diff --git a/src/hooks/prometheus-md-only/constants.ts b/src/hooks/prometheus-md-only/constants.ts
new file mode 100644
index 00000000..062a7414
--- /dev/null
+++ b/src/hooks/prometheus-md-only/constants.ts
@@ -0,0 +1,9 @@
+export const HOOK_NAME = "prometheus-md-only"
+
+export const PROMETHEUS_AGENTS = ["Prometheus (Planner)"]
+
+export const ALLOWED_EXTENSIONS = [".md"]
+
+export const ALLOWED_PATH_PREFIX = ".sisyphus/"
+
+export const BLOCKED_TOOLS = ["Write", "Edit", "write", "edit"]
diff --git a/src/hooks/prometheus-md-only/index.test.ts b/src/hooks/prometheus-md-only/index.test.ts
new file mode 100644
index 00000000..526cc3f6
--- /dev/null
+++ b/src/hooks/prometheus-md-only/index.test.ts
@@ -0,0 +1,162 @@
+import { describe, expect, test } from "bun:test"
+import { createPrometheusMdOnlyHook } from "./index"
+
+describe("prometheus-md-only", () => {
+  function createMockPluginInput() {
+    return {
+      client: {},
+      directory: "/tmp/test",
+    } as any
+  }
+
+  test("should block Prometheus from writing non-.md files", async () => {
+    // #given
+    const hook = createPrometheusMdOnlyHook(createMockPluginInput())
+    const input = {
+      tool: "Write",
+      sessionID: "test-session",
+      callID: "call-1",
+      agent: "Prometheus (Planner)",
+    }
+    const output = {
+      args: { filePath: "/path/to/file.ts" },
+    }
+
+    // #when / #then
+    await expect(
+      hook["tool.execute.before"](input, output)
+    ).rejects.toThrow("can only write/edit .md files")
+  })
+
+  test("should allow Prometheus to write .md files inside .sisyphus/", async () => {
+    // #given
+    const hook = createPrometheusMdOnlyHook(createMockPluginInput())
+    const input = {
+      tool: "Write",
+      sessionID: "test-session",
+      callID: "call-1",
+      agent: "Prometheus (Planner)",
+    }
+    const output = {
+      args: { filePath: "/project/.sisyphus/plans/work-plan.md" },
+    }
+
+    // #when / #then
+    await expect(
+      hook["tool.execute.before"](input, output)
+    ).resolves.toBeUndefined()
+  })
+
+  test("should block Prometheus from writing .md files outside .sisyphus/", async () => {
+    // #given
+    const hook = createPrometheusMdOnlyHook(createMockPluginInput())
+    const input = {
+      tool: "Write",
+      sessionID: "test-session",
+      callID: "call-1",
+      agent: "Prometheus (Planner)",
+    }
+    const output = {
+      args: { filePath: "/path/to/README.md" },
+    }
+
+    // #when / #then
+    await expect(
+      hook["tool.execute.before"](input, output)
+    ).rejects.toThrow("can only write/edit .md files inside .sisyphus/")
+  })
+
+  test("should not affect non-Prometheus agents", async () => {
+    // #given
+    const hook = createPrometheusMdOnlyHook(createMockPluginInput())
+    const input = {
+      tool: "Write",
+      sessionID: "test-session",
+      callID: "call-1",
+      agent: "Sisyphus",
+    }
+    const output = {
+      args: { filePath: "/path/to/file.ts" },
+    }
+
+    // #when / #then
+    await expect(
+      hook["tool.execute.before"](input, output)
+    ).resolves.toBeUndefined()
+  })
+
+  test("should not affect non-Write/Edit tools", async () => {
+    // #given
+    const hook = createPrometheusMdOnlyHook(createMockPluginInput())
+    const input = {
+      tool: "Read",
+      sessionID: "test-session",
+      callID: "call-1",
+      agent: "Prometheus (Planner)",
+    }
+    const output = {
+      args: { filePath: "/path/to/file.ts" },
+    }
+
+    // #when / #then
+    await expect(
+      hook["tool.execute.before"](input, output)
+    ).resolves.toBeUndefined()
+  })
+
+  test("should block Edit tool for non-.md files", async () => {
+    // #given
+    const hook = createPrometheusMdOnlyHook(createMockPluginInput())
+    const input = {
+      tool: "Edit",
+      sessionID: "test-session",
+      callID: "call-1",
+      agent: "Prometheus (Planner)",
+    }
+    const output = {
+      args: { filePath: "/path/to/code.py" },
+    }
+
+    // #when / #then
+    await expect(
+      hook["tool.execute.before"](input, output)
+    ).rejects.toThrow("can only write/edit .md files")
+  })
+
+  test("should handle missing filePath gracefully", async () => {
+    // #given
+    const hook = createPrometheusMdOnlyHook(createMockPluginInput())
+    const input = {
+      tool: "Write",
+      sessionID: "test-session",
+      callID: "call-1",
+      agent: "Prometheus (Planner)",
+    }
+    const output = {
+      args: {},
+    }
+
+    // #when / #then
+    await expect(
+      hook["tool.execute.before"](input, output)
+    ).resolves.toBeUndefined()
+  })
+
+  test("should handle missing agent gracefully", async () => {
+    // #given
+    const hook = createPrometheusMdOnlyHook(createMockPluginInput())
+    const input = {
+      tool: "Write",
+      sessionID: "test-session",
+      callID: "call-1",
+    }
+    const output = {
+      args: { filePath: "/path/to/file.ts" },
+    }
+
+    // #when / #then
+    await expect(
+      hook["tool.execute.before"](input, output)
+    ).resolves.toBeUndefined()
+  })
+})
diff --git a/src/hooks/prometheus-md-only/index.ts b/src/hooks/prometheus-md-only/index.ts
new file mode 100644
index 00000000..52e7c83c
--- /dev/null
+++ b/src/hooks/prometheus-md-only/index.ts
@@ -0,0 +1,57 @@
+import type { PluginInput } from "@opencode-ai/plugin"
+import { HOOK_NAME, PROMETHEUS_AGENTS, ALLOWED_EXTENSIONS, ALLOWED_PATH_PREFIX, BLOCKED_TOOLS } from "./constants"
+import { log } from "../../shared/logger"
+
+export * from "./constants"
+
+function isAllowedFile(filePath: string): boolean {
+  const hasAllowedExtension = ALLOWED_EXTENSIONS.some(ext => filePath.endsWith(ext))
+  const isInAllowedPath = filePath.includes(ALLOWED_PATH_PREFIX)
+  return hasAllowedExtension && isInAllowedPath
+}
+
+export function createPrometheusMdOnlyHook(_ctx: PluginInput) {
+  return {
+    "tool.execute.before": async (
+      input: { tool: string; sessionID: string; callID: string; agent?: string },
+      output: { args: Record<string, unknown>; message?: string }
+    ): Promise<void> => {
+      const agentName = input.agent
+      
+      if (!agentName || !PROMETHEUS_AGENTS.includes(agentName)) {
+        return
+      }
+
+      const toolName = input.tool
+      if (!BLOCKED_TOOLS.includes(toolName)) {
+        return
+      }
+
+      const filePath = (output.args.filePath ?? output.args.path ?? output.args.file) as string | undefined
+      if (!filePath) {
+        return
+      }
+
+      if (!isAllowedFile(filePath)) {
+        log(`[${HOOK_NAME}] Blocked: Prometheus can only write to .sisyphus/*.md`, {
+          sessionID: input.sessionID,
+          tool: toolName,
+          filePath,
+          agent: agentName,
+        })
+        throw new Error(
+          `[${HOOK_NAME}] Prometheus (Planner) can only write/edit .md files inside .sisyphus/ directory. ` +
+          `Attempted to modify: ${filePath}. ` +
+          `Prometheus is a READ-ONLY planner. Use /start-work to execute the plan.`
+        )
+      }
+
+      log(`[${HOOK_NAME}] Allowed: .sisyphus/*.md write permitted`, {
+        sessionID: input.sessionID,
+        tool: toolName,
+        filePath,
+        agent: agentName,
+      })
+    },
+  }
+}