Ch 4: Tools — Under the Hood

Ch 4 — Tools Under the Hood

JSON Schema validation, argument completion, error codes, cancellation, pagination, and SDK patterns

Index ← High Level

Under the Hood

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AJSON Schema for Tool InputsHow inputSchema defines and validates arguments

schema

inputSchemaJSON Schema object
defines arg types

validates

data_object

ArgumentsLLM-generated
JSON object

checked

verified

ValidatedServer validates
before execution

arrow_downward Argument auto-completion for better UX

BArgument CompletionServer suggests values as the user types

edit

User TypesPartial value
for an argument

complete

auto_awesome

completion/completeClient asks server
for suggestions

returns

list

SuggestionsMatching values
for the argument

arrow_downward Error handling: what happens when tools fail

CError HandlingTool errors vs protocol errors — two different things

warning

Tool ErrorisError: true
in result content

error

Protocol ErrorJSON-RPC error
response object

codes

tag

Error Codes-32600 to -32603
+ MCP-specific

arrow_downward Cancellation: aborting a running tool call

DCancellationAborting a long-running tool call mid-execution

hourglass_top

Tool RunningLong operation
in progress

cancel

cancelledClient sends
cancel notification

cleanup

cleaning_services

Server Cleans UpStops work, frees
resources

arrow_downward Pagination: handling large tool lists

EPaginationCursor-based pagination for large tool lists

first_page

First Pagetools/list with
no cursor

nextCursor

navigate_next

Next Pagetools/list with
cursor param

repeat

last_page

Last PageNo nextCursor
= done

arrow_downward SDK patterns: defining tools in Python and TypeScript

FSDK PatternsHow you actually define tools in code

code

Python SDK@mcp.tool()
decorator pattern

javascript

TypeScript SDKserver.tool()
method pattern

both

auto_fix_high

Auto SchemaSDK generates
inputSchema from types