diff --git a/.github/workflows/ci.yaml b/.github/workflows/ci.yaml
index 322e003..68aa6b8 100644
--- a/.github/workflows/ci.yaml
+++ b/.github/workflows/ci.yaml
@@ -10,7 +10,7 @@ permissions:
     contents: read
 
 env:
-    SPEAKEASY_VERSION: v2.959.0
+    SPEAKEASY_VERSION: v2.959.1
 
 jobs:
     non-live-validation:
diff --git a/internal/client/client.go b/internal/client/client.go
index 710fba8..7ace0ab 100644
--- a/internal/client/client.go
+++ b/internal/client/client.go
@@ -79,10 +79,39 @@ func NewClient(cmd *cobra.Command, allowedSecurityFields ...string) (*sdk.Gemini
 		httpClient = testClient
 	}
 	httpClient = WrapClientForDiagnostics(cmd, httpClient)
+	if hdrs, _ := flagutil.GetStringArrayFlag(cmd, "header"); len(hdrs) > 0 {
+		headers, err := flagutil.ParseHeaderFlags(hdrs)
+		if err != nil {
+			return nil, err
+		}
+		if repeated := flagutil.RepeatedHeaders(headers); len(repeated) > 0 {
+			httpClient = &repeatedHeaderClient{next: httpClient, headers: repeated}
+		}
+	}
 	sdkOpts = append(sdkOpts, sdk.WithClient(httpClient))
 	return sdk.New(sdkOpts...), nil
 }
 
+// repeatedHeaderClient sends every value of a header repeated with --header on
+// requests tagged with flagutil.RepeatedHeadersMarker.
+// operations.WithSetHeaders carries only one value per header.
+type repeatedHeaderClient struct {
+	next    HTTPClient
+	headers http.Header
+}
+
+func (c *repeatedHeaderClient) Do(req *http.Request) (*http.Response, error) {
+	if req.Header.Get(flagutil.RepeatedHeadersMarker) == "" {
+		return c.next.Do(req)
+	}
+	req = req.Clone(req.Context())
+	req.Header.Del(flagutil.RepeatedHeadersMarker)
+	for name, values := range c.headers {
+		req.Header[name] = append([]string(nil), values...)
+	}
+	return c.next.Do(req)
+}
+
 // SameOriginRedirect refuses redirects to another scheme or host, and
 // redirects that change the request method: net/http forwards custom
 // credential headers (API keys) across origins and turns a POST answered with
diff --git a/internal/flagutil/flags.go b/internal/flagutil/flags.go
index 0946002..0a35119 100644
--- a/internal/flagutil/flags.go
+++ b/internal/flagutil/flags.go
@@ -24,6 +24,7 @@ import (
 	"fmt"
 	"io"
 	"net"
+	"net/http"
 	"net/url"
 	"os"
 	"path/filepath"
@@ -611,6 +612,72 @@ func (e serverURLValidationError) Error() string {
 	return fmt.Sprintf("invalid --server-url %q: %v", e.value, e.error)
 }
 
+// ParseHeaderFlags parses "Name: value" --header values into canonical header
+// names, keeping repeated values in the order given.
+func ParseHeaderFlags(values []string) (http.Header, error) {
+	headers := http.Header{}
+	for _, h := range values {
+		k, v, ok := strings.Cut(h, ":")
+		if !ok {
+			return nil, WithCLIValidation(fmt.Errorf("invalid --header value: expected \"Key: Value\" with a colon after the header name"))
+		}
+		name, value := strings.TrimSpace(k), strings.TrimSpace(v)
+		if !validHeaderName(name) {
+			return nil, WithCLIValidation(fmt.Errorf("invalid header name %q: use letters, digits and !#$%%&'*+-.^_`|~", name))
+		}
+		if !validHeaderValue(value) {
+			return nil, WithCLIValidation(fmt.Errorf("invalid value for header %q: control characters such as CR or LF are not allowed", name))
+		}
+		if http.CanonicalHeaderKey(name) == RepeatedHeadersMarker {
+			return nil, WithCLIValidation(fmt.Errorf("invalid header name %q: reserved for internal use", name))
+		}
+		headers.Add(name, value)
+	}
+	return headers, nil
+}
+
+// RepeatedHeadersMarker tags the requests an operation builds itself, so the
+// remaining values of a repeated --header reach only those requests and not
+// the requests SDK hooks send through the same client, such as OAuth token
+// calls.
+const RepeatedHeadersMarker = "X-Cli-Repeated-Headers"
+
+// RepeatedHeaders returns the headers given more than once.
+func RepeatedHeaders(headers http.Header) http.Header {
+	repeated := http.Header{}
+	for name, values := range headers {
+		if len(values) > 1 {
+			repeated[name] = values
+		}
+	}
+	return repeated
+}
+
+func validHeaderName(name string) bool {
+	if name == "" {
+		return false
+	}
+	for i := 0; i < len(name); i++ {
+		c := name[i]
+		switch {
+		case 'a' <= c && c <= 'z', 'A' <= c && c <= 'Z', '0' <= c && c <= '9':
+		case strings.IndexByte("!#$%&'*+-.^_`|~", c) >= 0:
+		default:
+			return false
+		}
+	}
+	return true
+}
+
+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
+}
+
 func ValidateServerURL(value string) error {
 	u, err := url.Parse(value)
 	if err != nil {
diff --git a/internal/output/output.go b/internal/output/output.go
index 927c39e..ac63527 100644
--- a/internal/output/output.go
+++ b/internal/output/output.go
@@ -364,18 +364,23 @@ func renderPathEnvelope(cmd *cobra.Command, absPath string, envelope map[string]
 }
 
 // PrepareCallOpts builds common SDK call options from CLI flags.
-// Parses --header flags into operations.WithSetHeaders options.
+// Parses --header flags into operations.WithSetHeaders options carrying each
+// header's first value; client.NewClient sends the remaining values of a
+// repeated header.
 // Server resolution and skip-deserialization are handled per-operation.
 func PrepareCallOpts(cmd *cobra.Command) ([]operations.Option, error) {
 	var opts []operations.Option
 	if hdrs, _ := flagutil.GetStringArrayFlag(cmd, "header"); len(hdrs) > 0 {
-		headerMap := make(map[string]string, len(hdrs))
-		for _, h := range hdrs {
-			k, v, ok := strings.Cut(h, ":")
-			if !ok {
-				return nil, flagutil.WithCLIValidation(fmt.Errorf("invalid header format %q: expected \"Key: Value\"", h))
-			}
-			headerMap[strings.TrimSpace(k)] = strings.TrimSpace(v)
+		headers, err := flagutil.ParseHeaderFlags(hdrs)
+		if err != nil {
+			return nil, err
+		}
+		headerMap := make(map[string]string, len(headers))
+		for name, values := range headers {
+			headerMap[name] = values[0]
+		}
+		if len(flagutil.RepeatedHeaders(headers)) > 0 {
+			headerMap[flagutil.RepeatedHeadersMarker] = "1"
 		}
 		opts = append(opts, operations.WithSetHeaders(headerMap))
 	}