diff --git a/internal/ai/llm/openai/openai.go b/internal/ai/llm/openai/openai.go
index 4f21be7..70c94e9 100644
--- a/internal/ai/llm/openai/openai.go
+++ b/internal/ai/llm/openai/openai.go
@@ -53,6 +53,7 @@ type Config struct {
 	BaseURL string
 	// APIKey is sent as "Authorization: Bearer <APIKey>". Empty sends no
 	// Authorization header, for endpoints such as Ollama that need none.
+	// It must not have a control character other than tab.
 	APIKey string
 	// Model is the model Generate requests. ListModels and Verify do not
 	// need it.
@@ -102,6 +103,9 @@ func newFromConfig(cfg Config) (*Provider, error) {
 	if u.User != nil || strings.ContainsAny(cfg.BaseURL, "?#") {
 		return nil, errors.New("openai: base URL must not carry credentials, a query or a fragment")
 	}
+	if !validHeaderValue(cfg.APIKey) {
+		return nil, errors.New("openai: API key has a control character")
+	}
 	// No error from New quotes caller-supplied text or a name derived from
 	// it: any such text may equal or contain the API key, and redaction
 	// cannot catch a fragment or a case-changed copy of it.
diff --git a/internal/ai/llm/openai/openai_test.go b/internal/ai/llm/openai/openai_test.go
index c953fe1..12f2c09 100644
--- a/internal/ai/llm/openai/openai_test.go
+++ b/internal/ai/llm/openai/openai_test.go
@@ -628,6 +628,20 @@ func TestNewRejectsBadHeaderWithoutEchoingValue(t *testing.T) {
 	assertNoSecret(t, err)
 }
 
+func TestNewRejectsKeyWithControlCharacter(t *testing.T) {
+	for _, key := range []string{testKey + "\n", testKey + "\r\nX-Injected: 1", "\x00" + testKey, testKey + "\x7f"} {
+		_, err := New(Config{BaseURL: "https://example.com/v1", APIKey: key, StructuredOutput: JSONSchema})
+		if err == nil {
+			t.Errorf("New(%q) = nil error", key)
+			continue
+		}
+		assertNoSecret(t, err)
+		if strings.Contains(err.Error()+fmt.Sprintf("%#v", err), key) {
+			t.Errorf("error leaks the key: %v", err)
+		}
+	}
+}
+
 // TestNewConfigErrorsNeverLeakKey covers every New error that names
 // caller-supplied text, each with the API key embedded in that text.
 func TestNewConfigErrorsNeverLeakKey(t *testing.T) {
diff --git a/internal/ai/llm/presets/presets.go b/internal/ai/llm/presets/presets.go
index a05c675..d33d613 100644
--- a/internal/ai/llm/presets/presets.go
+++ b/internal/ai/llm/presets/presets.go
@@ -164,7 +164,7 @@ var all = map[string]Preset{
 
 // FromSettings returns the Provider s describes, using key as its API key.
 // It fails with a configuration error from package llm — ErrUnknownProvider,
-// ErrMissingKey, ErrUnexpectedKey, ErrMissingBaseURL, ErrMissingModel,
+// ErrMissingKey, ErrUnexpectedKey, ErrInvalidKey, ErrMissingBaseURL, ErrMissingModel,
 // ErrMissingPublicURL or ErrInvalidSetting — before contacting any vendor.
 //
 // The Provider sends s.Temperature with every request whose Temperature is
@@ -230,6 +230,8 @@ func check(p Preset, s llm.Settings, key string) error {
 		return llm.ErrMissingKey
 	case p.Key == KeyNone && key != "":
 		return llm.ErrUnexpectedKey
+	case !validHeaderValue(key):
+		return llm.ErrInvalidKey
 	}
 	switch {
 	case p.BaseURL == "" && s.BaseURL == "":
@@ -252,6 +254,17 @@ func check(p Preset, s llm.Settings, key string) error {
 	return nil
 }
 
+// validHeaderValue reports whether value has no control character other
+// than tab, so it can be sent as an HTTP header value.
+func validHeaderValue(value string) bool {
+	for i := 0; i < len(value); i++ {
+		if c := value[i]; (c < 0x20 && c != '\t') || c == 0x7f {
+			return false
+		}
+	}
+	return true
+}
+
 // configured applies the household temperature and the preset's
 // temperature quirk to every Generate.
 type configured struct {
diff --git a/internal/ai/llm/presets/presets_test.go b/internal/ai/llm/presets/presets_test.go
index 78c6b2c..47f98c5 100644
--- a/internal/ai/llm/presets/presets_test.go
+++ b/internal/ai/llm/presets/presets_test.go
@@ -337,7 +337,7 @@ func TestPresetTable(t *testing.T) {
 
 func TestConfigErrors(t *testing.T) {
 	sentinels := []error{
-		llm.ErrUnknownProvider, llm.ErrMissingKey, llm.ErrUnexpectedKey, llm.ErrMissingBaseURL,
+		llm.ErrUnknownProvider, llm.ErrMissingKey, llm.ErrUnexpectedKey, llm.ErrInvalidKey, llm.ErrMissingBaseURL,
 		llm.ErrMissingModel, llm.ErrMissingPublicURL, llm.ErrInvalidSetting,
 	}
 	cases := []struct {
@@ -351,6 +351,10 @@ func TestConfigErrors(t *testing.T) {
 		{"openai no key", llm.Settings{Provider: "openai", Model: "m"}, "", llm.ErrMissingKey},
 		{"openrouter no key", llm.Settings{Provider: "openrouter", Model: "m", PublicURL: "https://x.example"}, "", llm.ErrMissingKey},
 		{"ollama with key", llm.Settings{Provider: "ollama", BaseURL: "http://o:11434", Model: "m"}, testKey, llm.ErrUnexpectedKey},
+		{"anthropic key with newline", llm.Settings{Provider: "anthropic", Model: "m"}, testKey + "\n", llm.ErrInvalidKey},
+		{"openai key with newline", llm.Settings{Provider: "openai", Model: "m"}, testKey + "\n", llm.ErrInvalidKey},
+		{"openrouter key with CR LF", llm.Settings{Provider: "openrouter", Model: "m", PublicURL: "https://x.example"}, testKey + "\r\nX-Injected: 1", llm.ErrInvalidKey},
+		{"openai_compatible key with NUL", llm.Settings{Provider: "openai_compatible", BaseURL: "http://l/v1", Model: "m"}, "\x00" + testKey, llm.ErrInvalidKey},
 		{"ollama no base URL", llm.Settings{Provider: "ollama", Model: "m"}, "", llm.ErrMissingBaseURL},
 		{"openai_compatible no base URL", llm.Settings{Provider: "openai_compatible", Model: "m"}, "", llm.ErrMissingBaseURL},
 		{"anthropic no model", llm.Settings{Provider: "anthropic"}, testKey, llm.ErrMissingModel},
@@ -449,6 +453,8 @@ func TestConfigErrorsNeverLeakKey(t *testing.T) {
 		{"key as temperature", llm.Settings{Provider: "openai", Model: "m", Temperature: -0.1}, "-0.1", llm.ErrInvalidSetting},
 		{"key in bad base URL", llm.Settings{Provider: "openai_compatible", BaseURL: "ftp://k3y", Model: "m"}, "k3y", llm.ErrInvalidSetting},
 		{"ollama with key", llm.Settings{Provider: "ollama", BaseURL: "http://o:11434", Model: "m"}, "key", llm.ErrUnexpectedKey},
+		{"key with newline", llm.Settings{Provider: "openai", Model: "m"}, "sk-test\n", llm.ErrInvalidKey},
+		{"key with newline as text", llm.Settings{Provider: "openai", Model: "m"}, "header\n", llm.ErrInvalidKey},
 	}
 	for _, c := range cases {
 		t.Run(c.name, func(t *testing.T) {
diff --git a/internal/ai/llm/settings.go b/internal/ai/llm/settings.go
index a9333a7..adde36c 100644
--- a/internal/ai/llm/settings.go
+++ b/internal/ai/llm/settings.go
@@ -50,6 +50,9 @@ var (
 	// ErrUnexpectedKey means an API key was given to a preset that sends
 	// none.
 	ErrUnexpectedKey = errors.New("llm: provider takes no API key")
+	// ErrInvalidKey means the API key is not a valid HTTP header value:
+	// it has a control character such as CR or LF.
+	ErrInvalidKey = errors.New("llm: API key is not a valid header value")
 	// ErrMissingBaseURL means the preset requires a base URL and none was
 	// given.
 	ErrMissingBaseURL = errors.New("llm: base URL is required")