diff --git a/.changes/unreleased/Changed-20261004-020848.yaml b/.changes/unreleased/Changed-20261004-020848.yaml
new file mode 100644
index 0000000..59d4d14
--- /dev/null
+++ b/.changes/unreleased/Changed-20261004-020848.yaml
@@ -0,0 +1,3 @@
+kind: Changed
+body: Implement spec 0.3.0. It adds the postcode field and its messages, and results stay the same.
+time: 2026-10-04T02:08:48.403352+01:00
diff --git a/AGENTS.md b/AGENTS.md
index bbb5e2e..df59905 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -11,7 +11,7 @@ This repo is part of Gatepost, unofficial open-source developer tools for Nigeri
 
 ## Rules that no config file shows
 
-- If the task needs a public symbol that the Interface section of `spec/grammar.md` does not list, add nothing, not even an internal helper. Stop, and say that a spec change must come first (API-1, CS-8).
+- If the task needs a public symbol that no Interface section of the spec lists, in `spec/grammar.md`, `spec/client.md` or `spec/field.md`, add nothing, not even an internal helper. Stop, and say that a spec change must come first (API-1, CS-8).
 - Call only the documented NIPOST gateway endpoints.
 - Commit only NIPOST's published test codes and synthetic values, such as `FC-01-Z99-ZZ-01`.
 - Sign off each commit with `git commit -s`. The person who opens the pull request is the only author. Leave out co-author lines for AI tools.
diff --git a/CONTEXT.md b/CONTEXT.md
index fcc8182..4b82f5f 100644
--- a/CONTEXT.md
+++ b/CONTEXT.md
@@ -169,6 +169,18 @@ _Avoid_: input, widget, picker
 To show the user what a postcode points to before the app stores it.
 _Avoid_: verify, validate (for this step)
 
+**Check status**:
+What the WooCommerce plugin learned about the postcode of an order: `valid`, `invalid`, `unchecked` or `error`. The order meta `_gatepost_postcode_check` holds it.
+_Avoid_: verification status, validation result
+
+**Form value**:
+The text that a field gives its form when the form is sent: the canonical form of a postcode, the 6 digits of a legacy postcode, or the text without white space at the start and the end.
+_Avoid_: submitted value, output
+
+**Message catalogue**:
+A data file that holds the text of a UI in one language, one message for each key.
+_Avoid_: strings file, translations, locale file
+
 **Tell**:
 A habit that makes code look machine-written, such as very long lines.
 _Avoid_: smell (smells are design problems), anti-pattern
diff --git a/README.md b/README.md
index f0a6d52..ae0d3ca 100644
--- a/README.md
+++ b/README.md
@@ -109,7 +109,7 @@ The example uses `FC-01-Z99-ZZ-01`, a synthetic postcode, and shows the answer o
 |---|---|
 | PHP | 8.1 or later |
 | HTTP, for the client | a PSR-18 client and a PSR-17 request factory |
-| Gatepost spec | 0.2.0 |
+| Gatepost spec | 0.3.0 |
 
 ## Docs
 
diff --git a/docs/api-dump.md b/docs/api-dump.md
index ce4859c..6a99b8f 100644
--- a/docs/api-dump.md
+++ b/docs/api-dump.md
@@ -125,7 +125,7 @@ final class ParseResult
 
 final class Postcode
 {
-    public const SPEC_VERSION = '0.2.0';
+    public const SPEC_VERSION = '0.3.0';
     public readonly string $compact;
     public readonly string $canonical;
     public readonly string $display;
diff --git a/docs/developing.md b/docs/developing.md
index 62ababe..38694a4 100644
--- a/docs/developing.md
+++ b/docs/developing.md
@@ -15,6 +15,8 @@ composer check
 
 The BC check tool lists ext-intl as a requirement. Its check runs without that extension, so the install line ignores the requirement. The line also works on a machine that has ext-intl.
 
+`composer api:check` compares the public API with the newest release tag, so run `git fetch --tags` first. It ignores one finding: the value of `Postcode::SPEC_VERSION` changes with each spec version by design, so that change is not a break of the API.
+
 `composer run` lists the other scripts. `composer test` runs the tests without coverage. `composer mutation` runs the mutation tests, and it needs Xdebug.
 
 ## Contract scenarios
diff --git a/scripts/check-api.sh b/scripts/check-api.sh
index 7f0e4af..bf00b23 100755
--- a/scripts/check-api.sh
+++ b/scripts/check-api.sh
@@ -3,6 +3,10 @@
 # SPDX-License-Identifier: Apache-2.0
 # Compares the public API with the newest release tag (GIT-7, API-13). Before the first tag,
 # no released API exists, so the script says so and stops.
+#
+# The value of Postcode::SPEC_VERSION changes with each spec version by design. That change is
+# not a break of the API, so the script drops that one finding. It keeps every other finding,
+# and it fails on every other failure of the tool.
 set -euo pipefail
 
 # Git sorts v0.1.0-alpha.0 above v0.1.0 unless the config names the suffix. Without the option,
@@ -12,4 +16,32 @@ if [ -z "$tag" ]; then
   echo "No release tag yet, so there is no released API to compare."
   exit 0
 fi
-exec tools/bc-check/vendor/bin/roave-backward-compatibility-check --from="$tag"
+
+spec_version_change='[BC] CHANGED: Value of constant '
+spec_version_change+='Gatepost\Postcode\Postcode::SPEC_VERSION changed'
+status=0
+tool=tools/bc-check/vendor/bin/roave-backward-compatibility-check
+output="$("$tool" --from="$tag" 2>&1)" || status=$?
+
+dropped=0
+breaks=0
+while IFS= read -r line; do
+  if [[ $line == "$spec_version_change"* ]]; then
+    dropped=1
+  elif [[ $line =~ ^[0-9]+\ backwards-incompatible\ changes\ detected$ ]]; then
+    : # The tool counts the dropped finding, so the script prints its own count.
+  else
+    echo "$line"
+    if [[ $line == '[BC]'* ]]; then breaks=$((breaks + 1)); fi
+  fi
+done <<<"$output"
+
+if [ "$breaks" -gt 0 ]; then
+  echo "$breaks backwards-incompatible changes detected"
+  exit 3
+fi
+# Code 3 is the code of the tool for a break. Another code, or code 3 with no finding to
+# explain it, is a failure of the tool itself.
+if [ "$status" -ne 0 ] && { [ "$status" -ne 3 ] || [ "$dropped" -eq 0 ]; }; then
+  exit "$status"
+fi
diff --git a/spec b/spec
index 359db01..db359cd 160000
--- a/spec
+++ b/spec
@@ -1 +1 @@
-Subproject commit 359db01aa97f2d5b183db431d14dbccb793af9f3
+Subproject commit db359cde28a7c5c549f1a3069f20078f19f05a58
diff --git a/src/Postcode.php b/src/Postcode.php
index eb45f64..7e6453d 100644
--- a/src/Postcode.php
+++ b/src/Postcode.php
@@ -24,7 +24,7 @@ final class Postcode
     /**
      * The version of the Gatepost spec that this package implements.
      */
-    public const SPEC_VERSION = '0.2.0';
+    public const SPEC_VERSION = '0.3.0';
 
     // In UTF-8, a code point takes at most 4 bytes. Longer input is over the limit without a
     // count, so a 10 MB string costs no more than a short one.
diff --git a/tests/Unit/CheckApiTest.php b/tests/Unit/CheckApiTest.php
index e9fc5c6..43b5894 100644
--- a/tests/Unit/CheckApiTest.php
+++ b/tests/Unit/CheckApiTest.php
@@ -16,7 +16,7 @@
 /**
  * scripts/check-api.sh starts the BC check from the newest release tag. These tests run the
  * script in a temporary repo with the tags of one case. A stub takes the place of the BC check
- * tool and prints the tag that the script gave it.
+ * tool. It prints the tag that the script gave it, or the output of the case.
  */
 final class CheckApiTest extends TestCase
 {
@@ -83,6 +83,59 @@ public function stopsBeforeTheFirstTag(): void
         );
     }
 
+    /**
+     * @return array<string, array{string, int, int, string}>
+     */
+    public static function toolOutputs(): array
+    {
+        $constant = '[BC] CHANGED: Value of constant Gatepost\\Postcode\\Postcode::SPEC_VERSION '
+            . "changed from '0.2.0' to '0.3.0'";
+        $method = '[BC] REMOVED: Method Gatepost\\Postcode\\Postcode::check() was removed';
+        $summary = static fn(int $count): string
+            => "{$count} backwards-incompatible changes detected";
+
+        return [
+            'the constant alone' => [
+                "{$constant}\n{$summary(1)}",
+                3,
+                0,
+                '',
+            ],
+            'the constant and another break' => [
+                "{$constant}\n{$method}\n{$summary(2)}",
+                3,
+                3,
+                "{$method}\n{$summary(1)}",
+            ],
+            'another break alone' => [
+                "{$method}\n{$summary(1)}",
+                3,
+                3,
+                "{$method}\n{$summary(1)}",
+            ],
+            'a failure of the tool' => ['Could not read the tag', 1, 1, 'Could not read the tag'],
+            'a break code with no finding' => ['Odd output', 3, 3, 'Odd output'],
+        ];
+    }
+
+    #[Test]
+    #[DataProvider('toolOutputs')]
+    public function dropsOnlyTheChangeOfTheSpecVersion(
+        string $toolOutput,
+        int $toolCode,
+        int $exitCode,
+        string $output,
+    ): void {
+        $this->startRepo('v0.1.0');
+        $stub = "{$this->repo}/tools/bc-check/vendor/bin/roave-backward-compatibility-check";
+        \file_put_contents($stub, "#!/bin/sh\ncat <<'END'\n{$toolOutput}\nEND\nexit {$toolCode}\n");
+
+        $run = $this->checkApi();
+
+        self::assertSame($exitCode, $run->exitCode, $run->output);
+        self::assertSame($output, $run->output);
+    }
+
     // The repo starts in each test, so that tearDown() removes it even when a git command fails.
     private function startRepo(string ...$tags): void
     {