diff --git a/jsonschema/infer.go b/jsonschema/infer.go
index 9ff0ddd5..7b6b7e2b 100644
--- a/jsonschema/infer.go
+++ b/jsonschema/infer.go
@@ -8,15 +8,40 @@ package jsonschema
 
 import (
 	"fmt"
+	"log/slog"
+	"math/big"
 	"reflect"
 	"regexp"
+	"time"
 
 	"github.com/modelcontextprotocol/go-sdk/internal/util"
 )
 
+// ForOptions are options for the [For] function.
+type ForOptions struct {
+	// If IgnoreInvalidTypes is true, fields that can't be represented as a JSON Schema
+	// are ignored instead of causing an error.
+	// This allows callers to adjust the resulting schema using custom knowledge.
+	// For example, an interface type where all the possible implementations are
+	// known can be described with "oneof".
+	IgnoreInvalidTypes bool
+
+	// TypeSchemas maps types to their schemas.
+	// If [For] encounters a type equal to a type of a key in this map, the
+	// corresponding value is used as the resulting schema (after cloning to
+	// ensure uniqueness).
+	// Types in this map override the default translations, as described
+	// in [For]'s documentation.
+	TypeSchemas map[any]*Schema
+}
+
 // For constructs a JSON schema object for the given type argument.
+// If non-nil, the provided options configure certain aspects of this contruction,
+// described below.
+
+// It translates Go types into compatible JSON schema types, as follows.
+// These defaults can be overridden by [ForOptions.TypeSchemas].
 //
-// It translates Go types into compatible JSON schema types, as follows:
 //   - Strings have schema type "string".
 //   - Bools have schema type "boolean".
 //   - Signed and unsigned integer types have schema type "integer".
@@ -29,48 +54,51 @@ import (
 //     Their properties are derived from exported struct fields, using the
 //     struct field JSON name. Fields that are marked "omitempty" are
 //     considered optional; all other fields become required properties.
+//   - Some types in the standard library that implement json.Marshaler
+//     translate to schemas that match the values to which they marshal.
+//     For example, [time.Time] translates to the schema for strings.
+//
+// For will return an error if there is a cycle in the types.
 //
-// For returns an error if t contains (possibly recursively) any of the following Go
-// types, as they are incompatible with the JSON schema spec.
+// By default, For returns an error if t contains (possibly recursively) any of the
+// following Go types, as they are incompatible with the JSON schema spec.
+// If [ForOptions.IgnoreInvalidTypes] is true, then these types are ignored instead.
 //   - maps with key other than 'string'
 //   - function types
 //   - channel types
 //   - complex numbers
 //   - unsafe pointers
 //
-// It will return an error if there is a cycle in the types.
-//
 // This function recognizes struct field tags named "jsonschema".
 // A jsonschema tag on a field is used as the description for the corresponding property.
 // For future compatibility, descriptions must not start with "WORD=", where WORD is a
 // sequence of non-whitespace characters.
-func For[T any]() (*Schema, error) {
-	// TODO: consider skipping incompatible fields, instead of failing.
-	seen := make(map[reflect.Type]bool)
-	s, err := forType(reflect.TypeFor[T](), seen, false)
-	if err != nil {
-		var z T
-		return nil, fmt.Errorf("For[%T](): %w", z, err)
+func For[T any](opts *ForOptions) (*Schema, error) {
+	if opts == nil {
+		opts = &ForOptions{}
 	}
-	return s, nil
-}
-
-// ForLax behaves like [For], except that it ignores struct fields with invalid types instead of
-// returning an error. That allows callers to adjust the resulting schema using custom knowledge.
-// For example, an interface type where all the possible implementations are known
-// can be described with "oneof".
-func ForLax[T any]() (*Schema, error) {
-	// TODO: consider skipping incompatible fields, instead of failing.
-	seen := make(map[reflect.Type]bool)
-	s, err := forType(reflect.TypeFor[T](), seen, true)
+	schemas := make(map[reflect.Type]*Schema)
+	// Add types from the standard library that have MarshalJSON methods.
+	ss := &Schema{Type: "string"}
+	schemas[reflect.TypeFor[time.Time]()] = ss
+	schemas[reflect.TypeFor[slog.Level]()] = ss
+	schemas[reflect.TypeFor[big.Int]()] = &Schema{Types: []string{"null", "string"}}
+	schemas[reflect.TypeFor[big.Rat]()] = ss
+	schemas[reflect.TypeFor[big.Float]()] = ss
+
+	// Add types from the options. They override the default ones.
+	for v, s := range opts.TypeSchemas {
+		schemas[reflect.TypeOf(v)] = s
+	}
+	s, err := forType(reflect.TypeFor[T](), map[reflect.Type]bool{}, opts.IgnoreInvalidTypes, schemas)
 	if err != nil {
 		var z T
-		return nil, fmt.Errorf("ForLax[%T](): %w", z, err)
+		return nil, fmt.Errorf("For[%T](): %w", z, err)
 	}
 	return s, nil
 }
 
-func forType(t reflect.Type, seen map[reflect.Type]bool, lax bool) (*Schema, error) {
+func forType(t reflect.Type, seen map[reflect.Type]bool, ignore bool, schemas map[reflect.Type]*Schema) (*Schema, error) {
 	// Follow pointers: the schema for *T is almost the same as for T, except that
 	// an explicit JSON "null" is allowed for the pointer.
 	allowNull := false
@@ -89,6 +117,10 @@ func forType(t reflect.Type, seen map[reflect.Type]bool, lax bool) (*Schema, err
 		defer delete(seen, t)
 	}
 
+	if s := schemas[t]; s != nil {
+		return s.CloneSchemas(), nil
+	}
+
 	var (
 		s   = new(Schema)
 		err error
@@ -111,7 +143,7 @@ func forType(t reflect.Type, seen map[reflect.Type]bool, lax bool) (*Schema, err
 
 	case reflect.Map:
 		if t.Key().Kind() != reflect.String {
-			if lax {
+			if ignore {
 				return nil, nil // ignore
 			}
 			return nil, fmt.Errorf("unsupported map key type %v", t.Key().Kind())
@@ -119,22 +151,22 @@ func forType(t reflect.Type, seen map[reflect.Type]bool, lax bool) (*Schema, err
 		if t.Key().Kind() != reflect.String {
 		}
 		s.Type = "object"
-		s.AdditionalProperties, err = forType(t.Elem(), seen, lax)
+		s.AdditionalProperties, err = forType(t.Elem(), seen, ignore, schemas)
 		if err != nil {
 			return nil, fmt.Errorf("computing map value schema: %v", err)
 		}
-		if lax && s.AdditionalProperties == nil {
+		if ignore && s.AdditionalProperties == nil {
 			// Ignore if the element type is invalid.
 			return nil, nil
 		}
 
 	case reflect.Slice, reflect.Array:
 		s.Type = "array"
-		s.Items, err = forType(t.Elem(), seen, lax)
+		s.Items, err = forType(t.Elem(), seen, ignore, schemas)
 		if err != nil {
 			return nil, fmt.Errorf("computing element schema: %v", err)
 		}
-		if lax && s.Items == nil {
+		if ignore && s.Items == nil {
 			// Ignore if the element type is invalid.
 			return nil, nil
 		}
@@ -160,11 +192,11 @@ func forType(t reflect.Type, seen map[reflect.Type]bool, lax bool) (*Schema, err
 			if s.Properties == nil {
 				s.Properties = make(map[string]*Schema)
 			}
-			fs, err := forType(field.Type, seen, lax)
+			fs, err := forType(field.Type, seen, ignore, schemas)
 			if err != nil {
 				return nil, err
 			}
-			if lax && fs == nil {
+			if ignore && fs == nil {
 				// Skip fields of invalid type.
 				continue
 			}
@@ -184,7 +216,7 @@ func forType(t reflect.Type, seen map[reflect.Type]bool, lax bool) (*Schema, err
 		}
 
 	default:
-		if lax {
+		if ignore {
 			// Ignore.
 			return nil, nil
 		}
@@ -194,6 +226,7 @@ func forType(t reflect.Type, seen map[reflect.Type]bool, lax bool) (*Schema, err
 		s.Types = []string{"null", s.Type}
 		s.Type = ""
 	}
+	schemas[t] = s
 	return s, nil
 }
 
diff --git a/jsonschema/infer_test.go b/jsonschema/infer_test.go
index 8c8feec0..1a0895b4 100644
--- a/jsonschema/infer_test.go
+++ b/jsonschema/infer_test.go
@@ -5,22 +5,30 @@
 package jsonschema_test
 
 import (
+	"log/slog"
+	"math/big"
 	"strings"
 	"testing"
+	"time"
 
 	"github.com/google/go-cmp/cmp"
 	"github.com/google/go-cmp/cmp/cmpopts"
 	"github.com/modelcontextprotocol/go-sdk/jsonschema"
 )
 
-func forType[T any](lax bool) *jsonschema.Schema {
+type custom int
+
+func forType[T any](ignore bool) *jsonschema.Schema {
 	var s *jsonschema.Schema
 	var err error
-	if lax {
-		s, err = jsonschema.ForLax[T]()
-	} else {
-		s, err = jsonschema.For[T]()
+
+	opts := &jsonschema.ForOptions{
+		IgnoreInvalidTypes: ignore,
+		TypeSchemas: map[any]*jsonschema.Schema{
+			custom(0): {Type: "custom"},
+		},
 	}
+	s, err = jsonschema.For[T](opts)
 	if err != nil {
 		panic(err)
 	}
@@ -40,19 +48,23 @@ func TestFor(t *testing.T) {
 		want *jsonschema.Schema
 	}
 
-	tests := func(lax bool) []test {
+	tests := func(ignore bool) []test {
 		return []test{
-			{"string", forType[string](lax), &schema{Type: "string"}},
-			{"int", forType[int](lax), &schema{Type: "integer"}},
-			{"int16", forType[int16](lax), &schema{Type: "integer"}},
-			{"uint32", forType[int16](lax), &schema{Type: "integer"}},
-			{"float64", forType[float64](lax), &schema{Type: "number"}},
-			{"bool", forType[bool](lax), &schema{Type: "boolean"}},
-			{"intmap", forType[map[string]int](lax), &schema{
+			{"string", forType[string](ignore), &schema{Type: "string"}},
+			{"int", forType[int](ignore), &schema{Type: "integer"}},
+			{"int16", forType[int16](ignore), &schema{Type: "integer"}},
+			{"uint32", forType[int16](ignore), &schema{Type: "integer"}},
+			{"float64", forType[float64](ignore), &schema{Type: "number"}},
+			{"bool", forType[bool](ignore), &schema{Type: "boolean"}},
+			{"time", forType[time.Time](ignore), &schema{Type: "string"}},
+			{"level", forType[slog.Level](ignore), &schema{Type: "string"}},
+			{"bigint", forType[big.Int](ignore), &schema{Types: []string{"null", "string"}}},
+			{"custom", forType[custom](ignore), &schema{Type: "custom"}},
+			{"intmap", forType[map[string]int](ignore), &schema{
 				Type:                 "object",
 				AdditionalProperties: &schema{Type: "integer"},
 			}},
-			{"anymap", forType[map[string]any](lax), &schema{
+			{"anymap", forType[map[string]any](ignore), &schema{
 				Type:                 "object",
 				AdditionalProperties: &schema{},
 			}},
@@ -66,7 +78,7 @@ func TestFor(t *testing.T) {
 					NoSkip      string `json:",omitempty"`
 					unexported  float64
 					unexported2 int `json:"No"`
-				}](lax),
+				}](ignore),
 				&schema{
 					Type: "object",
 					Properties: map[string]*schema{
@@ -81,7 +93,7 @@ func TestFor(t *testing.T) {
 			},
 			{
 				"no sharing",
-				forType[struct{ X, Y int }](lax),
+				forType[struct{ X, Y int }](ignore),
 				&schema{
 					Type: "object",
 					Properties: map[string]*schema{
@@ -97,7 +109,7 @@ func TestFor(t *testing.T) {
 				forType[struct {
 					A S
 					S
-				}](lax),
+				}](ignore),
 				&schema{
 					Type: "object",
 					Properties: map[string]*schema{
@@ -165,7 +177,7 @@ func TestFor(t *testing.T) {
 }
 
 func forErr[T any]() error {
-	_, err := jsonschema.For[T]()
+	_, err := jsonschema.For[T](nil)
 	return err
 }
 
@@ -209,7 +221,7 @@ func TestForWithMutation(t *testing.T) {
 		D [3]S
 		E *bool
 	}
-	s, err := jsonschema.For[T]()
+	s, err := jsonschema.For[T](nil)
 	if err != nil {
 		t.Fatalf("For: %v", err)
 	}
@@ -220,7 +232,7 @@ func TestForWithMutation(t *testing.T) {
 	s.Properties["D"].MinItems = jsonschema.Ptr(10)
 	s.Properties["E"].Types[0] = "mutated"
 
-	s2, err := jsonschema.For[T]()
+	s2, err := jsonschema.For[T](nil)
 	if err != nil {
 		t.Fatalf("For: %v", err)
 	}
@@ -266,13 +278,13 @@ func TestForWithCycle(t *testing.T) {
 		shouldErr bool
 		fn        func() error
 	}{
-		{"slice alias (a)", true, func() error { _, err := jsonschema.For[a](); return err }},
-		{"unexported self cycle (b1)", false, func() error { _, err := jsonschema.For[b1](); return err }},
-		{"exported self cycle (b2)", true, func() error { _, err := jsonschema.For[b2](); return err }},
-		{"unexported map self cycle (c1)", false, func() error { _, err := jsonschema.For[c1](); return err }},
-		{"exported map self cycle (c2)", true, func() error { _, err := jsonschema.For[c2](); return err }},
-		{"cross-cycle x -> y -> x", true, func() error { _, err := jsonschema.For[x](); return err }},
-		{"cross-cycle y -> x -> y", true, func() error { _, err := jsonschema.For[y](); return err }},
+		{"slice alias (a)", true, func() error { _, err := jsonschema.For[a](nil); return err }},
+		{"unexported self cycle (b1)", false, func() error { _, err := jsonschema.For[b1](nil); return err }},
+		{"exported self cycle (b2)", true, func() error { _, err := jsonschema.For[b2](nil); return err }},
+		{"unexported map self cycle (c1)", false, func() error { _, err := jsonschema.For[c1](nil); return err }},
+		{"exported map self cycle (c2)", true, func() error { _, err := jsonschema.For[c2](nil); return err }},
+		{"cross-cycle x -> y -> x", true, func() error { _, err := jsonschema.For[x](nil); return err }},
+		{"cross-cycle y -> x -> y", true, func() error { _, err := jsonschema.For[y](nil); return err }},
 	}
 
 	for _, test := range tests {
diff --git a/jsonschema/schema.go b/jsonschema/schema.go
index 4b1d6eed..1d60de12 100644
--- a/jsonschema/schema.go
+++ b/jsonschema/schema.go
@@ -152,6 +152,42 @@ func (s *Schema) String() string {
 	return "<anonymous schema>"
 }
 
+// CloneSchemas returns a copy of s.
+// The copy is shallow except for sub-schemas, which are themelves copied with CloneSchemas.
+// This allows both s and s.CloneSchemas() to appear as sub-schemas in the same parent.
+func (s *Schema) CloneSchemas() *Schema {
+	if s == nil {
+		return nil
+	}
+	s2 := *s
+	v := reflect.ValueOf(&s2)
+	for _, info := range schemaFieldInfos {
+		fv := v.Elem().FieldByIndex(info.sf.Index)
+		switch info.sf.Type {
+		case schemaType:
+			sscss := fv.Interface().(*Schema)
+			fv.Set(reflect.ValueOf(sscss.CloneSchemas()))
+
+		case schemaSliceType:
+			slice := fv.Interface().([]*Schema)
+			slice = slices.Clone(slice)
+			for i, ss := range slice {
+				slice[i] = ss.CloneSchemas()
+			}
+			fv.Set(reflect.ValueOf(slice))
+
+		case schemaMapType:
+			m := fv.Interface().(map[string]*Schema)
+			m = maps.Clone(m)
+			for k, ss := range m {
+				m[k] = ss.CloneSchemas()
+			}
+			fv.Set(reflect.ValueOf(m))
+		}
+	}
+	return &s2
+}
+
 func (s *Schema) basicChecks() error {
 	if s.Type != "" && s.Types != nil {
 		return errors.New("both Type and Types are set; at most one should be")
diff --git a/jsonschema/schema_test.go b/jsonschema/schema_test.go
index 4ceb1ee1..4b6df511 100644
--- a/jsonschema/schema_test.go
+++ b/jsonschema/schema_test.go
@@ -142,3 +142,34 @@ func (s *Schema) jsonIndent() string {
 	}
 	return string(data)
 }
+
+func TestCloneSchemas(t *testing.T) {
+	ss1 := &Schema{Type: "string"}
+	ss2 := &Schema{Type: "integer"}
+	ss3 := &Schema{Type: "boolean"}
+	ss4 := &Schema{Type: "number"}
+	ss5 := &Schema{Contains: ss4}
+
+	s1 := Schema{
+		Contains:    ss1,
+		PrefixItems: []*Schema{ss2, ss3},
+		Properties:  map[string]*Schema{"a": ss5},
+	}
+	s2 := s1.CloneSchemas()
+
+	// The clones should appear identical.
+	if g, w := s1.json(), s2.json(); g != w {
+		t.Errorf("\ngot  %s\nwant %s", g, w)
+	}
+	// None of the schemas should overlap.
+	schemas1 := map[*Schema]bool{ss1: true, ss2: true, ss3: true, ss4: true, ss5: true}
+	for ss := range s2.all() {
+		if schemas1[ss] {
+			t.Errorf("uncloned schema %s", ss.json())
+		}
+	}
+	// s1's original schemas should be intact.
+	if s1.Contains != ss1 || s1.PrefixItems[0] != ss2 || s1.PrefixItems[1] != ss3 || ss5.Contains != ss4 || s1.Properties["a"] != ss5 {
+		t.Errorf("s1 modified")
+	}
+}
diff --git a/mcp/tool.go b/mcp/tool.go
index fc154991..ed80b660 100644
--- a/mcp/tool.go
+++ b/mcp/tool.go
@@ -89,7 +89,7 @@ func newServerTool[In, Out any](t *Tool, h ToolHandlerFor[In, Out]) (*serverTool
 func setSchema[T any](sfield **jsonschema.Schema, rfield **jsonschema.Resolved) error {
 	var err error
 	if *sfield == nil {
-		*sfield, err = jsonschema.For[T]()
+		*sfield, err = jsonschema.For[T](nil)
 	}
 	if err != nil {
 		return err