# Build checkpoint subtree paths via path.Join helper (no tree change)

`a13bcc5`·

Soph·2w ago·2 files·+117 added/-66 removed

A reviewer flagged `fmt.Sprintf("%s%d/%s", basePath, idx, paths.MetadataFileName)` as looking like a missing slash — it's correct only because basePath carries a trailing "/". Replace the string-concat path construction with a checkpointSubtreePath(base, segs...) helper over stdlib path.Join, so paths join correctly without relying on that invariant.

- Helper uses the `path` package (NEVER path/filepath — git tree paths are always "/"; filepath.Join would emit "\\" on Windows and corrupt tree keys). path.Join cleans separators, so base may be "" (ref root), "<shard>/<id>/" (v1, trailing slash), or a clean dir — all join identically.
- Converted every full-path site and routed the directory-prefix intermediates through the helper as CLEAN dirs (sessionDir, taskDir), updating their consumers (writeSessionToSubdirectory, writeTranscript, writeCompactTranscript, replaceTranscript, replaceSkillEvents, copyMetadataDir, writeTaskCheckpoint*).
- The two prefix-scoping deletes that relied on the trailing slash now append it explicitly: HasPrefix(key, sessionDir+"/") (so "1" doesn't match sibling "10") and the transcript chunk cleanup. Leading-slash SessionFilePaths values become "/" + checkpointSubtreePath(...).

No behavior change: committed-tree assertions (checkpoint write/read, tripwire, update), integration (378), and both canary modes (git-branch 59/59, git-refs 58/59 +1 skip) are unchanged.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

## Sessions

973f8248e820View transcript

## Changes

2

- cmd/entire/cli/checkpoint

- Mpersistent.go+84/-66

- Asubtree_path_test.go+33

```go
package checkpoint

import "testing"

func TestCheckpointSubtreePath(t *testing.T) {
	t.Parallel()
	tests := []struct {
		name string
		base string
		segs []string
		want string
	}{
		// Per-checkpoint-ref root (basePath == ").
		{"ref root metadata", "", []string{"metadata.json"}, "metadata.json"},
		{"ref root session meta", "", []string{"0", "metadata.json"}, "0/metadata.json"},
		// v1 branch layout (basePath has a trailing slash — path.Join cleans it).
		{"v1 root metadata", "a3/b2c4d5e6f7/", []string{"metadata.json"}, "a3/b2c4d5e6f7/metadata.json"},
		{"v1 session meta", "a3/b2c4d5e6f7/", []string{"0", "metadata.json"}, "a3/b2c4d5e6f7/0/metadata.json"},
		{"v1 task file", "a3/b2c4d5e6f7/", []string{"tasks", "tool-1", "checkpoint.json"}, "a3/b2c4d5e6f7/tasks/tool-1/checkpoint.json"},
		// A clean dir base (no trailing slash) joins identically.
		{"clean session dir", "a3/b2c4d5e6f7/0", []string{"full.jsonl"}, "a3/b2c4d5e6f7/0/full.jsonl"},
		// No segments returns the cleaned base (trailing slash stripped).
		{"base only trailing slash", "a3/b2c4d5e6f7/", nil, "a3/b2c4d5e6f7"},
	}
	for _, tt := range tests {
		t.Run(tt.name, func(t *testing.T) {
			if got := checkpointSubtreePath(tt.base, tt.segs...); got != tt.want {
				t.Errorf("checkpointSubtreePath(%q, %v) = %q, want %q", tt.base, tt.segs, got, tt.want)
			}
		})
	}
}
```