diff --git a/filter/kyofilter b/filter/kyofilter
index 545bf4a..5045a7c 100755
--- a/filter/kyofilter
+++ b/filter/kyofilter
@@ -24,5 +24,24 @@ def get_account_code(options):
     return code.zfill(8)
 
 
+def process_stream(lines, account_code):
+    if not account_code:
+        yield from lines
+        return
+
+    inject = f"({account_code}) statusdict /setmanagementnumber get exec\n".encode()
+    injected = False
+
+    for line in lines:
+        stripped = line.rstrip(b'\r\n')
+        if not injected and (stripped == b'%%EndSetup' or stripped.startswith(b'%%Page:')):
+            yield inject
+            injected = True
+        yield line
+
+    if not injected:
+        sys.stderr.write("kyofilter: WARNING: no injection point found in PostScript stream\n")
+
+
 if __name__ == '__main__':
     pass
diff --git a/tests/test_kyofilter.py b/tests/test_kyofilter.py
index df1727e..dc66f2c 100644
--- a/tests/test_kyofilter.py
+++ b/tests/test_kyofilter.py
@@ -37,3 +37,45 @@ class TestGetAccountCode:
 
     def test_non_numeric_code_returns_none(self):
         assert kyofilter.get_account_code({"KmManagment": "MGabc45"}) is None
+
+
+class TestProcessStream:
+    def _stream(self, text):
+        return iter(line.encode() for line in text.splitlines(keepends=True))
+
+    def _collect(self, text, code):
+        return b"".join(kyofilter.process_stream(self._stream(text), code))
+
+    def test_passthrough_when_no_code(self):
+        ps = "%!PS\n%%BeginSetup\n%%EndSetup\n%%Page: 1 1\n"
+        assert self._collect(ps, None) == ps.encode()
+
+    def test_injects_before_end_setup(self):
+        ps = "%!PS\n%%BeginSetup\n%%EndSetup\n%%Page: 1 1\n"
+        result = self._collect(ps, "00012345")
+        assert b"(00012345) statusdict /setmanagementnumber get exec\n%%EndSetup\n" in result
+
+    def test_end_setup_preserved_after_injection(self):
+        ps = "%!PS\n%%BeginSetup\n%%EndSetup\n"
+        result = self._collect(ps, "00012345")
+        assert b"%%EndSetup\n" in result
+
+    def test_fallback_injects_before_first_page_when_no_end_setup(self):
+        ps = "%!PS\n%%Page: 1 1\nshowpage\n"
+        result = self._collect(ps, "00012345")
+        assert b"(00012345) statusdict /setmanagementnumber get exec\n%%Page: 1 1\n" in result
+
+    def test_injects_only_once(self):
+        ps = "%!PS\n%%BeginSetup\n%%EndSetup\n%%Page: 1 1\n%%Page: 2 1\n"
+        result = self._collect(ps, "00012345")
+        assert result.count(b"setmanagementnumber") == 1
+
+    def test_content_before_injection_point_preserved(self):
+        ps = "%!PS\n%%BeginSetup\n/mydict 10 dict def\n%%EndSetup\n"
+        result = self._collect(ps, "00012345")
+        assert b"/mydict 10 dict def\n" in result
+
+    def test_content_after_injection_point_preserved(self):
+        ps = "%!PS\n%%BeginSetup\n%%EndSetup\nshowpage\n"
+        result = self._collect(ps, "00012345")
+        assert b"showpage\n" in result