diff --git a/packages/code/README.md b/packages/code/README.md
index d421e444..acae9bb8 100644
--- a/packages/code/README.md
+++ b/packages/code/README.md
@@ -225,6 +225,21 @@ SRT with:
 - bounded time and aggregate output, with best-effort process-group termination
   on cancellation, timeout, and completion.
 
+Native command output keeps a prefix and rolling suffix for each stream, so late
+summaries and errors survive truncation. Each stream stores at most
+`maxOutputBytes` of copied raw bytes while the command runs, independent of output
+volume or chunk count. When both streams are noisy they split the existing combined
+response budget equally, with the odd byte reserved for stderr; a quiet stream gives
+its unused allowance to the other based on rendered UTF-8 bytes, including replacement
+characters for malformed input. Sandbox violation annotations enter the same
+stderr window before rendering. UTF-8 boundaries and inline
+`[... N bytes omitted ...]` markers count toward the combined byte limit. The count
+reports omitted raw bytes for that stream, including annotation bytes. If a stream's
+allowance cannot fit its marker, only the retained text and the existing `truncated`
+flag are returned. Truncation does not stop execution. Exit codes, timeout/signal
+fields, and cancellation errors keep their existing semantics, and no new request,
+result, or capability keys are introduced.
+
 The Git-metadata denies are applied to the registered root directly rather
 than relying on SRT's Linux mandatory denies, which are derived from the
 worker process's own current directory — the worker home, not the workspace —
diff --git a/packages/code/src/native-sandbox.test.ts b/packages/code/src/native-sandbox.test.ts
index 9001102f..90215156 100644
--- a/packages/code/src/native-sandbox.test.ts
+++ b/packages/code/src/native-sandbox.test.ts
@@ -33,6 +33,7 @@ import {
 } from './native-sandbox.js';
 import { restoreScratchTraversal } from './native-scratch.js';
 import { WorkspaceToolError } from './workspace.js';
+import { isWorkspaceToolResult } from './protocol.js';
 
 const request = {
   protocolVersion: 1 as const,
@@ -1355,14 +1356,151 @@ test('executes in the canonical workspace and bounds aggregate output', async t
       operation: 'execute_command',
       workspaceId: 'primary',
       exitCode: 0,
-      stdout: '1234567890',
-      stderr: 'ab',
+      stdout: '123890',
+      stderr: 'abchij',
       truncated: true,
       timedOut: false,
     },
   );
 });
 
+test('retains real command summaries on both streams under the legacy combined budget', async t => {
+    const root = await mkdtemp(join(tmpdir(), 'librechat-code-native-'));
+    t.after(() => rm(root, { recursive: true, force: true }));
+    const sandbox = new NativeSrtWorkspaceCommandSandbox({
+        workspaceRoot: root,
+        manager: fakeManager().manager,
+    });
+    t.after(() => sandbox.close());
+    const commandRequest = {
+        ...request,
+        maxOutputBytes: 256,
+        command:
+            "printf 'OUT\\n'; printf '%20000d' 0; printf '\\n42 tests passed\\n'; printf 'ERR\\n' >&2; printf '%20000d' 0 >&2; printf '\\nlate stderr summary\\n' >&2",
+    };
+    const result = await sandbox.execute(commandRequest);
+    assert.equal(result.exitCode, 0);
+    assert.equal(result.timedOut, false);
+    assert.equal(result.truncated, true);
+    assert.ok(result.stdout.startsWith('OUT\n'));
+    assert.ok(result.stderr.startsWith('ERR\n'));
+    assert.ok(result.stdout.endsWith('\n42 tests passed\n'));
+    assert.ok(result.stderr.endsWith('\nlate stderr summary\n'));
+    assert.ok(result.stdout.includes('bytes omitted'));
+    assert.ok(result.stderr.includes('bytes omitted'));
+    assert.equal(isWorkspaceToolResult(commandRequest, result), true);
+});
+
+test('partly filled command buffers retain final summaries regardless of stream order', async t => {
+    const root = await mkdtemp(join(tmpdir(), 'librechat-code-native-'));
+    t.after(() => rm(root, { recursive: true, force: true }));
+    const sandbox = new NativeSrtWorkspaceCommandSandbox({
+        workspaceRoot: root,
+        manager: fakeManager().manager,
+    });
+    t.after(() => sandbox.close());
+    const summary = 'late stderr summary\n';
+    const quiet = `printf '%109d' 0; printf 'late stderr summary\\n'`;
+    const noisy = `printf '%20000d' 0`;
+    for (const [command, stream] of [
+        [`{ ${quiet}; } >&2; ${noisy}`, 'stderr'],
+        [`${noisy}; { ${quiet}; } >&2`, 'stderr'],
+        [`{ ${quiet}; }; ${noisy} >&2`, 'stdout'],
+        [`${noisy} >&2; { ${quiet}; }`, 'stdout'],
+    ] as const) {
+        const commandRequest = { ...request, command, maxOutputBytes: 256 };
+        const result = await sandbox.execute(commandRequest);
+        assert.ok(result[stream].endsWith(summary));
+        assert.equal(result.truncated, true);
+        assert.equal(result.exitCode, 0);
+        assert.equal(isWorkspaceToolResult(commandRequest, result), true);
+    }
+});
+
+test('quiet malformed command output is retained beside a noisy stream', async t => {
+    const root = await mkdtemp(join(tmpdir(), 'librechat-code-native-'));
+    t.after(() => rm(root, { recursive: true, force: true }));
+    const sandbox = new NativeSrtWorkspaceCommandSandbox({
+        workspaceRoot: root,
+        manager: fakeManager().manager,
+    });
+    t.after(() => sandbox.close());
+    for (const [command, stream] of [
+        ["printf '%20000d' 0; printf '\\377\\377' >&2", 'stderr'],
+        ["printf '\\377\\377'; printf '%20000d' 0 >&2", 'stdout'],
+    ] as const) {
+        const commandRequest = { ...request, command, maxOutputBytes: 32 };
+        const result = await sandbox.execute(commandRequest);
+        assert.equal(result[stream], '\ufffd\ufffd');
+        assert.equal(result.truncated, true);
+        assert.equal(result.exitCode, 0);
+        assert.equal(isWorkspaceToolResult(commandRequest, result), true);
+    }
+});
+
+test('sandbox annotations survive full stdout and remain bounded when stderr is noisy', async t => {
+    const root = await mkdtemp(join(tmpdir(), 'librechat-code-native-'));
+    t.after(() => rm(root, { recursive: true, force: true }));
+    const fake = fakeManager();
+    fake.manager.annotateStderrWithSandboxFailures = (_commandId, stderr) =>
+        stderr + '\n<sandbox_violations>\ndenied write\n</sandbox_violations>';
+    const sandbox = new NativeSrtWorkspaceCommandSandbox({
+        workspaceRoot: root,
+        manager: fake.manager,
+    });
+    t.after(() => sandbox.close());
+    for (const stderrCommand of ['', "printf '%20000d' 0 >&2;"]) {
+        const commandRequest = {
+            ...request,
+            maxOutputBytes: 512,
+            command: `printf '%20000d' 0; ${stderrCommand} true`,
+        };
+        const result = await sandbox.execute(commandRequest);
+        assert.ok(
+            result.stderr.endsWith(
+                '<sandbox_violations>\ndenied write\n</sandbox_violations>'
+            )
+        );
+        assert.equal(isWorkspaceToolResult(commandRequest, result), true);
+        assert.equal(result.truncated, true);
+    }
+    fake.manager.annotateStderrWithSandboxFailures = () => {
+        throw new Error('annotation unavailable');
+    };
+    const result = await sandbox.execute({
+        ...request,
+        maxOutputBytes: 128,
+        command: "printf '%20000d' 0; printf 'child failure' >&2",
+    });
+    assert.equal(result.stderr, 'child failure');
+});
+
+test('timeout settlement keeps late diagnostics, signal, and truncation without widening the result', async t => {
+    const root = await mkdtemp(join(tmpdir(), 'librechat-code-native-'));
+    t.after(() => rm(root, { recursive: true, force: true }));
+    const sandbox = new NativeSrtWorkspaceCommandSandbox({
+        workspaceRoot: root,
+        manager: fakeManager().manager,
+    });
+    t.after(() => sandbox.close());
+    const commandRequest = {
+        ...request,
+        maxOutputBytes: 128,
+        timeoutMs: 100,
+        command:
+            "printf 'START\\n'; printf '%20000d' 0; printf '\\nlast progress\\n'; printf 'last failure' >&2; sleep 30",
+    };
+    const result = await sandbox.execute(commandRequest);
+    assert.ok(result.stdout.startsWith('START\n'));
+    assert.ok(result.stdout.endsWith('\nlast progress\n'));
+    assert.equal(result.stderr, 'last failure');
+    assert.equal(result.timedOut, true);
+    assert.equal(result.truncated, true);
+    assert.equal(result.exitCode, null);
+    assert.equal(result.signal, 'SIGKILL');
+    assert.equal(isWorkspaceToolResult(commandRequest, result), true);
+});
+
 test('rejects an escaping or unavailable command working directory', async t => {
   const root = await mkdtemp(join(tmpdir(), 'librechat-code-native-'));
   t.after(() => rm(root, { recursive: true, force: true }));
diff --git a/packages/code/src/native-sandbox.ts b/packages/code/src/native-sandbox.ts
index 5d39d405..26c848e5 100644
--- a/packages/code/src/native-sandbox.ts
+++ b/packages/code/src/native-sandbox.ts
@@ -32,6 +32,7 @@ import {
   removePrivateStorageAcl,
 } from './private-storage.js';
 import { WorkspaceToolError } from './workspace.js';
+import { OutputBuffer, renderCommandOutput } from './output.js';
 import { restoreScratchTraversal } from './native-scratch.js';
 import { writeLinkedWorktreeGitGuard } from './linked-worktree-git-guard.js';
 
@@ -1301,28 +1302,10 @@ export class NativeSrtWorkspaceCommandSandbox implements WorkspaceCommandSandbox
         }
         let settled = false;
         let timedOut = false;
-        let outputBytes = 0;
-        let truncated = false;
-        const stdout: Buffer[] = [];
-        const stderr: Buffer[] = [];
-        const append = (target: Buffer[], chunk: Buffer): void => {
-          const remaining = outputLimit - outputBytes;
-          if (remaining <= 0) {
-            truncated = true;
-            return;
-          }
-          const accepted = chunk.subarray(0, remaining);
-          target.push(accepted);
-          outputBytes += accepted.byteLength;
-                    if (accepted.byteLength !== chunk.byteLength)
-                        truncated = true;
-        };
-                child.stdout.on('data', (chunk: Buffer) =>
-                    append(stdout, chunk),
-                );
-                child.stderr.on('data', (chunk: Buffer) =>
-                    append(stderr, chunk),
-                );
+        const stdout = new OutputBuffer(outputLimit);
+        const stderr = new OutputBuffer(outputLimit);
+        child.stdout.on('data', (chunk: Buffer) => stdout.append(chunk));
+        child.stderr.on('data', (chunk: Buffer) => stderr.append(chunk));
         const abort = (): void => {
           if (settled) return;
           this.killCommandTree(child);
@@ -1371,29 +1354,17 @@ export class NativeSrtWorkspaceCommandSandbox implements WorkspaceCommandSandbox
             );
             return;
           }
-                    const stdoutValue = boundedUtf8(
-                        Buffer.concat(stdout),
-                        outputLimit,
-                    );
-          const stderrBudget = Math.max(
-            0,
-            outputLimit - Buffer.byteLength(stdoutValue),
-          );
-          const rawStderr = Buffer.concat(stderr).toString('utf8');
-          let annotatedStderr = rawStderr;
           try {
-                        annotatedStderr =
-                            this.manager.annotateStderrWithSandboxFailures(
-              commandId,
-              rawStderr,
+            // SRT appends violations to its input. Capture them in the same bounded stderr window.
+            stderr.append(
+              Buffer.from(
+                this.manager.annotateStderrWithSandboxFailures(commandId, ''),
+              ),
             );
           } catch {
             // Preserve the bounded child error if optional violation annotation fails.
           }
-          const stderrValue = boundedUtf8(
-            Buffer.from(annotatedStderr),
-            stderrBudget,
-          );
+          const output = renderCommandOutput(stdout, stderr, outputLimit);
           resolvePromise({
             protocolVersion: BRIDGE_PROTOCOL_VERSION,
             operation: 'execute_command',
@@ -1403,11 +1374,7 @@ export class NativeSrtWorkspaceCommandSandbox implements WorkspaceCommandSandbox
                                 ? null
                                 : this.protocolExitCode(code),
             ...(childSignal ? { signal: childSignal } : {}),
-            stdout: stdoutValue,
-            stderr: stderrValue,
-            truncated:
-                            truncated ||
-                            Buffer.byteLength(annotatedStderr) > stderrBudget,
+            ...output,
             timedOut,
           });
         });
diff --git a/packages/code/src/output.test.ts b/packages/code/src/output.test.ts
new file mode 100644
index 00000000..f78b0993
--- /dev/null
+++ b/packages/code/src/output.test.ts
@@ -0,0 +1,353 @@
+import assert from 'node:assert/strict';
+import test from 'node:test';
+import { OutputBuffer, renderCommandOutput } from './output.js';
+
+function collect(
+    content: Buffer,
+    budget: number,
+    chunkSize = content.length || 1
+): OutputBuffer {
+    const output = new OutputBuffer(budget);
+    for (let offset = 0; offset < content.length; offset += chunkSize) {
+        output.append(content.subarray(offset, offset + chunkSize));
+    }
+    return output;
+}
+
+function assertBounded(text: string, budget: number): void {
+    assert.ok(Buffer.byteLength(text) <= budget);
+    assert.equal(Buffer.from(text).toString('utf8'), text);
+}
+
+test('small output stays byte-for-byte unchanged, including split UTF-8 and exact limits', () => {
+    for (const content of ['', '1234567', 'hello\n世界🌍\n']) {
+        const bytes = Buffer.from(content);
+        const budget = Math.max(1, bytes.length);
+        for (const chunkSize of [1, 2, 7, 64]) {
+            assert.deepEqual(collect(bytes, budget, chunkSize).render(budget), {
+                text: content,
+                truncated: false,
+            });
+        }
+    }
+});
+
+test('a rolling suffix keeps the last summary and reports the exact dropped-byte count', () => {
+    const budget = 128;
+    const content = Buffer.from(
+        'START\n' + 'x'.repeat(10_000) + '\n42 tests passed\n'
+    );
+    for (const chunkSize of [1, 3, 63, 64, 65, 127, 1024, content.length]) {
+        const result = collect(content, budget, chunkSize).render(budget);
+        assert.equal(result.truncated, true);
+        assert.ok(result.text.startsWith('START\n'));
+        assert.ok(result.text.endsWith('\n42 tests passed\n'));
+        const marker = /\n\[\.\.\. (\d+) bytes omitted \.\.\.\]\n/.exec(
+            result.text
+        );
+        assert.ok(marker);
+        assert.equal(
+            Number(marker[1]),
+            content.length -
+                Buffer.byteLength(result.text.replace(marker[0], ''))
+        );
+        assertBounded(result.text, budget);
+    }
+});
+
+test('either quiet stream donates its budget, and stdout cannot starve stderr', () => {
+    const budget = 256;
+    const empty = collect(Buffer.alloc(0), budget);
+    const exact = collect(Buffer.alloc(budget, 'a'), budget, 1);
+    assert.deepEqual(renderCommandOutput(exact, empty, budget), {
+        stdout: 'a'.repeat(budget),
+        stderr: '',
+        truncated: false,
+    });
+    assert.deepEqual(renderCommandOutput(empty, exact, budget), {
+        stdout: '',
+        stderr: 'a'.repeat(budget),
+        truncated: false,
+    });
+    const stdout = collect(
+        Buffer.from('OUT\n' + 'x'.repeat(4096) + '\nstdout summary'),
+        budget
+    );
+    const stderr = collect(
+        Buffer.from('ERR\n' + 'y'.repeat(4096) + '\nstderr summary'),
+        budget
+    );
+    const result = renderCommandOutput(stdout, stderr, budget);
+    assert.ok(result.stdout.startsWith('OUT\n'));
+    assert.ok(result.stderr.startsWith('ERR\n'));
+    assert.ok(result.stdout.endsWith('stdout summary'));
+    assert.ok(result.stderr.endsWith('stderr summary'));
+    assert.ok(result.stdout.includes('bytes omitted'));
+    assert.ok(result.stderr.includes('bytes omitted'));
+    assert.ok(
+        Buffer.byteLength(result.stdout) + Buffer.byteLength(result.stderr) <=
+            budget
+    );
+    assert.equal(result.truncated, true);
+
+    const shortError = collect(Buffer.from('late failure'), budget);
+    assert.equal(
+        renderCommandOutput(stdout, shortError, budget).stderr,
+        'late failure'
+    );
+    assert.equal(
+        renderCommandOutput(shortError, stdout, budget).stdout,
+        'late failure'
+    );
+});
+
+test('partly filled buffers retain final summaries on either stream', () => {
+    const capacity = 256;
+    const summary = 'late stderr summary\n';
+    const content = Buffer.from('y'.repeat(129 - summary.length) + summary);
+    for (const chunkSize of [1, 7, 128, content.length]) {
+        const quiet = collect(content, capacity, chunkSize);
+        const noisy = collect(Buffer.alloc(20_000, 'x'), capacity, chunkSize);
+        for (const [stdout, stderr, stream] of [
+            [noisy, quiet, 'stderr'],
+            [quiet, noisy, 'stdout'],
+        ] as const) {
+            const result = renderCommandOutput(stdout, stderr, capacity);
+            assert.ok(result[stream].endsWith(summary));
+            assert.ok(
+                Buffer.byteLength(result.stdout) +
+                    Buffer.byteLength(result.stderr) <=
+                    capacity
+            );
+        }
+    }
+});
+
+test('rendered windows match logical stream ends across capture and allowance boundaries', () => {
+    for (const capacity of [32, 63, 64, 127, 128, 255, 256, 257]) {
+        const half = Math.floor(capacity / 2);
+        for (const length of [
+            0,
+            1,
+            half - 1,
+            half,
+            half + 1,
+            capacity - 1,
+            capacity,
+            capacity + 1,
+            capacity * 4,
+        ]) {
+            const content = Buffer.from(
+                Array.from({ length }, (_, index) =>
+                    String.fromCharCode(65 + (index % 26))
+                ).join('')
+            );
+            for (const budget of [
+                0,
+                1,
+                Math.floor(half / 2),
+                half,
+                capacity - 1,
+                capacity,
+            ]) {
+                let expected = content.toString('utf8');
+                if (length > budget) {
+                    const marker = (bytes: number): string =>
+                        `\n[... ${bytes} bytes omitted ...]\n`;
+                    const markerBytes = Buffer.byteLength(marker(length));
+                    const includeMarker = budget >= markerBytes;
+                    const remaining = includeMarker
+                        ? budget - markerBytes
+                        : budget;
+                    const prefix = Math.floor(remaining / 2);
+                    const suffix = Math.ceil(remaining / 2);
+                    expected =
+                        content.subarray(0, prefix).toString('utf8') +
+                        (includeMarker
+                            ? marker(length - prefix - suffix)
+                            : '') +
+                        content.subarray(length - suffix).toString('utf8');
+                }
+                for (const chunkSize of [1, half, capacity + 7]) {
+                    assert.deepEqual(
+                        collect(content, capacity, chunkSize).render(budget),
+                        {
+                            text: expected,
+                            truncated: length > budget,
+                        },
+                        `capacity ${capacity}, length ${length}, budget ${budget}, chunk ${chunkSize}`
+                    );
+                }
+            }
+        }
+    }
+});
+
+test('UTF-8 summaries survive capture transitions and split code-point chunks', () => {
+    const summary = 'end 世界🌍\n';
+    for (const capacity of [128, 255, 256, 257]) {
+        const half = Math.floor(capacity / 2);
+        for (const length of [
+            half - 1,
+            half,
+            half + 1,
+            capacity - 1,
+            capacity,
+            capacity + 1,
+        ]) {
+            const content = Buffer.from(
+                'x'.repeat(length - Buffer.byteLength(summary)) + summary
+            );
+            for (const chunkSize of [1, 2, content.length]) {
+                const quiet = collect(content, capacity, chunkSize);
+                const noisy = collect(
+                    Buffer.alloc(capacity * 4, 'y'),
+                    capacity,
+                    chunkSize
+                );
+                for (const [stdout, stderr, stream] of [
+                    [noisy, quiet, 'stderr'],
+                    [quiet, noisy, 'stdout'],
+                ] as const) {
+                    const result = renderCommandOutput(
+                        stdout,
+                        stderr,
+                        capacity
+                    );
+                    assert.ok(result[stream].endsWith(summary));
+                    assert.ok(!result[stream].includes('\ufffd'));
+                    assert.ok(
+                        Buffer.byteLength(result.stdout) +
+                            Buffer.byteLength(result.stderr) <=
+                            capacity
+                    );
+                }
+            }
+        }
+    }
+});
+
+test('budgeting includes UTF-8 boundaries and marker overhead for every small limit', () => {
+    const content = Buffer.from('世界🌍 café Ελληνικά\n'.repeat(100));
+    for (let budget = 1; budget <= 256; budget += 1) {
+        const result = collect(content, budget, 1).render(budget);
+        assertBounded(result.text, budget);
+        assert.equal(result.truncated, true);
+        assert.ok(
+            !result.text.includes('\ufffd'),
+            `split code point at budget ${budget}`
+        );
+        const marker = /\n\[\.\.\. (\d+) bytes omitted \.\.\.\]\n/.exec(
+            result.text
+        );
+        if (marker) {
+            assert.equal(
+                Number(marker[1]),
+                content.length -
+                    Buffer.byteLength(result.text.replace(marker[0], ''))
+            );
+        }
+        const streams = renderCommandOutput(
+            collect(content, budget, 7),
+            collect(content, budget, 3),
+            budget
+        );
+        assert.ok(
+            Buffer.byteLength(streams.stdout) +
+                Buffer.byteLength(streams.stderr) <=
+                budget
+        );
+        assert.ok(
+            !streams.stdout.includes('\ufffd') &&
+                !streams.stderr.includes('\ufffd')
+        );
+    }
+});
+
+test('malformed output cannot expand past the byte budget during decoding', () => {
+    for (const content of [
+        Buffer.alloc(1024, 0xff),
+        Buffer.from([0xf0, 0x80, 0xff, 0xc2, 0xa2, 0xe0, 0xa0]),
+        Buffer.from('a\ufffdb'.repeat(300)),
+    ]) {
+        for (let budget = 1; budget <= 128; budget += 1) {
+            const result = collect(content, budget, 1).render(budget);
+            assertBounded(result.text, budget);
+            if (content.length > budget) assert.equal(result.truncated, true);
+        }
+    }
+});
+
+test('short decoded output is preserved when replacement characters fit the combined budget', () => {
+    const budget = 32;
+    assert.deepEqual(
+        renderCommandOutput(
+            collect(Buffer.from([0xff, 0xff]), budget),
+            collect(Buffer.from('child failure'), budget),
+            budget
+        ),
+        { stdout: '\ufffd\ufffd', stderr: 'child failure', truncated: false }
+    );
+});
+
+test('quiet-stream donation counts decoded bytes beside noisy output', () => {
+    for (const bytes of [
+        Buffer.from([0xff, 0xff]),
+        Buffer.from([0x80, 0x80]),
+        Buffer.from([0xf0, 0x9f]),
+        Buffer.from([0x61, 0xff, 0x62]),
+    ]) {
+        const decoded = bytes.toString('utf8');
+        for (const budget of [16, 31, 32, 63, 64, 128]) {
+            const quiet = collect(bytes, budget, 1);
+            const noisy = collect(Buffer.alloc(budget * 4, 'x'), budget, 7);
+            for (const [stdout, stderr, stream] of [
+                [noisy, quiet, 'stderr'],
+                [quiet, noisy, 'stdout'],
+            ] as const) {
+                const result = renderCommandOutput(stdout, stderr, budget);
+                assert.equal(result[stream], decoded);
+                assert.equal(result.truncated, true);
+                assert.ok(
+                    Buffer.byteLength(result.stdout) +
+                        Buffer.byteLength(result.stderr) <=
+                        budget
+                );
+            }
+        }
+    }
+});
+
+test('tiny budgets still flag truncation and give stderr the odd byte', () => {
+    for (let budget = 1; budget < 32; budget += 1) {
+        const result = renderCommandOutput(
+            collect(Buffer.from('a'.repeat(256)), budget),
+            collect(Buffer.from('b'.repeat(256)), budget),
+            budget
+        );
+        assert.equal(result.truncated, true);
+        assert.ok(result.stderr.length > 0);
+        assert.ok(
+            Buffer.byteLength(result.stdout) +
+                Buffer.byteLength(result.stderr) <=
+                budget
+        );
+    }
+});
+
+test('copies bounded windows instead of retaining or repeatedly concatenating source chunks', () => {
+    const budget = 128;
+    const output = new OutputBuffer(budget);
+    const oversized = Buffer.alloc(1024 * 1024, 'a');
+    output.append(oversized);
+    oversized.fill('z');
+    const byte = Buffer.from('b');
+    for (let i = 0; i < 100_000; i += 1) output.append(byte);
+    byte[0] = 0x7a;
+    const result = output.render(budget);
+    assert.ok(result.text.startsWith('a'.repeat(40)));
+    assert.ok(result.text.endsWith('b'.repeat(40)));
+    assert.ok(!result.text.includes('z'));
+    assert.equal(output.bytes, 1024 * 1024 + 100_000);
+    assertBounded(result.text, budget);
+});
diff --git a/packages/code/src/output.ts b/packages/code/src/output.ts
new file mode 100644
index 00000000..4cb34211
--- /dev/null
+++ b/packages/code/src/output.ts
@@ -0,0 +1,182 @@
+interface OutputWindow {
+    text: string;
+    bytes: number;
+}
+
+function utf8Window(
+    buffer: Buffer,
+    length: number,
+    tail: boolean
+): OutputWindow {
+    let start = tail ? buffer.length - length : 0;
+    let end = tail ? buffer.length : length;
+    if (tail) {
+        while (start < end && (buffer[start] & 0xc0) === 0x80) start += 1;
+    } else if (end > 0) {
+        let last = end - 1;
+        while (last > 0 && (buffer[last] & 0xc0) === 0x80) last -= 1;
+        const lead = buffer[last];
+        const width =
+            lead >= 0xc2 && lead <= 0xdf
+                ? 2
+                : lead >= 0xe0 && lead <= 0xef
+                ? 3
+                : lead >= 0xf0 && lead <= 0xf4
+                ? 4
+                : 1;
+        if (end - last < width) end = last;
+    }
+    return {
+        text: buffer.subarray(start, end).toString('utf8'),
+        bytes: end - start,
+    };
+}
+
+function boundedWindow(
+    buffer: Buffer,
+    budget: number,
+    tail: boolean
+): OutputWindow {
+    const length = Math.min(buffer.length, budget);
+    const window = utf8Window(buffer, length, tail);
+    if (Buffer.byteLength(window.text) <= budget) return window;
+
+    // Malformed bytes expand to replacement characters. Valid UTF-8 takes the fast path.
+    let low = 0;
+    let high = length;
+    while (low < high) {
+        const middle = Math.ceil((low + high) / 2);
+        if (
+            Buffer.byteLength(utf8Window(buffer, middle, tail).text) <= budget
+        ) {
+            low = middle;
+        } else {
+            high = middle - 1;
+        }
+    }
+    return utf8Window(buffer, low, tail);
+}
+
+/** Copies a prefix and rolling suffix, never retaining child-process chunk buffers. */
+export class OutputBuffer {
+    private readonly headCapacity: number;
+    private readonly tailCapacity: number;
+    private head?: Buffer;
+    private tail?: Buffer;
+    private headLength = 0;
+    private tailLength = 0;
+    private tailOffset = 0;
+    private totalBytes = 0;
+
+    constructor(capacity: number) {
+        this.headCapacity = Math.floor(capacity / 2);
+        this.tailCapacity = capacity - this.headCapacity;
+    }
+
+    get bytes(): number {
+        return this.totalBytes;
+    }
+
+    append(chunk: Buffer): void {
+        this.totalBytes += chunk.length;
+        const headBytes = Math.min(
+            chunk.length,
+            this.headCapacity - this.headLength
+        );
+        if (headBytes > 0) {
+            this.head ??= Buffer.allocUnsafe(this.headCapacity);
+            chunk.copy(this.head, this.headLength, 0, headBytes);
+            this.headLength += headBytes;
+        }
+        const remaining = chunk.length - headBytes;
+        if (remaining === 0) return;
+        this.tail ??= Buffer.allocUnsafe(this.tailCapacity);
+        if (remaining >= this.tailCapacity) {
+            chunk.copy(this.tail, 0, chunk.length - this.tailCapacity);
+            this.tailOffset = 0;
+            this.tailLength = this.tailCapacity;
+            return;
+        }
+        const first = Math.min(remaining, this.tailCapacity - this.tailOffset);
+        chunk.copy(this.tail, this.tailOffset, headBytes, headBytes + first);
+        chunk.copy(this.tail, 0, headBytes + first);
+        this.tailOffset = (this.tailOffset + remaining) % this.tailCapacity;
+        this.tailLength = Math.min(
+            this.tailCapacity,
+            this.tailLength + remaining
+        );
+    }
+
+    private suffix(): Buffer {
+        if (!this.tail) return Buffer.alloc(0);
+        if (this.tailLength < this.tailCapacity)
+            return this.tail.subarray(0, this.tailLength);
+        if (this.tailOffset === 0) return this.tail;
+        return Buffer.concat([
+            this.tail.subarray(this.tailOffset),
+            this.tail.subarray(0, this.tailOffset),
+        ]);
+    }
+
+    render(budget: number): { text: string; truncated: boolean } {
+        const head = this.head?.subarray(0, this.headLength) ?? Buffer.alloc(0);
+        const tail = this.suffix();
+        // Before overflow the buffers are adjacent; either window may cross their boundary.
+        const contiguous =
+            this.totalBytes === head.length + tail.length
+                ? tail.length === 0
+                    ? head
+                    : Buffer.concat([head, tail])
+                : undefined;
+        if (contiguous && this.totalBytes <= budget) {
+            const text = contiguous.toString('utf8');
+            if (Buffer.byteLength(text) <= budget)
+                return { text, truncated: false };
+        }
+        const marker = (bytes: number): string =>
+            `\n[... ${bytes} bytes omitted ...]\n`;
+        const markerBytes = Buffer.byteLength(marker(this.totalBytes));
+        const includeMarker = budget >= markerBytes;
+        const contentBudget = includeMarker ? budget - markerBytes : budget;
+        const prefix = boundedWindow(
+            contiguous ?? head,
+            Math.floor(contentBudget / 2),
+            false
+        );
+        const suffix = boundedWindow(
+            contiguous ?? tail,
+            Math.ceil(contentBudget / 2),
+            true
+        );
+        const omitted = this.totalBytes - prefix.bytes - suffix.bytes;
+        return {
+            text:
+                prefix.text +
+                (includeMarker ? marker(omitted) : '') +
+                suffix.text,
+            truncated: omitted > 0,
+        };
+    }
+}
+
+/** Each stream can use the whole budget when the other is quiet; stderr cannot be starved. */
+export function renderCommandOutput(
+    stdout: OutputBuffer,
+    stderr: OutputBuffer,
+    budget: number
+): {
+    stdout: string;
+    stderr: string;
+    truncated: boolean;
+} {
+    let err = stderr.render(Math.ceil(budget / 2));
+    const out = stdout.render(budget - Buffer.byteLength(err.text));
+    if (!out.truncated) {
+        err = stderr.render(budget - Buffer.byteLength(out.text));
+    }
+    return {
+        stdout: out.text,
+        stderr: err.text,
+        truncated: out.truncated || err.truncated,
+    };
+}