diff --git a/.github/workflows/validate.yml b/.github/workflows/validate.yml
index a323102..37dc3e5 100644
--- a/.github/workflows/validate.yml
+++ b/.github/workflows/validate.yml
@@ -30,3 +30,5 @@ jobs:
           while IFS= read -r file; do
             node --check "$file"
           done < <(find scripts -type f -name '*.mjs' -print)
+      - name: Run unit tests
+        run: node --test scripts/test/*.test.mjs
diff --git a/scripts/collect-activity.mjs b/scripts/collect-activity.mjs
index 4bf4e2c..a90639a 100644
--- a/scripts/collect-activity.mjs
+++ b/scripts/collect-activity.mjs
@@ -1,61 +1,111 @@
 import { activityApi as api, getFile, putFile } from "./lib/github.mjs";
 
 const repository = process.env.GITHUB_REPOSITORY;
-const [owner, repo] = repository.split("/");
+if (!repository || !/^[^/]+\/[^/]+$/.test(repository)) {
+  throw new Error("GITHUB_REPOSITORY must be in owner/repository format.");
+}
 const configured = (process.env.GH_ACTIVITY_REPOS ?? "").split(/[\s,]+/).filter(Boolean);
 const repositories = [...new Set([repository, ...configured])];
 const until = new Date();
 const since = new Date(until.getTime() - 24 * 60 * 60 * 1000);
 const date = until.toISOString().slice(0, 10);
 const [year, month] = date.split("-");
-const path = `logs/${year}/${month}/${date}.md`;
+const path = "logs/" + year + "/" + month + "/" + date + ".md";
 const entry = await getFile(path);
-if (!entry) throw new Error(`Daily entry does not exist: ${path}`);
+if (!entry) throw new Error("Daily entry does not exist: " + path);
 
 const sections = [];
+const failures = [];
+let completedRepositories = 0;
+
 for (const fullName of repositories) {
-  const [repoOwner, repoName] = fullName.split("/");
-  if (!repoOwner || !repoName) {
-    console.warn(`Skipping invalid repository setting: ${fullName}`);
+  if (!/^[^/]+\/[^/]+$/.test(fullName)) {
+    failures.push({ repository: fullName, reason: "invalid owner/repository setting" });
+    console.warn("Skipping invalid repository setting: " + fullName);
     continue;
   }
+
+  const [repoOwner, repoName] = fullName.split("/");
   try {
-    const query = new URLSearchParams({ since: since.toISOString(), until: until.toISOString(), per_page: "100" });
+    const query = new URLSearchParams({
+      since: since.toISOString(),
+      until: until.toISOString(),
+      per_page: "100",
+    });
     const [commits, pulls, releases] = await Promise.all([
-      api(`/repos/${repoOwner}/${repoName}/commits?${query}`),
-      api(`/repos/${repoOwner}/${repoName}/pulls?state=closed&sort=updated&direction=desc&per_page=100`),
-      api(`/repos/${repoOwner}/${repoName}/releases?per_page=100`),
+      api("/repos/" + repoOwner + "/" + repoName + "/commits?" + query),
+      api("/repos/" + repoOwner + "/" + repoName + "/pulls?state=closed&sort=updated&direction=desc&per_page=100"),
+      api("/repos/" + repoOwner + "/" + repoName + "/releases?per_page=100"),
     ]);
+
+    completedRepositories += 1;
     const recentCommits = commits.filter((item) => {
-      const authored = item.commit?.author?.date;
+      const authored = item.commit?.author?.date ?? item.commit?.committer?.date;
       return authored && authored >= since.toISOString() && authored <= until.toISOString();
     });
-    const recentPulls = pulls.filter((item) => item.merged_at && item.merged_at >= since.toISOString() && item.merged_at <= until.toISOString());
-    const recentReleases = releases.filter((item) => item.published_at && item.published_at >= since.toISOString() && item.published_at <= until.toISOString());
+    const recentPulls = pulls.filter((item) =>
+      item.merged_at && item.merged_at >= since.toISOString() && item.merged_at <= until.toISOString()
+    );
+    const recentReleases = releases.filter((item) =>
+      item.published_at && item.published_at >= since.toISOString() && item.published_at <= until.toISOString()
+    );
+
     for (const item of recentCommits) {
       const message = item.commit.message.split("\n")[0].replaceAll("\r", "");
       const isLogNote = /\[log\]/i.test(message);
       const category = isLogNote ? " log note" : "";
-      sections.push(`- **${fullName}${category}** — [${escapeMarkdown(message)}](${item.html_url})`);
+      sections.push("- **" + fullName + category + "** — [" + escapeMarkdown(message) + "](" + item.html_url + ")");
+    }
+    for (const item of recentPulls) {
+      sections.push("- **" + fullName + " PR** — [#" + item.number + " " + escapeMarkdown(item.title) + "](" + item.html_url + ")");
+    }
+    for (const item of recentReleases) {
+      sections.push("- **" + fullName + " release** — [" + escapeMarkdown(item.name || item.tag_name) + "](" + item.html_url + ")");
     }
-    for (const item of recentPulls) sections.push(`- **${fullName} PR** — [#${item.number} ${escapeMarkdown(item.title)}](${item.html_url})`);
-    for (const item of recentReleases) sections.push(`- **${fullName} release** — [${escapeMarkdown(item.name || item.tag_name)}](${item.html_url})`);
   } catch (error) {
-    console.warn(`Activity unavailable for ${fullName}: ${error.message}`);
+    const reason = String(error?.message ?? error).split("\n")[0].slice(0, 240);
+    failures.push({ repository: fullName, reason });
+    console.warn("Activity unavailable for " + fullName + ": " + reason);
   }
 }
 
-const activity = sections.length ? sections.join("\n") : "_No GitHub commits, merged pull requests, or releases found in the last 24 hours._";
+if (completedRepositories === 0) {
+  throw new Error(
+    "Activity collection failed for every configured repository; preserving the existing activity block. " +
+    failures.map((item) => item.repository + ": " + item.reason).join("; ")
+  );
+}
+
+let activity = sections.length
+  ? sections.join("\n")
+  : failures.length
+    ? "_No activity found in repositories checked successfully._"
+    : "_No GitHub commits, merged pull requests, or releases found in the last 24 hours._";
+if (failures.length) {
+  activity += "\n\n> ⚠️ Incomplete activity collection for: " +
+    failures.map((item) => escapeMarkdown(item.repository) + " (" + escapeMarkdown(item.reason) + ")").join("; ");
+}
+
 const start = "<!-- ACTIVITY:START -->";
 const end = "<!-- ACTIVITY:END -->";
-const block = `${start}\n### Shipped Today (GitHub activity)\n${activity}\n${end}`;
-let content = entry.content;
+const block = start + "\n## Shipped Today (GitHub activity)\n" + activity + "\n" + end;
 const pattern = /<!-- ACTIVITY:START -->[\s\S]*?<!-- ACTIVITY:END -->/;
+let content = entry.content;
 if (pattern.test(content)) content = content.replace(pattern, block);
-else content = `${content.trimEnd()}\n\n${block}\n`;
-await putFile(path, content, `chore(log): enrich ${date} with GitHub activity`, entry.sha);
-console.log(`Recorded ${sections.length} activity item(s) from ${repositories.length} repository/repositories.`);
+else content = content.trimEnd() + "\n\n" + block + "\n";
+
+await putFile(path, content, "chore(log): enrich " + date + " with GitHub activity", entry.sha);
+console.log(
+  "Recorded " + sections.length + " activity item(s) across " + completedRepositories +
+  " successfully checked repository/repositories; " + failures.length + " failure(s)."
+);
 
 function escapeMarkdown(value) {
-  return String(value).replaceAll("\\", "\\\\").replaceAll("[", "\\[").replaceAll("]", "\\]").replaceAll("(", "\\(").replaceAll(")", "\\)").replaceAll("|", "\\|");
+  return String(value)
+    .replaceAll("\\", "\\\\")
+    .replaceAll("[", "\\[")
+    .replaceAll("]", "\\]")
+    .replaceAll("(", "\\(")
+    .replaceAll(")", "\\)")
+    .replaceAll("|", "\\|");
 }
diff --git a/scripts/issue-to-log.mjs b/scripts/issue-to-log.mjs
index 919ef6c..e84ecd2 100644
--- a/scripts/issue-to-log.mjs
+++ b/scripts/issue-to-log.mjs
@@ -1,39 +1,36 @@
 import { getFile, getIssue, putFile } from "./lib/github.mjs";
+import { replaceMarkdownSection } from "./lib/markdown.mjs";
 
 const number = Number(process.env.ISSUE_NUMBER);
 if (!Number.isInteger(number) || number < 1) throw new Error("ISSUE_NUMBER must be a positive integer.");
+
 const issue = await getIssue(number);
-const dateMatch = issue.body.match(/^### Log date \(UTC\)\s*\r?\n+([^\r\n]+)/m);
+const body = typeof issue.body === "string" ? issue.body : "";
+const dateMatch = body.match(/^### Log date \(UTC\)\s*\r?\n+([^\r\n]+)/m);
 const requestedDate = dateMatch?.[1]?.trim();
 const date = /^\d{4}-\d{2}-\d{2}$/.test(requestedDate ?? "") ? requestedDate : new Date().toISOString().slice(0, 10);
-const parsed = new Date(`${date}T00:00:00Z`);
-if (Number.isNaN(parsed.valueOf()) || parsed.toISOString().slice(0, 10) !== date) throw new Error(`Invalid log date: ${date}`);
-const [year, month] = date.split("-");
-const path = `logs/${year}/${month}/${date}.md`;
+const parsed = new Date(date + "T00:00:00Z");
+if (Number.isNaN(parsed.valueOf()) || parsed.toISOString().slice(0, 10) !== date) {
+  throw new Error("Invalid log date: " + date);
+}
 
+const [year, month] = date.split("-");
+const path = "logs/" + year + "/" + month + "/" + date + ".md";
 const fields = Object.fromEntries(["Shipped", "Learned", "Blockers", "Tomorrow"].map((label) => {
-  const escaped = label.replace(/[.*+?^${}()|[\]\\]/g, "\\$&");
-  const match = issue.body.match(new RegExp(`^### ${escaped}\\s*\\r?\\n+([\\s\\S]*?)(?=^### |$(?![\\s\\S]))`, "m"));
-  const value = (match?.[1] ?? "").trim();
+  const escaped = label.replace(/[.*+?^{}()|[\]\\]/g, "\\$&").replaceAll("$", "\\$");
+  const pattern = new RegExp("^### " + escaped + "\\s*\\r?\\n+([\\s\\S]*?)(?=^### |$(?![\\s\\S]))", "m");
+  const value = (body.match(pattern)?.[1] ?? "").trim();
   return [label, value && value !== "_No response_" ? value : "_Not provided._"];
 }));
 
-let file = await getFile(path);
-let content = file?.content ?? `# Daily Log — ${date}\n\n**Day:** Captured from issue [#${number}](${issue.html_url})\n`;
+const file = await getFile(path);
+let content = file?.content ?? "# Daily Log — " + date + "\n\n**Day:** Captured from issue [#" + number + "](" + issue.html_url + ")\n";
 for (const [heading, value] of Object.entries(fields)) {
-  content = replaceSection(content, heading, value);
+  content = replaceMarkdownSection(content, heading, value);
 }
-if (!content.includes(`[issue #${number}]`)) content += `\n_Source: [issue #${number}](${issue.html_url})_\n`;
-await putFile(path, content, `docs(log): save issue #${number} as ${date}`, file?.sha);
-console.log(`Saved issue #${number} to ${path}.`);
-
-function replaceSection(markdown, heading, value) {
-  const marker = `### ${heading}`;
-  const start = markdown.indexOf(marker);
-  if (start < 0) return `${markdown.trimEnd()}\n\n${marker}\n${value}\n`;
-  const contentStart = markdown.indexOf("\n", start) + 1;
-  if (contentStart === 0) throw new Error(`Malformed section heading: ${heading}`);
-  const nextHeading = markdown.indexOf("\n### ", contentStart);
-  const suffix = nextHeading < 0 ? "" : markdown.slice(nextHeading + 1);
-  return `${markdown.slice(0, contentStart)}${value}\n\n${suffix}`;
+if (!content.includes("[issue #" + number + "]")) {
+  content = content.trimEnd() + "\n\n_Source: [issue #" + number + "](" + issue.html_url + ")_\n";
 }
+
+await putFile(path, content, "docs(log): save issue #" + number + " as " + date, file?.sha);
+console.log("Saved issue #" + number + " to " + path + ".");
diff --git a/scripts/lib/markdown.mjs b/scripts/lib/markdown.mjs
new file mode 100644
index 0000000..fe0fbbd
--- /dev/null
+++ b/scripts/lib/markdown.mjs
@@ -0,0 +1,49 @@
+export function replaceMarkdownSection(markdown, heading, value) {
+  if (typeof markdown !== "string") throw new TypeError("markdown must be a string.");
+  if (typeof heading !== "string" || !heading.trim() || /[\r\n]/.test(heading)) {
+    throw new TypeError("heading must be a non-empty single line.");
+  }
+  if (typeof value !== "string") throw new TypeError("value must be a string.");
+
+  const escaped = heading.replace(/[.*+?^{}()|[\]\\]/g, "\\$&").replaceAll("$", "\\$");
+  const headingPattern = new RegExp("^(#{1,6})[ \\t]+" + escaped + "[ \\t]*\\r?$", "gm");
+  const matches = [...markdown.matchAll(headingPattern)];
+
+  for (const duplicate of matches.slice(1).reverse()) {
+    const level = duplicate[1].length;
+    let contentStart = duplicate.index + duplicate[0].length;
+    if (markdown.slice(contentStart, contentStart + 2) === "\r\n") contentStart += 2;
+    else if (markdown[contentStart] === "\n") contentStart += 1;
+
+    const boundaryPattern = new RegExp(
+      "^#{1," + level + "}[ \\t]+|^---[ \\t]*$|^<!-- ACTIVITY:START -->$|^<!-- ACTIVITY:END -->$",
+      "gm",
+    );
+    boundaryPattern.lastIndex = contentStart;
+    const boundary = boundaryPattern.exec(markdown);
+    const end = boundary ? boundary.index : markdown.length;
+    const before = markdown.slice(0, duplicate.index).replace(/\n+$/, "");
+    const after = markdown.slice(end).replace(/^\n+/, "");
+    markdown = before + (before ? "\n\n" : "") + after;
+  }
+
+  const match = headingPattern.exec(markdown);
+  if (!match) {
+    return markdown.trimEnd() + "\n\n## " + heading + "\n" + value.trim() + "\n";
+  }
+
+  const level = match[1].length;
+  let contentStart = match.index + match[0].length;
+  if (markdown.slice(contentStart, contentStart + 2) === "\r\n") contentStart += 2;
+  else if (markdown[contentStart] === "\n") contentStart += 1;
+
+  const boundaryPattern = new RegExp(
+    "^#{1," + level + "}[ \\t]+|^---[ \\t]*$|^<!-- ACTIVITY:START -->$|^<!-- ACTIVITY:END -->$",
+    "gm",
+  );
+  boundaryPattern.lastIndex = contentStart;
+  const boundary = boundaryPattern.exec(markdown);
+  const end = boundary ? boundary.index : markdown.length;
+
+  return markdown.slice(0, contentStart) + value.trim() + "\n\n" + markdown.slice(end).replace(/^\n+/, "");
+}
diff --git a/scripts/test/markdown.test.mjs b/scripts/test/markdown.test.mjs
new file mode 100644
index 0000000..49b7b17
--- /dev/null
+++ b/scripts/test/markdown.test.mjs
@@ -0,0 +1,76 @@
+import test from "node:test";
+import assert from "node:assert/strict";
+import { replaceMarkdownSection } from "../lib/markdown.mjs";
+
+test("updates an existing template section without duplicating its heading", () => {
+  const source = "# Daily Log\n\n## Shipped\n- \n\n## Learned\n- \n";
+  const result = replaceMarkdownSection(source, "Shipped", "- Delivered the capture fix.");
+
+  assert.equal((result.match(/^## Shipped$/gm) ?? []).length, 1);
+  assert.match(result, /## Shipped\n- Delivered the capture fix\.\n\n## Learned/);
+});
+
+test("keeps later sections and activity markers outside the replaced section", () => {
+  const source = [
+    "# Daily Log",
+    "",
+    "## Tomorrow",
+    "- ",
+    "",
+    "<!-- ACTIVITY:START -->",
+    "## Shipped Today (GitHub activity)",
+    "old activity",
+    "<!-- ACTIVITY:END -->",
+    "",
+    "---",
+    "",
+  ].join("\n");
+  const result = replaceMarkdownSection(source, "Tomorrow", "- Review the PR.");
+
+  assert.match(result, /## Tomorrow\n- Review the PR\./);
+  assert.match(result, /<!-- ACTIVITY:START -->[\s\S]*old activity/);
+  assert.match(result, /\n---\n/);
+});
+
+test("appends a missing section using a level-two heading", () => {
+  const result = replaceMarkdownSection("# Daily Log\n", "Learned", "- A lesson.");
+
+  assert.match(result, /## Learned\n- A lesson\.\n$/);
+});
+
+test("matches headings literally rather than treating punctuation as regex syntax", () => {
+  const source = "# Notes\n\n## What (I) built\n- old\n\n## Next\n- soon\n";
+  const result = replaceMarkdownSection(source, "What (I) built", "- new");
+
+  assert.match(result, /## What \(I\) built\n- new\n\n## Next/);
+  assert.doesNotMatch(result, /- old/);
+});
+
+test("removes duplicate sections left by older capture runs", () => {
+  const source = [
+    "# Daily Log",
+    "",
+    "## Shipped",
+    "- old original",
+    "",
+    "## Learned",
+    "- learned",
+    "",
+    "### Shipped",
+    "- stale duplicate",
+    "",
+    "### Learned",
+    "- stale duplicate lesson",
+    "",
+    "---",
+    "",
+  ].join("\n");
+  let result = source;
+  for (const heading of ["Shipped", "Learned", "Blockers", "Tomorrow"]) {
+    result = replaceMarkdownSection(result, heading, "- corrected " + heading.toLowerCase());
+  }
+
+  assert.equal((result.match(/^#{2,6} Shipped$/gm) ?? []).length, 1);
+  assert.doesNotMatch(result, /stale duplicate/);
+  assert.match(result, /## Learned\n- corrected learned/);
+});