fix(coding-agent): use local date in system prompt

Signed-off-by: Christian Stewart <christian@aperture.us>

packages/coding-agent/src/session/agent-session.ts

@@ -314,6 +314,7 @@ import { resolveFileDisplayMode } from "../utils/file-display-mode";
 import { extractFileMentions, generateFileMentionMessages } from "../utils/file-mentions";
 import { normalizeModelContextImages } from "../utils/image-loading";
 import { describeAttachedImagesForTextModel } from "../utils/image-vision-fallback";
+import { formatLocalCalendarDate } from "../utils/local-date";
 import { generateSessionTitle } from "../utils/title-generator";
 import { buildNamedToolChoice, isToolChoiceActive } from "../utils/tool-choice";
 import type { AuthStorage } from "./auth-storage";
@@ -6517,7 +6518,7 @@ export class AgentSession {
 			entries.sort();
 			instructionsSegment = entries.join("\u0006");
 		}
-		const date = new Date().toISOString().slice(0, 10);
+		const date = formatLocalCalendarDate();
 		return `${nameSegment}\u0003${descriptionSegment}\u0005${registrySegment}\u0007${instructionsSegment}|${date}`;
 	}
 

packages/coding-agent/src/system-prompt.ts

@@ -24,6 +24,7 @@ import projectPromptTemplate from "./prompts/system/project-prompt.md" with { ty
 import systemPromptTemplate from "./prompts/system/system-prompt.md" with { type: "text" };
 import { shortenPath } from "./tools/render-utils";
 import { type ActiveRepoContext, resolveActiveRepoContext } from "./utils/active-repo-context";
+import { formatLocalCalendarDate } from "./utils/local-date";
 import { normalizePromptPath } from "./utils/prompt-path";
 import { AGENTS_MD_LIMIT, buildWorkspaceTree, type WorkspaceTree } from "./workspace-tree";
 
@@ -677,7 +678,7 @@ export async function buildSystemPrompt(options: BuildSystemPromptOptions = {}):
 		}
 	}
 
-	const date = new Date().toISOString().slice(0, 10);
+	const date = formatLocalCalendarDate();
 	const dateTime = date;
 	const promptCwd = shortenPath(normalizePromptPath(resolvedCwd));
 	const activeRepoContextPrompt = renderActiveRepoContextPrompt(activeRepoContext);

packages/coding-agent/src/utils/local-date.ts

@@ -0,0 +1,7 @@
+/** formatLocalCalendarDate formats a Date as YYYY-MM-DD in the host local timezone. */
+export function formatLocalCalendarDate(date: Date = new Date()): string {
+	const year = date.getFullYear();
+	const month = String(date.getMonth() + 1).padStart(2, "0");
+	const day = String(date.getDate()).padStart(2, "0");
+	return `${year}-${month}-${day}`;
+}

packages/coding-agent/test/agent-session-tool-rebuild-skip.test.ts

@@ -388,11 +388,14 @@ describe("AgentSession refreshMCPTools rebuild skipping", () => {
 		await session.refreshMCPTools([dynamicTool]);
 		expect(rebuildCount).toBe(baseline + 1);
 	});
-	it("rebuilds when the calendar date rolls over between tool-stable MCP refreshes", async () => {
-		// `buildSystemPrompt` injects today's date into the prompt body.
-		// A session spanning midnight must not serve yesterday's date after an MCP
-		// reconnect that happens to bring an identical tool set.
-		setSystemTime(new Date("2025-01-01T23:59:58Z"));
+	it("rebuilds when the local calendar date rolls over between tool-stable MCP refreshes", async () => {
+		// `buildSystemPrompt` injects today's date into the prompt body. A session
+		// spanning local midnight must not serve yesterday's date after an MCP
+		// reconnect that happens to bring an identical tool set, even when UTC has
+		// not rolled over.
+		const originalTimezone = process.env.TZ;
+		process.env.TZ = "America/Los_Angeles";
+		setSystemTime(new Date("2026-07-01T06:59:58Z"));
 		try {
 			let rebuildCount = 0;
 			const { session } = newSession(async toolNames => {
@@ -405,22 +408,27 @@ describe("AgentSession refreshMCPTools rebuild skipping", () => {
 			await session.refreshMCPTools([tool]);
 			expect(rebuildCount).toBe(1);
 
-			// Same tools, same day: signature matches, skip.
+			// Same tools, same local day: signature matches, skip.
 			await session.refreshMCPTools([tool]);
 			expect(rebuildCount).toBe(1);
 
-			// Advance past midnight.
-			setSystemTime(new Date("2025-01-02T00:00:01Z"));
+			// Advance past local midnight while the UTC date remains 2026-07-01.
+			setSystemTime(new Date("2026-07-01T07:00:01Z"));
 
-			// Same tools, new calendar day: date segment changed, must rebuild.
+			// Same tools, new local calendar day: date segment changed, must rebuild.
 			await session.refreshMCPTools([tool]);
 			expect(rebuildCount).toBe(2);
 
-			// Same tools, same new day: skip again.
+			// Same tools, same new local day: skip again.
 			await session.refreshMCPTools([tool]);
 			expect(rebuildCount).toBe(2);
 		} finally {
-			setSystemTime(); // restore real time
+			setSystemTime();
+			if (originalTimezone === undefined) {
+				delete process.env.TZ;
+			} else {
+				process.env.TZ = originalTimezone;
+			}
 		}
 	});
 	it("does not rebuild when MCP server instructions change only beyond the 4000-char truncation boundary", async () => {

packages/coding-agent/test/system-prompt-model.test.ts

@@ -1,4 +1,4 @@
-import { afterEach, beforeEach, describe, expect, it } from "bun:test";
+import { afterEach, beforeEach, describe, expect, it, setSystemTime } from "bun:test";
 import * as fs from "node:fs";
 import * as os from "node:os";
 import * as path from "node:path";
@@ -49,6 +49,33 @@ describe("system prompt model identifier", () => {
 		expect(systemPrompt.join("\n\n")).toContain("Model: anthropic/claude-opus-4");
 	});
 
+	it("renders the prompt date from the local timezone rather than UTC", async () => {
+		const originalTimezone = process.env.TZ;
+		process.env.TZ = "America/Los_Angeles";
+		setSystemTime(new Date("2026-07-01T03:15:00Z"));
+		try {
+			const { systemPrompt } = await buildSystemPrompt({
+				cwd: tempDir,
+				contextFiles: [],
+				skills: [],
+				rules: [],
+				toolNames: [],
+				workspaceTree: { ...EMPTY_TREE, rootPath: tempDir },
+			});
+			const rendered = systemPrompt.join("\n\n");
+
+			expect(rendered).toContain("Today is 2026-06-30");
+			expect(rendered).not.toContain("Today is 2026-07-01");
+		} finally {
+			setSystemTime();
+			if (originalTimezone === undefined) {
+				delete process.env.TZ;
+			} else {
+				process.env.TZ = originalTimezone;
+			}
+		}
+	});
+
 	it("omits the model line when no model is provided", async () => {
 		const { systemPrompt } = await buildSystemPrompt({
 			cwd: tempDir,