diff --git a/packages/agent/src/agent.ts b/packages/agent/src/agent.ts
index c668d10c4..5861b86b5 100644
--- a/packages/agent/src/agent.ts
+++ b/packages/agent/src/agent.ts
@@ -657,8 +657,8 @@ export class Agent {
 	}
 
 	// State mutators
-	setSystemPrompt(v: string[]) {
-		this.#state.systemPrompt = v;
+	setSystemPrompt(v: string[] | string) {
+		this.#state.systemPrompt = typeof v === "string" ? [v] : v;
 	}
 
 	setModel(m: Model) {
diff --git a/packages/coding-agent/CHANGELOG.md b/packages/coding-agent/CHANGELOG.md
index 55beb8bf8..d39ef17b6 100644
--- a/packages/coding-agent/CHANGELOG.md
+++ b/packages/coding-agent/CHANGELOG.md
@@ -15,6 +15,7 @@
 - Fixed `omp dry-balance --bench` to recover from 401 token failures by re-minting the failing OAuth credential in place before switching accounts
 - Fixed selector-style UI components to honor `tui.select.up` and `tui.select.down` keybindings instead of hard-coding raw Up/Down arrow bytes ([#1535](https://github.com/can1357/oh-my-pi/issues/1535)).
 - Fixed a collapsed, still-streaming tool preview (an `eval`/`bash`/`ssh` box with output streaming in) reading as "weirdly truncated" — top border and head rows missing — once its box outgrew the viewport, snapping back to whole only while expanded with `ctrl+o` and breaking again when collapsed. A streaming preview was classified commit-unstable whenever collapsed, so the transcript offered none of its rows to native scrollback; once the box outgrew the window its head fell into the gap between the commit boundary and the window top, committed nowhere and repainted nowhere. The `provisionalPendingPreview` flag now applies only to the pending call preview (before any result) — once a streaming result exists the result renderer is the live, top-anchored shape and the block is commit-stable in both collapsed and expanded states, so its durable head always reaches scrollback.
+- Fixed a crash in subagent task execution and extensions when a string (instead of a string array) was returned or set for the system prompt. Gracefully wrap string values in arrays.
 
 ## [15.13.0] - 2026-06-14
 
diff --git a/packages/coding-agent/src/extensibility/extensions/runner.ts b/packages/coding-agent/src/extensibility/extensions/runner.ts
index 31a3f1697..3a23d0e1f 100644
--- a/packages/coding-agent/src/extensibility/extensions/runner.ts
+++ b/packages/coding-agent/src/extensibility/extensions/runner.ts
@@ -907,7 +907,8 @@ export class ExtensionRunner {
 						messages.push(result.message);
 					}
 					if (result.systemPrompt !== undefined) {
-						currentSystemPrompt = result.systemPrompt;
+						currentSystemPrompt =
+							typeof result.systemPrompt === "string" ? [result.systemPrompt] : result.systemPrompt;
 						systemPromptModified = true;
 					}
 				}
diff --git a/packages/coding-agent/src/sdk.ts b/packages/coding-agent/src/sdk.ts
index 1744adc20..d68354df5 100644
--- a/packages/coding-agent/src/sdk.ts
+++ b/packages/coding-agent/src/sdk.ts
@@ -404,7 +404,7 @@ export interface CreateAgentSessionOptions {
 	scopedModels?: Array<{ model: Model; thinkingLevel?: ThinkingLevel }>;
 
 	/** System prompt blocks. Array replaces default, function receives default blocks and returns final blocks. */
-	systemPrompt?: string[] | ((defaultPrompt: string[]) => string[]);
+	systemPrompt?: string | string[] | ((defaultPrompt: string[]) => string | string[]);
 	/** Optional provider-facing session identifier for prompt caches and sticky auth selection.
 	 * Keeps persisted session files isolated while reusing provider-side caches. */
 	providerSessionId?: string;
@@ -2176,11 +2176,12 @@ export async function createAgentSession(options: CreateAgentSessionOptions = {}
 			if (options.systemPrompt === undefined) {
 				return defaultPrompt;
 			}
-			if (Array.isArray(options.systemPrompt)) {
-				return { systemPrompt: options.systemPrompt };
-			}
+			const customPrompt =
+				typeof options.systemPrompt === "function"
+					? options.systemPrompt(defaultPrompt.systemPrompt)
+					: options.systemPrompt;
 			return {
-				systemPrompt: options.systemPrompt(defaultPrompt.systemPrompt),
+				systemPrompt: typeof customPrompt === "string" ? [customPrompt] : customPrompt,
 			};
 		};
 
diff --git a/python/robomp/tests/test_worker.py b/python/robomp/tests/test_worker.py
index a9b070599..f2de9e17d 100644
--- a/python/robomp/tests/test_worker.py
+++ b/python/robomp/tests/test_worker.py
@@ -70,6 +70,7 @@ class _FakeRpcClient:
             messages: list = []
             events: list = []
             assistant_text: str = "ok"
+            assistant_message: dict | None = None
 
         return _Turn()