diff --git a/apps/server/agent/prompts/chat.ts b/apps/server/agent/prompts/chat.ts
index 9e2af87..49158c2 100644
--- a/apps/server/agent/prompts/chat.ts
+++ b/apps/server/agent/prompts/chat.ts
@@ -11,6 +11,7 @@ export interface ChatHistoryEntry {
 
 export function buildChatPrompt(
   targetType: 'phase' | 'task',
+  targetId: string,
   chatHistory: ChatHistoryEntry[],
   userInstruction: string,
 ): string {
@@ -18,10 +19,46 @@ export function buildChatPrompt(
     ? `<chat_history>\n${chatHistory.map(m => `[${m.role}]: ${m.content}`).join('\n\n')}\n</chat_history>`
     : '';
 
+  const scopeBlock = targetType === 'phase'
+    ? `<scope>
+Your **primary target** is the phase described in \`.cw/input/phase.md\` (ID: ${targetId}).
+
+**What you may modify:**
+- The target phase itself (update its name, description, dependencies)
+- Tasks that belong to the target phase — these are in \`.cw/input/context/tasks/\` with \`phaseId: ${targetId}\` in their frontmatter. You may create, update, delete, reorder, or restructure these tasks.
+- Create new tasks under the target phase (set \`phaseId: ${targetId}\` in frontmatter)
+
+**Context (modify only if necessary, e.g. to fix dependencies or references):**
+- \`.cw/input/initiative.md\` — the parent initiative. Prefer read-only.
+- \`.cw/input/context/phases/\` — other phases in the initiative. Prefer read-only — only modify if dependencies or references require it.
+- Tasks in \`.cw/input/context/tasks/\` where \`phaseId\` is NOT \`${targetId}\` — these belong to other phases. Only modify if dependencies or references require it.
+- \`.cw/input/pages/\` — initiative pages. Prefer read-only.
+
+Always focus changes on the primary target and its tasks. Only touch context entities when strictly necessary (e.g. updating a dependency link). If unsure, ask the user.
+</scope>`
+    : `<scope>
+Your **primary target** is the task described in \`.cw/input/task.md\` (ID: ${targetId}).
+
+**What you may modify:**
+- The target task itself (update its name, description, category, priority, dependencies)
+- Child tasks (subtasks) of the target task — these are in \`.cw/input/context/tasks/\` with \`parentTaskId: ${targetId}\` in their frontmatter. You may create, update, delete, or restructure these subtasks.
+- Create new child tasks under the target task (set \`parentTaskId: ${targetId}\` in frontmatter)
+
+**Context (modify only if necessary, e.g. to fix dependencies or references):**
+- \`.cw/input/initiative.md\` — the parent initiative. Prefer read-only.
+- \`.cw/input/phase.md\` — the phase this task belongs to (if any). Prefer read-only.
+- \`.cw/input/context/phases/\` — all phases. Prefer read-only.
+- Tasks in \`.cw/input/context/tasks/\` where \`parentTaskId\` is NOT \`${targetId}\` and \`id\` is NOT \`${targetId}\` — these are sibling or unrelated tasks. Only modify if dependencies or references require it.
+- \`.cw/input/pages/\` — initiative pages. Prefer read-only.
+
+Always focus changes on the primary target and its child tasks. Only touch context entities when strictly necessary (e.g. updating a dependency link). If unsure, ask the user.
+</scope>`;
+
   return `<role>
 You are an Architect agent in chat mode. You iteratively refine ${targetType} structure and content through conversation with the user. You do NOT write code.
 </role>
 ${INPUT_FILES}
+${scopeBlock}
 ${ID_GENERATION}
 ${SIGNAL_FORMAT}
 
@@ -67,5 +104,6 @@ After writing output files, write \`.cw/output/SUMMARY.md\` with a brief descrip
 - Apply the minimal set of changes needed for the user's instruction
 - Preserve existing entity IDs — only use \`cw id\` for new entities
 - When updating, only include changed fields in frontmatter (plus required \`action\` and \`title\`)
+- Focus on the primary target — only modify context entities when strictly necessary
 </rules>`;
 }
diff --git a/apps/server/trpc/routers/chat-session.ts b/apps/server/trpc/routers/chat-session.ts
index 2defcdb..6f3fb5e 100644
--- a/apps/server/trpc/routers/chat-session.ts
+++ b/apps/server/trpc/routers/chat-session.ts
@@ -96,7 +96,7 @@ export function chatSessionProcedures(publicProcedure: ProcedureBuilder) {
           input.initiativeId,
         );
 
-        const prompt = buildChatPrompt(input.targetType, chatHistory, input.message);
+        const prompt = buildChatPrompt(input.targetType, input.targetId, chatHistory, input.message);
 
         // Create a task for the chat agent
         const targetName = input.targetType === 'phase'