The Moment the Entrypoint Vanished: Adapting to Vast.ai's SSH Mode Quirk

In the intricate dance of deploying a distributed proving system across rented GPU instances on Vast.ai, few moments are as revelatory as the one captured in message 923 of this coding session. The assistant, having just created a fresh instance (32710414) with the newly rebuilt Docker image, SSHes into the running container expecting to find its carefully crafted entrypoint.sh orchestrating the worker lifecycle. Instead, it finds something else entirely: PID 1 is a Vast.ai init script called /.launch, and the entrypoint is nowhere to be seen. The assistant's response to this discovery—a rapid burst of reasoning, reconsideration, and decisive action—is a masterclass in platform-specific debugging and adaptive system design.

The Context: A Carefully Orchestrated Deployment

To understand why this message matters, we must first appreciate what led to it. The assistant and user had been building a sophisticated management system for running Filecoin proof computations (CuZK proving engine) on rented Vast.ai GPU instances. The architecture involved a custom Docker image with an entrypoint.sh that would: fetch proof parameters, start a portavailc tunnel for secure connectivity, run benchmark proofs to calibrate performance, and register the instance with a central vast-manager service. The manager would then monitor instances, kill stale ones, and maintain a dashboard of available compute.

The Docker image had been rebuilt and pushed. The old instance (32709851) had been destroyed. A new instance (32710414) was created using vastai create instance ... --ssh --direct, which the assistant had been using throughout the project as the standard way to launch instances with SSH access for debugging. The environment variables (PAVAIL and PAVAIL_SERVER) were correctly set. The image loaded successfully. Everything looked perfect.

Then the instance failed to register with the manager.

The Discovery: SSH Mode Replaces the Entrypoint

In message 921, the assistant SSHes into the new instance and runs ps aux. The output is shocking: PID 1 is /bin/sh -c while [ ! -e /.launch ]; do sleep 1; done; bash /.launch. The Docker ENTRYPOINT—the carefully written entrypoint.sh—is not running at all. In message 922, the assistant investigates further by reading /.launch and discovers the documented behavior: "The SSH and Jupyter launch modes inject setup scripts into your existing Docker image, which means your image's original entrypoint is replaced."

This is the critical realization. The --ssh flag, which the assistant had been using throughout the project, does not simply add SSH to the container—it fundamentally alters the container's lifecycle by replacing the ENTRYPOINT with Vast.ai's own initialization script. The assistant's entire deployment strategy was built on an incorrect assumption.

The Reasoning Process in Message 923

Message 923 is the assistant's response to this discovery, and it reveals a remarkable thinking process in real time. The message contains three distinct phases of reasoning, each building on the previous one:

Phase 1: The Initial Hypothesis. The assistant reads the .launch script and notices it runs /root/onstart.sh in the background if it exists. The first thought is: "So we need to put our entrypoint logic in /root/onstart.sh." This is a natural, surface-level response—the .launch script has a hook, and the obvious fix is to use that hook.

Phase 2: The Reconsideration. Almost immediately, the assistant catches itself: "But wait—this instance was created with --ssh mode which replaces our entrypoint." This is the key insight. The problem isn't just that the entrypoint isn't running—it's that the entire deployment model is wrong. The assistant briefly considers the alternative: "The better approach: destroy this and recreate without --ssh, using the entrypoint mode." But then it identifies a trade-off: "But then we lose SSH access for debugging."

This moment of hesitation is crucial. The assistant is weighing two competing values: the need for the entrypoint to run as designed (which requires --entrypoint mode) versus the need for SSH access to debug issues (which --ssh mode provides). It's a classic engineering trade-off, and the assistant is visibly working through it.

Phase 3: The Synthesis. The assistant re-examines the .launch script more carefully and notices something it missed: the script does run onstart.sh if it exists. This opens up a third path: "We can use a different approach: set the onstart command to call our entrypoint. Or better yet, just add an --onstart-cmd that calls our entrypoint."

This is the synthesis. The --onstart-cmd flag allows specifying a command that Vast.ai runs after its own initialization, preserving both the SSH access (from --ssh mode) and the entrypoint logic (via the onstart hook). It's not either/or—it's both/and, using the platform's own mechanism to achieve the desired outcome.

The assistant then acts decisively: "Let me destroy this instance and recreate with the right approach." The bash command to destroy instance 32710414 is issued immediately.

Assumptions Made and Corrected

This message reveals several assumptions that the assistant had been operating under:

The --ssh flag is additive. The assistant assumed that --ssh adds SSH access on top of the existing Docker ENTRYPOINT. In reality, it replaces the ENTRYPOINT entirely. This is a subtle but critical distinction that the Vast.ai documentation does make, but the assistant had not fully internalized.
The entrypoint would always run. The entire deployment architecture depended on entrypoint.sh executing on container start. The assistant had designed the system around this assumption without verifying it against Vast.ai's SSH mode behavior.
Previous instances were working correctly. Looking back at the conversation, earlier instances (like 32705217) were also created with --ssh and presumably had the same issue. The assistant may have been lucky that those instances worked (perhaps because they were created differently, or the entrypoint happened to be invoked through some other mechanism), or the problem had simply gone unnoticed.
The .launch script is a black box. Initially, the assistant treats the .launch script as an obstacle. Only by reading its contents does the assistant discover the onstart.sh hook, which becomes the solution.

Input Knowledge Required

To fully understand this message, one needs:

Vast.ai platform knowledge: Understanding that --ssh mode replaces the Docker ENTRYPOINT with Vast's own init script, and that --onstart-cmd exists as a mechanism to run custom commands after initialization.
Docker lifecycle knowledge: Understanding what an ENTRYPOINT is, how it differs from a CMD, and how container initialization works.
The project architecture: Knowing that entrypoint.sh is the heart of the worker lifecycle—it handles parameter fetching, tunnel setup, benchmarking, and registration with the vast-manager.
The immediate context: The previous failed attempt with mangled environment variables (instance 32710355), the correct recreation (32710414), and the growing frustration of an instance that starts but never registers.

Output Knowledge Created

This message produces several valuable pieces of knowledge:

A verified deployment pattern: --ssh + --onstart-cmd is the correct way to deploy instances that need both SSH access and custom entrypoint logic on Vast.ai. This becomes the standard pattern for all future instance creation.
A corrected mental model: The assistant now understands that --ssh mode is not additive but substitutive. This understanding will inform all future deployment decisions.
A reusable debugging technique: The pattern of SSHing into a non-responsive instance, checking ps aux to see what's actually running as PID 1, and then examining the init scripts to understand the platform's lifecycle, is a generalizable approach to debugging container orchestration issues.
Documentation of the .launch script's behavior: The assistant discovers that /.launch runs /root/onstart.sh in the background if it exists. This is a platform-specific detail that may not be obvious from Vast.ai's public documentation.

The Broader Significance

This message is a turning point in the segment. Before it, the assistant was operating under a flawed mental model of how Vast.ai instances work. After it, the assistant has a corrected understanding and a viable path forward. The --onstart-cmd approach becomes the standard for all subsequent instance creation in the conversation, including the instances on hosts 93197 and 88910 that follow.

More broadly, this message illustrates a fundamental truth about distributed systems deployment: the gap between how you think a platform works and how it actually works is where all the debugging time goes. The assistant's willingness to SSH into the instance, inspect the running processes, read the init scripts, and iteratively refine its understanding is what separates successful deployment from endless frustration. The moment of realization—"our entrypoint.sh is NOT running"—is the pivot point around which the entire deployment strategy rotates.

The message also demonstrates the value of reading platform documentation critically. The Vast.ai documentation states that SSH mode replaces the entrypoint, but the assistant had to see it with ps aux to truly believe it. And even then, the solution came not from the documentation but from carefully reading the .launch script itself—a reminder that in platform-specific engineering, the source of truth is often the running system, not the documentation.

Conclusion

Message 923 is a small but perfect example of adaptive engineering under real-world constraints. The assistant encounters an unexpected platform behavior, rapidly works through three alternative solutions, identifies the optimal synthesis, and acts on it—all within a single message. The reasoning process is visible, the assumptions are corrected in real time, and the output knowledge directly shapes the next phase of the project. It is, in essence, the kind of debugging insight that turns a failing deployment into a working one, and it deserves careful study by anyone building distributed systems on third-party infrastructure.