Agent Routing

Agent Routing — model + effort selection for subagents

Before spawning any subagent, answer one question: **is the task mechanically

checkable, or does it require judgment?** Checkable → Haiku at effort: 'low'

behind a verifier. Judgment → up-tier. The routing table refines this split; the

rest of the skill governs cascades and loops.

Do not up-tier checkable work "to be safe." Measured Haiku 4.5 failure on

every checkable task family tested is ≈0, at effort: 'low', with

chain-of-thought suppressed (evidence: references/calibration-2026-07-15.md).

The burden of proof is on routing *up*: reflexive up-tiering costs 3–5× for

safety that the data says is imaginary. Spend effort, not tier, and only where

reasoning depth demonstrably falls short.

To turn a judgment-shaped task INTO a Haiku-executable one (explicit procedures,

n-shot examples), use the sibling down-skilling skill. This skill decides the

routing; that one engineers the prompt.

Context handoff — routing picks the tier; the prompt carries the context

Subagents inherit nothing: not the conversation, not loaded skills, not the

existence of artifacts already on disk. Every index, scan output, artifact

path, or tool-invocation recipe the orchestrator relies on must be serialized

into the subagent's prompt (or written to a file the prompt points at).

Otherwise the agent falls back to blind rediscovery — and the tier premium is

spent on crawling, not judgment. A Sonnet with no handoff wastes more than a

Haiku with a good procedure.

This is the context-side twin of down-skilling: that skill engineers the

*procedure* into the prompt; this rule makes the *artifacts and tools* travel

with it. Checklist per spawn: (1) artifact paths + how to query them, (2) tool

commands verbatim (interpreter path included — subagents don't know your

venv), (3) explicit anti-patterns ("no ls/glob discovery"), (4) an output

spec. Skills that orchestrate fan-outs should embed this (see

exploring-codebases for the worked exploration case).

Evidence: 2026-07-16, four Sonnet Explore agents launched onto a 2,300-file

repo without the handoff opened with ls crawls despite a full tree-sitter

symbol index sitting on disk; relaunched with per-agent index slices + the

verbatim tool command + anti-crawl rules, discovery cost dropped to ~zero.

Routing table

| Task shape | Model | Effort | Verify with |

|---|---|---|---|

| Extraction, classification, format transforms, schema-bound output | haiku | low | schema / spot-check |

| Closed-form computation, state tracking, multi-hop lookup | haiku | low | deterministic check |

| Constraint-bound generation (exact counts, required tokens, lipograms) | haiku | low | mechanical checker |

| Bulk scans/greps, per-file summaries, fan-out reads | haiku | low | sample audit |

| Code edits with tests available | haiku first | low–medium | run the tests |

| Judging / scoring another model's output | sonnet+ | medium | — (judge ≠ worker) |

| Ambiguity resolution, novel synthesis, architecture, taste | sonnet/opus | high | human or panel |

| Long-horizon multi-step agentic work, cross-file reasoning | sonnet/opus | high/xhigh | milestone checks |

Effort mechanics (Managed Agents)

Effort is set on the agent, not per session — an effort inside a

per-session model override is silently ignored. Levels are low, medium,

high, xhigh, max (bare string or {"type": "high"}); not every model

accepts every level, and an invalid pair is rejected at agent-create. The create

response echoes the resolved config — if effort comes back None, the org's

beta header (managed-agents-2026-04-01) doesn't carry the feature: the field

was dropped, not rejected, so check the echo before assuming a level took.

Effort is the per-role cost lever — higher levels let Claude spend more tokens

per inference call — so buy depth only for judgment-heavy roles and drop triage

or formatting roles to low without touching the expensive role's budget. This

is the same split the table encodes; effort tunes within a tier, tier is the

coarse knob. Calibration null result: effort had no measurable effect on Haiku's

checkable tasks (Exp. 2 — low and high both 20/20), which is why the rule is

spend effort only where reasoning depth *demonstrably* falls short, never

by default.

Cascade (default composition)

When a near-free verifier exists, compose instead of choosing a tier up front:

result = haiku(task, effort=low)
if verify(result) fails:  result = sonnet(task)      # escalate on evidence
if verify(result) fails:  result = opus(task)         # rare

Input-cost ratio is Haiku 1× · Sonnet 3× · Opus 5×. Expected cascade cost ≈

c_haiku + p_fail × c_sonnet, so the cascade beats Sonnet-direct while Haiku's

failure rate stays below ~2/3, and beats Opus-direct below ~4/5 (derivation in

the reference). Every checkable task measured has Haiku failure ≈0, so the

cascade is nearly pure savings.

No verifier ⇒ no cascade. Route by the table instead, because silent Haiku

errors compound downstream with nothing to catch them.

Shared-prefix caching cuts the fan-out multiplier (unmeasured, conditional).

When N subagents share a byte-stable prefix — the *fixed* part of the handoff

(tool commands, procedure, anti-patterns), not the per-agent index slices, which

by design differ — that prefix is reusable by prefix caching *where the

orchestration surface exposes it*, pulling the shared portion's input cost toward

a read-discount rate and strengthening the cheap-first / fan-out bias. Keep

per-agent dynamic content at the tail so it doesn't invalidate the shared lead.

This is prompt-construction economics, not from the calibration battery, and it

assumes the surface caches subagent prefixes — verify that before relying on it

(the effort/pricing numbers above are measured; this is not).

Loop discipline

Never blind-loop. Re-applying the same prompt to a model's own output is the

identity at best (an LLM call already unrolls its reasoning depth internally) and

regression-then-freeze at worst — in calibration, a re-looped haiku broke its

own middle line on iteration 2 and froze on the broken text for every iteration

after. Loops only pay when an out-of-band evaluator scores each iteration.

• Loop only with an out-of-band evaluator — ground truth, mechanical

checker, or an up-tier judge scoring every iteration.

• Select, don't trust the last: final = argmax_r eval(answer_r). Never

ship iteration N just because it's newest.

• Stop on first regression. If eval(r) < eval(r-1), stop — loops froze on

degraded output rather than recovering, so early-stop risk beats drift risk.

• Loop for diversity, not depth. Vary the prompt/angle per iteration to

explore; identical re-application converges instantly.

• "Improve this" with no headroom is the danger zone. It pressures the model

to change something; without a selector, that change ships.

Judge rules

• Judge model ≠ worker model; judge at least one tier up (Sonnet judging Haiku,

Opus judging Sonnet). Same-model self-assessment is untested — don't assume it.

• Prefer mechanical checkers over judges wherever a spec can be executed (word

counts, schemas, tests, regex): free, deterministic, zero judge tokens.

• Judges are for rubric quality, not arithmetic — don't ask Sonnet to verify a

sum a Python one-liner can check.

Escalation triggers (route up despite the table)

• The verifier fails twice at the same tier.
• The task requires weighing trade-offs with no checkable ground truth.
• Output will be shipped verbatim to a human without review.
• The subagent must plan its own multi-step tool strategy over many turns.

Observing the fan-out — you can't govern what you can't watch

Loop discipline (stop-on-regression, select-don't-trust-last) and the escalation

triggers (verifier failed twice) all assume you can see a subagent's work *while

it runs*. By default you can't: the session-level stream previews only the

primary (coordinator) thread, and a subagent's output lands only after its whole

turn buffers — a two-minute researcher shows nothing for two minutes.

Close the gap with one stream per thread. Read the session stream for the

coordinator; on every session.thread_created (it carries session_thread_id

and agent_name), attach a watcher to that child's stream

(GET /v1/sessions/{id}/threads/{thread_id}/stream, with event_deltas) in a

background thread. Four rules keep it honest:

• Preview is a scratch buffer; the buffered event is the record. Deltas are

best-effort and shed under load, so concatenated deltas are a *prefix* of the

final text, not necessarily the whole. Reconcile by a single replace when the

buffered agent.message arrives; the SDK's accumulate_managed_agents_event

folds start / delta / record into one snapshot. One accumulator per connection.

• No replay. A stream opened after a request started gets no deltas for that

in-flight request, and reconnects never replay missed deltas — attach on

thread_created or miss the child's first response.

• Coordination events live on the primary thread — session.thread_created

(spawn), agent.thread_message_sent (handoff), agent.thread_message_received

(child reports back). Child tool calls cross-posted to the primary carry

session_thread_id; skip them, the child's own watcher shows them.

• Terminate cleanly. Watchers exit on session.thread_status_idle; the main

loop on session.status_idle — print the stop reason when it isn't end_turn,

and break on terminated-status events, so a session stuck on a tool

confirmation or out of retries ends the run instead of hanging it.

Operational, not calibrated (multi-turn agentic tool use is on the "not

measured" list below). Source: Anthropic Managed Agents notebook

CMA_watch_subagents_live (beta managed-agents-2026-04-01); full streaming

contract in events and streaming.

Recalibrate when

• The task is off the measured battery. No deterministic task has made Haiku

fail yet, so the cliff is past what's been probed — test harder instances

before trusting Haiku on a genuinely novel task family.

• The work is multi-turn agentic tool use. That was not calibrated; treat the

table's up-tier rows there as prior, not measurement.

• A model rev bumps. Re-run the battery (generators + scorer described in

references/calibration-2026-07-15.md) before trusting the table.