diff --git a/crates/brokerd/src/grants.rs b/crates/brokerd/src/grants.rs
index c244fcd..a36809c 100644
--- a/crates/brokerd/src/grants.rs
+++ b/crates/brokerd/src/grants.rs
@@ -260,6 +260,14 @@ fn check_grant(grant: &LoadedGrant, problems: &mut Vec<GrantProblem>) {
                 None,
                 format!("{:?} is not a valid absolute path", path),
             );
+        } else if path.contains([':', ',']) {
+            // Mounted as `--volume=<path>:<path>:ro`, where both are separators.
+            push(
+                problems,
+                file.clone(),
+                None,
+                format!("{:?} cannot be mounted: it contains ':' or ','", path),
+            );
         }
     }
     // Rule 8: every host must be a valid host pattern.
diff --git a/crates/brokerd/tests/grants_mount.rs b/crates/brokerd/tests/grants_mount.rs
new file mode 100644
index 0000000..00041ac
--- /dev/null
+++ b/crates/brokerd/tests/grants_mount.rs
@@ -0,0 +1,45 @@
+//! A grant path is mounted into the tool container as `--volume=<path>:<path>:ro`, so a path with
+//! `:` or `,` in it cannot be granted: the set is invalid, as for any other bad grant (M3b spec,
+//! section 3). Do not edit.
+
+#[path = "support/tmp.rs"]
+mod tmp;
+
+use brokerd::grants::load;
+use tmp::TempDir;
+
+fn grant_with_path(path: &str) -> String {
+    format!(
+        "tool = \"read_file\"\nmode = \"auto\"\nmax_taint = \"secret\"\n[constraints]\npaths = [{path:?}]\n"
+    )
+}
+
+#[test]
+fn a_path_with_a_colon_or_a_comma_makes_the_set_invalid() {
+    for path in ["/home/kyle/a:b", "/home/kyle/a,b", "/x:/y", "/n,ro"] {
+        let dir = TempDir::new("mount-bad");
+        dir.write("notes.toml", &grant_with_path(path));
+        let problems = load(dir.path()).expect_err(path);
+        assert_eq!(problems.len(), 1, "{path}: {problems:?}");
+        assert_eq!(problems[0].file, "notes.toml");
+        assert!(
+            problems[0].problem.contains("cannot be mounted"),
+            "{path}: {}",
+            problems[0].problem
+        );
+    }
+}
+
+#[test]
+fn other_punctuation_is_still_fine() {
+    for path in [
+        "/home/kyle/a b",
+        "/home/kyle/a;b",
+        "/home/kyle/a=b",
+        "/home/kyle/a.b-c_d",
+    ] {
+        let dir = TempDir::new("mount-ok");
+        dir.write("notes.toml", &grant_with_path(path));
+        assert!(load(dir.path()).is_ok(), "{path}");
+    }
+}
diff --git a/docs/implementer-log.md b/docs/implementer-log.md
index 931f4cc..b5afd48 100644
--- a/docs/implementer-log.md
+++ b/docs/implementer-log.md
@@ -419,4 +419,5 @@ each task's commit. Three rows were malformed by the first run's orchestrator: t
 six cells, and its notes had been pasted into task 05's row; task 10's notes into task 11's; and
 task 01's stopped row carried the notes of M2b task 11. Each is moved back or removed, with a
 bracketed mark, and pipes inside code are escaped so every row has its eight cells.
+| M3b/03-brokerd-grant-mount-rule | 2026-09-22 | done | 1 | pass | none | Copied `tests/grants_mount.rs` from the plan's `files/`. Added a third arm to the path loop in `check_grant` (grants.rs:263), an `else if path.contains([':', ','])` checked only when the first two arms did not apply, reporting `"{:?} cannot be mounted: it contains ':' or ','"`. The `else if` chain means a path already reported as invalid is not reported twice. `cargo fmt --all` kept the `push` multi-line (the single-line form in the task exceeds 100 columns); the wording matches the task verbatim. Both suites pass (17 grants, 2 mount); `make gate` prints `gate: ok` on the first run. | ? |