diff --git a/src/hooks/atlas/tool-execute-before.ts b/src/hooks/atlas/tool-execute-before.ts
index f4e03ad65..d9ede7484 100644
--- a/src/hooks/atlas/tool-execute-before.ts
+++ b/src/hooks/atlas/tool-execute-before.ts
@@ -1,4 +1,5 @@
 import { log } from "../../shared/logger"
+import { replaceToolArgs } from "../../shared/replace-tool-args"
 import { SYSTEM_DIRECTIVE_PREFIX } from "../../shared/system-directive"
 import { isCallerOrchestrator } from "../../shared/session-utils"
 import type { PluginInput } from "@opencode-ai/plugin"
@@ -179,7 +180,7 @@ export function createToolExecuteBeforeHandler(input: {
 
       const prompt = toolOutput.args.prompt as string | undefined
       if (prompt && !prompt.includes(SYSTEM_DIRECTIVE_PREFIX)) {
-        toolOutput.args.prompt = `<system-reminder>${SINGLE_TASK_DIRECTIVE}</system-reminder>\n` + prompt
+        replaceToolArgs(toolOutput, { prompt: `<system-reminder>${SINGLE_TASK_DIRECTIVE}</system-reminder>\n` + prompt })
         log(`[${HOOK_NAME}] Injected single-task directive to task`, {
           sessionID: toolInput.sessionID,
         })
diff --git a/src/hooks/claude-code-hooks/handlers/tool-execute-before-handler.ts b/src/hooks/claude-code-hooks/handlers/tool-execute-before-handler.ts
index 412cc2c23..612a63696 100644
--- a/src/hooks/claude-code-hooks/handlers/tool-execute-before-handler.ts
+++ b/src/hooks/claude-code-hooks/handlers/tool-execute-before-handler.ts
@@ -8,7 +8,7 @@ import {
 import { appendTranscriptEntry } from "../transcript"
 import { cacheToolInput } from "../tool-input-cache"
 import type { PluginConfig } from "../types"
-import { isHookDisabled, log } from "../../../shared"
+import { isHookDisabled, log, replaceToolArgs } from "../../../shared"
 
 export function createToolExecuteBeforeHandler(ctx: PluginInput, config: PluginConfig) {
 	return async (
@@ -39,7 +39,7 @@ export function createToolExecuteBeforeHandler(ctx: PluginInput, config: PluginC
 				)
 			}
 
-			output.args.todos = parsed
+			replaceToolArgs(output, { todos: parsed })
 			log("todowrite: parsed todos string to array", { sessionID: input.sessionID })
 		}
 
@@ -87,7 +87,7 @@ export function createToolExecuteBeforeHandler(ctx: PluginInput, config: PluginC
 		}
 
 		if (result.modifiedInput) {
-			Object.assign(output.args, result.modifiedInput)
+			replaceToolArgs(output, result.modifiedInput as Record<string, unknown>)
 		}
 	}
 }
diff --git a/src/hooks/compaction-todo-preserver/hook.ts b/src/hooks/compaction-todo-preserver/hook.ts
index 63e744c9c..dd2ea5c82 100644
--- a/src/hooks/compaction-todo-preserver/hook.ts
+++ b/src/hooks/compaction-todo-preserver/hook.ts
@@ -1,6 +1,7 @@
 import type { PluginInput } from "@opencode-ai/plugin"
 import { resolveSessionEventID } from "../../shared/event-session-id"
 import { log } from "../../shared/logger"
+import { replaceToolArgs } from "../../shared/replace-tool-args"
 
 interface TodoSnapshot {
   id?: string
@@ -233,7 +234,7 @@ export function createCompactionTodoPreserverHook(
       return
     }
 
-    output.args.todos = snapshot
+    replaceToolArgs(output, { todos: snapshot })
     log(`[${HOOK_NAME}] Replaced late Atlas bootstrap todowrite with restored snapshot`, {
       sessionID: input.sessionID,
       count: snapshot.length,
diff --git a/src/hooks/non-interactive-env/non-interactive-env-hook.ts b/src/hooks/non-interactive-env/non-interactive-env-hook.ts
index 6fc42aea9..42cb25122 100644
--- a/src/hooks/non-interactive-env/non-interactive-env-hook.ts
+++ b/src/hooks/non-interactive-env/non-interactive-env-hook.ts
@@ -1,6 +1,6 @@
 import type { PluginInput } from "@opencode-ai/plugin"
 import { HOOK_NAME, NON_INTERACTIVE_ENV, SHELL_COMMAND_PATTERNS } from "./constants"
-import { log, buildEnvPrefix } from "../../shared"
+import { log, buildEnvPrefix, replaceToolArgs } from "../../shared"
 import { detectShellType, type ShellType } from "../../shared/shell-env"
 
 export * from "./constants"
@@ -97,7 +97,7 @@ export function createNonInteractiveEnvHook(_ctx: PluginInput) {
         return
       }
 
-      output.args.command = `${envPrefix} ${command}`
+      replaceToolArgs(output, { command: `${envPrefix} ${command}` })
 
       log(`[${HOOK_NAME}] Prepended non-interactive env vars to git command`, {
         sessionID: input.sessionID,
diff --git a/src/hooks/prometheus-md-only/hook.ts b/src/hooks/prometheus-md-only/hook.ts
index 96f5093bd..57ee04a3d 100644
--- a/src/hooks/prometheus-md-only/hook.ts
+++ b/src/hooks/prometheus-md-only/hook.ts
@@ -1,6 +1,7 @@
 import type { PluginInput } from "@opencode-ai/plugin"
 import { HOOK_NAME, BLOCKED_TOOLS, PLANNING_CONSULT_WARNING, PROMETHEUS_WORKFLOW_REMINDER } from "./constants"
 import { log } from "../../shared/logger"
+import { replaceToolArgs } from "../../shared/replace-tool-args"
 import { SYSTEM_DIRECTIVE_PREFIX } from "../../shared/system-directive"
 import { getAgentDisplayName } from "../../shared/agent-display-names"
 import { getAgentFromSession } from "./agent-resolution"
@@ -27,7 +28,7 @@ export function createPrometheusMdOnlyHook(ctx: PluginInput) {
        if (TASK_TOOLS.includes(toolName)) {
          const prompt = output.args.prompt as string | undefined
          if (prompt && !prompt.includes(SYSTEM_DIRECTIVE_PREFIX)) {
-           output.args.prompt = PLANNING_CONSULT_WARNING + prompt
+           replaceToolArgs(output, { prompt: PLANNING_CONSULT_WARNING + prompt })
           log(`[${HOOK_NAME}] Injected planning warning to ${toolName}`, {
             sessionID: input.sessionID,
             tool: toolName,
diff --git a/src/hooks/question-label-truncator/hook.ts b/src/hooks/question-label-truncator/hook.ts
index a43fa4fc7..ed28a394a 100644
--- a/src/hooks/question-label-truncator/hook.ts
+++ b/src/hooks/question-label-truncator/hook.ts
@@ -1,3 +1,5 @@
+import { replaceToolArgs } from "../../shared/replace-tool-args"
+
 const MAX_LABEL_LENGTH = 30;
 
 interface QuestionOption {
@@ -56,7 +58,7 @@ export function createQuestionLabelTruncatorHook() {
       if (toolName === "askuserquestion" || toolName === "ask_user_question") {
         if (hasQuestions(output.args)) {
           const truncatedArgs = truncateQuestionLabels(output.args);
-          Object.assign(output.args, truncatedArgs);
+          replaceToolArgs(output, { questions: truncatedArgs.questions });
         }
       }
     },
diff --git a/src/hooks/sisyphus-junior-notepad/hook.ts b/src/hooks/sisyphus-junior-notepad/hook.ts
index 28a284e6f..5e9c00456 100644
--- a/src/hooks/sisyphus-junior-notepad/hook.ts
+++ b/src/hooks/sisyphus-junior-notepad/hook.ts
@@ -3,6 +3,7 @@ import type { PluginInput } from "@opencode-ai/plugin"
 import { isCallerOrchestrator } from "../../shared/session-utils"
 import { SYSTEM_DIRECTIVE_PREFIX } from "../../shared/system-directive"
 import { log } from "../../shared/logger"
+import { replaceToolArgs } from "../../shared/replace-tool-args"
 import { HOOK_NAME, NOTEPAD_DIRECTIVE } from "./constants"
 
 export function createSisyphusJuniorNotepadHook(ctx: PluginInput) {
@@ -33,7 +34,7 @@ export function createSisyphusJuniorNotepadHook(ctx: PluginInput) {
       }
 
       // 5. Prepend directive
-      output.args.prompt = NOTEPAD_DIRECTIVE + prompt
+      replaceToolArgs(output, { prompt: NOTEPAD_DIRECTIVE + prompt })
 
       // 6. Log injection
       log(`[${HOOK_NAME}] Injected notepad directive to task`, {
diff --git a/src/hooks/webfetch-redirect-guard/hook.ts b/src/hooks/webfetch-redirect-guard/hook.ts
index 1599bc1eb..055498fe6 100644
--- a/src/hooks/webfetch-redirect-guard/hook.ts
+++ b/src/hooks/webfetch-redirect-guard/hook.ts
@@ -1,5 +1,5 @@
 import type { PluginInput } from "@opencode-ai/plugin"
-import { log } from "../../shared"
+import { log, replaceToolArgs } from "../../shared"
 import {
   MAX_WEBFETCH_REDIRECTS,
   WEBFETCH_REDIRECT_ERROR_PATTERNS,
@@ -85,7 +85,7 @@ export function createWebFetchRedirectGuardHook(_ctx: PluginInput) {
         })
 
         if (resolution.type === "resolved") {
-          output.args.url = resolution.url
+          replaceToolArgs(output, { url: resolution.url })
           return
         }
 
diff --git a/src/plugin/tool-execute-before.ts b/src/plugin/tool-execute-before.ts
index 3b66aa2c9..508f40dde 100644
--- a/src/plugin/tool-execute-before.ts
+++ b/src/plugin/tool-execute-before.ts
@@ -3,7 +3,7 @@ import { randomUUID } from "node:crypto"
 
 import { getMainSessionID } from "../features/claude-code-session-state"
 import { clearBoulderState } from "../features/boulder-state"
-import { log } from "../shared"
+import { log, replaceToolArgs } from "../shared"
 import { stripInvisibleAgentCharacters } from "../shared/agent-display-names"
 import { resolveSessionAgent } from "./session-agent-resolver"
 import { parseRalphLoopArguments } from "../hooks/ralph-loop/command-arguments"
@@ -54,7 +54,7 @@ export function createToolExecuteBeforeHandler(args: {
   return async (input, output): Promise<void> => {
     if (input.tool.toLowerCase() === "bash" && typeof output.args.command === "string") {
       if (output.args.command.includes("\x00")) {
-        output.args.command = output.args.command.replace(/\x00/g, "")
+        replaceToolArgs(output, { command: output.args.command.replace(/\x00/g, "") })
         log("[tool-execute-before] Stripped null bytes from bash command", {
           sessionID: input.sessionID,
           callID: input.callID,
@@ -100,21 +100,20 @@ export function createToolExecuteBeforeHandler(args: {
     }
 
     if (input.tool === "task") {
-      const argsObject = output.args
-      const category = typeof argsObject.category === "string" ? argsObject.category : undefined
-      const subagentType = typeof argsObject.subagent_type === "string" ? argsObject.subagent_type : undefined
-      const taskId = typeof argsObject.task_id === "string" ? argsObject.task_id : undefined
+      const category = typeof output.args.category === "string" ? output.args.category : undefined
+      const subagentType = typeof output.args.subagent_type === "string" ? output.args.subagent_type : undefined
+      const taskId = typeof output.args.task_id === "string" ? output.args.task_id : undefined
 
       if (category) {
-        argsObject.subagent_type = "sisyphus-junior"
+        replaceToolArgs(output, { subagent_type: "sisyphus-junior" })
       } else if (!subagentType && taskId) {
         const resolvedAgent = await resolveSessionAgent(ctx.client, taskId)
-        argsObject.subagent_type = resolvedAgent ?? "continue"
+        replaceToolArgs(output, { subagent_type: resolvedAgent ?? "continue" })
       }
 
       const normalizedSubagentType =
-        typeof argsObject.subagent_type === "string" ? stripInvisibleAgentCharacters(argsObject.subagent_type) : undefined
-      const prompt = typeof argsObject.prompt === "string" ? argsObject.prompt : ""
+        typeof output.args.subagent_type === "string" ? stripInvisibleAgentCharacters(output.args.subagent_type) : undefined
+      const prompt = typeof output.args.prompt === "string" ? output.args.prompt : ""
       const loopState = typeof ctx.directory === "string" ? readState(ctx.directory) : null
       const shouldInjectOracleVerification =
         normalizedSubagentType === "oracle"
@@ -136,12 +135,14 @@ export function createToolExecuteBeforeHandler(args: {
           verification_attempt_id: verificationAttemptId,
           verification_session_id: undefined,
         })
-        argsObject.run_in_background = false
-        argsObject.prompt = buildUltraworkOracleVerificationPrompt(
-          prompt,
-          loopState.prompt,
-          verificationAttemptId,
-        )
+        replaceToolArgs(output, {
+          run_in_background: false,
+          prompt: buildUltraworkOracleVerificationPrompt(
+            prompt,
+            loopState.prompt,
+            verificationAttemptId,
+          ),
+        })
       }
     }
 
diff --git a/src/shared/index.ts b/src/shared/index.ts
index 07103d094..17ebfd0c9 100644
--- a/src/shared/index.ts
+++ b/src/shared/index.ts
@@ -84,3 +84,4 @@ export * from "./task-system-enabled"
 export * from "./parse-tools-config"
 export { parseModelString } from "./model-string-parser"
 export { EXCLUDED_DIRS } from "./excluded-dirs"
+export * from "./replace-tool-args"
diff --git a/src/shared/replace-tool-args.audit.test.ts b/src/shared/replace-tool-args.audit.test.ts
new file mode 100644
index 000000000..1c85ee6d9
--- /dev/null
+++ b/src/shared/replace-tool-args.audit.test.ts
@@ -0,0 +1,62 @@
+import { describe, it, expect } from "bun:test"
+import { readdir, readFile } from "node:fs/promises"
+import { join, relative } from "node:path"
+
+const SRC_DIR = join(import.meta.dir, "..")
+
+async function collectTsFiles(dir: string): Promise<string[]> {
+	const results: string[] = []
+	const entries = await readdir(dir, { withFileTypes: true })
+
+	for (const entry of entries) {
+		const fullPath = join(dir, entry.name)
+		if (entry.isDirectory()) {
+			if (entry.name === "node_modules" || entry.name === "dist") continue
+			results.push(...(await collectTsFiles(fullPath)))
+		} else if (entry.name.endsWith(".ts") && !entry.name.endsWith(".test.ts") && !entry.name.endsWith(".audit.test.ts")) {
+			results.push(fullPath)
+		}
+	}
+
+	return results
+}
+
+const HELPER_FILE = "shared/replace-tool-args.ts"
+
+// Matches direct mutations like `output.args.foo =` or `toolOutput.args.foo =`
+// but excludes comparisons (===, !==, ==)
+const DIRECT_MUTATION_PATTERN = /\w*[Oo]utput\.args\.\w+\s*=[^=]/g
+const OBJECT_ASSIGN_PATTERN = /Object\.assign\(\s*\w*[Oo]utput\.args/g
+
+describe("replace-tool-args audit", () => {
+	it("#given src/**/*.ts files #when scanning for direct output.args mutation #then no matches found outside the helper", async () => {
+		// given
+		const files = await collectTsFiles(SRC_DIR)
+		const violations: string[] = []
+
+		// when
+		for (const file of files) {
+			const relPath = relative(SRC_DIR, file)
+			if (relPath === HELPER_FILE) continue
+
+			const content = await readFile(file, "utf-8")
+			const lines = content.split("\n")
+
+			for (let i = 0; i < lines.length; i++) {
+				const line = lines[i]
+				if (DIRECT_MUTATION_PATTERN.test(line)) {
+					violations.push(`${relPath}:${i + 1}: ${line.trim()}`)
+				}
+				DIRECT_MUTATION_PATTERN.lastIndex = 0
+
+				if (OBJECT_ASSIGN_PATTERN.test(line)) {
+					violations.push(`${relPath}:${i + 1}: ${line.trim()}`)
+				}
+				OBJECT_ASSIGN_PATTERN.lastIndex = 0
+			}
+		}
+
+		// then
+		expect(violations).toEqual([])
+	})
+})
diff --git a/src/shared/replace-tool-args.test.ts b/src/shared/replace-tool-args.test.ts
new file mode 100644
index 000000000..113d8ab65
--- /dev/null
+++ b/src/shared/replace-tool-args.test.ts
@@ -0,0 +1,148 @@
+import { describe, it, expect } from "bun:test"
+import { replaceToolArgs } from "./replace-tool-args"
+
+describe("replaceToolArgs", () => {
+	describe("#given a mutable output.args object", () => {
+		it("#when patching a single property #then the output.args contains the patched value", () => {
+			// given
+			const output = { args: { command: "git status", timeout: 30 } as Record<string, unknown> }
+
+			// when
+			replaceToolArgs(output, { command: "git log" })
+
+			// then
+			expect(output.args.command).toBe("git log")
+			expect(output.args.timeout).toBe(30)
+		})
+
+		it("#when patching multiple properties #then all patched values are present", () => {
+			// given
+			const output = { args: { url: "http://old.com", format: "text" } as Record<string, unknown> }
+
+			// when
+			replaceToolArgs(output, { url: "http://new.com", format: "markdown" })
+
+			// then
+			expect(output.args.url).toBe("http://new.com")
+			expect(output.args.format).toBe("markdown")
+		})
+
+		it("#when patching #then the original args object is not the same reference", () => {
+			// given
+			const originalArgs = { command: "echo hi" } as Record<string, unknown>
+			const output = { args: originalArgs }
+
+			// when
+			replaceToolArgs(output, { command: "echo bye" })
+
+			// then
+			expect(output.args).not.toBe(originalArgs)
+			expect(originalArgs.command).toBe("echo hi")
+		})
+	})
+
+	describe("#given a frozen output.args object", () => {
+		it("#when patching a single property #then no TypeError is thrown and the value is updated", () => {
+			// given
+			const output = { args: Object.freeze({ command: "git status", timeout: 30 }) as Record<string, unknown> }
+
+			// when / then
+			expect(() => replaceToolArgs(output, { command: "git log" })).not.toThrow()
+			expect(output.args.command).toBe("git log")
+			expect(output.args.timeout).toBe(30)
+		})
+
+		it("#when patching with Object-typed value #then no TypeError is thrown", () => {
+			// given
+			const output = { args: Object.freeze({ todos: "[]" }) as Record<string, unknown> }
+			const parsed = [{ id: "1", content: "test", status: "pending" }]
+
+			// when / then
+			expect(() => replaceToolArgs(output, { todos: parsed })).not.toThrow()
+			expect(output.args.todos).toEqual(parsed)
+		})
+
+		it("#when patching url on frozen webfetch args #then no TypeError is thrown", () => {
+			// given
+			const output = { args: Object.freeze({ url: "http://old.com", format: "markdown" }) as Record<string, unknown> }
+
+			// when / then
+			expect(() => replaceToolArgs(output, { url: "http://redirected.com" })).not.toThrow()
+			expect(output.args.url).toBe("http://redirected.com")
+			expect(output.args.format).toBe("markdown")
+		})
+
+		it("#when patching command with env prefix on frozen bash args #then no TypeError is thrown", () => {
+			// given
+			const output = { args: Object.freeze({ command: "git rebase --continue" }) as Record<string, unknown> }
+
+			// when / then
+			expect(() => replaceToolArgs(output, { command: "GIT_EDITOR=: git rebase --continue" })).not.toThrow()
+			expect(output.args.command).toBe("GIT_EDITOR=: git rebase --continue")
+		})
+
+		it("#when patching prompt on frozen task args #then no TypeError is thrown", () => {
+			// given
+			const output = { args: Object.freeze({ prompt: "Do the thing", category: "quick" }) as Record<string, unknown> }
+
+			// when / then
+			expect(() => replaceToolArgs(output, { prompt: "[DIRECTIVE] Do the thing" })).not.toThrow()
+			expect(output.args.prompt).toBe("[DIRECTIVE] Do the thing")
+			expect(output.args.category).toBe("quick")
+		})
+
+		it("#when stripping null bytes from frozen command #then no TypeError is thrown", () => {
+			// given
+			const frozenCommand = "echo \x00hello"
+			const output = { args: Object.freeze({ command: frozenCommand }) as Record<string, unknown> }
+
+			// when / then
+			expect(() => replaceToolArgs(output, { command: "echo hello" })).not.toThrow()
+			expect(output.args.command).toBe("echo hello")
+		})
+
+		it("#when replacing truncated question labels on frozen args #then no TypeError is thrown", () => {
+			// given
+			const output = {
+				args: Object.freeze({
+					questions: [{ question: "Pick", options: [{ label: "A very long label that should be truncated" }] }],
+				}) as Record<string, unknown>,
+			}
+			const truncated = {
+				questions: [{ question: "Pick", options: [{ label: "A very long label that sho..." }] }],
+			}
+
+			// when / then
+			expect(() => replaceToolArgs(output, truncated)).not.toThrow()
+			expect((output.args.questions as Array<{ options: Array<{ label: string }> }>)[0].options[0].label).toBe(
+				"A very long label that sho...",
+			)
+		})
+
+		it("#when replacing modifiedInput from PreToolUse hook on frozen args #then no TypeError is thrown", () => {
+			// given
+			const output = { args: Object.freeze({ filePath: "/old/path.ts" }) as Record<string, unknown> }
+
+			// when / then
+			expect(() => replaceToolArgs(output, { filePath: "/new/path.ts" })).not.toThrow()
+			expect(output.args.filePath).toBe("/new/path.ts")
+		})
+
+		it("#when replacing todo snapshot on frozen args #then no TypeError is thrown", () => {
+			// given
+			const output = {
+				args: Object.freeze({
+					todos: [{ content: "bootstrap", status: "pending" }],
+				}) as Record<string, unknown>,
+			}
+			const snapshot = [
+				{ content: "Real task 1", status: "in_progress" },
+				{ content: "Real task 2", status: "pending" },
+			]
+
+			// when / then
+			expect(() => replaceToolArgs(output, { todos: snapshot })).not.toThrow()
+			expect(output.args.todos).toEqual(snapshot)
+		})
+	})
+})
diff --git a/src/shared/replace-tool-args.ts b/src/shared/replace-tool-args.ts
new file mode 100644
index 000000000..0a0566ad3
--- /dev/null
+++ b/src/shared/replace-tool-args.ts
@@ -0,0 +1,16 @@
+/**
+ * Safely replace tool arguments without mutating frozen objects.
+ *
+ * opencode >=1.14 may freeze `output.args` via Immer before plugin hooks run.
+ * Direct property assignment (`output.args.key = value`) or `Object.assign(output.args, patch)`
+ * throws `TypeError: Attempted to assign to readonly property` on a frozen object.
+ *
+ * This helper replaces `output.args` with a shallow clone containing the patch,
+ * which works regardless of whether the original args object is frozen.
+ */
+export function replaceToolArgs(
+	output: { args: Record<string, unknown> },
+	patch: Record<string, unknown>,
+): void {
+	output.args = { ...output.args, ...patch }
+}