diff --git a/packages/cli/src/run/v1.ts b/packages/cli/src/run/v1.ts
index 966d85e087..b5bd6022aa 100644
--- a/packages/cli/src/run/v1.ts
+++ b/packages/cli/src/run/v1.ts
@@ -2,6 +2,7 @@ import type { Endpoint } from "@opencode/client/effect/service"
 import { Effect } from "effect"
 import path from "node:path"
 import { Standalone } from "../services/standalone"
+import { errorMessage } from "../util/error"
 import { reportRunError, runNonInteractiveWithOptions, type RunCommandInput } from "./run"
 
 export type V1RunCommandInput = {
@@ -53,7 +54,7 @@ export function runV1Bridge(input: V1RunCommandInput) {
         )
       }),
     ),
-  ).catch((error) => reportRunError(input, error instanceof Error ? error.message : String(error)))
+  ).catch((error) => reportRunError(input, errorMessage(error)))
 }
 
 function nativeInput(input: V1RunCommandInput, endpoint: Endpoint): RunCommandInput {
diff --git a/packages/cli/src/session-target.ts b/packages/cli/src/session-target.ts
index 9394de89a5..3d56ad0a1c 100644
--- a/packages/cli/src/session-target.ts
+++ b/packages/cli/src/session-target.ts
@@ -1,5 +1,6 @@
 import type { LocationGetOutput, ModelRef, OpenCodeClient, SessionInfo } from "@opencode/client/promise"
 import { Model } from "@opencode/schema/model"
+import { errorMessage } from "./util/error"
 
 const SESSION_PAGE_LIMIT = 50
 
@@ -24,7 +25,7 @@ export class SessionTargetMutationError extends Error {
   override readonly name = "SessionTargetMutationError"
 
   constructor(cause: unknown) {
-    super(cause instanceof Error ? cause.message : "Session target mutation failed", { cause })
+    super(errorMessage(cause), { cause })
   }
 }
 
@@ -125,11 +126,9 @@ async function selectSession(input: {
   if (!selected) return { session: undefined, location }
   return {
     session: input.fork
-      ? await input.client.session
-          .fork({ sessionID: selected.id }, ...requestOptions(input.signal))
-          .catch((error) => {
-            throw new SessionTargetMutationError(error)
-          })
+      ? await input.client.session.fork({ sessionID: selected.id }, ...requestOptions(input.signal)).catch((error) => {
+          throw new SessionTargetMutationError(error)
+        })
       : selected,
   }
 }
@@ -156,11 +155,7 @@ async function latestSession(
   return latestSession(client, location, page.cursor.next, signal)
 }
 
-function resolveLocation(
-  client: OpenCodeClient,
-  location?: { directory?: string },
-  signal?: AbortSignal,
-) {
+function resolveLocation(client: OpenCodeClient, location?: { directory?: string }, signal?: AbortSignal) {
   if (!location && !signal) return client.location.get()
   if (!location) return client.location.get(undefined, { signal })
   return client.location.get({ location }, ...requestOptions(signal))
diff --git a/packages/tui/src/component/migration-overlay.tsx b/packages/tui/src/component/migration-overlay.tsx
index 92e35aa638..fc73e36b85 100644
--- a/packages/tui/src/component/migration-overlay.tsx
+++ b/packages/tui/src/component/migration-overlay.tsx
@@ -3,6 +3,7 @@ import { useClient } from "../context/client"
 import { useTheme } from "../context/theme"
 import { SplitBorder } from "../ui/border"
 import { useToast } from "../ui/toast"
+import { errorMessage } from "../util/error"
 import { Spinner } from "./spinner"
 
 type Progress = { label: string; numerator?: number; denominator?: number }
@@ -30,7 +31,7 @@ export function MigrationOverlay() {
       toast.show({
         variant: "error",
         title: "Data migration failed",
-        message: error instanceof Error ? error.message : String(error),
+        message: errorMessage(error),
         duration: 10_000,
       })
     })
diff --git a/packages/tui/src/feature-plugins/system/plugins.tsx b/packages/tui/src/feature-plugins/system/plugins.tsx
index 1235a92594..de7de15b34 100644
--- a/packages/tui/src/feature-plugins/system/plugins.tsx
+++ b/packages/tui/src/feature-plugins/system/plugins.tsx
@@ -7,6 +7,7 @@ import { Spinner } from "../../component/spinner"
 import { usePlugin } from "../../plugin/context"
 import { DialogSelect, type DialogSelectOption } from "../../ui/dialog-select"
 import { useDialog } from "../../ui/dialog"
+import { errorMessage } from "../../util/error"
 
 const id = "opencode.plugins"
 
@@ -134,7 +135,7 @@ export function PluginsDialog(props: {
       .catch((cause) => {
         props.context.ui.toast.show({
           variant: "error",
-          message: cause instanceof Error ? cause.message : String(cause),
+          message: errorMessage(cause),
         })
       })
       .finally(() => setLocked(false))
@@ -152,7 +153,7 @@ export function PluginsDialog(props: {
       .catch((cause) => {
         props.context.ui.toast.show({
           variant: "error",
-          message: cause instanceof Error ? cause.message : String(cause),
+          message: errorMessage(cause),
         })
       })
       .finally(() => setPending((keys) => keys.filter((key) => key !== entry.key)))
@@ -173,7 +174,7 @@ export function PluginsDialog(props: {
       .catch((cause) => {
         props.context.ui.toast.show({
           variant: "error",
-          message: cause instanceof Error ? cause.message : String(cause),
+          message: errorMessage(cause),
         })
       })
       .finally(() => setChecking(false))
diff --git a/packages/tui/src/mini/runtime.ts b/packages/tui/src/mini/runtime.ts
index f01c8e7003..5125619222 100644
--- a/packages/tui/src/mini/runtime.ts
+++ b/packages/tui/src/mini/runtime.ts
@@ -12,6 +12,7 @@ import { SessionMessage } from "@opencode/schema/session-message"
 import type { LocationRef } from "@opencode/client/promise"
 import type { Config } from "../config"
 import { newSessionLocation } from "../config/new-session-location"
+import { errorMessage } from "../util/error"
 import { loadRunAgents, loadRunCommands, loadRunReferences } from "./catalog.shared"
 import {
   resolveMiniSettings,
@@ -863,7 +864,7 @@ async function runInteractiveRuntime(input: RunRuntimeInput, deps: RunRuntimeDep
     if (signal?.aborted || footer.isClosed) return
     const text =
       (await state.stream?.then((item) => item.mod).catch(() => undefined))?.formatUnknownError(error) ??
-      (error instanceof Error ? error.message : String(error))
+      errorMessage(error)
     const commit = {
       kind: "error",
       text,
@@ -1007,7 +1008,7 @@ async function runInteractiveRuntime(input: RunRuntimeInput, deps: RunRuntimeDep
               })
               const commit = {
                 kind: "error",
-                text: error instanceof Error ? error.message : String(error),
+                text: errorMessage(error),
                 phase: "start",
                 source: "system",
                 messageID: SessionMessage.ID.create(),