Scaffold a new MCP tool definition. Use when the user asks to add a tool, create a new tool, or implement a new capability for the server.
Installation
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Tools use the tool() builder from @cyanheads/mcp-ts-core. Each tool lives in src/mcp-server/tools/definitions/ with a .tool.ts suffix. The standard registration pattern uses a definitions/index.ts barrel that collects all tools into an allToolDefinitions array for createApp(). Fresh scaffolds from init start with direct imports in src/index.ts — the barrel is introduced as definitions grow. Match the pattern already used by the project you're editing.
Steps
Gather the tool's name, purpose, and input/output shape from the user's request — ask only if genuinely absent
Determine if long-running — if the tool involves streaming, polling, or
multi-step async work, it should use task: true
Create the file at src/mcp-server/tools/definitions/{{tool-name}}.tool.ts
Register the tool in the project's existing createApp() tool list (directly in src/index.ts for fresh scaffolds, or via a barrel if the repo already has one)
Run bun run devcheck to verify — if Biome reports formatting issues, run bun run format to auto-fix, then re-run devcheck
Smoke-test with bun run rebuild && bun run start:stdio (or start:http)
Naming
Tools use lowercase snake_case with a canonical server/domain prefix: {server}_{verb}_{noun} — 3 words.
The server prefix uses the canonical platform/brand name, not an abbreviation (patentsview_ not patents_, clinicaltrials_ not ct_). When a name resists the schema — can't pick a verb, noun feels generic, wants 4+ segments — that's usually a signal the scope is fuzzy; split the tool, rename, or reconsider.
For shape selection (Workflow or Instruction variants — standard single-action tools are the default), see the design-mcp-server skill's Tool shapes section.
Template
/**
* @fileoverview {{TOOL_DESCRIPTION}}
* @modulemcp-server/tools/definitions/{{TOOL_NAME}}
*/import { tool, z } from'@cyanheads/mcp-ts-core';
import { JsonRpcErrorCode } from'@cyanheads/mcp-ts-core/errors';
exportconst {{TOOL_EXPORT}} = tool('{{tool_name}}', {
title: '{{TOOL_TITLE}}',
// Single cohesive paragraph — pack operational guidance into prose sentences,// not bullet lists or blank-line-separated sections. Descriptions render inline.description: '{{TOOL_DESCRIPTION}}',
annotations: { readOnlyHint: true },
input: z.object({
// All fields need .describe(). Only JSON-Schema-serializable Zod types allowed.
}),
output: z.object({
// All fields need .describe(). Only JSON-Schema-serializable Zod types allowed.
}),
// Agent-facing context on the success path — empty-result notices, the query as// the server parsed it, pagination totals. The counterpart to errors[]: merged// into structuredContent AND mirrored into content[] automatically (no format()// entry needed, never touched by format-parity). Populate via ctx.enrich(...) in// the handler or service layer. Keys must be disjoint from output. Delete if unused.enrichment: {
effectiveQuery: z.string().describe('The query as the server parsed it.'),
totalCount: z.number().describe('Total matches before any limit was applied.'),
},
// auth: ['tool:{{tool_name}}:read'],// Each entry declares a domain-specific failure mode and types// `ctx.fail(reason, …)` against the declared union. Baseline codes// (InternalError, ServiceUnavailable, Timeout, ValidationError,// SerializationError) bubble freely — only declare domain-specific reasons.// Delete this block if no domain failures apply.//// Keep contracts inline on this tool, even when other tools have similar// entries. The contract is part of the tool's documented public surface —// don't extract a shared `errors[]` constant; per-tool repetition is the// intended cost of self-contained tool defs.//// `recovery` is required (≥ 5 words) — it's the agent's next move when this// failure fires. Forcing function for thoughtful guidance: placeholders like// "Try again." get flagged by the linter. The contract `recovery` is the// single source of truth for what flows to the wire — opt in at the throw// site by spreading `ctx.recoveryFor('reason')` into the `data` arg.errors: [
{ reason: 'queue_full', code: JsonRpcErrorCode.RateLimited,
when: 'Local queue at capacity.', retryable: true,
recovery: 'Wait a few seconds before retrying or reduce batch size.' },
],
asynchandler(input, ctx) {
ctx.log.info('Processing', { /* relevant input fields */ });
// Pure logic — throw on failure, no try/catch.// With an `errors[]` contract: `throw ctx.fail('reason_id', message?, data?)`.// Without: throw via factories (`notFound`, `validationError`, …) or plain `Error`.const items = awaitsearch(input);
if (queue.full()) {
// Static recovery — resolve from the contract via ctx.recoveryFor('reason').// Single source of truth: the string lives in errors[] above; this spread// pulls it onto the wire so format()-only clients see the recovery hint.throw ctx.fail('queue_full', undefined, { ...ctx.recoveryFor('queue_full') });
}
// Surface what the agent reasons with — echoed query, true total — on BOTH// client surfaces, with no format() plumbing. An empty result is a notice,// not a throw: reserve ctx.fail for genuine failures (queue full, upstream down).
ctx.enrich.echo(input.query);
ctx.enrich.total(items.length);
if (items.length === 0) {
ctx.enrich.notice(`No items matched "${input.query}". Try broader terms or check the spelling.`);
}
return { items };
},
// format() populates MCP content[] — the markdown twin of structuredContent.// Different clients read different surfaces (Claude Code → structuredContent,// Claude Desktop → content[]), so both must carry the same data.// Enforced at lint time: every field in `output` must appear in the rendered text.format: (result) => {
constlines: string[] = [];
// Render each item with all relevant fields — not just a count or title.// A thin one-liner (e.g., "Found 5 items") leaves the model blind to the data.for (const item of result.items) {
lines.push(`## ${item.name}`);
lines.push(`**ID:** ${item.id} | **Status:** ${item.status}`);
if (item.description) lines.push(item.description);
}
return [{ type: 'text', text: lines.join('\n') }];
},
});
Task tool variant
Add task: true and use ctx.progress for long-running operations:
exportconst {{TOOL_EXPORT}} = tool('{{tool_name}}', {
description: '{{TOOL_DESCRIPTION}}',
task: true,
input: z.object({ /* ... */ }),
output: z.object({ /* ... */ }),
asynchandler(input, ctx) {
// ctx.progress is guaranteed non-null when task: true — the ! assertion is safe here.await ctx.progress!.setTotal(totalSteps);
for (const step of steps) {
if (ctx.signal.aborted) break;
await ctx.progress!.update(`Processing: ${step}`);
// ... do work ...await ctx.progress!.increment();
}
return { /* output */ };
},
});
If the repo already uses src/mcp-server/tools/definitions/index.ts, update that barrel instead of switching patterns midstream.
Feature-flagged tools (disabledTool wrapper)
When a tool is gated behind config (e.g., BRAPI_ENABLE_WRITES, FOO_PRO_FEATURES), the gate has two failure modes when wired naively. Excluding the tool from the array hides it from MCP registration and from the HTTP landing page — operators see a smaller catalog than the README documents and have no in-page hint that the tool exists at all. Always registering it lets clients call the tool and forces handler-side forbidden throws, which keeps the dangerous surface in the LLM's reach.
disabledTool() resolves this: the wrapped tool is present in the manifest and rendered on the landing page (muted card, with a reason and an optional hint for how to enable it), but skipped during MCP server registration so clients cannot call it.
DisabledMetadata shape: { reason: string; hint?: string; since?: string }. The reason renders as a sentence on the disabled card; hint (when present) renders as a code-styled block — use whatever the gate is (env var line, config key, doc reference). since annotates the card with a small "since vX" tag — useful when phasing a tool out behind a flag before removal.
Yes, with disabled field — discovery agents see them as present-but-uncallable
/ (HTML landing page)
Yes, in a 4th muted bucket after read | write | destructive
The wrapper composes with both standard and task tools, and preserves all original definition fields (handler, schemas, auth scopes, error contracts) — when re-enabled, the tool already conforms to every lint rule.
Tool Response Design
Tool responses are the LLM's only window into what happened. Every response should leave the agent informed about outcome, current state, and what to do next. This applies to success, partial success, empty results, and errors alike.
Agent-facing context belongs in enrichment
Empty-result notices, the query/filter as the server parsed it, pagination totals — the context an agent reasons with, as opposed to the domain payload itself — must reach both client surfaces: structuredContent (from output) and content[] (from format()). Hand-authored into format() text alone, this context reaches content[] but is invisible to structuredContent-only clients (Claude Code, MCP-SDK API callers).
Declare it as an enrichment block — the success-path counterpart to errors[] — and populate it via ctx.enrich(...) (or the kind-tagged helpers ctx.enrich.notice() / .total() / .echo()). The framework merges enrichment into structuredContent, advertises output.extend(enrichment) as the tool's outputSchema, and mirrors it into a content[] trailer — both surfaces, no format() entry, never touched by format-parity. ctx.enrich lives on the base Context (like ctx.log), so the service layer can populate it too.
enrichment: {
effectiveQuery: z.string().describe('The query as the server parsed it.'),
totalCount: z.number().describe('Total matches before the limit.'),
notice: z.string().optional().describe('Guidance when nothing matched.'),
},
asynchandler(input, ctx) {
const res = awaitsearch(input.query, input.limit);
ctx.enrich.echo(res.parsed); // → structuredContent.effectiveQuery + "Query: …" trailer
ctx.enrich.total(res.total); // → structuredContent.totalCount + "N total" trailerif (res.items.length === 0) ctx.enrich.notice(`No matches for "${input.query}".`);
return { items: res.items }; // enrichment never rides in the domain return
},
A required enrichment field the handler never populates fails the effective-output parse — surfacing the bug rather than dropping it silently. Enrichment keys must be disjoint from output keys (lint-enforced). The sections below are applications of this rule.
Trailer rendering is a per-field call. Each field's content[] trailer line resolves as: its kind-tag if set (notice/total/echo/delta), else the definition's per-field enrichmentTrailer.render/label, else the generic **key:** value (objects/arrays JSON.stringify'd). A structured (object/array) field with no render ships as a one-line JSON blob — the enrichment-trailer-render lint rule errors on that. Give it a renderer, or a label to relabel a scalar key:
enrichment: {
totalFound: z.number().describe('Matches before the page limit.'),
appliedFilters: z.object({ /* … */ }).describe('Filters the server applied.'),
},
enrichmentTrailer: {
totalFound: { label: 'Total Found' }, // → "**Total Found:** 2990"appliedFilters: { render: (f) =>`### Filters\n- Range: ${f.dateRange}` }, // markdown, not JSON
},
structuredContent always keeps the full structured value; enrichmentTrailer only controls the human-facing content[] line.
Image / audio output belongs in ctx.content
When a tool produces image or audio bytes for the calling model to see or hear — a rendered chart, a generated frame, synthesized speech — emit them via ctx.content, not an output field. ctx.content.image(data, mimeType) / .audio(data, mimeType) prepend a content block to content[] after format() runs and never write to structuredContent, so the base64 is carried once instead of duplicating into the typed output. Like ctx.enrich, it lives on the base Context and is callable from the service layer.
asynchandler(input, ctx) {
const png = awaitrender(input.spec); // base64 PNG
ctx.content.image(png, 'image/png'); // → content[] block, not structuredContentreturn { width: input.spec.w, height: input.spec.h }; // typed result stays small
},
The alternative — declaring previewData: z.string() in output and emitting the block from format() — ships the bytes twice (once in structuredContent, once in the block). Reserve output for data the agent reasons over; route raw media through ctx.content. Test with getContentBlocks(ctx). Full reference: skills/api-context § ctx.content.
Capped lists must disclose truncation
When a tool accepts a cap-like input (limit, per_page, page_size, max_results, max_items) and returns an array, disclose when the cap was hit — the agent otherwise treats a partial set as complete.
The one-liner: ctx.enrich.truncated({ shown, cap }). Declare the fields in the enrichment block:
enrichment: {
truncated: z.boolean().describe('True when the list was capped at the limit.'),
shown: z.number().describe('Number of items returned.'),
cap: z.number().describe('The limit that was applied.'),
},
asynchandler(input, ctx) {
const items = awaitfetchItems(input.limit);
if (items.length >= input.limit) {
ctx.enrich.truncated({ shown: items.length, cap: input.limit });
}
return { items };
},
Alternatively, if the upstream total is known, ctx.enrich.total(n) (writes totalCount) also satisfies the lint rule.
Threshold bound — when the upstream total is unknowable but the list is sorted by the cap key, the smallest shown value is a rigorous upper bound on all omitted items (Fagin Threshold Algorithm). Pass it as ceiling:
// items is sorted descending by count; anything hidden has count ≤ items.at(-1).count
ctx.enrich.truncated({
shown: items.length,
cap: input.limit,
ceiling: items.at(-1)?.count,
guidance: 'Narrow with filters or raise per_page (max 200).',
});
Declare truncationCeiling: z.number().optional() in the enrichment block to surface it. The capped-list-no-truncation lint rule warns when this disclosure is absent — see api-linter.
Communicate filtering and exclusions
If the tool omitted, truncated, or filtered anything, say what and how to get it back. Silent omission is invisible to the agent — it can't act on what it doesn't know about.
output: z.object({
items: z.array(ItemSchema).describe('Matching items (up to limit).'),
totalCount: z.number().describe('Total matches before pagination.'),
excludedCategories: z.array(z.string()).optional()
.describe('Categories filtered out by default. Use includeCategories to override.'),
}),
Batch input and partial success
When a tool accepts an array of items, some may succeed while others fail. Report both — don't silently return successes and swallow failures.
// Output schema — design for per-item resultsoutput: z.object({
succeeded: z.array(ItemResultSchema).describe('Items that completed successfully.'),
failed: z.array(z.object({
id: z.string().describe('Item ID that failed.'),
error: z.string().describe('What went wrong and how to resolve it.'),
})).describe('Items that failed with per-item error details.'),
}),
// Handler — collect results, don't throw on individual failuresasynchandler(input, ctx) {
constsucceeded: ItemResult[] = [];
constfailed: { id: string; error: string }[] = [];
for (const id of input.ids) {
try {
succeeded.push(awaitprocessItem(id));
} catch (err) {
failed.push({ id, error: err instanceofError ? err.message : String(err) });
}
}
return { succeeded, failed };
},
Note on the try/catch: this is the deliberate exception to the "logic throws, framework catches" rule. Per-item isolation is the whole point of partial-success batch tools — one failed item must not abort the batch, and the framework's partial-success telemetry (below) depends on seeing a populated failed array. Don't remove it to conform to the handler-level rule.
Single-item tools don't need this — they either succeed or throw. The partial success question only arises with array inputs.
Telemetry: The framework automatically detects this pattern — when a handler result contains a non-empty failed array, the span gets mcp.tool.partial_success, mcp.tool.batch.succeeded_count, and mcp.tool.batch.failed_count attributes. No manual instrumentation needed.
Empty results need context
An empty array with no explanation is a dead end. Echo back the criteria that produced zero results and suggest how to broaden. This is the canonical enrichment case — a notice is agent-facing context, not domain payload, and an empty result is a notice, not a throw:
// 1. Declare the notice as enrichment — reaches structuredContent AND content[],// no output field, no format() entry, no format-parity concern.enrichment: {
notice: z.string().optional()
.describe('Recovery hint when results are empty — echoes filters and suggests how to broaden.'),
},
// 2. Handler — populate via ctx.enrich.notice() when the result is empty.asynchandler(input, ctx) {
const results = awaitsearch(input);
if (results.length === 0) {
ctx.enrich.notice(
`No items matched status="${input.status}" in project "${input.project}". `
+ `Try a broader status filter or verify the project name.`,
);
}
return { items: results, totalCount: results.length };
},
The notice lands in structuredContent.notice and renders as a content[] blockquote automatically — both surfaces, zero format() plumbing.
Mutator response design
Mutators (write/update/delete/append/patch verbs, or destructiveHint: true) surface raw pre- and post-mutation observable state — not a synthetic verdict. The server can detect anomalies but can't classify them as problems; only the agent knows whether file shrunk is intentional truncation or a bug.
output: z.object({
path: z.string().describe('Resolved target path.'),
created: z.boolean().describe('True when the operation created a new target.'),
previousSizeInBytes: z.number().describe('Byte size before the mutation. Zero when created is true.'),
currentSizeInBytes: z.number().describe('Byte size after the mutation. Equals previous when no-op.'),
}),
The agent reads created: true, previousSizeInBytes: 0, currentSizeInBytes: 68 and knows: brand new target, the full file content is the body. If that matches intent, fine; if not (typo path, uninitialized periodic note), the agent self-corrects without re-fetching. Anti-pattern: server-side >= integrity throws on mutators — the server can't distinguish intentional shrink from bug, so it throws on every shrink, including the deliberate ones.
When the before/after is agent-facing context rather than primary payload, the enrichment-native form is ctx.enrich.delta({ field, before, after }) — it writes { before, after } to structuredContent and renders **field:** before → after in the content[] trailer. Declare the field in the enrichment block as z.object({ before, after }); the linter recognizes the shape, so it needs no custom enrichmentTrailer.render. Same stance — surface raw state, never a verdict:
enrichment: {
sizeInBytes: z.object({
before: z.number().describe('Byte size before the mutation.'),
after: z.number().describe('Byte size after the mutation.'),
}).describe('Raw size before/after — the agent judges whether a shrink was intended.'),
},
// handler:
ctx.enrich.delta({ field: 'sizeInBytes', before: prev, after: next });
Sparse upstream data must stay honest
When tool output comes from a third-party API, don't overstate certainty. Upstream systems often omit fields entirely; the tool schema and format() should preserve that uncertainty instead of collapsing it into fake false, 0, or empty-string facts.
Guidance:
Use optional output fields when the upstream source is sparse.
Render unknown values explicitly (Not available, Unknown) instead of inventing a concrete value.
Only render booleans, badges, counts, and summary facts when they are actually known.
output: z.object({
repos: z.array(z.object({
id: z.string().describe('Repository ID.'),
name: z.string().describe('Repository name.'),
archived: z.boolean().optional()
.describe('Archived status when provided by the upstream API. Omitted when unknown.'),
stars: z.number().optional()
.describe('Star count when provided by the upstream API. Omitted when unknown.'),
})).describe('Repositories returned by the search.'),
}),
format: (result) => [{
type: 'text',
text: result.repos.map((repo) => [
`## ${repo.name}`,
`**ID:** ${repo.id}`,
typeof repo.archived === 'boolean'
? `**Archived:** ${repo.archived ? 'Yes' : 'No'}`
: '**Archived:** Not available',
repo.stars != null
? `**Stars:** ${repo.stars}`
: '**Stars:** Not available',
].join('\n')).join('\n\n'),
}],
Error classification and messaging
Recommended: declare an errors[] contract. A typed contract surfaces in tools/list and gives the handler a typed ctx.fail(reason, …) keyed by the declared reason union — TypeScript catches ctx.fail('typo') at compile time, data.reason is auto-populated and tamper-proof, and the linter enforces conformance against the handler body.
import { JsonRpcErrorCode } from'@cyanheads/mcp-ts-core/errors';
exportconst fetchArticles = tool('fetch_articles', {
description: 'Fetch articles by PMID.',
errors: [
{ reason: 'no_pmid_match', code: JsonRpcErrorCode.NotFound,
when: 'None of the requested PMIDs returned data.',
recovery: 'Try pubmed_search_articles to discover valid PMIDs first.' },
{ reason: 'queue_full', code: JsonRpcErrorCode.RateLimited,
when: 'Local request queue at capacity.', retryable: true,
recovery: 'Wait 30 seconds and retry, or reduce batch size.' },
],
input: z.object({ pmids: z.array(z.string()).describe('PMIDs to fetch') }),
output: z.object({ articles: z.array(ArticleSchema).describe('Resolved articles') }),
asynchandler(input, ctx) {
// Static recovery — ctx.recoveryFor pulls the contract recovery onto the wire.// The contract is the single source of truth; this spread surfaces it on the// wire so format()-only clients see the hint mirrored into content[] text.if (queue.full()) throw ctx.fail('queue_full', undefined, { ...ctx.recoveryFor('queue_full') });
const articles = awaitfetch(input.pmids);
if (articles.length === 0) {
// Dynamic recovery — interpolate runtime context, override the contract default.throw ctx.fail('no_pmid_match', `No data for ${input.pmids.length} PMIDs`, {
pmids: input.pmids,
recovery: { hint: `Use pubmed_search_articles to discover valid PMIDs.` },
});
}
return { articles };
},
});
ctx.recoveryFor(reason) resolves the contract's recovery string into the wire shape { recovery: { hint } } — safe to spread into data so format()-only clients see the same recovery hint that structuredContent clients read. Always available on Context (no-op {} when no contract), strictly typed on HandlerContext<R> against the declared reasons. Use it for static recovery; pass { recovery: { hint: \…${dynamic}…` } }` directly when you need runtime context. The contract is the single source of truth — write the recovery once, lint validates it ≥5 words, the resolver carries it to every throw site.
Baseline codes (InternalError, ServiceUnavailable, Timeout, ValidationError, SerializationError) bubble freely and don't need declaring. Wire-level behavior is identical when the contract is omitted, but you lose the type-checked ctx.fail, the tools/list advertisement, and conformance lint coverage — declare a contract whenever the tool has a domain-specific failure mode.
ctx.fail accepts an optional 4th options argument for ES2022 cause chaining: throw ctx.fail('upstream_error', 'Upstream returned 500', { url }, { cause: e }).
Service-layer throws
API-wrapping tools usually delegate to a service: await ncbi.fetch(input, ctx). The throw lives in the service, not the handler. Services accept ctx (the unified Context) so they can call ctx.log, ctx.recoveryFor, etc. The handler doesn't catch — it just bubbles, and the framework's auto-classifier preserves data on the wire.
The contract entry on the tool and the data: { reason } on the service throw need to use the same reason string so the two sides line up. ctx.recoveryFor('reason') resolves the contract recovery from the calling tool's errors[] — same single-source-of-truth pattern that works in handlers.
// service — receives ctx; passes data.reason and spreads ctx.recoveryForimporttype { Context } from'@cyanheads/mcp-ts-core';
import { serviceUnavailable } from'@cyanheads/mcp-ts-core/errors';
exportclassNcbiService {
asyncfetch(pmids: string[], ctx: Context) {
const response = awaitfetchWithRetry(...);
if (!response.ok) {
throwserviceUnavailable(`NCBI returned HTTP ${response.status}`, {
reason: 'ncbi_unreachable',
status: response.status,
...ctx.recoveryFor('ncbi_unreachable'), // resolves from caller's contract
});
}
return response.json();
}
}
// tool — declares the matching contract entry, calls the service, doesn't catchexportconst fetchArticles = tool('fetch_articles', {
errors: [
{ reason: 'ncbi_unreachable', code: JsonRpcErrorCode.ServiceUnavailable,
when: 'NCBI E-utilities is unreachable.', retryable: true,
recovery: 'NCBI is degraded; retry in a few minutes.' },
],
asynchandler(input, ctx) {
return { articles: await ncbi.fetch(input.pmids, ctx) }; // throws bubble unchanged
},
});
ctx.recoveryFor returns {} when the calling tool has no contract or the reason isn't declared, so the spread is always safe — services don't have to know which tool called them.
See add-service for the full pattern.
Ad-hoc factory throws (fallback)
When no contract entry fits — prototype code, one-off throws, or service-layer fallbacks — use error factories or plain throw new Error(). The framework auto-classifies plain Error from message patterns as a last resort.
// Client input error — agent can fix and retryimport { validationError, notFound } from'@cyanheads/mcp-ts-core/errors';
throwvalidationError(`Invalid date format: "${input.date}". Expected YYYY-MM-DD.`);
// Not found — valid input but entity doesn't existthrownotFound(
`Project "${input.slug}" not found. Check the slug or use project_list to see available projects.`
);
// Upstream API — transient, may resolve on retryimport { serviceUnavailable } from'@cyanheads/mcp-ts-core/errors';
throwserviceUnavailable(`arXiv API returned HTTP ${status}. Retry in a few seconds.`);
// Recovery hint via the canonical `data.recovery.hint` shape — the framework// auto-mirrors it into the content[] text as `Recovery: <hint>`, so format()-only// clients (Claude Desktop) see the same guidance that structuredContent clients// (Claude Code) read from `error.data.recovery.hint`. Other `data` keys reach// structuredContent only.import { invalidParams } from'@cyanheads/mcp-ts-core/errors';
throwinvalidParams(
`Date range exceeds 90-day API limit.`,
{
maxDays: 90,
requestedDays: daysBetween,
recovery: { hint: 'Narrow the range or split into multiple queries.' },
},
);
Error messages are recovery instructions. Name what went wrong, why, and what action to take. The message is the agent's only signal — a bare "Not found" is a dead end. See skills/api-errors/SKILL.md for the full contract pattern, factories list, auto-classification table, and error-path parity (how data.recovery.hint reaches both client surfaces).
Include operational metadata
Counts, applied filters, truncation notices, and chaining IDs help the agent decide its next action without extra round trips.
Counts, applied-filter summaries, and query echo that describe the result set (rather than being the result) are textbook enrichment — ctx.enrich.total(n), ctx.enrich.echo(parsedQuery), or ctx.enrich({ appliedFilters }) put them on both client surfaces with no format() entry (a structured field like appliedFilters also needs an enrichmentTrailer.render so its trailer line is markdown, not a JSON blob — see Tool Response Design). Reserve domain output for the payload itself and post-action state (e.g. currentStatus after a write), as below:
return {
commits: formattedCommits,
total: allCommits.length,
shown: formattedCommits.length,
fromRef: input.from,
toRef: input.to,
// Post-write state — saves a follow-up status call
...(input.operation === 'commit' && { currentStatus: awaitgetStatus() }),
};
Seed orientation context when the next moves are predictable. Piggybacking a compact snapshot alongside the primary result — recent activity, tracked state, a few reference items — does two things: cuts a predictable follow-up call and primes the LLM on the project's conventions (recent commits teach the commit-message style the agent should match; recent tags teach the versioning format; reference records teach the naming format). Natural fits include session open/close tools, state-changing verbs where post-action confirmation helps, and entry points that drop the agent into a new scope. Gather sub-operations with Promise.allSettled and surface per-component failures as a warnings array rather than failing the outer call. See design-mcp-server's Output design for the full principle.
Defend against empty values from form-based clients
LLM clients (Claude, Cursor, etc.) only send populated fields. Form-based clients (MCP Inspector, web UIs) submit the full schema shape — optional object fields arrive with empty-string inner values instead of undefined. Zod's .optional() only rejects undefined, so { minDate: "", maxDate: "" } passes validation and reaches the handler.
Don't reject empty strings on optional fields — that punishes form clients for valid MCP behavior. Instead, guard for meaningful values in the handler:
// Schema: keep permissive — accepts empty strings from form clientsinput: z.object({
query: z.string().describe('Search terms'),
dateRange: z.object({
minDate: z.string().describe('Start date (YYYY-MM-DD)'),
maxDate: z.string().describe('End date (YYYY-MM-DD)'),
}).optional().describe('Restrict results to a date range.'),
}),
// Handler: check for meaningful values, not just object presenceasynchandler(input, ctx) {
constparams: Record<string, string> = { query: input.query };
if (input.dateRange?.minDate && input.dateRange?.maxDate) {
params.minDate = input.dateRange.minDate;
params.maxDate = input.dateRange.maxDate;
}
// ...
},
The same applies to optional arrays — use ?.length guards so empty arrays are skipped, not passed through.
Required fields are different. If a string field is required and must be non-empty to be meaningful, .min(1) is correct — the client shouldn't have submitted the form without filling it in.
Match response density to context budget
Large payloads burn the agent's context window. Default to curated summaries; offer full data via opt-in parameters.
Lists: Return top N with a total count and pagination cursor, not unbounded arrays
Large objects: Return key fields by default; accept a fields or verbose parameter for full data
Binary/blob content: Return metadata and a reference, not the raw content
Analytical working sets: When upstream returns more analytical rows (data an agent would SQL — aggregate, group, join) than fit in context, DataCanvas (ctx.core.canvas?, Tier 3 — opt-in via CANVAS_PROVIDER_TYPE=duckdb) lets you register the rows and return the canvas_id plus a preview so the agent can run SQL to slice down without a re-fetch. The spillover() helper (@cyanheads/mcp-ts-core/canvas) automates the overflow case: drain rows up to a character budget for the inline preview, auto-register the full source on overflow, return both as a discriminated union. Two gates: it must be analytical, not a discovery/search surface of categorical metadata (those don't earn a canvas regardless of row count — use MCP-side list filtering or pagination); and a tool emitting a canvas_id MUST be paired with a registered dataframe_query tool, or the handle is unreachable. Compute distributions or refinement hints across the full result — not the preview — so the agent gets honest aggregate signal on the rows it didn't read. See api-canvas for the register / query / export pattern and the spillover flow.
One large document: When a single call returns one document-shaped record (not a row set) that can overflow context, return a section outline — top-level keys + per-section byte size — and let the agent re-call with sections: [...] for only what it needs, instead of truncating one surface. outlineOnOverflow() with OUTLINE_VARIANT / selectSections() / formatOutline() (@cyanheads/mcp-ts-core/utils) measures the payload and returns a full | outline result. Declare the tool's output as a flat z.object with a kind discriminator and presence-based optional arms (fold in OUTLINE_VARIANT.shape.sections / .notice) — tool() rejects a z.discriminatedUnion output — and render each arm on field presence in format() so parity holds. Pure measure + key-slice — Workers-portable, unlike canvas spillover(). Use for one fat record; use spillover() for a row collection. See the techniques skill's outline-on-overflow reference.
MCP-side list filtering
When an upstream API has no native search but the relevant set is bounded (fits one or a few fetches), fetch it in full and filter on the server so an agent resolves a name → opaque ID in one call instead of scanning a blob. The design-mcp-server skill covers when to reach for this (the earns-its-keep gate, the query-vs-local-filter split); this is the how.
Name the local param for the mechanic — filter or nameContains, distinct from an upstream query. Filter the complete set, not the page (fetch up to the cap first). Strict token match is the default — normalize, then require every query token to appear; that handles word order and partials, needs no fuzzy library, and is too small to extract into a shared helper:
constnormalize = (s: string) =>
s.toLowerCase().normalize('NFKD').replace(/[̀-ͯ]/g, '').replace(/[^a-z0-9\s]/g, ' ');
// Filter the full bounded set — not a single page.const tokens = normalize(input.nameContains).split(/\s+/).filter(Boolean);
const hits = items.filter((it) => {
const hay = normalize(it.name);
return tokens.every((t) => hay.includes(t));
});
if (hits.length === 0) {
ctx.enrich.notice(
`No name matched "${input.nameContains}". Call the tool without a filter to browse the full list.`,
);
}
return { items: hits };
Add a fuzzy fallback only when a caller genuinely needs typo tolerance — an LLM caller rarely does. If you do: fire it only when the strict match is empty, score against the best-matching token in each name (not the whole string) and cap the results, and label hits approximate. Test it against a full-scale fixture with a deliberate near-miss — a small fixture has no long-name noise floor, so a unit test won't catch a fallback that returns dozens of bogus matches. A bare "no match — browse the unfiltered list" often beats an approximate guess: it lets the model self-correct rather than commit to the wrong record.
Checklist
File created at src/mcp-server/tools/definitions/{{tool-name}}.tool.ts
Tool name passed to tool() uses snake_case
title field set
annotations set correctly — readOnlyHint: false for write tools, destructiveHint: true for delete/overwrite tools
All Zod schema fields have .describe() annotations
Numeric output fields carry units in the field name (sizeInBytes, durationInMs, priceInCents, latencyInMs) — .describe() may be summarized away or truncated, but the field name persists into the JSON the agent reads. Exempt: dimensionless counts (totalCount, itemCount), indices (index, position)
Schemas use only JSON-Schema-serializable types (no z.custom(), z.date(), z.transform(), z.bigint(), z.symbol(), z.void(), z.map(), z.set())
JSDoc @fileoverview and @module header present
Optional nested objects guarded for empty inner values from form-based clients (check ?.field truthiness, not just object presence)
No console calls — use ctx.log for handler logging
handler(input, ctx) is pure — throws on failure, no try/catch (exception: batch tools with per-item isolation use try/catch inside the loop — that's intentional, don't remove it)
format() renders every field in the output schema — enforced at lint time via sentinel injection, startup fails with format-parity errors otherwise. Different clients forward different surfaces (Claude Code → structuredContent, Claude Desktop → content[]); both must carry the same data. Primary fix: render the missing field in format() (use z.discriminatedUnion for list/detail variants). Escape hatch: if the output schema was over-typed for a genuinely dynamic upstream API, relax it (z.object({}).passthrough()) rather than maintaining aspirational typing
Agent-facing context (empty-result notices, query/filter echo, pagination totals) declared in an enrichment block and populated via ctx.enrich(...) — reaches both structuredContent and content[] automatically, not authored solely in format() text. Enrichment keys disjoint from output keys
If wrapping external API: output schema and format() preserve uncertainty from sparse upstream payloads instead of inventing concrete values
auth scopes declared if the tool needs authorization
errors: [...] contract declared for the tool's domain-specific failure modes — or block deleted if no domain failures apply (baseline codes bubble freely)
Error contract declared inline on this tool — not imported from a shared module, even when other tools have near-identical entries
task: true added if the tool is long-running
If task: true: handler checks ctx.signal.aborted in its loop for cancellation support
If tool returns unbounded arrays: pagination with total count, or spillover() / DataCanvas for analytical working sets (an agent would SQL them — not a discovery/search surface). If any tool emits a canvas_id, a dataframe_query tool is registered in the same server — a token with no query tool is dead output
If tool returns one large document (not a row set) that can overflow context: outlineOnOverflow() returns a full | outline union so the agent re-calls with sections: [...] — not one-sided truncation
If tool is feature-gated: evaluated whether disabledTool() wrapper is appropriate (present in manifest but uncallable)
If the tool filters a bounded list locally (no upstream search): a distinct local param (filter/nameContains, not query), filters the full set (not one page), strict token match by default
Registered in the project's existing createApp() tool list (directly or via barrel)
Test file created via add-test skill, or handler tested directly with createMockContext()
bun run devcheck passes
Smoke-tested with bun run rebuild && bun run start:stdio (or start:http)
