diff --git a/c/openai_server.py b/c/openai_server.py
index 1dd987b..ae1032c 100644
--- a/c/openai_server.py
+++ b/c/openai_server.py
@@ -468,6 +468,9 @@ def generation_options(body, limit):
             raise APIError(400, "`response_format.type` must be \"text\", \"json_object\", "
                                 "\"json_schema\" or \"gbnf\".",
                            "response_format", "unsupported_value")
+        if grammar is not None and len(grammar.encode("utf-8")) > (1 << 20):
+            raise APIError(400, "`response_format` grammar/schema exceeds 1 MiB.",
+                           "response_format", "invalid_value")
 
     maximum = body.get("max_completion_tokens")
     maximum_param = "max_completion_tokens"
diff --git a/c/tests/test_openai_server.py b/c/tests/test_openai_server.py
index 7cbbc04..8000b40 100644
--- a/c/tests/test_openai_server.py
+++ b/c/tests/test_openai_server.py
@@ -99,6 +99,16 @@ class TemplateTest(unittest.TestCase):
             generation_options({"response_format": {"type": "yaml"}}, 8)
         with self.assertRaises(APIError):
             generation_options({"response_format": {"type": "json_schema", "json_schema": {}}}, 8)
+        with self.assertRaises(APIError):   # non-dict response_format
+            generation_options({"response_format": "json"}, 8)
+        with self.assertRaises(APIError):   # empty gbnf
+            generation_options({"response_format": {"type": "gbnf", "grammar": "  "}}, 8)
+        with self.assertRaises(APIError):   # oversized grammar (> 1 MiB pre-check)
+            generation_options({"response_format": {"type": "gbnf", "grammar": "x" * ((1 << 20) + 1)}}, 8)
+        # malformed GBNF passes the gateway by design: the ENGINE fail-softs it
+        # (draft source only — bad grammar costs the speedup, never the request)
+        opts = generation_options({"response_format": {"type": "gbnf", "grammar": "not a grammar ::="}}, 8)
+        self.assertEqual(opts[3], "not a grammar ::=")
 
 
 class ProtocolTest(unittest.TestCase):
diff --git a/docs/grammar-draft.md b/docs/grammar-draft.md
index 707fe5c..6140cbb 100644
--- a/docs/grammar-draft.md
+++ b/docs/grammar-draft.md
@@ -83,3 +83,27 @@ this implementation adds: forced spans as a **draft source verified in the targe
 own forward** (lossless even under a wrong grammar, composes with MTP/n-gram in one
 union batch), deployed where the win is denominated in expert I/O rather than
 forward passes.
+
+## Server usage: `response_format` (OpenAI API)
+
+The gateway (`openai_server.py`) turns `response_format` into a **per-request**
+grammar carried to the engine over the `SUBMIT` protocol (optional 7th header
+field, `gbytes`; 6-field headers from older clients remain valid — the field is
+additive and back-compatible in both directions):
+
+```jsonc
+{"response_format": {"type": "json_object"}}                       // generic JSON grammar
+{"response_format": {"type": "json_schema",
+                     "json_schema": {"schema": { ... }}}}          // compiled by schema_gbnf.h
+{"response_format": {"type": "gbnf", "grammar": "root ::= ..."}}   // raw GBNF (extension)
+```
+
+Semantics are identical to `GRAMMAR=`/`SCHEMA=`: a **draft source, never a
+sampling constraint**. A schema outside the supported subset, or malformed GBNF,
+costs the speedup — never the request, never the output. Drafting engages for
+greedy requests (`temperature: 0`); sampled requests run undrafted. Compile
+overhead is negligible: ~8 µs/request for a typical schema, ~18 µs at the
+32-level nesting cap (measured, M3 Max). Grammar payloads are capped at 1 MiB.
+As with MTP (#100), a drafted greedy run may differ from an undrafted one in
+near-tie tokens (the verify forward has a different batch shape); each output
+is a valid greedy stream of its own forward shapes.