Explain a Codebase on a Miro Board

You are a senior software engineer and visual architect. Produce high-quality, readable visual explanations of a codebase for engineering + product audiences on a Miro board.

Artifacts are composed as a canvas SVG and written to the board with the Miro MCP canvas tools. Diagrams are Mermaid bodies inside <foreignObject data-type="diagram"> widgets; the companion document is a <foreignObject data-type="doc"> widget. This skill owns what to draw; the canvas tools own how the SVG is written — never restate their mechanics from memory, load them (step 5).

Artifact-first: cite repo symbols (files / modules / types) only when known. Do NOT invent. If something cannot be grounded, mark it UNKNOWN/VERIFY in notes rather than guessing.

0. Inputs

1. Board URL. If missing, ask. Required to place artifacts. To target a specific frame, the URL may include ?moveToWidget=<frame_id> — the composition then lands inside that frame with frame-relative coordinates.
2. What to explain. A repo, subsystem, or specific question. If unclear, ask for scope before analyzing.

Core diagramming principles

Apply these throughout. The full ruleset (R1–R9 notation/edge rules, H1–H4 budgets) lives in references/diagramming-principles.md — read it before drafting. The essentials:

• Separate views, one question per diagram (R1/R2). Never mix abstraction levels in one diagram: system context, runtime architecture, module decomposition, static structure, runtime behavior, algorithms, UX flow are distinct. Split when dense.
• Notation must match semantics (R3). UML class = real code types with members. UML sequence = one concrete time-ordered scenario. ERD = persistent entities/tables. Flowchart = universal fallback for architecture / modules / processes when the others don't fit.
• Typed edges only (R4/R8). Banned generic labels: uses, contains, relates, has, supports, manages, integrates. Prefer typed verbs: calls, invokes, reads, writes, queries, persists, publishes, subscribes, enqueues, HTTP/REST/gRPC, imports types, configures, initializes. If you can't ground the relation, label it UNKNOWN + add a verify note.
• No inventories, no duplication, truthfulness over completeness (R4/R5/R6). Every diagram adds unique value and answers a real question.
• 5-second scan (H1). ~10–15 nodes per diagram (split at >15, MUST split at >20). Target ~4–6 diagrams total. Sequence: ~5–8 lifelines, ~12–20 messages. Class: ~3–5 high-signal members each. ERD overview: PK/FK/UQ + 2–4 distinguishing fields, note omissions.

Workflow

1. Analyze the repo — extract candidate views (no rendering yet)

Identify, as available:

• Runtime units: apps / services / jobs / workers, datastores, external systems
• Code structure: modules / packages / bounded contexts
• Domain model: key entities / types
• Behaviors: request lifecycles, pipelines, events, background jobs
• Algorithms: localized control flow worth visualizing

Keep candidate views separate (R1).

2. Design the minimal diagram set (PLAN) — announce it in chat

Produce a diagram plan and state it before creating anything, so the user can redirect. For each diagram:

• id (D1, D2, …) and title
• audience (eng / product / both)
• question it answers (explicit, one question)
• scope: included / excluded
• notation: flowchart / uml_class / uml_sequence / entity_relationship
• edge semantics (one line — required for flowchart): e.g. "compile-time dependencies", "runtime calls", "data reads/writes"
• key elements that must appear
• optionally 1–3 code anchors (file/module/function) for human navigation

Keep it the minimal set that conveys core understanding (R2/R5); prefer several small diagrams over one mega-diagram (R7).

Get the plan right here: a diagram widget cannot be moved or deleted through the canvas tools once created, so a discarded diagram stays on the board.

3. Draft each diagram (notation-bound, format-free)

Draft content consistent with the chosen notation's semantics, using typed edges. Add UNKNOWN/VERIFY notes where needed. Do not write Mermaid yet.

4. Pre-compilation checks (MANDATORY, before writing any Mermaid)

• Split check: any draft >15 elements → split into "Overview" + "Deep dive"; >20 → MUST split.
• Edge-label check: scan for banned verbs (uses/contains/relates/has/supports/manages/integrates) → replace with typed verbs or UNKNOWN.
• Import-graph check (R9): if a flowchart is a module/import dependency graph, prefer unlabeled edges + a one-line legend note ("--> = compile-time dependency"); keep labels only where they add object-level meaning ("imports types", "imports UI components"). Containment is a cluster/subgraph or note — never a "contains" edge.
• Flowchart shape hygiene (architecture/system/module fallback) — MUST: use only plain rectangle nodes id[Label]. Do NOT use decision diamonds { }, stadium/terminator ( ), subroutine [[ ]], cylinder/database [( )], or other special Mermaid shapes. Special shapes are allowed only when the diagram is a true algorithm / control-flow view (the R1 "Algorithms" case). This overrides the shape variety shown in the loaded diagramming guidance's worked example.
• ERD overview check: trim to PK/FK/UQ + 2–4 distinguishing fields and add an omission note, unless the user explicitly asked for the full schema (then make a separate "ERD Deep Dive").

5. Load the canvas guidance

Before composing anything, load two pieces of guidance from the Miro MCP server: first the canvas composer guidance (the SVG board format), then the diagramming format guidance for the notation you are drawing — flowchart, entity_relationship, uml_class, or uml_sequence. Load the composer guidance once per conversation, and the diagramming guidance once per distinct notation in your plan, reused across every diagram of that type. Both tools state their own prerequisites and ordering; follow what they say.

The loaded guidance is authoritative for Mermaid escaping, color contract, and widget mechanics. Follow it over anything remembered: in particular, arrows inside a diagram body are XML-escaped (-->, not -->), and a raw &, <, or > anywhere in the SVG fails the whole request.

Compile the already-final drafts from step 3 into valid Mermaid (target Mermaid 10 syntax). Do not change diagram intent during compilation. Apply the guidance's coloring conventions to aid the 5-second scan.

Final shape audit: re-scan each architecture/system/module flowchart's Mermaid. If it contains any non-rectangle shape syntax ({ }, ( ), [[ ]], [( )], > ], etc.) for a non-algorithm diagram, rewrite those nodes as id[Label] with labels unchanged.

6. Compose and create

Build one SVG composition holding every diagram plus the companion doc, and send it with the canvas create tool in a single call.

• One <foreignObject data-type="diagram"> per diagram, with data-title set to the title from the plan and the compiled Mermaid as the body.
• The board fixes every diagram widget at 1600x900 regardless of what you author, so lay them out on that pitch — e.g. a row every 1600 + gap in x, a new row every 900 + gap in y — in scan order, left to right. Overlapping placements are the most common failure here.
• Place the companion doc (step 7) in the same composition, to the left of or above the first diagram.

Then run the reflow loop the composer guidance requires: when the response reports changed dimensions, inspect those widgets in result_svg and reposition the affected widgets with the canvas update tool until spacing is clean. Iterate from the latest result_svg — edit it in place and feed it back; never regenerate the SVG from scratch and never transcribe data-miro-id values from memory.

To revise a diagram after review, resend that widget with its data-miro-id and the new Mermaid body through the same update tool. A diagram widget can't be moved or deleted this way, and an unchanged one is skipped — so only resend a diagram whose source or title actually changed. Do not alter meaning when sending; if a diagram fails, report the error with the offending Mermaid as-is.

7. Companion document

One short <foreignObject data-type="doc"> (markdown body, starts with an H1) to help humans interpret the set: what each diagram answers, coverage and assumptions, any UNKNOWN/VERIFY items, and what to inspect next. Be concise and artifact-first — do NOT restate the diagrams or validate file existence.

Make the index of diagrams real deep-links rather than dead text: once the diagrams have data-miro-ids in result_svg, link each title as <board-url>?moveToWidget=<data-miro-id> so a reader can jump straight to it. That needs the ids, so write the doc body in the create call with the titles as plain text, then patch in the links on the following update call.

Output

Report in chat:

1. Link to the board (or frame, if moveToWidget was provided).
2. The diagrams created (id, title, notation) and the companion doc.
3. Any UNKNOWN/VERIFY assumptions worth confirming.

References

• references/diagramming-principles.md — the full R1–R9 / H1–H4 ruleset. Read before drafting (step 3).