Rendering a Custom MCP Tool Output With a Widget

Coordinate two object-based Custom Lightning Types (CLTs) and an HXL widget to render the output of a custom MCP server tool whose implementation is an Apex @InvocableMethod. This skill never authors content directly — it loads and invokes leaf skills in dependency order, gates progress on user approval, and runs validation gates before reporting completion.

Ownership boundary — this skill owns only:

1. the MCP-tool use case — resolving the tool's output shape and orchestrating the leaf skills in the right order; and
2. the envelope CLT's default renderer.json — the one artifact it authors inline, bridging the envelope to the widget.

The CLTs are authored by platform-custom-lightning-type-generate, and all widget metadata (schema + body + .uiwidget-meta.xml) is authored and validated by platform-widget-generate. This skill never writes widget metadata and never modifies the Apex class — it supplies each leaf skill its inputs and wires the result together.

Scope

Custom MCP server tools backed by an Apex Invocable Action only. The MCP tool returns the platform's invocable-action result envelope — an object with actionName, isSuccess, and an outputValues node that carries the tool's real payload. To render this envelope with a widget, model it as two object-based CLTs (lightning__objectType) of equal standing — the only reason there are two is that one must reference the other by name (a CLT cannot reference itself), so they need distinct deployed names. Name and describe each by what it actually models — never by an invented role-label pair like "Payload CLT"/"Envelope CLT" or "Inner CLT"/"Outer CLT":

• The CLT that mimics the tool-result envelope, named <toolApiName>. Its outputValues property is typed to the other CLT via c__<responseCLT> (the CLT-reference prefix — see the namespace note below).
• The CLT that is the exact shape of the Invocable Action's response (@InvocableVariable fields on the @InvocableMethod response class), named <toolApiName>Response — "Response" is not an invented role word; it is the word the Apex source itself uses for that class.

The widget grounds on the response fields (flat), and the default renderer.json in the envelope CLT bridges the envelope nesting to the flat widget via {!$attrs.outputValues.<field>}.

Namespace prefix provenance (c__ / @apexClassType/<ns>__…): the Actions REST describe returns bare, unprefixed class names (Outer$Inner) and no namespace — the prefix is the org's CLT namespace, added by this skill. It is c__ in a namespace-less org (the common case, used literally throughout this doc) or the package namespace <ns>__ in a packaged org (read from the class's NamespacePrefix; default to c only when the org has none). Every c__ below is this prefix.

Out of scope, route elsewhere:

• Customizing an Apex-backed agent action output (Apex-backed CLT @apexClassType/..., single CLT, surface-specific renderer) → platform-lightning-type-widget-coordinate.
• A standalone widget with no MCP tool / Lightning Type → platform-widget-generate.
• Authoring only a CLT or only an Apex class → platform-custom-lightning-type-generate / platform-apex-generate.

Beta cardinality: the invocable-action result is a bulk array (content[]). For the beta release this skill models and renders a single response — the first element of content[]. The CLT envelope models one result object, not the content[] wrapper.

How this differs from platform-lightning-type-widget-coordinate

Phase Graph

Per-phase pattern: load the skill fresh → execute its workflow → verify outputs → checkpoint before the next phase. Even if you remember a leaf skill's content, skills evolve — always load fresh.

Phase 1 — Input selection

Determine where the payload shape comes from. Prefer the sources top-to-bottom:

Capture the tool API name (used to name all artifacts — see the naming convention below).

Source priority: live/authoritative schema sources beat parsing a local class, which beats a pasted example. In order:

1. action if an action API name and an authenticated org are available. The Actions REST API describe is the same schema the platform itself exposes, so it needs no request/helper filtering and gives real field types.
2. sample if a runtime JSON sample is pasted (runtime response — explicit and current).
3. apex if an Apex class exists locally AND none of the above apply (fallback only — may be stale relative to what's actually deployed behind the action).

If none are available, STOP and ask the user for an action name, a class, a sample, or a schema.

Phase 2 — Payload discovery

FIRST Read references/mcp-tool-output-discovery.md (REQUIRED — do NOT run Phase 2 from this summary alone), then execute the procedure for the chosen source. The reference is authoritative for the full per-source procedures, the field-type mapping tables, and the nested/list handling; the pointers below are only a map to it:

• action (preferred): describe via sf api request rest '/services/data/v<APIVER>/actions/custom/apex/<ActionApiName>' -o <org>; use the outputs array only (ignore inputs — the request wrapper); map each type → CLT lightning:type case-insensitively. An entry with "type": null and an "apexClass": "<Outer>$<Inner>" key is an Apex-class-typed field (not a describe gap): maxOccurs: 1 → single nested object, maxOccurs > 1 → top-level list. If the describe 404s, fall back to sample then apex.
• apex (fallback): locate the class, identify the response class (the @InvocableMethod return List<...> element type), enumerate its @InvocableVariable fields (exclude the request class and private helpers), map Apex type → CLT type.
• sample (fallback): parse the outputValues object; infer each field's lightning:type from its JSON value.

Nested-object and list fields (every source, additive to the flat-primitive case): a field typed as another Apex class — a single object (maxOccurs: 1), a top-level list (maxOccurs > 1), or a list inside a wrapper object (the describe returns one maxOccurs: 1 apexClass output and hides the interior list) — is in scope and is never modeled as a bare {"type":"object"} or an inlined lightning__objectType. Type it @apexClassType/c__<Outer>$<Inner> in the response CLT (never a CLT-level lightning__listType/items), enumerate a referenced/inner class by its public / @AuraEnabled members (Hard Rule 6), and recurse when a leaf is itself a class. The CLT typing and renderer binding depth per shape live in references/mcp-tool-output-discovery.md and references/two-clt-modeling.md ("Top-level list vs list-inside-wrapper") — also Hard Rules 4, 6, 14 — and the examples/nested-object-*-source-prompt.md walkthroughs.

Capture payloadFields — the ordered list of { name, title, lightning:type } that defines the response CLT and the widget schema. Record which source produced it in the build plan.

Staleness: do NOT maintain a cross-session cache. Read the local project fresh and re-retrieve from the org per session.

Phase 3 — Build plan + approval gate

Print a build plan using the template in references/build-plan-format.md. The plan must list:

• A one-line developer-facing summary (the PLAN: line).
• The tool API name and the response class FQN (or "from pasted sample").
• The two CLT names (envelope + payload) and the widget name, with absolute paths.
• The envelope CLT carries exactly actionName (text), isSuccess (boolean), and outputValues (typed c__<responseCLT>, the load-bearing field the renderer bridges through) — actionName/isSuccess are envelope-only and never appear on the widget (Hard Rule 5) — plus the response fields the response CLT + widget will carry.
• The validations that will run after generation.

Print the plan in full, then proceed unless the user's next reply explicitly pushes back. Explicit pushback = no, stop, wait, change X, use Y instead, or an equivalent rejection / revision request. Explicit approval is welcome but NOT required — silence, an unrelated follow-up, or the natural continuation of a single-turn eval all count as implicit approval. The invariant is the plan being visible in the transcript. If pushback arrives, revise and re-print before moving on.

Phase 4 — Generation

Load and invoke leaf skills in this order. For each: load the skill, execute its workflow against the Phase 3 spec, verify the outputs, checkpoint before the next.

1. Response CLT — load platform-custom-lightning-type-generate. Author an object-based CLT <responseCLT> (convention: <toolApiName>Response) whose properties are the payloadFields from Phase 2. Root is lightning__objectType, with root-level "lightning:tags": ["mcp"].

   • The response CLT's top-level properties are 1:1 with the describe's outputs[] names (or, for apex/sample, the response class's @InvocableVariable fields — the same set the describe would surface). A describe with N sibling outputs → N flat properties; a describe with one output → one property named after that output. A single-property CLT is correct only when the describe itself returns a single output.
   • Never collapse multiple sibling outputs into one invented wrapper property. Naming a lone property to hold several outputs invents a key that is in no describe output and is unresolvable under the action source — the response-class name never appears in outputs[]. Each response CLT property name must trace to a describe output name (see field-trace / clt-reference-integrity).
   • A top-level list output is typed directly to its element class (@apexClassType/c__<Outer>$<ElementClass>), not wrapped — see "List of nested objects" above.

2. Envelope CLT — load platform-custom-lightning-type-generate. Author an object-based CLT <toolCLT> (convention: <toolApiName>, the envelope), also with root-level "lightning:tags": ["mcp"], and:

   • actionName → lightning__textType
   • isSuccess → lightning__booleanType
   • outputValues → c__<responseCLT> (the referenced-CLT pattern; the response CLT must be deployed before the envelope CLT)

3. Widget — load platform-widget-generate. Author a flat widget whose schema.json properties are the payloadFields (name + primitive type) — a standalone widget contract, not derived from or coupled to any Lightning Type. It renders only outputValues data fields: never actionName/isSuccess (envelope-only). The widget body binds each field via {!$attrs.<field>} — the widget is envelope-agnostic and never references outputValues itself.

4. Default renderer (authored inline in the ENVELOPE CLT — never optional). FIRST Read platform-custom-lightning-type-generate/references/widget-rendition.md (REQUIRED — do NOT author from memory or copy an existing sample, which may use a deprecated shape). Then author <pkgDir>/lightningTypes/<toolCLT>/renderer.json — the default renderer, at the bundle root, parallel to schema.json (NOT under lightningDesktopGenAi/). Its renderer.componentOverrides["$"] sets definition to @widget/c/<widgetName> and maps every widget schema property to the matching payload field nested under the envelope's outputValues node via {!$attrs.outputValues.<payloadField>} — the nested binding that bridges the envelope CLT to the flat widget. Do NOT duplicate the widget body inside renderer.json. The worked JSON is in references/two-clt-modeling.md.

Existing-renderer handling: if renderer.json already exists at the target path, read it first. If it references the same widget with the same bindings, leave it. If it references a different widget or a custom-LWC root override (c/<component>), STOP and surface the conflict before overwriting.

Phase 5 — Validation gates

Read references/validation-gates.md (REQUIRED — it holds the RUN procedure and exact pass/fail predicates for each gate) and run every gate. Widget-bundle-internal checks (schema parse, root keys, leaf types, {!$attrs.X} resolution, .uiwidget-meta.xml well-formedness) are owned by platform-widget-generate and run in its own self-validation.

Hard — block on failure:

1. clt-reference-integrity — envelope outputValues typed c__<responseCLT>, the response CLT exists and both parse, no $schema/items, every non-primitive response property an @apexClassType/... reference (never a bare object or CLT-level lightning__listType).
2. renderer-wires-widget — the bundle-root renderer.json wires @widget/c/<widgetName> and binds every widget property under outputValues at the depth the describe dictates (two segments for a top-level primitive or list output, three for a leaf/list inside a wrapper object). Bidirectional: missing or extra bindings both fail.
3. nested-list-coverage — every nested-object and list field discovered in Phase 2 is rendered by the widget and bound at the correct depth. "Out of scope" / "beta single-response" are NOT valid drop rationales.

Warn — advisory:

1. field-trace — RUN the trace in references/validation-gates.md: enumerate response fields, jq the widget schema keys, classify INVENTED vs OMITTED. Invented widget fields fail; an omitted response data field warns.

Report each gate result by name in Phase 6 (pass, fail (<reason>), warn (<reason>), not run). Do not summarize as "all passed". This skill produces metadata only — it does not deploy; deployment is the caller's responsibility.

Phase 6 — Summary

MCP Tool Widget Build Complete: <widgetName>

FILES GENERATED:
  Response CLT:
    <pkgDir>/lightningTypes/<responseCLT>/schema.json
  Envelope CLT:
    <pkgDir>/lightningTypes/<toolCLT>/schema.json
    <pkgDir>/lightningTypes/<toolCLT>/renderer.json          # default renderer — wires the widget
  Widget bundle:
    <pkgDir>/uiWidgets/<widgetName>/<widgetName>.json
    <pkgDir>/uiWidgets/<widgetName>/schema.json
    <pkgDir>/uiWidgets/<widgetName>/<widgetName>.uiwidget-meta.xml

VALIDATIONS:
  widget self-validation (platform-widget-generate gates): <pass | fail — see sub-skill report>
  clt-reference-integrity (envelope.outputValues → c__<responseCLT>; nested → @apexClassType): <pass | fail (<reason>)>
  renderer-wires-widget (nested {!$attrs.outputValues.X} bindings): <pass | fail (<reason>)>
  nested-list-coverage (every discovered nested/list field rendered): <pass | fail (<reason>)>
  field-trace (INVENTED + OMITTED lists printed): <pass | warn (<reason>) | fail (invented: <list>)>

Hard Rules (always apply)

1. Plan-first, then proceed. Print the full Phase 3 build plan before writing any file. Explicit rejection or a change request → stop and revise; otherwise continue. The invariant is the plan being visible in the transcript, not an interactive human approval — this holds in manual chat, agent-to-agent flows, and single-turn evals.
2. Two object-based CLTs, never one. The envelope and the payload are separate CLTs. The envelope's outputValues is typed via c__<responseCLT>, never inlined as a nested lightning__objectType. Both CLTs carry root-level "lightning:tags": ["mcp"] (see platform-custom-lightning-type-generate/assets/primitive-types-and-constraints.md).
3. Renderer lives in the ENVELOPE CLT, at the bundle root. lightningTypes/<toolCLT>/renderer.json — the default renderer, parallel to schema.json. Never lightningDesktopGenAi/renderer.json (that is the agent-action flow's surface-specific path), never in the response CLT.
4. Renderer bindings are nested. Every widget attribute maps to {!$attrs.outputValues.<field>}, not {!$attrs.<field>}. The widget schema stays flat; the renderer does the bridging.
5. Widget grounds on the payload, not the envelope. The widget schema properties are the payload (outputValues) fields. The widget never references actionName or isSuccess — those are envelope-only fields on the envelope CLT.
6. Field source depends on class role. The top-level response class is gated by @InvocableVariable — the describe surfaces exactly those fields, so a top-level field carrying only @AuraEnabled (or no annotation) is correctly not an output; enumerate the top-level response class by @InvocableVariable and exclude the request class and private helper classes. An inner/referenced class reached via @apexClassType/<ns>__<Outer>$<Inner> is enumerated by its public / @AuraEnabled members instead — its leaves are never in the describe and @InvocableVariable is not used there, so grepping it for @InvocableVariable yields zero leaves (empty CLT + widget).
7. No invented fields. The widget schema (and the response CLT) must be a subset of the fields the tool actually exposes — top-level @InvocableVariable outputs plus the public/@AuraEnabled leaves of any referenced inner class — never a property no class exposes. field-trace prints both lists.
8. Single response for beta. Model one result object, not the content[] bulk wrapper.
9. Always load the leaf skill before generation. Do not author from memory.
10. Run gates, do not describe them. Reporting pass without executing a gate is a hard violation; report not run instead.
11. No shell metacharacters that trigger the Vibes safe-shell filter. In every Bash tool call emitted by this orchestrator and by any leaf skill it invokes, do NOT use command substitution ($(…) or backticks), process substitution (<(…), >(…)), brace expansion ({a,b,c} or {1..N}), or eval / exec. These force manual approval even under Bypass mode and stall the eval. Run separate commands (mkdir -p a && mkdir -p b), print each intermediate value with its own command and reason about the result, and use plain shell variables (X=literal) or here-strings when a value must be reused.
12. Resolve action schema from the Actions REST API describe, never from raw HTTP to the MCP endpoint or from credential extraction. Use sf api request rest against the org's Actions REST API (which uses the existing sf org auth). Never read a4d_mcp_settings.json or any MCP settings file, never extract an org access token, never curl an MCP server endpoint directly — that requires credentials the session doesn't have and targets a URL the runtime doesn't actually expose that way.
13. Never invoke an MCP tool to discover its output shape. Describing the payload must never execute the underlying action. Resolve the schema via the Actions REST API describe of the backing action — never by calling the tool with sample/guessed input to observe a response. If no action name is resolvable, ask the user for a pasted sample instead of invoking anything.
14. A response field typed as another Apex class is never a bare {"type":"object"}. Type it @apexClassType/<ns>__<OuterClass>$<InnerClass> in the response CLT, flatten to its leaf fields in the widget, and bind the renderer two levels deep ({!$attrs.outputValues.<objectField>.<leaf>}). This is additive to the flat-primitive case (Hard Rule 5), not a replacement for it — see references/two-clt-modeling.md.

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