diff --git a/booth/app.py b/booth/app.py
index 1e6d841..098a1f5 100644
--- a/booth/app.py
+++ b/booth/app.py
@@ -355,6 +355,35 @@ def record_seen(booth: Path, rel: str, items: Sequence[Item]) -> None:
         pass
 
 
+def wants_json(accept: str | None) -> bool:
+    """Whether a mark POST asked for the in-place answer (R2 C3).
+
+    True ONLY when the Accept header lists `application/json` exactly —
+    parameters stripped — with a q-value that is absent or above zero. Absent,
+    empty, wildcard, `application/*`, a near miss like `application/jsonx`, an
+    explicit `q=0`, a malformed q: all False. It FAILS TOWARD THE 303, because
+    the plain form's redirect is the no-JS guarantee and a mis-parse must land
+    there, never on a 204 a browser would render as nothing happening.
+    """
+    if not accept:
+        return False
+    try:
+        for entry in accept.split(","):
+            mtype, *params = entry.split(";")
+            if mtype.strip().lower() != "application/json":
+                continue
+            q = 1.0
+            for param in params:
+                key, _, value = param.partition("=")
+                if key.strip().lower() == "q":
+                    q = float(value.strip())
+            if q > 0:
+                return True
+    except ValueError:
+        return False
+    return False
+
+
 HOLD_UNREADABLE = "unreadable"
 HOLD_OPEN = "open"
 
@@ -1183,8 +1212,32 @@ def create_app(
         base = f"/b/{quote(name, safe='')}/"
         if form.get("back") == "marks":
             base = f"/b/{quote(name, safe='')}/marks"
+        elif form.get("back") == "view":
+            # R2 C3: judgment made at full size lands back at full size — the
+            # JS-off fix for being thrown out to the grid. Only for a MEDIA item
+            # of this booth; anything else takes the no-`back` landing above.
+            # Built from the resolved rel, never echoed from the form.
+            f = form.get("f")
+            if isinstance(f, str) and f:
+                try:
+                    ring = review_chain(booth_items(resolve_booth(name)))
+                except HTTPException:
+                    ring = []
+                if f in ring:
+                    return RedirectResponse(
+                        url=f"/b/{quote(name, safe='')}/view?f={quote(f, safe='/')}#rail",
+                        status_code=303)
         return RedirectResponse(url=f"{base}#{anchor}", status_code=303)
 
+    def _mark_done(request: Request, name: str, form, anchor: str) -> Response:
+        """The one exit for every mark route (R2 C3). A request that asked for
+        the in-place answer gets 204 and no body — the page fetches its own
+        fresh regions. Everything else gets `_mark_redirect`, byte for byte what
+        it got before R2 (INV-4)."""
+        if wants_json(request.headers.get("accept")):
+            return Response(status_code=204)
+        return _mark_redirect(name, form, anchor)
+
     @app.post("/b/{name}/answer")
     async def booth_answer(request: Request, name: str):
         """Record the operator's pick — one of N options a session declared in
@@ -1235,7 +1288,7 @@ def create_app(
                                         _form_text(form, "choice"), notes, who=who)
         except AskError as exc:
             raise HTTPException(status_code=400, detail=str(exc))
-        return _mark_redirect(name, form, f"mark-{quote(mark_id, safe='')}")
+        return _mark_done(request, name, form, f"mark-{quote(mark_id, safe='')}")
 
     @app.post("/b/{name}/note")
     async def booth_note(request: Request, name: str):
@@ -1256,7 +1309,7 @@ def create_app(
                 who=request.client.host if request.client else "")
         except AskError as exc:
             raise HTTPException(status_code=400, detail=str(exc))
-        return _mark_redirect(name, form, f"mark-{quote(mark.id, safe='')}")
+        return _mark_done(request, name, form, f"mark-{quote(mark.id, safe='')}")
 
     @app.post("/b/{name}/flag")
     async def booth_flag(request: Request, name: str):
@@ -1278,7 +1331,7 @@ def create_app(
                 who=request.client.host if request.client else "")
         except AskError as exc:
             raise HTTPException(status_code=400, detail=str(exc))
-        return _mark_redirect(name, form, f"item-{quote(target, safe='')}")
+        return _mark_done(request, name, form, f"item-{quote(target, safe='')}")
 
     @app.post("/b/{name}/unmark")
     async def booth_unmark(request: Request, name: str):
@@ -1290,7 +1343,7 @@ def create_app(
         if not isinstance(mark_id, str) or not mark_id:
             raise HTTPException(status_code=400, detail="which mark?")
         await run_in_threadpool(delete_mark, booth, mark_id)
-        return _mark_redirect(name, form, "marks")
+        return _mark_done(request, name, form, "marks")
 
     @app.post("/b/{name}/import-asks")
     async def booth_import_asks(request: Request, name: str):
diff --git a/docs/contracts/r2_flow.contract.md b/docs/contracts/r2_flow.contract.md
index dafd6c4..0aef900 100644
--- a/docs/contracts/r2_flow.contract.md
+++ b/docs/contracts/r2_flow.contract.md
@@ -129,10 +129,13 @@ no route derives it.
 ### C3 — in-place judgment (app.py, base.html)
 
 **`wants_json(request) -> bool`** is True **only** when the `Accept` header,
-split on commas with parameters stripped, contains the exact media type
-`application/json`.
+split on commas, contains an entry whose media type, parameters stripped, is
+exactly `application/json` and whose q-value is absent or greater than 0.
 
 - Absent, empty, `*/*` or `application/*` → False.
+- `application/json;q=0` → False. A client that explicitly refuses JSON gets
+  the redirect.
+- A near miss such as `application/jsonx` → False.
 - Any header that fails to parse → False.
 - **It fails toward the 303.**
 
diff --git a/tests/golden/r2_mark_303.json b/tests/golden/r2_mark_303.json
new file mode 100644
index 0000000..aa9d3e6
--- /dev/null
+++ b/tests/golden/r2_mark_303.json
@@ -0,0 +1,2180 @@
+[
+ {
+  "path": "/b/g/answer",
+  "form": {
+   "ask": "q",
+   "choice": "x"
+  },
+  "accept": null,
+  "status": 303,
+  "headers": [
+   [
+    "content-length",
+    "0"
+   ],
+   [
+    "location",
+    "/b/g/#mark-q"
+   ]
+  ],
+  "body": ""
+ },
+ {
+  "path": "/b/g/answer",
+  "form": {
+   "ask": "q",
+   "choice": "x"
+  },
+  "accept": "",
+  "status": 303,
+  "headers": [
+   [
+    "content-length",
+    "0"
+   ],
+   [
+    "location",
+    "/b/g/#mark-q"
+   ]
+  ],
+  "body": ""
+ },
+ {
+  "path": "/b/g/answer",
+  "form": {
+   "ask": "q",
+   "choice": "x"
+  },
+  "accept": "*/*",
+  "status": 303,
+  "headers": [
+   [
+    "content-length",
+    "0"
+   ],
+   [
+    "location",
+    "/b/g/#mark-q"
+   ]
+  ],
+  "body": ""
+ },
+ {
+  "path": "/b/g/answer",
+  "form": {
+   "ask": "q",
+   "choice": "x"
+  },
+  "accept": "text/html",
+  "status": 303,
+  "headers": [
+   [
+    "content-length",
+    "0"
+   ],
+   [
+    "location",
+    "/b/g/#mark-q"
+   ]
+  ],
+  "body": ""
+ },
+ {
+  "path": "/b/g/answer",
+  "form": {
+   "ask": "q",
+   "choice": "x"
+  },
+  "accept": "application/*",
+  "status": 303,
+  "headers": [
+   [
+    "content-length",
+    "0"
+   ],
+   [
+    "location",
+    "/b/g/#mark-q"
+   ]
+  ],
+  "body": ""
+ },
+ {
+  "path": "/b/g/answer",
+  "form": {
+   "ask": "q",
+   "choice": "x"
+  },
+  "accept": "text/html,application/xhtml+xml;q=0.9,*/*;q=0.8",
+  "status": 303,
+  "headers": [
+   [
+    "content-length",
+    "0"
+   ],
+   [
+    "location",
+    "/b/g/#mark-q"
+   ]
+  ],
+  "body": ""
+ },
+ {
+  "path": "/b/g/answer",
+  "form": {
+   "ask": "q",
+   "choice": "x"
+  },
+  "accept": "application/jsonx",
+  "status": 303,
+  "headers": [
+   [
+    "content-length",
+    "0"
+   ],
+   [
+    "location",
+    "/b/g/#mark-q"
+   ]
+  ],
+  "body": ""
+ },
+ {
+  "path": "/b/g/answer",
+  "form": {
+   "ask": "q",
+   "choice": "x"
+  },
+  "accept": ";;;",
+  "status": 303,
+  "headers": [
+   [
+    "content-length",
+    "0"
+   ],
+   [
+    "location",
+    "/b/g/#mark-q"
+   ]
+  ],
+  "body": ""
+ },
+ {
+  "path": "/b/g/answer",
+  "form": {
+   "ask": "q",
+   "choice": "x"
+  },
+  "accept": "application/json;q=0",
+  "status": 303,
+  "headers": [
+   [
+    "content-length",
+    "0"
+   ],
+   [
+    "location",
+    "/b/g/#mark-q"
+   ]
+  ],
+  "body": ""
+ },
+ {
+  "path": "/b/g/answer",
+  "form": {
+   "ask": "q",
+   "choice": "x",
+   "back": "marks"
+  },
+  "accept": null,
+  "status": 303,
+  "headers": [
+   [
+    "content-length",
+    "0"
+   ],
+   [
+    "location",
+    "/b/g/marks#mark-q"
+   ]
+  ],
+  "body": ""
+ },
+ {
+  "path": "/b/g/answer",
+  "form": {
+   "ask": "q",
+   "choice": "x",
+   "back": "marks"
+  },
+  "accept": "",
+  "status": 303,
+  "headers": [
+   [
+    "content-length",
+    "0"
+   ],
+   [
+    "location",
+    "/b/g/marks#mark-q"
+   ]
+  ],
+  "body": ""
+ },
+ {
+  "path": "/b/g/answer",
+  "form": {
+   "ask": "q",
+   "choice": "x",
+   "back": "marks"
+  },
+  "accept": "*/*",
+  "status": 303,
+  "headers": [
+   [
+    "content-length",
+    "0"
+   ],
+   [
+    "location",
+    "/b/g/marks#mark-q"
+   ]
+  ],
+  "body": ""
+ },
+ {
+  "path": "/b/g/answer",
+  "form": {
+   "ask": "q",
+   "choice": "x",
+   "back": "marks"
+  },
+  "accept": "text/html",
+  "status": 303,
+  "headers": [
+   [
+    "content-length",
+    "0"
+   ],
+   [
+    "location",
+    "/b/g/marks#mark-q"
+   ]
+  ],
+  "body": ""
+ },
+ {
+  "path": "/b/g/answer",
+  "form": {
+   "ask": "q",
+   "choice": "x",
+   "back": "marks"
+  },
+  "accept": "application/*",
+  "status": 303,
+  "headers": [
+   [
+    "content-length",
+    "0"
+   ],
+   [
+    "location",
+    "/b/g/marks#mark-q"
+   ]
+  ],
+  "body": ""
+ },
+ {
+  "path": "/b/g/answer",
+  "form": {
+   "ask": "q",
+   "choice": "x",
+   "back": "marks"
+  },
+  "accept": "text/html,application/xhtml+xml;q=0.9,*/*;q=0.8",
+  "status": 303,
+  "headers": [
+   [
+    "content-length",
+    "0"
+   ],
+   [
+    "location",
+    "/b/g/marks#mark-q"
+   ]
+  ],
+  "body": ""
+ },
+ {
+  "path": "/b/g/answer",
+  "form": {
+   "ask": "q",
+   "choice": "x",
+   "back": "marks"
+  },
+  "accept": "application/jsonx",
+  "status": 303,
+  "headers": [
+   [
+    "content-length",
+    "0"
+   ],
+   [
+    "location",
+    "/b/g/marks#mark-q"
+   ]
+  ],
+  "body": ""
+ },
+ {
+  "path": "/b/g/answer",
+  "form": {
+   "ask": "q",
+   "choice": "x",
+   "back": "marks"
+  },
+  "accept": ";;;",
+  "status": 303,
+  "headers": [
+   [
+    "content-length",
+    "0"
+   ],
+   [
+    "location",
+    "/b/g/marks#mark-q"
+   ]
+  ],
+  "body": ""
+ },
+ {
+  "path": "/b/g/answer",
+  "form": {
+   "ask": "q",
+   "choice": "x",
+   "back": "marks"
+  },
+  "accept": "application/json;q=0",
+  "status": 303,
+  "headers": [
+   [
+    "content-length",
+    "0"
+   ],
+   [
+    "location",
+    "/b/g/marks#mark-q"
+   ]
+  ],
+  "body": ""
+ },
+ {
+  "path": "/b/g/note",
+  "form": {
+   "target": "a.png",
+   "text": "hello"
+  },
+  "accept": null,
+  "status": 303,
+  "headers": [
+   [
+    "content-length",
+    "0"
+   ],
+   [
+    "location",
+    "/b/g/#mark-note-2"
+   ]
+  ],
+  "body": ""
+ },
+ {
+  "path": "/b/g/note",
+  "form": {
+   "target": "a.png",
+   "text": "hello"
+  },
+  "accept": "",
+  "status": 303,
+  "headers": [
+   [
+    "content-length",
+    "0"
+   ],
+   [
+    "location",
+    "/b/g/#mark-note-2"
+   ]
+  ],
+  "body": ""
+ },
+ {
+  "path": "/b/g/note",
+  "form": {
+   "target": "a.png",
+   "text": "hello"
+  },
+  "accept": "*/*",
+  "status": 303,
+  "headers": [
+   [
+    "content-length",
+    "0"
+   ],
+   [
+    "location",
+    "/b/g/#mark-note-2"
+   ]
+  ],
+  "body": ""
+ },
+ {
+  "path": "/b/g/note",
+  "form": {
+   "target": "a.png",
+   "text": "hello"
+  },
+  "accept": "text/html",
+  "status": 303,
+  "headers": [
+   [
+    "content-length",
+    "0"
+   ],
+   [
+    "location",
+    "/b/g/#mark-note-2"
+   ]
+  ],
+  "body": ""
+ },
+ {
+  "path": "/b/g/note",
+  "form": {
+   "target": "a.png",
+   "text": "hello"
+  },
+  "accept": "application/*",
+  "status": 303,
+  "headers": [
+   [
+    "content-length",
+    "0"
+   ],
+   [
+    "location",
+    "/b/g/#mark-note-2"
+   ]
+  ],
+  "body": ""
+ },
+ {
+  "path": "/b/g/note",
+  "form": {
+   "target": "a.png",
+   "text": "hello"
+  },
+  "accept": "text/html,application/xhtml+xml;q=0.9,*/*;q=0.8",
+  "status": 303,
+  "headers": [
+   [
+    "content-length",
+    "0"
+   ],
+   [
+    "location",
+    "/b/g/#mark-note-2"
+   ]
+  ],
+  "body": ""
+ },
+ {
+  "path": "/b/g/note",
+  "form": {
+   "target": "a.png",
+   "text": "hello"
+  },
+  "accept": "application/jsonx",
+  "status": 303,
+  "headers": [
+   [
+    "content-length",
+    "0"
+   ],
+   [
+    "location",
+    "/b/g/#mark-note-2"
+   ]
+  ],
+  "body": ""
+ },
+ {
+  "path": "/b/g/note",
+  "form": {
+   "target": "a.png",
+   "text": "hello"
+  },
+  "accept": ";;;",
+  "status": 303,
+  "headers": [
+   [
+    "content-length",
+    "0"
+   ],
+   [
+    "location",
+    "/b/g/#mark-note-2"
+   ]
+  ],
+  "body": ""
+ },
+ {
+  "path": "/b/g/note",
+  "form": {
+   "target": "a.png",
+   "text": "hello"
+  },
+  "accept": "application/json;q=0",
+  "status": 303,
+  "headers": [
+   [
+    "content-length",
+    "0"
+   ],
+   [
+    "location",
+    "/b/g/#mark-note-2"
+   ]
+  ],
+  "body": ""
+ },
+ {
+  "path": "/b/g/note",
+  "form": {
+   "target": "a.png",
+   "text": "hello",
+   "back": "marks"
+  },
+  "accept": null,
+  "status": 303,
+  "headers": [
+   [
+    "content-length",
+    "0"
+   ],
+   [
+    "location",
+    "/b/g/marks#mark-note-2"
+   ]
+  ],
+  "body": ""
+ },
+ {
+  "path": "/b/g/note",
+  "form": {
+   "target": "a.png",
+   "text": "hello",
+   "back": "marks"
+  },
+  "accept": "",
+  "status": 303,
+  "headers": [
+   [
+    "content-length",
+    "0"
+   ],
+   [
+    "location",
+    "/b/g/marks#mark-note-2"
+   ]
+  ],
+  "body": ""
+ },
+ {
+  "path": "/b/g/note",
+  "form": {
+   "target": "a.png",
+   "text": "hello",
+   "back": "marks"
+  },
+  "accept": "*/*",
+  "status": 303,
+  "headers": [
+   [
+    "content-length",
+    "0"
+   ],
+   [
+    "location",
+    "/b/g/marks#mark-note-2"
+   ]
+  ],
+  "body": ""
+ },
+ {
+  "path": "/b/g/note",
+  "form": {
+   "target": "a.png",
+   "text": "hello",
+   "back": "marks"
+  },
+  "accept": "text/html",
+  "status": 303,
+  "headers": [
+   [
+    "content-length",
+    "0"
+   ],
+   [
+    "location",
+    "/b/g/marks#mark-note-2"
+   ]
+  ],
+  "body": ""
+ },
+ {
+  "path": "/b/g/note",
+  "form": {
+   "target": "a.png",
+   "text": "hello",
+   "back": "marks"
+  },
+  "accept": "application/*",
+  "status": 303,
+  "headers": [
+   [
+    "content-length",
+    "0"
+   ],
+   [
+    "location",
+    "/b/g/marks#mark-note-2"
+   ]
+  ],
+  "body": ""
+ },
+ {
+  "path": "/b/g/note",
+  "form": {
+   "target": "a.png",
+   "text": "hello",
+   "back": "marks"
+  },
+  "accept": "text/html,application/xhtml+xml;q=0.9,*/*;q=0.8",
+  "status": 303,
+  "headers": [
+   [
+    "content-length",
+    "0"
+   ],
+   [
+    "location",
+    "/b/g/marks#mark-note-2"
+   ]
+  ],
+  "body": ""
+ },
+ {
+  "path": "/b/g/note",
+  "form": {
+   "target": "a.png",
+   "text": "hello",
+   "back": "marks"
+  },
+  "accept": "application/jsonx",
+  "status": 303,
+  "headers": [
+   [
+    "content-length",
+    "0"
+   ],
+   [
+    "location",
+    "/b/g/marks#mark-note-2"
+   ]
+  ],
+  "body": ""
+ },
+ {
+  "path": "/b/g/note",
+  "form": {
+   "target": "a.png",
+   "text": "hello",
+   "back": "marks"
+  },
+  "accept": ";;;",
+  "status": 303,
+  "headers": [
+   [
+    "content-length",
+    "0"
+   ],
+   [
+    "location",
+    "/b/g/marks#mark-note-2"
+   ]
+  ],
+  "body": ""
+ },
+ {
+  "path": "/b/g/note",
+  "form": {
+   "target": "a.png",
+   "text": "hello",
+   "back": "marks"
+  },
+  "accept": "application/json;q=0",
+  "status": 303,
+  "headers": [
+   [
+    "content-length",
+    "0"
+   ],
+   [
+    "location",
+    "/b/g/marks#mark-note-2"
+   ]
+  ],
+  "body": ""
+ },
+ {
+  "path": "/b/g/note",
+  "form": {
+   "text": "booth note"
+  },
+  "accept": null,
+  "status": 303,
+  "headers": [
+   [
+    "content-length",
+    "0"
+   ],
+   [
+    "location",
+    "/b/g/#mark-note-2"
+   ]
+  ],
+  "body": ""
+ },
+ {
+  "path": "/b/g/note",
+  "form": {
+   "text": "booth note"
+  },
+  "accept": "",
+  "status": 303,
+  "headers": [
+   [
+    "content-length",
+    "0"
+   ],
+   [
+    "location",
+    "/b/g/#mark-note-2"
+   ]
+  ],
+  "body": ""
+ },
+ {
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+   "on": "0",
+   "back": "marks"
+  },
+  "accept": ";;;",
+  "status": 303,
+  "headers": [
+   [
+    "content-length",
+    "0"
+   ],
+   [
+    "location",
+    "/b/g/marks#item-a.png"
+   ]
+  ],
+  "body": ""
+ },
+ {
+  "path": "/b/g/flag",
+  "form": {
+   "target": "a.png",
+   "on": "0",
+   "back": "marks"
+  },
+  "accept": "application/json;q=0",
+  "status": 303,
+  "headers": [
+   [
+    "content-length",
+    "0"
+   ],
+   [
+    "location",
+    "/b/g/marks#item-a.png"
+   ]
+  ],
+  "body": ""
+ },
+ {
+  "path": "/b/g/unmark",
+  "form": {
+   "mark": "note-1"
+  },
+  "accept": null,
+  "status": 303,
+  "headers": [
+   [
+    "content-length",
+    "0"
+   ],
+   [
+    "location",
+    "/b/g/#marks"
+   ]
+  ],
+  "body": ""
+ },
+ {
+  "path": "/b/g/unmark",
+  "form": {
+   "mark": "note-1"
+  },
+  "accept": "",
+  "status": 303,
+  "headers": [
+   [
+    "content-length",
+    "0"
+   ],
+   [
+    "location",
+    "/b/g/#marks"
+   ]
+  ],
+  "body": ""
+ },
+ {
+  "path": "/b/g/unmark",
+  "form": {
+   "mark": "note-1"
+  },
+  "accept": "*/*",
+  "status": 303,
+  "headers": [
+   [
+    "content-length",
+    "0"
+   ],
+   [
+    "location",
+    "/b/g/#marks"
+   ]
+  ],
+  "body": ""
+ },
+ {
+  "path": "/b/g/unmark",
+  "form": {
+   "mark": "note-1"
+  },
+  "accept": "text/html",
+  "status": 303,
+  "headers": [
+   [
+    "content-length",
+    "0"
+   ],
+   [
+    "location",
+    "/b/g/#marks"
+   ]
+  ],
+  "body": ""
+ },
+ {
+  "path": "/b/g/unmark",
+  "form": {
+   "mark": "note-1"
+  },
+  "accept": "application/*",
+  "status": 303,
+  "headers": [
+   [
+    "content-length",
+    "0"
+   ],
+   [
+    "location",
+    "/b/g/#marks"
+   ]
+  ],
+  "body": ""
+ },
+ {
+  "path": "/b/g/unmark",
+  "form": {
+   "mark": "note-1"
+  },
+  "accept": "text/html,application/xhtml+xml;q=0.9,*/*;q=0.8",
+  "status": 303,
+  "headers": [
+   [
+    "content-length",
+    "0"
+   ],
+   [
+    "location",
+    "/b/g/#marks"
+   ]
+  ],
+  "body": ""
+ },
+ {
+  "path": "/b/g/unmark",
+  "form": {
+   "mark": "note-1"
+  },
+  "accept": "application/jsonx",
+  "status": 303,
+  "headers": [
+   [
+    "content-length",
+    "0"
+   ],
+   [
+    "location",
+    "/b/g/#marks"
+   ]
+  ],
+  "body": ""
+ },
+ {
+  "path": "/b/g/unmark",
+  "form": {
+   "mark": "note-1"
+  },
+  "accept": ";;;",
+  "status": 303,
+  "headers": [
+   [
+    "content-length",
+    "0"
+   ],
+   [
+    "location",
+    "/b/g/#marks"
+   ]
+  ],
+  "body": ""
+ },
+ {
+  "path": "/b/g/unmark",
+  "form": {
+   "mark": "note-1"
+  },
+  "accept": "application/json;q=0",
+  "status": 303,
+  "headers": [
+   [
+    "content-length",
+    "0"
+   ],
+   [
+    "location",
+    "/b/g/#marks"
+   ]
+  ],
+  "body": ""
+ },
+ {
+  "path": "/b/g/unmark",
+  "form": {
+   "mark": "note-1",
+   "back": "marks"
+  },
+  "accept": null,
+  "status": 303,
+  "headers": [
+   [
+    "content-length",
+    "0"
+   ],
+   [
+    "location",
+    "/b/g/marks#marks"
+   ]
+  ],
+  "body": ""
+ },
+ {
+  "path": "/b/g/unmark",
+  "form": {
+   "mark": "note-1",
+   "back": "marks"
+  },
+  "accept": "",
+  "status": 303,
+  "headers": [
+   [
+    "content-length",
+    "0"
+   ],
+   [
+    "location",
+    "/b/g/marks#marks"
+   ]
+  ],
+  "body": ""
+ },
+ {
+  "path": "/b/g/unmark",
+  "form": {
+   "mark": "note-1",
+   "back": "marks"
+  },
+  "accept": "*/*",
+  "status": 303,
+  "headers": [
+   [
+    "content-length",
+    "0"
+   ],
+   [
+    "location",
+    "/b/g/marks#marks"
+   ]
+  ],
+  "body": ""
+ },
+ {
+  "path": "/b/g/unmark",
+  "form": {
+   "mark": "note-1",
+   "back": "marks"
+  },
+  "accept": "text/html",
+  "status": 303,
+  "headers": [
+   [
+    "content-length",
+    "0"
+   ],
+   [
+    "location",
+    "/b/g/marks#marks"
+   ]
+  ],
+  "body": ""
+ },
+ {
+  "path": "/b/g/unmark",
+  "form": {
+   "mark": "note-1",
+   "back": "marks"
+  },
+  "accept": "application/*",
+  "status": 303,
+  "headers": [
+   [
+    "content-length",
+    "0"
+   ],
+   [
+    "location",
+    "/b/g/marks#marks"
+   ]
+  ],
+  "body": ""
+ },
+ {
+  "path": "/b/g/unmark",
+  "form": {
+   "mark": "note-1",
+   "back": "marks"
+  },
+  "accept": "text/html,application/xhtml+xml;q=0.9,*/*;q=0.8",
+  "status": 303,
+  "headers": [
+   [
+    "content-length",
+    "0"
+   ],
+   [
+    "location",
+    "/b/g/marks#marks"
+   ]
+  ],
+  "body": ""
+ },
+ {
+  "path": "/b/g/unmark",
+  "form": {
+   "mark": "note-1",
+   "back": "marks"
+  },
+  "accept": "application/jsonx",
+  "status": 303,
+  "headers": [
+   [
+    "content-length",
+    "0"
+   ],
+   [
+    "location",
+    "/b/g/marks#marks"
+   ]
+  ],
+  "body": ""
+ },
+ {
+  "path": "/b/g/unmark",
+  "form": {
+   "mark": "note-1",
+   "back": "marks"
+  },
+  "accept": ";;;",
+  "status": 303,
+  "headers": [
+   [
+    "content-length",
+    "0"
+   ],
+   [
+    "location",
+    "/b/g/marks#marks"
+   ]
+  ],
+  "body": ""
+ },
+ {
+  "path": "/b/g/unmark",
+  "form": {
+   "mark": "note-1",
+   "back": "marks"
+  },
+  "accept": "application/json;q=0",
+  "status": 303,
+  "headers": [
+   [
+    "content-length",
+    "0"
+   ],
+   [
+    "location",
+    "/b/g/marks#marks"
+   ]
+  ],
+  "body": ""
+ }
+]
\ No newline at end of file
diff --git a/tests/test_flow.py b/tests/test_flow.py
index ce616eb..a872fdf 100644
--- a/tests/test_flow.py
+++ b/tests/test_flow.py
@@ -151,3 +151,76 @@ def test_a_look_that_cannot_be_recorded_still_serves_the_page(tmp_path):
         b.chmod(0o755)
     assert r.status_code == 200
     assert not (b / ".seen").exists()
+
+
+# ---- C3: in-place judgment --------------------------------------------------
+
+GOLDEN = pathlib.Path(__file__).parent / "golden" / "r2_mark_303.json"
+
+
+def _seed(root: pathlib.Path) -> TestClient:
+    """The golden's fixture, byte for byte (see golden_gen in the R2 notes)."""
+    from booth.marks import declare_pick, write_note
+    root.mkdir(parents=True, exist_ok=True)
+    b = _booth(root, "g", {"a.png": PNG, "b b.png": PNG, "c.md": PNG})
+    declare_pick(b, "q", {"prompt": "Which?", "options": ["x", "y"]}, target="a.png")
+    set_flag(b, "a.png", True)
+    write_note(b, "a.png", "seed")
+    return TestClient(create_app(root, ttl_hours=24, start_sweeper=False),
+                      follow_redirects=False)
+
+
+def test_every_pre_r2_request_shape_gets_a_byte_identical_303(tmp_path):
+    """INV-4. The golden was recorded from the PRE-R2 code: every mark route,
+    with `back` absent and `back=marks`, under nine Accept headers that must
+    NOT count as asking for JSON. Status, every header, and the body must match
+    exactly — the no-JS guarantee lives in these bytes."""
+    import json
+    cases = json.loads(GOLDEN.read_text())
+    assert len(cases) == 108
+    for i, case in enumerate(cases):
+        c = _seed(tmp_path / str(i))
+        headers = {} if case["accept"] is None else {"accept": case["accept"]}
+        r = c.post(case["path"], data=case["form"], headers=headers)
+        got = {"status": r.status_code,
+               "headers": sorted([k.lower(), v] for k, v in r.headers.items()),
+               "body": r.content.decode("latin-1")}
+        want = {k: case[k] for k in ("status", "headers", "body")}
+        assert got == want, (case["path"], case["form"], case["accept"])
+
+
+@pytest.mark.parametrize("path,form", [
+    ("/b/g/answer", {"ask": "q", "choice": "y"}),
+    ("/b/g/note", {"target": "a.png", "text": "in place"}),
+    ("/b/g/flag", {"target": "b b.png", "on": "1"}),
+    ("/b/g/unmark", {"mark": "note-1"}),
+])
+@pytest.mark.parametrize("accept", ["application/json", "text/html, application/json;q=0.5"])
+def test_an_explicit_json_accept_gets_204_and_the_write_still_lands(tmp_path, path, form, accept):
+    """The in-place path: same write as the form, no redirect, no body."""
+    from booth.marks import marks_for
+    c = _seed(tmp_path)
+    before = [(m.id, m.shape, m.answer, m.text) for m in marks_for(tmp_path / "g")]
+    r = c.post(path, data=form, headers={"accept": accept})
+    assert r.status_code == 204 and r.content == b""
+    assert "location" not in r.headers
+    after = [(m.id, m.shape, m.answer, m.text) for m in marks_for(tmp_path / "g")]
+    assert after != before, "the write must happen exactly as for the form"
+
+
+@pytest.mark.parametrize("f,landing", [
+    ("a.png", "/b/g/view?f=a.png#rail"),
+    ("b b.png", "/b/g/view?f=b%20b.png#rail"),
+    ("c.md", "/b/g/#item-b%20b.png"),          # a doc is not in the review ring
+    ("gone.png", "/b/g/#item-b%20b.png"),      # not an item
+    ("", "/b/g/#item-b%20b.png"),
+    ("../../etc/passwd", "/b/g/#item-b%20b.png"),
+])
+def test_back_view_lands_on_the_review_only_for_a_media_item(tmp_path, f, landing):
+    """The JS-off fix for the bounce: a flag set at full size lands back at
+    full size. Anything that is not a media item in this booth falls back to
+    the booth page exactly as a form with no `back` does."""
+    c = _seed(tmp_path)
+    r = c.post("/b/g/flag", data={"target": "b b.png", "on": "1", "back": "view", "f": f})
+    assert r.status_code == 303
+    assert r.headers["location"] == landing