Route models by FSM state

ReasonBlocks tracks each step’s difficulty through a finite state machine. By passing model_routing to the ReasonBlocks constructor, you can map any of the four post-INIT states (FAST, NORMAL, SLOW, SKIP) to a model identifier. The middleware swaps the model on the active ModelRequest inside wrap_model_call, before pattern injections render — so format-routing pattern selection sees the post-routing model family.

Model routing is opt-in. Without model_routing, the agent’s configured model runs every step.

How states map to routing

State	Trigger	Routing
`INIT`	First call only	No routing override
`FAST`	All scores in `fast_window` (default 6) below `fast_threshold` (default `0.2`)	Routed if mapped; also skips E1/E2/E3 retrieval
`NORMAL`	Default working state	Routed if mapped
`SLOW`	All scores in `slow_window` (default 5) above `slow_threshold` (default `0.6`)	Routed if mapped
`SKIP`	All scores in `skip_window` (default 35) above `skip_threshold` (default `0.85`)	Routed if mapped

FAST is the only state that affects E-trace retrieval — it skips E1, E2, and E3 because the agent is sailing through. Monitor scoring still runs in FAST so loop detection stays active. Any state can be mapped or unmapped independently; unmapped states leave the agent’s configured model untouched.

Configure routing

Keys must match FSMState names: "FAST", "NORMAL", "SLOW", "SKIP". Values follow LangChain’s init_chat_model format ("provider:model-name").

from reasonblocks import ReasonBlocks

rb = ReasonBlocks(
    api_key="rb_live_...",
    model_routing={
        "FAST":   "anthropic:claude-haiku-4-5-20251001",
        "NORMAL": "anthropic:claude-haiku-4-5-20251001",
        "SLOW":   "anthropic:claude-sonnet-4-6",
        "SKIP":   "anthropic:claude-sonnet-4-6",
    },
)

You don’t have to map every state. Omit any you don’t want to override:

# Only escalate when struggling — every other state uses the agent's default model
rb = ReasonBlocks(
    api_key="rb_live_...",
    model_routing={
        "SLOW": "anthropic:claude-sonnet-4-6",
    },
)

The middleware caches resolved chat models per id (init_chat_model is called once per unique id and reused).

Inspect routing decisions

StepLogEntry.model_id holds the resolved model id when routing fired on that step. Steps with no routing override leave it empty.

mw = rb.middleware(agent_name="bugfixer")
agent = create_agent(..., middleware=[mw])

with mw:
    result = agent.invoke({"messages": [("user", "Fix the bug.")]})

for entry in mw.step_log:
    routed = f" -> {entry.model_id}" if entry.model_id else ""
    print(f"step {entry.step}: {entry.fsm_state}{routed} difficulty={entry.difficulty}")

Sample output:

step 0: INIT (first call)
step 1: NORMAL difficulty=0.512
step 2: NORMAL difficulty=0.489
step 3: NORMAL difficulty=0.141
step 4: NORMAL difficulty=0.098
step 5: NORMAL difficulty=0.077
step 6: NORMAL difficulty=0.061
step 7: FAST -> anthropic:claude-haiku-4-5-20251001 difficulty=0.055
step 8: FAST -> anthropic:claude-haiku-4-5-20251001 difficulty=0.043
step 9: NORMAL difficulty=0.731
step 10: SLOW -> anthropic:claude-sonnet-4-6 difficulty=0.812

Tune the FSM thresholds

Pass fsm_thresholds to adjust when routing transitions happen. See Advanced configuration for the full set of knobs and the hysteresis rule that prevents thrashing at boundaries.

rb = ReasonBlocks(
    api_key="rb_live_...",
    model_routing={
        "FAST": "anthropic:claude-haiku-4-5-20251001",
        "SLOW": "anthropic:claude-sonnet-4-6",
    },
    fsm_thresholds={
        "fast_threshold": 0.15,   # require very-easy steps to drop to FAST
        "slow_threshold": 0.65,   # require clearly-hard steps to escalate
        "fast_window":    4,      # react sooner to easy runs
        "slow_window":    3,      # react sooner to struggling runs
    },
)

Lowering fast_threshold keeps the agent on the cheap model longer. Raising slow_threshold delays escalation. Either change reduces cost; both can affect quality on genuinely hard work.

Getting Started

Concepts

Using ReasonBlocks

Connectors and sync

Route models by FSM state

How states map to routing

Configure routing

Inspect routing decisions

Tune the FSM thresholds

​How states map to routing

​Configure routing

​Inspect routing decisions

​Tune the FSM thresholds

How states map to routing

Configure routing

Inspect routing decisions

Tune the FSM thresholds