FastAPI converts endpoint signatures into a Dependant tree, recursively solves dependencies, validates request parameters, and returns values ready for the endpoint function.
5parameter sources
2dependency scopes
1shared cache
2async exit stacks
01
1Analyze
PLAN
Build Dependant
FastAPI inspects the endpoint signature and classifies each parameter before any request is handled.
analysis result
SIG
Read typed signature
get_dependant calls get_typed_signature and iterates every parameter in the endpoint.
PATH
Detect path params
Path parameter names are extracted so matching function arguments become path fields.
DEP
Recurse through Depends
Parameters with Depends create nested Dependant objects, carrying cache and scope settings.
analysis result
OK
Tree built once per route APIRoute stores dependant and flat dependant structures for later execution.
!
Scope rules are enforced Request-scoped generator dependencies cannot depend on function-scoped dependencies.
02
2Solve
RUN
Resolve Values
At request time solve_dependencies recursively resolves sub-dependencies, applies overrides, and reuses cached dependency values.
solving gates
1
Apply dependency overrides
If overrides are configured, FastAPI rebuilds the dependant for the replacement call.
OVR
2
Solve children first
Each sub_dependant is solved recursively before its parent function receives arguments.
REC
3
Use dependency cache
Cache keys prevent repeated dependency calls when use_cache is enabled.
CAC
Generators get cleanup stacks
Request and function scopes select the appropriate AsyncExitStack.
Async and sync both work
Coroutine dependencies are awaited; sync dependencies run in a threadpool.
03
3Execute
CALL
Call Endpoint
FastAPI merges dependency values with request data and hands a validated values map to the route handler.
endpoint payload
REQ01
Parse request parameters
Path, query, header, cookie, and body parameters are converted into values and validation errors.
CTX02
Inject request helpers
Request, WebSocket, response, background tasks, and security scopes are inserted when declared.
END03
Run endpoint function
routing.py passes solved values to the endpoint, awaiting async functions or using a threadpool for sync ones.
endpoint payload
Response payload
OK
Errors stay structured
SolvedDependency carries values, errors, background tasks, response, and dependency cache.
i
Security scopes travel
SecurityScopes is populated from the dependant oauth scopes when requested.