fix(config): clear ZERO_PROVIDER when switching to an unnamed/default profile

SetActiveProviderEnv("") now unsets ZERO_PROVIDER instead of leaving the previous
value in the environment. Switching from a named provider back to an unnamed/default
profile otherwise kept exporting the stale provider into child zero exec processes,
which would resolve the old credentials. Extended the round-trip test with the
stale-env regression case: seed a value, switch to blank, assert both os.Getenv and
applyEnv no longer see it.

Addresses CodeRabbit review on #348.

internal/config/active_provider_env_test.go

@@ -0,0 +1,49 @@
+package config
+
+import (
+	"os"
+	"testing"
+)
+
+// SetActiveProviderEnv must export the provider name so a child process (whose
+// resolution reads the inherited environment via the nil-map -> os.Getenv path in
+// applyEnv) resolves the SAME active provider. A blank name CLEARS it.
+func TestSetActiveProviderEnvRoundTrips(t *testing.T) {
+	t.Setenv(ActiveProviderEnv, "") // register cleanup; restores the original after
+
+	if err := os.Unsetenv(ActiveProviderEnv); err != nil {
+		t.Fatalf("unset: %v", err)
+	}
+	SetActiveProviderEnv("") // blank with nothing set stays unset
+	if got := os.Getenv(ActiveProviderEnv); got != "" {
+		t.Fatalf("empty name set the env to %q, want unset", got)
+	}
+
+	// Regression: switching from a named provider back to an unnamed/default profile
+	// must CLEAR a previously exported value, not leave it stale for children.
+	if err := os.Setenv(ActiveProviderEnv, "stale-provider"); err != nil {
+		t.Fatalf("seed stale: %v", err)
+	}
+	SetActiveProviderEnv("") // blank must unset the stale value
+	if got := os.Getenv(ActiveProviderEnv); got != "" {
+		t.Fatalf("blank name left stale env %q, want cleared", got)
+	}
+	cfgStale := &FileConfig{}
+	applyEnv(cfgStale, nil)
+	if cfgStale.ActiveProvider != "" {
+		t.Fatalf("applyEnv still resolved stale provider %q after clear", cfgStale.ActiveProvider)
+	}
+
+	SetActiveProviderEnv("ollama-cloud")
+	if got := os.Getenv(ActiveProviderEnv); got != "ollama-cloud" {
+		t.Fatalf("env = %q, want ollama-cloud", got)
+	}
+
+	// applyEnv with a nil map is the exact path a child uses: it falls back to
+	// os.Getenv, so the inherited value selects the active provider.
+	cfg := &FileConfig{}
+	applyEnv(cfg, nil)
+	if cfg.ActiveProvider != "ollama-cloud" {
+		t.Fatalf("applyEnv ActiveProvider = %q, want ollama-cloud (inherited env not read)", cfg.ActiveProvider)
+	}
+}

internal/config/resolver.go

@@ -431,8 +431,28 @@ func mergeProfile(base ProviderProfile, next ProviderProfile) ProviderProfile {
 	return base
 }
 
+// ActiveProviderEnv selects the active provider profile by name (read in applyEnv).
+const ActiveProviderEnv = "ZERO_PROVIDER"
+
+// SetActiveProviderEnv exports the active provider name to the process environment
+// so a spawned child process (which inherits the environment) resolves the SAME
+// provider profile — and therefore the same credentials (env key / stored key /
+// OAuth) — as its parent. Without this a sub-agent re-resolves config.json's
+// default provider and can land on one whose credentials don't match the parent's
+// live selection, failing auth the instant it spawns. A blank name CLEARS the
+// variable: switching back to an unnamed/default profile must not keep exporting a
+// stale provider to children.
+func SetActiveProviderEnv(name string) {
+	name = strings.TrimSpace(name)
+	if name == "" {
+		_ = os.Unsetenv(ActiveProviderEnv)
+		return
+	}
+	_ = os.Setenv(ActiveProviderEnv, name)
+}
+
 func applyEnv(cfg *FileConfig, env map[string]string) {
-	activeProvider := strings.TrimSpace(envValue(env, "ZERO_PROVIDER"))
+	activeProvider := strings.TrimSpace(envValue(env, ActiveProviderEnv))
 	if activeProvider != "" {
 		cfg.ActiveProvider = activeProvider
 	}

internal/tui/command_center.go

@@ -441,6 +441,8 @@ func (m model) handleModelCommand(args string) (model, string) {
 	m.providerProfile = nextProfile
 	m.provider = nextProvider
 	m.providerName = displayValue(nextProfile.Name, string(metadata.ProviderKind))
+	// Keep sub-agent child processes on the same provider we just switched to.
+	config.SetActiveProviderEnv(nextProfile.Name)
 	m.modelName = target.modelID
 	resetEffort := false
 	if m.reasoningEffort != "" && !reasoningEffortAllowed(target.reasoningEfforts, m.reasoningEffort) {
@@ -516,6 +518,8 @@ func (m model) switchProviderModel(providerName, modelID string) (model, string)
 	m.providerProfile = target
 	m.providerName = target.Name
 	m.modelName = target.Model
+	// Keep sub-agent child processes on the same provider we just switched to.
+	config.SetActiveProviderEnv(target.Name)
 	if strings.TrimSpace(m.userConfigPath) != "" {
 		_, _ = config.SetActiveProvider(m.userConfigPath, target.Name)
 		_, _ = config.SetProviderModel(m.userConfigPath, target.Name, target.Model)