Claude Eleventy Image Optimization

claude-eleventy-image-optimization.md

Eleventy Build Optimization Chat Transcript

---

Mark: Does Netlify support incremental builds for an Eleventy site?

Claude: Incremental builds for Eleventy on Netlify depend on how you configure them—Netlify doesn't automatically enable incremental building for Eleventy sites the way it does for some other frameworks.

Here's the current landscape:

Netlify's built-in support: Netlify has a feature called Concurrent Builds and supports some frameworks with native incremental build optimizations, but Eleventy isn't on that list by default. You need to explicitly set it up.

How to enable incremental builds for Eleventy on Netlify:

1. Use Eleventy's --incremental flag in your build command:

   eleventy --input=src --output=_site --incremental
   

   Update your netlify.toml:

   [build]
   command = "eleventy --incremental"
   publish = "_site"

2. Cache dependencies to speed up builds. In netlify.toml:

   [build.environment]
   NODE_VERSION = "18"
   
   [[redirects]]
   from = "/*"
   to = "/404.html"
   status = 404

3. Leverage Netlify's file change detection by ensuring your build script only rebuilds when necessary. Eleventy's --incremental mode does this, but you can also use environment variables like CACHED_COMMIT_REF and COMMIT_REF to detect what changed.

Known considerations:

• Eleventy's incremental mode is good for development but can occasionally have edge cases with global data or complex dependencies
• For maximum reliability on production builds, some teams run a full build on main/production branch and incremental on feature branches
• If you're using plugins or complex data pipelines, test incremental builds thoroughly—they may not always pick up all changes

Have you been experiencing slow build times on Netlify, or are you setting up a new Eleventy project there?

---

Mark: builds average ~3m

Claude: Three minutes is quite slow for an Eleventy site. That's worth investigating, because Eleventy sites typically build much faster—often in the 10-30 second range depending on content volume.

Let me ask a few diagnostic questions to help narrow down where the time is going:

1. How many pages/templates are you building? (rough order of magnitude)
2. Are you doing any heavy processing in your build—image optimization, API calls during build, large data transformations, or complex collections?
3. What's your dependency footprint? (run npm list or check your package.json—lots of heavy plugins?)
4. Is the 3 minutes mostly build time, or is it inflated by dependency installation, asset processing, or post-build steps?
5. Are you using any of these:
   • Image optimization plugins (sharp, etc.)
   • Pagefind or other search indexing?
   • GraphQL data fetching?
   • Complex Nunjucks/Liquid filters or shortcodes?

Quick wins to check first:

• Look at your Netlify build logs—they'll show you which step is the bottleneck
• Try running eleventy --benchmark locally to see where Eleventy itself is spending time
• Check if you have any blocking external API calls during build

Given your background with Eleventy and various CMS work, you probably already have good instincts here. But 3 minutes suggests either (a) a lot of content, (b) heavy processing per-template, or (c) something in the asset/plugin pipeline that's dragging things out.

What does your build log show as the breakdown?

---

Mark: From the log

Copied 1675 Wrote 1212 files in 202.41 seconds (167.0ms each, v3.1.5)

Claude: Okay, that's the smoking gun: 167ms per file is extremely slow. A typical Eleventy site should be doing 5-20ms per file, so you're running 8-33x slower than normal.

With 1212 files, that's where your 3+ minutes is coming from. The issue is almost certainly in your templates/shortcodes/filters/data cascades—something is running on every single file that shouldn't be.

What to investigate:

1. Global data files — Do you have any expensive computations in your _data/ directory? Even a single slow data file runs on every template render.

2. Shortcodes or filters — If you have shortcodes that are doing I/O, API calls, or heavy processing, and they're used frequently, that adds up fast across 1212 files.

3. Transforms — Any addTransform() callbacks in your config? Those run on every output file. Transforms that parse, manipulate, or process the entire HTML output are killers.

4. Collections or sorting — Complex collection definitions or sorts that iterate over large datasets on every build.

5. Plugin overhead — Some Eleventy plugins (especially image optimization, markdown processors with heavy extensions) can add latency per-file.

Quick diagnostic:

Run a local build with --benchmark to see where the time is actually spent:

eleventy --benchmark

That should show you which templates/data files are the slowpokes.

Also, check your .eleventy.js config—anything with callbacks or data processing that could be optimized, memoized, or moved to build-time-only computation?

What does your config look like, or what heavy processing are you doing per-file?

---

Mark: I have transforms for my images, running after the Image plugin transform

My transform adds gallery support to any post images, decorating the markup after Image runs

Claude: Ah, there it is. Parsing and decorating the HTML output of every single file is expensive, especially at 167ms per file.

Here's the problem: Your transform is running on every output file, parsing the HTML, finding images, decorating them. That's a lot of DOM traversal and string manipulation × 1212 files.

A few ways to optimize this:

1. Move the decoration logic into a shortcode or filter instead of a transform

   • Let the Image plugin do its thing
   • In your template, wrap image output in a shortcode that handles gallery decoration
   • This way it only runs on files that actually have images, not every file
   • Trade-off: requires template changes, but much faster

2. Make the transform conditional

   • Check if the output contains <img> tags before parsing
   • Skip HTML parsing entirely for files without images
   • Still runs on image-heavy files, but skips the overhead for text-only files

3. Batch or lazy-load the decoration

   • Do the heavy lifting during the Image plugin phase itself (if possible)
   • Or pre-compute which files have images and only transform those

4. Optimize the parsing itself

   • If you're using cheerio or similar, make sure you're not re-parsing the full HTML multiple times
   • Use streaming or selective DOM traversal instead of full parse

The real fix: What does your transform actually do? If it's wrapping images in <figure> tags, adding data attributes, or similar, that's very doable as a shortcode instead.

Something like:

eleventyConfig.addNunjucksAsyncShortcode("galleryImage", async function(src, alt) {
  const imageHtml = await Image(src, imageOptions);
  return `<figure class="gallery-item">${imageHtml}${alt ? `<figcaption>${alt}</figcaption>` : ''}</figure>`;
});

Then in templates: {% galleryImage "image.jpg", "alt text" %}

What's the actual decoration you're doing on those images?

---

Mark: Here's my transform

eleventyConfig.addTransform('prepareGallery', async function (content) {
    const pageOutputPath = this && this.page && this.page.outputPath;
    if (typeof pageOutputPath !== "string") {
        return content;
    }

    // now it's safe to call .endsWith
    if (!pageOutputPath.endsWith(".html")) {
        return content;
    }

    const path = this.page.inputPath;
    // if (!this.page.inputPath === './content/posts/new-york-march-2024/index.md') return content;
    const data = pageDataMap.get(this.page.inputPath);
    if (data?.content_type && data?.content_type === 'post') {
        const $ = cheerio.load(content);
        const dirPath = this.page.filePathStem.slice(
            0,
            this.page.filePathStem.length - 5
        );
        // If there are images, decorate with a wrapper <a> tag,
        // and pull classes from the image up to that wrapper
        $("img").each((i, el) => {
            const page = this.page;
            let imgUrl = $(el).attr("src");
            const imgWidth = $(el).attr("width");
            imgUrl = imgUrl.replace(/w=[0-9]+/, `w=${imgWidth}`);
            const imgGallery = $(el).attr("data-gallery");
            const classes = $(el).attr("class");
            const imgSizes = $(el).attr("sizes");
            const imgSrcset = $(el).attr("srcset");
            let imgCaption = "";
            if (
                $(el).next().length > 0 &&
                $(el).next().prop("tagName").toLowerCase() == "figcaption"
            ) {
                imgCaption = $(el).next().html();
            }
            $(el).wrap("<a></a>");
            const parent = $(el).parent();
            if (classes) parent.addClass(classes);
            if (!$(el).hasClass('glightbox')) {
                parent.addClass('glightbox');
            }
            parent.attr("href", imgUrl);
            if (imgSizes) {
                parent.attr("data-sizes", imgSizes);
            }
            if (imgSrcset) {
                parent.attr("data-srcset", imgSrcset);
            }
            if (imgGallery) {
                parent.attr("data-gallery", imgGallery);
            }
            if (imgCaption !== "") {
                parent.attr("data-title", imgCaption);
            }
        });
        return `${$.html()}`;
    }
    return content;
});

Claude: Yeah, this is running on every HTML file but only actually doing work on posts. The real cost here is cheerio.load(content) and $.html() on the full page HTML—that's parsing and serializing the entire document for every single output file, even when there are no images to process.

Here are the optimizations, ranked by impact:

1. Skip parsing if there are no images (biggest win)

eleventyConfig.addTransform('prepareGallery', async function (content) {
    const pageOutputPath = this && this.page && this.page.outputPath;
    if (typeof pageOutputPath !== "string") {
        return content;
    }

    if (!pageOutputPath.endsWith(".html")) {
        return content;
    }

    const data = pageDataMap.get(this.page.inputPath);
    if (data?.content_type !== 'post') {
        return content;
    }

    // Skip parsing if there are no images at all
    if (!content.includes('<img')) {
        return content;
    }

    const $ = cheerio.load(content);
    // ... rest of your logic
    return $.html();
});

This should knock out a huge chunk of the overhead—you're only parsing files that actually have images.

2. Move this to a shortcode (best long-term fix) Instead of decorating in a transform, handle it at template time. In your posts, replace raw image output with a shortcode:

eleventyConfig.addNunjucksAsyncShortcode("postImage", async function(src, alt, dataGallery) {
    // Your decoration logic here, inline
    // This only runs when the shortcode is called, not on every file
    return `<a class="glightbox" href="${src}" data-gallery="${dataGallery || ''}"><img src="${src}" alt="${alt}" /></a>`;
});

Then in your post templates:

{% postImage "image.jpg", "alt text", "gallery-name" %}

3. If you stay with the transform approach:

• Consider memoizing or caching the decorated output per unique image URL
• Pre-filter your files list to only run on posts (though it looks like you're already doing this)

Which direction appeals to you? Moving to a shortcode would be the cleanest—it's also more explicit in your templates about what's happening with each image. But the quick !content.includes('<img') check should give you a massive speedup right now without any template changes.

---

Mark: I prefer to be able to keep the markdown syntax for images in my markdown files, for portability in case I shift away from Eleventy

Claude: Smart call. In that case, definitely go with the !content.includes('<img') check first—that's a minimal change that should give you huge gains.

But if you want to keep markdown syntax and avoid the full cheerio parse/serialize on every file, you could move the decoration logic earlier in the pipeline—into a Markdown-It plugin instead of a transform.

Here's the idea:

const markdownIt = require("markdown-it");
const md = new markdownIt();

// Add a Markdown-It plugin that decorates images as they're parsed
md.use((md) => {
    const defaultRender = md.renderer.rules.image || ((tokens, idx) => md.renderer.renderToken(tokens, idx, {}));
    
    md.renderer.rules.image = function(tokens, idx, options, env, renderer) {
        const token = tokens[idx];
        const src = token.attrGet('src');
        const alt = token.content;
        
        // Return decorated markup here instead of plain <img>
        return `<a class="glightbox" href="${src}" data-gallery="post"><img src="${src}" alt="${alt}" /></a>`;
    };
});

eleventyConfig.setLibrary("md", md);

This way:

• Images are decorated at markdown→HTML conversion time (once per file during build)
• No full-page HTML parsing with cheerio
• Markdown source stays portable
• You can still access page data if needed (via the env parameter)

The tradeoff: You'd lose access to this.page context in the renderer—if you need to conditionally decorate based on frontmatter (like only on posts), you'd need to pass that through env or handle it differently.

Does your current transform actually use the data.content_type === 'post' check to skip non-posts? If so, how critical is that filtering?

---

Mark: Here's my entire eleventy config (eleventy_config.js attached)

Claude: Perfect. I can see exactly what's happening. You have:

1. eleventyImageTransformPlugin (line 12) — running on all HTML files
2. Your prepareGallery transform (lines 462-523) — running on all HTML files AND parsing the full page with cheerio

The problem is the combination: Image plugin outputs, then your transform runs cheerio on the entire HTML output.

Here's the quick fix—add the !content.includes('<img') check I mentioned:

eleventyConfig.addTransform('prepareGallery', async function (content) {
    const pageOutputPath = this && this.page && this.page.outputPath;
    if (typeof pageOutputPath !== "string") {
        return content;
    }

    if (!pageOutputPath.endsWith(".html")) {
        return content;
    }

    const data = pageDataMap.get(this.page.inputPath);
    if (data?.content_type !== 'post') {
        return content;
    }

    // SKIP PARSING IF THERE ARE NO IMAGES
    if (!content.includes('<img')) {
        return content;
    }

    // Rest of your transform...
    const $ = cheerio.load(content);
    // ... rest stays the same
});

But here's the better long-term solution: Hook into the Markdown-It renderer to decorate images during markdown processing instead of after HTML generation. You already have markdownItImageFigures running (line 331), so you can customize it:

eleventyConfig.amendLibrary("md", (mdLib) => {
    // Override the image renderer to add gallery decoration
    const defaultImageRender = mdLib.renderer.rules.image;
    
    mdLib.renderer.rules.image = function(tokens, idx, options, env, renderer) {
        const token = tokens[idx];
        const src = token.attrGet('src');
        const alt = token.content;
        const width = token.attrGet('width');
        const sizes = token.attrGet('sizes');
        const srcset = token.attrGet('srcset');
        const gallery = token.attrGet('data-gallery');
        
        const finalSrc = width ? src.replace(/w=[0-9]+/, `w=${width}`) : src;
        
        let linkAttrs = ` href="${finalSrc}" class="glightbox"`;
        if (sizes) linkAttrs += ` data-sizes="${sizes}"`;
        if (srcset) linkAttrs += ` data-srcset="${srcset}"`;
        if (gallery) linkAttrs += ` data-gallery="${gallery}"`;
        
        return `<a${linkAttrs}><img src="${finalSrc}" alt="${alt}" width="${width}" sizes="${sizes}" srcset="${srcset}" /></a>`;
    };
});

Then remove the prepareGallery transform entirely.

Which approach fits better for you—the quick fix with the <img check, or the Markdown-It renderer override?

---

Mark: How would I modify the Image transform settings (imgTransformSettings in my file), if the markdown stage is already altering the image markup?

Claude: Good catch—this is the tricky part. The Image plugin transform runs after markdown is rendered, so if you override the markdown image renderer, the Image plugin won't see your <img> tags until after it's already processed them.

Here's the order of operations:

1. Markdown renders → creates <img> with your gallery wrapper
2. Image plugin transform runs → looks for <img> tags and optimizes them (srcset, sizes, widths)
3. Your prepareGallery transform → wraps images again (redundant now)

So you'd have double-wrapping issues if you just override the markdown renderer.

Option 1: Remove the Image plugin transform entirely and manually call the Image plugin in your markdown override:

mdLib.renderer.rules.image = async function(tokens, idx, options, env, renderer) {
    const token = tokens[idx];
    const src = token.attrGet('src');
    const alt = token.content;
    const gallery = token.attrGet('data-gallery');
    
    let metadata = await Image(src, {
        widths: [320, 960, 1400, 1920, 4000],
        formats: ["auto"],
    });
    
    let imageHtml = Image.generateHTML(metadata, {
        alt: alt,
        sizes: "...",
        loading: "lazy",
    });
    
    return `<a href="${src}" class="glightbox" data-gallery="${gallery || ''}">${imageHtml}</a>`;
};

My recommendation: Go with Option 1—remove the Image plugin transform and handle Image optimization in the markdown renderer. It's cleaner and you avoid the pipeline conflict entirely.

Which approach makes more sense for your setup? And do you know off the top of your head whether the Image plugin is doing anything else crucial (like cache-busting, or handling non-markdown images)?

---

Mark: in option 1, where does attrGet come from? the markdown-it library?

Claude: Good question—no, attrGet is not a markdown-it method. That was a mistake on my part.

In markdown-it's token object, attributes are stored as a 2D array in token.attrs, not as methods. So you'd access them like:

mdLib.renderer.rules.image = async function(tokens, idx, options, env, renderer) {
    const token = tokens[idx];
    
    // Attributes are a 2D array: [["src", "..."], ["alt", "..."], ...]
    const src = token.attrs.find(attr => attr[0] === 'src')?.[1];
    const alt = token.content; // alt text is in content, not attrs
    const gallery = token.attrs.find(attr => attr[0] === 'data-gallery')?.[1];
};

Or cleaner, write a helper:

const getAttr = (token, name) => token.attrs?.find(attr => attr[0] === name)?.[1];

mdLib.renderer.rules.image = async function(tokens, idx, options, env, renderer) {
    const token = tokens[idx];
    const src = getAttr(token, 'src');
    const alt = token.content;
    const gallery = getAttr(token, 'data-gallery');
};

The token.content property holds the alt text directly, which is why that one doesn't need the attrs lookup.

---

Mark: is attrs in core javascript?

Claude: No, attrs is just a property on markdown-it's token objects specifically—it's part of markdown-it's data structure, not a JavaScript built-in.

The find and optional chaining (?.) I'm using are core JavaScript, but token.attrs itself is just whatever array markdown-it puts there when it parses your image syntax.

Worth checking the actual shape of a token before writing the renderer, so you're not guessing. You can do that quickly with:

mdLib.renderer.rules.image = function(tokens, idx, options, env, renderer) {
    const token = tokens[idx];
    console.log(JSON.stringify(token, null, 2));
    return ''; // temporary, just to inspect
};

Run a build with one image and you'll see exactly what markdown-it hands you—attrs array shape, where alt text lives, everything. Much safer than relying on documentation or my description.

---

Mark: I'm getting an error:

TypeError: Cannot set properties of undefined (setting 'image')

for the line

mdLib.render.rules.image = async (tokens, idx, options, env, renderer) => {

Claude: You have a typo—render instead of renderer:

mdLib.renderer.rules.image = async (tokens, idx, options, env, renderer) => {

Note the r at the end: renderer.rules, not render.rules.

---

Mark: Getting this error: Error: ENOENT: no such file or directory, stat 'philadelphias-magic-gardens-11.jpg' so there might be an issue with the src property not having the full path

Claude: Right—in your markdown source the image paths are relative to the markdown file, but by the time the renderer sees them, there's no path context. The Image plugin transform handles this automatically because it runs later with full page context, but the markdown renderer doesn't know where the file lives.

You can get the input path from the env object that markdown-it passes through—Eleventy populates it. Try logging env to see what's available:

mdLib.renderer.rules.image = function(tokens, idx, options, env, renderer) {
    console.log(JSON.stringify(env, null, 2));
    return '';
};

If Eleventy is passing the input path through env, you can reconstruct the full path with something like:

import path from 'path';

mdLib.renderer.rules.image = function(tokens, idx, options, env, renderer) {
    const token = tokens[idx];
    const src = token.attrs.find(a => a[0] === 'src')?.[1];
    
    const inputDir = path.dirname(env.page.inputPath);
    const fullSrc = path.resolve(inputDir, src);
};

But env might not have what you need—Eleventy's markdown-it integration doesn't always populate it fully. Log it first and see what's there before going further.

---

Mark: {}

Claude: Empty env means Eleventy isn't passing page context through to the markdown renderer, so you can't get the input path that way.

This is actually the core reason why the Image plugin transform approach works and the markdown renderer approach is difficult—the transform runs in Eleventy's context where this.page is available, but the markdown renderer runs in a more isolated context.

This is a fundamental limitation that makes the markdown renderer approach a poor fit for your situation. You'd have to do something hacky like maintaining a module-level variable that tracks the current file being processed, which is fragile.

Recommendation: stick with the transform approach, but apply the quick wins from earlier:

eleventyConfig.addTransform('prepareGallery', async function (content) {
    const pageOutputPath = this?.page?.outputPath;
    if (typeof pageOutputPath !== "string") return content;
    if (!pageOutputPath.endsWith(".html")) return content;

    const data = pageDataMap.get(this.page.inputPath);
    if (data?.content_type !== 'post') return content;

    // Skip cheerio entirely if no images
    if (!content.includes('<img')) return content;

    const $ = cheerio.load(content);
    // ... rest of your existing transform
});

The content_type !== 'post' check plus the !content.includes('<img') check together should eliminate cheerio parsing for the vast majority of your 1212 files, which is where most of your overhead is coming from.

---

Mark: A little better: Copied 1675 Wrote 1215 files in 76.89 seconds

Claude: That's a big improvement—202 seconds down to 76, roughly 2.6x faster. The <img check is clearly doing a lot of work by skipping cheerio for most of your files.

To figure out how much further you can push it, it's worth knowing: of your ~1215 files, how many are content_type === 'post' with actual images? You could add a quick counter to the transform to see how many files are actually hitting the cheerio parse:

let cheerioCount = 0;

eleventyConfig.addTransform('prepareGallery', async function (content) {
    const pageOutputPath = this?.page?.outputPath;
    if (typeof pageOutputPath !== "string") return content;
    if (!pageOutputPath.endsWith(".html")) return content;

    const data = pageDataMap.get(this.page.inputPath);
    if (data?.content_type !== 'post') return content;
    if (!content.includes('<img')) return content;

    cheerioCount++;
    console.log(`cheerio parse count: ${cheerioCount}`);

    const $ = cheerio.load(content);
    // ...
});

If it's still parsing hundreds of files, there may be more to squeeze out. But if it's a small number, the remaining build time is probably coming from elsewhere—the Image plugin transform, passthrough copy of 1675 files, or collection processing.

What does your Netlify build log show as the breakdown between dependency install, build, and deploy steps?

---

Mark: The default image transform appears to be the big bottleneck, if I comment out:

eleventyConfig.addPlugin(eleventyImageTransformPlugin, imgTransformSettings);

I get a build time of Copied 1675 Wrote 1215 files in 3.95 seconds

Claude: Wow, yes—that's the culprit. 3.95 seconds is what Eleventy should be doing. The Image transform is consuming ~73 of your ~77 seconds.

That's actually expected behavior though—the Image plugin is doing real work: reading every image, generating multiple widths and formats, writing them to disk. The question is whether it's doing more work than necessary.

A few things to check:

1. Is the image cache working? The Image plugin has a built-in disk cache. If it's not persisting between builds on Netlify, it's reprocessing every image every time. In your imgTransformSettings, make sure you have:

cacheOptions: {
    duration: "1y",
    directory: ".cache",
},

And in netlify.toml, cache that directory between builds:

[build]
  command = "npx @11ty/eleventy"
  publish = "_site"

[[plugins]]
  package = "netlify-plugin-cache"
  
  [plugins.inputs]
  paths = [".cache"]

2. How many images are actually being processed? You're passing through images with addPassthroughCopy and running the Image transform—you may be processing images that don't need optimization. Check if the transform is picking up images it shouldn't.

3. Check your truncated config Lines 263-304 of your config were truncated in the file view—that's right where imgTransformSettings is defined. Can you share what's in there?

---

Mark: I'm checking this locally for now

Claude: Good call—much faster feedback loop than waiting for Netlify deploys.

What does your local build time look like with the Image transform enabled? And is .cache already present in your project, or has caching not been set up yet?

---

Mark: how can I verify caching

Claude: Run two consecutive builds locally and compare the image output. If caching is working, the second build should be significantly faster because the Image plugin skips reprocessing images it's already handled.

You can also check directly:

ls -la .cache

If the directory exists and has content after a build, caching is active. If it's empty or missing, it's not persisting.

You can also add useCache: true explicitly to your imgTransformSettings (it should be on by default, but worth being explicit) and point it at a specific directory:

cacheOptions: {
    duration: "1y",
    directory: ".cache",
},

Then after a build, check:

ls -la .cache/eleventy-cache-assets-*

What does your .cache directory look like right now?

---

Mark: does not exist

Claude: That's your problem for both local and Netlify builds—every build is reprocessing every image from scratch.

Add the cache config to your imgTransformSettings:

let imgTransformSettings = {
    extensions: "html",
    formats: ["auto"],
    widths: [320, 960, 1400, 1920, 4000],
    cacheOptions: {
        duration: "1y",
        directory: ".cache",
    },
};

Then add .cache to your .gitignore (you don't want to commit processed images):

.cache

Run a full build, then run it again and compare times. The second build should be dramatically faster since all the image processing is already cached.

For Netlify, once caching is working locally, you'll need to persist .cache between deploys. The easiest way is the netlify-plugin-cache package—add it to your netlify.toml:

[[plugins]]
  package = "netlify-plugin-cache"
  
  [plugins.inputs]
  paths = [".cache"]

And install it:

npm install -D netlify-plugin-cache

Run two local builds first though and confirm the cache is actually helping before wiring up the Netlify side.

---

Mark: no .cache directory exists after build

Claude: The cacheOptions config may not be the right key for the Image transform plugin—that API is for the standalone Image() function. The transform plugin has a different config surface.

Check your truncated config section (lines 263-304)—can you share what's in there? That's where your imgTransformSettings is fully defined and it'll show what you're actually passing to the plugin.

Also worth checking the Eleventy Image docs for the transform-specific caching config, since it may use a different key than cacheOptions. Do you want me to look that up?

---

Mark: Yes

Claude: Good news—Zach Leatherman himself wrote about this exact problem and the fix is straightforward: point outputDir to .cache instead of _site, then copy the processed images to the output directory after the build completes.

Here's the pattern, adapted for your config:

import fs from "node:fs";
import path from "node:path";

let imgTransformSettings = {
    extensions: "html",
    formats: ["auto"],
    widths: [320, 960, 1400, 1920, 4000],
    urlPath: "/img/built/",
    outputDir: ".cache/@11ty/img/",
    // ... rest of your existing settings
};

eleventyConfig.addPlugin(eleventyImageTransformPlugin, imgTransformSettings);

// After build, copy cached images to output dir
eleventyConfig.on("eleventy.after", () => {
    fs.cpSync(
        ".cache/@11ty/img/",
        path.join(eleventyConfig.directories.output, "/img/built/"),
        { recursive: true }
    );
});

Then add .cache to .gitignore if it isn't already, and add it to your netlify.toml to persist it between deploys:

[build]
  command = "npx @11ty/eleventy"
  publish = "_site"

[[plugins]]
  package = "netlify-plugin-cache"
  
  [plugins.inputs]
  paths = [".cache"]

Zach reported this dropped his build time from 9:40 to 3:56. Your first build will still be slow while the cache is being populated, but subsequent builds should be dramatically faster.

One thing to check: your existing imgTransformSettings has some conditional logic for serve mode (lines 255-303 in your config). Make sure the outputDir and urlPath changes are applied in both branches.

---

Mark: should my config use /img/built/?

Claude: Good question—no, you should use whatever urlPath and outputDir match your existing setup. /img/built/ is just what Zach used on his site.

Check your truncated config (lines 263-304) to see what outputDir and urlPath you're currently using, if any. If you're not setting them explicitly, the Image plugin defaults to writing optimized images into _site/img/ with a urlPath of /img/.

Can you share those truncated lines?

---

Mark:

imgTransformSettings = {
    extensions: "html",
    // Output formats for each image.
    formats: ["auto"],
    widths: [320, 960, 1400, 1920, 4000],
    transformOnRequest: false,
    urlFormat: function ({
        hash, // not included for `statsOnly` images
        src,
        width,
        format,
    }) {
        return src.replace("content", "");
    },
    defaultAttributes: {
        loading: "lazy",
        decoding: "async",
        sizes: "(max-width: 45em) 640px, 100vw",
    },
    urlPath: "/img/built/",
    outputDir: ".cache/@11ty/img/"
};

Claude: You already have outputDir and urlPath set correctly—so the caching pattern is already in place. The missing piece is just the eleventy.after copy step.

But there's a wrinkle: your urlFormat function is doing something custom—it's stripping content from the src path and using the original filename rather than the hashed filename the Image plugin normally generates. That means the cpSync destination needs to match whatever path urlFormat is producing, not just /img/built/.

Can you check what your current output looks like in _site? Specifically, where are the optimized images ending up right now, and what do their filenames look like? That'll confirm the right destination path for cpSync.

---

Mark: the built images remain colocated within the content folder for each post

Claude: That's the conflict—your urlFormat is returning the original src path with content stripped, which means the Image plugin is writing images back into the content directory structure rather than into a centralized /img/built/. So the outputDir: ".cache/@11ty/img/" you have set isn't actually being used, because urlFormat overrides where the images end up.

This is why .cache never gets created—the custom urlFormat bypasses the normal output/cache mechanism entirely.

You have two paths forward:

Option 1: Remove urlFormat and use the standard output path Let the Image plugin write to .cache/@11ty/img/ as intended, then cpSync to _site/img/built/. Images get hashed filenames but caching works properly.

Option 2: Keep urlFormat but cache differently Keep your colocated structure (which is nice for portability), but accept that the Image plugin will reprocess on every build unless you find another way to persist the output. Since Netlify persists .cache but not _site, you'd need to point outputDir somewhere inside .cache that mirrors your content structure.

The reason you probably have urlFormat in the first place is to keep image URLs clean and colocated—is that important to preserve, or is the URL structure flexible?

---

Mark: I prefer to keep the URL structure colocated

Claude: Then the caching approach from Zach's post won't work as-is for you, because it relies on the standard outputDir/urlPath flow.

Your best option is to persist the _site output directory between Netlify builds instead of .cache. Netlify doesn't do this by default, but netlify-plugin-cache can cache any directory—it doesn't have to be .cache.

[[plugins]]
  package = "netlify-plugin-cache"
  
  [plugins.inputs]
  paths = ["_site"]

The tradeoff is that _site will be much larger than a dedicated image cache, but it means your already-processed images survive between deploys.

The cleaner alternative is to change your urlFormat to write into .cache but mirror your content structure, then cpSync the entire .cache/@11ty/img/ tree into _site in eleventy.after. Your URLs stay the same, but the processed images live in .cache between builds.

Which feels more manageable for your setup?

---

Mark: The latter option

Claude: Okay. The key is making sure cpSync mirrors your content directory structure into _site. Here's how it would look:

import fs from "node:fs";
import path from "node:path";

imgTransformSettings = {
    extensions: "html",
    formats: ["auto"],
    widths: [320, 960, 1400, 1920, 4000],
    transformOnRequest: false,
    urlFormat: function ({ src, width, format }) {
        return src.replace("content", "");
    },
    defaultAttributes: {
        loading: "lazy",
        decoding: "async",
        sizes: "(max-width: 45em) 640px, 100vw",
    },
    urlPath: "/",
    outputDir: ".cache/@11ty/img/",
};

eleventyConfig.addPlugin(eleventyImageTransformPlugin, imgTransformSettings);

eleventyConfig.on("eleventy.after", () => {
    fs.cpSync(
        ".cache/@11ty/img/",
        eleventyConfig.directories.output,
        { recursive: true }
    );
});

The critical change is urlPath: "/" — since your urlFormat is already computing the full public path, urlPath just needs to be the root so it doesn't prepend anything unexpected.

Before committing to this, I'd recommend doing a test build and checking:

1. Does .cache/@11ty/img/ get created and populated?
2. Do the paths inside it match what you expect?
3. After cpSync, do images appear in the right places in _site?

What does your current urlFormat output look like for a typical image path—do you know what src looks like at that point?

---

Mark: I'm using Netlify's image CDN so on production the image looks like:

<img loading="lazy" decoding="async" src="/.netlify/images?url=/posts/haverford-college-walk-13/haverford-college-walk-13-1.jpg&amp;w=320&amp;fit=contain" alt="Cherry blossom tree." width="2667" height="4000" srcset="/.netlify/images?url=/posts/haverford-college-walk-13/haverford-college-walk-13-1.jpg&amp;w=320&amp;fit=contain 320w, /.netlify/images?url=/posts/haverford-college-walk-13/haverford-college-walk-13-1.jpg&amp;w=960&amp;fit=contain 960w, /.netlify/images?url=/posts/haverford-college-walk-13/haverford-college-walk-13-1.jpg&amp;w=1400&amp;fit=contain 1400w, /.netlify/images?url=/posts/haverford-college-walk-13/haverford-college-walk-13-1.jpg&amp;w=1920&amp;fit=contain 1920w, /.netlify/images?url=/posts/haverford-college-walk-13/haverford-college-walk-13-1.jpg&amp;w=2667&amp;fit=contain 2667w" sizes="(max-width: 45em) 640px, 100vw">

Claude: That changes everything. You're using Netlify Image CDN, which means Netlify is handling the image transformation at the CDN level—the Image plugin transform is just generating the /.netlify/images?url=... URLs, not actually processing and writing image files to disk.

So the slow build times aren't from writing processed image files—they're from the Image plugin parsing every HTML file to rewrite <img> tags into the /.netlify/images?url= format.

That means:

• The .cache approach won't help—there are no image files being written to cache
• The bottleneck is purely the HTML transform overhead across 1215 files

Is the Netlify Image CDN integration coming from a specific Eleventy plugin or is that what your custom urlFormat is doing?

---

Mark: that's from my custom urlFormat:

urlFormat: function ({
    hash, // not included for `statsOnly` images
    src,
    width,
    format,
}) {
    return `/.netlify/images?url=${src.replace(
        "content",
        ""
    )}&w=${width}&fit=contain`;
},

Claude: That's the key insight then. Your urlFormat is generating URLs that point to Netlify's CDN—so the Image plugin isn't writing any image files at all. It's purely doing HTML transformation work: parsing every HTML file, finding <img> tags, and rewriting their src/srcset attributes with /.netlify/images?url=... URLs.

That means the entire processing cost is cheerio parsing × 1215 files, just like your prepareGallery transform.

The good news is this is actually a much simpler problem than image optimization caching—you don't need to persist anything to disk. You just need to make the HTML rewriting faster.

Given that your urlFormat is already doing simple string manipulation, replacing the Image plugin transform with a lightweight custom transform seems like the most direct path. What do you think?

---

Mark: I would only want that transform to run on netlify

this is why I had branching urlFormat code for local testing vs Netlify

Claude: Right, that makes sense—locally you want the Image plugin to write actual files for dev purposes, on Netlify you just want the URL rewriting.

You're already branching on process.env.ELEVENTY_RUN_MODE for serve mode. You can do the same for Netlify using the NETLIFY environment variable that Netlify automatically sets. Is that roughly how your current branching is structured, or is it different?

---

Mark: Is this accurate to how Netlify would identify itself as a process?

Claude: (searches Netlify docs)

Good news—your original code was correct. NETLIFY is always true on Netlify builds and can be used to check if the build is running on Netlify. However the value is the string "true", not a boolean, so your check should be:

const isNetlify = process.env.NETLIFY === "true";

---

Mark: that code was from you. here's my logic:

if (process.env.ELEVENTY_RUN_MODE === "serve") {
    imgTransformSettings = {
        extensions: "html",
        formats: ["auto"],
        widths: [320, 960, 1400, 1920, 4000],
        transformOnRequest: false,
        urlFormat: function ({ hash, src, width, format }) {
            return src.replace("content", "");
        },
        defaultAttributes: {
            loading: "lazy",
            decoding: "async",
            sizes: "(max-width: 45em) 640px, 100vw",
        },
    };
} else {
    imgTransformSettings = {
        extensions: "html",
        formats: ["auto"],
        widths: [320, 960, 1400, 1920, 4000],
        transformOnRequest: false,
        urlFormat: function ({ hash, src, width, format }) {
            return `/.netlify/images?url=${src.replace("content", "")}&w=${width}&fit=contain`;
        },
        defaultAttributes: {
            loading: "lazy",
            decoding: "async",
            sizes: "(max-width: 45em) 640px, 100vw",
        },
    };
}

Claude: Ah, so you're branching on serve vs everything else—which means your Netlify Image CDN urlFormat is also running during local build mode, not just on Netlify. That's probably fine, but it means process.env.NETLIFY isn't currently in your logic at all.

The question is: what should happen during a local eleventy build (not serve, not Netlify)? Right now it uses the Netlify CDN urlFormat, which would produce broken /.netlify/images?url=... URLs locally. Is that intentional, or do you only ever run serve locally and leave full builds to Netlify?

---

Mark: I only run serve locally

Claude: Then your current two-branch logic is already correct for your workflow—serve locally, everything else is Netlify. You don't need a third branch.

So for adding the optimized transform, you'd just use that existing split:

if (process.env.ELEVENTY_RUN_MODE === "serve") {
    eleventyConfig.addPlugin(eleventyImageTransformPlugin, imgTransformSettings);
} else {
    // On Netlify: skip the Image plugin transform entirely,
    // use a lightweight combined transform instead
    eleventyConfig.addTransform("prepareImages", ...);
}

Though looking at this more carefully, the regex approach for reconstructing srcset gets complicated fast. Is the Image plugin transform actually slow on Netlify, or does Netlify's build infrastructure make it fast enough that the cheerio overhead is acceptable?

---

Mark: It's equally slow on netlify. Even with the changes to my prepareGallery transform the build is still over 4 mins

Claude: So even with the <img check in prepareGallery, the Image plugin transform is still the dominant bottleneck—which tracks with your local test showing 3.95s without it vs 76s with it.

But before going down the regex road—the original point of prepareGallery was to wrap images for GLightbox. Your two transforms are doing related but distinct jobs:

1. Image plugin transform — rewrites <img src="..."> to add srcset with Netlify CDN URLs
2. prepareGallery transform — wraps <img> in <a class="glightbox"> for lightbox support

Both are running cheerio on every HTML file. Can you combine them into a single cheerio pass? Want to work through what that would look like?

---

Mark: Yes

Claude: Looking at your existing prepareGallery transform, it's already doing most of the work. We just need to fold in the Netlify CDN URL rewriting. Here's what the combined transform would look like:

const WIDTHS = [320, 960, 1400, 1920, 4000];

eleventyConfig.addTransform("prepareImages", async function(content) {
    if (!this.page.outputPath?.endsWith(".html")) return content;
    if (!content.includes("<img")) return content;

    const data = pageDataMap.get(this.page.inputPath);
    if (data?.content_type !== "post") return content;

    const $ = cheerio.load(content);

    $("img").each((i, el) => {
        let imgUrl = $(el).attr("src");
        const imgWidth = $(el).attr("width");
        const imgGallery = $(el).attr("data-gallery");
        const classes = $(el).attr("class");
        let imgCaption = "";

        if (
            $(el).next().length > 0 &&
            $(el).next().prop("tagName").toLowerCase() === "figcaption"
        ) {
            imgCaption = $(el).next().html();
        }

        const srcset = WIDTHS.map(w =>
            `/.netlify/images?url=${imgUrl}&w=${w}&fit=contain ${w}w`
        ).join(", ");
        const cdnSrc = `/.netlify/images?url=${imgUrl}&w=${imgWidth || 960}&fit=contain`;

        $(el).attr("src", cdnSrc);
        $(el).attr("srcset", srcset);
        $(el).attr("sizes", "(max-width: 45em) 640px, 100vw");

        $(el).wrap("<a></a>");
        const parent = $(el).parent();
        if (classes) parent.addClass(classes);
        parent.addClass("glightbox");
        parent.attr("href", `/.netlify/images?url=${imgUrl}&w=${imgWidth || 4000}&fit=contain`);
        if (imgGallery) parent.attr("data-gallery", imgGallery);
        if (imgCaption !== "") parent.attr("data-title", imgCaption);
    });

    return $.html();
});

A few things to verify:

1. Does your current Image plugin output width/height attributes on the <img> tag?
2. The href on the lightbox anchor—is that the right behavior for the full-size lightbox view?
3. This transform would only run on content_type === "post" pages—are there images on other page types that need either treatment?

---

Mark:

1. Yes
2. I am using the data-src attribute so that glightbox serves up an image and the browser picks the right size
3. This is fine

Claude: Good clarifications. For point 1, the Image plugin generates width/height from the actual image dimensions—we'd need to replicate that without the plugin. For point 2, using data-srcset on the anchor rather than a fixed href is the right approach—let me correct that.

Can you share what the current full output of a processed <img> tag looks like in your local build—including all attributes the Image plugin adds?

---

Mark:

<figure><a class="glightbox" href="/.netlify/images?url=/posts/madison-cunningham-ardmore-music-hall-april-2026/madison-cunningham-ardmore-music-hall-april-2026-1.jpg&amp;w=2667&amp;fit=contain" data-sizes="(max-width: 45em) 640px, 100vw" data-srcset="/.netlify/images?url=/posts/madison-cunningham-ardmore-music-hall-april-2026/madison-cunningham-ardmore-music-hall-april-2026-1.jpg&amp;w=320&amp;fit=contain 320w, ...2667w"><img loading="lazy" decoding="async" src="/.netlify/images?url=/posts/madison-cunningham-ardmore-music-hall-april-2026/madison-cunningham-ardmore-music-hall-april-2026-1.jpg&amp;w=320&amp;fit=contain" alt="Marquee reading &quot;Ardmore Music Hall: Madison Cunninham&quot;" width="2667" height="4000" srcset="...320w, ...960w, ...1400w, ...1920w, ...2667w" sizes="(max-width: 45em) 640px, 100vw"></a></figure>

Claude: This is really helpful. A few observations from the actual output:

1. The href on the anchor uses the original image width (2667) as the w= value—so the lightbox is serving the full original size via CDN
2. data-srcset and data-sizes are on the anchor, srcset and sizes are on the <img>—both are present
3. width and height are the original image dimensions (2667×4000), not a resized value
4. The smallest width (320) is used as the src on the <img>

The width and height attributes are the tricky part—those come from the Image plugin reading the actual image file dimensions. To replicate that in a custom transform you'd need to pull in something like sharp or image-size to get dimensions without processing.

Before going further—is the width/height on the <img> something you're willing to drop, or is CLS prevention important enough to keep?

---

Mark: I'm ok dropping width/height

Claude: Good. Then the combined transform is straightforward. Here's the full thing, replacing both the Image plugin and prepareGallery:

const WIDTHS = [320, 960, 1400, 1920, 4000];
const SIZES = "(max-width: 45em) 640px, 100vw";

eleventyConfig.addTransform("prepareImages", function(content) {
    if (!this.page.outputPath?.endsWith(".html")) return content;
    if (!content.includes("<img")) return content;

    const data = pageDataMap.get(this.page.inputPath);
    if (data?.content_type !== "post") return content;

    const $ = cheerio.load(content);

    $("img").each((i, el) => {
        const imgUrl = $(el).attr("src");
        const imgGallery = $(el).attr("data-gallery");
        const classes = $(el).attr("class");

        let imgCaption = "";
        if (
            $(el).next().length > 0 &&
            $(el).next().prop("tagName").toLowerCase() === "figcaption"
        ) {
            imgCaption = $(el).next().html();
        }

        const cdnUrl = (w) => `/.netlify/images?url=${imgUrl}&w=${w}&fit=contain`;
        const srcset = WIDTHS.map(w => `${cdnUrl(w)} ${w}w`).join(", ");

        $(el).attr("src", cdnUrl(WIDTHS[0]));
        $(el).attr("srcset", srcset);
        $(el).attr("sizes", SIZES);
        $(el).removeAttr("width");
        $(el).removeAttr("height");

        $(el).wrap("<a></a>");
        const parent = $(el).parent();
        if (classes) parent.addClass(classes);
        parent.addClass("glightbox");
        parent.attr("href", cdnUrl(WIDTHS[WIDTHS.length - 1]));
        parent.attr("data-srcset", srcset);
        parent.attr("data-sizes", SIZES);
        if (imgGallery) parent.attr("data-gallery", imgGallery);
        if (imgCaption !== "") parent.attr("data-title", imgCaption);
    });

    return $.html();
});

Then in your branching logic:

if (process.env.ELEVENTY_RUN_MODE === "serve") {
    eleventyConfig.addPlugin(eleventyImageTransformPlugin, imgTransformSettings);
} else {
    eleventyConfig.addTransform("prepareImages", ...);
}

And remove the prepareGallery transform entirely—it's now folded in.

---

Mark: Local build ran clean, Netlify build had an error:

[11ty] 1. Having trouble writing to "false" from "./content/reading/empire-of-ai-karen-hao/index.md" (via EleventyTemplateError)
[11ty] 2. Transform `prepareImages` encountered an error when transforming ./content/reading/empire-of-ai-karen-hao/index.md. (via EleventyTransformError)
[11ty] 3. this.page.outputPath?.endsWith is not a function (via TypeError)

Claude: The error is this.page.outputPath?.endsWith is not a function—which means outputPath is the boolean false rather than a string on that particular file. That's Eleventy setting outputPath to false for files it's not writing to disk (like data files or permalinks set to false).

The optional chaining ?. only guards against null/undefined, not false. Fix it by checking the type explicitly, same as your original prepareGallery did:

eleventyConfig.addTransform("prepareImages", function(content) {
    const pageOutputPath = this?.page?.outputPath;
    if (typeof pageOutputPath !== "string") return content;
    if (!pageOutputPath.endsWith(".html")) return content;
    // ...rest of transform
});

That typeof !== "string" check handles false, undefined, and null in one go.

---

Mark: When I run a build I am seeing the src only rendering the image name, without the content path prepending it:

/.netlify/images?url=barnes-foundation-february-2026-1.jpg&w=320&fit=contain

Claude: That's because the Image plugin transform was previously rewriting the src to include the full path, but now we're taking the src directly from the raw <img> tag that markdown-it outputs—which is just the bare filename as written in the markdown.

What does the image reference look like in your markdown source? And what did the src look like on the <img> tag before the Image plugin processed it? You may need to use this.page.filePathStem to build the full path, similar to what your original prepareGallery transform was doing with dirPath.

---

Mark: If I comment out my transform this is what gets rendered for an image:

<img src="madison-cunningham-ardmore-music-hall-april-2026-1.jpg" alt="Marquee reading &quot;Ardmore Music Hall: Madison Cunninham&quot;">

Claude: So the Image plugin was doing two things you now need to handle manually:

1. Resolving the bare filename to a full path
2. Rewriting to the Netlify CDN URL

You have this.page.filePathStem available in the transform, which gives you something like /posts/madison-cunningham-ardmore-music-hall-april-2026/index. You can use that to reconstruct the full path:

$("img").each((i, el) => {
    const imgSrc = $(el).attr("src");
    
    // filePathStem: /posts/madison-cunningham-ardmore-music-hall-april-2026/index
    // we want:      /posts/madison-cunningham-ardmore-music-hall-april-2026/image.jpg
    const dir = this.page.filePathStem.replace(/\/index$/, "");
    const imgUrl = `${dir}/${imgSrc}`;
    
    const cdnUrl = (w) => `/.netlify/images?url=${imgUrl}&w=${w}&fit=contain`;
    // ...rest of transform
});

That should produce /posts/madison-cunningham-ardmore-music-hall-april-2026/madison-cunningham-ardmore-music-hall-april-2026-1.jpg which matches what your original output was using.

Worth logging this.page.filePathStem on a known post to confirm the exact format before relying on it.

---

Mark: What is this line doing? const cdnUrl = (w) => \/.netlify/images?url=${imgUrl}&w=${w}&fit=contain`;`

Claude: It's defining a small helper function that takes a width value w and returns the full Netlify CDN URL string for that width. It's an arrow function assigned to a const.

So when you call cdnUrl(320) it returns:

/.netlify/images?url=/posts/madison-cunningham-ardmore-music-hall-april-2026/madison-cunningham-ardmore-music-hall-april-2026-1.jpg&w=320&fit=contain

And cdnUrl(960) returns the same URL but with w=960, etc. It avoids repeating the template literal every time you need a CDN URL at a different width.