diff --git a/.github/workflows/diff.yaml b/.github/workflows/diff.yaml
new file mode 100644
index 0000000..8459ec2
--- /dev/null
+++ b/.github/workflows/diff.yaml
@@ -0,0 +1,32 @@
+name: check diff
+
+on:
+  push:
+    tags:
+      - '*'
+
+permissions:
+  contents: write
+
+jobs:
+  goreleaser:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+
+      - name: Set up Go
+        uses: actions/setup-go@v5
+        with:
+          go-version: stable
+          check-latest: true
+
+      - name: Generate test
+        run: |
+          make generate
+
+      - name: Check diff
+        run: |
+          git diff --exit-code
diff --git a/analyzer.go b/analyzer.go
index 8b244d8..c05f784 100644
--- a/analyzer.go
+++ b/analyzer.go
@@ -34,6 +34,8 @@ func run(pass *analysis.Pass) (any, error) {
 		return nil, fmt.Errorf("want %T, got %T", spctor, pass.ResultOf[inspect.Analyzer])
 	}
 
+	wellKnownHeaders := initialism()
+
 	nodeFilter := []ast.Node{
 		(*ast.CallExpr)(nil),
 	}
@@ -97,8 +99,8 @@ func run(pass *analysis.Pass) (any, error) {
 			return
 		}
 
-		headerKeyCanonical := http.CanonicalHeaderKey(headerKeyOriginal)
-		if headerKeyOriginal == headerKeyCanonical {
+		headerKeyCanonical, isWellKnown := canonicalHeaderKey(headerKeyOriginal, wellKnownHeaders)
+		if headerKeyOriginal == headerKeyCanonical || isWellKnown {
 			return
 		}
 
@@ -131,6 +133,17 @@ func run(pass *analysis.Pass) (any, error) {
 	return nil, outerErr
 }
 
+func canonicalHeaderKey(s string, m map[string]string) (string, bool) {
+	canonical := http.CanonicalHeaderKey(s)
+
+	wellKnown, ok := m[canonical]
+	if !ok {
+		return canonical, ok
+	}
+
+	return wellKnown, ok
+}
+
 func isHTTPHeader(named *types.Named) bool {
 	return named.Obj() != nil &&
 		named.Obj().Pkg() != nil &&
diff --git a/analyzer_test.go b/analyzer_test.go
index b9a78ff..56a8153 100644
--- a/analyzer_test.go
+++ b/analyzer_test.go
@@ -22,6 +22,7 @@ func TestAnalyzer(t *testing.T) {
 		"common",
 		"embedded",
 		"global",
+		"initialism",
 		"struct",
 	}
 
diff --git a/cmd/initialismer/main.go b/cmd/initialismer/main.go
new file mode 100644
index 0000000..2441730
--- /dev/null
+++ b/cmd/initialismer/main.go
@@ -0,0 +1,390 @@
+package main
+
+import (
+	"cmp"
+	"flag"
+	"fmt"
+	"html/template"
+	"log/slog"
+	"net/http"
+	"os"
+	"slices"
+)
+
+var mustBeIgnore = [...]string{
+	"A-IM",
+	"Accept",
+	"Accept-Additions",
+	"Accept-CH",
+	"Accept-Charset",
+	"Accept-Datetime",
+	"Accept-Encoding",
+	"Accept-Features",
+	"Accept-Language",
+	"Accept-Patch",
+	"Accept-Post",
+	"Accept-Ranges",
+	"Accept-Signature",
+	"Access-Control",
+	"Access-Control-Allow-Credentials",
+	"Access-Control-Allow-Headers",
+	"Access-Control-Allow-Methods",
+	"Access-Control-Allow-Origin",
+	"Access-Control-Expose-Headers",
+	"Access-Control-Max-Age",
+	"Access-Control-Request-Headers",
+	"Access-Control-Request-Method",
+	"Age",
+	"Allow",
+	"ALPN",
+	"Alt-Svc",
+	"Alt-Used",
+	"Alternates",
+	"AMP-Cache-Transform",
+	"Apply-To-Redirect-Ref",
+	"Authentication-Control",
+	"Authentication-Info",
+	"Authorization",
+	"C-Ext",
+	"C-Man",
+	"C-Opt",
+	"C-PEP",
+	"C-PEP-Info",
+	"Cache-Control",
+	"Cache-Status",
+	"Cal-Managed-ID",
+	"CalDAV-Timezones",
+	"Capsule-Protocol",
+	"CDN-Cache-Control",
+	"CDN-Loop",
+	"Cert-Not-After",
+	"Cert-Not-Before",
+	"Clear-Site-Data",
+	"Client-Cert",
+	"Client-Cert-Chain",
+	"Close",
+	"Configuration-Context",
+	"Connection",
+	"Content-Base",
+	"Content-Digest",
+	"Content-Disposition",
+	"Content-Encoding",
+	"Content-ID",
+	"Content-Language",
+	"Content-Length",
+	"Content-Location",
+	"Content-MD5",
+	"Content-Range",
+	"Content-Script-Type",
+	"Content-Security-Policy",
+	"Content-Security-Policy-Report-Only",
+	"Content-Style-Type",
+	"Content-Type",
+	"Content-Version",
+	"Cookie",
+	"Cookie2",
+	"Cross-Origin-Embedder-Policy",
+	"Cross-Origin-Embedder-Policy-Report-Only",
+	"Cross-Origin-Opener-Policy",
+	"Cross-Origin-Opener-Policy-Report-Only",
+	"Cross-Origin-Resource-Policy",
+	"DASL",
+	"Date",
+	"DAV",
+	"Default-Style",
+	"Delta-Base",
+	"Depth",
+	"Derived-From",
+	"Destination",
+	"Differential-ID",
+	"Digest",
+	"DPoP",
+	"DPoP-Nonce",
+	"Early-Data",
+	"EDIINT-Features",
+	"Expect",
+	"Expect-CT",
+	"X-Correlation-ID",
+	"X-UA-Compatible",
+	"X-XSS-Protection",
+	"Expires",
+	"Ext",
+	"Forwarded",
+	"From",
+	"GetProfile",
+	"Hobareg",
+	"Host",
+	"HTTP2-Settings",
+	"If",
+	"If-Match",
+	"If-Modified-Since",
+	"If-None-Match",
+	"If-Range",
+	"If-Schedule-Tag-Match",
+	"If-Unmodified-Since",
+	"IM",
+	"Include-Referred-Token-Binding-ID",
+	"Isolation",
+	"Keep-Alive",
+	"Label",
+	"Last-Event-ID",
+	"Last-Modified",
+	"Link",
+	"Location",
+	"Lock-Token",
+	"Man",
+	"Max-Forwards",
+	"Memento-Datetime",
+	"Meter",
+	"Method-Check",
+	"Method-Check-Expires",
+	"MIME-Version",
+	"Negotiate",
+	"NEL",
+	"OData-EntityId",
+	"OData-Isolation",
+	"OData-MaxVersion",
+	"OData-Version",
+	"Opt",
+	"Optional-WWW-Authenticate",
+	"Ordering-Type",
+	"Origin",
+	"Origin-Agent-Cluster",
+	"OSCORE",
+	"OSLC-Core-Version",
+	"Overwrite",
+	"P3P",
+	"PEP",
+	"PEP-Info",
+	"Permissions-Policy",
+	"PICS-Label",
+	"Ping-From",
+	"Ping-To",
+	"Position",
+	"Pragma",
+	"Prefer",
+	"Preference-Applied",
+	"Priority",
+	"ProfileObject",
+	"Protocol",
+	"Protocol-Info",
+	"Protocol-Query",
+	"Protocol-Request",
+	"Proxy-Authenticate",
+	"Proxy-Authentication-Info",
+	"Proxy-Authorization",
+	"Proxy-Features",
+	"Proxy-Instruction",
+	"Proxy-Status",
+	"Public",
+	"Public-Key-Pins",
+	"Public-Key-Pins-Report-Only",
+	"Range",
+	"Redirect-Ref",
+	"Referer",
+	"Referer-Root",
+	"Refresh",
+	"Repeatability-Client-ID",
+	"Repeatability-First-Sent",
+	"Repeatability-Request-ID",
+	"Repeatability-Result",
+	"Replay-Nonce",
+	"Reporting-Endpoints",
+	"Repr-Digest",
+	"Retry-After",
+	"Safe",
+	"Schedule-Reply",
+	"Schedule-Tag",
+	"Sec-GPC",
+	"Sec-Purpose",
+	"Sec-Token-Binding",
+	"Sec-WebSocket-Accept",
+	"Sec-WebSocket-Extensions",
+	"Sec-WebSocket-Key",
+	"Sec-WebSocket-Protocol",
+	"Sec-WebSocket-Version",
+	"Security-Scheme",
+	"Server",
+	"Server-Timing",
+	"Set-Cookie",
+	"Set-Cookie2",
+	"SetProfile",
+	"Signature",
+	"Signature-Input",
+	"SLUG",
+	"SoapAction",
+	"Status-URI",
+	"Strict-Transport-Security",
+	"Sunset",
+	"Surrogate-Capability",
+	"Surrogate-Control",
+	"TCN",
+	"TE",
+	"Timeout",
+	"Timing-Allow-Origin",
+	"Topic",
+	"Traceparent",
+	"Tracestate",
+	"Trailer",
+	"Transfer-Encoding",
+	"TTL",
+	"Upgrade",
+	"Urgency",
+	"URI",
+	"User-Agent",
+	"Variant-Vary",
+	"Vary",
+	"Via",
+	"Want-Content-Digest",
+	"Want-Digest",
+	"Want-Repr-Digest",
+	"Warning",
+	"X-Content-Type-Options",
+	"X-Frame-Options",
+	"ETag",
+	"DNT",
+	"X-Request-ID",
+	"X-XSS",
+	"X-DNS-Prefetch-Control",
+	"WWW-Authenticate",
+	"X-WebKit-CSP",
+	"X-Real-IP",
+}
+
+type generateTarget uint8
+
+func (t generateTarget) String() string {
+	switch t {
+	case generateTest:
+		return "test"
+	case generateTestGolden:
+		return "test-golden"
+	case generateMapping:
+		return "mapping"
+	default:
+		return "unknown"
+	}
+}
+
+const (
+	generateUnknown generateTarget = iota
+	generateTest
+	generateTestGolden
+	generateMapping
+)
+
+func parseTarget() (generateTarget, error) {
+	var t string
+	flag.StringVar(&t, "target", "", "test or mapping")
+	flag.Parse()
+
+	switch t {
+	case generateTest.String():
+		return generateTest, nil
+
+	case generateMapping.String():
+		return generateMapping, nil
+
+	case generateTestGolden.String():
+		return generateTestGolden, nil
+
+	default:
+		return generateUnknown, fmt.Errorf("unknown target %q", t)
+	}
+}
+
+func main() {
+	filtered := make(map[string]struct{}, len(mustBeIgnore))
+
+	type ignore struct {
+		Canonical,
+		Original string
+	}
+
+	results := make([]ignore, 0, len(mustBeIgnore))
+
+	for _, s := range mustBeIgnore {
+		_, ok := filtered[s]
+		if ok {
+			slog.Error("has duplicate:", slog.String("duplicate", s))
+			os.Exit(1)
+		}
+		filtered[s] = struct{}{}
+
+		canonical := http.CanonicalHeaderKey(s)
+		if canonical != s {
+			results = append(results, ignore{
+				Canonical: canonical,
+				Original:  s,
+			})
+		}
+	}
+	slices.SortFunc(results, func(a, b ignore) int {
+		return cmp.Compare(a.Canonical, b.Canonical)
+	})
+
+	genTarget, err := parseTarget()
+	if err != nil {
+		slog.Error("parse target:", slog.Any("error", err))
+	}
+
+	var tmpl string
+	switch genTarget {
+	case generateTest:
+		tmpl = tmplTest
+
+	case generateMapping:
+		tmpl = tmplMapping
+
+	case generateTestGolden:
+		tmpl = tmplTestGolden
+	}
+
+	t := template.Must(template.New("").Parse(tmpl))
+
+	err = t.Execute(os.Stdout, results)
+	if err == nil {
+		return
+	}
+
+	slog.Error("execute template:", slog.Any("error", err))
+	os.Exit(1)
+}
+
+const tmplMapping = `// Code generated by initialismer; DO NOT EDIT.
+package canonicalheader
+
+// initialism mapping of not canonical headers from
+// https://en.wikipedia.org/wiki/List_of_HTTP_header_fields
+// https://www.iana.org/assignments/http-fields/http-fields.xhtml.
+func initialism() map[string]string {
+    return map[string]string{
+        {{range .}}"{{.Canonical}}": "{{.Original}}",
+        {{end}}
+    }
+}
+`
+
+const tmplTest = `// Code generated by initialismer; DO NOT EDIT.
+package initialism
+
+import "net/http"
+
+func _() {
+    h := http.Header{}
+    {{range .}}
+    h.Get("{{.Original}}"){{end}}
+}
+`
+
+const tmplTestGolden = `// Code generated by initialismer; DO NOT EDIT.
+package initialism
+
+import "net/http"
+
+func _() {
+    h := http.Header{}
+    {{range .}}
+    h.Get("{{.Original}}"){{end}}
+}
+`
diff --git a/initialism.go b/initialism.go
new file mode 100644
index 0000000..c3d91c2
--- /dev/null
+++ b/initialism.go
@@ -0,0 +1,75 @@
+// Code generated by initialismer; DO NOT EDIT.
+package canonicalheader
+
+// initialism mapping of not canonical headers from
+// https://en.wikipedia.org/wiki/List_of_HTTP_header_fields
+// https://www.iana.org/assignments/http-fields/http-fields.xhtml.
+func initialism() map[string]string {
+	return map[string]string{
+		"A-Im":                              "A-IM",
+		"Accept-Ch":                         "Accept-CH",
+		"Alpn":                              "ALPN",
+		"Amp-Cache-Transform":               "AMP-Cache-Transform",
+		"C-Pep":                             "C-PEP",
+		"C-Pep-Info":                        "C-PEP-Info",
+		"Cal-Managed-Id":                    "Cal-Managed-ID",
+		"Caldav-Timezones":                  "CalDAV-Timezones",
+		"Cdn-Cache-Control":                 "CDN-Cache-Control",
+		"Cdn-Loop":                          "CDN-Loop",
+		"Content-Id":                        "Content-ID",
+		"Content-Md5":                       "Content-MD5",
+		"Dasl":                              "DASL",
+		"Dav":                               "DAV",
+		"Differential-Id":                   "Differential-ID",
+		"Dnt":                               "DNT",
+		"Dpop":                              "DPoP",
+		"Dpop-Nonce":                        "DPoP-Nonce",
+		"Ediint-Features":                   "EDIINT-Features",
+		"Etag":                              "ETag",
+		"Expect-Ct":                         "Expect-CT",
+		"Getprofile":                        "GetProfile",
+		"Http2-Settings":                    "HTTP2-Settings",
+		"Im":                                "IM",
+		"Include-Referred-Token-Binding-Id": "Include-Referred-Token-Binding-ID",
+		"Last-Event-Id":                     "Last-Event-ID",
+		"Mime-Version":                      "MIME-Version",
+		"Nel":                               "NEL",
+		"Odata-Entityid":                    "OData-EntityId",
+		"Odata-Isolation":                   "OData-Isolation",
+		"Odata-Maxversion":                  "OData-MaxVersion",
+		"Odata-Version":                     "OData-Version",
+		"Optional-Www-Authenticate":         "Optional-WWW-Authenticate",
+		"Oscore":                            "OSCORE",
+		"Oslc-Core-Version":                 "OSLC-Core-Version",
+		"P3p":                               "P3P",
+		"Pep":                               "PEP",
+		"Pep-Info":                          "PEP-Info",
+		"Pics-Label":                        "PICS-Label",
+		"Profileobject":                     "ProfileObject",
+		"Repeatability-Client-Id":           "Repeatability-Client-ID",
+		"Repeatability-Request-Id":          "Repeatability-Request-ID",
+		"Sec-Gpc":                           "Sec-GPC",
+		"Sec-Websocket-Accept":              "Sec-WebSocket-Accept",
+		"Sec-Websocket-Extensions":          "Sec-WebSocket-Extensions",
+		"Sec-Websocket-Key":                 "Sec-WebSocket-Key",
+		"Sec-Websocket-Protocol":            "Sec-WebSocket-Protocol",
+		"Sec-Websocket-Version":             "Sec-WebSocket-Version",
+		"Setprofile":                        "SetProfile",
+		"Slug":                              "SLUG",
+		"Soapaction":                        "SoapAction",
+		"Status-Uri":                        "Status-URI",
+		"Tcn":                               "TCN",
+		"Te":                                "TE",
+		"Ttl":                               "TTL",
+		"Uri":                               "URI",
+		"Www-Authenticate":                  "WWW-Authenticate",
+		"X-Correlation-Id":                  "X-Correlation-ID",
+		"X-Dns-Prefetch-Control":            "X-DNS-Prefetch-Control",
+		"X-Real-Ip":                         "X-Real-IP",
+		"X-Request-Id":                      "X-Request-ID",
+		"X-Ua-Compatible":                   "X-UA-Compatible",
+		"X-Webkit-Csp":                      "X-WebKit-CSP",
+		"X-Xss":                             "X-XSS",
+		"X-Xss-Protection":                  "X-XSS-Protection",
+	}
+}
diff --git a/makefile b/makefile
index 2b91254..a96cb62 100644
--- a/makefile
+++ b/makefile
@@ -5,3 +5,8 @@ test:
 
 linter:
 	golangci-lint -v run ./...
+
+generate:
+	go run ./cmd/initialismer/*.go -target="mapping" > ./initialism.go
+	go run ./cmd/initialismer/*.go -target="test" > ./testdata/src/initialism/initialism.go
+	gofmt -w ./initialism.go ./testdata/src/initialism/initialism.go
diff --git a/testdata/src/initialism/initialism.go b/testdata/src/initialism/initialism.go
new file mode 100644
index 0000000..a4cccc1
--- /dev/null
+++ b/testdata/src/initialism/initialism.go
@@ -0,0 +1,74 @@
+// Code generated by initialismer; DO NOT EDIT.
+package initialism
+
+import "net/http"
+
+func _() {
+	h := http.Header{}
+
+	h.Get("A-IM")
+	h.Get("Accept-CH")
+	h.Get("ALPN")
+	h.Get("AMP-Cache-Transform")
+	h.Get("C-PEP")
+	h.Get("C-PEP-Info")
+	h.Get("Cal-Managed-ID")
+	h.Get("CalDAV-Timezones")
+	h.Get("CDN-Cache-Control")
+	h.Get("CDN-Loop")
+	h.Get("Content-ID")
+	h.Get("Content-MD5")
+	h.Get("DASL")
+	h.Get("DAV")
+	h.Get("Differential-ID")
+	h.Get("DNT")
+	h.Get("DPoP")
+	h.Get("DPoP-Nonce")
+	h.Get("EDIINT-Features")
+	h.Get("ETag")
+	h.Get("Expect-CT")
+	h.Get("GetProfile")
+	h.Get("HTTP2-Settings")
+	h.Get("IM")
+	h.Get("Include-Referred-Token-Binding-ID")
+	h.Get("Last-Event-ID")
+	h.Get("MIME-Version")
+	h.Get("NEL")
+	h.Get("OData-EntityId")
+	h.Get("OData-Isolation")
+	h.Get("OData-MaxVersion")
+	h.Get("OData-Version")
+	h.Get("Optional-WWW-Authenticate")
+	h.Get("OSCORE")
+	h.Get("OSLC-Core-Version")
+	h.Get("P3P")
+	h.Get("PEP")
+	h.Get("PEP-Info")
+	h.Get("PICS-Label")
+	h.Get("ProfileObject")
+	h.Get("Repeatability-Client-ID")
+	h.Get("Repeatability-Request-ID")
+	h.Get("Sec-GPC")
+	h.Get("Sec-WebSocket-Accept")
+	h.Get("Sec-WebSocket-Extensions")
+	h.Get("Sec-WebSocket-Key")
+	h.Get("Sec-WebSocket-Protocol")
+	h.Get("Sec-WebSocket-Version")
+	h.Get("SetProfile")
+	h.Get("SLUG")
+	h.Get("SoapAction")
+	h.Get("Status-URI")
+	h.Get("TCN")
+	h.Get("TE")
+	h.Get("TTL")
+	h.Get("URI")
+	h.Get("WWW-Authenticate")
+	h.Get("X-Correlation-ID")
+	h.Get("X-DNS-Prefetch-Control")
+	h.Get("X-Real-IP")
+	h.Get("X-Request-ID")
+	h.Get("X-UA-Compatible")
+	h.Get("X-WebKit-CSP")
+	h.Get("X-XSS")
+	h.Get("X-XSS-Protection")
+}