diff --git a/src/hooks/agent-usage-reminder/hook.ts b/src/hooks/agent-usage-reminder/hook.ts
index bc7f3243fd..90928fdf11 100644
--- a/src/hooks/agent-usage-reminder/hook.ts
+++ b/src/hooks/agent-usage-reminder/hook.ts
@@ -6,6 +6,7 @@ import {
 } from "./storage";
 import { TARGET_TOOLS, AGENT_TOOLS, REMINDER_MESSAGE } from "./constants";
 import type { AgentUsageState } from "./types";
+import { appendToOutput } from "../hook-output-guard";
 
 interface ToolExecuteInput {
   tool: string;
@@ -77,7 +78,7 @@ export function createAgentUsageReminderHook(_ctx: PluginInput) {
       return;
     }
 
-    output.output += REMINDER_MESSAGE;
+    appendToOutput(output, REMINDER_MESSAGE);
     state.reminderCount++;
     state.updatedAt = Date.now();
     saveAgentUsageState(state);
diff --git a/src/hooks/category-skill-reminder/hook.ts b/src/hooks/category-skill-reminder/hook.ts
index b15715cda0..65a82d8a8f 100644
--- a/src/hooks/category-skill-reminder/hook.ts
+++ b/src/hooks/category-skill-reminder/hook.ts
@@ -3,6 +3,7 @@ import type { AvailableSkill } from "../../agents/dynamic-agent-prompt-builder"
 import { getSessionAgent } from "../../features/claude-code-session-state"
 import { log } from "../../shared"
 import { buildReminderMessage } from "./formatter"
+import { appendToOutput } from "../hook-output-guard"
 
 /**
  * Target agents that should receive category+skill reminders.
@@ -106,7 +107,7 @@ export function createCategorySkillReminderHook(
     state.toolCallCount++
 
     if (state.toolCallCount >= 3 && !state.delegationUsed && !state.reminderShown) {
-      output.output += reminderMessage
+      appendToOutput(output, reminderMessage)
       state.reminderShown = true
       log("[category-skill-reminder] Reminder injected", {
         sessionID,
diff --git a/src/hooks/claude-code-hooks/handlers/tool-execute-after-handler.ts b/src/hooks/claude-code-hooks/handlers/tool-execute-after-handler.ts
index 0dc934ea22..4fc6b93029 100644
--- a/src/hooks/claude-code-hooks/handlers/tool-execute-after-handler.ts
+++ b/src/hooks/claude-code-hooks/handlers/tool-execute-after-handler.ts
@@ -10,6 +10,7 @@ import { getToolInput } from "../tool-input-cache"
 import { appendTranscriptEntry, getTranscriptPath } from "../transcript"
 import type { PluginConfig } from "../types"
 import { isHookDisabled, log } from "../../../shared"
+import { appendToOutput } from "../../../hooks/hook-output-guard"
 
 export function createToolExecuteAfterHandler(ctx: PluginInput, config: PluginConfig) {
 	return async (
@@ -80,11 +81,11 @@ export function createToolExecuteAfterHandler(ctx: PluginInput, config: PluginCo
 		}
 
 		if (result.warnings && result.warnings.length > 0) {
-			output.output = `${output.output}\n\n${result.warnings.join("\n")}`
+			appendToOutput(output, `\n\n${result.warnings.join("\n")}`)
 		}
 
 		if (result.message) {
-			output.output = `${output.output}\n\n${result.message}`
+			appendToOutput(output, `\n\n${result.message}`)
 		}
 
 		if (result.hookName) {
diff --git a/src/hooks/comment-checker/cli-runner.ts b/src/hooks/comment-checker/cli-runner.ts
index b546125fc7..7365c5324c 100644
--- a/src/hooks/comment-checker/cli-runner.ts
+++ b/src/hooks/comment-checker/cli-runner.ts
@@ -2,6 +2,7 @@ import type { PendingCall } from "./types"
 import { existsSync } from "fs"
 
 import { runCommentChecker, getCommentCheckerPath, startBackgroundInit, type HookInput } from "./cli"
+import { appendToOutput } from "../hook-output-guard"
 
 let cliPathPromise: Promise<string | null> | null = null
 
@@ -52,7 +53,7 @@ export async function processWithCli(
 
   if (result.hasComments && result.message) {
     debugLog("CLI detected comments, appending message")
-    output.output += `\n\n${result.message}`
+    appendToOutput(output, `\n\n${result.message}`)
   } else {
     debugLog("CLI: no comments detected")
   }
@@ -92,7 +93,7 @@ export async function processApplyPatchEditsWithCli(
 
     if (result.hasComments && result.message) {
       debugLog("CLI detected comments for apply_patch file:", edit.filePath)
-      output.output += `\n\n${result.message}`
+      appendToOutput(output, `\n\n${result.message}`)
     }
   }
 }
diff --git a/src/hooks/context-window-monitor.ts b/src/hooks/context-window-monitor.ts
index 3b92191146..507186ddd7 100644
--- a/src/hooks/context-window-monitor.ts
+++ b/src/hooks/context-window-monitor.ts
@@ -1,5 +1,6 @@
 import type { PluginInput } from "@opencode-ai/plugin"
 import { createSystemDirective, SystemDirectiveTypes } from "../shared/system-directive"
+import { appendToOutput } from "./hook-output-guard"
 
 const ANTHROPIC_DISPLAY_LIMIT = 1_000_000
 const ANTHROPIC_ACTUAL_LIMIT =
@@ -74,8 +75,8 @@ export function createContextWindowMonitorHook(ctx: PluginInput) {
       const usedTokens = totalInputTokens.toLocaleString()
       const limitTokens = ANTHROPIC_DISPLAY_LIMIT.toLocaleString()
 
-      output.output += `\n\n${CONTEXT_REMINDER}
-[Context Status: ${usedPct}% used (${usedTokens}/${limitTokens} tokens), ${remainingPct}% remaining]`
+      appendToOutput(output, `\n\n${CONTEXT_REMINDER}
+[Context Status: ${usedPct}% used (${usedTokens}/${limitTokens} tokens), ${remainingPct}% remaining]`)
     } catch {
       // Graceful degradation - do not disrupt tool execution
     }
diff --git a/src/hooks/delegate-task-retry/hook.ts b/src/hooks/delegate-task-retry/hook.ts
index 915da323d8..31730d5a20 100644
--- a/src/hooks/delegate-task-retry/hook.ts
+++ b/src/hooks/delegate-task-retry/hook.ts
@@ -2,6 +2,7 @@ import type { PluginInput } from "@opencode-ai/plugin"
 
 import { buildRetryGuidance } from "./guidance"
 import { detectDelegateTaskError } from "./patterns"
+import { appendToOutput } from "../hook-output-guard"
 
 export function createDelegateTaskRetryHook(_ctx: PluginInput) {
   return {
@@ -11,10 +12,10 @@ export function createDelegateTaskRetryHook(_ctx: PluginInput) {
     ) => {
       if (input.tool.toLowerCase() !== "task") return
 
-      const errorInfo = detectDelegateTaskError(output.output)
+      const errorInfo = detectDelegateTaskError(output.output ?? "")
       if (errorInfo) {
         const guidance = buildRetryGuidance(errorInfo)
-        output.output += `\n${guidance}`
+        appendToOutput(output, `\n${guidance}`)
       }
     },
   }
diff --git a/src/hooks/directory-agents-injector/injector.ts b/src/hooks/directory-agents-injector/injector.ts
index dc6ca0810d..9b14195502 100644
--- a/src/hooks/directory-agents-injector/injector.ts
+++ b/src/hooks/directory-agents-injector/injector.ts
@@ -5,6 +5,7 @@ import { dirname } from "node:path";
 import type { createDynamicTruncator } from "../../shared/dynamic-truncator";
 import { findAgentsMdUp, resolveFilePath } from "./finder";
 import { loadInjectedPaths, saveInjectedPaths } from "./storage";
+import { appendToOutput } from "../hook-output-guard";
 
 type DynamicTruncator = ReturnType<typeof createDynamicTruncator>;
 
@@ -46,7 +47,7 @@ export async function processFilePathForAgentsInjection(input: {
       const truncationNotice = truncated
         ? `\n\n[Note: Content was truncated to save context window space. For full context, please read the file directly: ${agentsPath}]`
         : "";
-      input.output.output += `\n\n[Directory Context: ${agentsPath}]\n${result}${truncationNotice}`;
+      appendToOutput(input.output, `\n\n[Directory Context: ${agentsPath}]\n${result}${truncationNotice}`);
       cache.add(agentsDir);
     } catch {}
   }
diff --git a/src/hooks/directory-readme-injector/injector.ts b/src/hooks/directory-readme-injector/injector.ts
index 082165844b..0bfb75f3f8 100644
--- a/src/hooks/directory-readme-injector/injector.ts
+++ b/src/hooks/directory-readme-injector/injector.ts
@@ -5,6 +5,7 @@ import { dirname } from "node:path";
 import type { createDynamicTruncator } from "../../shared/dynamic-truncator";
 import { findReadmeMdUp, resolveFilePath } from "./finder";
 import { loadInjectedPaths, saveInjectedPaths } from "./storage";
+import { appendToOutput } from "../hook-output-guard";
 
 type DynamicTruncator = ReturnType<typeof createDynamicTruncator>;
 
@@ -46,7 +47,7 @@ export async function processFilePathForReadmeInjection(input: {
       const truncationNotice = truncated
         ? `\n\n[Note: Content was truncated to save context window space. For full context, please read the file directly: ${readmePath}]`
         : "";
-      input.output.output += `\n\n[Project README: ${readmePath}]\n${result}${truncationNotice}`;
+      appendToOutput(input.output, `\n\n[Project README: ${readmePath}]\n${result}${truncationNotice}`);
       cache.add(readmeDir);
     } catch {}
   }
diff --git a/src/hooks/edit-error-recovery/hook.ts b/src/hooks/edit-error-recovery/hook.ts
index 7df8aa0ffd..6de9ef8673 100644
--- a/src/hooks/edit-error-recovery/hook.ts
+++ b/src/hooks/edit-error-recovery/hook.ts
@@ -1,4 +1,5 @@
 import type { PluginInput } from "@opencode-ai/plugin"
+import { appendToOutput } from "../hook-output-guard"
 
 /**
  * Known Edit tool error patterns that indicate the AI made a mistake
@@ -50,7 +51,7 @@ export function createEditErrorRecoveryHook(_ctx: PluginInput) {
       )
 
       if (hasEditError) {
-        output.output += `\n${EDIT_ERROR_REMINDER}`
+        appendToOutput(output, `\n${EDIT_ERROR_REMINDER}`)
       }
     },
   }
diff --git a/src/hooks/hook-output-guard.test.ts b/src/hooks/hook-output-guard.test.ts
new file mode 100644
index 0000000000..3145522657
--- /dev/null
+++ b/src/hooks/hook-output-guard.test.ts
@@ -0,0 +1,71 @@
+import { describe, it, expect } from "bun:test"
+import { appendToOutput } from "./hook-output-guard"
+
+describe("appendToOutput", () => {
+	describe("#given output.output is a normal string", () => {
+		it("#then should append text to existing output", () => {
+			const output = { output: "original" }
+			appendToOutput(output, " appended")
+			expect(output.output).toBe("original appended")
+		})
+	})
+
+	describe("#given output.output is an empty string", () => {
+		it("#then should append text directly", () => {
+			const output = { output: "" }
+			appendToOutput(output, "new text")
+			expect(output.output).toBe("new text")
+		})
+	})
+
+	describe("#given output.output is undefined (MCP tool response)", () => {
+		it("#then should initialize with the text instead of crashing", () => {
+			const output = { output: undefined as unknown as string }
+			appendToOutput(output, "reminder message")
+			expect(output.output).toBe("reminder message")
+		})
+	})
+
+	describe("#given output.output is null", () => {
+		it("#then should initialize with the text", () => {
+			const output = { output: null as unknown as string }
+			appendToOutput(output, "context injection")
+			expect(output.output).toBe("context injection")
+		})
+	})
+
+	describe("#given output.output is a non-string object", () => {
+		it("#then should not modify the output to preserve structured data", () => {
+			const structured = { key: "value" }
+			const output = { output: structured as unknown as string }
+			appendToOutput(output, "should not corrupt")
+			expect(output.output).toBe(structured as unknown as string)
+		})
+	})
+
+	describe("#given output.output is a number", () => {
+		it("#then should not modify the output", () => {
+			const output = { output: 42 as unknown as string }
+			appendToOutput(output, " text")
+			expect(output.output).toBe(42 as unknown as string)
+		})
+	})
+
+	describe("#given multiple sequential appends", () => {
+		it("#then should accumulate all text", () => {
+			const output = { output: "base" }
+			appendToOutput(output, "\nfirst")
+			appendToOutput(output, "\nsecond")
+			expect(output.output).toBe("base\nfirst\nsecond")
+		})
+	})
+
+	describe("#given multiple appends starting from undefined", () => {
+		it("#then should initialize once and accumulate", () => {
+			const output = { output: undefined as unknown as string }
+			appendToOutput(output, "first")
+			appendToOutput(output, " second")
+			expect(output.output).toBe("first second")
+		})
+	})
+})
diff --git a/src/hooks/hook-output-guard.ts b/src/hooks/hook-output-guard.ts
new file mode 100644
index 0000000000..120b43bce8
--- /dev/null
+++ b/src/hooks/hook-output-guard.ts
@@ -0,0 +1,23 @@
+/**
+ * Safely appends text to a hook output's `output` field.
+ *
+ * MCP tools (Atlassian, Exa, Chrome DevTools, grep.app, etc.) can return
+ * responses where `output.output` is `undefined` instead of a string.
+ * This helper normalizes `null`/`undefined` to an empty string before
+ * appending, preserving context injection while avoiding TypeError crashes.
+ *
+ * Non-string values (objects, arrays) are left untouched to prevent
+ * corrupting structured tool responses.
+ */
+export function appendToOutput(
+	output: { output: string },
+	text: string,
+): void {
+	if (output.output == null) {
+		output.output = text
+		return
+	}
+	if (typeof output.output === "string") {
+		output.output += text
+	}
+}
diff --git a/src/hooks/interactive-bash-session/hook.ts b/src/hooks/interactive-bash-session/hook.ts
index 1d45a6b0a9..2bf3b19ee6 100644
--- a/src/hooks/interactive-bash-session/hook.ts
+++ b/src/hooks/interactive-bash-session/hook.ts
@@ -5,6 +5,7 @@ import type { InteractiveBashSessionState } from "./types";
 import { tokenizeCommand, findSubcommand, extractSessionNameFromTokens } from "./parser";
 import { getOrCreateState, isOmoSession, killAllTrackedSessions } from "./state-manager";
 import { subagentSessions } from "../../features/claude-code-session-state";
+import { appendToOutput } from "../hook-output-guard";
 
 interface ToolExecuteInput {
   tool: string;
@@ -97,7 +98,7 @@ export function createInteractiveBashSessionHook(ctx: PluginInput) {
         Array.from(state.tmuxSessions),
       );
       if (reminder) {
-        output.output += reminder;
+        appendToOutput(output, reminder);
       }
     }
   };
diff --git a/src/hooks/interactive-bash-session/interactive-bash-session-hook.ts b/src/hooks/interactive-bash-session/interactive-bash-session-hook.ts
index b3ba0976bc..448adcc2ed 100644
--- a/src/hooks/interactive-bash-session/interactive-bash-session-hook.ts
+++ b/src/hooks/interactive-bash-session/interactive-bash-session-hook.ts
@@ -1,6 +1,7 @@
 import type { PluginInput } from "@opencode-ai/plugin";
 import { createInteractiveBashSessionTracker } from "./interactive-bash-session-tracker";
 import { parseTmuxCommand } from "./tmux-command-parser";
+import { appendToOutput } from "../hook-output-guard";
 
 interface ToolExecuteInput {
   tool: string;
@@ -57,7 +58,7 @@ export function createInteractiveBashSessionHook(ctx: PluginInput) {
       toolOutput,
     })
     if (reminderToAppend) {
-      output.output += reminderToAppend
+      appendToOutput(output, reminderToAppend)
     }
   };
 
diff --git a/src/hooks/rules-injector/injector.ts b/src/hooks/rules-injector/injector.ts
index 9ba0324bab..38b29ffd11 100644
--- a/src/hooks/rules-injector/injector.ts
+++ b/src/hooks/rules-injector/injector.ts
@@ -11,6 +11,7 @@ import {
 import { parseRuleFrontmatter } from "./parser";
 import { saveInjectedRules } from "./storage";
 import type { SessionInjectedRulesCache } from "./cache";
+import { appendToOutput } from "../hook-output-guard";
 
 type ToolExecuteOutput = {
   title: string;
@@ -116,7 +117,7 @@ export function createRuleInjectionProcessor(deps: {
       const truncationNotice = truncated
         ? `\n\n[Note: Content was truncated to save context window space. For full context, please read the file directly: ${rule.relativePath}]`
         : "";
-      output.output += `\n\n[Rule: ${rule.relativePath}]\n[Match: ${rule.matchReason}]\n${result}${truncationNotice}`;
+      appendToOutput(output, `\n\n[Rule: ${rule.relativePath}]\n[Match: ${rule.matchReason}]\n${result}${truncationNotice}`);
     }
 
     saveInjectedRules(sessionID, cache);
diff --git a/src/hooks/task-reminder/hook.ts b/src/hooks/task-reminder/hook.ts
index 4e795018d0..254df0c8c9 100644
--- a/src/hooks/task-reminder/hook.ts
+++ b/src/hooks/task-reminder/hook.ts
@@ -1,4 +1,5 @@
 import type { PluginInput } from "@opencode-ai/plugin"
+import { appendToOutput } from "../hook-output-guard"
 
 const TASK_TOOLS = new Set([
   "task",
@@ -39,7 +40,7 @@ export function createTaskReminderHook(_ctx: PluginInput) {
     const newCount = currentCount + 1
 
     if (newCount >= TURN_THRESHOLD) {
-      output.output += REMINDER_MESSAGE
+      appendToOutput(output, REMINDER_MESSAGE)
       sessionCounters.set(sessionID, 0)
     } else {
       sessionCounters.set(sessionID, newCount)