Validate extra headers in New and honour a configured Accept header

Config.Headers names must be RFC 9110 tokens and values must hold no
control characters other than horizontal tab, so bad configuration fails
at construction instead of on the first request. Headers the adapter or
net/http manage (Authorization, Content-Type, Content-Length,
Transfer-Encoding, Host, Connection) are rejected. A configured Accept
header is sent as given; without one the adapter sends application/json.
Header values are never echoed in errors.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/ai/llm/openai/openai.go

@@ -54,7 +54,9 @@ type Config struct {
 	// need it.
 	Model string
 	// Headers are extra request headers, e.g. OpenRouter's HTTP-Referer and
-	// X-Title. They must not set Authorization or Content-Type.
+	// X-Title. An Accept header replaces the default "application/json".
+	// Headers the adapter or net/http set themselves (see managedHeaders)
+	// are rejected.
 	Headers map[string]string
 	// StructuredOutput is how JSON is requested when a Request has a Schema.
 	StructuredOutput StructuredOutput
@@ -91,17 +93,24 @@ func New(cfg Config) (*Provider, error) {
 	default:
 		return nil, fmt.Errorf("openai: unknown structured output mode %q", cfg.StructuredOutput)
 	}
-	headers := make(http.Header, len(cfg.Headers))
+	headers := make(http.Header, len(cfg.Headers)+1)
 	for name, value := range cfg.Headers {
+		// Values are never echoed: one may carry a credential.
+		if !validHeaderName(name) {
+			return nil, fmt.Errorf("openai: extra header name %q is not a valid HTTP token", name)
+		}
 		canonical := textproto.CanonicalMIMEHeaderKey(name)
-		if canonical == "Authorization" || canonical == "Content-Type" {
+		if managedHeaders[canonical] {
 			return nil, fmt.Errorf("openai: extra header %q is set by the adapter", canonical)
 		}
-		if name == "" || strings.ContainsAny(name, " \t\r\n:") || strings.ContainsAny(value, "\r\n") {
-			return nil, fmt.Errorf("openai: extra header %q is malformed", name)
+		if !validHeaderValue(value) {
+			return nil, fmt.Errorf("openai: extra header %q has a control character in its value", canonical)
 		}
 		headers.Set(canonical, value)
 	}
+	if headers.Get("Accept") == "" {
+		headers.Set("Accept", "application/json")
+	}
 	return &Provider{
 		base:    strings.TrimRight(cfg.BaseURL, "/"),
 		apiKey:  cfg.APIKey,
@@ -112,6 +121,46 @@ func New(cfg Config) (*Provider, error) {
 	}, nil
 }
 
+// managedHeaders are the request headers the adapter or net/http set; a
+// configured value for one would be overwritten or ignored.
+var managedHeaders = map[string]bool{
+	"Authorization":     true,
+	"Content-Type":      true,
+	"Content-Length":    true,
+	"Transfer-Encoding": true,
+	"Host":              true,
+	"Connection":        true,
+}
+
+// validHeaderName reports whether name is an RFC 9110 field-name: a
+// non-empty token of letters, digits and !#$%&'*+-.^_`|~.
+func validHeaderName(name string) bool {
+	if name == "" {
+		return false
+	}
+	for i := 0; i < len(name); i++ {
+		c := name[i]
+		switch {
+		case 'a' <= c && c <= 'z', 'A' <= c && c <= 'Z', '0' <= c && c <= '9':
+		case strings.IndexByte("!#$%&'*+-.^_`|~", c) >= 0:
+		default:
+			return false
+		}
+	}
+	return true
+}
+
+// validHeaderValue reports whether value is an RFC 9110 field-value: no
+// control characters other than horizontal tab, so no CR, LF or DEL.
+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
+}
+
 // maxResponseBody caps a successful response body; a larger one is an error.
 const maxResponseBody = 16 << 20
 
@@ -339,7 +388,6 @@ func (p *Provider) do(ctx context.Context, method, path string, payload []byte)
 	if payload != nil {
 		httpReq.Header.Set("Content-Type", "application/json")
 	}
-	httpReq.Header.Set("Accept", "application/json")
 	if p.apiKey != "" {
 		httpReq.Header.Set("Authorization", "Bearer "+p.apiKey)
 	}

internal/ai/llm/openai/openai_test.go

@@ -619,6 +619,40 @@ func TestGenerateRejectsInvalidRequests(t *testing.T) {
 	}
 }
 
+func TestNewRejectsBadHeaderWithoutEchoingValue(t *testing.T) {
+	_, err := New(Config{BaseURL: "https://example.com/v1", StructuredOutput: JSONSchema, Headers: map[string]string{"X-Title": testKey + "\u0019"}})
+	if err == nil {
+		t.Fatal("New() = nil error")
+	}
+	assertNoSecret(t, err)
+}
+
+func TestAcceptHeader(t *testing.T) {
+	for name, tc := range map[string]struct {
+		headers map[string]string
+		want    string
+	}{
+		"default":    {nil, "application/json"},
+		"configured": {map[string]string{"accept": "application/json, text/event-stream"}, "application/json, text/event-stream"},
+	} {
+		vs := newVendorServer(t, reply{status: 200, fixture: "openai_models.json"})
+		p := newProvider(t, vs.URL+"/v1", JSONSchema, tc.headers)
+		if _, err := p.ListModels(context.Background()); err != nil {
+			t.Fatalf("%s: %v", name, err)
+		}
+		if got := vs.only(t).header.Values("Accept"); len(got) != 1 || got[0] != tc.want {
+			t.Errorf("%s: Accept = %q, want %q", name, got, tc.want)
+		}
+	}
+}
+
+func TestNewAcceptsValidHeaders(t *testing.T) {
+	headers := map[string]string{"X-Trace_ID.v2!#$%&'*+^`|~": "tab\tand obs-text \xe2\x9c\x93", "X-Empty": ""}
+	if _, err := New(Config{BaseURL: "https://example.com/v1", StructuredOutput: JSONSchema, Headers: headers}); err != nil {
+		t.Fatal(err)
+	}
+}
+
 func TestNewRejectsBadConfig(t *testing.T) {
 	cases := map[string]Config{
 		"empty base":         {StructuredOutput: JSONSchema},
@@ -636,6 +670,15 @@ func TestNewRejectsBadConfig(t *testing.T) {
 		"content-type hdr":   {BaseURL: "https://example.com/v1", StructuredOutput: JSONSchema, Headers: map[string]string{"Content-Type": "text/plain"}},
 		"header newline":     {BaseURL: "https://example.com/v1", StructuredOutput: JSONSchema, Headers: map[string]string{"X-Title": "a\r\nb"}},
 		"header bad name":    {BaseURL: "https://example.com/v1", StructuredOutput: JSONSchema, Headers: map[string]string{"X Title": "a"}},
+		"header empty name":  {BaseURL: "https://example.com/v1", StructuredOutput: JSONSchema, Headers: map[string]string{"": "a"}},
+		"header bracket":     {BaseURL: "https://example.com/v1", StructuredOutput: JSONSchema, Headers: map[string]string{"X[Trace]": "a"}},
+		"header colon":       {BaseURL: "https://example.com/v1", StructuredOutput: JSONSchema, Headers: map[string]string{"X-Title:": "a"}},
+		"header non-ASCII":   {BaseURL: "https://example.com/v1", StructuredOutput: JSONSchema, Headers: map[string]string{"X-Tïtle": "a"}},
+		"header U+0019":      {BaseURL: "https://example.com/v1", StructuredOutput: JSONSchema, Headers: map[string]string{"X-Title": "a\u0019b"}},
+		"header NUL":         {BaseURL: "https://example.com/v1", StructuredOutput: JSONSchema, Headers: map[string]string{"X-Title": "a\x00b"}},
+		"header DEL":         {BaseURL: "https://example.com/v1", StructuredOutput: JSONSchema, Headers: map[string]string{"X-Title": "a\x7fb"}},
+		"host hdr":           {BaseURL: "https://example.com/v1", StructuredOutput: JSONSchema, Headers: map[string]string{"host": "other.example"}},
+		"content-length hdr": {BaseURL: "https://example.com/v1", StructuredOutput: JSONSchema, Headers: map[string]string{"Content-Length": "1"}},
 	}
 	for name, cfg := range cases {
 		cfg.APIKey = testKey