Inject current date into deep research planning and query prompts (#1347)

Deep research generated search queries from the LLM's training-cutoff
knowledge, so it emitted stale-year queries like "best Python tutorials
2025" when the actual year is later (issue #1341). The chat/agent path
already grounds the model with "Today is ..." (src/agent_loop.py); the
deep research planning and query-generation prompts had no equivalent.

Add a small current_date_context() helper and prepend it at the plan and
query-generation prompt sites (and the research_handler plan preview path
that reuses RESEARCH_PLAN_PROMPT). System-TZ local, portable strftime.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/test_deep_research_date_context.py

@@ -0,0 +1,68 @@
+"""Regression tests for issue #1341 — deep research used the model's
+training-cutoff year (e.g. "best Python tutorials 2025") because the
+query-generation and planning prompts never told the LLM the current date.
+
+The chat/agent path already injects "Today is ..." (src/agent_loop.py); deep
+research had no equivalent. These tests pin that the current year now reaches
+the LLM at both the planning and query-generation steps, without needing a live
+LLM or DB.
+"""
+import asyncio
+from datetime import datetime
+
+from src.deep_research import (
+    DeepResearcher,
+    current_date_context,
+    RESEARCH_PLAN_PROMPT,
+)
+
+
+def _this_year() -> str:
+    return datetime.now().astimezone().strftime("%Y")
+
+
+def test_current_date_context_names_the_real_year():
+    ctx = current_date_context()
+    assert _this_year() in ctx
+    # It must actively steer the model away from training-data years.
+    assert "training data" in ctx.lower()
+
+
+def test_generate_queries_prompt_carries_the_current_year():
+    # Build without the heavy __init__; _generate_queries only needs these.
+    r = DeepResearcher.__new__(DeepResearcher)
+    r.research_plan = ""
+    r.queries_used = set()
+
+    seen = {}
+
+    async def _fake_llm(messages, **kwargs):
+        seen["prompt"] = messages[0]["content"]
+        return '["python tutorials", "python guides"]'
+
+    r._llm = _fake_llm
+
+    queries = asyncio.run(r._generate_queries("best python tutorials", "", 1))
+
+    assert queries  # sanity: the JSON array parsed
+    # The fix: the real current year is in the prompt the LLM actually sees.
+    assert _this_year() in seen["prompt"]
+
+
+def test_plan_prompt_carries_the_current_year():
+    r = DeepResearcher.__new__(DeepResearcher)
+
+    seen = {}
+
+    async def _fake_llm(messages, **kwargs):
+        seen["prompt"] = messages[0]["content"]
+        return "{}"
+
+    r._llm = _fake_llm
+
+    asyncio.run(r._create_plan("what changed this year"))
+
+    assert _this_year() in seen["prompt"]
+    # The base template itself stays year-agnostic; the year comes from the
+    # prepended context, proving the wiring (not a hard-coded prompt edit).
+    assert _this_year() not in RESEARCH_PLAN_PROMPT