Decode WebMCP output Chromium leaves as Latin-1 JSON text

server/lib/webmcpclient/client.go

@@ -191,6 +191,7 @@ func (c *connection) handleProtocolEvent(message cdpclient.Message) {
 			if response.ErrorText == "" && response.Exception != nil {
 				response.ErrorText = exceptionText(response.Exception)
 			}
+			response.Output = decodeLatin1Output(response.Output)
 			key := invocationKey{sessionID: message.SessionID, invocationID: response.InvocationID}
 			c.stateMu.Lock()
 			c.pruneAbandonedInvocationsLocked()
@@ -231,6 +232,35 @@ func exceptionText(exception *exceptionDetails) string {
 	return text
 }
 
+// decodeLatin1Output parses JSON output that Chromium left as a string.
+// Before Chromium 158, a result whose characters all fit in Latin-1, with at
+// least one in U+0080-U+00FF, was handed to a UTF-8 JSON parser as raw Latin-1
+// bytes; the parse failed and toolResponded carried the JSON text instead of
+// the value. Later versions parse the same text, so this matches them.
+func decodeLatin1Output(output any) any {
+	text, ok := output.(string)
+	if !ok {
+		return output
+	}
+	latin1 := false
+	for _, r := range text {
+		if r > 0xFF {
+			return output
+		}
+		if r >= 0x80 {
+			latin1 = true
+		}
+	}
+	if !latin1 {
+		return output
+	}
+	var decoded any
+	if json.Unmarshal([]byte(text), &decoded) != nil {
+		return output
+	}
+	return decoded
+}
+
 // customToolIdentity decodes the hidden name used for a custom registration.
 // The browser registers custom.<ct_CUID2>.<name> to avoid collisions with page
 // tools, while discovery exposes the original name and the generated ID separately.

server/lib/webmcpclient/client_test.go

@@ -669,6 +669,32 @@ func TestToolResponsesAreScopedBySession(t *testing.T) {
 	require.Equal(t, "iframe-session", client.invocations[invocationKey{sessionID: "iframe-session", invocationID: "invocation-1"}].Output)
 }
 
+func TestToolResponseDecodesLatin1JSONOutput(t *testing.T) {
+	client := &connection{
+		invocations:          make(map[invocationKey]invocationResponse),
+		waitingInvocations:   make(map[invocationKey]string),
+		abandonedInvocations: make(map[invocationKey]time.Time),
+		stateChangedCh:       make(chan struct{}, 1),
+	}
+	cases := map[string]struct {
+		output any
+		want   any
+	}{
+		"latin1 json":        {`{"content":[{"type":"text","text":"600×400"}]}`, map[string]any{"content": []any{map[string]any{"type": "text", "text": "600×400"}}}},
+		"latin1 json string": {`"Café"`, "Café"},
+		"latin1 text":        {"Café", "Café"},
+		"ascii json text":    {`{"a":1}`, `{"a":1}`},
+		"wider than latin1":  {`{"text":"Café—"}`, `{"text":"Café—"}`},
+		"object":             {map[string]any{"text": "Café"}, map[string]any{"text": "Café"}},
+	}
+	for name, tc := range cases {
+		params, err := json.Marshal(invocationResponse{InvocationID: name, Status: "Completed", Output: tc.output})
+		require.NoError(t, err)
+		client.handleProtocolEvent(cdpclient.Message{Method: "WebMCP.toolResponded", SessionID: "page-session", Params: params})
+		require.Equal(t, tc.want, client.invocations[invocationKey{sessionID: "page-session", invocationID: name}].Output, name)
+	}
+}
+
 func TestManagerReconnectsWhenChromiumUpstreamChanges(t *testing.T) {
 	first := newFakeCDP(t, false)
 	second := newFakeCDP(t, false)