fix: address PR review feedback for PicoLM provider

- Add os.Stat validation on binary path in constructor
- Return error from NewPicoLMProvider for invalid config
- Inject tool definitions into system prompt via buildToolsPrompt
- Fix stdout-on-error
- Remove unused template config field
- Fix expandHome to propagate os.UserHomeDir errors
- Add unit tests

pkg/config/config.go

@@ -271,7 +271,6 @@ type PicoLMProviderConfig struct {
 	Model     string `json:"model" env:"PICOCLAW_PROVIDERS_PICOLM_MODEL"`
 	MaxTokens int    `json:"max_tokens" env:"PICOCLAW_PROVIDERS_PICOLM_MAX_TOKENS"`
 	Threads   int    `json:"threads" env:"PICOCLAW_PROVIDERS_PICOLM_THREADS"`
-	Template  string `json:"template" env:"PICOCLAW_PROVIDERS_PICOLM_TEMPLATE"`
 }
 
 type ProviderConfig struct {

pkg/providers/factory.go

@@ -361,7 +361,7 @@ func CreateProvider(cfg *config.Config) (LLMProvider, error) {
 	case providerTypeGitHubCopilot:
 		return NewGitHubCopilotProvider(sel.apiBase, sel.connectMode, sel.model)
 	case providerTypePicoLM:
-		return NewPicoLMProvider(cfg.Providers.PicoLM), nil
+		return NewPicoLMProvider(cfg.Providers.PicoLM)
 	default:
 		return NewHTTPProvider(sel.apiKey, sel.apiBase, sel.proxy), nil
 	}

pkg/providers/picolm_provider.go

@@ -3,6 +3,7 @@ package providers
 import (
 	"bytes"
 	"context"
+	"encoding/json"
 	"fmt"
 	"os"
 	"os/exec"
@@ -17,13 +18,33 @@ type PicoLMProvider struct {
 	model     string
 	maxTokens int
 	threads   int
-	template  string
 }
 
 // NewPicoLMProvider creates a new PicoLM provider from config.
-func NewPicoLMProvider(cfg config.PicoLMProviderConfig) *PicoLMProvider {
-	binary := expandHome(cfg.Binary)
-	model := expandHome(cfg.Model)
+// Returns an error if the binary path does not point to an executable file.
+func NewPicoLMProvider(cfg config.PicoLMProviderConfig) (*PicoLMProvider, error) {
+	binary, err := expandHome(cfg.Binary)
+	if err != nil {
+		return nil, fmt.Errorf("picolm: failed to expand binary path: %w", err)
+	}
+	model, err := expandHome(cfg.Model)
+	if err != nil {
+		return nil, fmt.Errorf("picolm: failed to expand model path: %w", err)
+	}
+
+	if binary != "" {
+		info, err := os.Stat(binary)
+		if err != nil {
+			return nil, fmt.Errorf("picolm: binary not found at %q: %w", binary, err)
+		}
+		if info.IsDir() {
+			return nil, fmt.Errorf("picolm: binary path %q is a directory, not an executable", binary)
+		}
+		if info.Mode()&0111 == 0 {
+			return nil, fmt.Errorf("picolm: binary %q is not executable", binary)
+		}
+	}
+
 	maxTokens := cfg.MaxTokens
 	if maxTokens <= 0 {
 		maxTokens = 256
@@ -32,17 +53,12 @@ func NewPicoLMProvider(cfg config.PicoLMProviderConfig) *PicoLMProvider {
 	if threads <= 0 {
 		threads = 4
 	}
-	template := cfg.Template
-	if template == "" {
-		template = "chatml"
-	}
 	return &PicoLMProvider{
 		binary:    binary,
 		model:     model,
 		maxTokens: maxTokens,
 		threads:   threads,
-		template:  template,
-	}
+	}, nil
 }
 
 // Chat implements LLMProvider.Chat by executing the picolm binary.
@@ -74,9 +90,24 @@ func (p *PicoLMProvider) Chat(ctx context.Context, messages []Message, tools []T
 	cmd.Stderr = &stderr
 
 	err := cmd.Run()
+	if err != nil {
+		if ctx.Err() == context.Canceled {
+			return nil, ctx.Err()
+		}
+		if stderrStr := stderr.String(); stderrStr != "" {
+			return nil, fmt.Errorf("picolm error: %s", stderrStr)
+		}
+		return nil, fmt.Errorf("picolm error: %w", err)
+	}
+
+	output := strings.TrimSpace(stdout.String())
+	if output == "" {
+		return &LLMResponse{
+			Content:      "",
+			FinishReason: "stop",
+		}, nil
+	}
 
-	// Try to parse stdout even on non-zero exit, as picolm may print diagnostics to stderr.
-	if output := strings.TrimSpace(stdout.String()); output != "" {
 	toolCalls := extractToolCallsFromText(output)
 	finishReason := "stop"
 	content := output
@@ -89,22 +120,6 @@ func (p *PicoLMProvider) Chat(ctx context.Context, messages []Message, tools []T
 		ToolCalls:    toolCalls,
 		FinishReason: finishReason,
 	}, nil
-	}
-
-	if err != nil {
-		if ctx.Err() == context.Canceled {
-			return nil, ctx.Err()
-		}
-		if stderrStr := stderr.String(); stderrStr != "" {
-			return nil, fmt.Errorf("picolm error: %s", stderrStr)
-		}
-		return nil, fmt.Errorf("picolm error: %w", err)
-	}
-
-	return &LLMResponse{
-		Content:      "",
-		FinishReason: "stop",
-	}, nil
 }
 
 // GetDefaultModel returns the default model identifier.
@@ -116,12 +131,16 @@ func (p *PicoLMProvider) GetDefaultModel() string {
 func (p *PicoLMProvider) buildPrompt(messages []Message, tools []ToolDefinition) string {
 	var sb strings.Builder
 
+	// Collect system message parts and append tool definitions.
 	var systemParts []string
 	for _, msg := range messages {
 		if msg.Role == "system" {
 			systemParts = append(systemParts, msg.Content)
 		}
 	}
+	if len(tools) > 0 {
+		systemParts = append(systemParts, buildToolsPrompt(tools))
+	}
 
 	if len(systemParts) > 0 {
 		sb.WriteString("<|system|>\n")
@@ -152,17 +171,50 @@ func (p *PicoLMProvider) buildPrompt(messages []Message, tools []ToolDefinition)
 	return sb.String()
 }
 
+// buildToolsPrompt creates the tool definitions section for injection into the system prompt.
+func buildToolsPrompt(tools []ToolDefinition) string {
+	var sb strings.Builder
+
+	sb.WriteString("## Available Tools\n\n")
+	sb.WriteString("When you need to use a tool, respond with ONLY a JSON object:\n\n")
+	sb.WriteString("```json\n")
+	sb.WriteString(`{"tool_calls":[{"id":"call_xxx","type":"function","function":{"name":"tool_name","arguments":"{...}"}}]}`)
+	sb.WriteString("\n```\n\n")
+	sb.WriteString("CRITICAL: The 'arguments' field MUST be a JSON-encoded STRING.\n\n")
+	sb.WriteString("### Tool Definitions:\n\n")
+
+	for _, tool := range tools {
+		if tool.Type != "function" {
+			continue
+		}
+		sb.WriteString(fmt.Sprintf("#### %s\n", tool.Function.Name))
+		if tool.Function.Description != "" {
+			sb.WriteString(fmt.Sprintf("Description: %s\n", tool.Function.Description))
+		}
+		if len(tool.Function.Parameters) > 0 {
+			paramsJSON, _ := json.Marshal(tool.Function.Parameters)
+			sb.WriteString(fmt.Sprintf("Parameters:\n```json\n%s\n```\n", string(paramsJSON)))
+		}
+		sb.WriteString("\n")
+	}
+
+	return sb.String()
+}
+
 // expandHome expands ~ to the user's home directory.
-func expandHome(path string) string {
+func expandHome(path string) (string, error) {
 	if path == "" {
-		return path
+		return path, nil
 	}
 	if path[0] == '~' {
-		home, _ := os.UserHomeDir()
+		home, err := os.UserHomeDir()
+		if err != nil {
+			return "", fmt.Errorf("failed to resolve home directory: %w", err)
+		}
 		if len(path) > 1 && path[1] == '/' {
-			return home + path[1:]
+			return home + path[1:], nil
 		}
-		return home
+		return home, nil
 	}
-	return path
+	return path, nil
 }

pkg/providers/picolm_provider_test.go

@@ -0,0 +1,676 @@
+package providers
+
+import (
+	"context"
+	"fmt"
+	"os"
+	"path/filepath"
+	"runtime"
+	"strings"
+	"testing"
+	"time"
+
+	"github.com/sipeed/picoclaw/pkg/config"
+)
+
+// --- Compile-time interface check ---
+
+var _ LLMProvider = (*PicoLMProvider)(nil)
+
+// --- Helper: create mock picolm binary ---
+
+// createMockPicoLM creates a temporary script that simulates the picolm binary.
+// It reads stdin (the prompt) and writes stdout/stderr, exiting with the given code.
+func createMockPicoLM(t *testing.T, stdout, stderr string, exitCode int) string {
+	t.Helper()
+	if runtime.GOOS == "windows" {
+		t.Skip("mock CLI scripts not supported on Windows")
+	}
+
+	dir := t.TempDir()
+
+	if stdout != "" {
+		if err := os.WriteFile(filepath.Join(dir, "stdout.txt"), []byte(stdout), 0644); err != nil {
+			t.Fatal(err)
+		}
+	}
+	if stderr != "" {
+		if err := os.WriteFile(filepath.Join(dir, "stderr.txt"), []byte(stderr), 0644); err != nil {
+			t.Fatal(err)
+		}
+	}
+
+	var sb strings.Builder
+	sb.WriteString("#!/bin/sh\n")
+	// Consume stdin to avoid broken pipe
+	sb.WriteString("cat > /dev/null\n")
+	if stderr != "" {
+		sb.WriteString(fmt.Sprintf("cat '%s/stderr.txt' >&2\n", dir))
+	}
+	if stdout != "" {
+		sb.WriteString(fmt.Sprintf("cat '%s/stdout.txt'\n", dir))
+	}
+	sb.WriteString(fmt.Sprintf("exit %d\n", exitCode))
+
+	script := filepath.Join(dir, "picolm")
+	if err := os.WriteFile(script, []byte(sb.String()), 0755); err != nil {
+		t.Fatal(err)
+	}
+	return script
+}
+
+// createStdinCaptureMockPicoLM creates a mock that captures stdin to a file.
+func createStdinCaptureMockPicoLM(t *testing.T, captureFile string, stdout string) string {
+	t.Helper()
+	if runtime.GOOS == "windows" {
+		t.Skip("mock CLI scripts not supported on Windows")
+	}
+
+	dir := t.TempDir()
+	if stdout != "" {
+		if err := os.WriteFile(filepath.Join(dir, "stdout.txt"), []byte(stdout), 0644); err != nil {
+			t.Fatal(err)
+		}
+	}
+
+	var sb strings.Builder
+	sb.WriteString("#!/bin/sh\n")
+	sb.WriteString(fmt.Sprintf("cat > '%s'\n", captureFile))
+	if stdout != "" {
+		sb.WriteString(fmt.Sprintf("cat '%s/stdout.txt'\n", dir))
+	}
+	sb.WriteString("exit 0\n")
+
+	script := filepath.Join(dir, "picolm")
+	if err := os.WriteFile(script, []byte(sb.String()), 0755); err != nil {
+		t.Fatal(err)
+	}
+	return script
+}
+
+// createSlowMockPicoLM creates a script that sleeps before responding.
+func createSlowMockPicoLM(t *testing.T, sleepSeconds int) string {
+	t.Helper()
+	if runtime.GOOS == "windows" {
+		t.Skip("mock CLI scripts not supported on Windows")
+	}
+
+	dir := t.TempDir()
+	script := filepath.Join(dir, "picolm")
+	content := fmt.Sprintf("#!/bin/sh\ncat > /dev/null\nsleep %d\necho 'late response'\n", sleepSeconds)
+	if err := os.WriteFile(script, []byte(content), 0755); err != nil {
+		t.Fatal(err)
+	}
+	return script
+}
+
+// --- Constructor tests ---
+
+func TestNewPicoLMProvider(t *testing.T) {
+	binary := createMockPicoLM(t, "", "", 0)
+	p, err := NewPicoLMProvider(config.PicoLMProviderConfig{
+		Binary:    binary,
+		Model:     "/tmp/model.gguf",
+		MaxTokens: 128,
+		Threads:   2,
+	})
+	if err != nil {
+		t.Fatalf("NewPicoLMProvider() error = %v", err)
+	}
+	if p.binary != binary {
+		t.Errorf("binary = %q, want %q", p.binary, binary)
+	}
+	if p.maxTokens != 128 {
+		t.Errorf("maxTokens = %d, want 128", p.maxTokens)
+	}
+	if p.threads != 2 {
+		t.Errorf("threads = %d, want 2", p.threads)
+	}
+}
+
+func TestNewPicoLMProvider_Defaults(t *testing.T) {
+	binary := createMockPicoLM(t, "", "", 0)
+	p, err := NewPicoLMProvider(config.PicoLMProviderConfig{
+		Binary: binary,
+		Model:  "/tmp/model.gguf",
+	})
+	if err != nil {
+		t.Fatalf("NewPicoLMProvider() error = %v", err)
+	}
+	if p.maxTokens != 256 {
+		t.Errorf("maxTokens = %d, want 256 (default)", p.maxTokens)
+	}
+	if p.threads != 4 {
+		t.Errorf("threads = %d, want 4 (default)", p.threads)
+	}
+}
+
+func TestNewPicoLMProvider_BinaryNotFound(t *testing.T) {
+	_, err := NewPicoLMProvider(config.PicoLMProviderConfig{
+		Binary: "/nonexistent/path/picolm",
+		Model:  "/tmp/model.gguf",
+	})
+	if err == nil {
+		t.Fatal("expected error for nonexistent binary")
+	}
+	if !strings.Contains(err.Error(), "binary not found") {
+		t.Errorf("error = %q, want to contain 'binary not found'", err.Error())
+	}
+}
+
+func TestNewPicoLMProvider_BinaryIsDirectory(t *testing.T) {
+	dir := t.TempDir()
+	_, err := NewPicoLMProvider(config.PicoLMProviderConfig{
+		Binary: dir,
+		Model:  "/tmp/model.gguf",
+	})
+	if err == nil {
+		t.Fatal("expected error when binary is a directory")
+	}
+	if !strings.Contains(err.Error(), "is a directory") {
+		t.Errorf("error = %q, want to contain 'is a directory'", err.Error())
+	}
+}
+
+func TestNewPicoLMProvider_BinaryNotExecutable(t *testing.T) {
+	if runtime.GOOS == "windows" {
+		t.Skip("executable bit not meaningful on Windows")
+	}
+	dir := t.TempDir()
+	notExec := filepath.Join(dir, "picolm")
+	if err := os.WriteFile(notExec, []byte("not executable"), 0644); err != nil {
+		t.Fatal(err)
+	}
+	_, err := NewPicoLMProvider(config.PicoLMProviderConfig{
+		Binary: notExec,
+		Model:  "/tmp/model.gguf",
+	})
+	if err == nil {
+		t.Fatal("expected error for non-executable binary")
+	}
+	if !strings.Contains(err.Error(), "not executable") {
+		t.Errorf("error = %q, want to contain 'not executable'", err.Error())
+	}
+}
+
+func TestNewPicoLMProvider_EmptyBinaryAllowed(t *testing.T) {
+	// Empty binary is allowed at construction time (caught at Chat time)
+	p, err := NewPicoLMProvider(config.PicoLMProviderConfig{
+		Model: "/tmp/model.gguf",
+	})
+	if err != nil {
+		t.Fatalf("NewPicoLMProvider() error = %v", err)
+	}
+	if p.binary != "" {
+		t.Errorf("binary = %q, want empty", p.binary)
+	}
+}
+
+// --- GetDefaultModel tests ---
+
+func TestPicoLMProvider_GetDefaultModel(t *testing.T) {
+	binary := createMockPicoLM(t, "", "", 0)
+	p, _ := NewPicoLMProvider(config.PicoLMProviderConfig{Binary: binary})
+	if got := p.GetDefaultModel(); got != "picolm-local" {
+		t.Errorf("GetDefaultModel() = %q, want %q", got, "picolm-local")
+	}
+}
+
+// --- Chat() tests ---
+
+func TestPicoLMChat_Success(t *testing.T) {
+	script := createMockPicoLM(t, "Photosynthesis is the process by which plants convert sunlight.", "", 0)
+
+	p := &PicoLMProvider{
+		binary:    script,
+		model:     "/tmp/model.gguf",
+		maxTokens: 256,
+		threads:   4,
+	}
+
+	resp, err := p.Chat(context.Background(), []Message{
+		{Role: "user", Content: "What is photosynthesis?"},
+	}, nil, "", nil)
+
+	if err != nil {
+		t.Fatalf("Chat() error = %v", err)
+	}
+	if resp.Content != "Photosynthesis is the process by which plants convert sunlight." {
+		t.Errorf("Content = %q", resp.Content)
+	}
+	if resp.FinishReason != "stop" {
+		t.Errorf("FinishReason = %q, want %q", resp.FinishReason, "stop")
+	}
+	if len(resp.ToolCalls) != 0 {
+		t.Errorf("ToolCalls len = %d, want 0", len(resp.ToolCalls))
+	}
+}
+
+func TestPicoLMChat_WithToolCallsInResponse(t *testing.T) {
+	mockOutput := `Checking weather.
+{"tool_calls":[{"id":"call_1","type":"function","function":{"name":"get_weather","arguments":"{\"location\":\"NYC\"}"}}]}`
+	script := createMockPicoLM(t, mockOutput, "", 0)
+
+	p := &PicoLMProvider{
+		binary:    script,
+		model:     "/tmp/model.gguf",
+		maxTokens: 256,
+		threads:   4,
+	}
+
+	resp, err := p.Chat(context.Background(), []Message{
+		{Role: "user", Content: "What's the weather?"},
+	}, []ToolDefinition{{
+		Type: "function",
+		Function: ToolFunctionDefinition{
+			Name:        "get_weather",
+			Description: "Get weather",
+			Parameters:  map[string]interface{}{"type": "object"},
+		},
+	}}, "", nil)
+
+	if err != nil {
+		t.Fatalf("Chat() error = %v", err)
+	}
+	if resp.FinishReason != "tool_calls" {
+		t.Errorf("FinishReason = %q, want %q", resp.FinishReason, "tool_calls")
+	}
+	if len(resp.ToolCalls) != 1 {
+		t.Fatalf("ToolCalls len = %d, want 1", len(resp.ToolCalls))
+	}
+	if resp.ToolCalls[0].Name != "get_weather" {
+		t.Errorf("ToolCalls[0].Name = %q, want %q", resp.ToolCalls[0].Name, "get_weather")
+	}
+	if resp.ToolCalls[0].Arguments["location"] != "NYC" {
+		t.Errorf("ToolCalls[0].Arguments[location] = %v, want NYC", resp.ToolCalls[0].Arguments["location"])
+	}
+}
+
+func TestPicoLMChat_EmptyOutput(t *testing.T) {
+	script := createMockPicoLM(t, "", "", 0)
+
+	p := &PicoLMProvider{
+		binary:    script,
+		model:     "/tmp/model.gguf",
+		maxTokens: 256,
+		threads:   4,
+	}
+
+	resp, err := p.Chat(context.Background(), []Message{
+		{Role: "user", Content: "Hello"},
+	}, nil, "", nil)
+
+	if err != nil {
+		t.Fatalf("Chat() error = %v", err)
+	}
+	if resp.Content != "" {
+		t.Errorf("Content = %q, want empty", resp.Content)
+	}
+	if resp.FinishReason != "stop" {
+		t.Errorf("FinishReason = %q, want %q", resp.FinishReason, "stop")
+	}
+}
+
+func TestPicoLMChat_StderrError(t *testing.T) {
+	script := createMockPicoLM(t, "", "model load failed: out of memory", 1)
+
+	p := &PicoLMProvider{
+		binary:    script,
+		model:     "/tmp/model.gguf",
+		maxTokens: 256,
+		threads:   4,
+	}
+
+	_, err := p.Chat(context.Background(), []Message{
+		{Role: "user", Content: "Hello"},
+	}, nil, "", nil)
+
+	if err == nil {
+		t.Fatal("Chat() expected error")
+	}
+	if !strings.Contains(err.Error(), "out of memory") {
+		t.Errorf("error = %q, want to contain 'out of memory'", err.Error())
+	}
+}
+
+func TestPicoLMChat_NonZeroExitNoStderr(t *testing.T) {
+	script := createMockPicoLM(t, "", "", 1)
+
+	p := &PicoLMProvider{
+		binary:    script,
+		model:     "/tmp/model.gguf",
+		maxTokens: 256,
+		threads:   4,
+	}
+
+	_, err := p.Chat(context.Background(), []Message{
+		{Role: "user", Content: "Hello"},
+	}, nil, "", nil)
+
+	if err == nil {
+		t.Fatal("Chat() expected error for non-zero exit")
+	}
+	if !strings.Contains(err.Error(), "picolm error") {
+		t.Errorf("error = %q, want to contain 'picolm error'", err.Error())
+	}
+}
+
+func TestPicoLMChat_NonZeroExitWithStdout(t *testing.T) {
+	// When the process fails with non-zero exit, stdout should NOT be treated as valid output.
+	script := createMockPicoLM(t, "partial garbage output", "segfault", 139)
+
+	p := &PicoLMProvider{
+		binary:    script,
+		model:     "/tmp/model.gguf",
+		maxTokens: 256,
+		threads:   4,
+	}
+
+	_, err := p.Chat(context.Background(), []Message{
+		{Role: "user", Content: "Hello"},
+	}, nil, "", nil)
+
+	if err == nil {
+		t.Fatal("Chat() expected error when process exits non-zero, even with stdout")
+	}
+}
+
+func TestPicoLMChat_ContextCancellation(t *testing.T) {
+	script := createSlowMockPicoLM(t, 30)
+
+	p := &PicoLMProvider{
+		binary:    script,
+		model:     "/tmp/model.gguf",
+		maxTokens: 256,
+		threads:   4,
+	}
+
+	ctx, cancel := context.WithTimeout(context.Background(), 200*time.Millisecond)
+	defer cancel()
+
+	_, err := p.Chat(ctx, []Message{
+		{Role: "user", Content: "Hello"},
+	}, nil, "", nil)
+
+	if err == nil {
+		t.Fatal("Chat() expected error on context cancellation")
+	}
+}
+
+func TestPicoLMChat_NoBinary(t *testing.T) {
+	p := &PicoLMProvider{model: "/tmp/model.gguf"}
+	_, err := p.Chat(context.Background(), []Message{
+		{Role: "user", Content: "Hello"},
+	}, nil, "", nil)
+	if err == nil || !strings.Contains(err.Error(), "binary path not configured") {
+		t.Errorf("expected 'binary path not configured' error, got: %v", err)
+	}
+}
+
+func TestPicoLMChat_NoModel(t *testing.T) {
+	p := &PicoLMProvider{binary: "/usr/bin/echo"}
+	_, err := p.Chat(context.Background(), []Message{
+		{Role: "user", Content: "Hello"},
+	}, nil, "", nil)
+	if err == nil || !strings.Contains(err.Error(), "model path not configured") {
+		t.Errorf("expected 'model path not configured' error, got: %v", err)
+	}
+}
+
+// --- buildPrompt tests ---
+
+func TestPicoLMBuildPrompt_SimpleUserMessage(t *testing.T) {
+	p := &PicoLMProvider{}
+	prompt := p.buildPrompt([]Message{
+		{Role: "user", Content: "Hello"},
+	}, nil)
+
+	if !strings.Contains(prompt, "<|user|>\nHello</s>") {
+		t.Errorf("prompt missing user message, got:\n%s", prompt)
+	}
+	if !strings.HasSuffix(prompt, "<|assistant|>\n") {
+		t.Errorf("prompt should end with assistant turn, got:\n%s", prompt)
+	}
+}
+
+func TestPicoLMBuildPrompt_SystemAndUser(t *testing.T) {
+	p := &PicoLMProvider{}
+	prompt := p.buildPrompt([]Message{
+		{Role: "system", Content: "You are helpful."},
+		{Role: "user", Content: "Hello"},
+	}, nil)
+
+	if !strings.Contains(prompt, "<|system|>\nYou are helpful.</s>") {
+		t.Errorf("prompt missing system message, got:\n%s", prompt)
+	}
+	if !strings.Contains(prompt, "<|user|>\nHello</s>") {
+		t.Errorf("prompt missing user message, got:\n%s", prompt)
+	}
+}
+
+func TestPicoLMBuildPrompt_MultipleSystemMessages(t *testing.T) {
+	p := &PicoLMProvider{}
+	prompt := p.buildPrompt([]Message{
+		{Role: "system", Content: "Part one."},
+		{Role: "system", Content: "Part two."},
+		{Role: "user", Content: "Hello"},
+	}, nil)
+
+	if !strings.Contains(prompt, "Part one.\n\nPart two.") {
+		t.Errorf("system parts should be joined, got:\n%s", prompt)
+	}
+	// Should have exactly one <|system|> block
+	if strings.Count(prompt, "<|system|>") != 1 {
+		t.Errorf("expected exactly 1 system block, got %d", strings.Count(prompt, "<|system|>"))
+	}
+}
+
+func TestPicoLMBuildPrompt_AssistantMessage(t *testing.T) {
+	p := &PicoLMProvider{}
+	prompt := p.buildPrompt([]Message{
+		{Role: "user", Content: "Hi"},
+		{Role: "assistant", Content: "Hello!"},
+		{Role: "user", Content: "How are you?"},
+	}, nil)
+
+	if !strings.Contains(prompt, "<|assistant|>\nHello!</s>") {
+		t.Errorf("prompt missing assistant message, got:\n%s", prompt)
+	}
+}
+
+func TestPicoLMBuildPrompt_ToolResult(t *testing.T) {
+	p := &PicoLMProvider{}
+	prompt := p.buildPrompt([]Message{
+		{Role: "user", Content: "What's the weather?"},
+		{Role: "tool", ToolCallID: "call_1", Content: `{"temp": 72}`},
+	}, nil)
+
+	if !strings.Contains(prompt, "[Tool Result for call_1]: {\"temp\": 72}") {
+		t.Errorf("prompt missing tool result, got:\n%s", prompt)
+	}
+	// Tool results are wrapped in user tags
+	if !strings.Contains(prompt, "<|user|>\n[Tool Result for call_1]") {
+		t.Errorf("tool result should be in user block, got:\n%s", prompt)
+	}
+}
+
+func TestPicoLMBuildPrompt_WithTools(t *testing.T) {
+	p := &PicoLMProvider{}
+	tools := []ToolDefinition{
+		{
+			Type: "function",
+			Function: ToolFunctionDefinition{
+				Name:        "get_weather",
+				Description: "Get weather for a city",
+				Parameters: map[string]interface{}{
+					"type": "object",
+					"properties": map[string]interface{}{
+						"city": map[string]interface{}{"type": "string"},
+					},
+					"required": []interface{}{"city"},
+				},
+			},
+		},
+	}
+
+	prompt := p.buildPrompt([]Message{
+		{Role: "system", Content: "You are helpful."},
+		{Role: "user", Content: "What's the weather in NYC?"},
+	}, tools)
+
+	// Tool definitions should be in the system block
+	if !strings.Contains(prompt, "## Available Tools") {
+		t.Errorf("prompt missing tool definitions header, got:\n%s", prompt)
+	}
+	if !strings.Contains(prompt, "#### get_weather") {
+		t.Errorf("prompt missing tool name, got:\n%s", prompt)
+	}
+	if !strings.Contains(prompt, "Get weather for a city") {
+		t.Errorf("prompt missing tool description, got:\n%s", prompt)
+	}
+	if !strings.Contains(prompt, `"tool_calls"`) {
+		t.Errorf("prompt missing tool call format example, got:\n%s", prompt)
+	}
+	// Tool definitions should be part of the system block
+	systemIdx := strings.Index(prompt, "<|system|>")
+	systemEnd := strings.Index(prompt, "</s>")
+	toolsIdx := strings.Index(prompt, "## Available Tools")
+	if toolsIdx < systemIdx || toolsIdx > systemEnd {
+		t.Errorf("tool definitions should be inside system block")
+	}
+}
+
+func TestPicoLMBuildPrompt_WithTools_NoSystem(t *testing.T) {
+	p := &PicoLMProvider{}
+	tools := []ToolDefinition{
+		{
+			Type: "function",
+			Function: ToolFunctionDefinition{
+				Name:        "search",
+				Description: "Search the web",
+			},
+		},
+	}
+
+	prompt := p.buildPrompt([]Message{
+		{Role: "user", Content: "Search for cats"},
+	}, tools)
+
+	// Should still create a system block for tools even without system messages
+	if !strings.Contains(prompt, "<|system|>") {
+		t.Errorf("expected system block for tools, got:\n%s", prompt)
+	}
+	if !strings.Contains(prompt, "#### search") {
+		t.Errorf("prompt missing tool name, got:\n%s", prompt)
+	}
+}
+
+func TestPicoLMBuildPrompt_ToolsFilterNonFunction(t *testing.T) {
+	p := &PicoLMProvider{}
+	tools := []ToolDefinition{
+		{
+			Type: "not_a_function",
+			Function: ToolFunctionDefinition{
+				Name: "should_be_skipped",
+			},
+		},
+		{
+			Type: "function",
+			Function: ToolFunctionDefinition{
+				Name:        "included",
+				Description: "This one is included",
+			},
+		},
+	}
+
+	prompt := p.buildPrompt([]Message{
+		{Role: "user", Content: "Hello"},
+	}, tools)
+
+	if strings.Contains(prompt, "should_be_skipped") {
+		t.Errorf("non-function tools should be filtered out")
+	}
+	if !strings.Contains(prompt, "#### included") {
+		t.Errorf("function tools should be included")
+	}
+}
+
+// --- Stdin prompt verification ---
+
+func TestPicoLMChat_PromptSentViaStdin(t *testing.T) {
+	captureFile := filepath.Join(t.TempDir(), "captured_stdin.txt")
+	script := createStdinCaptureMockPicoLM(t, captureFile, "Response from model")
+
+	p := &PicoLMProvider{
+		binary:    script,
+		model:     "/tmp/model.gguf",
+		maxTokens: 256,
+		threads:   4,
+	}
+
+	_, err := p.Chat(context.Background(), []Message{
+		{Role: "user", Content: "What is 2+2?"},
+	}, nil, "", nil)
+
+	if err != nil {
+		t.Fatalf("Chat() error = %v", err)
+	}
+
+	captured, err := os.ReadFile(captureFile)
+	if err != nil {
+		t.Fatalf("failed to read captured stdin: %v", err)
+	}
+
+	stdinContent := string(captured)
+	if !strings.Contains(stdinContent, "What is 2+2?") {
+		t.Errorf("stdin should contain user message, got:\n%s", stdinContent)
+	}
+	if !strings.Contains(stdinContent, "<|user|>") {
+		t.Errorf("stdin should contain ChatML tags, got:\n%s", stdinContent)
+	}
+}
+
+// --- expandHome tests ---
+
+func TestExpandHome_Empty(t *testing.T) {
+	result, err := expandHome("")
+	if err != nil {
+		t.Fatalf("expandHome(\"\") error = %v", err)
+	}
+	if result != "" {
+		t.Errorf("expandHome(\"\") = %q, want empty", result)
+	}
+}
+
+func TestExpandHome_NoTilde(t *testing.T) {
+	result, err := expandHome("/usr/bin/picolm")
+	if err != nil {
+		t.Fatalf("expandHome error = %v", err)
+	}
+	if result != "/usr/bin/picolm" {
+		t.Errorf("expandHome = %q, want %q", result, "/usr/bin/picolm")
+	}
+}
+
+func TestExpandHome_WithTilde(t *testing.T) {
+	result, err := expandHome("~/bin/picolm")
+	if err != nil {
+		t.Fatalf("expandHome error = %v", err)
+	}
+	home, _ := os.UserHomeDir()
+	expected := home + "/bin/picolm"
+	if result != expected {
+		t.Errorf("expandHome = %q, want %q", result, expected)
+	}
+}
+
+func TestExpandHome_TildeOnly(t *testing.T) {
+	result, err := expandHome("~")
+	if err != nil {
+		t.Fatalf("expandHome error = %v", err)
+	}
+	home, _ := os.UserHomeDir()
+	if result != home {
+		t.Errorf("expandHome = %q, want %q", result, home)
+	}
+}