fix: prevent write_interim_draft context overflow (465K → ≤300K chars)

Two bugs caused all 5 interim blocks to fail with "Claude CLI failed
(exit 1): unknown error":

1. source_context was embedded BOTH inside the prompt template (via
   {source_context}) AND prepended again in write_block — doubling every
   block's context size (232K chars × 2 = 465K chars).

2. _build_source_context loaded all 9 case documents for every block
   regardless of relevance.

Fixes:
- Remove the duplicate source_context prepend in write_block; the
  template already contains it via {source_context}
- Add per-block document filtering (_BLOCK_DOC_TYPES): block-he/zayin →
  empty, block-chet → protocol only, block-tet → appraisals only
- Add 400K char guard before calling claude -p with a descriptive error
  (vs opaque "exit 1: unknown error")
- Add prompt-size warning and size info in claude_session error messages

Result: block-he 0 chars, block-zayin 0 chars, block-vav ~172K,
block-chet ~45K, block-tet ~300K (all under 400K limit)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

mcp-server/src/legal_mcp/services/block_writer.py

@@ -360,13 +360,9 @@ async def write_block(
         post_hearing_context=post_hearing_context,
     )
 
-    # Restructure: sources first, then instructions
-    prompt = (
-        f"## חומרי מקור (מסמכים מלאים — צטט מהם מילה במילה כשאפשר):\n\n"
-        f"{source_context}\n\n"
-        f"---\n\n"
-        f"{formatted_prompt}"
-    )
+    # source_context is already embedded inside formatted_prompt via {source_context} in the
+    # template. Do NOT prepend it again — doing so doubles the prompt size (was 465K chars).
+    prompt = formatted_prompt
 
     if instructions:
         prompt += f"\n\n## הנחיות נוספות:\n{instructions}"
@@ -377,6 +373,19 @@ async def write_block(
         if not dir_doc.get("approved"):
             raise ValueError("לא ניתן לכתוב בלוק דיון ללא כיוון מאושר. הפעל brainstorm → approve_direction קודם.")
 
+    # Guard against context overflow before calling claude -p.
+    # Sonnet: 200K context → ~800K chars max; Opus: 200K context → same.
+    # In practice the CLI has crashed on prompts above ~400K chars, so use
+    # that as a conservative ceiling (well below the token limit).
+    _MAX_PROMPT_CHARS = 400_000
+    if len(prompt) > _MAX_PROMPT_CHARS:
+        raise RuntimeError(
+            f"Prompt too large for {block_id}: {len(prompt):,} chars "
+            f"(limit {_MAX_PROMPT_CHARS:,}). "
+            f"source_context: {len(source_context):,} chars. "
+            f"Reduce documents or call extract_appraiser_facts first."
+        )
+
     # Call Claude via Claude Code session (no API)
     model_key = block_cfg["model"]
     timeout = claude_session.LONG_TIMEOUT if model_key == "opus" else claude_session.DEFAULT_TIMEOUT
@@ -414,16 +423,35 @@ def _build_case_context(case: dict, decision: dict | None) -> str:
 - תוצאה: {outcome_heb}"""
 
 
+# Which doc_types are relevant per block.
+# None  → skip source docs entirely (block uses other context, e.g. claims_context)
+# []    → include all doc types (default for unspecified blocks)
+# [..] → include only the listed doc_type values
+_BLOCK_DOC_TYPES: dict[str, list[str] | None] = {
+    "block-he":    None,                      # only case_context needed; no full docs
+    "block-vav":   ["appeal", "protocol"],    # כתב ערר + פרוטוקול ועדה
+    "block-zayin": None,                      # claims_context is sufficient
+    "block-chet":  ["protocol"],              # פרוטוקול + השלמות טיעון
+    "block-tet":   ["appraisal"],             # שומות בלבד
+    # block-yod, block-yod-alef, block-he etc. default → all docs
+}
+
+
 async def _build_source_context(case_id: UUID, block_id: str) -> str:
-    """Get full document texts for the block.
+    """Get document texts for the block, filtered by relevance.
 
     Per Anthropic best practices: send full source documents, not truncated excerpts.
-    Place documents at the TOP of the prompt (before instructions) for 30% better recall.
-    For grounding: instruct Claude to cite word-for-word from these documents.
+    Per-block filtering prevents context overflow on large cases (9+ docs).
     """
+    allowed = _BLOCK_DOC_TYPES.get(block_id, [])  # [] sentinel = not in map → all docs
+    if allowed is None:
+        return ""  # this block doesn't need raw source docs
+
     docs = await db.list_documents(case_id)
     context_parts = []
     for doc in docs:
+        if allowed and doc["doc_type"] not in allowed:
+            continue
         text = await db.get_document_text(UUID(doc["id"]))
         if text:
             context_parts.append(f"--- מסמך: {doc['title']} ({doc['doc_type']}) ---\n{text}")

mcp-server/src/legal_mcp/services/claude_session.py

@@ -72,6 +72,9 @@ async def query(
     """
     full_prompt = f"{system}\n\n{prompt}" if system else prompt
 
+    if len(full_prompt) > 150_000:
+        logger.warning("Large prompt: %d chars — may hit context limits", len(full_prompt))
+
     cmd = [
         "claude", "-p",
         "--output-format", "json",
@@ -110,7 +113,8 @@ async def query(
 
     if proc.returncode != 0:
         stderr = stderr_b.decode("utf-8", errors="replace").strip()[:500] or "unknown error"
-        raise RuntimeError(f"Claude CLI failed (exit {proc.returncode}): {stderr}")
+        size_info = f"; prompt_len={len(full_prompt):,} chars" if len(full_prompt) > 100_000 else ""
+        raise RuntimeError(f"Claude CLI failed (exit {proc.returncode}): {stderr}{size_info}")
 
     stdout = stdout_b.decode("utf-8", errors="replace").strip()
     if not stdout: