One agent in three shapes.
What a provider SDK gives you
A tool-calling API, as most providers describe it, is a loop you drive yourself. The general shape (field names differ between providers and change over time, so check the docs of the one you use):
You send a system prompt, the messages so far, and the list of tools (each with a name, a description and a JSON schema for its arguments).
You get back content blocks and a stop reason . If the reason says the model wants a tool, one of the blocks is a tool call : an id, a tool name and the arguments.
You run the tool. Nothing runs by itself.
You append the model's message, then a new user message holding a tool result that carries the same id, and you call again.
When the stop reason says the model is finished, the text blocks are the answer.
That is the whole contract. Everything else (when to stop, what to do with an error, how many steps, what to record) is yours . In sdk_loop.py the loop is about 80 lines of code, and most of them are the limits and the trace from modules 6 and 7.
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One agent written three ways: same tools, same limits, same model Raw loop kernel.py while True: ask model, run tool, append result, check limits 74 lines of code (without comments) SDK-shaped loop sdk_loop.py same, but messages and tool_use / tool_result blocks as an API sends them 83 lines of code (without comments) State graph graph_agent.py + graphlite.py nodes: model, tools, stop; conditional edges decide the next node 95 lines of code (without comments) 10 of 10 tasks: identical status, answer and tool calls in all three The shape changes how you write and read the agent. It does not change what the agent does. Line counts are for these toy files; real code with real error handling is longer. One agent written three ways
One agent, three shapes
The same agent, written three ways, on the same ten tasks, with the same model, tools and limits:
Shape
File
Lines (no comments)
What it looks like
Raw loop
kernel.py
74
a while True: ask the model, run the tool, append the result, check the limits
SDK-shaped loop
sdk_loop.py
83
the same, but with messages, tool-call blocks and tool-result blocks as an API sends them
State graph
graph_agent.py + graphlite.py
95
nodes model, tools, finish, stop_*; conditional edges pick the next node
Result: 10 of 10 tasks give the same status, the same answer and the same sequence of tool calls in all three. The word counts differ by a few words in the SDK version, because the message envelope is counted a little differently; the behaviour does not.
This is the central claim of the module, and it is easy to check in the example: the shape changes how you write and read the agent, not what it does. A graph is a loop whose "if" statements have been drawn as arrows. That gives you something real (you can see the flow, you can save the state after every node, you can pause at a node and wait for a person) and costs you something real (a new vocabulary, and the flow now lives in wiring instead of in a function you can read top to bottom).
The graph toy has the four ideas that graph frameworks are built around:
Idea
In graphlite.py
Why it exists
State that nodes read and update
a dictionary passed through the run
one place for everything the loop needs
Reducers
messages is appended, other fields are replaced
so two nodes updating the same field do not overwrite each other
Conditional edges
route_model returns tools, finish or stop_*
the agent's decisions as visible branches
A recursion limit and a checkpoint hook
RecursionLimit; on_checkpoint(node, state) after every node
a hard stop on node visits (module 6), and a place to save, resume or ask a person (module 7)
Tip: put the limits in the router, not in the model
In all three shapes the limit checks sit outside the model call : the step limit, the word budget and the repeat check each end the run from code the model cannot argue with. In the graph they are the conditional edges after model and after tools. Whatever shape you pick, keep the guards in code.
Where every piece came from
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Every piece of the loop is something you built earlier; frameworks give it another name Piece you wrote Where Names you may meet (general knowledge, varies by framework) Loop: ask, act, observe, repeat Module 1 agent executor, run loop, 'agent runner' Tool schema and argument checks Module 2 tool decorator, function tool, tool registry Plan first, then act Module 3 planner node, plan-and-execute agent Critique and revise Module 4 reflection node, critic agent Step, word and repeat limits Module 6 max iterations, recursion limit, budget settings Events, replay, versions Module 7 tracing, callbacks, checkpoints, threads Check the current documentation of the framework you use; names and defaults change between versions. Pieces of the loop and the names frameworks give them
Nothing in the final agent is new. The loop is module 1; the tool schema and argument checks are module 2; planning (3) and reflection (4) are optional layers on top; the step, word and repeat limits and the partial answer are module 6; the event log, versions and replay are module 7. The right-hand column of the picture lists names you may meet. That column is general knowledge , not something I checked against a particular version: frameworks rename things, and one framework's "agent" is another's "node". Use the table as a translation key, then read the documentation.