diff --git a/AGENTS.md b/AGENTS.md
index d1b2554a..f00bcce3 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -14,11 +14,15 @@ applyTo: '**'
 **For contributing:**
 - [CONTRIBUTING.md](CONTRIBUTING.md): Development setup, dargstack guidelines, code style, git workflow
 
+**Cross-service contracts** (policies two or more services must implement identically, with no code shared between them):
+- [docs/password-strength.md](docs/password-strength.md): Password strength requirements enforced by `vibetype` and `postgraphile`
+
 ## Code Style
 
 - Do not use abbreviations in naming, except where omitting them would look unnatural
 - Use natural language in any non-code text instead of referring to code directly, e.g. "the database's password" instead of "the `postgres_password`", except when a code reference is needed
 - Use backticks in any non-code text to refer to code, e.g. "`postgres`" instead of "postgres"
+- In markdown prose, start each sentence on its own line (semantic line breaks); renders the same, but keeps diffs scoped to the sentence that changed
 - Sort YAML keys lexicographically except where order is semantically significant
 - Code formatting is done by the editor via `.editorconfig`
 
diff --git a/docs/password-strength.md b/docs/password-strength.md
new file mode 100644
index 00000000..c3cac1fe
--- /dev/null
+++ b/docs/password-strength.md
@@ -0,0 +1,63 @@
+# Password strength policy
+
+This document is the single source of truth for the password strength policy that any implementation setting an account password must satisfy.
+It lives here, rather than in `vibetype` or `postgraphile`, because both services implement it independently (code sharing between them is not an option), and a policy shared across services belongs in `stack` rather than being duplicated per repo.
+When either service's implementation changes, check it against this document rather than against the other service's code.
+
+## Scope
+
+Applies to every operation that sets a password a user will authenticate with:
+
+| Operation               | Field carrying the new password | Covered |
+| ------------------------ | -------------------------------- | ------- |
+| `accountRegistration`    | `input.password`                 | yes     |
+| `accountPasswordReset`   | `input.password`                 | yes     |
+| `accountPasswordChange`  | `input.passwordNew`              | yes     |
+
+`accountPasswordChange`'s `input.passwordCurrent` is explicitly **out of scope**: it authenticates an existing password, which may predate this policy, and must never be strength-checked.
+
+## Requirements
+
+Both of the following must hold.
+
+1. **Minimum length**: 8 characters.
+   This matches NIST SP 800-63B's own floor.
+   It is a cheap sanity backstop, not the control doing the real work, see [Why keep a length floor](#why-keep-a-length-floor).
+2. **Minimum strength**: a [zxcvbn](https://github.com/zxcvbn-ts/zxcvbn) score of at least 3 ("safely unguessable", resists an offline, slow-hash attack; see the library's own scoring guidance).
+   This is the control that actually determines whether a password is accepted in practice.
+
+### Why keep a length floor
+
+NIST SP 800-63B requires a minimum length (>= 8) plus screening against common or compromised passwords.
+It does not separately mandate a guessability-estimator score on top of that.
+zxcvbn's score already factors in length as one of its inputs, so once score >= 3 is required, an 8 character floor rarely does independent work: empirically, the shortest fully random password (e.g. `xK9#mL2qP`, drawn from a large character set) needed to reach score 3 is 9 characters, one above this floor.
+The floor is kept anyway as a structural backstop that does not depend on zxcvbn's heuristics being correct for a given input, and as a small margin against future improvements in offline hash-cracking speed, which erode a short password's safety margin fastest regardless of how patternless it is.
+
+## Algorithm and configuration
+
+Both implementations must use identical configuration, or they will disagree on borderline passwords (a password accepted by the client but rejected by the server, or vice versa).
+
+- **Library**: `@zxcvbn-ts/core`, via `new ZxcvbnFactory(options).check(password).score`.
+- **Dictionaries**: `@zxcvbn-ts/language-common` (common passwords plus adjacency graphs for keyboard-pattern detection) merged with `@zxcvbn-ts/language-de` and `@zxcvbn-ts/language-en` (both dictionaries only; German and English are the platform's supported locales).
+- **Translations**: `@zxcvbn-ts/language-en`.
+  This only affects zxcvbn's internal feedback strings.
+  Neither implementation surfaces them to the user, so the specific language here is not user-visible, but the `ZxcvbnFactory` constructor requires a non-empty value.
+- **Package versions**: pinned independently in each repo's `package.json`.
+  Keep `@zxcvbn-ts/core`, `@zxcvbn-ts/language-common`, `@zxcvbn-ts/language-de`, and `@zxcvbn-ts/language-en` at the same version in both repos.
+  A dictionary update can change which side of the score-3 boundary a given password falls on.
+
+## Current implementation status
+
+| Layer                          | Minimum length (8) | zxcvbn score (>= 3) |
+| ------------------------------- | -------------------- | --------------------- |
+| `vibetype` (client)             | enforced, all 3 operations | enforced, all 3 operations |
+| `postgraphile` (server)         | not this layer's job, see below | enforced, all 3 operations |
+| `sqitch` (database)             | enforced, all 3 operations (`char_length(...) < 8` in each function) | not applicable, zxcvbn cannot run in SQL |
+
+`postgraphile` intentionally does not re-check length: since every underlying sqitch function already rejects anything shorter than 8 characters, and that is exactly this policy's floor, duplicating the check in `postgraphile` would add no protection.
+
+## Where each side implements this
+
+- `vibetype`: `src/app/utils/passwordStrength.ts` (scoring), `src/app/utils/validation.ts` (`SCHEMA_PASSWORD_V2`, length), `src/app/composables/useAuthPasswordValidation.ts` and `usePasswordPairValidation.ts` (live field validation wiring).
+- `postgraphile`: `src/presets/passwordStrength.ts` (`PasswordStrengthPlugin`, a Grafserv middleware that inspects `accountRegistration`, `accountPasswordReset`, and `accountPasswordChange` mutations before they execute).
+- `sqitch`: the `char_length(...) < 8` check in `function_account_registration.sql`, `function_account_password_reset.sql`, and `function_account_password_change.sql`.