The Onstart-Cmd Breakthrough: Adapting to Vast.ai's SSH Mode Entrypoint Replacement

Introduction

In the complex dance between custom Docker containers and cloud GPU platforms, few things are more frustrating than discovering that a carefully crafted entrypoint has been silently replaced by the platform's own initialization scripts. This is precisely the challenge that message [msg 925] addresses. In this single, decisive message, the assistant resolves a fundamental architectural conflict between the need for SSH access to a Vast.ai GPU instance and the requirement that a custom lifecycle entrypoint (entrypoint.sh) run automatically on container start. The solution—using Vast.ai's --onstart-cmd flag in combination with --ssh mode—represents a critical adaptation to the platform's specific behavior, and it marks the moment when the entire deployment pipeline finally comes together.

The Message

The assistant writes:

There's --onstart-cmd for running a script on start with SSH mode. Let me use --ssh --onstart-cmd to run our entrypoint in the background:

>

``bash ssh 10.1.2.104 "vastai create instance 29854362 --image theuser/curio-cuzk:latest --disk 250 --env '-e PAVAIL=portavail1:[REDACTED] -e PAVAIL_SERVER=45.33.141.226:22222' --ssh --direct --onstart-cmd 'nohup /usr/local/bin/entrypoint.sh > /var/log/entrypoint.log 2>&1 &'" ``

>

Started. {'success': True, 'new_contract': 32710471, 'instance_api_key': '[REDACTED]'}

This is a creation command for a new Vast.ai instance, but it is far more than a routine provisioning call. It encapsulates the lessons learned from two previous failed attempts and embodies a deep understanding of how Vast.ai's launch modes interact with Docker container lifecycle management.

The Journey to This Point

To understand why this message was written, one must trace the path that led to it. The assistant had been working on deploying a custom Docker image (theuser/curio-cuzk:latest) containing a Filecoin proof-of-spacetime (PoSt) proving pipeline. This image had a carefully designed entrypoint.sh that handled:

  1. Establishing a portavailc tunnel for secure communication
  2. Registering the instance with a central manager service (vast-manager)
  3. Running benchmark and worker processes The first attempt at creating an instance (ID 32710355) failed because the PAVAIL environment variable, which contains the authentication secret for the port forwarding tunnel, was mangled during creation. The shell expression $(cat /etc/portavaild/secret) was passed literally rather than being evaluated, resulting in a malformed value. The second attempt (ID 32710414) corrected the environment variable issue, but introduced a more subtle problem: the instance was created with the --ssh flag. On Vast.ai, --ssh mode replaces the Docker image's original ENTRYPOINT with the platform's own initialization script (/.launch). This script sets up SSH access and runs any user-specified onstart commands, but it completely bypasses the Docker entrypoint. When the assistant SSH'd into the running instance, they discovered that entrypoint.sh was not running at all—PID 1 was the Vast init script, not the custom entrypoint.

The Reasoning Process

The thinking visible in this message is concise but powerful. The assistant states: "There's --onstart-cmd for running a script on start with SSH mode." This sentence reveals a moment of recognition—the assistant has connected the platform's documented feature (--onstart-cmd) with the observed problem (entrypoint not running). The key insight is that --onstart-cmd is designed precisely for this scenario: it allows users to inject custom startup commands that run alongside Vast's SSH initialization, rather than replacing it.

The assistant then constructs the command with careful attention to detail:

Decisions Made

Several critical decisions are embedded in this message:

  1. Retain --ssh mode: The assistant could have abandoned SSH mode entirely and used --entrypoint mode instead, which would preserve the Docker ENTRYPOINT. However, SSH access was deemed too valuable for debugging and monitoring to sacrifice.
  2. Use --onstart-cmd rather than modifying the image: Another option would have been to modify the Docker image to detect whether it was running under Vast's SSH mode and adapt accordingly. The assistant chose the more immediate, platform-level solution.
  3. Background the entrypoint: The nohup and backgrounding pattern ensures the entrypoint runs as a daemon process, not blocking Vast's initialization.
  4. Log redirection: Directing output to /var/log/entrypoint.log preserves the ability to debug later, acknowledging that the entrypoint's output would otherwise be lost.

Assumptions and Corrections

This message corrects a significant incorrect assumption from earlier in the session: that --ssh mode would still somehow run the Docker ENTRYPOINT. The assistant had assumed that Vast's SSH mode would layer SSH access on top of the existing entrypoint, rather than replacing it entirely. This assumption was natural—many container platforms do layer SSH on top of existing entrypoints—but Vast.ai does not.

The correction came through direct observation: SSH'ing into instance 32710414 and examining the process list revealed PID 1 was Vast's init script, not the custom entrypoint. This empirical debugging approach—creating an instance, inspecting it, learning from the failure, and adapting—is the hallmark of the entire session.

Input Knowledge Required

To understand this message, one needs knowledge of:

Output Knowledge Created

This message produces several important outcomes:

  1. A working instance template: The command serves as a reusable pattern for creating Vast.ai instances that need both SSH access and custom startup logic
  2. Instance 32710471: A running GPU instance with the correct configuration, which will eventually validate the entire deployment pipeline
  3. A documented workaround: The --ssh --onstart-cmd pattern becomes the standard operating procedure for all future instance creation in this project
  4. Validation of the entrypoint design: By ensuring entrypoint.sh runs, the assistant confirms that the earlier design decisions (log shipping, registration, benchmark automation) remain viable

The Broader Significance

This message sits at a turning point in the conversation. The previous chunks had been consumed with platform-specific quirks: environment variable visibility, entrypoint replacement, and instance lifecycle management. Message [msg 925] is where all these threads converge into a single, correct command. From this point forward, the deployment process stabilizes, and the focus shifts from "how do we get the container running?" to "how do we optimize the workload for different hardware configurations?"

The --onstart-cmd workaround is elegant in its simplicity. Rather than fighting Vast.ai's platform design or rebuilding the Docker image, the assistant works with the platform's existing mechanisms. The onstart hook is exactly the escape hatch that Vast provides for this scenario, and using it properly demonstrates a mature understanding of platform integration.

Conclusion

Message [msg 925] is a masterclass in adaptive problem-solving. It shows how a single command can encapsulate the lessons from multiple failures: the env var mangling of instance 32710355, the entrypoint replacement of instance 32710414, and the discovery of Vast's /.launch mechanism. The assistant's thinking process—"There's --onstart-cmd for running a script on start with SSH mode"—represents the moment of synthesis where all the pieces finally fit together. The result is a robust, repeatable deployment pattern that will serve as the foundation for all subsequent work in this session.