diff --git a/__tests__/comment.test.ts b/__tests__/comment.test.ts
new file mode 100644
index 00000000..4df76955
--- /dev/null
+++ b/__tests__/comment.test.ts
@@ -0,0 +1,143 @@
+import { describe, expect, test } from "bun:test";
+import {
+  buildCommentBody,
+  type CommentParts,
+  parseDiff,
+  parseSummary,
+  truncate,
+} from "../src/comment";
+
+describe("parseDiff", () => {
+  test("prefixes each action keyword with its diff symbol", () => {
+    const show = [
+      "Terraform will perform the following actions:",
+      "  # aws_s3_bucket.a will be created",
+      "  # aws_instance.b will be destroyed",
+      "  # aws_iam_role.c will be updated in-place",
+      "  # aws_db.d must be replaced",
+      "  # data.aws_ami.e will be read during apply",
+      "  # module.f.x will be moved to module.g.x",
+      "    some non-comment line",
+    ].join("\n");
+    const { lines, count } = parseDiff(show);
+    expect(lines).toEqual([
+      "+ aws_s3_bucket.a will be created",
+      "- aws_instance.b will be destroyed",
+      "! aws_iam_role.c will be updated in-place",
+      "! aws_db.d must be replaced",
+      "~ data.aws_ami.e will be read during apply",
+      "# module.f.x will be moved to module.g.x",
+    ]);
+    // the fallback `#` line is not counted as a change
+    expect(count).toBe(5);
+  });
+
+  test("returns no lines when there is no diff", () => {
+    expect(
+      parseDiff("No changes. Your infrastructure matches the configuration."),
+    ).toEqual({
+      lines: [],
+      count: 0,
+    });
+  });
+});
+
+describe("parseSummary", () => {
+  test("returns the last matching status line", () => {
+    const out =
+      "Initializing...\nPlan: 1 to add, 0 to change, 0 to destroy.\ntrailing noise";
+    expect(parseSummary(out)).toBe(
+      "Plan: 1 to add, 0 to change, 0 to destroy.",
+    );
+  });
+
+  test("matches apply/no-changes/error forms", () => {
+    expect(parseSummary("Apply complete! Resources: 1 added.")).toBe(
+      "Apply complete! Resources: 1 added.",
+    );
+    expect(
+      parseSummary(
+        "No changes. Your infrastructure matches the configuration.",
+      ),
+    ).toBe("No changes. Your infrastructure matches the configuration.");
+    expect(parseSummary("Error: Invalid resource type")).toBe(
+      "Error: Invalid resource type",
+    );
+  });
+
+  test("falls back to 'View output.' when nothing matches", () => {
+    expect(parseSummary("just some logs\nmore logs")).toBe("View output.");
+  });
+});
+
+describe("truncate", () => {
+  test("returns the text unchanged when within the limit", () => {
+    expect(truncate("short", 10)).toBe("short");
+  });
+
+  test("truncates by code point and appends the sentinel", () => {
+    expect(truncate("abcdef", 3)).toBe("abc\n…");
+  });
+
+  test("counts astral code points as one (never splits a character)", () => {
+    // four 😀 (each one code point); limit 2 keeps two, not a broken surrogate
+    expect(truncate("😀😀😀😀", 2)).toBe("😀😀\n…");
+  });
+});
+
+function makeParts(overrides: Partial<CommentParts> = {}): CommentParts {
+  return {
+    positions: ["P1", "P2", "P3", "P4", "P5", "P6"],
+    command: "terraform plan",
+    diff: { lines: [], count: 0 },
+    console: "Plan: 1 to add",
+    diffText: "",
+    summary: "Plan: 1 to add, 0 to change, 0 to destroy.",
+    syntax: "hcl",
+    actorTag: "",
+    actor: "octocat",
+    timestamp: "2026-06-28T00:00:00Z",
+    runUrl: "https://example/run",
+    marker: "terraform-7-abc.tfplan",
+    expandDiff: false,
+    expandSummary: false,
+    ...overrides,
+  };
+}
+
+describe("buildCommentBody", () => {
+  test("includes the command block, placeholders, marker, and actor footer", () => {
+    const body = buildCommentBody(makeParts({ actorTag: "@" }));
+    expect(body).toContain("```fish\nterraform plan\n```");
+    expect(body).toContain("<!-- terraform-7-abc.tfplan -->");
+    expect(body).toContain(
+      "By @octocat at 2026-06-28T00:00:00Z [(view log)](https://example/run).",
+    );
+    for (const p of ["P1", "P2", "P3", "P4", "P5", "P6"])
+      expect(body).toContain(p);
+  });
+
+  test("omits the diff block when there are no diff lines", () => {
+    expect(buildCommentBody(makeParts())).not.toContain("<summary>Diff of");
+  });
+
+  test("renders a collapsible diff block with the right count and noun", () => {
+    const body = buildCommentBody(
+      makeParts({
+        diff: { lines: ["+ a will be created"], count: 1 },
+        diffText: "+ a will be created",
+        expandDiff: true,
+      }),
+    );
+    expect(body).toContain(
+      "<details open><summary>Diff of 1 change.</summary>",
+    );
+    expect(body).toContain("```diff\n+ a will be created\n```");
+  });
+
+  test("expand-summary opens the summary details", () => {
+    expect(buildCommentBody(makeParts({ expandSummary: true }))).toContain(
+      "<details open><summary>",
+    );
+  });
+});
diff --git a/src/comment.ts b/src/comment.ts
new file mode 100644
index 00000000..9d92b57f
--- /dev/null
+++ b/src/comment.ts
@@ -0,0 +1,141 @@
+/**
+ * PR-comment and job-summary rendering (Phase 5 of the TypeScript migration).
+ *
+ * Pure functions that reproduce the composite action's `post`-step markdown: the
+ * diff symbol-prefixing, the summary line extraction, length truncation, and the
+ * final comment body. The orchestrator supplies the already-resolved command
+ * string, console output, and context; this module only formats.
+ *
+ * Truncation is by Unicode code point (not bytes, per the migration decision),
+ * so it never splits a multi-byte character.
+ */
+
+/** A parsed plan diff: symbol-prefixed lines and the count of real changes. */
+export interface PlanDiff {
+  lines: string[];
+  /** Number of changed resources (lines not starting with `# `). */
+  count: number;
+}
+
+/**
+ * Turn `terraform show` output into diff-highlighted lines, mirroring the
+ * composite action's `grep '^  # ' | sed` symbol-prefixing. Each `  # …` line
+ * becomes `+`/`-`/`!`/`~`/`#`-prefixed by its action keyword. First match wins,
+ * matching the sequential `sed` rewrite.
+ */
+export function parseDiff(showOutput: string): PlanDiff {
+  const lines: string[] = [];
+  for (const raw of showOutput.split("\n")) {
+    if (!raw.startsWith("  # ")) continue;
+    const rest = raw.slice(4);
+    let symbol: string;
+    if (rest.includes(" be created")) symbol = "+";
+    else if (rest.includes(" be destroyed")) symbol = "-";
+    else if (rest.includes(" be updated") || rest.includes(" be replaced"))
+      symbol = "!";
+    else if (rest.includes(" be read")) symbol = "~";
+    else symbol = "#";
+    lines.push(`${symbol} ${rest}`);
+  }
+  const count = lines.filter((line) => !line.startsWith("# ")).length;
+  return { lines, count };
+}
+
+/**
+ * The last meaningful status line from console output, mirroring the composite
+ * action's `awk` over `^(Error:|Plan:|Apply complete!|No changes.|Success)`;
+ * falls back to "View output." when none is present.
+ */
+export function parseSummary(consoleOutput: string): string {
+  // `No changes.` keeps the composite's unescaped `.` (any char) so a future
+  // "No changes:"/"!" variant matches identically to the bash awk.
+  const pattern = /^(Error:|Plan:|Apply complete!|No changes.|Success)/;
+  let summary = "";
+  for (const line of consoleOutput.split("\n")) {
+    if (pattern.test(line)) summary = line;
+  }
+  return summary !== "" ? summary : "View output.";
+}
+
+/**
+ * Truncate by Unicode code point, appending a `\n…` sentinel when shortened
+ * (the composite used a byte limit + `…`; we use code points so a multi-byte
+ * character is never split).
+ */
+export function truncate(text: string, maxCodePoints: number): string {
+  const codePoints = Array.from(text);
+  if (codePoints.length <= maxCodePoints) return text;
+  return `${codePoints.slice(0, maxCodePoints).join("")}\n…`;
+}
+
+export interface CommentParts {
+  /** `comment-pos-1..6` markdown placeholders, in order. */
+  positions: readonly [string, string, string, string, string, string];
+  /** The (hide/show-args filtered, secret-scrubbed) command line. */
+  command: string;
+  /** Parsed diff; rendered as a collapsible block when it has lines. */
+  diff: PlanDiff;
+  /** Console output (already truncated by the caller). */
+  console: string;
+  /** Diff text (already truncated by the caller). */
+  diffText: string;
+  /** Summary line for the collapsible header. */
+  summary: string;
+  /** Syntax highlight for the console block: `hcl` normally, `diff` on fmt failure. */
+  syntax: "hcl" | "diff";
+  /** `@` to tag the actor, or `""`. */
+  actorTag: string;
+  actor: string;
+  timestamp: string;
+  runUrl: string;
+  /** The hidden identifier marker (without the `<!-- -->` wrapper). */
+  marker: string;
+  expandDiff: boolean;
+  expandSummary: boolean;
+}
+
+/** The collapsible diff block, or "" when there are no changed-resource lines. */
+function renderDiffBlock(parts: CommentParts): string {
+  if (parts.diff.lines.length === 0) return "";
+  const noun = parts.diff.count === 1 ? "change" : "changes";
+  const open = parts.expandDiff ? " open" : "";
+  return `
+<details${open}><summary>Diff of ${parts.diff.count} ${noun}.</summary>
+
+\`\`\`diff
+${parts.diffText}
+\`\`\`
+</details>`;
+}
+
+/**
+ * Assemble the full PR-comment / job-summary body, reproducing the composite
+ * action's heredoc layout (the six `comment-pos` placeholders, the ```fish
+ * command block, the collapsible diff, the collapsible summary with the actor
+ * footer and view-log link, the console block, and the trailing identifier
+ * marker that the upsert logic keys off).
+ */
+export function buildCommentBody(parts: CommentParts): string {
+  const [pos1, pos2, pos3, pos4, pos5, pos6] = parts.positions;
+  const summaryOpen = parts.expandSummary ? " open" : "";
+  return `${pos1}
+\`\`\`fish
+${parts.command}
+\`\`\`
+${pos2}
+${renderDiffBlock(parts)}
+${pos3}
+<details${summaryOpen}><summary>${parts.summary}</br>
+
+${pos4}
+###### By ${parts.actorTag}${parts.actor} at ${parts.timestamp} [(view log)](${parts.runUrl}).
+</summary>
+
+\`\`\`${parts.syntax}
+${parts.console}
+\`\`\`
+</details>
+${pos5}
+<!-- ${parts.marker} -->
+${pos6}`;
+}