diff --git a/src/agents/builtin-agents.ts b/src/agents/builtin-agents.ts
index b20c166ef..dd91e019f 100644
--- a/src/agents/builtin-agents.ts
+++ b/src/agents/builtin-agents.ts
@@ -171,6 +171,7 @@ export async function createBuiltinAgents(
     availableAgents,
     availableSkills,
     mergedCategories,
+    directory,
     userCategories: categories,
   })
   if (atlasConfig) {
diff --git a/src/agents/builtin-agents/agent-overrides.ts b/src/agents/builtin-agents/agent-overrides.ts
index ad80e8d6f..89873def6 100644
--- a/src/agents/builtin-agents/agent-overrides.ts
+++ b/src/agents/builtin-agents/agent-overrides.ts
@@ -2,6 +2,7 @@ import type { AgentConfig } from "@opencode-ai/sdk"
 import type { AgentOverrideConfig } from "../types"
 import type { CategoryConfig } from "../../config/schema"
 import { deepMerge, migrateAgentConfig } from "../../shared"
+import { resolvePromptAppend } from "./resolve-file-uri"
 
 /**
  * Expands a category reference from an agent override into concrete config properties.
@@ -28,19 +29,23 @@ export function applyCategoryOverride(
   if (categoryConfig.maxTokens !== undefined) result.maxTokens = categoryConfig.maxTokens
 
   if (categoryConfig.prompt_append && typeof result.prompt === "string") {
-    result.prompt = result.prompt + "\n" + categoryConfig.prompt_append
+    result.prompt = result.prompt + "\n" + resolvePromptAppend(categoryConfig.prompt_append)
   }
 
   return result as AgentConfig
 }
 
-export function mergeAgentConfig(base: AgentConfig, override: AgentOverrideConfig): AgentConfig {
+export function mergeAgentConfig(
+  base: AgentConfig,
+  override: AgentOverrideConfig,
+  directory?: string
+): AgentConfig {
   const migratedOverride = migrateAgentConfig(override as Record<string, unknown>) as AgentOverrideConfig
   const { prompt_append, ...rest } = migratedOverride
   const merged = deepMerge(base, rest as Partial<AgentConfig>)
 
   if (prompt_append && merged.prompt) {
-    merged.prompt = merged.prompt + "\n" + prompt_append
+    merged.prompt = merged.prompt + "\n" + resolvePromptAppend(prompt_append, directory)
   }
 
   return merged
@@ -49,7 +54,8 @@ export function mergeAgentConfig(base: AgentConfig, override: AgentOverrideConfi
 export function applyOverrides(
   config: AgentConfig,
   override: AgentOverrideConfig | undefined,
-  mergedCategories: Record<string, CategoryConfig>
+  mergedCategories: Record<string, CategoryConfig>,
+  directory?: string
 ): AgentConfig {
   let result = config
   const overrideCategory = (override as Record<string, unknown> | undefined)?.category as string | undefined
@@ -58,7 +64,7 @@ export function applyOverrides(
   }
 
   if (override) {
-    result = mergeAgentConfig(result, override)
+    result = mergeAgentConfig(result, override, directory)
   }
 
   return result
diff --git a/src/agents/builtin-agents/atlas-agent.ts b/src/agents/builtin-agents/atlas-agent.ts
index e9c3d17ef..f1658ebc9 100644
--- a/src/agents/builtin-agents/atlas-agent.ts
+++ b/src/agents/builtin-agents/atlas-agent.ts
@@ -16,6 +16,7 @@ export function maybeCreateAtlasConfig(input: {
   availableAgents: AvailableAgent[]
   availableSkills: AvailableSkill[]
   mergedCategories: Record<string, CategoryConfig>
+  directory?: string
   userCategories?: CategoriesConfig
   useTaskSystem?: boolean
 }): AgentConfig | undefined {
@@ -28,6 +29,7 @@ export function maybeCreateAtlasConfig(input: {
     availableAgents,
     availableSkills,
     mergedCategories,
+    directory,
     userCategories,
   } = input
 
@@ -58,7 +60,7 @@ export function maybeCreateAtlasConfig(input: {
     orchestratorConfig = { ...orchestratorConfig, variant: atlasResolvedVariant }
   }
 
-  orchestratorConfig = applyOverrides(orchestratorConfig, orchestratorOverride, mergedCategories)
+  orchestratorConfig = applyOverrides(orchestratorConfig, orchestratorOverride, mergedCategories, directory)
 
   return orchestratorConfig
 }
diff --git a/src/agents/builtin-agents/general-agents.ts b/src/agents/builtin-agents/general-agents.ts
index 491e7be11..c84dea2e8 100644
--- a/src/agents/builtin-agents/general-agents.ts
+++ b/src/agents/builtin-agents/general-agents.ts
@@ -84,7 +84,7 @@ export function collectPendingBuiltinAgents(input: {
       config = applyEnvironmentContext(config, directory)
     }
 
-    config = applyOverrides(config, override, mergedCategories)
+    config = applyOverrides(config, override, mergedCategories, directory)
 
     // Store for later - will be added after sisyphus and hephaestus
     pendingAgentConfigs.set(name, config)
diff --git a/src/agents/builtin-agents/hephaestus-agent.ts b/src/agents/builtin-agents/hephaestus-agent.ts
index 649ee293d..dc1ac25d3 100644
--- a/src/agents/builtin-agents/hephaestus-agent.ts
+++ b/src/agents/builtin-agents/hephaestus-agent.ts
@@ -85,7 +85,7 @@ export function maybeCreateHephaestusConfig(input: {
   }
 
   if (hephaestusOverride) {
-    hephaestusConfig = mergeAgentConfig(hephaestusConfig, hephaestusOverride)
+    hephaestusConfig = mergeAgentConfig(hephaestusConfig, hephaestusOverride, directory)
   }
   return hephaestusConfig
 }
diff --git a/src/agents/builtin-agents/resolve-file-uri.test.ts b/src/agents/builtin-agents/resolve-file-uri.test.ts
new file mode 100644
index 000000000..9c045babd
--- /dev/null
+++ b/src/agents/builtin-agents/resolve-file-uri.test.ts
@@ -0,0 +1,109 @@
+import { afterAll, beforeAll, describe, expect, test } from "bun:test"
+import { mkdirSync, rmSync, writeFileSync } from "node:fs"
+import { homedir, tmpdir } from "node:os"
+import { join } from "node:path"
+import { resolvePromptAppend } from "./resolve-file-uri"
+
+describe("resolvePromptAppend", () => {
+  const fixtureRoot = join(tmpdir(), `resolve-file-uri-${Date.now()}`)
+  const configDir = join(fixtureRoot, "config")
+  const homeFixtureDir = join(homedir(), `.resolve-file-uri-home-${Date.now()}`)
+
+  const absoluteFilePath = join(fixtureRoot, "absolute.txt")
+  const relativeFilePath = join(configDir, "relative.txt")
+  const spacedFilePath = join(fixtureRoot, "with space.txt")
+  const homeFilePath = join(homeFixtureDir, "home.txt")
+
+  beforeAll(() => {
+    mkdirSync(fixtureRoot, { recursive: true })
+    mkdirSync(configDir, { recursive: true })
+    mkdirSync(homeFixtureDir, { recursive: true })
+
+    writeFileSync(absoluteFilePath, "absolute-content", "utf8")
+    writeFileSync(relativeFilePath, "relative-content", "utf8")
+    writeFileSync(spacedFilePath, "encoded-content", "utf8")
+    writeFileSync(homeFilePath, "home-content", "utf8")
+  })
+
+  afterAll(() => {
+    rmSync(fixtureRoot, { recursive: true, force: true })
+    rmSync(homeFixtureDir, { recursive: true, force: true })
+  })
+
+  test("returns non-file URI strings unchanged", () => {
+    //#given
+    const input = "append this text"
+
+    //#when
+    const resolved = resolvePromptAppend(input)
+
+    //#then
+    expect(resolved).toBe(input)
+  })
+
+  test("resolves absolute file URI to file contents", () => {
+    //#given
+    const input = `file://${absoluteFilePath}`
+
+    //#when
+    const resolved = resolvePromptAppend(input)
+
+    //#then
+    expect(resolved).toBe("absolute-content")
+  })
+
+  test("resolves relative file URI using configDir", () => {
+    //#given
+    const input = "file://./relative.txt"
+
+    //#when
+    const resolved = resolvePromptAppend(input, configDir)
+
+    //#then
+    expect(resolved).toBe("relative-content")
+  })
+
+  test("resolves home directory URI path", () => {
+    //#given
+    const input = `file://~/${homeFixtureDir.split("/").pop()}/home.txt`
+
+    //#when
+    const resolved = resolvePromptAppend(input)
+
+    //#then
+    expect(resolved).toBe("home-content")
+  })
+
+  test("resolves percent-encoded URI path", () => {
+    //#given
+    const input = `file://${encodeURIComponent(spacedFilePath)}`
+
+    //#when
+    const resolved = resolvePromptAppend(input)
+
+    //#then
+    expect(resolved).toBe("encoded-content")
+  })
+
+  test("returns warning for malformed percent-encoding", () => {
+    //#given
+    const input = "file://%E0%A4%A"
+
+    //#when
+    const resolved = resolvePromptAppend(input)
+
+    //#then
+    expect(resolved).toContain("[WARNING: Malformed file URI")
+  })
+
+  test("returns warning when file does not exist", () => {
+    //#given
+    const input = "file:///path/does/not/exist.txt"
+
+    //#when
+    const resolved = resolvePromptAppend(input)
+
+    //#then
+    expect(resolved).toContain("[WARNING: Could not resolve file URI")
+  })
+})
diff --git a/src/agents/builtin-agents/resolve-file-uri.ts b/src/agents/builtin-agents/resolve-file-uri.ts
new file mode 100644
index 000000000..56c3ace5f
--- /dev/null
+++ b/src/agents/builtin-agents/resolve-file-uri.ts
@@ -0,0 +1,30 @@
+import { existsSync, readFileSync } from "node:fs"
+import { homedir } from "node:os"
+import { isAbsolute, resolve } from "node:path"
+
+export function resolvePromptAppend(promptAppend: string, configDir?: string): string {
+  if (!promptAppend.startsWith("file://")) return promptAppend
+
+  const encoded = promptAppend.slice(7)
+
+  let filePath: string
+  try {
+    const decoded = decodeURIComponent(encoded)
+    const expanded = decoded.startsWith("~/") ? decoded.replace(/^~\//, `${homedir()}/`) : decoded
+    filePath = isAbsolute(expanded)
+      ? expanded
+      : resolve(configDir ?? process.cwd(), expanded)
+  } catch {
+    return `[WARNING: Malformed file URI (invalid percent-encoding): ${promptAppend}]`
+  }
+
+  if (!existsSync(filePath)) {
+    return `[WARNING: Could not resolve file URI: ${promptAppend}]`
+  }
+
+  try {
+    return readFileSync(filePath, "utf8")
+  } catch {
+    return `[WARNING: Could not read file: ${promptAppend}]`
+  }
+}
diff --git a/src/agents/builtin-agents/sisyphus-agent.ts b/src/agents/builtin-agents/sisyphus-agent.ts
index 11e34e22a..11b313e21 100644
--- a/src/agents/builtin-agents/sisyphus-agent.ts
+++ b/src/agents/builtin-agents/sisyphus-agent.ts
@@ -77,7 +77,7 @@ export function maybeCreateSisyphusConfig(input: {
     sisyphusConfig = { ...sisyphusConfig, variant: sisyphusResolvedVariant }
   }
 
-  sisyphusConfig = applyOverrides(sisyphusConfig, sisyphusOverride, mergedCategories)
+  sisyphusConfig = applyOverrides(sisyphusConfig, sisyphusOverride, mergedCategories, directory)
   sisyphusConfig = applyEnvironmentContext(sisyphusConfig, directory)
 
   return sisyphusConfig
diff --git a/src/agents/sisyphus-junior/default.ts b/src/agents/sisyphus-junior/default.ts
index fea9e7150..85d919556 100644
--- a/src/agents/sisyphus-junior/default.ts
+++ b/src/agents/sisyphus-junior/default.ts
@@ -7,6 +7,8 @@
  * - Extended reasoning context for complex tasks
  */
 
+import { resolvePromptAppend } from "../builtin-agents/resolve-file-uri"
+
 export function buildDefaultSisyphusJuniorPrompt(
   useTaskSystem: boolean,
   promptAppend?: string
@@ -40,7 +42,7 @@ Task NOT complete without:
 </Style>`
 
   if (!promptAppend) return prompt
-  return prompt + "\n\n" + promptAppend
+  return prompt + "\n\n" + resolvePromptAppend(promptAppend)
 }
 
 function buildConstraintsSection(useTaskSystem: boolean): string {
diff --git a/src/agents/sisyphus-junior/gpt.ts b/src/agents/sisyphus-junior/gpt.ts
index e4a849651..1db0e5666 100644
--- a/src/agents/sisyphus-junior/gpt.ts
+++ b/src/agents/sisyphus-junior/gpt.ts
@@ -16,6 +16,8 @@
  * - Explicit decision criteria needed (model won't infer)
  */
 
+import { resolvePromptAppend } from "../builtin-agents/resolve-file-uri"
+
 export function buildGptSisyphusJuniorPrompt(
   useTaskSystem: boolean,
   promptAppend?: string
@@ -85,7 +87,7 @@ Task NOT complete without evidence:
 </style_spec>`
 
   if (!promptAppend) return prompt
-  return prompt + "\n\n" + promptAppend
+  return prompt + "\n\n" + resolvePromptAppend(promptAppend)
 }
 
 function buildGptBlockedActionsSection(useTaskSystem: boolean): string {
diff --git a/src/config/schema/agent-overrides.ts b/src/config/schema/agent-overrides.ts
index 8fd48e330..876560ec6 100644
--- a/src/config/schema/agent-overrides.ts
+++ b/src/config/schema/agent-overrides.ts
@@ -12,6 +12,7 @@ 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(),
diff --git a/src/plugin-handlers/prometheus-agent-config-builder.ts b/src/plugin-handlers/prometheus-agent-config-builder.ts
index 6e3129add..fa6c12a48 100644
--- a/src/plugin-handlers/prometheus-agent-config-builder.ts
+++ b/src/plugin-handlers/prometheus-agent-config-builder.ts
@@ -1,5 +1,6 @@
 import type { CategoryConfig } from "../config/schema";
 import { PROMETHEUS_PERMISSION, PROMETHEUS_SYSTEM_PROMPT } from "../agents/prometheus";
+import { resolvePromptAppend } from "../agents/builtin-agents/resolve-file-uri";
 import { AGENT_MODEL_REQUIREMENTS } from "../shared/model-requirements";
 import {
   fetchAvailableModels,
@@ -92,7 +93,7 @@ export async function buildPrometheusAgentConfig(params: {
   const { prompt_append, ...restOverride } = override;
   const merged = { ...base, ...restOverride };
   if (prompt_append && typeof merged.prompt === "string") {
-    merged.prompt = merged.prompt + "\n" + prompt_append;
+    merged.prompt = merged.prompt + "\n" + resolvePromptAppend(prompt_append);
   }
   return merged;
 }