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Add support for Mermaid diagrams to be embedded in blog posts using markdown code fence syntax. This will allow blog posts covering various topics to include visual diagrams (flowcharts, sequence diagrams, class diagrams, state diagrams, etc.) to enhance content clarity and engagement.
Blog posts that discuss technical topics benefit from inline visual diagrams — flowcharts, sequence diagrams, architecture overviews, and more. Currently, authors would need to create and link external static images, which is fragile and hard to maintain.
This feature enables authors to embed Mermaid diagrams directly in blog post markdown using standard code fence syntax (```mermaid). The diagrams are rendered client-side as SVG elements within the blog post body, appearing inline with the surrounding prose. Non-Mermaid code fences and all other markdown content remain unaffected.
Design Intent
Seamless content integration: Diagrams should feel like a natural part of the blog post body — not a separate widget or embedded application. They sit within the standard content column and follow the same vertical rhythm as other block-level elements.
Content-first presentation: Align with the design system's editorial, typography-led approach. Diagrams enhance understanding of prose; they should not dominate or distract from surrounding content.
Visual restraint: Use Mermaid's default theme without custom styling. The diagrams should feel professional and neutral, consistent with the site's calm aesthetic.
Graceful degradation: When a diagram cannot render (malformed syntax), the user sees a clear, polite inline error message rather than a broken layout or blank space.
Minimal loading disruption: Because Mermaid requires the DOM and renders client-side, a lightweight placeholder should be shown while the library initialises, avoiding layout shift and providing visual continuity.
Lifecycle Check
Task ID: T014
Run State: harness/run-state.md
Current Stage: Design
Proceeding Validly: Yes
Brief note:
The run state confirms T014 is in progress with Design as the current stage (RUN-2026-03-31-T014-DESIGN). The approved feature specification at harness/artifacts/specs/T014-feature-spec.md has been reviewed. All prerequisite artifacts are available. This design pass is valid.
Page / Feature Structure
Mermaid diagrams appear within existing blog post pages. No new pages or routes are introduced.
The affected region is the blog post body content area — the markdown-rendered zone that already contains headings, paragraphs, lists, images, and syntax-highlighted code blocks.
The structural hierarchy within the blog post body is unchanged:
Blog post page
Post header (title, date, metadata)
Post body content (markdown-rendered region) ← diagrams appear here
Prose blocks
Code blocks (unchanged)
Mermaid diagram blocks (new)
Post footer / sidebar
Mermaid diagram blocks are block-level elements that replace specific code fences. They occupy the full width of the content column, just as a code block or image would.
Section Breakdown
Mermaid Diagram Block
Purpose: Render a single Mermaid diagram from a ```mermaid ``` code fence.
Expected content: A rendered SVG diagram produced by the Mermaid library from the author's Mermaid syntax source.
Visual guidance:
The diagram block sits within the content column at the same horizontal width as other block-level elements (paragraphs, code blocks).
Standard vertical spacing separates the diagram from adjacent content — consistent with the spacing used between other block-level elements in the post body (the existing space-y rhythm).
No additional border, shadow, or background decoration surrounds the diagram. The SVG output stands on its own, consistent with the design system's visual restraint principle.
The diagram is horizontally centered within the content column.
Interaction expectations: Diagrams are static rendered SVGs. No click, hover, or zoom interactions are provided. The user reads the diagram as they would read an inline image.
Loading Placeholder
Purpose: Provide visual continuity while the Mermaid library loads and renders the diagram client-side.
Expected content: A low-profile placeholder occupying the diagram's vertical space to prevent layout shift.
Visual guidance:
A subtle neutral-toned container (using the border colour #E5E5E5 or equivalent design-system divider tone) with a minimum height to reserve space.
No spinner or animated indicator — the placeholder should be calm and unobtrusive, consistent with the site's minimal interaction design. A simple muted text label such as "Loading diagram…" in secondary text colour (#6B6B6B) is sufficient.
Once rendering completes, the placeholder is replaced by the rendered SVG with no abrupt layout change.
Error State
Purpose: Inform the reader when a Mermaid code fence contains invalid syntax and cannot be rendered.
Expected content: A user-visible inline message explaining that the diagram could not be rendered.
Visual guidance:
Displayed in the same block-level position the diagram would have occupied.
Uses the existing content column width and standard block spacing.
A bordered container using the site's border colour (#E5E5E5) with a subtle background tint (very light neutral, not red or alarming) to distinguish it from normal content.
Error text in secondary text colour (#6B6B6B) at body font size. Example text: "This diagram could not be rendered."
The error state should feel informational and calm, not alarming. No icons, no bold red, no exclamation marks.
The raw Mermaid source is not displayed to the reader (it would not be meaningful to most audiences).
Component Recommendations
Mermaid Diagram Renderer
A single client-side component responsible for:
Receiving Mermaid source text as input
Rendering it to SVG via the Mermaid library
Displaying the loading placeholder before rendering completes
Displaying the error state if rendering fails
This component should be used wherever a ```mermaid ``` code fence is detected in blog post markdown processing. It replaces the default code-block rendering for the mermaid language identifier only.
Existing Blog Markdown Pipeline
The current blog markdown rendering pipeline already supports custom rendering for code fences (e.g., syntax highlighting). The Mermaid diagram renderer should integrate into this pipeline by intercepting code fences with the mermaid language tag and delegating them to the Mermaid diagram renderer component instead of the standard code block renderer.
All other code fence languages continue to use standard code block rendering without any change.
No New Layout Components
No new layout primitives, section wrappers, or page-level components are needed. The diagram renderer is a leaf-level content component within the existing post body layout.
Responsive Behaviour
Mobile
Diagrams render within the full width of the content column.
Wide diagrams (e.g., Gantt charts, sequence diagrams with many participants) that exceed the viewport width are contained within a horizontally scrollable region. The scroll container prevents the diagram from breaking the page layout or causing full-page horizontal scroll.
The scroll container should have no visible chrome — it scrolls naturally on touch.
Diagram text and labels remain at the size Mermaid renders them; no additional scaling is applied at the mobile breakpoint.
The loading placeholder and error state occupy the full content width, same as other block elements.
Tablet
Same behaviour as mobile. The wider viewport accommodates more diagrams at native size, but wide diagrams still use the horizontal scroll container if they exceed the content column width.
Desktop
Most diagrams fit comfortably within the max-w-3xl / max-w-4xl content column and require no scrolling.
Exceptionally wide diagrams use the same horizontal scroll container as mobile and tablet.
Diagrams remain horizontally centered within the content column.
Cross-Breakpoint Rules
No diagram should cause horizontal page overflow at any viewport width.
The horizontal scroll container is the consistent mechanism at all breakpoints — it activates only when the rendered SVG exceeds the content column width.
Vertical spacing around diagram blocks remains consistent across breakpoints, following the existing post body spacing rhythm.
Layout shift is minimised by the loading placeholder reserving vertical space.
Accessibility Considerations
Semantic structure: Mermaid diagram blocks are embedded within the post body flow. They should be wrapped in a container with role="img" and a generic aria-label (e.g., "Diagram") to identify the SVG as a meaningful image for assistive technologies. (The specification notes that custom text descriptions per diagram are out of scope.)
Heading hierarchy: Diagrams do not introduce headings. The post's heading hierarchy is unaffected.
Keyboard accessibility: Diagrams are static images and do not require keyboard interaction. If the horizontal scroll container is used, it should be scrollable via keyboard (standard browser behaviour for overflow-x: auto containers with focusable content or tabindex="0").
Focus states: The horizontal scroll container, if keyboard-scrollable, should have a visible focus ring consistent with the site's existing focus styling.
Contrast: Mermaid's default theme provides adequate contrast for diagram text and shapes. No custom theme modifications are applied.
Error state: The error message text meets contrast requirements using secondary text colour (#6B6B6B) on a white or near-white background.
Reduced motion: Mermaid rendering is a one-time paint, not an animation. No motion concerns apply.
Dependencies
Approved feature specification: harness/artifacts/specs/T014-feature-spec.md — reviewed and used as the basis for this design.
Design system: harness/design-system/design-system.md — spacing, typography, colour, layout, and component guidance followed throughout.
Existing blog post markdown pipeline: The current rendering pipeline that processes markdown content and renders code fences. The Mermaid renderer integrates into this pipeline.
Mermaid JavaScript library: Required as a runtime dependency for client-side SVG rendering. Dynamic/lazy loading is expected to manage bundle size.
Risks
Layout shift during client-side rendering: Because Mermaid renders only on the client, there is a window between page load and diagram appearance. The loading placeholder mitigates this, but the exact rendered height of a diagram is unknown until Mermaid processes it, so minor shift may still occur. Implementation should size the placeholder reasonably.
Wide diagram overflow: Some diagram types produce very wide SVGs. The horizontal scroll container addresses this, but authors should be aware that extremely wide diagrams may not be ideal on mobile. This is a content authoring concern rather than a design defect.
Mermaid library bundle size: Mermaid is a large library. If it is not lazily loaded, it could impact page load performance for all blog posts, including those with no diagrams. The design assumes dynamic/lazy loading so the library is only fetched when a post contains Mermaid fences.
Open Questions
Loading placeholder height: Should the placeholder use a fixed minimum height, or attempt to estimate height from the source length? A fixed minimum height is simpler and recommended; implementation planning can finalize the value.
Scroll container indicator: On desktop, should there be any visual hint (e.g., a subtle fade or shadow at the right edge) that a diagram is horizontally scrollable? The current recommendation is no — rely on standard browser scrollbar behaviour — but this can be revisited during implementation if usability testing reveals issues.
Enable Mermaid diagram rendering within blog posts so that authors can embed visual diagrams using standard markdown code fence syntax (```mermaid) and have them rendered as interactive SVG diagrams for readers.
User Value
Blog posts that discuss technical topics benefit significantly from visual diagrams — flowcharts, sequence diagrams, architecture diagrams, and more. Currently, authors cannot include diagrams directly in markdown content; they would need to produce and link static images, which is fragile and hard to maintain.
By supporting Mermaid code fences natively, authors can:
Write diagrams inline using a widely adopted text-based syntax
Keep diagram source alongside prose in the same markdown file
Update diagrams by editing text rather than regenerating images
Provide readers with clear, responsive, professional visuals that enhance understanding
In Scope
Detection and rendering of ```mermaid ``` code fences within blog post markdown content
Support for all default Mermaid diagram types: flowchart, sequence, class, state, ER, Gantt, pie, git, mindmap, timeline, sankey, block, and any other types supported by the Mermaid library version adopted
Client-side rendering of Mermaid diagrams (Mermaid requires a DOM and renders to SVG)
Responsive diagram display that adapts to mobile and desktop viewports (e.g., horizontal scrolling or scaling for wide diagrams)
Graceful error handling: when Mermaid code is invalid or malformed, a visible, user-friendly error message is displayed in place of the diagram
Mermaid's default theme and styling are used without customization
Non-Mermaid code fences continue to render as standard code blocks with no change in behavior
Blog posts with zero Mermaid fences are unaffected
Blog posts may contain multiple Mermaid diagrams intermixed with other content
Unit tests covering Mermaid component rendering and error handling
Integration tests verifying Mermaid diagrams render correctly within full blog post content
Out of Scope
Custom Mermaid themes or styling beyond Mermaid defaults
Mermaid diagram editing or live-preview capabilities for authors
Server-side rendering or static SVG generation of Mermaid diagrams
Export or download of rendered diagrams as images
Mermaid configuration overrides per diagram or per post
Accessibility enhancements beyond what Mermaid provides by default (e.g., custom aria labels or text descriptions)
Performance optimization for posts with very large numbers of diagrams (beyond reasonable use)
Mermaid version pinning strategy or upgrade policy
Acceptance Criteria
A blog post containing a ```mermaid ``` code fence renders a visible Mermaid diagram (SVG) in the browser at the code fence location.
All default Mermaid diagram types render correctly when valid syntax is provided, including at minimum: flowchart, sequence, class, state, ER, Gantt, pie, mindmap, and timeline.
A blog post containing multiple Mermaid code fences renders each diagram independently and correctly.
Non-Mermaid code fences (e.g., ```javascript ```, ```bash ```) in the same post continue to render as syntax-highlighted code blocks.
Markdown content surrounding Mermaid code fences (headings, paragraphs, lists, images, other code blocks) renders correctly and is not disrupted.
When a Mermaid code fence contains invalid or malformed syntax, a user-visible error message is displayed in place of the diagram, indicating the diagram could not be rendered.
Rendered diagrams display responsively: on narrow viewports, wide diagrams are scrollable or scaled to remain usable without breaking the page layout.
Diagrams use Mermaid's default theme and styling — no custom CSS is applied to diagram internals.
Blog posts with no Mermaid code fences render identically to their current behavior (no regression).
Unit tests exist that verify: a valid Mermaid code fence produces a rendered diagram element, and an invalid Mermaid code fence produces an error message element.
Integration tests exist that verify a blog post containing Mermaid diagrams alongside other markdown content renders all content correctly.
Dependencies
Mermaid library: A compatible version of the Mermaid JavaScript library (or a React wrapper such as mermaid npm package) must be added as a project dependency.
Existing blog markdown pipeline: The current blog post rendering pipeline (markdown parsing + React component rendering) must support extension to intercept ```mermaid ``` code fences and delegate them to a Mermaid renderer component. The pipeline exists and has been extended before (e.g., for code syntax highlighting).
Client-side rendering environment: Mermaid renders to SVG using the DOM, so the Mermaid component must execute on the client side (e.g., using Next.js dynamic import with SSR disabled, or a useEffect-based approach).
Risks
Mermaid library size: Mermaid is a large JavaScript library. Adding it to the client bundle may noticeably increase page load time for blog posts. The design stage should consider lazy loading or dynamic imports to mitigate this.
Client-only rendering: Since Mermaid requires the DOM, diagrams will not render during server-side rendering. There may be a flash of unstyled content or a loading state before diagrams appear. The design should account for this.
Mermaid version compatibility: Different Mermaid versions support different diagram types and syntax. The adopted version determines which diagram types are actually available.
Wide diagrams on mobile: Some diagram types (e.g., Gantt charts, sequence diagrams with many participants) can be very wide. Responsive handling needs to ensure these do not break the page layout.
Open Questions
Should a loading indicator (e.g., skeleton or spinner) be displayed while Mermaid initializes and renders on the client, or is a brief flash acceptable?
Is there a preferred Mermaid library version or React wrapper to adopt, or should the design stage determine this?
Lifecycle Check
Task ID: T014
Current Status: Ready
Active Task Check: T014 is the active task in run-state.md
Proceeding Validly: Yes
Brief note:
Task T014 is in Ready status, run-state confirms Specification as the current stage with run ID RUN-2026-03-31-T014-SPEC. All required inputs were reviewed. The specification is written to the canonical path. No blockers identified.
Task ID: T014
Run State: harness/run-state.md
Current Stage: Implementation Planning
Proceeding Validly: Yes
Brief note:
Run state confirms T014 is in progress with Implementation Planning as the current stage (RUN-2026-03-31-T014-PLAN). The approved feature specification and approved design note are both available. Specification and Design stages are recorded as complete. All prerequisite artifacts are present and this planning pass is valid.
Technical Approach
The blog post page (src/app/blog/(post)/[slug]/page.tsx) renders markdown content via ReactMarkdown with remark/rehype plugins. Code fences are rendered as <pre><code> blocks by default.
The implementation strategy has three parts:
Mermaid client component — Create a new React client component (src/components/blog/mermaid-diagram.tsx) that accepts Mermaid source text, dynamically imports the mermaid library, renders it to SVG, and handles loading/error states per the design note. The component uses useEffect and a ref to invoke Mermaid's render() API against the DOM. Dynamic import of mermaid ensures the ~1 MB library is only fetched when a post actually contains a Mermaid diagram.
Custom code component for ReactMarkdown — Pass a components prop to the existing ReactMarkdown instance with a custom code renderer. When a code block has the language class mermaid, delegate to the Mermaid client component instead of the default <code> element. All other language classes pass through to the existing code block rendering unchanged.
Rehype-sanitize allowlist update — The current pipeline uses rehype-sanitize which strips attributes and elements not in the allowlist. The custom components prop in react-markdown operates at the React level (after rehype), so Mermaid-language code fences should pass through as <code className="language-mermaid"> blocks. The existing sanitization schema may need a minor tweak to preserve the className attribute on code elements so the language class survives to the component layer.
This approach:
Requires no changes to the markdown content pipeline or remark/rehype plugin chain beyond sanitization
Keeps the Mermaid library lazily loaded so posts without diagrams pay no bundle cost
Confines all Mermaid-specific logic to a single leaf component
Preserves all existing code fence and markdown rendering behavior
Implementation Slices
Slice 1 — Mermaid Diagram Client Component
Purpose: Create the MermaidDiagram client component that renders Mermaid source to SVG with loading placeholder and error state handling.
Likely areas affected:
src/components/blog/mermaid-diagram.tsx (new file)
Validation focus:
Component renders valid Mermaid source to an SVG element
Component shows a loading placeholder before rendering completes
Component shows a calm error message for invalid Mermaid syntax
Component wraps diagram in an accessible container (role="img", aria-label)
Wide diagrams are wrapped in an overflow-x-auto scroll container
The mermaid library is dynamically imported (not statically bundled)
Slice 2 — ReactMarkdown Integration
Purpose: Wire the Mermaid component into the blog post markdown pipeline by providing a custom code component to ReactMarkdown, and adjust rehype-sanitize config if needed.
Likely areas affected:
src/app/blog/(post)/[slug]/page.tsx
Validation focus:
Mermaid code fences render using the MermaidDiagram component
Non-Mermaid code fences render identically to current behavior
Surrounding markdown content (headings, paragraphs, lists, images) is unaffected
Blog posts with no Mermaid fences have no behavioral change
Slice 3 — Dependency Installation
Purpose: Add the mermaid npm package as a project dependency.
Likely areas affected:
src/package.json
src/pnpm-lock.yaml
Validation focus:
mermaid package is listed in dependencies
Project builds and starts without errors
Slice 4 — Tests
Purpose: Add unit tests for the MermaidDiagram component and integration tests for Mermaid diagrams within blog post content.
Likely areas affected:
src/app/tests/unit/mermaid-diagram.test.tsx (new file)
src/app/tests/integration/blog-mermaid.test.tsx (new file)
Validation focus:
Unit tests verify valid diagram rendering, error state rendering, and loading state
Integration tests verify Mermaid diagrams alongside other markdown content
All existing tests continue to pass
Affected Areas of the Codebase
src/components/blog/mermaid-diagram.tsx — new client component for Mermaid rendering
src/app/tests/unit/mermaid-diagram.test.tsx — new unit test file
src/app/tests/integration/blog-mermaid.test.tsx — new integration test file
Implementation Steps
Step 1 — Install the Mermaid dependency
Add the mermaid npm package to the project dependencies using pnpm add mermaid from within the src/ directory.
Step 2 — Create the MermaidDiagram client component
Create src/components/blog/mermaid-diagram.tsx as a "use client" component that:
Accepts a code string prop containing Mermaid source text
Uses useEffect to dynamically import mermaid and call its render API with a unique element ID
Manages three states: loading, rendered (SVG HTML), and error (error message string)
Renders a loading placeholder (neutral bordered container with "Loading diagram…" text in muted colour) while the library loads
On success, injects the rendered SVG into the DOM via a ref container
On failure, renders a calm bordered container with "This diagram could not be rendered." in secondary text colour
Wraps the diagram in a container with role="img" and aria-label="Diagram" for accessibility
Wraps the SVG output in an overflow-x-auto container with tabindex="0" for keyboard-scrollable wide diagrams
Horizontally centers the diagram within the content column
Step 3 — Integrate into the blog post page
In src/app/blog/(post)/[slug]/page.tsx:
Define a custom code component function that checks for the language-mermaid className; if matched, render <MermaidDiagram code={children} /> instead of the default <code> element; otherwise, render the default <code> element with existing behavior
Pass the custom component via the components prop on ReactMarkdown
Verify that rehype-sanitize's schema allows className on code elements so the language class is preserved; if not, extend the schema's attributes to include className for code tags
A test that renders a blog post page (or the ReactMarkdown pipeline) with markdown containing a Mermaid code fence and asserts the Mermaid diagram component is rendered at the code fence location
A test that renders markdown containing both Mermaid and non-Mermaid code fences and asserts non-Mermaid fences still render as standard code blocks
A test that renders markdown with Mermaid fences alongside headings, paragraphs, and other content and asserts all content renders correctly
Step 6 — Validate all existing tests pass
Run the full test suite to confirm no regressions in existing blog post rendering, code block handling, or any other functionality.
Test Plan
Test Type
Description
Aligns With
Unit
Valid Mermaid source renders a diagram element
AC 1
Unit
Invalid Mermaid source displays error message
AC 6
Unit
Accessible container with role="img" is present
Design: Accessibility
Unit
Overflow scroll container is present for wide diagrams
AC 7
Unit
Loading placeholder is shown before render completes
Design: Loading placeholder
Integration
Mermaid code fence within blog markdown renders diagram component
AC 1, 3
Integration
Non-Mermaid code fences render as standard code blocks
AC 4
Integration
Surrounding markdown content is not disrupted
AC 5
Integration
Posts with no Mermaid fences render identically to current behavior
AC 9
E2E (optional stretch)
Blog post with Mermaid diagram loads and displays SVG in browser
AC 1, 2
Risks
Mermaid library bundle size: Mermaid is approximately 1 MB. Dynamic import mitigates impact for posts without diagrams, but posts with diagrams will incur this fetch cost. This is an accepted trade-off per the specification.
Client-only rendering flash: Diagrams render only after JavaScript loads and Mermaid initializes. The loading placeholder minimizes layout shift, but a brief delay before diagram appearance is expected. This is noted as acceptable in the design note.
Rehype-sanitize stripping language classes: The sanitization step runs before ReactMarkdown's component layer. If className on code elements is stripped, the custom component will not be able to detect Mermaid fences. This requires verification and a minor schema tweak if needed.
Mermaid render API stability: Mermaid's JavaScript API (mermaid.render()) has had breaking changes across major versions. The implementation should use the current stable API pattern and pin to the installed version.
Unique ID generation for concurrent diagrams: Mermaid's render() API requires a unique element ID per diagram call. Posts with multiple diagrams need a reliable unique ID strategy (e.g., useId() or a counter).
Test environment DOM mocking: Mermaid requires a real DOM to render SVGs. Unit tests in Vitest with jsdom may need to mock the mermaid module rather than testing actual SVG output. Integration/E2E tests in a real browser provide the true rendering validation.
Assumptions
The react-markdowncomponents prop receives className on code elements, allowing language detection. This is the standard behavior but depends on rehype-sanitize not stripping it.
Mermaid's npm package (mermaid) supports dynamic import in a Next.js client component environment.
The current space-y-6 vertical rhythm on the blog post body section naturally spaces Mermaid diagram blocks consistently with other block elements without additional margin overrides.
A fixed minimum height for the loading placeholder (e.g., 100–200px) is acceptable; exact rendered diagram height is unknown before Mermaid processes the source.
Blog post markdown content is trusted author content; Mermaid source within code fences does not require additional sanitization beyond Mermaid's own rendering scope (Mermaid sanitizes its input by default).
Task ID: T014
Run State: harness/run-state.md
Current Stage: QA
Proceeding Validly: Yes
Brief note:
harness/run-state.md exists and references task T014. The current stage is QA with run ID RUN-2026-03-31-T014-QA. All four upstream stages (Specification, Design, Implementation Planning, Build) are recorded as complete with their respective artifacts present at the canonical paths. The task is in a valid lifecycle position for QA review. No blockers.
Validation Result
Pass
Acceptance Criteria Review
A blog post containing a ```mermaid ``` code fence renders a visible Mermaid diagram (SVG) in the browser at the code fence location.
Status: Pass
Notes: MermaidDiagram dynamically imports Mermaid, calls mermaid.render(), and injects the resulting SVG via dangerouslySetInnerHTML. MermaidCodeBlock intercepts language-mermaid code fences and delegates to MermaidDiagram. The integration test (blog-mermaid.test.tsx, test 3) confirms the full ReactMarkdown pipeline renders the mermaid-diagram test ID for a Mermaid code fence. Build validation confirms Next.js builds successfully with all 253 pages including 68 blog posts.
All default Mermaid diagram types render correctly when valid syntax is provided.
Status: Pass
Notes: The implementation passes the raw Mermaid source to mermaid.render() without filtering diagram types. All types supported by mermaid@11.13.0 are available. No diagram-type restrictions exist in the code. This is a library capability and is verified by the dependency version.
A blog post containing multiple Mermaid code fences renders each diagram independently and correctly.
Status: Pass
Notes: Each MermaidDiagram instance generates a unique ID via useId() (with colons stripped for Mermaid compatibility). Multiple instances operate independently with isolated state. The implementation correctly supports concurrent diagrams.
Non-Mermaid code fences continue to render as syntax-highlighted code blocks.
Status: Pass
Notes: MermaidCodeBlock checks for language-mermaid className; all other code fences fall through to the default <code> element. Integration test 3 verifies console.log("hello"); appears as rendered text in a non-Mermaid code fence alongside a Mermaid diagram.
Markdown content surrounding Mermaid code fences renders correctly and is not disrupted.
Status: Pass
Notes: Integration test 3 verifies headings, paragraphs, code blocks, and text surrounding Mermaid fences all render correctly in the ReactMarkdown pipeline. The component is a leaf-level replacement that does not affect surrounding content.
Invalid or malformed Mermaid syntax displays a user-visible error message.
Status: Pass
Notes: The catch block in renderDiagram() sets state to { status: "error" }, rendering a bordered container with "This diagram could not be rendered." Unit test confirms error state for invalid Mermaid source. Error message is calm and informational per design note.
Rendered diagrams display responsively with horizontal scrolling for wide diagrams.
Status: Pass
Notes: The SVG is wrapped in a div with overflow-x-auto class and tabIndex={0} for keyboard scrollability. Unit test verifies the scroll container and tabindex attribute. This prevents wide diagrams from breaking page layout at any viewport width.
Diagrams use Mermaid's default theme — no custom CSS applied to diagram internals.
Status: Pass
Notes: mermaid.initialize({ startOnLoad: false }) is the only configuration. No theme overrides or custom CSS are applied. The [&>svg]:mx-auto utility only centers the SVG horizontally; it does not alter diagram internals.
Blog posts with no Mermaid code fences render identically to current behavior (no regression).
Status: Pass
Notes: MermaidCodeBlock passes non-Mermaid code fences through unchanged. Mermaid is dynamically imported only when MermaidDiagram mounts, so posts without diagrams incur no library fetch. All 114 existing tests pass with no regressions. Build produces all 253 pages successfully.
Unit tests verify valid diagram rendering and invalid diagram error message.
Status: Pass
Notes: mermaid-diagram.test.tsx contains 5 unit tests: loading state, valid render, error state, accessible container, and scrollable container. All tests pass.
Integration tests verify Mermaid diagrams alongside other markdown content render correctly.
Status: Pass
Notes: blog-mermaid.test.tsx contains 3 integration tests: blog post rendering, non-Mermaid code fences, and the full ReactMarkdown pipeline with mixed content (headings, paragraphs, Mermaid fences, JavaScript code fences, surrounding text). All tests pass.
Design Alignment Review
Structural alignment: The implementation follows the design note precisely. Mermaid diagrams are block-level elements within the blog post body content area. No new pages, routes, or layout components are introduced. The MermaidDiagram component is a leaf-level content component within the existing post body layout.
Visual/system alignment: The loading placeholder uses border-border, text-muted-foreground, and a minimum height — consistent with the design note's guidance for a neutral-toned, calm placeholder with "Loading diagram…" text. The error state uses border-border, bg-muted/30, and text-muted-foreground with the exact text "This diagram could not be rendered." — matching the design note's specification for a calm, non-alarming error state. These align with the design system's colour palette (borders #E5E5E5, secondary text #6B6B6B).
Responsive behavior: The overflow-x-auto container with tabIndex={0} handles wide diagrams at all viewport widths. SVGs are centered via [&>svg]:mx-auto. This matches the design note's cross-breakpoint rules.
Accessibility: The diagram is wrapped in role="img" with aria-label="Diagram" per the design note. The scroll container has tabindex="0" and focus-visible styling for keyboard accessibility. These match the design note's accessibility considerations.
Component/pattern alignment: Mermaid's default theme is used without customization. No custom CSS is applied to diagram internals. This matches the design note's visual restraint principle and the design system's minimal aesthetic.
No deviations from the design note were found.
Implementation Plan Alignment Review
Slice 1 — Mermaid Diagram Client Component: Completed. src/components/blog/mermaid-diagram.tsx implements all planned functionality: dynamic import of Mermaid, useEffect-based rendering, three states (loading/rendered/error), accessible container, overflow scroll container, unique ID generation via useId().
Slice 2 — ReactMarkdown Integration: Completed. src/app/blog/(post)/[slug]/page.tsx passes MermaidCodeBlock as the code component via ReactMarkdown's components prop. The rehype-sanitize schema is extended to preserve className on code elements. An additional src/components/blog/mermaid-code-block.tsx was created to separate the code-fence detection logic from the diagram renderer — this is a minor structural refinement beyond the plan that improves separation of concerns and is within scope.
Slice 3 — Dependency Installation: Completed. mermaid@11.13.0 added to src/package.json and lockfile updated.
Slice 4 — Tests: Completed. 5 unit tests and 3 integration tests created at the planned file paths. All tests pass. All 114 existing tests continue to pass (no regressions).
Step 6 — Full suite validation: Completed. The build run report records: typecheck pass, lint pass (no new errors), 122 tests pass (8 new + 114 existing), Next.js build pass (253 pages generated).
Scope: The implementation stays within the approved scope. No out-of-scope features were added.
Deviation: The plan described a single custom code component function defined in the blog post page file. The implementation instead created a separate MermaidCodeBlock component in its own file. This is a minor structural improvement that improves reusability and separation of concerns. It does not affect functionality and is a reasonable refinement.
Test Coverage Review
Tests added:
src/app/tests/unit/mermaid-diagram.test.tsx — 5 unit tests covering loading state, valid render, error state, accessible container (role="img"), and scrollable container (tabindex).
src/app/tests/integration/blog-mermaid.test.tsx — 3 integration tests covering blog post rendering pipeline, non-Mermaid code fence preservation, and full ReactMarkdown pipeline integration with mixed content.
Alignment with acceptance criteria: Unit tests address AC 1, 6, 7 and design requirements (accessibility, loading state). Integration tests address AC 1, 3, 4, 5, and 11. Coverage is thorough.
Test quality: Tests are behavior-focused, using screen.getByTestId, screen.getByRole, and screen.getByText queries. Mermaid is properly mocked in the test environment (jsdom). Tests verify user-visible outcomes rather than implementation internals. The integration test for the full ReactMarkdown pipeline is particularly strong, testing the complete content pathway with real markdown input.
Gaps: No E2E (Playwright) test was added. The implementation plan listed E2E as "optional stretch." Given that the integration tests exercise the full ReactMarkdown pipeline with real components and the build produced all pages successfully, this is acceptable. Mermaid requires a real browser DOM for actual SVG rendering, which is correctly mocked in the Vitest environment. A future E2E test could provide additional confidence but is not blocking.
Build validation evidence:
Lint: Pass (no new errors; 3 pre-existing errors noted, unrelated to T014)
E2E tests: Not recorded in build report (not blocking)
Issues Identified
No blocking or non-blocking issues found.
The implementation is complete, well-tested, and aligned with all upstream artifacts. Code is clean, idiomatic, and follows project conventions.
Governance / Process Issues
No governance or process issues found.
Run state is valid and correctly references the QA stage with the correct run ID.
All four upstream artifacts exist at their canonical paths.
The build run report documents complete validation evidence (typecheck, lint, tests, build).
Run IDs follow the standard naming convention.
The task lifecycle is in the correct position for QA review.
Recommended Follow-up Actions
Proceed to Awaiting Approval. All acceptance criteria are satisfied, the implementation is aligned with the specification, design note, and implementation plan, tests are comprehensive, and build validation is clean. The feature is ready for human review.
