diff --git a/server/services/assistant_chat_session.py b/server/services/assistant_chat_session.py
index 73a17e6..b0051b8 100755
--- a/server/services/assistant_chat_session.py
+++ b/server/services/assistant_chat_session.py
@@ -242,6 +242,13 @@ class AssistantChatSession:
         # Get system prompt with project context
         system_prompt = get_system_prompt(self.project_name, self.project_dir)
 
+        # Write system prompt to CLAUDE.md file to avoid Windows command line length limit
+        # The SDK will read this via setting_sources=["project"]
+        claude_md_path = self.project_dir / "CLAUDE.md"
+        with open(claude_md_path, "w", encoding="utf-8") as f:
+            f.write(system_prompt)
+        logger.info(f"Wrote assistant system prompt to {claude_md_path}")
+
         # Use system Claude CLI
         system_cli = shutil.which("claude")
 
@@ -253,7 +260,9 @@ class AssistantChatSession:
                 options=ClaudeAgentOptions(
                     model="claude-opus-4-5-20251101",
                     cli_path=system_cli,
-                    system_prompt=system_prompt,
+                    # System prompt loaded from CLAUDE.md via setting_sources
+                    # This avoids Windows command line length limit (~8191 chars)
+                    setting_sources=["project"],
                     allowed_tools=[*READONLY_BUILTIN_TOOLS, *ASSISTANT_FEATURE_TOOLS],
                     mcp_servers=mcp_servers,
                     permission_mode="bypassPermissions",
diff --git a/server/services/spec_chat_session.py b/server/services/spec_chat_session.py
index e073a4e..1f0d8cc 100644
--- a/server/services/spec_chat_session.py
+++ b/server/services/spec_chat_session.py
@@ -154,6 +154,13 @@ class SpecChatSession:
         project_path = str(self.project_dir.resolve())
         system_prompt = skill_content.replace("$ARGUMENTS", project_path)
 
+        # Write system prompt to CLAUDE.md file to avoid Windows command line length limit
+        # The SDK will read this via setting_sources=["project"]
+        claude_md_path = self.project_dir / "CLAUDE.md"
+        with open(claude_md_path, "w", encoding="utf-8") as f:
+            f.write(system_prompt)
+        logger.info(f"Wrote system prompt to {claude_md_path}")
+
         # Create Claude SDK client with limited tools for spec creation
         # Use Opus for best quality spec generation
         # Use system Claude CLI to avoid bundled Bun runtime crash (exit code 3) on Windows
@@ -167,7 +174,9 @@ class SpecChatSession:
                 options=ClaudeAgentOptions(
                     model="claude-opus-4-5-20251101",
                     cli_path=system_cli,
-                    system_prompt=system_prompt,
+                    # System prompt loaded from CLAUDE.md via setting_sources
+                    # This avoids Windows command line length limit (~8191 chars)
+                    setting_sources=["project"],
                     allowed_tools=[
                         "Read",
                         "Write",