feat: add harness model selection

Author: easel

cmd/dun/main.go

@@ -32,6 +32,8 @@ var respondFn = dun.Respond
 var installRepo = dun.InstallRepo
 var callHarnessFn = callHarnessImpl
 var callHarnessStreamingFn = callHarnessStreamingImpl
+var harnessModel string
+var harnessModelOverrides map[string]string
 
 func main() {
 	code := run(os.Args[1:], os.Stdout, os.Stderr)
@@ -99,6 +101,8 @@ REVIEW MODE:
     --principles  Path to principles document (default docs/helix/01-frame/principles.md)
     --harnesses   Comma-separated list of review harnesses (default: codex,claude,gemini)
     --synth-harness Harness to synthesize final review (default: first harness)
+    --model       Model override for selected harness(es)
+    --models      Per-harness model overrides (e.g., codex:o3,claude:sonnet)
     --automation  Mode: manual, plan, auto, yolo (default: auto)
     --dry-run     Print prompt without calling harnesses
     --verbose     Print individual harness reviews
@@ -121,6 +125,8 @@ LOOP MODE:
 
   Options:
     --harness     Agent to use: codex, claude, gemini, opencode (default: from config)
+    --model       Model override for selected harness(es)
+    --models      Per-harness model overrides (e.g., codex:o3,claude:sonnet)
     --automation  Mode: manual, plan, auto, yolo (default: auto)
     --max-iterations  Safety limit (default: 100)
     --dry-run     Show prompt without calling agent
@@ -501,6 +507,8 @@ func runLoop(args []string, stdout io.Writer, stderr io.Writer) int {
 	fs.SetOutput(stderr)
 	configPath := fs.String("config", explicitConfig, "path to config file")
 	harness := fs.String("harness", "", "agent harness (codex|claude|gemini|opencode); default from config")
+	model := fs.String("model", opts.AgentModel, "model override for selected harness(es)")
+	models := fs.String("models", "", "per-harness model overrides (e.g., codex:o3,claude:sonnet)")
 	automation := fs.String("automation", opts.AutomationMode, "automation mode (manual|plan|auto|yolo)")
 	maxIterations := fs.Int("max-iterations", 100, "maximum iterations before stopping")
 	dryRun := fs.Bool("dry-run", false, "print prompt without calling harness")
@@ -529,6 +537,30 @@ func runLoop(args []string, stdout io.Writer, stderr io.Writer) int {
 		}
 	}
 
+	modelOverrides := make(map[string]string)
+	for harnessName, modelName := range opts.AgentModels {
+		if modelName == "" {
+			continue
+		}
+		modelOverrides[harnessName] = modelName
+	}
+	if *models != "" {
+		parsed, parseErr := parseHarnessModelOverrides(*models)
+		if parseErr != nil {
+			fmt.Fprintf(stderr, "dun loop failed: invalid models: %v\n", parseErr)
+			return dun.ExitUsageError
+		}
+		for harnessName, modelName := range parsed {
+			modelOverrides[harnessName] = modelName
+		}
+	}
+	harnessModel = strings.TrimSpace(*model)
+	if len(modelOverrides) == 0 {
+		harnessModelOverrides = nil
+	} else {
+		harnessModelOverrides = modelOverrides
+	}
+
 	// Parse quorum configuration if specified
 	var quorumCfg dun.QuorumConfig
 	if *quorumFlag != "" || *harnessesFlag != "" {
@@ -737,6 +769,40 @@ func callHarnessStreaming(harness, prompt, automation string, stdout, stderr io.
 	return callHarnessStreamingFn(harness, prompt, automation, stdout, stderr)
 }
 
+func resolveHarnessModel(harness string) string {
+	if len(harnessModelOverrides) > 0 {
+		if model, ok := harnessModelOverrides[harness]; ok && model != "" {
+			return model
+		}
+	}
+	return harnessModel
+}
+
+func parseHarnessModelOverrides(raw string) (map[string]string, error) {
+	if raw == "" {
+		return nil, nil
+	}
+	overrides := make(map[string]string)
+	parts := strings.Split(raw, ",")
+	for _, part := range parts {
+		item := strings.TrimSpace(part)
+		if item == "" {
+			continue
+		}
+		fields := strings.SplitN(item, ":", 2)
+		if len(fields) != 2 {
+			return nil, fmt.Errorf("invalid model override %q (expected harness:model)", item)
+		}
+		harness := strings.TrimSpace(fields[0])
+		model := strings.TrimSpace(fields[1])
+		if harness == "" || model == "" {
+			return nil, fmt.Errorf("invalid model override %q (expected harness:model)", item)
+		}
+		overrides[harness] = model
+	}
+	return overrides, nil
+}
+
 func callHarnessImpl(harnessName, prompt, automation string) (string, error) {
 	// Convert automation string to AutomationMode
 	var mode dun.AutomationMode
@@ -757,7 +823,8 @@ func callHarnessImpl(harnessName, prompt, automation string) (string, error) {
 	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Minute)
 	defer cancel()
 
-	result, err := dun.ExecuteHarness(ctx, harnessName, prompt, mode, ".")
+	model := resolveHarnessModel(harnessName)
+	result, err := dun.ExecuteHarness(ctx, harnessName, prompt, mode, ".", model)
 	if err != nil {
 		return "", err
 	}
@@ -784,7 +851,8 @@ func callHarnessStreamingImpl(harnessName, prompt, automation string, stdout, st
 	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Minute)
 	defer cancel()
 
-	result, err := dun.ExecuteHarnessWithOutput(ctx, harnessName, prompt, mode, ".", stdout, stderr)
+	model := resolveHarnessModel(harnessName)
+	result, err := dun.ExecuteHarnessWithOutput(ctx, harnessName, prompt, mode, ".", model, stdout, stderr)
 	if err != nil {
 		return "", err
 	}

cmd/dun/main_test.go

@@ -1317,6 +1317,9 @@ func TestRunHelpIncludesLoop(t *testing.T) {
 	if !strings.Contains(output, "--harness") {
 		t.Fatalf("help should document harness option")
 	}
+	if !strings.Contains(output, "--model") {
+		t.Fatalf("help should document model option")
+	}
 	if !strings.Contains(output, "--max-iterations") {
 		t.Fatalf("help should document max-iterations option")
 	}

cmd/dun/review.go

@@ -43,6 +43,8 @@ func runReview(args []string, stdout io.Writer, stderr io.Writer) int {
 	principlesPath := fs.String("principles", "docs/helix/01-frame/principles.md", "path to principles document")
 	harnessesFlag := fs.String("harnesses", "codex,claude,gemini", "comma-separated list of review harnesses")
 	synthHarness := fs.String("synth-harness", "", "harness used to synthesize final review (default: first harness)")
+	model := fs.String("model", opts.AgentModel, "model override for selected harness(es)")
+	models := fs.String("models", "", "per-harness model overrides (e.g., codex:o3,claude:sonnet)")
 	automation := fs.String("automation", opts.AutomationMode, "automation mode (manual|plan|auto|yolo)")
 	dryRun := fs.Bool("dry-run", false, "print review prompt without calling harnesses")
 	verbose := fs.Bool("verbose", false, "print individual harness reviews")
@@ -90,6 +92,30 @@ func runReview(args []string, stdout io.Writer, stderr io.Writer) int {
 		synth = reviewCfg.Harnesses[0]
 	}
 
+	modelOverrides := make(map[string]string)
+	for harnessName, modelName := range opts.AgentModels {
+		if modelName == "" {
+			continue
+		}
+		modelOverrides[harnessName] = modelName
+	}
+	if *models != "" {
+		parsed, parseErr := parseHarnessModelOverrides(*models)
+		if parseErr != nil {
+			fmt.Fprintf(stderr, "dun review failed: invalid models: %v\n", parseErr)
+			return dun.ExitUsageError
+		}
+		for harnessName, modelName := range parsed {
+			modelOverrides[harnessName] = modelName
+		}
+	}
+	harnessModel = strings.TrimSpace(*model)
+	if len(modelOverrides) == 0 {
+		harnessModelOverrides = nil
+	} else {
+		harnessModelOverrides = modelOverrides
+	}
+
 	for _, doc := range docs {
 		reviewPrompt := buildReviewPrompt(doc, principles, *principlesPath)
 		if *dryRun {

docs/design/contracts/API-001-dun-cli.md

@@ -208,6 +208,8 @@ $ dun check --prompt
 **Options**:
 - `--config` : Path to config file (default `.dun/config.yaml` if present)
 - `--harness` : Agent harness (`codex`, `claude`, `gemini`, `opencode`, default from config)
+- `--model` : Model override for selected harness(es)
+- `--models` : Per-harness model overrides (e.g., `codex:o3,claude:sonnet`)
 - `--automation` : Automation mode (`manual`, `plan`, `auto`, `yolo`, default `auto`)
 - `--max-iterations` : Maximum iterations before stopping (default `100`)
 - `--dry-run` : Print prompt without calling harness

internal/dun/config.go

@@ -16,11 +16,13 @@ type Config struct {
 }
 
 type AgentConfig struct {
-	Cmd        string `yaml:"cmd"`
-	Harness    string `yaml:"harness"`
-	TimeoutMS  int    `yaml:"timeout_ms"`
-	Mode       string `yaml:"mode"`
-	Automation string `yaml:"automation"`
+	Cmd        string            `yaml:"cmd"`
+	Harness    string            `yaml:"harness"`
+	Model      string            `yaml:"model"`
+	Models     map[string]string `yaml:"models"`
+	TimeoutMS  int               `yaml:"timeout_ms"`
+	Mode       string            `yaml:"mode"`
+	Automation string            `yaml:"automation"`
 }
 
 type GoConfig struct {
@@ -35,6 +37,8 @@ agent:
   automation: auto
   mode: auto
   timeout_ms: 300000
+  model: ""
+  models: {}
 go:
   coverage_threshold: 80
 `
@@ -54,6 +58,18 @@ func ApplyConfig(opts Options, cfg Config) Options {
 	if cfg.Agent.Harness != "" {
 		opts.AgentHarness = cfg.Agent.Harness
 	}
+	if cfg.Agent.Model != "" {
+		opts.AgentModel = cfg.Agent.Model
+	}
+	if len(cfg.Agent.Models) > 0 {
+		opts.AgentModels = make(map[string]string, len(cfg.Agent.Models))
+		for harness, model := range cfg.Agent.Models {
+			if model == "" {
+				continue
+			}
+			opts.AgentModels[harness] = model
+		}
+	}
 	if cfg.Agent.TimeoutMS > 0 {
 		opts.AgentTimeout = time.Duration(cfg.Agent.TimeoutMS) * time.Millisecond
 	}

internal/dun/config_test.go

@@ -13,7 +13,7 @@ func TestLoadConfigDefaultPath(t *testing.T) {
 	if err := os.MkdirAll(filepath.Dir(cfgPath), 0755); err != nil {
 		t.Fatalf("mkdir config dir: %v", err)
 	}
-	content := "agent:\n  cmd: echo hi\n  harness: codex\n  timeout_ms: 120000\n  mode: auto\n  automation: plan\n" +
+	content := "agent:\n  cmd: echo hi\n  harness: codex\n  model: o3\n  models:\n    claude: sonnet\n  timeout_ms: 120000\n  mode: auto\n  automation: plan\n" +
 		"go:\n  coverage_threshold: 95\n"
 	if err := os.WriteFile(cfgPath, []byte(content), 0644); err != nil {
 		t.Fatalf("write config: %v", err)
@@ -34,6 +34,12 @@ func TestLoadConfigDefaultPath(t *testing.T) {
 	if opts.AgentHarness != "codex" {
 		t.Fatalf("expected agent harness codex, got %q", opts.AgentHarness)
 	}
+	if opts.AgentModel != "o3" {
+		t.Fatalf("expected agent model o3, got %q", opts.AgentModel)
+	}
+	if opts.AgentModels["claude"] != "sonnet" {
+		t.Fatalf("expected agent model override for claude, got %q", opts.AgentModels["claude"])
+	}
 	if opts.AgentTimeout != 120*time.Second {
 		t.Fatalf("expected timeout 120s, got %s", opts.AgentTimeout)
 	}

internal/dun/harness.go

@@ -58,6 +58,9 @@ type HarnessConfig struct {
 	// Command is the base command to execute (optional, uses default if empty)
 	Command string
 
+	// Model selects the model for the harness (optional, uses harness default if empty)
+	Model string
+
 	// WorkDir is the working directory for command execution
 	WorkDir string
 
@@ -184,6 +187,9 @@ func (h *ClaudeHarness) Execute(ctx context.Context, prompt string) (string, err
 		"--input-format", "text",
 		"--output-format", "text",
 	}
+	if h.config.Model != "" {
+		args = append(args, "--model", h.config.Model)
+	}
 	switch h.config.AutomationMode {
 	case AutomationPlan:
 		args = append(args, "--permission-mode", "plan")
@@ -233,6 +239,9 @@ func (h *GeminiHarness) Execute(ctx context.Context, prompt string) (string, err
 		"--prompt", "",
 		"--output-format", "text",
 	}
+	if h.config.Model != "" {
+		args = append(args, "--model", h.config.Model)
+	}
 	switch h.config.AutomationMode {
 	case AutomationPlan:
 		args = append(args, "--approval-mode", "plan")
@@ -280,7 +289,11 @@ func (h *CodexHarness) Name() string {
 // Uses exec --full-auto for autonomous execution.
 // Reference: ralph-orchestrator/crates/ralph-adapters/src/cli_backend.rs
 func (h *CodexHarness) Execute(ctx context.Context, prompt string) (string, error) {
-	args := []string{"exec"}
+	args := []string{}
+	if h.config.Model != "" {
+		args = append(args, "--model", h.config.Model)
+	}
+	args = append(args, "exec")
 	switch h.config.AutomationMode {
 	case AutomationPlan:
 		args = append(args, "--sandbox", "read-only")
@@ -325,7 +338,11 @@ func (h *OpenCodeHarness) Name() string {
 // Execute runs the OpenCode CLI with the given prompt.
 // OpenCode expects the prompt as a positional message for `opencode run`.
 func (h *OpenCodeHarness) Execute(ctx context.Context, prompt string) (string, error) {
-	args := []string{"run", prompt}
+	args := []string{"run"}
+	if h.config.Model != "" {
+		args = append(args, "--model", h.config.Model)
+	}
+	args = append(args, prompt)
 
 	return h.runCommand(ctx, h.config.Command, prompt, args...)
 }
@@ -471,13 +488,14 @@ func formatEnv(env map[string]string) []string {
 var DefaultRegistry = NewHarnessRegistry()
 
 // ExecuteHarness is a convenience function that executes a prompt using a harness from the default registry.
-func ExecuteHarness(ctx context.Context, harnessName, prompt string, automationMode AutomationMode, workDir string) (HarnessResult, error) {
+func ExecuteHarness(ctx context.Context, harnessName, prompt string, automationMode AutomationMode, workDir string, model string) (HarnessResult, error) {
 	start := time.Now()
 
 	config := HarnessConfig{
 		Name:           harnessName,
 		WorkDir:        workDir,
 		AutomationMode: automationMode,
+		Model:          model,
 	}
 
 	harness, err := DefaultRegistry.Get(harnessName, config)
@@ -511,13 +529,14 @@ func ExecuteHarness(ctx context.Context, harnessName, prompt string, automationM
 }
 
 // ExecuteHarnessWithOutput streams harness output while capturing the full response.
-func ExecuteHarnessWithOutput(ctx context.Context, harnessName, prompt string, automationMode AutomationMode, workDir string, stdoutWriter io.Writer, stderrWriter io.Writer) (HarnessResult, error) {
+func ExecuteHarnessWithOutput(ctx context.Context, harnessName, prompt string, automationMode AutomationMode, workDir string, model string, stdoutWriter io.Writer, stderrWriter io.Writer) (HarnessResult, error) {
 	start := time.Now()
 
 	config := HarnessConfig{
 		Name:           harnessName,
 		WorkDir:        workDir,
 		AutomationMode: automationMode,
+		Model:          model,
 		StdoutWriter:   stdoutWriter,
 		StderrWriter:   stderrWriter,
 	}