feat(config): add {file:...} syntax support for agent markdown prompts

Enable file content substitution in agent configuration prompts using the
existing `ConfigPaths.substitute` utility. This allows agent markdown files
to reference external file contents via `{file:path}` syntax, with missing
files resolving to empty strings.

Export `substitute` function from ConfigPaths to support agent config
processing.

packages/opencode/src/config/agent.ts

@@ -13,6 +13,7 @@ import * as ConfigMarkdown from "./markdown"
 import { ConfigModelID } from "./model-id"
 import { ConfigParse } from "./parse"
 import { ConfigPermission } from "./permission"
+import { ConfigVariable } from "./variable"
 // kilocode_change start
 import { KilocodeConfig } from "@/kilocode/config/config"
 import type { Warning } from "./config"
@@ -162,10 +163,18 @@ export async function load(dir: string, warnings?: Warning[]) {
     // kilocode_change end
     const name = configEntryNameFromPath(item, patterns)
 
+    const prompt = await ConfigVariable.substitute({
+      text: md.content.trim(),
+      type: "virtual",
+      dir: path.dirname(item),
+      source: item,
+      missing: "empty",
+    })
+
     const config = {
       name,
       ...md.data,
-      prompt: md.content.trim(),
+      prompt,
     }
     // kilocode_change start - use Effect schema (propertyOrder: original) + non-fatal handleInvalid
     try {

packages/opencode/test/config/agent-file-syntax.test.ts

@@ -0,0 +1,88 @@
+import { describe, expect, test } from "bun:test"
+import path from "node:path"
+import fs from "node:fs/promises"
+import { tmpdir } from "../fixture/fixture"
+import { Instance } from "../../src/project/instance"
+import { Agent } from "../../src/agent/agent"
+
+describe("agent {file:...} syntax in markdown body", () => {
+  test("loads file content when {file:...} is used in agent body", async () => {
+    await using tmp = await tmpdir({
+      git: true,
+      init: async (dir) => {
+        await Bun.write(path.join(dir, "prompts", "guide.md"), "Guidelines content")
+
+        const agentsDir = path.join(dir, ".kilo", "agents")
+        await fs.mkdir(agentsDir, { recursive: true })
+        await Bun.write(
+          path.join(agentsDir, "test.md"),
+          ["---", "description: Test Agent", "---", "Base prompt", "", "{file:../../prompts/guide.md}", ""].join("\n"),
+        )
+      },
+    })
+
+    await Instance.provide({
+      directory: tmp.path,
+      fn: async () => {
+        const agent = await Agent.get("test")
+
+        expect(agent).toBeDefined()
+        expect(agent?.prompt).toContain("Base prompt")
+        expect(agent?.prompt).toContain("Guidelines content")
+      },
+    })
+  })
+
+  test("replaces missing file reference with empty string", async () => {
+    await using tmp = await tmpdir({
+      git: true,
+      init: async (dir) => {
+        const agentsDir = path.join(dir, ".kilo", "agents")
+        await fs.mkdir(agentsDir, { recursive: true })
+        await Bun.write(
+          path.join(agentsDir, "test.md"),
+          ["---", "description: Test Agent", "---", "Base prompt", "", "{file:missing.txt}", ""].join("\n"),
+        )
+      },
+    })
+
+    await Instance.provide({
+      directory: tmp.path,
+      fn: async () => {
+        const agent = await Agent.get("test")
+
+        expect(agent).toBeDefined()
+        expect(agent?.prompt).toContain("Base prompt")
+        expect(agent?.prompt).not.toContain("{file:")
+      },
+    })
+  })
+
+  test("resolves multiple file references in same agent", async () => {
+    await using tmp = await tmpdir({
+      git: true,
+      init: async (dir) => {
+        await Bun.write(path.join(dir, "a.md"), "Content A")
+        await Bun.write(path.join(dir, "b.md"), "Content B")
+
+        const agentsDir = path.join(dir, ".kilo", "agents")
+        await fs.mkdir(agentsDir, { recursive: true })
+        await Bun.write(
+          path.join(agentsDir, "test.md"),
+          ["---", "description: Test", "---", "Base", "", "{file:../../a.md}", "{file:../../b.md}", ""].join("\n"),
+        )
+      },
+    })
+
+    await Instance.provide({
+      directory: tmp.path,
+      fn: async () => {
+        const agent = await Agent.get("test")
+
+        expect(agent).toBeDefined()
+        expect(agent?.prompt).toContain("Content A")
+        expect(agent?.prompt).toContain("Content B")
+      },
+    })
+  })
+})