fix(core): respect model input limits (#39797)

packages/core/test/session-compaction.test.ts

@@ -19,9 +19,11 @@ import { Session } from "@opencode-ai/core/session"
 import { Project } from "@opencode-ai/core/project"
 import { ProjectTable } from "@opencode-ai/core/project/sql"
 import { App } from "@opencode-ai/core/app"
+import { Agent } from "@opencode-ai/core/agent"
+import { Location } from "@opencode-ai/core/location"
 import { AbsolutePath } from "@opencode-ai/core/schema"
 import { Money } from "@opencode-ai/schema/money"
-import { DateTime, Effect, Fiber, Layer, Stream } from "effect"
+import { DateTime, Effect, Fiber, Layer, Schema, Stream } from "effect"
 import { asc, eq } from "drizzle-orm"
 import { testEffect } from "./lib/effect"
 
@@ -130,6 +132,52 @@ test("compaction prompt requires the checkpoint headings in order", () => {
   expect(prompt).toContain("Keep every section, even when empty.")
 })
 
+it.effect("auto compaction reserves a buffer below the prompt ceiling", () =>
+  Effect.gen(function* () {
+    const compaction = yield* SessionCompaction.Service
+    const session = Session.Info.make({
+      id: Session.ID.make("ses_input_limit"),
+      projectID: Project.ID.global,
+      cost: Money.USD.zero,
+      tokens: { input: 0, output: 0, reasoning: 0, cache: { read: 0, write: 0 } },
+      time: { created: DateTime.makeUnsafe(0), updated: DateTime.makeUnsafe(0) },
+      location: Location.Ref.make({ directory: AbsolutePath.make("/tmp") }),
+    })
+    const input = (tokens: number, limits: { context: number; input?: number; output: number }) => ({
+      session,
+      model: Model.make({
+        id: "test-model",
+        provider: "test-provider",
+        route: OpenAIChat.route.with({ limits }),
+      }),
+      cost: [],
+      messages: [
+        Schema.decodeUnknownSync(SessionMessage.Assistant)({
+          id: SessionMessage.ID.make("msg_assistant"),
+          type: "assistant",
+          agent: Agent.defaultID,
+          model: { id: "test-model", providerID: "test-provider" },
+          content: [],
+          tokens: { input: tokens, output: 0, reasoning: 0, cache: { read: 0, write: 0 } },
+          time: { created: 0, completed: 0 },
+        }),
+      ],
+    })
+
+    const inputLimited = { context: 400_000, input: 272_000, output: 128_000 }
+    expect(compaction.required(input(251_999, inputLimited))).toBe(false)
+    expect(compaction.required(input(252_000, inputLimited))).toBe(true)
+
+    const contextLimited = { context: 100_000, output: 10_000 }
+    expect(compaction.required(input(79_999, contextLimited))).toBe(false)
+    expect(compaction.required(input(80_000, contextLimited))).toBe(true)
+
+    const outputLimited = { context: 100_000, output: 30_000 }
+    expect(compaction.required(input(69_999, outputLimited))).toBe(false)
+    expect(compaction.required(input(70_000, outputLimited))).toBe(true)
+  }),
+)
+
 it.effect("manual compaction summarizes short context instead of no-op", () =>
   Effect.gen(function* () {
     requests = []