diff --git a/docs/implementer-lessons.md b/docs/implementer-lessons.md
index ec84b1c..e199ea1 100644
--- a/docs/implementer-lessons.md
+++ b/docs/implementer-lessons.md
@@ -67,6 +67,8 @@ How it is used:
 
 | T29 | Check a skeleton's comments, not only its code. Print every `todo!()` beside its signature and read each pair before hand-over. Filling the bodies from your own code proves the split can pass; it cannot show that a comment sits on the wrong function. Edit comments by function, never by "the next comment after this name". And tell the implementer to stop and quote a comment that does not fit. | M4a task 14, second session: `post`'s comment was `now_ms`'s text, put there by a script; Ornith saw the contradiction and deliberated over it until the turn ran out. M4a task 16: a comment naming the function it replaced made the task's own `grep` fail; and a given test copied into the plan before `cargo fmt` came back reformatted. Run the task's checks, and `cargo fmt`, on the end state before hand-over. |
 
+| T30 | Every behaviour a task writes must be reachable by that task's own tests. When a function's effect shows only after a later task (here: a refusal's reason, which only task 15 could send), give a unit test in the first task, or write that function in the later one. And never let one name mean two things in a comment: say which `reason`. | M4b task 14 dropped a refusal's reason, reading `Refused -> Ended::Refused { by, reason }` as the `DenyReason` in scope; its tests passed, and task 15's caught it, in a file task 15 could not touch. |
+
 ## What worked and should be kept
 
 - Byte-exact fixtures, compared in both directions. No wire-format defect reached review.
diff --git a/docs/plans/M4b/15-gatewayd-answers.md b/docs/plans/M4b/15-gatewayd-answers.md
index 9b6ec88..757402d 100644
--- a/docs/plans/M4b/15-gatewayd-answers.md
+++ b/docs/plans/M4b/15-gatewayd-answers.md
@@ -24,7 +24,8 @@ The last three pieces (M4b spec, section 7):
   pending here"), `crates/gatewayd/tests/support/gateway.rs` (`Tuning` has `notice`)
 - Modify: `crates/gatewayd/src/approvals/mod.rs`, `crates/gatewayd/src/deliver.rs`,
   `crates/gatewayd/src/serve/mod.rs`, `crates/gatewayd/src/serve/handle.rs`,
-  `crates/gatewayd/src/sessions.rs`, `docs/implementer-log.md`
+  `crates/gatewayd/src/serve/approvals.rs` (one fix, below), `crates/gatewayd/src/sessions.rs`,
+  `docs/implementer-log.md`
 
 ## The changes, exactly
 
@@ -128,6 +129,32 @@ parameter of `Gateway::new`, which `run` passes as `tuning.notice`.
 
 **`sessions.rs`**: delete `pub const M4B_COMMAND`.
 
+**`serve/approvals.rs`, a fix to task 14's `ended`.** `ended` has two different values that were
+both called `reason`: the refusal's text, in `Ending::Answered { reason, .. }`, and the
+`DenyReason` in `GatewayOutcome::Denied { reason }`. Task 14's comment did not say which went
+where, and its tests could not send a refusal, so the refusal's text was dropped (`reason: _`, and
+`Ended::Refused { by, reason: None }`). This task's `refused_with_a_reason_the_post_says_so` shows
+it. Name the refusal's text `refusal`:
+
+```rust
+            Ending::Answered {
+                by,
+                reason: refusal,
+                outcome,
+            } => {
+```
+
+and in the `match outcome` inside it, the `Refused` arm becomes:
+
+```rust
+                    GatewayOutcome::Refused {} => Ended::Refused {
+                        by,
+                        reason: refusal,
+                    },
+```
+
+The `Denied { reason }` arm stays as it is: that `reason` is the `DenyReason`.
+
 ## Steps
 
 - [ ] **1. Copy.** `git switch m4b`, then
diff --git a/docs/plans/M4b/README.md b/docs/plans/M4b/README.md
index 21db49f..cefbb52 100644
--- a/docs/plans/M4b/README.md
+++ b/docs/plans/M4b/README.md
@@ -65,6 +65,20 @@ signature (tip T29).
 
 At the end of task 15: about 845 tests (768 before task 01).
 
+## Changes during the run
+
+- 2026-09-24, task 15: tasks 01 to 14 committed, each on its first session. Task 15's session made
+  every change the task asks for, and one given test still failed: a refusal's reason was missing
+  from the post. It traced the cause to task 14's `ended` in `serve/approvals.rs`, a file task 15
+  did not list, and stopped instead of editing it, which was right. The fault was the plan's: in
+  `ended`, the refusal's text and a `DenyReason` were both called `reason`, and the comment said
+  `Refused -> Ended::Refused { by, reason }` without saying which; and task 14's own tests could
+  not send a refusal, so nothing there could catch it. Task 14 took the `DenyReason` reading, then
+  silenced the unused-variable warning on the other with `reason: _` (and said so in its log).
+  Task 15 now lists `serve/approvals.rs` with the exact fix. Checked in a scratch copy: the saved
+  attempt (`.state/runs/M4b/15-first-attempt*`) plus that fix passes every suite three times and
+  the gate, 845 tests. Resume from task 15.
+
 ## Running it
 
 ```sh