Add -o flag for output file

.gitignore

@@ -1,2 +1,3 @@
 # Binary output
 coding-context
+coding-agent-context-cli

README.md

@@ -33,15 +33,19 @@ Usage:
 Options:
   -C string
     	Change to directory before doing anything. (default ".")
+  -S value
+    	Exclude rules with matching frontmatter. Can be specified multiple times as key=value.
+  -o string
+    	Output file. If unspecified, output is written to stdout.
   -p value
     	Parameter to substitute in the prompt. Can be specified multiple times as key=value.
   -s value
     	Include rules with matching frontmatter. Can be specified multiple times as key=value.
-  -S value
-    	Exclude rules with matching frontmatter. Can be specified multiple times as key=value.
 ```
 
-### Example
+### Examples
+
+**Example 1: Pipe to another program**
 
 ```bash
 coding-agent-context-cli -p jira_issue_key=PROJ-1234 fix-bug | llm -m gemini-pro
@@ -56,6 +60,17 @@ This command will:
 6. Print the combined context (rules + task) to `stdout`.
 7. Pipe the output to another program (in this case, `llm`).
 
+**Example 2: Save to a file**
+
+```bash
+coding-agent-context-cli -p jira_issue_key=PROJ-1234 -o context.txt fix-bug
+```
+
+This command will:
+1. Generate the combined context as in Example 1.
+2. Write the output to `context.txt` instead of stdout.
+3. Print diagnostic messages (file inclusions, token counts) to stderr.
+
 ### Example Tasks
 
 The `<task-name>` is the name of the task you want the agent to perform. Here are some common examples:

integration_test.go

@@ -72,7 +72,7 @@ Please help with this task.
 	outputStr := string(output)
 	bootstrapIdx := strings.Index(outputStr, "Running bootstrap")
 	setupIdx := strings.Index(outputStr, "# Development Setup")
-	
+
 	if bootstrapIdx == -1 {
 		t.Errorf("bootstrap output not found in stdout")
 	}
@@ -535,3 +535,174 @@ Please help with this task.
 		t.Errorf(".mdc file content not found in stdout")
 	}
 }
+
+func TestOutputFileFlag(t *testing.T) {
+	// Build the binary
+	binaryPath := filepath.Join(t.TempDir(), "coding-context")
+	cmd := exec.Command("go", "build", "-o", binaryPath, ".")
+	if output, err := cmd.CombinedOutput(); err != nil {
+		t.Fatalf("failed to build binary: %v\n%s", err, output)
+	}
+
+	// Create a temporary directory structure
+	tmpDir := t.TempDir()
+	rulesDir := filepath.Join(tmpDir, ".agents", "rules")
+	tasksDir := filepath.Join(tmpDir, ".agents", "tasks")
+
+	if err := os.MkdirAll(rulesDir, 0755); err != nil {
+		t.Fatalf("failed to create rules dir: %v", err)
+	}
+	if err := os.MkdirAll(tasksDir, 0755); err != nil {
+		t.Fatalf("failed to create tasks dir: %v", err)
+	}
+
+	// Create a rule file
+	ruleFile := filepath.Join(rulesDir, "info.md")
+	ruleContent := `---
+---
+# Project Info
+
+General information about the project.
+`
+	if err := os.WriteFile(ruleFile, []byte(ruleContent), 0644); err != nil {
+		t.Fatalf("failed to write rule file: %v", err)
+	}
+
+	// Create a task file
+	taskFile := filepath.Join(tasksDir, "test-task.md")
+	taskContent := `---
+---
+# Test Task
+
+Please help with this task.
+`
+	if err := os.WriteFile(taskFile, []byte(taskContent), 0644); err != nil {
+		t.Fatalf("failed to write task file: %v", err)
+	}
+
+	// Create output file path
+	outputFile := filepath.Join(tmpDir, "output.txt")
+
+	// Run the binary with -o flag
+	cmd = exec.Command(binaryPath, "-C", tmpDir, "-o", outputFile, "test-task")
+	output, err := cmd.CombinedOutput()
+	if err != nil {
+		t.Fatalf("failed to run binary: %v\n%s", err, output)
+	}
+
+	// Check that stdout is empty (since output went to file)
+	stdoutStr := string(output)
+	if strings.Contains(stdoutStr, "# Project Info") {
+		t.Errorf("rule content should not be in stdout when using -o flag")
+	}
+	if strings.Contains(stdoutStr, "# Test Task") {
+		t.Errorf("task content should not be in stdout when using -o flag")
+	}
+
+	// Check that diagnostic messages are still in stderr (CombinedOutput combines stdout and stderr)
+	if !strings.Contains(stdoutStr, "Including rule file") {
+		t.Errorf("diagnostic messages should still be in stderr")
+	}
+
+	// Read the output file and verify content
+	fileContent, err := os.ReadFile(outputFile)
+	if err != nil {
+		t.Fatalf("failed to read output file: %v", err)
+	}
+
+	fileStr := string(fileContent)
+	if !strings.Contains(fileStr, "# Project Info") {
+		t.Errorf("rule content not found in output file")
+	}
+	if !strings.Contains(fileStr, "# Test Task") {
+		t.Errorf("task content not found in output file")
+	}
+
+	// Verify diagnostic messages are NOT in the output file
+	if strings.Contains(fileStr, "Including rule file") {
+		t.Errorf("diagnostic messages should not be in output file")
+	}
+}
+
+func TestOutputFileWithBootstrap(t *testing.T) {
+	// Build the binary
+	binaryPath := filepath.Join(t.TempDir(), "coding-context")
+	cmd := exec.Command("go", "build", "-o", binaryPath, ".")
+	if output, err := cmd.CombinedOutput(); err != nil {
+		t.Fatalf("failed to build binary: %v\n%s", err, output)
+	}
+
+	// Create a temporary directory structure
+	tmpDir := t.TempDir()
+	rulesDir := filepath.Join(tmpDir, ".agents", "rules")
+	tasksDir := filepath.Join(tmpDir, ".agents", "tasks")
+
+	if err := os.MkdirAll(rulesDir, 0755); err != nil {
+		t.Fatalf("failed to create rules dir: %v", err)
+	}
+	if err := os.MkdirAll(tasksDir, 0755); err != nil {
+		t.Fatalf("failed to create tasks dir: %v", err)
+	}
+
+	// Create a rule file with bootstrap
+	ruleFile := filepath.Join(rulesDir, "setup.md")
+	ruleContent := `---
+---
+# Setup
+
+Setup instructions.
+`
+	if err := os.WriteFile(ruleFile, []byte(ruleContent), 0644); err != nil {
+		t.Fatalf("failed to write rule file: %v", err)
+	}
+
+	bootstrapFile := filepath.Join(rulesDir, "setup-bootstrap")
+	bootstrapContent := `#!/bin/bash
+echo "Running setup bootstrap"
+`
+	if err := os.WriteFile(bootstrapFile, []byte(bootstrapContent), 0755); err != nil {
+		t.Fatalf("failed to write bootstrap file: %v", err)
+	}
+
+	// Create a task file
+	taskFile := filepath.Join(tasksDir, "test-task.md")
+	taskContent := `---
+---
+# Test Task
+
+Please help with this task.
+`
+	if err := os.WriteFile(taskFile, []byte(taskContent), 0644); err != nil {
+		t.Fatalf("failed to write task file: %v", err)
+	}
+
+	// Create output file path
+	outputFile := filepath.Join(tmpDir, "output.txt")
+
+	// Run the binary with -o flag
+	cmd = exec.Command(binaryPath, "-C", tmpDir, "-o", outputFile, "test-task")
+	output, err := cmd.CombinedOutput()
+	if err != nil {
+		t.Fatalf("failed to run binary: %v\n%s", err, output)
+	}
+
+	// Check that bootstrap output is in stderr (part of CombinedOutput)
+	stdoutStr := string(output)
+	if !strings.Contains(stdoutStr, "Running setup bootstrap") {
+		t.Errorf("bootstrap output should still be in stderr")
+	}
+
+	// Read the output file
+	fileContent, err := os.ReadFile(outputFile)
+	if err != nil {
+		t.Fatalf("failed to read output file: %v", err)
+	}
+
+	fileStr := string(fileContent)
+	if !strings.Contains(fileStr, "# Setup") {
+		t.Errorf("rule content not found in output file")
+	}
+	if !strings.Contains(fileStr, "# Test Task") {
+		t.Errorf("task content not found in output file")
+	}
+}

main.go

@@ -5,6 +5,7 @@ import (
 	_ "embed"
 	"flag"
 	"fmt"
+	"io"
 	"os"
 	"os/exec"
 	"os/signal"
@@ -14,17 +15,19 @@ import (
 )
 
 var (
-	workDir  string
-	params   = make(paramMap)
-	includes = make(selectorMap)
-	excludes = make(selectorMap)
+	workDir    string
+	outputFile string
+	params     = make(paramMap)
+	includes   = make(selectorMap)
+	excludes   = make(selectorMap)
 )
 
 func main() {
 	ctx, cancel := signal.NotifyContext(context.Background(), os.Interrupt, syscall.SIGTERM)
 	defer cancel()
 
 	flag.StringVar(&workDir, "C", ".", "Change to directory before doing anything.")
+	flag.StringVar(&outputFile, "o", "", "Output file. If unspecified, output is written to stdout.")
 	flag.Var(&params, "p", "Parameter to substitute in the prompt. Can be specified multiple times as key=value.")
 	flag.Var(&includes, "s", "Include rules with matching frontmatter. Can be specified multiple times as key=value.")
 	flag.Var(&excludes, "S", "Exclude rules with matching frontmatter. Can be specified multiple times as key=value.")
@@ -48,6 +51,18 @@ func main() {
 }
 
 func run(ctx context.Context, args []string) error {
+	// Setup output writer
+	var output io.Writer
+	if outputFile != "" {
+		f, err := os.Create(outputFile)
+		if err != nil {
+			return fmt.Errorf("failed to create output file: %w", err)
+		}
+		defer f.Close()
+		output = f
+	} else {
+		output = os.Stdout
+	}
 	if len(args) != 1 {
 		return fmt.Errorf("invalid usage")
 	}
@@ -188,7 +203,7 @@ func run(ctx context.Context, args []string) error {
 			tokens := estimateTokens(content)
 			totalTokens += tokens
 			fmt.Fprintf(os.Stderr, "⪢ Including rule file: %s (~%d tokens)\n", path, tokens)
-			fmt.Println(content)
+			fmt.Fprintln(output, content)
 
 			return nil
 
@@ -216,7 +231,7 @@ func run(ctx context.Context, args []string) error {
 	totalTokens += tokens
 	fmt.Fprintf(os.Stderr, "⪢ Including task file: %s (~%d tokens)\n", taskPromptPath, tokens)
 
-	fmt.Println(expanded)
+	fmt.Fprintln(output, expanded)
 
 	// Print total token count
 	fmt.Fprintf(os.Stderr, "⪢ Total estimated tokens: %d\n", totalTokens)

token_counter_test.go

@@ -4,9 +4,9 @@ import "testing"
 
 func TestEstimateTokens(t *testing.T) {
 	tests := []struct {
-		name  string
-		text  string
-		want  int
+		name string
+		text string
+		want int
 	}{
 		{
 			name: "empty string",