diff --git a/src/hooks/prometheus-md-only/constants.ts b/src/hooks/prometheus-md-only/constants.ts
index 062a74149..b25db57f4 100644
--- a/src/hooks/prometheus-md-only/constants.ts
+++ b/src/hooks/prometheus-md-only/constants.ts
@@ -7,3 +7,24 @@ export const ALLOWED_EXTENSIONS = [".md"]
 export const ALLOWED_PATH_PREFIX = ".sisyphus/"
 
 export const BLOCKED_TOOLS = ["Write", "Edit", "write", "edit"]
+
+export const PLANNING_CONSULT_WARNING = `
+
+---
+
+[SYSTEM DIRECTIVE - READ-ONLY PLANNING CONSULTATION]
+
+You are being invoked by Prometheus (Planner), a READ-ONLY planning agent.
+
+**CRITICAL CONSTRAINTS:**
+- DO NOT modify any files (no Write, Edit, or any file mutations)
+- DO NOT execute commands that change system state
+- DO NOT create, delete, or rename files
+- ONLY provide analysis, recommendations, and information
+
+**YOUR ROLE**: Provide consultation, research, and analysis to assist with planning.
+Return your findings and recommendations. The actual implementation will be handled separately after planning is complete.
+
+---
+
+`
diff --git a/src/hooks/prometheus-md-only/index.test.ts b/src/hooks/prometheus-md-only/index.test.ts
index 526cc3f6c..2a61b477b 100644
--- a/src/hooks/prometheus-md-only/index.test.ts
+++ b/src/hooks/prometheus-md-only/index.test.ts
@@ -159,4 +159,109 @@ describe("prometheus-md-only", () => {
       hook["tool.execute.before"](input, output)
     ).resolves.toBeUndefined()
   })
+
+  test("should inject read-only warning when Prometheus calls sisyphus_task", async () => {
+    // #given
+    const hook = createPrometheusMdOnlyHook(createMockPluginInput())
+    const input = {
+      tool: "sisyphus_task",
+      sessionID: "test-session",
+      callID: "call-1",
+      agent: "Prometheus (Planner)",
+    }
+    const output = {
+      args: { prompt: "Analyze this codebase" },
+    }
+
+    // #when
+    await hook["tool.execute.before"](input, output)
+
+    // #then
+    expect(output.args.prompt).toContain("[SYSTEM DIRECTIVE - READ-ONLY PLANNING CONSULTATION]")
+    expect(output.args.prompt).toContain("DO NOT modify any files")
+  })
+
+  test("should inject read-only warning when Prometheus calls task", async () => {
+    // #given
+    const hook = createPrometheusMdOnlyHook(createMockPluginInput())
+    const input = {
+      tool: "task",
+      sessionID: "test-session",
+      callID: "call-1",
+      agent: "Prometheus (Planner)",
+    }
+    const output = {
+      args: { prompt: "Research this library" },
+    }
+
+    // #when
+    await hook["tool.execute.before"](input, output)
+
+    // #then
+    expect(output.args.prompt).toContain("[SYSTEM DIRECTIVE - READ-ONLY PLANNING CONSULTATION]")
+  })
+
+  test("should inject read-only warning when Prometheus calls call_omo_agent", async () => {
+    // #given
+    const hook = createPrometheusMdOnlyHook(createMockPluginInput())
+    const input = {
+      tool: "call_omo_agent",
+      sessionID: "test-session",
+      callID: "call-1",
+      agent: "Prometheus (Planner)",
+    }
+    const output = {
+      args: { prompt: "Find implementation examples" },
+    }
+
+    // #when
+    await hook["tool.execute.before"](input, output)
+
+    // #then
+    expect(output.args.prompt).toContain("[SYSTEM DIRECTIVE - READ-ONLY PLANNING CONSULTATION]")
+  })
+
+  test("should not inject warning for non-Prometheus agents calling sisyphus_task", async () => {
+    // #given
+    const hook = createPrometheusMdOnlyHook(createMockPluginInput())
+    const input = {
+      tool: "sisyphus_task",
+      sessionID: "test-session",
+      callID: "call-1",
+      agent: "Sisyphus",
+    }
+    const originalPrompt = "Implement this feature"
+    const output = {
+      args: { prompt: originalPrompt },
+    }
+
+    // #when
+    await hook["tool.execute.before"](input, output)
+
+    // #then
+    expect(output.args.prompt).toBe(originalPrompt)
+    expect(output.args.prompt).not.toContain("[SYSTEM DIRECTIVE - READ-ONLY PLANNING CONSULTATION]")
+  })
+
+  test("should not double-inject warning if already present", async () => {
+    // #given
+    const hook = createPrometheusMdOnlyHook(createMockPluginInput())
+    const input = {
+      tool: "sisyphus_task",
+      sessionID: "test-session",
+      callID: "call-1",
+      agent: "Prometheus (Planner)",
+    }
+    const promptWithWarning = "Some prompt [SYSTEM DIRECTIVE - READ-ONLY PLANNING CONSULTATION] already here"
+    const output = {
+      args: { prompt: promptWithWarning },
+    }
+
+    // #when
+    await hook["tool.execute.before"](input, output)
+
+    // #then
+    const occurrences = (output.args.prompt as string).split("[SYSTEM DIRECTIVE - READ-ONLY PLANNING CONSULTATION]").length - 1
+    expect(occurrences).toBe(1)
+  })
 })
diff --git a/src/hooks/prometheus-md-only/index.ts b/src/hooks/prometheus-md-only/index.ts
index 52e7c83c4..2ff8c71bb 100644
--- a/src/hooks/prometheus-md-only/index.ts
+++ b/src/hooks/prometheus-md-only/index.ts
@@ -1,5 +1,5 @@
 import type { PluginInput } from "@opencode-ai/plugin"
-import { HOOK_NAME, PROMETHEUS_AGENTS, ALLOWED_EXTENSIONS, ALLOWED_PATH_PREFIX, BLOCKED_TOOLS } from "./constants"
+import { HOOK_NAME, PROMETHEUS_AGENTS, ALLOWED_EXTENSIONS, ALLOWED_PATH_PREFIX, BLOCKED_TOOLS, PLANNING_CONSULT_WARNING } from "./constants"
 import { log } from "../../shared/logger"
 
 export * from "./constants"
@@ -10,6 +10,8 @@ function isAllowedFile(filePath: string): boolean {
   return hasAllowedExtension && isInAllowedPath
 }
 
+const TASK_TOOLS = ["sisyphus_task", "task", "call_omo_agent"]
+
 export function createPrometheusMdOnlyHook(_ctx: PluginInput) {
   return {
     "tool.execute.before": async (
@@ -23,6 +25,21 @@ export function createPrometheusMdOnlyHook(_ctx: PluginInput) {
       }
 
       const toolName = input.tool
+
+      // Inject read-only warning for task tools called by Prometheus
+      if (TASK_TOOLS.includes(toolName)) {
+        const prompt = output.args.prompt as string | undefined
+        if (prompt && !prompt.includes("[SYSTEM DIRECTIVE - READ-ONLY PLANNING CONSULTATION]")) {
+          output.args.prompt = prompt + PLANNING_CONSULT_WARNING
+          log(`[${HOOK_NAME}] Injected read-only planning warning to ${toolName}`, {
+            sessionID: input.sessionID,
+            tool: toolName,
+            agent: agentName,
+          })
+        }
+        return
+      }
+
       if (!BLOCKED_TOOLS.includes(toolName)) {
         return
       }