feat: session prompt, clipboard fix, output batching, and dep cleanup by datasciencemonkey · Pull Request #119 · datasciencemonkey/coding-agents-databricks-apps

datasciencemonkey · 2026-04-14T10:43:19Z

Summary

Closes #118, closes #120, closes #121, closes #122

Addresses multiple issues discovered during investigation of a user-reported crash on the coda Databricks App:

1. OOM crash prevention (#118)

The app crashed suddenly with clean logs on a Medium instance (6GB RAM / 2 vCPUs) — pointing to an OOM kill. Users unknowingly accumulated terminal sessions (each with PTY + bash + AI agent subprocesses) until the container exceeded its memory limit.

2. Copy-paste broken inside Claude Code (#120)

Claude Code uses OSC 52 escape sequences for clipboard operations. The xterm.js ClipboardAddon was never loaded, so OSC 52 sequences were silently ignored.

3. Garbled rendering in Claude Code no-flicker mode (#121)

Claude Code's fullscreen/no-flicker mode (CLAUDE_CODE_NO_FLICKER=1) uses dense cursor-positioning sequences that get split when the PTY reader sends fixed 4096-byte chunks. Split escape sequences cause text fragments to render at wrong positions.

4. Garbled screen on reattach after alternate-screen apps (#122)

After exiting Claude Code (or vim/htop), reattaching to the session showed a screen full of repeated bash prompts. The output buffer contained raw alternate-screen escape sequences that produced garbage on replay.

5. Dependency cleanup

Replaced mlflow-tracing with mlflow-skinny==3.10.1 — the app only needs the lightweight MLflow client, not the tracing package.

6. Docs fix

Corrected repo name in deployment docs (coding-agents-in-databricks → coding-agents-databricks-apps).

Changes

File	Change
`app.py`	`MAX_CONCURRENT_SESSIONS` backend cap (env var, default 5) with TOCTOU-safe double-check; PTY read chunk 4096→65536 bytes
`app.yaml`	`MAX_CONCURRENT_SESSIONS` env var
`static/index.html`	Session creation prompt; `createWriteBatcher()` for output batching; ClipboardAddon; session count label in tab bar; alternate screen exit detection; SIGWINCH-based reattach; 429 error message; escape key handling
`static/lib/addon-clipboard.js`	New: xterm.js ClipboardAddon for OSC 52
`pyproject.toml`	Replaced `mlflow-tracing` with `mlflow-skinny==3.10.1`
`requirements.txt`	Regenerated
`docs/deployment.md`	Fixed repo name
`README.md`	Fixed repo name
`tests/test_session_limit.py`	New: 8 tests for session limit enforcement
`tests/test_clipboard_addon.py`	New: 8 tests for clipboard addon integration

Test plan

Automated (all pass locally)

uv run python -m pytest tests/ -q — 163/163 pass (excluding 2 pre-existing flaky network tests in test_gateway_discovery.py and test_npm_version_pinning.py)

Manual testing on Databricks Apps

Session prompt (#118):

Fresh deploy, no sessions — page load creates session normally (no prompt)
Reload with 1+ sessions alive — shows prompt listing existing sessions
Select session number — reattaches (scrollback replayed)
Press n — creates new session
Press q or Esc — creates new session (cancel = default to new)
New tab / split pane with existing sessions — shows prompt
Session picker (Ctrl+Shift+S) — works as before, no double-prompt

Session cap (#118):

Create 5 sessions, try 6th — red error in terminal: "Maximum 5 concurrent sessions reached"
Error message includes hint to increase MAX_CONCURRENT_SESSIONS or use Ctrl+Shift+S

Session count label (tab bar):

Text label visible on right side of tab bar (e.g., "2 active sessions")
Increments when new session created
Decrements when session exits (typing exit)
Decrements when session killed via picker (dN)
Updates to "No active sessions" when all closed
Updates after kill-all ("x" in picker)
Updates after cancelled picker (q/Esc)

Clipboard / OSC 52 (#120):

Open Claude Code inside terminal session
Copy text from Claude Code output — works via OSC 52
Paste into Claude Code — works
Regular terminal copy-paste still works

Output rendering / no-flicker mode (#121):

Run CLAUDE_CODE_NO_FLICKER=1 claude inside a session
Ask Claude to run bash commands / edit files
Text renders correctly — no garbled fragments or mispositioned words
No longer need to resize window to fix rendering
Normal (non-no-flicker) mode still renders correctly
Rapid output (cat large file) renders smoothly
Exiting Claude Code no-flicker clears screen cleanly (150ms auto-clear)

Reattach after alternate-screen apps (#122):

Reattach to a session running Claude Code — UI redraws via forced SIGWINCH
Reattach to a plain bash session — clean prompt (no garbled replay)
No blank screen on reattach (SIGWINCH forces redraw without manual resize)

Dependencies:

pip list | grep mlflow shows only mlflow-skinny==3.10.1
App starts without import errors

Docs:

Deployment docs show correct repo URL (coding-agents-databricks-apps)

#118) Prevent OOM crashes on 6GB Databricks Apps containers by reducing unnecessary session accumulation. Users now see their active sessions and can reuse one instead of blindly spawning a new one each time. - Add session-aware prompt on page load, new tab, and split pane - Add MAX_CONCURRENT_SESSIONS backend cap (env var, default 5) - Add session count badge on the toolbar sessions button - Add 8 tests covering the session limit enforcement

Claude Code uses OSC 52 escape sequences for clipboard operations. Without the ClipboardAddon, xterm.js silently ignores these sequences, making copy-paste inside Claude Code sessions non-functional. - Add addon-clipboard.js to static/lib/ - Load and initialize ClipboardAddon in createPane() - Add 8 tests verifying addon presence and integration

Align with the lockfile pin to prevent accidental upgrades that could pull in incompatible transitive dependencies.

No Python code imports mlflow. Removing unused deps cuts the package count from 65 to 50, reducing container image size and attack surface.

Keep only mlflow-skinny (lightweight client) — mlflow-tracing and the full mlflow package are not needed. Pins to 3.10.1.

…ation Claude Code's no-flicker mode uses dense cursor-positioning sequences that get split when the PTY reader sends fixed-size chunks. This causes garbled rendering (text fragments at wrong positions). - Add requestAnimationFrame write batching (~16ms) for PTY output - Increase PTY read chunk size from 4096 to 65536 bytes - Only PTY stream is batched; direct UI writes (splash, picker) unaffected

After replaying the output buffer on reattach, send ESC[?1049l to ensure the terminal is on the main screen buffer. Prevents garbled display when reattaching after exiting Claude Code no-flicker mode or other alternate-screen apps (vim, htop).

When Claude Code no-flicker mode (or vim/htop) exits the alternate screen, the restored main buffer has stale content overlapping with exit output. Detect ESC[?1049l in the output stream and clear the screen after 150ms to let exit output finish, then show a clean prompt.

The output buffer contains raw escape sequences captured at different terminal dimensions or from alternate-screen apps. Replaying them produces garbled output. Clear the screen after replay to guarantee a clean terminal, then resize triggers the shell to redraw its prompt.

…122) Buffer replay produces garbled output from stale escape sequences and conflicts with alternate-screen apps (Claude Code no-flicker mode). Replace with term.reset() + resize — the resize sends SIGWINCH so the running app (bash, Claude Code, vim) redraws its own UI correctly.

The kernel only sends SIGWINCH when the PTY size actually changes. On reattach, if the terminal dimensions are unchanged, no SIGWINCH fires and the app (Claude Code in no-flicker mode) never redraws. Fix by sending a 1-column shrink then restoring the real size, guaranteeing SIGWINCH regardless of current dimensions.

- Add q/Esc/Q cancel handler to promptExistingSessions so the Promise can't hang forever if user presses an unrecognized key - Add cancel() method to write batcher to clear pending data and timers - Call batchWrite.cancel() from cleanupPane to prevent writes to disposed terminals when altExitTimer fires after pane teardown

Backend: - Fix TOCTOU race in session cap: authoritative len(sessions) check inside the same sessions_lock as dict insertion. Early approximate check still rejects before forking a PTY. - Use len(sessions) instead of exited flag filter (exited is dead code — sessions are popped, never marked) Tests: - Fix env var test to patch the actual module constant - Fix bare open() calls in clipboard tests (use context managers) - Use re.search for more specific OSC 52 handler assertion - Replace exited-flag tests with removal-based tests matching real behavior

When the backend returns 429 (max sessions), display a clear error in the terminal with the message and a hint to increase MAX_CONCURRENT_SESSIONS in app.yaml or close sessions via Ctrl+Shift+S.

coding-agents-in-databricks → coding-agents-databricks-apps

Add overflow:visible to sessions button and reposition badge to top-right corner (-4px offset) so it sits above the button edge instead of being clipped by the flex container.

The red badge on the hamburger button was clipped and invisible. Replace with a plain text label on the right side of the tab bar showing "1 active session" / "2 active sessions" / etc.

Add updateSessionBadge() to cleanupPane (covers all exit paths) and after individual session kills in the session picker. Count now decrements when sessions are closed, exited, or killed.

Traced all session create/close/exit/switch paths and found 2 gaps: - Cancelled session picker (Esc/q) early-returns before badge update - Kill-all ("x") in picker didn't update badge before creating new session Both now call updateSessionBadge() at the right point.

When a session exits (user types "exit"), cleanupPane fetches the session count immediately — but the backend hasn't finished removing the session yet (3s SIGKILL wait). Add a 4s delayed retry so the count reflects the removal after backend cleanup completes.

datasciencemonkey · 2026-04-14T18:31:29Z

All tests pass.

Session management, clipboard, rendering, and dependency improvements.

datasciencemonkey · 2026-04-14T18:33:43Z

v0.17.0 Ship Summary

What shipped

Session creation prompt — users see existing sessions before creating new ones, reducing OOM risk on 6GB containers
MAX_CONCURRENT_SESSIONS cap — backend hard limit (env var, default 5) with TOCTOU-safe double-check under lock
Session count label — tab bar shows "N active sessions" with updates on every create/close/exit/kill path, including a 4s delayed retry for backend cleanup lag
ClipboardAddon (OSC 52) — copy-paste now works inside Claude Code sessions
Write batching — requestAnimationFrame coalescing prevents escape sequence fragmentation in no-flicker mode
Alternate screen exit auto-clear — 150ms debounced clear after vim/Claude Code exits
SIGWINCH-based reattach — forced size toggle guarantees app redraw on session switch
429 error feedback — clear terminal message when session limit is hit
Dependency cleanup — mlflow-tracing → mlflow-skinny 3.10.1
Docs fix — corrected repo name in deployment docs

Test results

163/163 automated tests pass (2 pre-existing flaky network tests excluded)
16 new tests added (8 session limit + 8 clipboard addon)
3 independent code reviews completed (frontend, backend, tests) — all critical/high issues resolved

Issues closed

Prompt users to reuse existing sessions before creating new ones #118 (OOM crash prevention)
Copy-paste broken inside Claude Code sessions (missing OSC 52 support) #120 (clipboard broken in Claude Code)
Garbled rendering in Claude Code no-flicker/fullscreen mode #121 (garbled rendering in no-flicker mode)
Garbled screen on reattach after exiting alternate screen buffer apps (Claude Code, vim) #122 (garbled reattach after alternate-screen apps)

datasciencemonkey self-assigned this Apr 14, 2026

datasciencemonkey added the enhancement New feature or request label Apr 14, 2026

datasciencemonkey added 5 commits April 14, 2026 07:15

chore: pin mlflow-tracing to 3.10.1 in pyproject.toml

6286d0d

Align with the lockfile pin to prevent accidental upgrades that could pull in incompatible transitive dependencies.

chore: remove mlflow-tracing and opentelemetry dependencies

7438f56

No Python code imports mlflow. Removing unused deps cuts the package count from 65 to 50, reducing container image size and attack surface.

chore(deps): replace mlflow-tracing with mlflow-skinny 3.10.1

80c9e34

Keep only mlflow-skinny (lightweight client) — mlflow-tracing and the full mlflow package are not needed. Pins to 3.10.1.

datasciencemonkey changed the title ~~feat: prompt users to reuse sessions, add concurrent session cap~~ feat: session prompt, clipboard fix, output batching, and dep cleanup Apr 14, 2026

This was referenced Apr 14, 2026

Copy-paste broken inside Claude Code sessions (missing OSC 52 support) #120

Closed

Garbled rendering in Claude Code no-flicker/fullscreen mode #121

Closed

Garbled screen on reattach after exiting alternate screen buffer apps (Claude Code, vim) #122

Closed

datasciencemonkey added 14 commits April 14, 2026 09:23

fix: show error message when session limit is reached

e35ebaa

When the backend returns 429 (max sessions), display a clear error in the terminal with the message and a hint to increase MAX_CONCURRENT_SESSIONS in app.yaml or close sessions via Ctrl+Shift+S.

fix: correct repo name in deployment docs and README

e656490

coding-agents-in-databricks → coding-agents-databricks-apps

fix: session count badge clipped by button overflow

098a9c4

Add overflow:visible to sessions button and reposition badge to top-right corner (-4px offset) so it sits above the button edge instead of being clipped by the flex container.

fix: replace badge with text label in tab bar for session count

99bf99a

The red badge on the hamburger button was clipped and invisible. Replace with a plain text label on the right side of the tab bar showing "1 active session" / "2 active sessions" / etc.

fix: update session count on close/exit, not just create

b4ac7b0

Add updateSessionBadge() to cleanupPane (covers all exit paths) and after individual session kills in the session picker. Count now decrements when sessions are closed, exited, or killed.

chore: bump version to 0.17.0

bc7ee10

Session management, clipboard, rendering, and dependency improvements.

datasciencemonkey merged commit 291f8fb into main Apr 14, 2026
1 check passed

datasciencemonkey mentioned this pull request Apr 14, 2026

fix: case-insensitive email auth and AI Gateway fallback #123

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feat: session prompt, clipboard fix, output batching, and dep cleanup#119

feat: session prompt, clipboard fix, output batching, and dep cleanup#119
datasciencemonkey merged 21 commits intomainfrom
feat/session-creation-prompt

datasciencemonkey commented Apr 14, 2026 •

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datasciencemonkey commented Apr 14, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

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Summary

1. OOM crash prevention (#118)

2. Copy-paste broken inside Claude Code (#120)

3. Garbled rendering in Claude Code no-flicker mode (#121)

4. Garbled screen on reattach after alternate-screen apps (#122)

5. Dependency cleanup

6. Docs fix

Changes

Test plan

Automated (all pass locally)

Manual testing on Databricks Apps

Uh oh!

datasciencemonkey commented Apr 14, 2026

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datasciencemonkey commented Apr 14, 2026

v0.17.0 Ship Summary

What shipped

Test results

Issues closed

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datasciencemonkey commented Apr 14, 2026 •

edited

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