diff --git a/README.md b/README.md
index a23e3fdc0e..023cc55235 100644
--- a/README.md
+++ b/README.md
@@ -886,6 +886,26 @@ Use `prompt_append` to add extra instructions without replacing the default syst
 }
 ```
 
+You can also reference external files using `file://` URIs:
+
+```json
+{
+  "agents": {
+    "oracle": {
+      "prompt_append": "file://~/.config/opencode/my-oracle-instructions.md"
+    },
+    "Sisyphus": {
+      "prompt_append": "file://./project-context.md"
+    }
+  }
+}
+```
+
+Supported URI formats:
+- `file:///absolute/path` - Absolute path
+- `file://relative/path` - Relative to project root
+- `file://~/path` - Relative to home directory
+
 You can also override settings for `Sisyphus` (the main orchestrator) and `build` (the default agent) using the same options.
 
 #### Permission Options
diff --git a/src/agents/utils.test.ts b/src/agents/utils.test.ts
index 336ed628a3..c2d3bb9bc4 100644
--- a/src/agents/utils.test.ts
+++ b/src/agents/utils.test.ts
@@ -1,7 +1,26 @@
-import { describe, test, expect } from "bun:test"
+import { describe, test, expect, beforeAll, afterAll } from "bun:test"
+import { mkdirSync, writeFileSync, rmSync } from "fs"
+import { join } from "path"
+import { tmpdir } from "os"
 import { createBuiltinAgents } from "./utils"
 import type { AgentConfig } from "@opencode-ai/sdk"
 
+const TEST_DIR = join(tmpdir(), "utils-test-prompts")
+const TEST_PROMPT_FILE = join(TEST_DIR, "custom-prompt.md")
+const TEST_PROMPT_CONTENT = "Custom prompt from file\nWith multiple lines"
+const TEST_ENCODED_FILE = join(TEST_DIR, "my prompt file.md")
+const TEST_ENCODED_CONTENT = "Prompt from percent-encoded path"
+
+beforeAll(() => {
+  mkdirSync(TEST_DIR, { recursive: true })
+  writeFileSync(TEST_PROMPT_FILE, TEST_PROMPT_CONTENT)
+  writeFileSync(TEST_ENCODED_FILE, TEST_ENCODED_CONTENT)
+})
+
+afterAll(() => {
+  rmSync(TEST_DIR, { recursive: true, force: true })
+})
+
 describe("createBuiltinAgents with model overrides", () => {
   test("Sisyphus with default model has thinking config", () => {
     // #given - no overrides
@@ -72,6 +91,71 @@ describe("createBuiltinAgents with model overrides", () => {
     expect(agents.oracle.textVerbosity).toBeUndefined()
   })
 
+  test("resolves file:// URI in prompt_append with absolute path", () => {
+    // #given
+    const overrides = {
+      oracle: { prompt_append: `file://${TEST_PROMPT_FILE}` },
+    }
+
+    // #when
+    const agents = createBuiltinAgents([], overrides)
+
+    // #then
+    expect(agents.oracle.prompt).toContain(TEST_PROMPT_CONTENT)
+  })
+
+  test("resolves file:// URI in prompt_append with relative path", () => {
+    // #given
+    const overrides = {
+      oracle: { prompt_append: "file://custom-prompt.md" },
+    }
+
+    // #when
+    const agents = createBuiltinAgents([], overrides, TEST_DIR)
+
+    // #then
+    expect(agents.oracle.prompt).toContain(TEST_PROMPT_CONTENT)
+  })
+
+  test("handles non-existent file:// URI in prompt_append with warning", () => {
+    // #given
+    const overrides = {
+      oracle: { prompt_append: "file://non-existent-prompt.md" },
+    }
+
+    // #when
+    const agents = createBuiltinAgents([], overrides, TEST_DIR)
+
+    // #then
+    expect(agents.oracle.prompt).toContain("[WARNING: Could not resolve file URI: file://non-existent-prompt.md]")
+  })
+
+  test("resolves file:// URI with percent-encoded path", () => {
+    // #given - filename has space: "my prompt file.md" → "my%20prompt%20file.md"
+    const overrides = {
+      oracle: { prompt_append: "file://my%20prompt%20file.md" },
+    }
+
+    // #when
+    const agents = createBuiltinAgents([], overrides, TEST_DIR)
+
+    // #then
+    expect(agents.oracle.prompt).toContain(TEST_ENCODED_CONTENT)
+  })
+
+  test("handles malformed percent-encoding in file URI with warning", () => {
+    // #given - malformed percent-encoding (incomplete escape sequence)
+    const overrides = {
+      oracle: { prompt_append: "file://my%prompt.md" },
+    }
+
+    // #when
+    const agents = createBuiltinAgents([], overrides, TEST_DIR)
+
+    // #then
+    expect(agents.oracle.prompt).toContain("[WARNING: Malformed file URI (invalid percent-encoding): file://my%prompt.md]")
+  })
+
   test("non-model overrides are still applied after factory rebuild", () => {
     // #given
     const overrides = {
diff --git a/src/agents/utils.ts b/src/agents/utils.ts
index d831caa871..2a8cf3247d 100644
--- a/src/agents/utils.ts
+++ b/src/agents/utils.ts
@@ -13,6 +13,9 @@ import { createOrchestratorSisyphusAgent, orchestratorSisyphusAgent } from "./or
 import { createMomusAgent } from "./momus"
 import type { AvailableAgent } from "./sisyphus-prompt-builder"
 import { deepMerge } from "../shared"
+import { existsSync, readFileSync } from "fs"
+import { resolve, isAbsolute } from "path"
+import { homedir } from "os"
 import { DEFAULT_CATEGORIES } from "../tools/sisyphus-task/constants"
 import { resolveMultipleSkills } from "../features/opencode-skill-loader/skill-content"
 
@@ -113,13 +116,35 @@ export function createEnvContext(): string {
 
 function mergeAgentConfig(
   base: AgentConfig,
-  override: AgentOverrideConfig
+  override: AgentOverrideConfig,
+  configDir?: string
 ): AgentConfig {
   const { prompt_append, ...rest } = override
   const merged = deepMerge(base, rest as Partial<AgentConfig>)
 
   if (prompt_append && merged.prompt) {
-    merged.prompt = merged.prompt + "\n" + prompt_append
+    let resolved = prompt_append
+    if (prompt_append.startsWith("file://")) {
+      let path: string
+      try {
+        path = decodeURIComponent(prompt_append.slice(7))
+      } catch {
+        resolved = `[WARNING: Malformed file URI (invalid percent-encoding): ${prompt_append}]`
+        path = ""
+      }
+
+      if (path) {
+        const abs = path.startsWith("~/")
+          ? resolve(homedir(), path.slice(2))
+          : isAbsolute(path)
+            ? path
+            : resolve(configDir ?? process.cwd(), path)
+        resolved = existsSync(abs)
+          ? readFileSync(abs, "utf-8")
+          : `[WARNING: Could not resolve file URI: ${prompt_append}]`
+      }
+    }
+    merged.prompt = merged.prompt + "\n" + resolved
   }
 
   return merged
@@ -157,7 +182,7 @@ export function createBuiltinAgents(
     }
 
     if (override) {
-      config = mergeAgentConfig(config, override)
+      config = mergeAgentConfig(config, override, directory)
     }
 
     result[name] = config
@@ -184,7 +209,7 @@ export function createBuiltinAgents(
     }
 
     if (sisyphusOverride) {
-      sisyphusConfig = mergeAgentConfig(sisyphusConfig, sisyphusOverride)
+      sisyphusConfig = mergeAgentConfig(sisyphusConfig, sisyphusOverride, directory)
     }
 
     result["Sisyphus"] = sisyphusConfig
@@ -199,7 +224,7 @@ export function createBuiltinAgents(
     })
 
     if (orchestratorOverride) {
-      orchestratorConfig = mergeAgentConfig(orchestratorConfig, orchestratorOverride)
+      orchestratorConfig = mergeAgentConfig(orchestratorConfig, orchestratorOverride, directory)
     }
 
     result["orchestrator-sisyphus"] = orchestratorConfig
diff --git a/src/config/schema.ts b/src/config/schema.ts
index d5fad7e014..685bc703b7 100644
--- a/src/config/schema.ts
+++ b/src/config/schema.ts
@@ -106,6 +106,7 @@ export const AgentOverrideConfigSchema = z.object({
   temperature: z.number().min(0).max(2).optional(),
   top_p: z.number().min(0).max(1).optional(),
   prompt: z.string().optional(),
+  /** Text to append to agent prompt. Supports file:// URIs (file:///abs, file://rel, file://~/home) */
   prompt_append: z.string().optional(),
   tools: z.record(z.string(), z.boolean()).optional(),
   disable: z.boolean().optional(),