Use actual agent for workflow model fallback (#470)

## Summary
- stop applying generic `default_model` based on the configured workflow
agent when the actual runnable agent differs
- use the actual resolved agent for fix, review, refine, security, and
design model fallback across CLI, daemon, CI, and batch paths
- add regressions for workflow-agent fallback and unavailable-agent
fallback cases

## Testing
- go fmt ./...
- go test ./...
- go vet ./...

Author: wesm

cmd/roborev/analyze.go

@@ -800,11 +800,6 @@ func runFixAgent(cmd *cobra.Command, repoPath, agentName, model, reasoning, prom
 	}
 
 	// Resolve agent and model via fix workflow config.
-	// When the agent is explicitly overridden but the model is not,
-	// skip generic default_model (it's paired with default_agent).
-	configAgent := config.ResolveAgentForWorkflow("", repoPath, cfg, "fix", reasoning)
-	cliAgentChanged := agentName != "" &&
-		agent.CanonicalName(agentName) != agent.CanonicalName(configAgent)
 	agentName = config.ResolveAgentForWorkflow(agentName, repoPath, cfg, "fix", reasoning)
 	backupAgent := config.ResolveBackupAgentForWorkflow(repoPath, cfg, "fix")
 
@@ -821,10 +816,8 @@ func runFixAgent(cmd *cobra.Command, repoPath, agentName, model, reasoning, prom
 		agent.CanonicalName(a.Name()) != agent.CanonicalName(preferredForAnalyze)
 	if usingBackup && model == "" {
 		model = config.ResolveBackupModelForWorkflow(repoPath, cfg, "fix")
-	} else if cliAgentChanged && model == "" {
-		model = config.ResolveWorkflowModel(repoPath, cfg, "fix", reasoning)
 	} else {
-		model = config.ResolveModelForWorkflow(model, repoPath, cfg, "fix", reasoning)
+		model = resolveFixModel(a.Name(), model, repoPath, cfg, reasoning)
 	}
 
 	// Configure agent: agentic mode, with model and reasoning

cmd/roborev/fix.go

@@ -308,21 +308,16 @@ func fixJobDirect(ctx context.Context, params fixJobParams, prompt string) (*fix
 }
 
 // resolveFixModel determines the model for a fix operation, skipping
-// generic default_model when the CLI agent differs from config default.
+// generic default_model when the actual fix agent that will run differs
+// from the generic default agent. In that case, an empty result lets
+// the fix agent keep its own built-in default model unless a
+// fix-specific model override is configured.
 func resolveFixModel(
-	cliAgent, cliModel, repoPath string,
+	selectedAgent, cliModel, repoPath string,
 	cfg *config.Config, reasoning string,
 ) string {
-	configAgent := config.ResolveAgentForWorkflow(
-		"", repoPath, cfg, "fix", reasoning,
-	)
-	cliAgentChanged := cliAgent != "" &&
-		agent.CanonicalName(cliAgent) != agent.CanonicalName(configAgent)
-	if cliAgentChanged && cliModel == "" {
-		return config.ResolveWorkflowModel(repoPath, cfg, "fix", reasoning)
-	}
-	return config.ResolveModelForWorkflow(
-		cliModel, repoPath, cfg, "fix", reasoning,
+	return agent.ResolveWorkflowModelForAgent(
+		selectedAgent, cliModel, repoPath, cfg, "fix", reasoning,
 	)
 }
 
@@ -356,7 +351,7 @@ func resolveFixAgent(repoPath string, opts fixOptions) (agent.Agent, error) {
 	if usingBackup && opts.model == "" {
 		modelStr = config.ResolveBackupModelForWorkflow(repoPath, cfg, "fix")
 	} else {
-		modelStr = resolveFixModel(opts.agentName, opts.model, repoPath, cfg, reasoning)
+		modelStr = resolveFixModel(a.Name(), opts.model, repoPath, cfg, reasoning)
 	}
 
 	reasoningLevel := agent.ParseReasoningLevel(reasoning)

cmd/roborev/fix_test.go

@@ -26,6 +26,7 @@ import (
 	"github.com/roborev-dev/roborev/internal/agent"
 	"github.com/roborev-dev/roborev/internal/config"
 	"github.com/roborev-dev/roborev/internal/storage"
+	"github.com/roborev-dev/roborev/internal/testutil"
 )
 
 func patchFixDaemonRetryForTest(t *testing.T, ensure func() error) {
@@ -2597,6 +2598,62 @@ fix_model = "gemini-2.5-pro"
 	}
 }
 
+func TestResolveFixAgentSkipsDefaultModelForConfiguredFixAgent(t *testing.T) {
+	// When fix_agent differs from default_agent and no fix_model is set,
+	// the fix agent should keep its own built-in default model.
+
+	tmpDir := t.TempDir()
+	t.Setenv("ROBOREV_DATA_DIR", tmpDir)
+
+	cfgPath := filepath.Join(tmpDir, "config.toml")
+	if err := os.WriteFile(cfgPath, []byte(`
+default_agent = "codex"
+default_model = "gpt-5.4"
+fix_agent = "claude"
+`), 0644); err != nil {
+		t.Fatal(err)
+	}
+
+	cfg, err := config.LoadGlobal()
+	if err != nil {
+		t.Fatalf("LoadGlobal: %v", err)
+	}
+
+	modelStr := resolveFixModel("claude-code", "", tmpDir, cfg, "standard")
+	if modelStr != "" {
+		t.Fatalf("expected empty model so claude keeps its default, got %q", modelStr)
+	}
+}
+
+func TestResolveFixAgentFallbackUsesDefaultModelForActualAgent(t *testing.T) {
+	t.Cleanup(testutil.MockExecutableIsolated(t, "codex", 0))
+
+	tmpDir := t.TempDir()
+	t.Setenv("ROBOREV_DATA_DIR", tmpDir)
+
+	cfgPath := filepath.Join(tmpDir, "config.toml")
+	if err := os.WriteFile(cfgPath, []byte(`
+default_agent = "codex"
+default_model = "gpt-5.4"
+fix_agent = "claude"
+`), 0644); err != nil {
+		t.Fatal(err)
+	}
+
+	selected, err := resolveFixAgent(tmpDir, fixOptions{})
+	if err != nil {
+		t.Fatalf("resolveFixAgent: %v", err)
+	}
+
+	codexAgent, ok := selected.(*agent.CodexAgent)
+	if !ok {
+		t.Fatalf("expected codex fallback agent, got %T", selected)
+	}
+	if codexAgent.Model != "gpt-5.4" {
+		t.Fatalf("expected fallback codex agent to use default_model, got %q", codexAgent.Model)
+	}
+}
+
 func TestResolveFixAgentUsesRepoWorkflowModel(t *testing.T) {
 	// Repo-level workflow-specific models should be used even when
 	// --agent is overridden on CLI.
@@ -2703,9 +2760,7 @@ func TestResolveFixAgentSameAsDefault(t *testing.T) {
 			}
 
 			// Call the production resolveFixModel function directly
-			modelStr := resolveFixModel(
-				tt.cliAgent, "", tmpDir, cfg, "fast",
-			)
+			modelStr := resolveFixModel(tt.cliAgent, "", tmpDir, cfg, "fast")
 
 			if modelStr != tt.wantModel {
 				t.Errorf("model = %q, want %q", modelStr, tt.wantModel)

cmd/roborev/refine.go

@@ -1189,8 +1189,8 @@ func applyModelForAgent(
 			repoPath, cfg, workflow,
 		)
 	} else {
-		model = config.ResolveModelForWorkflow(
-			cliModel, repoPath, cfg, workflow, reasoning,
+		model = agent.ResolveWorkflowModelForAgent(
+			a.Name(), cliModel, repoPath, cfg, workflow, reasoning,
 		)
 	}
 

cmd/roborev/refine_test.go

@@ -1087,6 +1087,41 @@ func TestApplyModelForAgent_SameAgentPrimaryAndBackup(t *testing.T) {
 	}
 }
 
+func TestApplyModelForAgentFallbackUsesDefaultModelForActualAgent(t *testing.T) {
+	a, err := agent.Get("codex")
+	if err != nil {
+		t.Fatalf("agent.Get: %v", err)
+	}
+
+	cfg := &config.Config{
+		DefaultAgent: "codex",
+		DefaultModel: "gpt-5.4",
+		ReviewAgent:  "claude",
+	}
+
+	result, model := applyModelForAgent(
+		a,
+		"claude", // preferred but unavailable
+		"",       // no backup
+		"",       // no CLI model
+		"",       // no repo path
+		cfg,
+		"review",
+		"standard",
+	)
+
+	codexAgent, ok := result.(*agent.CodexAgent)
+	if !ok {
+		t.Fatalf("expected *CodexAgent, got %T", result)
+	}
+	if model != "gpt-5.4" {
+		t.Fatalf("expected default model for actual fallback agent, got %q", model)
+	}
+	if codexAgent.Model != "gpt-5.4" {
+		t.Fatalf("expected codex fallback agent to use default_model, got %q", codexAgent.Model)
+	}
+}
+
 func TestRefineFlagValidation(t *testing.T) {
 	tests := []struct {
 		name    string

internal/agent/model_resolution.go

@@ -0,0 +1,51 @@
+package agent
+
+import (
+	"strings"
+
+	"github.com/roborev-dev/roborev/internal/config"
+)
+
+// ResolveWorkflowModelForAgent resolves a workflow model for the actual
+// agent that will run. If that agent differs from the generic default
+// agent and no explicit model was provided, generic default_model is
+// skipped so the selected agent can keep its own built-in default unless
+// a workflow-specific model override exists.
+func ResolveWorkflowModelForAgent(
+	selectedAgent, cliModel, repoPath string,
+	globalCfg *config.Config,
+	workflow, level string,
+) string {
+	if s := strings.TrimSpace(cliModel); s != "" {
+		return config.ResolveModelForWorkflow(
+			s, repoPath, globalCfg, workflow, level,
+		)
+	}
+
+	selectedAgent = strings.TrimSpace(selectedAgent)
+	if selectedAgent == "" {
+		return config.ResolveModelForWorkflow(
+			"", repoPath, globalCfg, workflow, level,
+		)
+	}
+
+	defaultAgent := config.ResolveAgent("", repoPath, globalCfg)
+	if workflowModelComparableAgentName(selectedAgent, globalCfg) !=
+		workflowModelComparableAgentName(defaultAgent, globalCfg) {
+		return config.ResolveWorkflowModel(
+			repoPath, globalCfg, workflow, level,
+		)
+	}
+
+	return config.ResolveModelForWorkflow(
+		"", repoPath, globalCfg, workflow, level,
+	)
+}
+
+func workflowModelComparableAgentName(name string, cfg *config.Config) string {
+	name = strings.TrimSpace(name)
+	if isConfiguredACPAgentName(name, cfg) {
+		return defaultACPName
+	}
+	return CanonicalName(name)
+}