diff --git a/make-pdf/src/browseClient.ts b/make-pdf/src/browseClient.ts
index 63cec7755..9c70c716c 100644
--- a/make-pdf/src/browseClient.ts
+++ b/make-pdf/src/browseClient.ts
@@ -268,6 +268,17 @@ export function loadHtml(opts: LoadHtmlOptions): void {
   }
 }
 
+/**
+ * Load an HTML file (already under browse's safe dirs, e.g. /tmp) into a tab
+ * by path. Cheaper than loadHtml for large pages — no JSON payload round-trip;
+ * browse reads the file directly (diagram-render bundle is ~9MB).
+ */
+export function loadHtmlFile(opts: { file: string; tabId: number; waitUntil?: "load" | "domcontentloaded" | "networkidle" }): void {
+  const args = ["load-html", opts.file, "--tab-id", String(opts.tabId)];
+  if (opts.waitUntil) args.push("--wait-until", opts.waitUntil);
+  runBrowse(args);
+}
+
 /**
  * Evaluate a JS expression in a tab. Returns the serialized result as string.
  */
diff --git a/make-pdf/src/cli.ts b/make-pdf/src/cli.ts
index 62a3b948e..f8e1fe52e 100644
--- a/make-pdf/src/cli.ts
+++ b/make-pdf/src/cli.ts
@@ -86,6 +86,12 @@ function printUsage(): void {
   lines.push("  --quiet                   Suppress progress on stderr.");
   lines.push("  --verbose                 Per-stage timings on stderr.");
   lines.push("");
+  lines.push("Diagrams & images:");
+  lines.push("  ```mermaid / ```excalidraw fences render as vector diagrams.");
+  lines.push("  Add render=false to a fence info string to keep it as a code block.");
+  lines.push("  Local images are inlined; oversized rasters downscale to print resolution.");
+  lines.push("  --strict                  Missing/remote images fail the run (CI mode).");
+  lines.push("");
   lines.push("Network:");
   lines.push("  --allow-network           Load external images (off by default).");
   lines.push("");
@@ -136,6 +142,7 @@ function generateOptionsFromFlags(parsed: ParsedArgs): GenerateOptions {
     quiet: f.quiet === true,
     verbose: f.verbose === true,
     allowNetwork: f["allow-network"] === true,
+    strict: f.strict === true,
     title: typeof f.title === "string" ? f.title : undefined,
     author: typeof f.author === "string" ? f.author : undefined,
     date: typeof f.date === "string" ? f.date : undefined,
diff --git a/make-pdf/src/diagram-prepass.ts b/make-pdf/src/diagram-prepass.ts
new file mode 100644
index 000000000..f0e2a2c4b
--- /dev/null
+++ b/make-pdf/src/diagram-prepass.ts
@@ -0,0 +1,610 @@
+/**
+ * Diagram + image pre-pass. Runs between "read markdown" and render() in the
+ * orchestrator, and owns everything that needs the diagram-render bundle.
+ *
+ *   markdown ─▶ extractDiagramFences() ──▶ render() (marked+sanitize+smarty)
+ *        │        fences → placeholder tokens        │
+ *        │                                           ▼
+ *        └─▶ renderFenceSlots() ───────────▶ substituteSlots(html, slots)
+ *             one browse render tab/run              │
+ *             error ⇒ diagnostic block + page reload ▼
+ *                                          inlineLocalImages(html)
+ *                                            data URIs, probe dims from bytes,
+ *                                            downscale >2x content box @300dpi,
+ *                                            remote warn / missing placeholder /
+ *                                            --strict hard-fail
+ *
+ * Placeholders survive marked, the sanitizer, and smartypants because they are
+ * plain hyphenated lowercase tokens with no quotes or HTML. Slot HTML is run
+ * through the same sanitizer as user content before substitution (the bundle
+ * renders with securityLevel strict — the sanitizer is the second layer).
+ *
+ * Reset contract (eng-review D6.2): each fence renders with a fresh
+ * mermaid.render id; after ANY render error the bundle page is reloaded before
+ * the next fence so a poisoned global can't corrupt diagram N+1.
+ */
+
+import * as fs from "node:fs";
+import * as os from "node:os";
+import * as path from "node:path";
+import * as crypto from "node:crypto";
+
+import * as browseClient from "./browseClient";
+import { sanitizeUntrustedHtml } from "./render";
+import { imageDims } from "./image-size";
+
+// ─── Types ────────────────────────────────────────────────────────────
+
+export interface DiagramFence {
+  /** "mermaid" | "excalidraw" */
+  lang: string;
+  /** Fence body (the diagram source). */
+  source: string;
+  /** Optional title="..." from the fence info string (a11y label, D6.4). */
+  title?: string;
+  /** render=false → leave as a plain code block (escape hatch, D6.3). */
+  render: boolean;
+  /** Placeholder token substituted into the markdown. */
+  token: string;
+  /** 1-based ordinal among rendered fences (unique ids, aria fallback). */
+  ordinal: number;
+}
+
+export interface FenceExtraction {
+  markdown: string;
+  fences: DiagramFence[];
+}
+
+export interface PrepassWarnings {
+  warn: (msg: string) => void;
+}
+
+export interface PrepassImageOptions {
+  /** Directory of the source markdown — relative image paths resolve here. */
+  inputDir: string;
+  /** Hard-fail on missing/remote images instead of warn (D6.1). */
+  strict: boolean;
+  /** Remote images are left untouched when network is explicitly allowed. */
+  allowNetwork: boolean;
+  /** Physical content-box width in inches (page width minus margins). */
+  contentWidthIn: number;
+  warn: (msg: string) => void;
+  /** Lazily provides a ready bundle tab (only opened when needed). */
+  getTab: () => RenderTab | null;
+}
+
+/** Print-resolution policy (eng-review D4): downscale rasters wider than
+ * 2 × contentWidth × 300dpi down to contentWidth × 300dpi. */
+const PRINT_DPI = 300;
+const DOWNSCALE_FACTOR = 2;
+
+export class StrictModeError extends Error {
+  constructor(msg: string) {
+    super(msg);
+    this.name = "StrictModeError";
+  }
+}
+
+// ─── Fence extraction (pure) ──────────────────────────────────────────
+
+const DIAGRAM_LANGS = new Set(["mermaid", "excalidraw"]);
+
+/**
+ * Extract top-level ```mermaid / ```excalidraw fences, replacing each with a
+ * unique placeholder token paragraph. Backtick and tilde fences, any length
+ * >= 3; closers must be at least as long as the opener (CommonMark). Fences
+ * with `render=false` in the info string are left untouched.
+ */
+export function extractDiagramFences(markdown: string): FenceExtraction {
+  const lines = markdown.split("\n");
+  const out: string[] = [];
+  const fences: DiagramFence[] = [];
+  const runId = crypto.randomBytes(4).toString("hex");
+
+  let i = 0;
+  let openFence: { char: string; len: number; info: string; body: string[] } | null = null;
+  let ordinal = 0;
+
+  while (i < lines.length) {
+    const line = lines[i];
+
+    if (openFence) {
+      const close = matchFenceLine(line);
+      if (close && close.char === openFence.char && close.len >= openFence.len && close.info === "") {
+        const info = parseInfoString(openFence.info);
+        if (DIAGRAM_LANGS.has(info.lang) && info.render) {
+          ordinal++;
+          const token = `gstack-diagram-slot-${runId}-${ordinal}`;
+          fences.push({
+            lang: info.lang,
+            source: openFence.body.join("\n"),
+            title: info.title,
+            render: true,
+            token,
+            ordinal,
+          });
+          out.push("", token, "");
+        } else {
+          // Not a diagram fence (or render=false): replay verbatim, but strip
+          // the render=false flag so it never leaks into highlighted output.
+          const infoOut = info.render ? openFence.info : info.lang;
+          out.push(`${openFence.char.repeat(openFence.len)}${infoOut}`);
+          out.push(...openFence.body);
+          out.push(line);
+        }
+        openFence = null;
+        i++;
+        continue;
+      }
+      openFence.body.push(line);
+      i++;
+      continue;
+    }
+
+    const open = matchFenceLine(line);
+    if (open && open.info !== "") {
+      openFence = { char: open.char, len: open.len, info: open.info, body: [] };
+      i++;
+      continue;
+    }
+    if (open) {
+      // Anonymous fence (plain code block) — copy through to its closer so a
+      // ```mermaid example INSIDE a plain fence is never extracted.
+      out.push(line);
+      i++;
+      while (i < lines.length) {
+        const l = lines[i];
+        const close = matchFenceLine(l);
+        out.push(l);
+        i++;
+        if (close && close.char === open.char && close.len >= open.len && close.info === "") break;
+      }
+      continue;
+    }
+
+    out.push(line);
+    i++;
+  }
+
+  // Unclosed fence at EOF: replay verbatim (CommonMark treats it as code to EOF).
+  if (openFence) {
+    out.push(`${openFence.char.repeat(openFence.len)}${openFence.info}`);
+    out.push(...openFence.body);
+  }
+
+  return { markdown: out.join("\n"), fences };
+}
+
+function matchFenceLine(line: string): { char: string; len: number; info: string } | null {
+  const m = line.match(/^ {0,3}(`{3,}|~{3,})\s*(.*)$/);
+  if (!m) return null;
+  return { char: m[1][0], len: m[1].length, info: m[2].trim() };
+}
+
+/** Parse a fence info string: `mermaid`, `mermaid render=false`, `mermaid title="Auth flow"`. */
+export function parseInfoString(info: string): { lang: string; render: boolean; title?: string } {
+  const lang = (info.match(/^\S+/)?.[0] ?? "").toLowerCase();
+  const render = !/\brender\s*=\s*false\b/i.test(info);
+  const title = info.match(/\btitle\s*=\s*"([^"]*)"/i)?.[1]
+    ?? info.match(/\btitle\s*=\s*'([^']*)'/i)?.[1];
+  return { lang, render, title };
+}
+
+// ─── Slot substitution (pure) ─────────────────────────────────────────
+
+/**
+ * Replace placeholder tokens in rendered HTML with their final slot HTML.
+ * marked wraps the bare token line in <p>…</p>; replace the wrapper too so
+ * the figure isn't nested inside a paragraph.
+ */
+export function substituteSlots(html: string, slots: Map<string, string>): string {
+  let s = html;
+  for (const [token, slotHtml] of slots) {
+    const wrapped = new RegExp(`<p>\\s*${token}\\s*</p>`, "g");
+    if (wrapped.test(s)) {
+      s = s.replace(new RegExp(`<p>\\s*${token}\\s*</p>`, "g"), slotHtml);
+    } else {
+      s = s.split(token).join(slotHtml);
+    }
+  }
+  return s;
+}
+
+/**
+ * Visible diagnostic block for a failed fence render — never silent raw code
+ * (eng-review: explicit error blocks). Sanitizer-safe: all dynamic content is
+ * HTML-escaped.
+ */
+export function buildDiagnosticBlock(fence: DiagramFence, errorMessage: string): string {
+  const excerpt = fence.source.split("\n").slice(0, 8).join("\n");
+  const truncated = fence.source.split("\n").length > 8 ? "\n…" : "";
+  return [
+    `<figure class="diagram diagram-error" role="img" aria-label="${escapeAttr(diagramLabel(fence))} (failed to render)">`,
+    `<figcaption class="diagram-error-title">Diagram failed to render (${escapeHtml(fence.lang)})</figcaption>`,
+    `<pre class="diagram-error-detail">${escapeHtml(errorMessage.trim())}\n\n${escapeHtml(excerpt + truncated)}</pre>`,
+    `</figure>`,
+  ].join("\n");
+}
+
+/** Wrap a rendered SVG in an accessible figure (D6.4). */
+export function buildDiagramFigure(fence: DiagramFence, svg: string): string {
+  const label = diagramLabel(fence);
+  const cleanSvg = sanitizeUntrustedHtml(svg);
+  const captioned = fence.title
+    ? `\n<figcaption class="diagram-caption">${escapeHtml(fence.title)}</figcaption>`
+    : "";
+  return [
+    `<figure class="diagram" role="img" aria-label="${escapeAttr(label)}">`,
+    `<!-- gstack-diagram-source lang=${escapeAttr(fence.lang)}`,
+    escapeHtmlComment(fence.source),
+    `-->`,
+    cleanSvg,
+    captioned,
+    `</figure>`,
+  ].join("\n");
+}
+
+function diagramLabel(fence: DiagramFence): string {
+  return fence.title ?? `diagram ${fence.ordinal}`;
+}
+
+// ─── Render tab (bundle page lifecycle) ───────────────────────────────
+
+const PAYLOAD_TMP_DIR = process.platform === "win32" ? os.tmpdir() : "/tmp";
+const READY_TIMEOUT_MS = 20_000;
+
+export class RenderTab {
+  private constructor(
+    public readonly tabId: number,
+    private readonly stagedBundlePath: string,
+  ) {}
+
+  /**
+   * Open a tab and load the diagram-render bundle. The bundle HTML is staged
+   * under /tmp (content-addressed, reused across runs — load-html only reads
+   * inside its safe dirs) and loaded by PATH, not --from-file: a 9MB JSON
+   * round-trip per run would be pure waste.
+   */
+  static open(): RenderTab {
+    const bundleSrc = resolveBundlePath();
+    const html = fs.readFileSync(bundleSrc);
+    const sha = crypto.createHash("sha256").update(html).digest("hex").slice(0, 16);
+    const staged = path.join(PAYLOAD_TMP_DIR, `gstack-diagram-render-${sha}.html`);
+    if (!fs.existsSync(staged)) {
+      // Concurrent-safe: write to a unique temp name, then atomic rename.
+      const tmp = `${staged}.${process.pid}.${crypto.randomBytes(4).toString("hex")}`;
+      fs.writeFileSync(tmp, html);
+      try {
+        fs.renameSync(tmp, staged);
+      } catch {
+        fs.unlinkSync(tmp); // another process won the race — theirs is identical
+      }
+    }
+    const tabId = browseClient.newtab();
+    const tab = new RenderTab(tabId, staged);
+    tab.loadBundle();
+    return tab;
+  }
+
+  /** (Re)load the bundle page — also the reset path after a render error. */
+  loadBundle(): void {
+    browseClient.loadHtmlFile({ file: this.stagedBundlePath, tabId: this.tabId });
+    const ready = browseClient.waitForExpression({
+      expression: "document.getElementById('status') !== null && document.getElementById('status').textContent === 'ready'",
+      tabId: this.tabId,
+      timeoutMs: READY_TIMEOUT_MS,
+    });
+    if (!ready) {
+      throw new Error(
+        "diagram-render bundle did not become ready in the browse tab " +
+        `(${READY_TIMEOUT_MS}ms). Check \`browse js "window.__errors"\` on tab ${this.tabId}.`,
+      );
+    }
+  }
+
+  /**
+   * Call one of the bundle's async window functions with JSON-safe string
+   * args. Errors come back as a recognizable ERR: prefix so a render failure
+   * is data, not a thrown browse exit.
+   */
+  call(fn: string, ...args: Array<string | number>): string {
+    const argList = args.map((a) => JSON.stringify(a)).join(",");
+    const expression =
+      `window.${fn}(${argList})` +
+      `.then(r => "OK:" + r)` +
+      `.catch(e => "ERR:" + String((e && e.message) || e))`;
+    const result = this.js(expression);
+    if (result.startsWith("OK:")) return result.slice(3);
+    if (result.startsWith("ERR:")) throw new RenderCallError(result.slice(4));
+    throw new RenderCallError(`unexpected bundle result: ${result.slice(0, 200)}`);
+  }
+
+  private js(expression: string): string {
+    // Large payloads (scene JSON, SVG text, data URIs) blow past argv limits —
+    // browseClient.js shells out with the expression as an argv element, so
+    // stage anything big through a tmp file the page can fetch? No: file URLs
+    // are unreachable from the page. Instead, chunk through a window buffer.
+    if (expression.length <= 100_000) {
+      return browseClient.js({ expression, tabId: this.tabId });
+    }
+    return this.jsViaBuffer(expression);
+  }
+
+  /**
+   * argv-safe path for big expressions: ship the expression into the page in
+   * 64KB chunks (window.__exprBuf), then eval it there. Used for multi-MB
+   * data URIs (photo downscaling) where a single argv would exceed OS limits.
+   */
+  private jsViaBuffer(expression: string): string {
+    browseClient.js({ expression: "window.__exprBuf = ''", tabId: this.tabId });
+    const CHUNK = 64_000;
+    for (let i = 0; i < expression.length; i += CHUNK) {
+      const chunk = expression.slice(i, i + CHUNK);
+      browseClient.js({
+        expression: `window.__exprBuf += ${JSON.stringify(chunk)}, window.__exprBuf.length`,
+        tabId: this.tabId,
+      });
+    }
+    // Eval the buffer as a single expression so the resulting promise is the
+    // statement value browse awaits. The buffer resets at the next call.
+    return browseClient.js({
+      expression: `(0, eval)(window.__exprBuf)`,
+      tabId: this.tabId,
+    });
+  }
+
+  close(): void {
+    try {
+      browseClient.closetab(this.tabId);
+    } catch {
+      // best-effort: orchestrator finally path
+    }
+  }
+}
+
+export class RenderCallError extends Error {
+  constructor(msg: string) {
+    super(msg);
+    this.name = "RenderCallError";
+  }
+}
+
+/** Resolve dist/diagram-render.html: env override → repo-relative (dev) → global install. */
+export function resolveBundlePath(env: NodeJS.ProcessEnv = process.env): string {
+  const candidates = [
+    env.GSTACK_DIAGRAM_BUNDLE,
+    // dev: make-pdf/src/* → repo root lib/. (In a compiled binary this is the
+    // virtual /$bunfs/root and simply never exists — harmless.)
+    path.resolve(import.meta.dir, "../../lib/diagram-render/dist/diagram-render.html"),
+    // compiled binary at <root>/make-pdf/dist/pdf → <root>/lib/… — same shape
+    // in the repo and in the ~/.claude/skills/gstack global install. argv[0]
+    // is the literal string "bun" in compiled binaries; execPath is real.
+    path.resolve(path.dirname(process.execPath), "../../lib/diagram-render/dist/diagram-render.html"),
+    path.join(os.homedir(), ".claude/skills/gstack/lib/diagram-render/dist/diagram-render.html"),
+  ].filter((p): p is string => !!p);
+  for (const p of candidates) {
+    if (fs.existsSync(p)) return p;
+  }
+  throw new Error(
+    "diagram-render bundle not found. Tried:\n" +
+    candidates.map((c) => `  - ${c}`).join("\n") +
+    "\nRun `bun run build:diagram-render` (repo) or re-run ./setup (install).",
+  );
+}
+
+// ─── Fence rendering ──────────────────────────────────────────────────
+
+/**
+ * Render every extracted fence to its slot HTML. One bundle tab serves all
+ * fences; a failed fence yields a diagnostic block and a bundle reload
+ * (reset contract) before the next fence renders.
+ */
+export function renderFenceSlots(
+  fences: DiagramFence[],
+  tab: RenderTab,
+  warn: (msg: string) => void,
+): Map<string, string> {
+  const slots = new Map<string, string>();
+  for (const fence of fences) {
+    try {
+      let svg: string;
+      if (fence.lang === "mermaid") {
+        svg = tab.call("__renderMermaid", `mermaid-fence-${fence.ordinal}`, fence.source);
+      } else {
+        JSON.parse(fence.source); // fail fast with a JSON diagnostic, not a bundle stack
+        svg = tab.call("__excalidrawToSvg", fence.source);
+      }
+      slots.set(fence.token, buildDiagramFigure(fence, svg));
+    } catch (err: any) {
+      const msg = err?.message ?? String(err);
+      warn(`diagram ${fence.ordinal} (${fence.lang}) failed to render: ${firstLine(msg)}`);
+      slots.set(fence.token, buildDiagnosticBlock(fence, msg));
+      // Reset contract: a poisoned page must not corrupt the next fence.
+      try {
+        tab.loadBundle();
+      } catch (reloadErr: any) {
+        warn(`bundle reload after render error failed: ${firstLine(reloadErr?.message ?? String(reloadErr))}`);
+      }
+    }
+  }
+  return slots;
+}
+
+// ─── Image inlining (eng-review D1 + D4 + D6.1) ───────────────────────
+
+const IMG_TAG_RE = /<img\b[^>]*>/gi;
+const SRC_RE = /\bsrc\s*=\s*("([^"]*)"|'([^']*)')/i;
+
+/**
+ * Inline every local <img> as a data URI, probe intrinsic dimensions from the
+ * bytes, and annotate the tag with data-gstack-px-width/-height for the width
+ * policy. Oversized rasters are downscaled to print resolution via the bundle
+ * tab. Missing files become visible placeholders (or throw under --strict);
+ * remote URLs warn (offline posture) unless --allow-network.
+ */
+export function inlineLocalImages(html: string, opts: PrepassImageOptions): string {
+  const maxPx = Math.round(opts.contentWidthIn * PRINT_DPI * DOWNSCALE_FACTOR);
+  const targetPx = Math.round(opts.contentWidthIn * PRINT_DPI);
+
+  return html.replace(IMG_TAG_RE, (tag) => {
+    const srcMatch = tag.match(SRC_RE);
+    if (!srcMatch) return tag;
+    const src = srcMatch[2] ?? srcMatch[3] ?? "";
+
+    if (src.startsWith("data:")) return annotateFromDataUri(tag, src);
+
+    if (/^[a-z][a-z0-9+.-]*:/i.test(src)) {
+      // Absolute URL with a scheme (http, https, file, …)
+      if (opts.allowNetwork && /^https?:/i.test(src)) return tag;
+      if (/^https?:/i.test(src)) {
+        const msg = `remote image not fetched (offline posture): ${src}`;
+        if (opts.strict) throw new StrictModeError(msg + " — re-run without --strict or pass --allow-network");
+        opts.warn(msg);
+        return tag;
+      }
+      // file:// and friends fall through to the local path branch
+      if (!src.startsWith("file:")) return tag;
+    }
+
+    const filePath = src.startsWith("file:")
+      ? decodeURIComponent(new URL(src).pathname)
+      : path.resolve(opts.inputDir, decodeURIComponent(src));
+
+    if (!fs.existsSync(filePath)) {
+      const msg = `image not found: ${src} (resolved to ${filePath})`;
+      if (opts.strict) throw new StrictModeError(msg);
+      opts.warn(msg);
+      return buildMissingImagePlaceholder(src);
+    }
+
+    let buf = fs.readFileSync(filePath);
+    let dims = imageDims(buf);
+    let mime = dims?.mime ?? mimeFromExtension(filePath);
+
+    // Print-resolution normalization (D4): rasters only — SVG scales free.
+    if (dims && mime !== "image/svg+xml" && dims.width > maxPx) {
+      const tab = opts.getTab();
+      if (tab) {
+        try {
+          const dataUri = `data:${mime};base64,${buf.toString("base64")}`;
+          const scaled = tab.call("__downscaleRaster", dataUri, targetPx, mime);
+          const scaledB64 = scaled.replace(/^data:[^,]*,/, "");
+          opts.warn(
+            `downscaled ${path.basename(filePath)} ${dims.width}px → ${targetPx}px ` +
+            `(print is ${PRINT_DPI}dpi; original exceeds ${maxPx}px content-box ceiling)`,
+          );
+          buf = Buffer.from(scaledB64, "base64");
+          mime = scaled.slice(5, scaled.indexOf(";"));
+          dims = { ...dims, height: Math.round((dims.height * targetPx) / dims.width), width: targetPx };
+        } catch (err: any) {
+          opts.warn(`downscale failed for ${src}, inlining at full size: ${firstLine(err?.message ?? String(err))}`);
+        }
+      }
+    }
+
+    const dataUri = `data:${mime};base64,${buf.toString("base64")}`;
+    let newTag = tag.replace(SRC_RE, `src="${dataUri}"`);
+    if (dims) {
+      newTag = newTag.replace(
+        /^<img\b/i,
+        `<img data-gstack-px-width="${Math.round(dims.width)}" data-gstack-px-height="${Math.round(dims.height)}"`,
+      );
+    }
+    return newTag;
+  });
+}
+
+function annotateFromDataUri(tag: string, src: string): string {
+  try {
+    const b64 = src.slice(src.indexOf(",") + 1);
+    const head = Buffer.from(b64.slice(0, 8192), "base64");
+    const dims = imageDims(head);
+    if (!dims) return tag;
+    return tag.replace(
+      /^<img\b/i,
+      `<img data-gstack-px-width="${Math.round(dims.width)}" data-gstack-px-height="${Math.round(dims.height)}"`,
+    );
+  } catch {
+    return tag;
+  }
+}
+
+function buildMissingImagePlaceholder(src: string): string {
+  return (
+    `<span class="image-missing" role="img" aria-label="missing image">` +
+    `[missing image: ${escapeHtml(src)}]</span>`
+  );
+}
+
+function mimeFromExtension(p: string): string {
+  switch (path.extname(p).toLowerCase()) {
+    case ".png": return "image/png";
+    case ".jpg":
+    case ".jpeg": return "image/jpeg";
+    case ".gif": return "image/gif";
+    case ".webp": return "image/webp";
+    case ".svg": return "image/svg+xml";
+    default: return "application/octet-stream";
+  }
+}
+
+// ─── Content-box math ─────────────────────────────────────────────────
+
+const PAGE_WIDTHS_IN: Record<string, number> = {
+  letter: 8.5,
+  a4: 8.27,
+  legal: 8.5,
+  tabloid: 11,
+};
+
+/** Parse a CSS dimension ("1in" | "72pt" | "25mm" | "2.54cm") to inches. */
+export function dimToInches(dim: string | undefined, fallbackIn: number): number {
+  if (!dim) return fallbackIn;
+  const m = dim.trim().match(/^([0-9.]+)\s*(in|pt|cm|mm|px)?$/i);
+  if (!m) return fallbackIn;
+  const v = parseFloat(m[1]);
+  switch ((m[2] ?? "in").toLowerCase()) {
+    case "in": return v;
+    case "pt": return v / 72;
+    case "cm": return v / 2.54;
+    case "mm": return v / 25.4;
+    case "px": return v / 96;
+    default: return fallbackIn;
+  }
+}
+
+export function contentWidthInches(opts: {
+  pageSize?: string;
+  margins?: string;
+  marginLeft?: string;
+  marginRight?: string;
+}): number {
+  const pageW = PAGE_WIDTHS_IN[opts.pageSize ?? "letter"] ?? 8.5;
+  const left = dimToInches(opts.marginLeft ?? opts.margins, 1);
+  const right = dimToInches(opts.marginRight ?? opts.margins, 1);
+  return Math.max(1, pageW - left - right);
+}
+
+// ─── tiny helpers ─────────────────────────────────────────────────────
+
+function escapeHtml(s: string): string {
+  return s
+    .replace(/&/g, "&amp;")
+    .replace(/</g, "&lt;")
+    .replace(/>/g, "&gt;")
+    .replace(/"/g, "&quot;")
+    .replace(/'/g, "&#39;");
+}
+
+function escapeAttr(s: string): string {
+  return escapeHtml(s);
+}
+
+/** Comments may not contain `--`; encode it so the raw source survives. */
+function escapeHtmlComment(s: string): string {
+  return s.replace(/--/g, "-‐");
+}
+
+function firstLine(s: string): string {
+  return s.split("\n")[0].slice(0, 200);
+}
diff --git a/make-pdf/src/image-size.ts b/make-pdf/src/image-size.ts
new file mode 100644
index 000000000..96b4f4fa5
--- /dev/null
+++ b/make-pdf/src/image-size.ts
@@ -0,0 +1,106 @@
+/**
+ * Intrinsic image dimensions from raw bytes. Pure, no DOM, no deps.
+ *
+ * The diagram pre-pass probes every local image it inlines (eng-review D1:
+ * "dimensions are probed from the bytes") so the width policy and landscape
+ * detector never need a browser round-trip. Formats: PNG, JPEG, GIF, WebP
+ * (VP8/VP8L/VP8X), and SVG (attribute/viewBox best-effort).
+ *
+ * Returns null when the format is unrecognized or the header is truncated —
+ * callers treat unknown dimensions as "no policy applied", never an error.
+ */
+
+export interface ImageDims {
+  width: number;
+  height: number;
+  mime: string;
+}
+
+export function imageDims(buf: Buffer): ImageDims | null {
+  if (buf.length < 12) return null;
+  return pngDims(buf) ?? jpegDims(buf) ?? gifDims(buf) ?? webpDims(buf) ?? svgDims(buf);
+}
+
+function pngDims(b: Buffer): ImageDims | null {
+  // 8-byte signature, then IHDR chunk: length(4) "IHDR"(4) width(4) height(4)
+  if (b.length < 24) return null;
+  if (b.readUInt32BE(0) !== 0x89504e47 || b.readUInt32BE(4) !== 0x0d0a1a0a) return null;
+  if (b.toString("ascii", 12, 16) !== "IHDR") return null;
+  return { width: b.readUInt32BE(16), height: b.readUInt32BE(20), mime: "image/png" };
+}
+
+function jpegDims(b: Buffer): ImageDims | null {
+  if (b[0] !== 0xff || b[1] !== 0xd8) return null;
+  let i = 2;
+  while (i + 9 < b.length) {
+    if (b[i] !== 0xff) { i++; continue; }
+    const marker = b[i + 1];
+    // Standalone markers without length payload
+    if (marker === 0xd8 || (marker >= 0xd0 && marker <= 0xd9)) { i += 2; continue; }
+    const len = b.readUInt16BE(i + 2);
+    if (len < 2) return null;
+    // SOF0-SOF15 except DHT(C4)/JPGA(C8)/DAC(CC) carry dimensions
+    if (marker >= 0xc0 && marker <= 0xcf && marker !== 0xc4 && marker !== 0xc8 && marker !== 0xcc) {
+      if (i + 9 >= b.length) return null;
+      return { height: b.readUInt16BE(i + 5), width: b.readUInt16BE(i + 7), mime: "image/jpeg" };
+    }
+    i += 2 + len;
+  }
+  return null;
+}
+
+function gifDims(b: Buffer): ImageDims | null {
+  const sig = b.toString("ascii", 0, 6);
+  if (sig !== "GIF87a" && sig !== "GIF89a") return null;
+  return { width: b.readUInt16LE(6), height: b.readUInt16LE(8), mime: "image/gif" };
+}
+
+function webpDims(b: Buffer): ImageDims | null {
+  if (b.toString("ascii", 0, 4) !== "RIFF" || b.toString("ascii", 8, 12) !== "WEBP") return null;
+  const fmt = b.toString("ascii", 12, 16);
+  if (fmt === "VP8X" && b.length >= 30) {
+    // 24-bit little-endian width-1 / height-1 at offsets 24 / 27
+    const w = 1 + (b[24] | (b[25] << 8) | (b[26] << 16));
+    const h = 1 + (b[27] | (b[28] << 8) | (b[29] << 16));
+    return { width: w, height: h, mime: "image/webp" };
+  }
+  if (fmt === "VP8 " && b.length >= 30) {
+    // Lossy: dimensions at offset 26, 14 bits each, little-endian
+    return {
+      width: b.readUInt16LE(26) & 0x3fff,
+      height: b.readUInt16LE(28) & 0x3fff,
+      mime: "image/webp",
+    };
+  }
+  if (fmt === "VP8L" && b.length >= 25) {
+    if (b[20] !== 0x2f) return null;
+    const bits = b.readUInt32LE(21);
+    return {
+      width: (bits & 0x3fff) + 1,
+      height: ((bits >> 14) & 0x3fff) + 1,
+      mime: "image/webp",
+    };
+  }
+  return null;
+}
+
+/**
+ * SVG: parse width/height attributes (px or unitless) off the root element,
+ * falling back to viewBox. CSS-unit widths (em, %, pt) are ignored — the
+ * width policy treats them as "no intrinsic size".
+ */
+function svgDims(b: Buffer): ImageDims | null {
+  const head = b.toString("utf8", 0, Math.min(b.length, 4096));
+  const tag = head.match(/<svg\b[^>]*>/i)?.[0];
+  if (!tag) return null;
+  const attr = (name: string): number | null => {
+    const m = tag.match(new RegExp(`\\b${name}\\s*=\\s*["']\\s*([0-9.]+)(px)?\\s*["']`, "i"));
+    return m ? parseFloat(m[1]) : null;
+  };
+  const w = attr("width");
+  const h = attr("height");
+  if (w && h) return { width: w, height: h, mime: "image/svg+xml" };
+  const vb = tag.match(/\bviewBox\s*=\s*["']\s*[-0-9.]+[\s,]+[-0-9.]+[\s,]+([0-9.]+)[\s,]+([0-9.]+)\s*["']/i);
+  if (vb) return { width: parseFloat(vb[1]), height: parseFloat(vb[2]), mime: "image/svg+xml" };
+  return null;
+}
diff --git a/make-pdf/src/orchestrator.ts b/make-pdf/src/orchestrator.ts
index cf8dffae6..be25941cc 100644
--- a/make-pdf/src/orchestrator.ts
+++ b/make-pdf/src/orchestrator.ts
@@ -24,6 +24,14 @@ import { render } from "./render";
 import type { GenerateOptions, PreviewOptions } from "./types";
 import { ExitCode } from "./types";
 import * as browseClient from "./browseClient";
+import {
+  RenderTab,
+  contentWidthInches,
+  extractDiagramFences,
+  inlineLocalImages,
+  renderFenceSlots,
+  substituteSlots,
+} from "./diagram-prepass";
 
 class ProgressReporter {
   private readonly quiet: boolean;
@@ -80,10 +88,14 @@ export async function generate(opts: GenerateOptions): Promise<string> {
   const markdown = fs.readFileSync(input, "utf8");
   progress.end("Reading markdown");
 
+  // Stage 1.5: diagram pre-pass — extract ```mermaid/```excalidraw fences and
+  // swap in placeholder tokens. Rendering happens after the tab opens below.
+  const extraction = extractDiagramFences(markdown);
+
   // Stage 2: render HTML
   progress.begin("Rendering HTML");
   const rendered = render({
-    markdown,
+    markdown: extraction.markdown,
     title: opts.title,
     author: opts.author,
     date: opts.date,
@@ -99,11 +111,66 @@ export async function generate(opts: GenerateOptions): Promise<string> {
   });
   progress.end("Rendering HTML", `${rendered.meta.wordCount} words`);
 
+  // Stage 2.5: render diagram fences in a dedicated bundle tab, substitute
+  // slots, then inline + probe + (if oversized) downscale local images.
+  // The bundle tab is lazy: image-only documents open it only when a raster
+  // actually needs print-resolution downscaling (eng-review D4).
+  const warn = (msg: string) => {
+    if (!opts.quiet) process.stderr.write(`\r\x1b[K[make-pdf] warning: ${msg}\n`);
+  };
+  let renderTab: RenderTab | null = null;
+  const getRenderTab = (): RenderTab | null => {
+    if (renderTab) return renderTab;
+    try {
+      renderTab = RenderTab.open();
+    } catch (err: any) {
+      warn(`diagram-render tab unavailable: ${String(err?.message ?? err).split("\n")[0]}`);
+      return null;
+    }
+    return renderTab;
+  };
+
+  let finalHtml = rendered.html;
+  try {
+    if (extraction.fences.length > 0) {
+      progress.begin(`Rendering ${extraction.fences.length} diagram(s)`);
+      const tab = getRenderTab();
+      if (tab) {
+        const slots = renderFenceSlots(extraction.fences, tab, warn);
+        finalHtml = substituteSlots(finalHtml, slots);
+      } else {
+        // No bundle/tab: visible diagnostic beats silent raw tokens.
+        const slots = new Map(
+          extraction.fences.map((f) => [
+            f.token,
+            `<figure class="diagram diagram-error" role="img" aria-label="diagram ${f.ordinal} (not rendered)">` +
+            `<figcaption class="diagram-error-title">Diagram not rendered (${f.lang}) — diagram-render bundle unavailable</figcaption></figure>`,
+          ]),
+        );
+        finalHtml = substituteSlots(finalHtml, slots);
+      }
+      progress.end(`Rendering ${extraction.fences.length} diagram(s)`);
+    }
+
+    progress.begin("Inlining images");
+    finalHtml = inlineLocalImages(finalHtml, {
+      inputDir: path.dirname(input),
+      strict: opts.strict === true,
+      allowNetwork: opts.allowNetwork === true,
+      contentWidthIn: contentWidthInches(opts),
+      warn,
+      getTab: getRenderTab,
+    });
+    progress.end("Inlining images");
+  } finally {
+    renderTab?.close();
+  }
+
   // Stage 3: write HTML to a tmp file browse can read
   // (We don't actually write it; we pass inline via --from-file JSON.)
   // But for preview mode and debugging, we still write to tmp.
   const htmlTmp = tmpFile("html");
-  fs.writeFileSync(htmlTmp, rendered.html, "utf8");
+  fs.writeFileSync(htmlTmp, finalHtml, "utf8");
 
   // Stage 4: spin up a dedicated tab, load HTML, (wait for Paged.js if TOC),
   // then emit PDF. Always close the tab.
@@ -114,7 +181,7 @@ export async function generate(opts: GenerateOptions): Promise<string> {
   try {
     progress.begin("Loading HTML into Chromium");
     browseClient.loadHtml({
-      html: rendered.html,
+      html: finalHtml,
       waitUntil: "domcontentloaded",
       tabId,
     });
diff --git a/make-pdf/src/print-css.ts b/make-pdf/src/print-css.ts
index 2366f42b9..db6f9c925 100644
--- a/make-pdf/src/print-css.ts
+++ b/make-pdf/src/print-css.ts
@@ -324,6 +324,18 @@ function figureRules(): string {
     `figure { margin: 12pt 0; }`,
     `figure img { display: block; max-width: 100%; height: auto; }`,
     `figcaption { font-size: 9pt; color: #666; margin-top: 6pt; font-style: italic; }`,
+    // Diagram figures (diagram-prepass): rendered mermaid/excalidraw SVG.
+    // SVGs scale to the content box and never split across pages.
+    `figure.diagram { break-inside: avoid; text-align: center; }`,
+    `figure.diagram > svg { max-width: 100%; height: auto; }`,
+    `figure.diagram .diagram-caption { text-align: center; }`,
+    // Diagnostic block for a fence that failed to render — loud, boxed,
+    // unmistakably an error (never silent raw code).
+    `figure.diagram-error { border: 1.5pt solid #b00020; padding: 8pt 10pt; text-align: left; }`,
+    `figure.diagram-error .diagram-error-title { font-weight: 700; color: #b00020; font-style: normal; margin: 0 0 6pt; }`,
+    `figure.diagram-error .diagram-error-detail { font-size: 8.5pt; white-space: pre-wrap; margin: 0; }`,
+    // Missing local image placeholder (non-strict mode).
+    `.image-missing { display: inline-block; border: 1pt dashed #b00020; color: #b00020; padding: 4pt 8pt; font-size: 9pt; }`,
   ].join("\n");
 }
 
diff --git a/make-pdf/src/types.ts b/make-pdf/src/types.ts
index 6d4e67108..aa6706548 100644
--- a/make-pdf/src/types.ts
+++ b/make-pdf/src/types.ts
@@ -44,6 +44,10 @@ export interface GenerateOptions {
   // Network
   allowNetwork?: boolean;         // default: false
 
+  // Strict mode (eng-review D6.1): missing/remote images hard-fail instead of
+  // warn + placeholder. For CI docs pipelines that need determinism.
+  strict?: boolean;               // default: false
+
   // Metadata
   title?: string;
   author?: string;