diff --git a/packages/opencode/src/session/prompt.ts b/packages/opencode/src/session/prompt.ts
index de62788200b..74f6565e413 100644
--- a/packages/opencode/src/session/prompt.ts
+++ b/packages/opencode/src/session/prompt.ts
@@ -45,6 +45,7 @@ import { LLM } from "./llm"
 import { iife } from "@/util/iife"
 import { Shell } from "@/shell/shell"
 import { Truncate } from "@/tool/truncation"
+import { Output } from "@/util/output"
 
 // @ts-ignore
 globalThis.AI_SDK_LOG_WARNINGS = false
@@ -1485,29 +1486,21 @@ NOTE: At any point in time through this workflow you should feel free to ask the
       },
     })
 
-    let output = ""
+    const state = Output.create()
 
-    proc.stdout?.on("data", (chunk) => {
-      output += chunk.toString()
+    const handleChunk = (chunk: Buffer) => {
+      Output.append(state, chunk)
       if (part.state.status === "running") {
         part.state.metadata = {
-          output: output,
+          output: Output.preview(state),
           description: "",
         }
         Session.updatePart(part)
       }
-    })
+    }
 
-    proc.stderr?.on("data", (chunk) => {
-      output += chunk.toString()
-      if (part.state.status === "running") {
-        part.state.metadata = {
-          output: output,
-          description: "",
-        }
-        Session.updatePart(part)
-      }
-    })
+    proc.stdout?.on("data", handleChunk)
+    proc.stderr?.on("data", handleChunk)
 
     let aborted = false
     let exited = false
@@ -1517,11 +1510,13 @@ NOTE: At any point in time through this workflow you should feel free to ask the
     if (abort.aborted) {
       aborted = true
       await kill()
+      Output.cleanup(state)
     }
 
     const abortHandler = () => {
       aborted = true
       void kill()
+      Output.cleanup(state)
     }
 
     abort.addEventListener("abort", abortHandler, { once: true })
@@ -1534,11 +1529,17 @@ NOTE: At any point in time through this workflow you should feel free to ask the
       })
     })
 
+    Output.close(state)
+
     if (aborted) {
-      output += "\n\n" + ["<metadata>", "User aborted the command", "</metadata>"].join("\n")
+      Output.appendMetadata(state, "\n\n" + ["<metadata>", "User aborted the command", "</metadata>"].join("\n"))
     }
+
     msg.time.completed = Date.now()
     await Session.updateMessage(msg)
+
+    const result = Output.finalize(state)
+
     if (part.state.status === "running") {
       part.state = {
         status: "completed",
@@ -1549,10 +1550,11 @@ NOTE: At any point in time through this workflow you should feel free to ask the
         input: part.state.input,
         title: "",
         metadata: {
-          output,
+          output: result.preview,
           description: "",
+          ...(result.truncated && { outputPath: result.path }),
         },
-        output,
+        output: result.output,
       }
       await Session.updatePart(part)
     }
diff --git a/packages/opencode/src/tool/bash.ts b/packages/opencode/src/tool/bash.ts
index bf7c524941f..4b211e36c07 100644
--- a/packages/opencode/src/tool/bash.ts
+++ b/packages/opencode/src/tool/bash.ts
@@ -9,15 +9,14 @@ import { lazy } from "@/util/lazy"
 import { Language } from "web-tree-sitter"
 
 import { $ } from "bun"
-import { Filesystem } from "@/util/filesystem"
 import { fileURLToPath } from "url"
 import { Flag } from "@/flag/flag.ts"
 import { Shell } from "@/shell/shell"
 
 import { BashArity } from "@/permission/arity"
 import { Truncate } from "./truncation"
+import { Output } from "@/util/output"
 
-const MAX_METADATA_LENGTH = 30_000
 const DEFAULT_TIMEOUT = Flag.OPENCODE_EXPERIMENTAL_BASH_DEFAULT_TIMEOUT_MS || 2 * 60 * 1000
 
 export const log = Log.create({ service: "bash-tool" })
@@ -164,7 +163,7 @@ export const BashTool = Tool.define("bash", async () => {
         detached: process.platform !== "win32",
       })
 
-      let output = ""
+      const state = Output.create()
 
       // Initialize metadata with empty output
       ctx.metadata({
@@ -175,11 +174,10 @@ export const BashTool = Tool.define("bash", async () => {
       })
 
       const append = (chunk: Buffer) => {
-        output += chunk.toString()
+        Output.append(state, chunk)
         ctx.metadata({
           metadata: {
-            // truncate the metadata to avoid GIANT blobs of data (has nothing to do w/ what agent can access)
-            output: output.length > MAX_METADATA_LENGTH ? output.slice(0, MAX_METADATA_LENGTH) + "\n\n..." : output,
+            output: Output.preview(state),
             description: params.description,
           },
         })
@@ -197,11 +195,13 @@ export const BashTool = Tool.define("bash", async () => {
       if (ctx.abort.aborted) {
         aborted = true
         await kill()
+        Output.cleanup(state)
       }
 
       const abortHandler = () => {
         aborted = true
         void kill()
+        Output.cleanup(state)
       }
 
       ctx.abort.addEventListener("abort", abortHandler, { once: true })
@@ -240,18 +240,28 @@ export const BashTool = Tool.define("bash", async () => {
         resultMetadata.push("User aborted the command")
       }
 
+      Output.close(state)
+
       if (resultMetadata.length > 0) {
-        output += "\n\n<bash_metadata>\n" + resultMetadata.join("\n") + "\n</bash_metadata>"
+        Output.appendMetadata(state, "\n\n<bash_metadata>\n" + resultMetadata.join("\n") + "\n</bash_metadata>")
       }
 
+      const result = Output.finalize(state, {
+        hint: state.file
+          ? `The command output was ${state.written} bytes and was truncated (inline limit: ${Output.THRESHOLD} bytes).\nFull output saved to: ${state.file.path}\nUse Grep to search the full content or Read with offset/limit to view specific sections.`
+          : undefined,
+      })
+
       return {
         title: params.description,
         metadata: {
-          output: output.length > MAX_METADATA_LENGTH ? output.slice(0, MAX_METADATA_LENGTH) + "\n\n..." : output,
+          output: result.preview,
           exit: proc.exitCode,
           description: params.description,
+          truncated: result.truncated,
+          outputPath: result.path,
         },
-        output,
+        output: result.output,
       }
     },
   }
diff --git a/packages/opencode/src/util/output.ts b/packages/opencode/src/util/output.ts
new file mode 100644
index 00000000000..b2e19670bb0
--- /dev/null
+++ b/packages/opencode/src/util/output.ts
@@ -0,0 +1,159 @@
+import fs from "fs"
+import path from "path"
+import { Identifier } from "../id/id"
+import { Truncate } from "../tool/truncation"
+import { Log } from "./log"
+
+const log = Log.create({ service: "output" })
+
+/**
+ * Handles streaming command output with automatic file spillover when threshold is exceeded.
+ * Avoids O(n²) memory usage from repeated string concatenation by:
+ * 1. Accumulating in memory up to a threshold
+ * 2. Streaming to a temp file once threshold is exceeded
+ */
+export namespace Output {
+  export const THRESHOLD = Truncate.MAX_BYTES
+  export const MAX_PREVIEW = 30_000
+
+  export interface StreamFile {
+    fd: number
+    path: string
+  }
+
+  export interface State {
+    /** In-memory buffer (only used when not streaming to file) */
+    buffer: string
+    /** Total bytes received */
+    bytes: number
+    /** Total lines received */
+    lines: number
+    /** File handle when streaming to disk */
+    file?: StreamFile
+    /** Bytes written to file */
+    written: number
+  }
+
+  export interface Result {
+    /** The output content (full if not truncated, preview + hint if truncated) */
+    output: string
+    /** Preview for UI display (always fits in memory) */
+    preview: string
+    /** Whether output was truncated/streamed to file */
+    truncated: boolean
+    /** Path to file if output was streamed */
+    path?: string
+    /** Total bytes of output */
+    bytes: number
+    /** Total lines of output */
+    lines: number
+  }
+
+  export function create(): State {
+    return {
+      buffer: "",
+      bytes: 0,
+      lines: 0,
+      written: 0,
+    }
+  }
+
+  function createFile(state: State): StreamFile | undefined {
+    let fd: number | undefined
+    try {
+      const dir = Truncate.DIR
+      fs.mkdirSync(dir, { recursive: true })
+      Truncate.cleanup().catch(() => {})
+      const filepath = path.join(dir, Identifier.ascending("tool"))
+      fd = fs.openSync(filepath, "w")
+      if (state.buffer) {
+        fs.writeSync(fd, state.buffer)
+        state.written += Buffer.byteLength(state.buffer, "utf-8")
+      }
+      state.buffer = ""
+      return { fd, path: filepath }
+    } catch (e) {
+      if (fd !== undefined) fs.closeSync(fd)
+      log.warn("failed to create stream file, continuing in memory", { error: e })
+      return undefined
+    }
+  }
+
+  export function append(state: State, chunk: Buffer | string): void {
+    const text = typeof chunk === "string" ? chunk : chunk.toString()
+    const size = typeof chunk === "string" ? Buffer.byteLength(chunk, "utf-8") : chunk.length
+
+    state.bytes += size
+    state.lines += (text.match(/\n/g) || []).length
+
+    if (!state.file && (state.bytes > THRESHOLD || state.lines > Truncate.MAX_LINES)) {
+      state.file = createFile(state)
+    }
+
+    if (state.file) {
+      fs.writeSync(state.file.fd, text)
+      state.written += Buffer.byteLength(text, "utf-8")
+    } else {
+      state.buffer += text
+    }
+  }
+
+  export function preview(state: State): string {
+    if (state.file) {
+      return `[streaming to file: ${state.written} bytes written...]\n`
+    }
+    if (state.buffer.length > MAX_PREVIEW) {
+      return state.buffer.slice(0, MAX_PREVIEW) + "\n\n..."
+    }
+    return state.buffer
+  }
+
+  export function close(state: State): void {
+    if (state.file) {
+      fs.closeSync(state.file.fd)
+    }
+  }
+
+  export function cleanup(state: State): void {
+    if (state.file) {
+      try {
+        fs.unlinkSync(state.file.path)
+      } catch {}
+    }
+  }
+
+  export function appendMetadata(state: State, metadata: string): void {
+    if (state.file) {
+      fs.appendFileSync(state.file.path, metadata)
+    } else {
+      state.buffer += metadata
+    }
+  }
+
+  export function finalize(state: State, options?: { hint?: string }): Result {
+    const truncated = !!state.file
+    const filepath = state.file?.path
+
+    if (truncated && filepath) {
+      const hint =
+        options?.hint ??
+        `The command output was ${state.written} bytes and was truncated (inline limit: ${THRESHOLD} bytes).\nFull output saved to: ${filepath}\nUse Grep to search the full content or Read with offset/limit to view specific sections.`
+      return {
+        output: hint,
+        preview: `[output streamed to file: ${state.written} bytes]`,
+        truncated: true,
+        path: filepath,
+        bytes: state.bytes,
+        lines: state.lines,
+      }
+    }
+
+    return {
+      output: state.buffer,
+      preview: state.buffer.length > MAX_PREVIEW ? state.buffer.slice(0, MAX_PREVIEW) + "\n\n..." : state.buffer,
+      truncated: false,
+      bytes: state.bytes,
+      lines: state.lines,
+    }
+  }
+}
diff --git a/packages/opencode/test/tool/bash.test.ts b/packages/opencode/test/tool/bash.test.ts
index 750ff8193e9..6c4f3ef4abc 100644
--- a/packages/opencode/test/tool/bash.test.ts
+++ b/packages/opencode/test/tool/bash.test.ts
@@ -248,7 +248,7 @@ describe("tool.bash truncation", () => {
         )
         expect((result.metadata as any).truncated).toBe(true)
         expect(result.output).toContain("truncated")
-        expect(result.output).toContain("The tool call succeeded but the output was truncated")
+        expect(result.output).toContain("Full output saved to:")
       },
     })
   })
@@ -268,7 +268,7 @@ describe("tool.bash truncation", () => {
         )
         expect((result.metadata as any).truncated).toBe(true)
         expect(result.output).toContain("truncated")
-        expect(result.output).toContain("The tool call succeeded but the output was truncated")
+        expect(result.output).toContain("Full output saved to:")
       },
     })
   })