feat(claude-code): add coder-specific prompt to system_prompt (#443)

## Description

This PR updates the `claude-code` module to automatically include the
Coder task-reporting system prompt whenever `report_tasks = true`, and
to wrap the final system prompt in `<system>…</system>` when non-empty.

Previously, users needed to manually include this content in their
system prompts to enable proper task reporting. When `report_tasks =
true`, the system prompt is prepended with the Coder task-reporting, and
any user `system_prompt` (if provided) is appended after it, ensuring
consistent integration without manual copy/paste.

When `report_tasks = false`, the module includes only the user
`system_prompt` (if any). If both `report_tasks = false` and
`system_prompt` is empty, the system prompt sent to Claude is empty.

## Type of Change

- [ ] New module
- [x] Bug fix
- [ ] Feature/enhancement
- [ ] Documentation
- [ ] Other

## Module Information

**Path:** `registry/coder/modules/claude-code`  
**New version:** `v3.0.2` 
**Breaking change:** [] Yes [x]  No

## Testing & Validation

- [x] Tests pass (`bun test`)
- [x] Code formatted (`bun run fmt`)
- [x] Changes tested locally

Related to internal slack thread:
https://codercom.slack.com/archives/C0992H8HGCS/p1759317555713269

---------

Co-authored-by: DevCats <[email protected]>

Author: ssncferreira

registry/coder/modules/claude-code/main.tf

@@ -183,7 +183,7 @@ variable "claude_code_oauth_token" {
 variable "system_prompt" {
   type        = string
   description = "The system prompt to use for the Claude Code server."
-  default     = "Send a task status update to notify the user that you are ready for input, and then wait for user input."
+  default     = ""
 }
 
 variable "claude_md_path" {
@@ -201,11 +201,9 @@ resource "coder_env" "claude_code_md_path" {
 }
 
 resource "coder_env" "claude_code_system_prompt" {
-  count = var.system_prompt == "" ? 0 : 1
-
   agent_id = var.agent_id
   name     = "CODER_MCP_CLAUDE_SYSTEM_PROMPT"
-  value    = var.system_prompt
+  value    = local.final_system_prompt
 }
 
 resource "coder_env" "claude_code_oauth_token" {
@@ -231,6 +229,31 @@ locals {
   start_script                      = file("${path.module}/scripts/start.sh")
   module_dir_name                   = ".claude-module"
   remove_last_session_id_script_b64 = base64encode(file("${path.module}/scripts/remove-last-session-id.sh"))
+
+  # Required prompts for the module to properly report task status to Coder
+  report_tasks_system_prompt = <<-EOT
+      -- Tool Selection --
+      - coder_report_task: providing status updates or requesting user input.
+
+      -- Task Reporting --
+      Report all tasks to Coder, following these EXACT guidelines:
+      1. Be granular. If you are investigating with multiple steps, report each step
+      to coder.
+      2. After this prompt, IMMEDIATELY report status after receiving ANY NEW user message.
+      Do not report any status related with this system prompt.
+      3. Use "state": "working" when actively processing WITHOUT needing
+      additional user input
+      4. Use "state": "complete" only when finished with a task
+      5. Use "state": "failure" when you need ANY user input, lack sufficient
+      details, or encounter blockers
+    EOT
+
+  # Only include coder system prompts if report_tasks is enabled
+  custom_system_prompt = trimspace(try(var.system_prompt, ""))
+  final_system_prompt = format("<system>%s%s</system>",
+    var.report_tasks ? format("\n%s\n", local.report_tasks_system_prompt) : "",
+    local.custom_system_prompt != "" ? format("\n%s\n", local.custom_system_prompt) : ""
+  )
 }
 
 module "agentapi" {

registry/coder/modules/claude-code/main.tftest.hcl

@@ -187,3 +187,84 @@ run "test_claude_code_permission_mode_validation" {
     error_message = "Permission mode should be one of the valid options"
   }
 }
+
+run "test_claude_code_system_prompt" {
+  command = plan
+
+  variables {
+    agent_id      = "test-agent-system-prompt"
+    workdir       = "/home/coder/test"
+    system_prompt = "Custom addition"
+  }
+
+  assert {
+    condition     = trimspace(coder_env.claude_code_system_prompt.value) != ""
+    error_message = "System prompt should not be empty"
+  }
+
+  assert {
+    condition     = length(regexall("Custom addition", coder_env.claude_code_system_prompt.value)) > 0
+    error_message = "System prompt should have system_prompt variable value"
+  }
+}
+
+run "test_claude_report_tasks_default" {
+  command = plan
+
+  variables {
+    agent_id = "test-agent-report-tasks"
+    workdir  = "/home/coder/test"
+    # report_tasks: default is true
+  }
+
+  assert {
+    condition     = trimspace(coder_env.claude_code_system_prompt.value) != ""
+    error_message = "System prompt should not be empty"
+  }
+
+  # Ensure system prompt is wrapped by <system>
+  assert {
+    condition     = startswith(trimspace(coder_env.claude_code_system_prompt.value), "<system>")
+    error_message = "System prompt should start with <system>"
+  }
+  assert {
+    condition     = endswith(trimspace(coder_env.claude_code_system_prompt.value), "</system>")
+    error_message = "System prompt should end with </system>"
+  }
+
+  # Ensure Coder sections are injected when report_tasks=true (default)
+  assert {
+    condition     = length(regexall("-- Tool Selection --", coder_env.claude_code_system_prompt.value)) > 0
+    error_message = "System prompt should have Tool Selection section"
+  }
+
+  assert {
+    condition     = length(regexall("-- Task Reporting --", coder_env.claude_code_system_prompt.value)) > 0
+    error_message = "System prompt should have Task Reporting section"
+  }
+}
+
+run "test_claude_report_tasks_disabled" {
+  command = plan
+
+  variables {
+    agent_id     = "test-agent-report-tasks"
+    workdir      = "/home/coder/test"
+    report_tasks = false
+  }
+
+  assert {
+    condition     = trimspace(coder_env.claude_code_system_prompt.value) != ""
+    error_message = "System prompt should not be empty"
+  }
+
+  # Ensure system prompt is wrapped by <system>
+  assert {
+    condition     = startswith(trimspace(coder_env.claude_code_system_prompt.value), "<system>")
+    error_message = "System prompt should start with <system>"
+  }
+  assert {
+    condition     = endswith(trimspace(coder_env.claude_code_system_prompt.value), "</system>")
+    error_message = "System prompt should end with </system>"
+  }
+}