ActionFusion fused tools bypass host arg validation, leaking cryptic TypeError on invalid input #67

Summary

When actionFusion is enabled, calling the fused write (same presumably applies to edit) with invalid arguments produces a cryptic low-level TypeError instead of the host's clean schema-validation error. The fused tool path appears to bypass the host's argument-validation layer that native tools go through. Fail-safe behavior is intact; this is about error quality.

Environment

• SoL-Pi 0.1.0 (2b79168), installed as a standalone extension (no Pi patches)
• Host: Pi-compatible agent v18.2.1 (same @earendil-works/pi-coding-agent extension API)
• Node v25.2.1, Windows

Repro

Call write with only path (deliberately omit required content), once with actionFusion: false and once with actionFusion: true, and compare the error text.

Observed

• AF off (native tool): clean validation error —
  Validation failed for tool "write": content must be file content (was missing) plus the received arguments.
• AF on (fused tool): undefined is not an object (evaluating 't.split') followed by the (correct) [then_run:skipped] The file mutation did not complete successfully; the command was not run.

Analysis

createActionFusionExtension (src/sol-pi/extensions/action-fusion/index.ts) re-registers edit/write via pi.registerTool, spreading the base template's parameters and adding optional then_run. Its execute calls the base definition's execute directly through executeMutationThenRun (then-run.ts). I verified by reading the sources (neither file contains any .split call) and by experiment: calling the host's own createWriteToolDefinition(ctx.cwd).execute(...) directly with valid args succeeds, so the t.split TypeError comes from the implementation receiving unvalidated input (content === undefined) — input the native path would have rejected up front with a schema error.

To be explicit: valid fused calls work fine on this host (verified end-to-end with the real executeMutationThenRun: file created byte-exact, [then_run:succeeded] marker present). Only the invalid-input error path degrades.

Expected

Fused tools should surface the same argument-validation error a native tool would for invalid input — e.g. validate required fields before mutate(), or route through the host's validation — instead of leaking an implementation-internal TypeError to the model.