Sandbox does not mount skills-store, so agents cannot read skill files

Sandbox does not mount skills-store, so agents cannot read skill files

Problem

The agent receives each skill <location> in the system prompt and is told to use read_file to open the corresponding SKILL.md.

However, in sandbox mode the container only mounts the workspace and does not mount the skills-store. This means that even if the Loader discovers the skill, the agent still cannot actually read the skill file inside the sandbox.

Verified behavior

The following has been confirmed:

• BuildSummary(...) emits <location> values pointing to host skills-store/.../SKILL.md paths
• sandbox startup mounts only the workspace into the container
• calling read_file on that absolute path inside the sandbox fails
• FsBridge.resolvePath(...) falls back when the absolute path is outside the sandbox workdir
• the agent never gets the real skill content

Root cause

The sandbox-visible filesystem does not match the skill paths returned by the Loader:

• Loader returns host absolute paths under skills-store
• sandbox mounts only the workspace
• the container does not contain the corresponding skills-store tree

So the prompt tells the agent to read a skill file that is not reachable inside the sandbox.

Relevant code

• internal/agent/systemprompt.go: instructs the agent to open SKILL.md via read_file using <location>
• internal/skills/loader.go: BuildSummary emits <location>
• internal/sandbox/docker.go: sandbox only mounts the workspace
• internal/sandbox/fsbridge.go: absolute paths outside the workdir are rejected/fallen back
• internal/tools/sandbox_hints.go: shows sandbox file-not-found hints
• internal/tools/use_skill.go: use_skill does not load content itself; it still depends on read_file

Impact

• skills are effectively unusable in sandbox mode
• the agent knows the skill name and location but cannot read the content
• prompt behavior and runtime filesystem do not match
• to users this appears as “the skill exists but the agent does not follow it”

Recommendation

At least one of the following must be true:

1. sandbox mounts the skills-store, and <location> is mapped to a real readable path inside the container
2. Loader returns container-accessible paths in sandbox mode instead of host paths

A more robust fix is to unify host/container path mapping for skill files so agents never see host absolute paths that are unreachable inside the container.

Start by tracing the skill path from internal/skills/loader.go and internal/agent/systemprompt.go into sandbox startup in internal/sandbox/docker.go. Read internal/sandbox/fsbridge.go and internal/tools/use_skill.go to understand how paths and skill loading interact. Done means a sandboxed agent can read the referenced SKILL.md through a container-accessible path, with the prompt and runtime filesystem aligned.