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fix(core): bound session title generation with a small output limit #54411

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fix(core): bound session title generation with a small output limit #54411

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843746d
fix(core): bound session title generation with a small output limit
eatmoreduck Oct 11, 2026
3f70b0d
fix(core): size the title cap to leave room for a thinking budget
eatmoreduck Oct 22, 2026
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13 changes: 10 additions & 3 deletions packages/core/src/session/model-request.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -48,6 +48,10 @@ const GENERATION_KEYS = new Set(Object.keys(GenerationOptions.fields))
const OUTPUT_TOKEN_FALLBACK = 32_000
// No reply needs more, however much the model allows.
const OUTPUT_TOKEN_MAX = 256_000
// A title never needs more; without a bound a local thinking model can reason in circles
// for minutes on the background title request while the user waits (#54401). Sized to leave
// room for a small thinking budget, so budget-aware providers don't reject the request.
const TITLE_OUTPUT_MAX = 4_096
// Prompt text is estimated at about 4 characters per token, which can run low on dense text such as code.
const ESTIMATE_ERROR = 0.15
// Never ask for less; only reachable with automatic compaction off, since it keeps the window from filling this far.
Expand Down Expand Up @@ -241,8 +245,9 @@ export const layer = Layer.effect(
const given = new Map(
tools.definitions.map((t) => [{ description: t.description, input: { ...t.inputSchema } }, t] as const),
)
// Hooks see the default output limit and may change or remove it. Titles and generate keep the provider default,
// because their reasoning is hard to budget.
// Hooks see the default output limit and may change or remove it. Titles get a small
// bound so background thinking-model runs cannot outpace the main request; generate
// keeps the provider default, because its reasoning is hard to budget.
const shaped = yield* shape(
{
sessionID: session.id,
Expand All @@ -252,7 +257,9 @@ export const layer = Layer.effect(
options:
kind === "primary" || kind === "compaction"
? { maxTokens: outputLimit(model.limit, input.inputTokens) }
: {},
: kind === "title"
? { maxTokens: Math.min(outputLimit(model.limit, input.inputTokens), TITLE_OUTPUT_MAX) }
: {},
},
Object.fromEntries(Array.from(given, ([d, t]) => [t.name, d])),
)
Expand Down
17 changes: 17 additions & 0 deletions packages/core/test/session-title.test.ts
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Expand Up @@ -674,3 +674,20 @@ it.effect("preserves a manual rename completed while generation is in flight", (
expect((yield* store.get(sessionID))?.title).toBe("Manual title")
}),
)

it.effect("bounds the title request with a small output limit (#54401)", () =>
Effect.gen(function* () {
yield* enableTitleAgent
const sessionID = Session.ID.make("ses_title_limit")
yield* insertSession(sessionID)
yield* prompt(sessionID, "Hello")

const title = yield* SessionTitle.Service
yield* title.generate(sessionID)

expect(requests).toHaveLength(1)
const maxTokens = requests[0]?.generation?.maxTokens
expect(maxTokens).toBeDefined()
expect(maxTokens!).toBeLessThanOrEqual(4_096)
}),
)
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