diff --git a/askcc/definitions.py b/askcc/definitions.py
index ff52f50..5550499 100644
--- a/askcc/definitions.py
+++ b/askcc/definitions.py
@@ -281,7 +281,19 @@
 On completion:
 - Run /simplify or /refactor to simplify and improve the code.
 - Commit, push the feature branch, and open a PR linked to the issue.
+- Update the test plan checklist in the PR description (see "Test plan update" below).
 - Add an issue comment summarizing what was implemented.
+
+Test plan update:
+- After opening the PR, read the PR description with `gh pr view <number> --json body -q .body`.
+- Look for a `## Test plan` section containing checklist items (`- [ ]` checkboxes).
+- For each test plan task, decide whether it is satisfied by the implementation, \
+the new tests you wrote, or the verification gate run (pytest/ruff/pyright).
+- Check off satisfied tasks by replacing `- [ ]` with `- [x]` in the PR body.
+- Leave items requiring manual/external verification (browser QA, deployment \
+validation, etc.) unchecked.
+- Update the PR description with `gh pr edit <number> --body "<updated body>"`.
+- If no `## Test plan` section exists, skip this step silently.
 """
 
 REVIEW_AGENT_PROMPT = (
diff --git a/tests/test_askcc.py b/tests/test_askcc.py
index 2d6e2e5..10a9bea 100644
--- a/tests/test_askcc.py
+++ b/tests/test_askcc.py
@@ -279,6 +279,34 @@ def test_removes_legacy_write_tests_line(self):
         assert "Write tests for every new or changed behavior" not in DEVELOP_AGENT_PROMPT
 
 
+class TestDevelopPromptTestPlanUpdate:
+    """The develop prompt must update the PR test plan after PR creation (issue #88).
+
+    Assertions are literal-substring matches: prompt-wording changes will surface
+    here intentionally. Don't "fix" them away — update both the prompt and the
+    asserted substrings together, mirroring the TestReviewprPromptMergeGuard precedent.
+    """
+
+    def test_includes_test_plan_update_section_heading(self):
+        assert "Test plan update:" in DEVELOP_AGENT_PROMPT
+
+    def test_includes_pr_body_read_command(self):
+        assert "gh pr view <number> --json body -q .body" in DEVELOP_AGENT_PROMPT
+
+    def test_includes_pr_body_edit_command(self):
+        assert 'gh pr edit <number> --body "<updated body>"' in DEVELOP_AGENT_PROMPT
+
+    def test_includes_manual_verification_carveout(self):
+        assert "manual/external verification" in DEVELOP_AGENT_PROMPT
+        assert "unchecked" in DEVELOP_AGENT_PROMPT
+
+    def test_includes_noop_when_no_test_plan_section(self):
+        assert "If no `## Test plan` section exists, skip this step" in DEVELOP_AGENT_PROMPT
+
+    def test_completion_step_references_test_plan_update(self):
+        assert "Update the test plan checklist in the PR description" in DEVELOP_AGENT_PROMPT
+
+
 class TestReviewprPromptMergeGuard:
     """The pr-review prompt must block merging when CHANGES_REQUESTED reviews exist (issue #86)."""