diff --git a/packages/codeplane/src/tui/routes/session/index.tsx b/packages/codeplane/src/tui/routes/session/index.tsx
index bcfa30a84..09dbba681 100644
--- a/packages/codeplane/src/tui/routes/session/index.tsx
+++ b/packages/codeplane/src/tui/routes/session/index.tsx
@@ -39,6 +39,7 @@ import type {
 } from "@/tui/_compat/sdk-v2"
 import { useLocal } from "@/tui/context/local"
 import { Locale } from "@/tui/_compat/locale"
+import { textValue } from "@/tui/util/text-value"
 import type { Tool } from "@/tui/_compat/tool-tool"
 import type { ReadTool } from "@/tool/read"
 import type { WriteTool } from "@/tool/write"
@@ -1720,7 +1721,7 @@ function AssistantMessage(props: { message: AssistantMessage; parts: Part[]; las
           customBorderChars={SplitBorder.customBorderChars}
           borderColor={theme.error}
         >
-          <text fg={theme.textMuted}>{props.message.error?.data.message}</text>
+          <text fg={theme.textMuted}>{textValue(props.message.error?.data.message)}</text>
         </box>
       </Show>
       <Switch>
@@ -2174,7 +2175,7 @@ function Shell(props: ToolProps<typeof ShellTool>) {
           onClick={overflow() ? () => setExpanded((prev) => !prev) : undefined}
         >
           <box gap={1}>
-            <text fg={theme.text}>$ {props.input.command}</text>
+            <text fg={theme.text}>$ {textValue(props.input.command)}</text>
             <Show when={output()}>
               <text fg={theme.text}>{limited()}</text>
             </Show>
@@ -2186,7 +2187,7 @@ function Shell(props: ToolProps<typeof ShellTool>) {
       </Match>
       <Match when={true}>
         <InlineTool icon="$" pending="Writing command..." complete={props.input.command} part={props.part}>
-          {props.input.command}
+          {textValue(props.input.command)}
         </InlineTool>
       </Match>
     </Switch>
@@ -2228,7 +2229,7 @@ function Write(props: ToolProps<typeof WriteTool>) {
 function Glob(props: ToolProps<typeof GlobTool>) {
   return (
     <InlineTool icon="✱" pending="Finding files..." complete={props.input.pattern} part={props.part}>
-      Glob "{props.input.pattern}" <Show when={props.input.path}>in {normalizePath(props.input.path)} </Show>
+      Glob "{textValue(props.input.pattern)}" <Show when={props.input.path}>in {normalizePath(props.input.path)} </Show>
       <Show when={props.metadata.count}>
         ({props.metadata.count} {props.metadata.count === 1 ? "match" : "matches"})
       </Show>
@@ -2273,7 +2274,7 @@ function Read(props: ToolProps<typeof ReadTool>) {
 function Grep(props: ToolProps<typeof GrepTool>) {
   return (
     <InlineTool icon="✱" pending="Searching content..." complete={props.input.pattern} part={props.part}>
-      Grep "{props.input.pattern}" <Show when={props.input.path}>in {normalizePath(props.input.path)} </Show>
+      Grep "{textValue(props.input.pattern)}" <Show when={props.input.path}>in {normalizePath(props.input.path)} </Show>
       <Show when={props.metadata.matches}>
         ({props.metadata.matches} {props.metadata.matches === 1 ? "match" : "matches"})
       </Show>
@@ -2284,7 +2285,7 @@ function Grep(props: ToolProps<typeof GrepTool>) {
 function WebFetch(props: ToolProps<typeof WebFetchTool>) {
   return (
     <InlineTool icon="%" pending="Fetching from the web..." complete={props.input.url} part={props.part}>
-      WebFetch {props.input.url}
+      WebFetch {textValue(props.input.url)}
     </InlineTool>
   )
 }
@@ -2293,7 +2294,7 @@ function WebSearch(props: ToolProps<typeof WebSearchTool>) {
   const metadata = props.metadata as { numResults?: number }
   return (
     <InlineTool icon="◈" pending="Searching web..." complete={props.input.query} part={props.part}>
-      Exa Web Search "{props.input.query}" <Show when={metadata.numResults}>({metadata.numResults} results)</Show>
+      Exa Web Search "{textValue(props.input.query)}" <Show when={metadata.numResults}>({metadata.numResults} results)</Show>
     </InlineTool>
   )
 }
@@ -2553,7 +2554,7 @@ function Question(props: ToolProps<typeof QuestionTool>) {
 function Skill(props: ToolProps<typeof SkillTool>) {
   return (
     <InlineTool icon="→" pending="Loading skill..." complete={props.input.name} part={props.part}>
-      Skill "{props.input.name}"
+      Skill "{textValue(props.input.name)}"
     </InlineTool>
   )
 }
@@ -2572,7 +2573,7 @@ function Diagnostics(props: { diagnostics?: Record<string, Record<string, any>[]
         <For each={errors()}>
           {(diagnostic) => (
             <text fg={theme.error}>
-              Error [{diagnostic.range.start.line + 1}:{diagnostic.range.start.character + 1}] {diagnostic.message}
+              Error [{diagnostic.range.start.line + 1}:{diagnostic.range.start.character + 1}] {textValue(diagnostic.message)}
             </text>
           )}
         </For>
diff --git a/packages/codeplane/src/tui/util/text-value.ts b/packages/codeplane/src/tui/util/text-value.ts
new file mode 100644
index 000000000..4187802b3
--- /dev/null
+++ b/packages/codeplane/src/tui/util/text-value.ts
@@ -0,0 +1,26 @@
+// Coerce an arbitrary value into a string that is safe to render as a child of
+// an opentui `<text>` element.
+//
+// Tool `state.input` / `state.metadata` and message `error.data` are typed
+// `unknown` and can hold raw, partially-streamed, or model-supplied JSON whose
+// fields are objects/arrays — not always the strings the renderer assumes. When
+// a non-string / non-StyledText child is mounted into a `<text>`, opentui's
+// `TextNodeRenderable.add()` throws:
+//
+//   "TextNodeRenderable only accepts strings, TextNodeRenderable instances, or
+//    StyledText instances"
+//
+// which crashed the entire TUI when switching into a session that contained
+// such a part. `textValue` guarantees a string is always returned.
+export function textValue(value: unknown): string {
+  if (typeof value === "string") return value
+  if (value === null || value === undefined) return ""
+  if (typeof value === "number" || typeof value === "bigint" || typeof value === "boolean") return String(value)
+  try {
+    return JSON.stringify(value) ?? ""
+  } catch {
+    return String(value)
+  }
+}
+
+export * as TextValue from "./text-value"
diff --git a/packages/codeplane/test/tui/text-value.test.ts b/packages/codeplane/test/tui/text-value.test.ts
new file mode 100644
index 000000000..c31b317e8
--- /dev/null
+++ b/packages/codeplane/test/tui/text-value.test.ts
@@ -0,0 +1,55 @@
+import { describe, expect, test } from "bun:test"
+import { textValue } from "@/tui/util/text-value"
+
+// Regression coverage for the TUI crash where switching into a session that
+// contained a tool part with a non-string `state.input` / `state.metadata`
+// field threw, in opentui's `TextNodeRenderable.add()`:
+//   "TextNodeRenderable only accepts strings, TextNodeRenderable instances, or
+//    StyledText instances"
+// `textValue` is the guard the session renderer uses to coerce those `unknown`
+// values before they are mounted as a `<text>` child. The invariant is simple:
+// it must ALWAYS return a string, regardless of input shape.
+describe("textValue", () => {
+  test("passes strings through unchanged", () => {
+    expect(textValue("")).toBe("")
+    expect(textValue("ls -la")).toBe("ls -la")
+    expect(textValue("multi\nline")).toBe("multi\nline")
+  })
+
+  test("renders nullish as empty string (no-op child)", () => {
+    expect(textValue(null)).toBe("")
+    expect(textValue(undefined)).toBe("")
+  })
+
+  test("coerces scalar non-strings", () => {
+    expect(textValue(42)).toBe("42")
+    expect(textValue(0)).toBe("0")
+    expect(textValue(true)).toBe("true")
+    expect(textValue(false)).toBe("false")
+    expect(textValue(10n)).toBe("10")
+  })
+
+  test("serializes objects and arrays instead of throwing (the crash case)", () => {
+    // The exact class of value that used to crash the TUI: a tool input field
+    // arriving as a partial/streamed object or array rather than a string.
+    expect(textValue({ command: "ls" })).toBe('{"command":"ls"}')
+    expect(textValue(["a", "b"])).toBe('["a","b"]')
+    expect(textValue({ nested: { a: 1 } })).toBe('{"nested":{"a":1}}')
+  })
+
+  test("never throws and always returns a string for hostile inputs", () => {
+    const circular: Record<string, unknown> = {}
+    circular.self = circular
+    const cases: unknown[] = [
+      circular,
+      Symbol("x"),
+      () => {},
+      new Map([["a", 1]]),
+      { toJSON() { throw new Error("boom") } },
+    ]
+    for (const value of cases) {
+      const result = textValue(value)
+      expect(typeof result).toBe("string")
+    }
+  })
+})