Layer	Source id	Writable	Wins
Inherited process environment	`env`	no	always
`$DSH_HOME/.credentials.yaml` document	`file`	yes (`set`/`unset`)	over both `.env` layers
`<invocation cwd>/.env`	`project-env`	not here	over the user `.env`
`$DSH_HOME/.env`	`user-env`	not here	otherwise

The launching environment wins because a per-run override (DEEPSEEK_API_KEY=… dsh, a CI secret, a container -e) is operator intent for this run — and because it cannot be edited from inside, it must be visibly read-only: describe() reports source: 'env', writable: false, and set/unset reject instead of writing a change the reader would never see.

Everything below it loses to the managed store, so a key written by the Models page takes effect immediately even when an older key sits in a .env. Those two layers still resolve when nothing is stored, and describe() names them project-env or user-env with writable: true — storing a key replaces them as the effective source.

Under the product CLI, resolution reads the launcher's frozen environment snapshot rather than process.env: only the snapshot can say whether a value came from the launching shell or from a file. A composition the product CLI did not boot has the inherited environment as its only layer, which keeps embedders on the semantics they already had.

Config

Field	Default	Meaning
`path`	`<harness home>/.credentials.yaml`	Credentials document location.
`dshHome`	`$DSH_HOME` or `~/.dsh`	Harness home used when `path` is omitted.
`watch`	`true`	Hot-publish external edits.
`debounceMs`	`100`	Watcher write-settle window.

The document

A versioned YAML document with one section per key space, and nothing else:

version: 1

refs:
  DEEPSEEK_API_KEY: sk-…
  OPENAI_API_KEY: sk-…

records:
  llm-pi-ai/openai-codex:
    kind: grant
    payload:                    # written verbatim; this provider does not interpret it
      type: oauth
      access: eyJhbGciOi…
      refresh: rft_9f8e7d…
      expires: 1786000000000
  llm-pi-ai/amazon-bedrock:
    kind: api-key               # environment values, no key: this route uses an AWS profile
    env:
      AWS_PROFILE: prod
  llm-pi-ai/amazon-bedrock-dev:
    kind: api-key               # neither: the owner confirmed the ambient credential chain

The document holds credentials only, so every deviation is a rejection rather than a skipped entry — a silently ignored key would read as "the credential I stored has no effect". A non-mapping root, an unknown top-level key, a key that is not addressable in its space, a wrong-typed value, an empty string, an unknown record tag or field, a duplicate key, and malformed YAML all fail: loud at boot, and warn-and-keep-the-last-good-snapshot on a live reload.

A grant payload must survive a JSON round trip, enforced in both directions. YAML spells values JSON has none for — .inf, alias cycles — and an owner may hand over a Date or a bigint; either way the store refuses rather than saving something it could not read back exactly as written.

The pre-release layout was a flat mapping with no version. A boot that recognizes it exactly — addressable names over non-empty string scalars, no directives — upgrades the document in place under the writer lock: the original lines nest verbatim under refs:, so values, comments, and spellings survive byte for byte. Any other flat shape is refused by name, with the entry count and the one edit needed (version: 1, nest under refs:) — never read as an empty store, which would surface as an authentication failure on the first request instead of at load. A live reload never migrates: a flat document restored mid-run keeps the last good snapshot until the next boot.

Writes patch the parsed document rather than rebuilding it, so comments and the formatting of every untouched entry survive. A comment directly above an entry is that entry's annotation and is removed with it. Every write first re-reads the document under the cross-process writer lock of dsh-atomic-write and publishes anything it had not observed, then commits atomically with mode 0600 under an owner-only (0700) directory — so a concurrent writer or an external edit inside the watcher's debounce window is folded in rather than overwritten. An on-disk document that no longer parses fails the write instead of overwriting content the provider could not understand.

Any string value round-trips, multi-line values included, so no entry is unwritable for want of a quoting style. An empty stored value is absent, per the seam rule — which is why an empty string in the document is rejected outright: unset removes a key, it does not blank it.

Permissions

The provider creates the directory 0700 and creates or atomically replaces the document 0600. It holds what it reads to that same bound: on POSIX a document carrying any group or other permission bit fails before its contents are parsed — at boot and on every reload — and the error names the chmod 600 repair. Windows has no mode to inspect, so the check is skipped there rather than faked.

Hot reload

External edits publish credentials/reference-updated per changed reference after the snapshot is replaced wholesale — an entry deleted on disk never lingers in memory. Before Chokidar opens the target, the provider realpaths its deepest existing ancestor and restores any missing suffix; file access and diagnostics retain the configured path, while Windows cannot mix an 8.3 alias with long-form libuv events. The provider's own writes are recognized by content and publish exactly their one commit event. An unreadable or invalid document at runtime keeps the last good snapshot and warns; an absent file is an empty store; an unreadable or invalid file at boot fails loud.

Security boundary

The document is 0600 under a 0700 directory, which stops other OS users — not the model. Tool processes (bash, the filesystem tools) run as the same user, and the shipped workspace-write file policy confines mutations rather than reads, so they can read this file exactly like any other file the user owns; no sandbox mode singles it out. What the harness does hold to is narrower: it never hands the model a resolved path to the document, and never loads it into the process environment — unlike $DSH_HOME/.env, which is the user's ordinary environment layer (see app-boot's Harness-home layers) — so reaching the value takes a deliberate read of a path the agent was not given.

That is discretion, not a boundary. A deployment that must keep provider keys away from its own agent cannot get there with file permissions; an OS-keychain provider — a store the model's processes cannot read at all — is the deferred answer and belongs beside this provider as a sibling package.

Model Experience

Indirectly, through the consuming LLM adapters: stored values authorize their provider requests, and the adapter owns every model-visible surface.

KV Cache effect

No direct invalidation; credentials never enter a request prefix.

Known Limitations and Deferred Work

Same-reference concurrent writes are last-write-wins — the writer lock and the read-modify-write keep concurrent writers from dropping each other's entries, but two writers editing one reference still resolve to the later write; there is no revision check.
A same-UID process can read the document — see Security boundary: the file-effect sandbox modes do not deny reads, and an OS-keychain provider is deferred.
Environment changes are invisible — the snapshot is frozen at launch, so a variable exported after startup reaches neither resolution nor describe; changing an environment-sourced credential takes a restart.
Atomic, not crash-durable — inherited from dsh-atomic-write; the store re-reads on boot.