 	// set through this field, if the model supports it.
 	Options map[string]any `json:"options"`
+
+	// Think controls whether thinking/reasoning models will think before
+	// responding. Needs to be a pointer so we can distinguish between false
+	// (request that thinking _not_ be used) and unset (use the old behavior
+	// before this option was introduced)
+	Think *bool `json:"think,omitempty"`
 }
 
 // ChatRequest describes a request sent by [Client.Chat].
@@ -108,6 +114,10 @@ type ChatRequest struct {
 
 	// Options lists model-specific options.
 	Options map[string]any `json:"options"`
+
+	// Think controls whether thinking/reasoning models will think before
+	// responding
+	Think *bool `json:"think,omitempty"`
 }
 
 type Tools []Tool
@@ -128,6 +138,9 @@ func (t Tool) String() string {
 type Message struct {
 	Role    string `json:"role"`
 	Content string `json:"content"`
+	// Thinking contains the text that was inside thinking tags in the
+	// original model output when ChatRequest.Think is enabled.
+	Thinking  string      `json:"thinking,omitempty"`
 	Images    []ImageData `json:"images,omitempty"`
 	ToolCalls []ToolCall  `json:"tool_calls,omitempty"`
 }
@@ -275,6 +288,8 @@ type Options struct {
 	MirostatTau      float32  `json:"mirostat_tau,omitempty"`
 	MirostatEta      float32  `json:"mirostat_eta,omitempty"`
 	Stop             []string `json:"stop,omitempty"`
+
+	Think bool `json:"think,omitempty"`
 }
 
 // Runner options which must be set when the model is loaded into memory

api/types_test.go

@@ -372,3 +372,50 @@ func TestPropertyType_MarshalJSON(t *testing.T) {
 		})
 	}
 }
+
+func TestThinking_UnmarshalJSON(t *testing.T) {
+	trueVal := true
+	falseVal := false
+
+	tests := []struct {
+		name             string
+		input            string
+		expectedThinking *bool
+		expectedError    bool
+	}{
+		{
+			name:             "true",
+			input:            `{ "think": true }`,
+			expectedThinking: &trueVal,
+		},
+		{
+			name:             "false",
+			input:            `{ "think": false }`,
+			expectedThinking: &falseVal,
+		},
+		{
+			name:             "unset",
+			input:            `{ }`,
+			expectedThinking: nil,
+		},
+		{
+			name:             "invalid",
+			input:            `{ "think": "true" }`,
+			expectedThinking: nil,
+			expectedError:    true,
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			var req GenerateRequest
+			err := json.Unmarshal([]byte(test.input), &req)
+			if test.expectedError {
+				require.Error(t, err)
+			} else {
+				require.NoError(t, err)
+				assert.Equal(t, test.expectedThinking, req.Think)
+			}
+		})
+	}
+}

cmd/cmd.go

@@ -38,12 +38,32 @@ import (
 	"github.com/ollama/ollama/format"
 	"github.com/ollama/ollama/parser"
 	"github.com/ollama/ollama/progress"
+	"github.com/ollama/ollama/readline"
 	"github.com/ollama/ollama/runner"
 	"github.com/ollama/ollama/server"
 	"github.com/ollama/ollama/types/model"
 	"github.com/ollama/ollama/version"
 )
 
+// warnMissingThinking emits a warning if the model does not advertise thinking
+// support and opts.Thinking is set. Failures to query the capability are
+// ignored so this does not impact regular usage.
+func warnMissingThinking(ctx context.Context, client *api.Client, name string) {
+	if name == "" {
+		return
+	}
+	resp, err := client.Show(ctx, &api.ShowRequest{Model: name})
+	if err != nil {
+		return
+	}
+	for _, cap := range resp.Capabilities {
+		if cap == model.CapabilityThinking {
+			return
+		}
+	}
+	fmt.Fprintf(os.Stderr, "warning: model %q does not support thinking output\n", name)
+}
+
 var errModelfileNotFound = errors.New("specified Modelfile wasn't found")
 
 func getModelfileName(cmd *cobra.Command) (string, error) {
@@ -240,9 +260,18 @@ func loadOrUnloadModel(cmd *cobra.Command, opts *runOptions) error {
 		return err
 	}
 
+	think := opts.Think
+	if think == nil {
+		falseVal := false
+		think = &falseVal
+	}
+
 	req := &api.GenerateRequest{
 		Model:     opts.Model,
 		KeepAlive: opts.KeepAlive,
+
+		// pass Think here so we fail before getting to the chat prompt if the model doesn't support it
+		Think: opts.Think,
 	}
 
 	return client.Generate(cmd.Context(), req, func(api.GenerateResponse) error { return nil })
@@ -277,6 +306,17 @@ func RunHandler(cmd *cobra.Command, args []string) error {
 	}
 	opts.Format = format
 
+	thinkFlag := cmd.Flags().Lookup("think")
+	if thinkFlag.Changed {
+		think, err := cmd.Flags().GetBool("think")
+		if err != nil {
+			return err
+		}
+		opts.Think = &think
+	} else {
+		opts.Think = nil
+	}
+
 	keepAlive, err := cmd.Flags().GetString("keepalive")
 	if err != nil {
 		return err
@@ -361,6 +401,7 @@ func RunHandler(cmd *cobra.Command, args []string) error {
 		if err := loadOrUnloadModel(cmd, &opts); err != nil {
 			return err
 		}
+		warnMissingThinking(cmd.Context(), client, opts.Model)
 
 		for _, msg := range info.Messages {
 			switch msg.Role {
@@ -876,6 +917,7 @@ type runOptions struct {
 	Options     map[string]any
 	MultiModal  bool
 	KeepAlive   *api.Duration
+	Think       *bool
 }
 
 type displayResponseState struct {
@@ -958,6 +1000,8 @@ func chat(cmd *cobra.Command, opts runOptions) (*api.Message, error) {
 	var latest api.ChatResponse
 	var fullResponse strings.Builder
 	var role string
+	var thinkTagOpened bool = false
+	var thinkTagClosed bool = false
 
 	fn := func(response api.ChatResponse) error {
 		p.StopAndClear()
@@ -965,7 +1009,23 @@ func chat(cmd *cobra.Command, opts runOptions) (*api.Message, error) {
 		latest = response
 
 		role = response.Message.Role
+		if response.Message.Thinking != "" {
+			if !thinkTagOpened {
+				fmt.Print(readline.ColorGrey + readline.ColorBold + "<think>" + readline.ColorDefault + readline.ColorGrey)
+				thinkTagOpened = true
+			}
+			displayResponse(response.Message.Thinking, opts.WordWrap, state)
+		}
+
 		content := response.Message.Content
+		if !thinkTagClosed && thinkTagOpened && content != "" {
+			fmt.Print(readline.ColorGrey + readline.ColorBold + "</think>" + readline.ColorDefault)
+			thinkTagClosed = true
+		}
+		// purposefully not putting thinking blocks in the response, which would
+		// only be needed if we later added tool calling to the cli (they get
+		// filtered out anyway since current models don't expect them unless you're
+		// about to finish some tool calls)
 		fullResponse.WriteString(content)
 
 		displayResponse(content, opts.WordWrap, state)
@@ -982,6 +1042,11 @@ func chat(cmd *cobra.Command, opts runOptions) (*api.Message, error) {
 		Messages: opts.Messages,
 		Format:   json.RawMessage(opts.Format),
 		Options:  opts.Options,
+		Think:    opts.Think,
+	}
+
+	if opts.Think != nil {
+		warnMissingThinking(cmd.Context(), client, opts.Model)
 	}
 
 	if opts.KeepAlive != nil {
@@ -1075,6 +1140,7 @@ func generate(cmd *cobra.Command, opts runOptions) error {
 		System:    opts.System,
 		Options:   opts.Options,
 		KeepAlive: opts.KeepAlive,
+		Think:     opts.Think,
 	}
 
 	if err := client.Generate(ctx, &request, fn); err != nil {
@@ -1290,6 +1356,7 @@ func NewCLI() *cobra.Command {
 	runCmd.Flags().Bool("insecure", false, "Use an insecure registry")
 	runCmd.Flags().Bool("nowordwrap", false, "Don't wrap words to the next line automatically")
 	runCmd.Flags().String("format", "", "Response format (e.g. json)")
+	runCmd.Flags().Bool("think", false, "Turn on thinking mode for supported models")
 
 	stopCmd := &cobra.Command{
 		Use:     "stop MODEL",

cmd/interactive.go

@@ -62,6 +62,8 @@ func generateInteractive(cmd *cobra.Command, opts runOptions) error {
 		fmt.Fprintln(os.Stderr, "  /set noformat          Disable formatting")
 		fmt.Fprintln(os.Stderr, "  /set verbose           Show LLM stats")
 		fmt.Fprintln(os.Stderr, "  /set quiet             Disable LLM stats")
+		fmt.Fprintln(os.Stderr, "  /set think             Enable thinking")
+		fmt.Fprintln(os.Stderr, "  /set nothink           Disable thinking")
 		fmt.Fprintln(os.Stderr, "")
 	}
 
@@ -260,6 +262,17 @@ func generateInteractive(cmd *cobra.Command, opts runOptions) error {
 						return err
 					}
 					fmt.Println("Set 'quiet' mode.")
+				case "think":
+					think := true
+					opts.Think = &think
+					if client, err := api.ClientFromEnvironment(); err == nil {
+						warnMissingThinking(cmd.Context(), client, opts.Model)
+					}
+					fmt.Println("Set 'think' mode.")
+				case "nothink":
+					think := false
+					opts.Think = &think
+					fmt.Println("Set 'nothink' mode.")
 				case "format":
 					if len(args) < 3 || args[2] != "json" {
 						fmt.Println("Invalid or missing format. For 'json' mode use '/set format json'")

cmd/templatefmt/main.go

@@ -0,0 +1,30 @@
+package main
+
+import (
+	"flag"
+	"fmt"
+	"io/ioutil"
+	"log"
+	"os"
+
+	"github.com/ollama/ollama/template"
+)
+
+func main() {
+	flag.Parse()
+	if flag.NArg() != 1 {
+		fmt.Fprintf(os.Stderr, "usage: %s <template.gotmpl>\n", os.Args[0])
+		os.Exit(2)
+	}
+	path := flag.Arg(0)
+	data, err := ioutil.ReadFile(path)
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	out, err := template.Format(string(data))
+	if err != nil {
+		log.Fatal(err)
+	}
+	fmt.Print(out)
+}

cmd/warn_thinking_test.go

@@ -0,0 +1,64 @@
+package cmd
+
+import (
+	"context"
+	"encoding/json"
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"os"
+	"strings"
+	"testing"
+
+	"github.com/ollama/ollama/api"
+	"github.com/ollama/ollama/types/model"
+)
+
+// Test that a warning is printed when thinking is requested but not supported.
+func TestWarnMissingThinking(t *testing.T) {
+	cases := []struct {
+		capabilities []model.Capability
+		expectWarn   bool
+	}{
+		{capabilities: []model.Capability{model.CapabilityThinking}, expectWarn: false},
+		{capabilities: []model.Capability{}, expectWarn: true},
+	}
+
+	for _, tc := range cases {
+		srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			if r.URL.Path != "/api/show" || r.Method != http.MethodPost {
+				t.Fatalf("unexpected request to %s %s", r.URL.Path, r.Method)
+			}
+			var req api.ShowRequest
+			if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
+				t.Fatalf("decode request: %v", err)
+			}
+			resp := api.ShowResponse{Capabilities: tc.capabilities}
+			if err := json.NewEncoder(w).Encode(resp); err != nil {
+				t.Fatalf("encode response: %v", err)
+			}
+		}))
+		defer srv.Close()
+
+		t.Setenv("OLLAMA_HOST", srv.URL)
+		client, err := api.ClientFromEnvironment()
+		if err != nil {
+			t.Fatal(err)
+		}
+		oldStderr := os.Stderr
+		r, w, _ := os.Pipe()
+		os.Stderr = w
+		warnMissingThinking(context.Background(), client, "m")
+		w.Close()
+		os.Stderr = oldStderr
+		out, _ := io.ReadAll(r)
+
+		warned := strings.Contains(string(out), "warning:")
+		if tc.expectWarn && !warned {
+			t.Errorf("expected warning, got none")
+		}
+		if !tc.expectWarn && warned {
+			t.Errorf("did not expect warning, got: %s", string(out))
+		}
+	}
+}

readline/types.go

@@ -61,6 +61,8 @@ const (
 	ColorGrey    = Esc + "[38;5;245m"
 	ColorDefault = Esc + "[0m"
 
+	ColorBold = Esc + "[1m"
+
 	StartBracketedPaste = Esc + "[?2004h"
 	EndBracketedPaste   = Esc + "[?2004l"
 )

server/images.go

@@ -37,6 +37,7 @@ var (
 	errCapabilityInsert     = errors.New("insert")
 	errCapabilityVision     = errors.New("vision")
 	errCapabilityEmbedding  = errors.New("embedding")
+	errCapabilityThinking   = errors.New("thinking")
 	errInsecureProtocol     = errors.New("insecure protocol http")
 )
 
@@ -106,6 +107,12 @@ func (m *Model) Capabilities() []model.Capability {
 		capabilities = append(capabilities, model.CapabilityInsert)
 	}
 
+	// Check for thinking capability
+	openingTag, closingTag := inferThinkingTags(m.Template.Template)
+	if openingTag != "" && closingTag != "" {
+		capabilities = append(capabilities, model.CapabilityThinking)
+	}
+
 	return capabilities
 }
 
@@ -122,6 +129,7 @@ func (m *Model) CheckCapabilities(want ...model.Capability) error {
 		model.CapabilityInsert:     errCapabilityInsert,
 		model.CapabilityVision:     errCapabilityVision,
 		model.CapabilityEmbedding:  errCapabilityEmbedding,
+		model.CapabilityThinking:   errCapabilityThinking,
 	}
 
 	for _, cap := range want {

server/prompt.go

@@ -22,7 +22,7 @@ var errTooManyImages = errors.New("vision model only supports a single image per
 // chatPrompt accepts a list of messages and returns the prompt and images that should be used for the next chat turn.
 // chatPrompt truncates any messages that exceed the context window of the model, making sure to always include 1) the
 // latest message and 2) system messages
-func chatPrompt(ctx context.Context, m *Model, tokenize tokenizeFunc, opts *api.Options, msgs []api.Message, tools []api.Tool) (prompt string, images []llm.ImageData, _ error) {
+func chatPrompt(ctx context.Context, m *Model, tokenize tokenizeFunc, opts *api.Options, msgs []api.Message, tools []api.Tool, think *bool) (prompt string, images []llm.ImageData, _ error) {
 	var system []api.Message
 
 	isMllama := checkMllamaModelFamily(m)
@@ -56,8 +56,12 @@ func chatPrompt(ctx context.Context, m *Model, tokenize tokenizeFunc, opts *api.
 			}
 		}
 
+		thinkVal := false
+		if think != nil {
+			thinkVal = *think
+		}
 		var b bytes.Buffer
-		if err := m.Template.Execute(&b, template.Values{Messages: append(system, msgs[i:]...), Tools: tools}); err != nil {
+		if err := m.Template.Execute(&b, template.Values{Messages: append(system, msgs[i:]...), Tools: tools, Think: thinkVal, IsThinkSet: think != nil}); err != nil {
 			return "", nil, err
 		}
 
@@ -142,7 +146,11 @@ func chatPrompt(ctx context.Context, m *Model, tokenize tokenizeFunc, opts *api.
 
 	// truncate any messages that do not fit into the context window
 	var b bytes.Buffer
-	if err := m.Template.Execute(&b, template.Values{Messages: append(system, msgs[currMsgIdx:]...), Tools: tools}); err != nil {
+	thinkVal := false
+	if think != nil {
+		thinkVal = *think
+	}
+	if err := m.Template.Execute(&b, template.Values{Messages: append(system, msgs[currMsgIdx:]...), Tools: tools, Think: thinkVal, IsThinkSet: think != nil}); err != nil {
 		return "", nil, err
 	}
 

server/prompt_test.go

@@ -318,7 +318,8 @@ func TestChatPrompt(t *testing.T) {
 		t.Run(tt.name, func(t *testing.T) {
 			model := tt.model
 			opts := api.Options{Runner: api.Runner{NumCtx: tt.limit}}
-			prompt, images, err := chatPrompt(context.TODO(), &model, mockRunner{}.Tokenize, &opts, tt.msgs, nil)
+			think := false
+			prompt, images, err := chatPrompt(context.TODO(), &model, mockRunner{}.Tokenize, &opts, tt.msgs, nil, &think)
 			if tt.error == nil && err != nil {
 				t.Fatal(err)
 			} else if tt.error != nil && err != tt.error {

server/routes.go

@@ -18,7 +18,6 @@ import (
 	"os"
 	"os/signal"
 	"path/filepath"
-	"regexp"
 	"slices"
 	"strings"
 	"syscall"
@@ -181,6 +180,12 @@ func (s *Server) GenerateHandler(c *gin.Context) {
 	if req.Suffix != "" {
 		caps = append(caps, model.CapabilityInsert)
 	}
+	if req.Think != nil {
+		// note that the capability is still required even if `Thinking` is false
+		// because turning off thinking requires the model to support it (e.g.,
+		// older qwen3 templates don't know how to turn off thinking)
+		caps = append(caps, model.CapabilityThinking)
+	}
 
 	r, m, opts, err := s.scheduleRunner(c.Request.Context(), name.String(), caps, req.Options, req.KeepAlive)
 	if errors.Is(err, errCapabilityCompletion) {
@@ -1475,6 +1480,9 @@ func (s *Server) ChatHandler(c *gin.Context) {
 	if len(req.Tools) > 0 {
 		caps = append(caps, model.CapabilityTools)
 	}
+	if req.Think != nil {
+		caps = append(caps, model.CapabilityThinking)
+	}
 
 	name := model.ParseName(req.Model)
 	if !name.IsValid() {
@@ -1515,7 +1523,7 @@ func (s *Server) ChatHandler(c *gin.Context) {
 	}
 	msgs = filterThinkTags(msgs, m)
 
-	prompt, images, err := chatPrompt(c.Request.Context(), m, r.Tokenize, opts, msgs, req.Tools)
+	prompt, images, err := chatPrompt(c.Request.Context(), m, r.Tokenize, opts, msgs, req.Tools, req.Think)
 	if err != nil {
 		slog.Error("chat prompt error", "error", err)
 		c.JSON(http.StatusInternalServerError, gin.H{"error": err.Error()})
@@ -1524,6 +1532,15 @@ func (s *Server) ChatHandler(c *gin.Context) {
 
 	slog.Debug("chat request", "images", len(images), "prompt", prompt)
 
+	var thinkingState *thinkingParser
+	openingTag, closingTag := inferThinkingTags(m.Template.Template)
+	if req.Think != nil && *req.Think && openingTag != "" && closingTag != "" {
+		thinkingState = &thinkingParser{
+			openingTag: openingTag,
+			closingTag: closingTag,
+		}
+	}
+
 	ch := make(chan any)
 	go func() {
 		defer close(ch)
@@ -1548,6 +1565,20 @@ func (s *Server) ChatHandler(c *gin.Context) {
 				},
 			}
 
+			if thinkingState != nil {
+				if openingTag == "" || closingTag == "" {
+					// TODO(drifkin): put warning here
+				} else {
+					thinkingContent, remainingContent := thinkingState.addContent(res.Message.Content)
+					if thinkingContent == "" && remainingContent == "" && !r.Done {
+						// need to accumulate more to decide what to send
+						return
+					}
+					res.Message.Content = remainingContent
+					res.Message.Thinking = thinkingContent
+				}
+			}
+
 			if r.Done {
 				res.DoneReason = r.DoneReason.String()
 				res.TotalDuration = time.Since(checkpointStart)
@@ -1565,7 +1596,7 @@ func (s *Server) ChatHandler(c *gin.Context) {
 			// Streaming tool calls:
 			// If tools are recognized, use a flag to track the sending of a tool downstream
 			// This ensures that content is cleared from the message on the last chunk sent
-			sb.WriteString(r.Content)
+			sb.WriteString(res.Message.Content)
 			if toolCalls, ok := m.parseToolCalls(sb.String()); ok {
 				res.Message.ToolCalls = toolCalls
 				for i := range toolCalls {
@@ -1613,9 +1644,12 @@ func (s *Server) ChatHandler(c *gin.Context) {
 		}
 
 		resp.Message.Content = sb.String()
+		if req.Think != nil && *req.Think {
+			resp.Message.Thinking, resp.Message.Content = extractThinking(resp.Message.Content)
+		}
 
 		if len(req.Tools) > 0 {
-			if toolCalls, ok := m.parseToolCalls(sb.String()); ok {
+			if toolCalls, ok := m.parseToolCalls(resp.Message.Content); ok {
 				resp.Message.ToolCalls = toolCalls
 				resp.Message.Content = ""
 			}
@@ -1643,7 +1677,16 @@ func handleScheduleError(c *gin.Context, name string, err error) {
 	}
 }
 
-var thinkTagRegexp = regexp.MustCompile(`<think>(?s).*?</think>(\n)*`)
+// returns (thinkingContent, content)
+func extractThinking(text string) (string, string) {
+	thinking := thinkingParser{
+		openingTag: "<think>",
+		closingTag: "</think>",
+	}
+
+	thinkingContent, content := thinking.addContent(text)
+	return thinkingContent, content
+}
 
 func filterThinkTags(msgs []api.Message, m *Model) []api.Message {
 	if m.Config.ModelFamily == "qwen3" || model.ParseName(m.Name).Model == "deepseek-r1" {
@@ -1656,7 +1699,9 @@ func filterThinkTags(msgs []api.Message, m *Model) []api.Message {
 
 		for i, msg := range msgs {
 			if msg.Role == "assistant" && i < finalUserIndex {
-				msgs[i].Content = thinkTagRegexp.ReplaceAllString(msg.Content, "")
+				thinkingContent, content := extractThinking(msg.Content)
+				msg.Content = content
+				msg.Thinking = thinkingContent
 			}
 		}
 	}

server/routes_generate_test.go

@@ -143,6 +143,25 @@ func TestGenerateChat(t *testing.T) {
 		}
 	})
 
+	t.Run("missing thinking capability", func(t *testing.T) {
+		think := true
+		w := createRequest(t, s.ChatHandler, api.ChatRequest{
+			Model: "test",
+			Messages: []api.Message{
+				{Role: "user", Content: "Hello!"},
+			},
+			Think: &think,
+		})
+
+		if w.Code != http.StatusBadRequest {
+			t.Errorf("expected status 400, got %d", w.Code)
+		}
+
+		if diff := cmp.Diff(w.Body.String(), `{"error":"registry.ollama.ai/library/test:latest does not support thinking"}`); diff != "" {
+			t.Errorf("mismatch (-got +want):\n%s", diff)
+		}
+	})
+
 	t.Run("missing model", func(t *testing.T) {
 		w := createRequest(t, s.ChatHandler, api.ChatRequest{})
 		if w.Code != http.StatusBadRequest {

server/thinking.go

@@ -0,0 +1,256 @@
+package server
+
+import (
+	"strings"
+	"text/template"
+	"text/template/parse"
+	"unicode"
+)
+
+type thinkingParseState int
+
+const (
+	thinkingParseState_LookingForOpening thinkingParseState = iota
+	thinkingParseState_Thinking
+	thinkingParseState_ThinkingDone
+)
+
+func (s thinkingParseState) String() string {
+	switch s {
+	case thinkingParseState_LookingForOpening:
+		return "LookingForOpening"
+	case thinkingParseState_Thinking:
+		return "Thinking"
+	case thinkingParseState_ThinkingDone:
+		return "ThinkingDone"
+	default:
+		return "Unknown"
+	}
+}
+
+type thinkingParser struct {
+	state      thinkingParseState
+	openingTag string
+	closingTag string
+	acc        strings.Builder
+}
+
+// returns the thinking content and the normal content that should be
+// immediately sent to the user. It will internally buffer if it needs to see
+// more content to disambiguate
+func (s *thinkingParser) addContent(content string) (string, string) {
+	s.acc.WriteString(content)
+
+	var thinkingAcc, remainingAcc strings.Builder
+
+	var thinking, remaining string
+	keepLooping := true
+	// we loop because we might pass through multiple parsing states in a single
+	// call to addContent, and we want to make sure callers don't have to wait for
+	// data that's already unambiguous
+	for keepLooping {
+		thinking, remaining, keepLooping = eat(s)
+		thinkingAcc.WriteString(thinking)
+		remainingAcc.WriteString(remaining)
+	}
+
+	return thinkingAcc.String(), remainingAcc.String()
+}
+
+// the additional bool return is true iff we should continue eating
+func eat(s *thinkingParser) (string, string, bool) {
+	switch s.state {
+	case thinkingParseState_LookingForOpening:
+		trimmed := strings.TrimLeftFunc(s.acc.String(), unicode.IsSpace)
+		if strings.HasPrefix(trimmed, s.openingTag) {
+			after := strings.Join(strings.Split(trimmed, s.openingTag)[1:], s.openingTag)
+			after = strings.TrimLeftFunc(after, unicode.IsSpace)
+			// after might contain more than just thinking tokens, so we continue
+			// parsing instead of returning it as thinking tokens here
+			s.acc.Reset()
+			s.acc.WriteString(after)
+			s.state = thinkingParseState_Thinking
+			return "", "", true
+		} else if strings.HasPrefix(s.openingTag, trimmed) {
+			// partial opening seen, so let's keep accumulating
+			return "", "", false
+		} else if trimmed == "" {
+			// saw whitespace only, so let's keep accumulating
+			return "", "", false
+		} else {
+			// didn't see an opening tag, but we have content, so thinking was skipped
+			s.state = thinkingParseState_ThinkingDone
+			// note that we use the original content, not the trimmed one because we
+			// don't want to eat any whitespace in the real content if there were no
+			// thinking tags
+			return "", s.acc.String(), false
+		}
+	case thinkingParseState_Thinking:
+		acc := s.acc.String()
+		if strings.Contains(acc, s.closingTag) {
+			split := strings.Split(acc, s.closingTag)
+			thinking := split[0]
+			remaining := strings.Join(split[1:], s.closingTag)
+			remaining = strings.TrimLeftFunc(remaining, unicode.IsSpace)
+			s.acc.Reset()
+			s.state = thinkingParseState_ThinkingDone
+			return thinking, remaining, false
+		} else if overlapLen := overlap(acc, s.closingTag); overlapLen > 0 {
+			thinking := acc[:len(acc)-overlapLen]
+			remaining := acc[len(acc)-overlapLen:]
+			s.acc.Reset()
+			// keep track of the candidate closing tag. We have to buffer it until it
+			// becomes disambiguated
+			s.acc.WriteString(remaining)
+			return thinking, "", false
+		} else {
+			// purely just thinking tokens, so we can return them
+			s.acc.Reset()
+			return acc, "", false
+		}
+	case thinkingParseState_ThinkingDone:
+		acc := s.acc.String()
+		s.acc.Reset()
+		return "", acc, false
+	default:
+		panic("unknown state")
+	}
+}
+
+// longest overlap between suffix of s and prefix of delim
+func overlap(s, delim string) int {
+	max := min(len(delim), len(s))
+	for i := max; i > 0; i-- {
+		if strings.HasSuffix(s, delim[:i]) {
+			return i
+		}
+	}
+	return 0
+}
+
+func templateVisit(n parse.Node, enterFn func(parse.Node) bool, exitFn func(parse.Node)) {
+	if n == nil {
+		return
+	}
+	shouldContinue := enterFn(n)
+	if !shouldContinue {
+		return
+	}
+	switch x := n.(type) {
+	case *parse.ListNode:
+		for _, c := range x.Nodes {
+			templateVisit(c, enterFn, exitFn)
+		}
+	case *parse.BranchNode:
+		if x.Pipe != nil {
+			templateVisit(x.Pipe, enterFn, exitFn)
+		}
+		if x.List != nil {
+			templateVisit(x.List, enterFn, exitFn)
+		}
+		if x.ElseList != nil {
+			templateVisit(x.ElseList, enterFn, exitFn)
+		}
+	case *parse.ActionNode:
+		templateVisit(x.Pipe, enterFn, exitFn)
+	case *parse.WithNode:
+		templateVisit(&x.BranchNode, enterFn, exitFn)
+	case *parse.RangeNode:
+		templateVisit(&x.BranchNode, enterFn, exitFn)
+	case *parse.IfNode:
+		templateVisit(&x.BranchNode, enterFn, exitFn)
+	case *parse.TemplateNode:
+		templateVisit(x.Pipe, enterFn, exitFn)
+	case *parse.PipeNode:
+		for _, c := range x.Cmds {
+			templateVisit(c, enterFn, exitFn)
+		}
+	case *parse.CommandNode:
+		for _, a := range x.Args {
+			templateVisit(a, enterFn, exitFn)
+		}
+		// text, field, number, etc. are leaves – nothing to recurse into
+	}
+	if exitFn != nil {
+		exitFn(n)
+	}
+}
+
+// We use a heuristic to infer the tags that surround thinking traces:
+// We look for a range node that iterates over "Messages" and then look for a
+// reference to "Thinking" like `{{.Thinking}}`. We then go up to the nearest
+// ListNode and take the first and last TextNodes as the opening and closing
+// tags.
+func inferThinkingTags(t *template.Template) (string, string) {
+	ancestors := []parse.Node{}
+
+	openingTag := ""
+	closingTag := ""
+
+	enterFn := func(n parse.Node) bool {
+		ancestors = append(ancestors, n)
+
+		switch x := n.(type) {
+		case *parse.FieldNode:
+			if len(x.Ident) > 0 && x.Ident[0] == "Thinking" {
+				var mostRecentRange *parse.RangeNode
+				for i := len(ancestors) - 1; i >= 0; i-- {
+					if r, ok := ancestors[i].(*parse.RangeNode); ok {
+						mostRecentRange = r
+						break
+					}
+				}
+				if mostRecentRange == nil || !rangeUsesField(mostRecentRange, "Messages") {
+					return true
+				}
+
+				// TODO(drifkin): to be more robust, check that it's in the action
+				// part, not the `if`'s pipeline part. We do match on the nearest list
+				// that starts and ends with text nodes, which makes this not strictly
+				// necessary for our heuristic
+
+				// go up to the nearest ancestor that is a *parse.ListNode
+				for i := len(ancestors) - 1; i >= 0; i-- {
+					if l, ok := ancestors[i].(*parse.ListNode); ok {
+						firstNode := l.Nodes[0]
+						if t, ok := firstNode.(*parse.TextNode); ok {
+							openingTag = strings.TrimSpace(t.String())
+						}
+						lastNode := l.Nodes[len(l.Nodes)-1]
+						if t, ok := lastNode.(*parse.TextNode); ok {
+							closingTag = strings.TrimSpace(t.String())
+						}
+
+						break
+					}
+				}
+			}
+		}
+
+		return true
+	}
+
+	exitFn := func(n parse.Node) {
+		ancestors = ancestors[:len(ancestors)-1]
+	}
+
+	templateVisit(t.Root, enterFn, exitFn)
+
+	return openingTag, closingTag
+}
+
+// checks to see if the given field name is present in the pipeline of the given range node
+func rangeUsesField(rangeNode *parse.RangeNode, field string) bool {
+	found := false
+	enterFn := func(n parse.Node) bool {
+		switch x := n.(type) {
+		case *parse.FieldNode:
+			if x.Ident[0] == field {
+				found = true
+			}
+		}
+		return true
+	}
+	templateVisit(rangeNode.BranchNode.Pipe, enterFn, nil)
+	return found
+}

server/thinking_test.go

@@ -0,0 +1,286 @@
+package server
+
+import (
+	"testing"
+	"text/template"
+)
+
+func TestExtractThinking(t *testing.T) {
+	tests := []struct {
+		in, wantContent, wantThink string
+	}{
+		{
+			in:          "<think> internal </think> world",
+			wantThink:   "internal ",
+			wantContent: "world",
+		},
+		{
+			in:          "<think>a</think><think>b</think>c",
+			wantThink:   "a",
+			wantContent: "<think>b</think>c",
+		},
+		{
+			in:          "no think",
+			wantThink:   "",
+			wantContent: "no think",
+		},
+	}
+	for i, tt := range tests {
+		gotThinking, gotContent := extractThinking(tt.in)
+		if gotContent != tt.wantContent || gotThinking != tt.wantThink {
+			t.Errorf("case %d: got (%q,%q), want (%q,%q)", i, gotThinking, gotContent, tt.wantThink, tt.wantContent)
+		}
+	}
+}
+
+func TestThinkingStreaming(t *testing.T) {
+
+	type step struct {
+		input          string
+		wantThinking   string
+		wantContent    string
+		wantStateAfter thinkingParseState
+	}
+
+	cases := []struct {
+		desc  string
+		skip  bool
+		steps []step
+	}{
+		{
+			desc: "content without a thinking tag",
+			steps: []step{
+				{
+					input:          "  abc",
+					wantThinking:   "",
+					wantContent:    "  abc",
+					wantStateAfter: thinkingParseState_ThinkingDone,
+				},
+			},
+		},
+		{
+			desc: "content before a thinking tag nerfs the thinking tag",
+			steps: []step{
+				{
+					input:          "  abc <think>def</think> ghi",
+					wantThinking:   "",
+					wantContent:    "  abc <think>def</think> ghi",
+					wantStateAfter: thinkingParseState_ThinkingDone,
+				},
+			},
+		},
+		{
+			desc: "building up a thinking tag partially",
+			// skip: true,
+			steps: []step{
+				{
+					input:          "  <th",
+					wantThinking:   "",
+					wantContent:    "",
+					wantStateAfter: thinkingParseState_LookingForOpening,
+				},
+				{
+					input:          "in",
+					wantThinking:   "",
+					wantContent:    "",
+					wantStateAfter: thinkingParseState_LookingForOpening,
+				},
+				{
+					input:          "k>a",
+					wantThinking:   "a",
+					wantContent:    "",
+					wantStateAfter: thinkingParseState_Thinking,
+				},
+			},
+		},
+		{
+			desc: "partial closing tag",
+			steps: []step{
+				{
+					input:          "<think>abc</th",
+					wantThinking:   "abc",
+					wantContent:    "",
+					wantStateAfter: thinkingParseState_Thinking,
+				},
+				{
+					input:          "ink>def",
+					wantThinking:   "",
+					wantContent:    "def",
+					wantStateAfter: thinkingParseState_ThinkingDone,
+				},
+			},
+		},
+		{
+			desc: "partial closing tag fakeout",
+			steps: []step{
+				{
+					input:          "<think>abc</th",
+					wantThinking:   "abc",
+					wantContent:    "",
+					wantStateAfter: thinkingParseState_Thinking,
+				},
+				{
+					input:          "ing>def",
+					wantThinking:   "</thing>def",
+					wantContent:    "",
+					wantStateAfter: thinkingParseState_Thinking,
+				},
+				{
+					input:          "ghi</thi",
+					wantThinking:   "ghi",
+					wantContent:    "",
+					wantStateAfter: thinkingParseState_Thinking,
+				},
+				{
+					input:          "nk>jkl",
+					wantThinking:   "",
+					wantContent:    "jkl",
+					wantStateAfter: thinkingParseState_ThinkingDone,
+				},
+			},
+		},
+	}
+
+	for _, c := range cases {
+		parser := thinkingParser{
+			openingTag: "<think>",
+			closingTag: "</think>",
+		}
+		if c.skip {
+			continue
+		}
+		for i, step := range c.steps {
+			thinking, content := parser.addContent(step.input)
+			if content != step.wantContent || thinking != step.wantThinking {
+				t.Errorf("case %q (step %d): got (%q,%q), want (%q,%q)", c.desc, i, content, thinking, step.wantContent, step.wantThinking)
+			}
+			if parser.state != step.wantStateAfter {
+				t.Errorf("case %q (step %d): got state %s, want %s", c.desc, i, parser.state.String(), step.wantStateAfter.String())
+			}
+		}
+	}
+}
+
+func TestInferThinkingTags(t *testing.T) {
+	cases := []struct {
+		desc           string
+		tmplString     string
+		wantOpeningTag string
+		wantClosingTag string
+	}{
+		{
+			desc: "basic",
+			tmplString: `
+			{{ if .Thinking}}
+				/think
+			{{ end }}
+			{{- range $i, $_ := .Messages }}
+				{{- $last := eq (len (slice $.Messages $i)) 1 -}}
+				{{ if and $last .Thinking }}
+					<think>{{ .Thinking }}</think>
+				{{ end }}
+			{{ end }}
+		`,
+			wantOpeningTag: "<think>",
+			wantClosingTag: "</think>",
+		},
+		{
+			desc: "doubly nested range",
+			tmplString: `
+			{{ if .Thinking}}
+				/think
+			{{ end }}
+			{{- range $i, $_ := .Messages }}
+				{{- range $j, $_ := .NotMessages }}
+					{{- $last := eq (len (slice $.Messages $i)) 1 -}}
+					{{ if and $last .Thinking }}
+						<think>{{ .Thinking }}</think>
+					{{ end }}
+				{{ end }}
+			{{ end }}
+		`,
+			wantOpeningTag: "",
+			wantClosingTag: "",
+		},
+		{
+			desc: "whitespace is trimmed",
+			tmplString: `
+			{{ if .Thinking}}
+				/think
+			{{ end }}
+			{{- range $i, $_ := .Messages }}
+				{{- $last := eq (len (slice $.Messages $i)) 1 -}}
+				{{ if and $last .Thinking }}
+					Some text before   {{ .Thinking }}    Some text after
+				{{ end }}
+			{{ end }}
+		`,
+			wantOpeningTag: "Some text before",
+			wantClosingTag: "Some text after",
+		},
+		{
+			desc: "qwen3",
+			tmplString: `
+{{- if or .System .Tools .Thinking }}<|im_start|>system
+{{- if .System }}
+{{ .System }}
+{{- end }}
+{{- if .Tools }}
+
+# Tools
+
+You may call one or more functions to assist with the user query.
+
+You are provided with function signatures within <tools></tools> XML tags:
+<tools>
+{{- range .Tools }}
+{"type": "function", "function": {{ .Function }}}
+{{- end }}
+</tools>
+
+For each function call, return a json object with function name and arguments within <tool_call></tool_call> XML tags:
+<tool_call>
+{"name": <function-name>, "arguments": <args-json-object>}
+</tool_call>
+{{- end }}
+{{- if .Thinking }}
+/think
+{{- else }}
+/no_think
+{{- end }}<|im_end|>
+{{ end }}
+{{- range $i, $_ := .Messages }}
+{{- $last := eq (len (slice $.Messages $i)) 1 -}}
+{{- if eq .Role "user" }}<|im_start|>user
+{{ .Content }}<|im_end|>
+{{ else if eq .Role "assistant" }}<|im_start|>assistant
+{{ if and $last .Thinking }}
+<think>{{ .Thinking }}</think>
+{{ end }}
+{{ if .Content }}{{ .Content }}
+{{- else if .ToolCalls }}<tool_call>
+{{ range .ToolCalls }}{"name": "{{ .Function.Name }}", "arguments": {{ .Function.Arguments }}}
+{{ end }}</tool_call>
+{{- end }}{{ if not $last }}<|im_end|>
+{{ end }}
+{{- else if eq .Role "tool" }}<|im_start|>user
+<tool_response>
+{{ .Content }}
+</tool_response><|im_end|>
+{{ end }}
+{{- if and (ne .Role "assistant") $last }}<|im_start|>assistant
+{{ end }}
+{{- end }}
+			`,
+			wantOpeningTag: "<think>",
+			wantClosingTag: "</think>",
+		},
+	}
+	for _, c := range cases {
+		tmpl := template.Must(template.New("test").Parse(c.tmplString))
+		openingTag, closingTag := inferThinkingTags(tmpl)
+		if openingTag != c.wantOpeningTag || closingTag != c.wantClosingTag {
+			t.Errorf("case %q: got (%q,%q), want (%q,%q)", c.desc, openingTag, closingTag, c.wantOpeningTag, c.wantClosingTag)
+		}
+	}
+}

template/pretty.go

@@ -0,0 +1,101 @@
+package template
+
+import (
+	"strings"
+	texttmpl "text/template"
+	"text/template/parse"
+)
+
+// Format returns a human-readable representation of the template.
+// The formatting indents nested sections such as if/else blocks.
+func Format(src string) (string, error) {
+	tmpl, err := texttmpl.New("pretty").Parse(src)
+	if err != nil {
+		return "", err
+	}
+	var sb strings.Builder
+	printNodes(tmpl.Tree.Root, 0, &sb)
+	return sb.String(), nil
+}
+
+func indent(sb *strings.Builder, level int) {
+	for i := 0; i < level; i++ {
+		sb.WriteString("  ")
+	}
+}
+
+func printNodes(list *parse.ListNode, level int, sb *strings.Builder) {
+	if list == nil {
+		return
+	}
+	for _, n := range list.Nodes {
+		printNode(n, level, sb)
+	}
+}
+
+func printNode(n parse.Node, level int, sb *strings.Builder) {
+	switch n := n.(type) {
+	case *parse.TextNode:
+		text := strings.TrimSpace(string(n.Text))
+		if text == "" {
+			return
+		}
+		indent(sb, level)
+		sb.WriteString(text)
+		sb.WriteByte('\n')
+	case *parse.ActionNode:
+		indent(sb, level)
+		// sb.WriteString("ACTION {{ ")
+		sb.WriteString(n.String())
+		// sb.WriteString(" }}\n")
+		sb.WriteByte('\n')
+	case *parse.IfNode:
+		indent(sb, level)
+		sb.WriteString("{{ if ")
+		sb.WriteString(n.Pipe.String())
+		sb.WriteString(" }}\n")
+		printNodes(n.List, level+1, sb)
+		if n.ElseList != nil {
+			indent(sb, level)
+			sb.WriteString("{{ else }}\n")
+			printNodes(n.ElseList, level+1, sb)
+		}
+		indent(sb, level)
+		sb.WriteString("{{ end }}\n")
+	case *parse.RangeNode:
+		indent(sb, level)
+		sb.WriteString("{{ range ")
+		sb.WriteString(n.Pipe.String())
+		sb.WriteString(" }}\n")
+		printNodes(n.List, level+1, sb)
+		if n.ElseList != nil {
+			indent(sb, level)
+			sb.WriteString("{{ else }}\n")
+			printNodes(n.ElseList, level+1, sb)
+		}
+		indent(sb, level)
+		sb.WriteString("{{ end }}\n")
+	case *parse.WithNode:
+		indent(sb, level)
+		sb.WriteString("{{ with ")
+		sb.WriteString(n.Pipe.String())
+		sb.WriteString(" }}\n")
+		printNodes(n.List, level+1, sb)
+		if n.ElseList != nil {
+			indent(sb, level)
+			sb.WriteString("{{ else }}\n")
+			printNodes(n.ElseList, level+1, sb)
+		}
+		indent(sb, level)
+		sb.WriteString("{{ end }}\n")
+	case *parse.TemplateNode:
+		indent(sb, level)
+		sb.WriteString("{{ template ")
+		sb.WriteString(n.Name)
+		sb.WriteString(" }}\n")
+	default:
+		indent(sb, level)
+		sb.WriteString(n.String())
+		sb.WriteByte('\n')
+	}
+}

template/pretty_test.go

@@ -0,0 +1,30 @@
+package template
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestFormatIndentation(t *testing.T) {
+	tmpl := "{{ if .Cond }}A{{ else }}B{{ end }}"
+	out, err := Format(tmpl)
+	if err != nil {
+		t.Fatal(err)
+	}
+	expectedLines := []string{
+		"{{ if .Cond }}",
+		"  A",
+		"{{ else }}",
+		"  B",
+		"{{ end }}",
+	}
+	got := strings.Split(strings.TrimSpace(out), "\n")
+	if len(got) != len(expectedLines) {
+		t.Fatalf("expected %d lines, got %d: %q", len(expectedLines), len(got), out)
+	}
+	for i, line := range expectedLines {
+		if strings.TrimSpace(got[i]) != strings.TrimSpace(line) {
+			t.Errorf("line %d = %q, want %q", i, got[i], line)
+		}
+	}
+}

template/template.go

@@ -167,6 +167,10 @@ type Values struct {
 	api.Tools
 	Prompt string
 	Suffix string
+	Think  bool
+	// whether or not the user explicitly set the thinking flag (vs. it being
+	// implicitly false). Templates can't see whether `Think` is nil
+	IsThinkSet bool
 
 	// forceLegacy is a flag used to test compatibility with legacy templates
 	forceLegacy bool
@@ -225,6 +229,8 @@ func (t *Template) Execute(w io.Writer, v Values) error {
 			"Prompt":     v.Prompt,
 			"Suffix":     v.Suffix,
 			"Response":   "",
+			"Think":      v.Think,
+			"IsThinkSet": v.IsThinkSet,
 		})
 	} else if !v.forceLegacy && slices.Contains(t.Vars(), "messages") {
 		return t.Template.Execute(w, map[string]any{
@@ -232,6 +238,8 @@ func (t *Template) Execute(w io.Writer, v Values) error {
 			"Messages":   messages,
 			"Tools":      v.Tools,
 			"Response":   "",
+			"Think":      v.Think,
+			"IsThinkSet": v.IsThinkSet,
 		})
 	}
 
@@ -244,6 +252,8 @@ func (t *Template) Execute(w io.Writer, v Values) error {
 				"System":     system,
 				"Prompt":     prompt,
 				"Response":   response,
+				"Think":      v.Think,
+				"IsThinkSet": v.IsThinkSet,
 			}); err != nil {
 				return err
 			}
@@ -289,6 +299,8 @@ func (t *Template) Execute(w io.Writer, v Values) error {
 		"System":     system,
 		"Prompt":     prompt,
 		"Response":   response,
+		"Think":      v.Think,
+		"IsThinkSet": v.IsThinkSet,
 	}); err != nil {
 		return err
 	}

types/model/capability.go

@@ -8,6 +8,7 @@ const (
 	CapabilityInsert     = Capability("insert")
 	CapabilityVision     = Capability("vision")
 	CapabilityEmbedding  = Capability("embedding")
+	CapabilityThinking   = Capability("thinking")
 )
 
 func (c Capability) String() string {