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fix(session): stabilize single-message prompt prefix for llama cache reuse #18710

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fix(session): stabilize single-message prompt prefix for llama cache reuse #18710

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5 changes: 3 additions & 2 deletions packages/opencode/src/session/prompt.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -655,9 +655,10 @@ export namespace SessionPrompt {
// Build system prompt, adding structured output instruction if needed
const skills = await SystemPrompt.skills(agent)
const system = [
...(await SystemPrompt.environment(model)),
...(skills ? [skills] : []),
...(await InstructionPrompt.system()),
// Put the most stable instructions first to maximize prompt-cache reuse.
...(await SystemPrompt.environment(model, session.time.created)),
...(skills ? [skills] : []),
]
const format = lastUser.format ?? { type: "text" }
if (format.type === "json_schema") {
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5 changes: 3 additions & 2 deletions packages/opencode/src/session/system.ts
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Expand Up @@ -25,7 +25,7 @@ export namespace SystemPrompt {
return [PROMPT_DEFAULT]
}

export async function environment(model: Provider.Model) {
export async function environment(model: Provider.Model, sessionCreatedAt: number) {
const project = Instance.project
return [
[
Expand All @@ -36,7 +36,8 @@ export namespace SystemPrompt {
` Workspace root folder: ${Instance.worktree}`,
` Is directory a git repo: ${project.vcs === "git" ? "yes" : "no"}`,
` Platform: ${process.platform}`,
` Today's date: ${new Date().toDateString()}`,
// Keep the session day stable so long-lived sessions do not churn the prompt at midnight.
` Today's date: ${new Date(sessionCreatedAt).toDateString()}`,
`</env>`,
`<directories>`,
` ${
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24 changes: 24 additions & 0 deletions packages/opencode/test/session/system.test.ts
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Expand Up @@ -6,6 +6,30 @@ import { SystemPrompt } from "../../src/session/system"
import { tmpdir } from "../fixture/fixture"

describe("session.system", () => {
test("environment uses the session creation day", async () => {
await using tmp = await tmpdir({
git: true,
})

await Instance.provide({
directory: tmp.path,
fn: async () => {
const environment = await SystemPrompt.environment(
{
providerID: "openai",
api: {
id: "gpt-5.2",
},
} as any,
Date.UTC(2026, 2, 22, 23, 59, 59),
)

expect(environment).toHaveLength(1)
expect(environment[0]).toContain("Today's date: Sun Mar 22 2026")
},
})
})

test("skills output is sorted by name and stable across calls", async () => {
await using tmp = await tmpdir({
git: true,
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