diff --git a/CHANGELOG.md b/CHANGELOG.md
index 3284c30..a6b5bb8 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -26,6 +26,8 @@ are marked **Breaking** and listed first in their section.
   release archives and the `http-assert` binary name are unchanged.
 - Assertion failures and evaluation errors are structured library data. The
   CLI owns human-readable formatting and preserves its existing output.
+- Assertion families use the exported `AssertionKind` type and constants
+  instead of requiring consumers to compare raw strings.
 - The library's zero-value client uses a 20-second total request timeout instead
   of the unbounded `http.DefaultClient`. Callers can still inject an HTTP client
   or apply a shorter request-context deadline.
diff --git a/api_test.go b/api_test.go
index 106f33d..6eb4949 100644
--- a/api_test.go
+++ b/api_test.go
@@ -13,6 +13,14 @@ func (f exampleTransport) RoundTrip(req *http.Request) (*http.Response, error) {
 	return f(req)
 }
 
+type customAssertion struct{}
+
+func (customAssertion) Kind() ha.AssertionKind { return "custom" }
+
+func (customAssertion) Check(*ha.Response) (*ha.Failure, error) { return nil, nil }
+
+var _ ha.Assertion = customAssertion{}
+
 func ExampleClient_Do() {
 	req, _ := http.NewRequest(http.MethodGet, "https://example.test/health", nil)
 	client := ha.Client{HTTPClient: &http.Client{Transport: exampleTransport(func(req *http.Request) (*http.Response, error) {
diff --git a/assertions.go b/assertions.go
index ce3b18a..73c25e7 100644
--- a/assertions.go
+++ b/assertions.go
@@ -11,6 +11,21 @@ import (
 	"github.com/itchyny/gojq"
 )
 
+// AssertionKind identifies an assertion family. Custom assertions may define
+// their own values; the constants below name the families built into this
+// package.
+type AssertionKind string
+
+const (
+	KindStatusOK  AssertionKind = "ok"
+	KindStatusNOK AssertionKind = "nok"
+	KindStatus    AssertionKind = "status"
+	KindHeader    AssertionKind = "header"
+	KindBody      AssertionKind = "body"
+	KindRedirect  AssertionKind = "redirect"
+	KindJQ        AssertionKind = "jq"
+)
+
 // Assertion checks one property of a response.
 //
 // Check separates the two things an assertion can report, which a single error
@@ -20,9 +35,8 @@ import (
 // failed run; the distinction exists so a machine-readable consumer can tell
 // "the service is wrong" from "we could not tell" (#45).
 type Assertion interface {
-	// Kind names the family this assertion belongs to: "ok", "nok",
-	// "status", "header", "body", "redirect" or "jq".
-	Kind() string
+	// Kind names the family this assertion belongs to.
+	Kind() AssertionKind
 
 	// Check reports (nil, nil) when the assertion holds.
 	Check(res *Response) (*Failure, error)
@@ -65,7 +79,7 @@ const (
 // Failure describes an assertion that was evaluated and did not hold. It is
 // deliberately data only: applications decide how (or whether) to format it.
 type Failure struct {
-	Kind     string // assertion family; Client and built-in assertions populate it
+	Kind     AssertionKind // assertion family; Client and built-in assertions populate it
 	Code     FailureCode
 	Target   string // header name, jq query, or "" when the kind needs no subject
 	Expected any
@@ -79,11 +93,11 @@ type Failure struct {
 // result needed to carry more than a string. Thirteen one-method structs would
 // have said the same thing at ten times the length.
 type assertionFunc struct {
-	kind  string
+	kind  AssertionKind
 	check func(res *Response) (*Failure, error)
 }
 
-func (a assertionFunc) Kind() string { return a.kind }
+func (a assertionFunc) Kind() AssertionKind { return a.kind }
 
 // Check stamps the failure with the assertion's kind, so Kind() and
 // Failure.Kind cannot disagree and no constructor has to repeat itself.
@@ -96,7 +110,7 @@ func (a assertionFunc) Check(res *Response) (*Failure, error) {
 	return f, err
 }
 
-func newAssertion(kind string, check func(res *Response) (*Failure, error)) Assertion {
+func newAssertion(kind AssertionKind, check func(res *Response) (*Failure, error)) Assertion {
 	return assertionFunc{kind: kind, check: check}
 }
 
@@ -228,7 +242,7 @@ func parseStatusCode(text string) (int, error) {
 
 // AssertStatusOK accepts any success or redirect status (2xx or 3xx).
 func AssertStatusOK() Assertion {
-	return newAssertion("ok", func(res *Response) (*Failure, error) {
+	return newAssertion(KindStatusOK, func(res *Response) (*Failure, error) {
 		if s := res.StatusCode; s < 200 || s >= 400 {
 			return &Failure{
 				Code:     FailureStatusOK,
@@ -243,7 +257,7 @@ func AssertStatusOK() Assertion {
 
 // AssertStatusNOK accepts any status outside the 2xx and 3xx ranges.
 func AssertStatusNOK() Assertion {
-	return newAssertion("nok", func(res *Response) (*Failure, error) {
+	return newAssertion(KindStatusNOK, func(res *Response) (*Failure, error) {
 		if s := res.StatusCode; s >= 200 && s < 400 {
 			return &Failure{
 				Code:     FailureStatusNOK,
@@ -269,7 +283,7 @@ func AssertStatus(text string) (Assertion, error) {
 }
 
 func assertStatus(spec statusSpec) Assertion {
-	return newAssertion("status", func(res *Response) (*Failure, error) {
+	return newAssertion(KindStatus, func(res *Response) (*Failure, error) {
 		if !spec.matches(res.StatusCode) {
 			return &Failure{
 				Code:     FailureStatus,
@@ -284,7 +298,7 @@ func assertStatus(spec statusSpec) Assertion {
 
 // AssertHeaderPresent requires at least one value for name.
 func AssertHeaderPresent(name string) Assertion {
-	return newAssertion("header", func(res *Response) (*Failure, error) {
+	return newAssertion(KindHeader, func(res *Response) (*Failure, error) {
 		if res.Header.Values(name) == nil {
 			return &Failure{
 				Code:     FailureHeaderPresent,
@@ -299,7 +313,7 @@ func AssertHeaderPresent(name string) Assertion {
 
 // AssertHeaderMissing requires name to be absent.
 func AssertHeaderMissing(name string) Assertion {
-	return newAssertion("header", func(res *Response) (*Failure, error) {
+	return newAssertion(KindHeader, func(res *Response) (*Failure, error) {
 		if vs := res.Header.Values(name); vs != nil {
 			return &Failure{
 				Code:     FailureHeaderMissing,
@@ -316,7 +330,7 @@ func AssertHeaderMissing(name string) Assertion {
 // AssertHeaderEqual accepts the response when any value of name equals
 // expValue.
 func AssertHeaderEqual(name, expValue string) Assertion {
-	return newAssertion("header", func(res *Response) (*Failure, error) {
+	return newAssertion(KindHeader, func(res *Response) (*Failure, error) {
 		vs := res.Header.Values(name)
 		if vs == nil {
 			return &Failure{
@@ -349,7 +363,7 @@ func AssertHeaderMatch(name, expPattern string) (Assertion, error) {
 		return nil, err
 	}
 
-	return newAssertion("header", func(res *Response) (*Failure, error) {
+	return newAssertion(KindHeader, func(res *Response) (*Failure, error) {
 		vs := res.Header.Values(name)
 		if vs == nil {
 			return &Failure{
@@ -385,7 +399,7 @@ func bodyOf(res *Response) ([]byte, error) {
 	if res.DecodeErr != nil {
 		return nil, &EvaluationError{
 			Code:     EvaluationBodyDecode,
-			Kind:     "body",
+			Kind:     KindBody,
 			Encoding: res.Encoding,
 			Cause:    res.DecodeErr,
 		}
@@ -396,7 +410,7 @@ func bodyOf(res *Response) ([]byte, error) {
 
 // AssertBodyEmpty requires the decoded response body to contain zero bytes.
 func AssertBodyEmpty() Assertion {
-	return newAssertion("body", func(res *Response) (*Failure, error) {
+	return newAssertion(KindBody, func(res *Response) (*Failure, error) {
 		body, err := bodyOf(res)
 		if err != nil {
 			return nil, err
@@ -450,7 +464,7 @@ func AssertJQ(query string) (Assertion, error) {
 		return nil, err
 	}
 
-	return newAssertion("jq", func(res *Response) (*Failure, error) {
+	return newAssertion(KindJQ, func(res *Response) (*Failure, error) {
 		return runJQ(code, query, res, jqTimeout)
 	}), nil
 }
@@ -486,7 +500,7 @@ func runJQ(code *gojq.Code, query string, res *Response, timeout time.Duration)
 		if e, isErr := v.(error); isErr {
 			return nil, &EvaluationError{
 				Code:   EvaluationJQ,
-				Kind:   "jq",
+				Kind:   KindJQ,
 				Target: query,
 				Cause:  e,
 			}
@@ -520,7 +534,7 @@ func runJQ(code *gojq.Code, query string, res *Response, timeout time.Duration)
 // AssertBodyNotEmpty requires the decoded response body to contain at least
 // one byte.
 func AssertBodyNotEmpty() Assertion {
-	return newAssertion("body", func(res *Response) (*Failure, error) {
+	return newAssertion(KindBody, func(res *Response) (*Failure, error) {
 		body, err := bodyOf(res)
 		if err != nil {
 			return nil, err
@@ -539,7 +553,7 @@ func AssertBodyNotEmpty() Assertion {
 
 // AssertBodyEqual requires the decoded response body to equal expContent.
 func AssertBodyEqual(expContent string) Assertion {
-	return newAssertion("body", func(res *Response) (*Failure, error) {
+	return newAssertion(KindBody, func(res *Response) (*Failure, error) {
 		body, err := bodyOf(res)
 		if err != nil {
 			return nil, err
@@ -576,7 +590,7 @@ func AssertBodyMatch(expPattern string) (Assertion, error) {
 		return nil, err
 	}
 
-	return newAssertion("body", func(res *Response) (*Failure, error) {
+	return newAssertion(KindBody, func(res *Response) (*Failure, error) {
 		body, err := bodyOf(res)
 		if err != nil {
 			return nil, err
@@ -631,7 +645,7 @@ func redirectPrecondition(res *Response, expected any) *Failure {
 // expLocation. Configure the HTTP client not to follow redirects when using
 // this assertion.
 func AssertRedirectEqual(expLocation string) Assertion {
-	return newAssertion("redirect", func(res *Response) (*Failure, error) {
+	return newAssertion(KindRedirect, func(res *Response) (*Failure, error) {
 		if f := redirectPrecondition(res, expLocation); f != nil {
 			return f, nil
 		}
@@ -658,7 +672,7 @@ func AssertRedirectMatch(expPattern string) (Assertion, error) {
 		return nil, err
 	}
 
-	return newAssertion("redirect", func(res *Response) (*Failure, error) {
+	return newAssertion(KindRedirect, func(res *Response) (*Failure, error) {
 		if f := redirectPrecondition(res, expPattern); f != nil {
 			return f, nil
 		}
diff --git a/assertions_test.go b/assertions_test.go
index bd3fdc7..f032d46 100644
--- a/assertions_test.go
+++ b/assertions_test.go
@@ -670,7 +670,7 @@ func Test_AssertionIdentity(t *testing.T) {
 		Name      string
 		Assertion Assertion
 		Res       *Response
-		Kind      string
+		Kind      AssertionKind
 		Target    string
 		Expected  any
 		Actual    any
@@ -678,43 +678,43 @@ func Test_AssertionIdentity(t *testing.T) {
 		{
 			Name: "ok", Assertion: AssertStatusOK(),
 			Res:  statusRes(500, "500 Internal Server Error"),
-			Kind: "ok", Expected: "2xx-3xx", Actual: 500,
+			Kind: KindStatusOK, Expected: "2xx-3xx", Actual: 500,
 		},
 		{
 			Name: "nok", Assertion: AssertStatusNOK(),
 			Res:  statusRes(200, "200 OK"),
-			Kind: "nok", Expected: "not 2xx-3xx", Actual: 200,
+			Kind: KindStatusNOK, Expected: "not 2xx-3xx", Actual: 200,
 		},
 		{
 			Name: "status", Assertion: mustStatusAssertion(t, "200"),
 			Res:  statusRes(500, "500 Internal Server Error"),
-			Kind: "status", Expected: "200", Actual: 500,
+			Kind: KindStatus, Expected: "200", Actual: 500,
 		},
 		{
 			Name: "header present", Assertion: AssertHeaderPresent("X-Absent"),
 			Res:  headerRes(http.Header{}),
-			Kind: "header", Target: "X-Absent", Expected: "present",
+			Kind: KindHeader, Target: "X-Absent", Expected: "present",
 		},
 		{
 			Name: "header equal", Assertion: AssertHeaderEqual("X-A", "want"),
 			Res:  headerRes(http.Header{"X-A": []string{"got"}}),
-			Kind: "header", Target: "X-A", Expected: "want", Actual: []string{"got"},
+			Kind: KindHeader, Target: "X-A", Expected: "want", Actual: []string{"got"},
 		},
 		{
 			Name: "body equal", Assertion: AssertBodyEqual("want"),
 			Res:  &Response{BodyBytes: []byte("got")},
-			Kind: "body", Expected: "want", Actual: "got",
+			Kind: KindBody, Expected: "want", Actual: "got",
 		},
 		{
 			Name: "redirect", Assertion: AssertRedirectEqual("/there"),
 			Res: headerRes(http.Header{"Location": []string{"/elsewhere"}}),
 			// A 200 never reaches the Location comparison, so this is the
 			// precondition failure, which reports the status it wanted.
-			Kind: "redirect", Expected: "3xx", Actual: 200,
+			Kind: KindRedirect, Expected: "3xx", Actual: 200,
 		},
 		{
 			Name: "jq", Assertion: jq, Res: jqResponse(`{"n":2}`),
-			Kind: "jq", Target: ".n == 1", Expected: true, Actual: false,
+			Kind: KindJQ, Target: ".n == 1", Expected: true, Actual: false,
 		},
 	}
 
diff --git a/client.go b/client.go
index aa775e4..35c65d6 100644
--- a/client.go
+++ b/client.go
@@ -41,7 +41,7 @@ const (
 // satisfy the assertion.
 type EvaluationError struct {
 	Code     EvaluationErrorCode
-	Kind     string
+	Kind     AssertionKind
 	Target   string
 	Encoding string
 	Cause    error
@@ -68,7 +68,7 @@ func (e *EvaluationError) Unwrap() error {
 // Outcome is the result of evaluating one assertion. Passed reports whether
 // both Failure and Err are nil.
 type Outcome struct {
-	Kind    string
+	Kind    AssertionKind
 	Failure *Failure
 	Err     error
 }
diff --git a/client_test.go b/client_test.go
index c4b3d99..03449a7 100644
--- a/client_test.go
+++ b/client_test.go
@@ -15,11 +15,11 @@ func (f roundTripFunc) RoundTrip(req *http.Request) (*http.Response, error) {
 }
 
 type testAssertion struct {
-	kind  string
+	kind  AssertionKind
 	check func(*Response) (*Failure, error)
 }
 
-func (a testAssertion) Kind() string { return a.kind }
+func (a testAssertion) Kind() AssertionKind { return a.kind }
 
 func (a testAssertion) Check(res *Response) (*Failure, error) {
 	return a.check(res)
@@ -27,7 +27,7 @@ func (a testAssertion) Check(res *Response) (*Failure, error) {
 
 type pointerAssertion struct{}
 
-func (*pointerAssertion) Kind() string { return "pointer" }
+func (*pointerAssertion) Kind() AssertionKind { return "pointer" }
 
 func (*pointerAssertion) Check(*Response) (*Failure, error) { return nil, nil }
 
diff --git a/response.go b/response.go
index e96f234..0cec283 100644
--- a/response.go
+++ b/response.go
@@ -53,7 +53,7 @@ func (r *Response) decodeJSON() (any, error) {
 	if err := json.Unmarshal(body, &r.jsonBody); err != nil {
 		r.jsonErr = &EvaluationError{
 			Code:  EvaluationJSON,
-			Kind:  "body",
+			Kind:  KindBody,
 			Cause: err,
 		}
 		return nil, r.jsonErr