Decision protocol for wrapping a REST / GraphQL / RPC API as a tool an LLM
agent can call. The load-bearing premise: the *tool surface* is an
LM-friendly subset of the *API surface* — one tool per user intent, not one
per endpoint. Activates when a coder agent must expose an external HTTP API
to a model (function calling, tool_use, MCP, LangChain `@tool`, CrewAI
`BaseTool`). Encodes the *what to surface, how to name, how to shape, how to
fail* — not any single framework's API. ~80% of agent tools in production are
HTTP wrappers; this is the SOP for getting them right.
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Decision protocol for wrapping a REST / GraphQL / RPC API as a tool an LLM
agent can call. The load-bearing premise: the *tool surface* is an
LM-friendly subset of the *API surface* — one tool per user intent, not one
per endpoint. Activates when a coder agent must expose an external HTTP API
to a model (function calling, tool_use, MCP, LangChain `@tool`, CrewAI
`BaseTool`). Encodes the *what to surface, how to name, how to shape, how to
fail* — not any single framework's API. ~80% of agent tools in production are
HTTP wrappers; this is the SOP for getting them right.
domain
coder-agent / tool-construction
audience
engineers wiring external APIs into LLM agents
trigger_keywords
["wrap an API as a tool","expose REST endpoint to agent","function calling for my API","MCP server for existing API","tool returns too much JSON","agent rate limited / 429","GraphQL / RPC as agent tool"]
when_to_use
["exposing a third-party or internal HTTP API to an LLM agent","deciding which of N endpoints deserve to become tools","an existing tool dumps raw JSON and the model hallucinates fields","tool calls fail on rate limits, timeouts, or pagination","porting the same tool across OpenAI / Anthropic / MCP / LangChain / CrewAI"]
when_not_to_use
["the API is already an MCP server you only consume (just connect)","no external I/O — pure local computation (write a plain function tool)","designing the upstream API itself (that's API design, not tool wrapping)"]
HTTP / External API → Agent Tool · SOP
Source posture: every non-trivial claim is cited inline with short tags like
[oai/fc], [anthropic/tooluse], [lc/tools], [mcp/spec], [apxml/schema].
Resolve them against references/R1-source-evidence.md for full URLs. Reusable
code shapes live in references/R2-pattern-library.md.
1. 何时激活 (When to Activate)
Activate when a coder agent must make an external HTTP API callable by an LLM.
Concrete triggers:
The task says "give the agent access to ", "add a tool that calls
", "wrap our REST/GraphQL/RPC endpoint as a function the model can use".
You are choosing which of N endpoints become tools, or how to name them.
An existing tool returns a huge JSON blob and the model hallucinates field
names, or burns context re-reading it.
Tool calls die on 429, timeouts, or unpaginated list endpoints.
You need the same tool to run under OpenAI function calling, Anthropic
tool_use, an MCP server, LangChain @tool, and CrewAI BaseTool.
Do not activate when: the API is already exposed as an MCP server you merely
consume (just connect it); the "tool" is pure local computation with no network
I/O (write a plain typed function); or you are designing the upstream API itself.
This is a tool-construction skill — sibling to the framework SOPs
(langgraph-sop, crewai-sop) which decide whether/where tools run. Once you
know you need a tool, this skill decides what shape it takes.
2. 核心心智模型 (Core Mental Model)
The tool surface is an LM-friendly subset of the API surface. One tool per
intent, not one per endpoint.
A REST API is designed for programmers who read docs, hold a mental model of
resources, and compose calls. An agent tool is designed for a language model
that sees only a name, a description, and a JSON schema — and must decide,
mid-reasoning, whether this is the thing to call. These are different audiences,
so the surface must be re-cut, not mirrored.
"Tool descriptions are often more important than code comments because the LLM
directly uses them for reasoning." [apxml/schema]
Four load-bearing consequences:
Intent, not CRUD. The unit of a tool is a thing the agent wants to
accomplish (cancel_order, find_customer_by_email), not an HTTP verb on a
resource (DELETE /orders/{id}). One intent may compose several endpoints;
one endpoint may serve zero intents (admin/batch/webhook-out endpoints get
dropped). Surface intent, not the verb table [zuplo/agent-ready].
The schema is the prompt. The model never sees your code. It sees the
tool name, the description, and each field's description=. Every field
needs units, format, enum values, and an example aimed at the model — "if a
field is a date, specify ISO 8601 vs Unix timestamp" [apxml/schema]. A
typed schema (Pydantic / JSON Schema) is non-negotiable because it is both
the validation layer and the documentation the model reads [lc/tools].
The response is context, and context is scarce. A 10 MB JSON payload is
not "data the agent has" — it is tokens the agent must pay for, re-read, and
can misquote. Shape the response down to the fields the agent needs to
reason or act on. Returning raw upstream JSON is the second most common
anti-pattern after 1:1 mapping.
The model cannot promise call discipline. It may emit zero, one, or
several calls — "best practice [is] to assume there are several"
[oai/fc] — retry on its own, or be resumed by the framework. So the
wrapper owns reliability (timeout, retry, rate-limit) and safety
(idempotency on mutations). You cannot prompt these guarantees into existence;
you build them into the tool. (Side-effect safety is deep enough to be its
own skill — cross-link llm-tool-idempotency for any mutating tool.)
The pre-LLM analog: you are writing an SDK for a non-deterministic, amnesiac
junior dev who reads only the function signature — generous docstrings, narrow
typed inputs, small clean returns, and total robustness to being called wrong.
3. SOP 工作流 (Standard Operating Procedure)
Walk top-down. Each step has a gate — if it fails, fix it before adding surface.
Step 1 · Triage: which endpoints deserve to be tools?
List every endpoint × verb. For each, ask: "what user/agent intent does this
serve?" Drop endpoints with no agent-facing intent (internal admin, batch
jobs, outbound webhooks). The MCP guidance is a useful first cut: GET-style
data reads often map to resources; create/update/delete map to tools[gun/mcp]. Target ≤10 surfaced operations for a first pass.
Gate: if you are about to create one tool per endpoint, stop — that is AP-1.
Auto-generated 1:1 servers from an OpenAPI spec "routinely under-perform
hand-curated tools" [stainless/mcp].
Step 2 · Name from intent
Tool name = verb_object describing intent: search_orders, cancel_order,
get_order_status. Notpost_orders_v2, delete_orders_id. Test: a model
that has never seen your API, reading only the name, should guess when to call
it. The name should be a verb; the description should explain when to call,
not how[oai/prompting].
Step 3 · Flatten params into a typed schema
Define a Pydantic model (or JSON Schema). Rules:
Type-annotated fields, each with a model-facing description= (units, format,
enum, example) [lc/tools][apxml/schema].
Flatten the API's wire format: filter[status]=open → status: Literal["open","closed"]. The model should never construct a query-string
fragment.
Explicit required vs optional. Defaults where the API has sensible ones.
Hide pagination/auth/internal knobs from the schema (Steps 5–6).
Gate: every field the model can set has a description=. Untyped **kwargs or
a free-form body: str is a smell — the model will fill it wrong.
Step 4 · Error handling: translate, never leak
Catch HTTPStatusError / ValidationError / network errors. Return a
structured, LM-readable error, never a raw stack trace:
{"error":"rate_limited","message":"...","retryable":true,"hint":"wait and retry"}
Use a small closed set of error codes (not_found, invalid_input,
auth_failed, rate_limited, server_error). LangChain's ToolException
converts a raised error into an LM-visible string for the same reason
[lc/structured]. The model reasons over the error like any other tool output —
give it something it can act on.
Step 5 · Shape the output
Define an output model with only the fields the agent needs. Drop audit
timestamps, internal mirrors, deprecated fields, ETags. Summarize blobs into
strings. Aim for a compact payload per call (rule of thumb: keep it small enough
that re-reading it 5 times in a loop is cheap). For lists, return items + a
next_cursor, not the whole dataset (Step 5b).
Step 5b · Pagination. Default: fetch one page, return items + next_cursor, let the agent decide to continue. Prefer cursor over offset —
"cursor-based pagination is more reliable than offset/limit for agentic
scrolling" [techops/rest]. Auto-loop only when total is small and bounded
(≤200); never loop unbounded — a single agent can "burst 20 sequential API
calls to complete one task" [zuplo/agent-ready] (cross-link bounded-loop skill).
Step 6 · Auth & secrets at the wrapper boundary
Read the key/token from env or a secret store inside the wrapper. Never
expose api_key as a tool parameter and never put a secret in the description —
the model doesn't need it and traces would leak it. Per-tenant tokens flow via a
closure or context object, not via tool args [northflank/mcp].
Gate: grep your tool schema and description for key, token, secret,
password. Zero hits.
Step 7 · Idempotency on mutations
If the tool does POST/PUT/DELETE, it will be retried by the model or the
framework. Generate an idempotency key per logical operation and pass it
(Idempotency-Key header) when the API supports it — the canonical Stripe
pattern [stripe/idem]. Tag the tool metadata mutating=True. For the full
decision tree (key derivation, dedup store, at-least-once vs exactly-once),
defer to the llm-tool-idempotency skill — that is its entire domain.
Action: Read secret inside the wrapper from env/secret store; never a tool
param; per-tenant via closure/context.
Output: Tool that authenticates with no secret in schema.
Evidence: [northflank/mcp].
OP-5 · Timeout + retry + jittered backoff
Trigger: Any outbound HTTP from a tool.
Action: Explicit per-attempt timeout=. Retry only on 429/5xx/network,
max 3–5, exponential backoff with jitter, honor Retry-After. Never retry
other 4xx.
DC-1 · Wide API (50+ endpoints): one mega-tool or many?
Scenario: A CRM API has 60 endpoints. Do you ship 60 tools, or one
crm_operation(operation: str, params: dict) mega-tool?
Trap (mega-tool): A single tool with a free-form operation string and a
dict of params pushes all routing into the model with no schema help. "A
mega-tool with a single instructions string invites hallucinations"
[medium/velorum] — the model invents operation names and param shapes, and the
wrapper can't validate them.
Trap (1:1, 60 tools): Flat catalogs degrade selection accuracy at scale —
beyond ~50 tools, "flat tool-list catalogs degrade selection accuracy;
hierarchical / graph organization helps" [arxiv/toolnet]. The model spends
reasoning budget scanning a wall of near-identical names.
Decision rule:
Triage to the ~10 endpoints with real agent intent (OP-1). Most wide APIs
collapse hard — 60 endpoints, ~8 intents.
If still >~15 after triage, group by sub-domain into a few medium tools,
each with a typedaction: Literal[...] enum (not a free string) plus a
discriminated-union params model. The enum keeps schema validation; the
grouping keeps the catalog short. This is the middle path between 1:1 and
one mega-blob.
Only consider a true mega-tool if the API is genuinely uniform (e.g. a
GraphQL endpoint where the single tool is graphql_query(query, variables)
with a documented schema) — and even then, constrain it.
Verdict: Neither extreme. Triage first, then typed grouping. The win is a
short catalog of validated tools, not raw endpoint count in either direction.
DC-2 · API returns 10 MB JSON: what to expose?
Scenario: get_customer_360 returns a 10 MB document — full order history,
event logs, nested addresses, internal flags.
Trap: Return it whole. The model pays ~2–3M tokens, can't fit it, and will
quote fields that aren't there. Truncating blindly loses the field the agent
needed.
Decision rule:
Ask what the agent will do with this. Usually it needs 5–15 fields, not
2,000. Define a thin output model of exactly those (OP-7).
For the long tails (order history, logs), don't inline them — return a
count + a summary + a follow-up tool: recent_orders_count: int,
last_order_summary: str, and a separate list_customer_orders(cursor) the
agent calls only if it needs more (OP-6 pagination).
For genuinely large text blobs the agent must read, store them and return a
reference/handle the agent can fetch on demand, rather than inlining.
Set a hard per-call byte budget in the wrapper; if the shaped output still
exceeds it, that's a signal the tool is doing too much — split it.
Verdict: Expose a thin reason/act slice; demote bulk to follow-up paginated
tools or references. The tool's job is to give the model enough to decide the
next step, not the whole record.
DC-3 · Async / long-running API (submit job → poll): one tool or two?
Scenario: A report API: POST /reports returns a job_id; you poll
GET /reports/{job_id} until status=done (can take minutes).
Options:
A. One tool that blocks — generate_report() submits then polls
internally until done. Simple mental model for the model, but holds the agent
(and its timeout) hostage for minutes, and a single per-attempt HTTP timeout
can't cover it.
B. Two tools — submit_report() -> job_id and
check_report(job_id) -> status|result. The agent submits, does other work,
polls. Robust to long waits; matches the agent loop; but the model must
remember to poll.
Decision rule:
If the job reliably finishes in seconds and well under one HTTP timeout →
one blocking tool (A) with internal bounded poll + jittered backoff (OP-5).
If it can run minutes+, or you need the agent to stay responsive →
two tools (B). Make check_report return a clear status enum so the model
knows whether to wait, and bound the agent's poll count (bounded-loop skill).
Either way, the submit call is a mutation — give it an idempotency key
(OP-9) so a retried submit doesn't queue two jobs.
Verdict: Match the tool shape to the latency. Sub-second → hide the poll
inside one tool; minutes → split, and make the agent's polling explicit and
bounded.
6. 反模式与边界 (Anti-Patterns & Boundaries)
#
Anti-pattern
Symptom
Fix
AP-1
1:1 endpoint→tool mapping
40+ near-identical tools; model picks wrong one
Triage to intents (OP-1); auto-gen 1:1 "under-performs hand-curated" [stainless/mcp]
Describe when to call, not how; one intent per tool [oai/prompting]
AP-7
Free-form body: str / params: dict
Model fills the wire format wrong
Typed flattened schema (OP-3)
AP-8
Raising stack traces to the model
Model parrots Python tracebacks at the user
Structured {error, retryable, hint} (OP-8)
Hard boundaries — this skill is the wrong frame when:
The API is already an MCP server / first-class SDK with model-friendly
surface — just connect it; don't re-wrap.
The "tool" has no network I/O — write a plain typed function tool.
You own and can change the upstream API — fix the API to be agent-ready
(intent endpoints, machine-readable errors [serghei/agent-ready]) rather than
papering over it in a wrapper.
Side-effect safety is the core problem (exactly-once, dedup store) — that's
the llm-tool-idempotency skill; this skill only flags the hook (OP-9).
7. 跨框架对照 (Cross-Framework Mapping)
The wrapper logic — triage, naming, typed schema, auth, retry, pagination,
shaping, errors — is framework-independent. The only framework-specific layer
is the registration call. One Pydantic v2 model feeds all five targets via
model_json_schema() and model_validate().
Two cross-framework heuristics carried in from the sibling SOPs:
Per-step / per-agent tool scoping (from CrewAI + OpenAI allowed_tools):
give each agent/step only the tools its role needs. Fewer tools = better
selection and less context [oai/tools][crewai/tools]. A wide wrapped API
should still be scoped per agent, not bound wholesale.
Tools are functions, not chains (from LangGraph): the wrapper does one
thing and returns; orchestration (retries across tools, branching, HITL) lives
in the graph/crew, not inside the tool. Keep the wrapper pure and bounded.
附录: 引用速查 (Citation Index)
Short tags → full sources in references/R1-source-evidence.md:
