diff --git a/src/features/background-agent/manager.test.ts b/src/features/background-agent/manager.test.ts
index 7bd7709f..8392e8fd 100644
--- a/src/features/background-agent/manager.test.ts
+++ b/src/features/background-agent/manager.test.ts
@@ -855,7 +855,7 @@ describe("BackgroundManager.notifyParentSession - dynamic message lookup", () =>
       .notifyParentSession(task)
 
     //#then
-    expect(capturedBody?.agent).toBe("sisyphus")
+    expect(capturedBody?.agent).toBe("Sisyphus (Ultraworker)")
     expect(capturedBody?.model).toEqual({ providerID: "anthropic", modelID: "claude-opus-4-6" })
 
     manager.shutdown()
diff --git a/src/features/background-agent/manager.ts b/src/features/background-agent/manager.ts
index 61e5d843..2233e0a2 100644
--- a/src/features/background-agent/manager.ts
+++ b/src/features/background-agent/manager.ts
@@ -15,6 +15,7 @@ import {
   resolveInheritedPromptTools,
   createInternalAgentTextPart,
 } from "../../shared"
+import { normalizeAgentForPrompt } from "../../shared/agent-display-names"
 import { setSessionTools } from "../../shared/session-tools-store"
 import { SessionCategoryRegistry } from "../../shared/session-category-registry"
 import { ConcurrencyManager } from "./concurrency"
@@ -1311,10 +1312,11 @@ Use \`background_output(task_id="${task.id}")\` to retrieve this result when rea
         }
 
         const resolvedTools = resolveInheritedPromptTools(task.parentSessionID, tools)
+        const promptAgent = normalizeAgentForPrompt(agent)
 
         log("[background-agent] notifyParentSession context:", {
           taskId: task.id,
-          resolvedAgent: agent,
+          resolvedAgent: promptAgent,
           resolvedModel: model,
         })
 
@@ -1323,7 +1325,7 @@ Use \`background_output(task_id="${task.id}")\` to retrieve this result when rea
             path: { id: task.parentSessionID },
             body: {
               noReply: !allComplete,
-              ...(agent !== undefined ? { agent } : {}),
+              ...(promptAgent !== undefined ? { agent: promptAgent } : {}),
               ...(model !== undefined ? { model } : {}),
               ...(resolvedTools ? { tools: resolvedTools } : {}),
               parts: [createInternalAgentTextPart(notification)],
diff --git a/src/hooks/atlas/boulder-continuation-injector.ts b/src/hooks/atlas/boulder-continuation-injector.ts
index 289668b4..acfa7661 100644
--- a/src/hooks/atlas/boulder-continuation-injector.ts
+++ b/src/hooks/atlas/boulder-continuation-injector.ts
@@ -1,5 +1,6 @@
 import type { PluginInput } from "@opencode-ai/plugin"
 import type { BackgroundManager } from "../../features/background-agent"
+import { normalizeAgentForPrompt } from "../../shared/agent-display-names"
 import { log } from "../../shared/logger"
 import { createInternalAgentTextPart, resolveInheritedPromptTools } from "../../shared"
 import { HOOK_NAME } from "./hook-name"
@@ -40,6 +41,7 @@ export async function injectBoulderContinuation(input: {
   const prompt =
     BOULDER_CONTINUATION_PROMPT.replace(/{PLAN_NAME}/g, planName) +
     `\n\n[Status: ${total - remaining}/${total} completed, ${remaining} remaining]`
+  const promptAgent = normalizeAgentForPrompt(agent ?? "atlas") ?? "atlas"
 
   try {
     log(`[${HOOK_NAME}] Injecting boulder continuation`, { sessionID, planName, remaining })
@@ -50,7 +52,7 @@ export async function injectBoulderContinuation(input: {
     await ctx.client.session.promptAsync({
       path: { id: sessionID },
       body: {
-        agent: agent ?? "atlas",
+        agent: promptAgent,
         ...(promptContext.model !== undefined ? { model: promptContext.model } : {}),
         ...(inheritedTools ? { tools: inheritedTools } : {}),
         parts: [createInternalAgentTextPart(prompt)],
diff --git a/src/hooks/atlas/index.test.ts b/src/hooks/atlas/index.test.ts
index 065f20b9..1d3b0084 100644
--- a/src/hooks/atlas/index.test.ts
+++ b/src/hooks/atlas/index.test.ts
@@ -997,7 +997,7 @@ describe("atlas hook", () => {
        // then - should call prompt for sisyphus
        expect(mockInput._promptMock).toHaveBeenCalled()
        const callArgs = mockInput._promptMock.mock.calls[0][0]
-       expect(callArgs.body.agent).toBe("sisyphus")
+       expect(callArgs.body.agent).toBe("Sisyphus (Ultraworker)")
      })
 
     test("should debounce rapid continuation injections (prevent infinite loop)", async () => {
diff --git a/src/hooks/ralph-loop/continuation-prompt-injector.ts b/src/hooks/ralph-loop/continuation-prompt-injector.ts
index 58f31953..0b71eebd 100644
--- a/src/hooks/ralph-loop/continuation-prompt-injector.ts
+++ b/src/hooks/ralph-loop/continuation-prompt-injector.ts
@@ -8,6 +8,7 @@ import {
 	normalizeSDKResponse,
 	resolveInheritedPromptTools,
 } from "../../shared"
+import { normalizeAgentForPrompt } from "../../shared/agent-display-names"
 
 type MessageInfo = {
 	agent?: string
@@ -68,11 +69,12 @@ export async function injectContinuationPrompt(
 	}
 
 	const inheritedTools = resolveInheritedPromptTools(sourceSessionID, tools)
+	const promptAgent = normalizeAgentForPrompt(agent)
 
 	await ctx.client.session.promptAsync({
 		path: { id: options.sessionID },
 		body: {
-			...(agent !== undefined ? { agent } : {}),
+			...(promptAgent ? { agent: promptAgent } : {}),
 			...(model !== undefined ? { model } : {}),
 			...(inheritedTools ? { tools: inheritedTools } : {}),
 			parts: [createInternalAgentTextPart(options.prompt)],
diff --git a/src/hooks/session-recovery/resume.ts b/src/hooks/session-recovery/resume.ts
index e5d187d7..168bb773 100644
--- a/src/hooks/session-recovery/resume.ts
+++ b/src/hooks/session-recovery/resume.ts
@@ -1,4 +1,5 @@
 import type { createOpencodeClient } from "@opencode-ai/sdk"
+import { normalizeAgentForPrompt } from "../../shared/agent-display-names"
 import type { MessageData, ResumeConfig } from "./types"
 import { createInternalAgentTextPart, resolveInheritedPromptTools } from "../../shared"
 
@@ -25,13 +26,15 @@ export function extractResumeConfig(userMessage: MessageData | undefined, sessio
 }
 
 export async function resumeSession(client: Client, config: ResumeConfig): Promise<boolean> {
+  const promptAgent = normalizeAgentForPrompt(config.agent)
+
   try {
     const inheritedTools = resolveInheritedPromptTools(config.sessionID, config.tools)
     await client.session.promptAsync({
       path: { id: config.sessionID },
       body: {
         parts: [createInternalAgentTextPart(RECOVERY_RESUME_TEXT)],
-        agent: config.agent,
+        ...(promptAgent ? { agent: promptAgent } : {}),
         model: config.model,
         ...(inheritedTools ? { tools: inheritedTools } : {}),
       },
diff --git a/src/hooks/unstable-agent-babysitter/unstable-agent-babysitter-hook.ts b/src/hooks/unstable-agent-babysitter/unstable-agent-babysitter-hook.ts
index 9bfdbb01..baf9d57c 100644
--- a/src/hooks/unstable-agent-babysitter/unstable-agent-babysitter-hook.ts
+++ b/src/hooks/unstable-agent-babysitter/unstable-agent-babysitter-hook.ts
@@ -1,5 +1,6 @@
 import type { BackgroundManager } from "../../features/background-agent"
 import { getMainSessionID, getSessionAgent } from "../../features/claude-code-session-state"
+import { normalizeAgentForPrompt } from "../../shared/agent-display-names"
 import { log } from "../../shared/logger"
 import { createInternalAgentTextPart, resolveInheritedPromptTools } from "../../shared"
 import {
@@ -80,7 +81,7 @@ async function resolveMainSessionTarget(
     log(`[${HOOK_NAME}] Failed to resolve main session agent`, { sessionID, error: String(error) })
   }
 
-  return { agent, model, tools: resolveInheritedPromptTools(sessionID, tools) }
+  return { agent: normalizeAgentForPrompt(agent), model, tools: resolveInheritedPromptTools(sessionID, tools) }
 }
 
 async function getThinkingSummary(ctx: BabysitterContext, sessionID: string): Promise<string | null> {
diff --git a/src/shared/agent-display-names.ts b/src/shared/agent-display-names.ts
index 26ec999a..4704c54d 100644
--- a/src/shared/agent-display-names.ts
+++ b/src/shared/agent-display-names.ts
@@ -53,4 +53,32 @@ export function getAgentConfigKey(agentName: string): string {
   if (reversed !== undefined) return reversed
   if (AGENT_DISPLAY_NAMES[lower] !== undefined) return lower
   return lower
-}
\ No newline at end of file
+}
+
+/**
+ * Normalize an agent name for prompt APIs.
+ * - Known display names -> canonical display names
+ * - Known config keys (any case) -> canonical display names
+ * - Unknown/custom names -> preserved as-is (trimmed)
+ */
+export function normalizeAgentForPrompt(agentName: string | undefined): string | undefined {
+  if (typeof agentName !== "string") {
+    return undefined
+  }
+
+  const trimmed = agentName.trim()
+  if (!trimmed) {
+    return undefined
+  }
+
+  const lower = trimmed.toLowerCase()
+  const reversed = REVERSE_DISPLAY_NAMES[lower]
+  if (reversed !== undefined) {
+    return AGENT_DISPLAY_NAMES[reversed] ?? trimmed
+  }
+  if (AGENT_DISPLAY_NAMES[lower] !== undefined) {
+    return AGENT_DISPLAY_NAMES[lower]
+  }
+
+  return trimmed
+}