diff --git a/cmd/cue/cmd/testdata/script/unknown_filetype_attrval.txtar b/cmd/cue/cmd/testdata/script/unknown_filetype_attrval.txtar
index ec517b109..994b3eeba 100644
--- a/cmd/cue/cmd/testdata/script/unknown_filetype_attrval.txtar
+++ b/cmd/cue/cmd/testdata/script/unknown_filetype_attrval.txtar
@@ -1,9 +1,8 @@
 # Check that we cannot specify an arbitary unknown key-value pair
 # in a filetype.
-# TODO fix this test.
 
-exec cue export cue+foo=bar: x.cue
-# cmp stderr expect-stderr
+! exec cue export cue+foo=bar: x.cue
+cmp stderr expect-stderr
 -- x.cue --
 true
 -- expect-stderr --
diff --git a/cue/build/file.go b/cue/build/file.go
index e9d000799..945fe31e4 100644
--- a/cue/build/file.go
+++ b/cue/build/file.go
@@ -16,14 +16,24 @@ package build
 
 import "cuelang.org/go/cue/errors"
 
+// Note: the json tags in File correspond directly to names
+// used in the encoding/filetypes package, which unmarshals
+// results from CUE into a build.File.
+
 // A File represents a file that is part of the build process.
 type File struct {
 	Filename string `json:"filename"`
 
-	Encoding       Encoding          `json:"encoding,omitempty"`
-	Interpretation Interpretation    `json:"interpretation,omitempty"`
-	Form           Form              `json:"form,omitempty"`
-	Tags           map[string]string `json:"tags,omitempty"` // code=go
+	Encoding       Encoding       `json:"encoding,omitempty"`
+	Interpretation Interpretation `json:"interpretation,omitempty"`
+	Form           Form           `json:"form,omitempty"`
+	// Tags holds key-value pairs relating to the encoding
+	// conventions to use for the file.
+	Tags map[string]string `json:"tags,omitempty"` // e.g. code+lang=go
+
+	// BoolTags holds boolean-valued tags relating to the
+	// encoding conventions to use for the file.
+	BoolTags map[string]bool `json:"boolTags,omitempty"`
 
 	ExcludeReason errors.Error `json:"-"`
 	Source        interface{}  `json:"-"` // TODO: swap out with concrete type.
diff --git a/internal/filetypes/filetypes.go b/internal/filetypes/filetypes.go
index f8b24be6b..ad9a2347c 100644
--- a/internal/filetypes/filetypes.go
+++ b/internal/filetypes/filetypes.go
@@ -15,7 +15,9 @@
 package filetypes
 
 import (
+	"fmt"
 	"path/filepath"
+	"strconv"
 	"strings"
 
 	"cuelang.org/go/cue"
@@ -313,21 +315,57 @@ func parseType(scope string, mode Mode) (modeVal, fileVal cue.Value, _ error) {
 	modeVal = typesValue.LookupPath(cue.MakePath(cue.Str("modes"), cue.Str(mode.String())))
 	fileVal = modeVal.LookupPath(cue.MakePath(cue.Str("FileInfo")))
 
-	if scope != "" {
-		for _, tag := range strings.Split(scope, "+") {
-			tagName, tagVal, ok := strings.Cut(tag, "=")
-			if ok {
-				fileVal = fileVal.FillPath(cue.MakePath(cue.Str("tags"), cue.Str(tagName)), tagVal)
-			} else {
-				info := typesValue.LookupPath(cue.MakePath(cue.Str("tagInfo"), cue.Str(tag)))
-				if !info.Exists() {
-					return cue.Value{}, cue.Value{}, errors.Newf(token.NoPos, "unknown filetype %s", tag)
-				}
+	if scope == "" {
+		return modeVal, fileVal, nil
+	}
+	var otherTags []string
+	for _, tag := range strings.Split(scope, "+") {
+		tagName, _, ok := strings.Cut(tag, "=")
+		if ok {
+			otherTags = append(otherTags, tag)
+		} else {
+			info := typesValue.LookupPath(cue.MakePath(cue.Str("tagInfo"), cue.Str(tagName)))
+			if info.Exists() {
 				fileVal = fileVal.Unify(info)
+			} else {
+				// The tag might only be available when all the
+				// other tags have been evaluated.
+				otherTags = append(otherTags, tag)
 			}
 		}
 	}
-
+	if len(otherTags) == 0 {
+		return modeVal, fileVal, nil
+	}
+	// There are tags that aren't mentioned in tagInfo.
+	// They might still be valid, but just only valid within the file types that
+	// have been specified above, so look at the schema that we've got
+	// and see if it specifies any tags.
+	allowedTags := fileVal.LookupPath(cue.MakePath(cue.Str("tags")))
+	allowedBoolTags := fileVal.LookupPath(cue.MakePath(cue.Str("boolTags")))
+	for _, tag := range otherTags {
+		tagName, tagVal, hasValue := strings.Cut(tag, "=")
+		tagNamePath := cue.MakePath(cue.Str(tagName)).Optional()
+		tagSchema := allowedTags.LookupPath(tagNamePath)
+		if tagSchema.Exists() {
+			fileVal = fileVal.FillPath(cue.MakePath(cue.Str("tags"), cue.Str(tagName)), tagVal)
+			continue
+		}
+		if !allowedBoolTags.LookupPath(tagNamePath).Exists() {
+			return cue.Value{}, cue.Value{}, errors.Newf(token.NoPos, "unknown filetype %s", tagName)
+		}
+		tagValBool := true
+		if hasValue {
+			// It's a boolean tag and an explicit value has been specified.
+			// Allow the usual boolean string values.
+			t, err := strconv.ParseBool(tagVal)
+			if err != nil {
+				return cue.Value{}, cue.Value{}, fmt.Errorf("invalid boolean value for tag %q", tagName)
+			}
+			tagValBool = t
+		}
+		fileVal = fileVal.FillPath(cue.MakePath(cue.Str("boolTags"), cue.Str(tagName)), tagValBool)
+	}
 	return modeVal, fileVal, nil
 }
 
diff --git a/internal/filetypes/filetypes_test.go b/internal/filetypes/filetypes_test.go
index f4cf4ac52..a675fe322 100644
--- a/internal/filetypes/filetypes_test.go
+++ b/internal/filetypes/filetypes_test.go
@@ -179,6 +179,11 @@ func TestFromFile(t *testing.T) {
 				Encoding:       build.JSON,
 				Interpretation: "jsonschema",
 				Form:           build.Schema,
+				BoolTags: map[string]bool{
+					"strict":         false,
+					"strictFeatures": false,
+					"strictKeywords": false,
+				},
 			},
 			Definitions:  true,
 			Data:         true,
@@ -324,8 +329,8 @@ func TestParseFile(t *testing.T) {
 		out: &build.File{
 			Filename:       "-",
 			Encoding:       build.JSON,
-			Form:           build.Schema,
 			Interpretation: build.OpenAPI,
+			Form:           build.Schema,
 		},
 	}, {
 		in: "cue:file.json",
@@ -347,6 +352,9 @@ func TestParseFile(t *testing.T) {
 			Encoding: build.Code,
 			Tags:     map[string]string{"lang": "js"},
 		},
+	}, {
+		in:  "json+lang=js:foo.x",
+		out: `unknown filetype lang`,
 	}, {
 		in:  "foo:file.bar",
 		out: `unknown filetype foo`,
@@ -409,6 +417,26 @@ func TestParseArgs(t *testing.T) {
 				Encoding:       build.JSON,
 				Form:           build.Schema,
 				Interpretation: "jsonschema",
+				BoolTags: map[string]bool{
+					"strict":         false,
+					"strictFeatures": false,
+					"strictKeywords": false,
+				},
+			},
+		},
+	}, {
+		in: "jsonschema+strict: bar.schema",
+		out: []*build.File{
+			{
+				Filename:       "bar.schema",
+				Encoding:       "json",
+				Interpretation: "jsonschema",
+				Form:           build.Schema,
+				BoolTags: map[string]bool{
+					"strict":         true,
+					"strictFeatures": true,
+					"strictKeywords": true,
+				},
 			},
 		},
 	}, {
diff --git a/internal/filetypes/types.cue b/internal/filetypes/types.cue
index 486f337d3..19c95c6cd 100644
--- a/internal/filetypes/types.cue
+++ b/internal/filetypes/types.cue
@@ -28,6 +28,7 @@ package build
 	form?:           #Form
 	// Note: tags includes values for non-boolean tags only.
 	tags?: [string]: string
+	boolTags?: [string]: bool
 }
 
 // Default is the file used for stdin and stdout. The settings depend
@@ -293,6 +294,11 @@ interpretations: auto: forms.schema
 interpretations: jsonschema: {
 	forms.schema
 	encoding: *"json" | _
+	boolTags: {
+		strict:         *false | bool
+		strictKeywords: *strict | bool
+		strictFeatures: *strict | bool
+	}
 }
 
 interpretations: openapi: {