feat(config): expose config defaults and provide sparse mapping functions

docs/adr/0018-sparse-config-subtraction.md

@@ -0,0 +1,65 @@
+# 0018. Sparse Config Subtraction
+
+**Status**: proposed
+**Date**: 2026-08-18
+
+## Problem Statement
+
+`config diff` (CLI-2156) and `config pull` (CLI-2064) compare a project's remote configuration against the local `config.toml` to surface *drift*: any difference between the project's effective remote configuration and the local file. The remote endpoint (`GET /v2/projects/{ref}/config`) returns the *effective* config — every setting reported, defaulted or not — and a locally decoded `ProjectConfig` likewise has every default filled in. Comparing these full objects directly would drown the user in hundreds of identical default values. CLI-2155 asks for a stored reference of config defaults and a mapping function that omits values matching them, so diffs stay readable and pulled files stay sparse.
+
+The trap is where that mapping recurses. A `[remotes.<label>]` block declares config overrides for a specific [persistent Supabase branch](https://supabase.com/docs/guides/local-development/cli/config#branching-config): the branch's project ref in `project_id` binds the block to the branch (the label is a user-chosen alias and is never matched on), any root config option can be overridden inside it, and unspecified options inherit from the *base config* — the root-scope fields of the file, before any remote block is overlaid. The block is therefore itself a sparse overlay over the merged base config. A value in a remote block that happens to equal a *global default* is not redundant: if the base config overrides the same property, removing the remote's value silently changes what that branch resolves to.
+
+## Decision
+
+`@supabase/config` exports a pure, parameterized subtraction core:
+
+- `getDefaultProjectConfig()` — the default config, derived by decoding `{}` through `ProjectConfigSchema` (memoized). The schema's `default` annotations and decoding defaults are the single source of truth; no hand-maintained defaults table exists.
+- `subtractProjectConfig(config, baseline)` — returns the sparse config `config − baseline`: every value strictly deep-equal (order-sensitive) to the baseline's is removed, then sections left empty are dropped recursively.
+- `omitDefaultValues(config)` — `subtractProjectConfig` with the default config as baseline.
+
+What the output *is* depends on the baseline. In the primary case — subtracting the default config (`omitDefaultValues`) — the result is itself a valid config document: re-decoding refills exactly what was removed, so it denotes the same effective config *under the current schema's defaults*. That parenthetical is load-bearing: a sparse file's meaning leans on the defaults reference, so a default that changes in a future schema version changes the file's effective meaning — the dependency the PRFAQ's versioned-defaults open question exists to manage. Subtracting any other baseline — a remote block against the merged base config — yields an overlay that is meaningful only relative to that baseline and is not a standalone config. At the type level, `SparseProjectConfig` is a deep-partial of `ProjectConfig` either way and must be re-decoded before use where a complete config is required.
+
+Subtraction never recurses into `remotes` when the baseline is the default config: the default config has no remote blocks, so under subtract semantics user remote blocks survive untouched. **The correct baseline for a remote block is the merged base config, never the default config.** Callers that want to sparsify a remote block (CLI-2156/2064) must call `subtractProjectConfig(remoteBlock, mergedBaseConfig)` explicitly.
+
+All functions are pure and synchronous, operating on decoded `ProjectConfig` values, with no Effect in the public signature.
+
+## Rationale
+
+- **One core instead of a cascade.** The merge-and-prune cascade sketched in CLI-2155's planning comment reduces algebraically to `subtract(merge(local, remote), defaults)`. Exporting the subtraction gives `config diff` and `config pull` the shared comparison core without binding this package to the Management API's response shape (translating that shape to `ProjectConfig` is CLI-2156's concern).
+- **Defaults derived, not duplicated.** Every field's default already lives in the schema (`default` annotations plus `withDecodingDefaultKey`). Decoding `{}` materializes them; a parallel hand-written defaults object would drift.
+- **Strict deep equality, array order matters.** Order is semantically load-bearing for values like `api.extra_search_path` (Postgres `search_path` resolution order). Treating reordered arrays as equal would prune values that behave differently from the default. The false-positive cost (a semantically-default-but-reordered array survives as harmless noise) is far cheaper than wrongly deleting meaningful config.
+- **Dropping empty sections is lossless.** Every section carries a section-level decoding default, so an absent section and an empty section decode identically; empty carcass headers are pure diff noise.
+- **Pure functions.** The subtraction has no failure channel, no resources, and no concurrency — the repo's Effect-native policy explicitly exempts such leaf primitives. Purity also keeps the module portable if the package later grows browser-compatible entry points.
+
+## Consequences
+
+### Positive
+
+- Diffs and pulled configs contain only values that differ from their baseline.
+- One walk implementation serves defaults-stripping and remote-block sparsification.
+- Schema remains the single source of truth for defaults; changing a default in the schema changes the subtraction behavior with no second edit.
+- A unit test pins the invariant that all schema defaults are mutually valid (decoding `{}` succeeds), so a conflicting default fails CI loudly.
+
+### Negative
+
+- Fields declared `optionalKey` without a `default` annotation can never be pruned; if a platform default exists for such a field, it must be added to the schema before subtraction can see it.
+- A user's explicitly-written default value (`max_rows = 1000` typed by hand) is indistinguishable from an omitted one and will be pruned; intent is not preserved.
+- Callers must know the remote-block baseline rule; misusing the default config as a remote block's baseline reintroduces the override-erasure bug this ADR exists to prevent.
+
+## Alternatives Considered
+
+1. **Merge-and-prune cascade as a single function** (`(apiResponse, configToml) → massaged config`): binds `@supabase/config` to the Management API response shape and entangles this package with remote-to-local translation, which belongs to the diff core (CLI-2156).
+2. **Recursing into `remotes` against global defaults**: looks obviously correct, is subtly wrong — pruning a remote's `api.max_rows = 1000` (global default) under a base that sets `500` changes the branch's effective value from 1000 to 500.
+3. **Hand-written defaults reference object**: duplicates ~100 defaults already declared in the schema and drifts silently.
+4. **Order-insensitive array comparison**: prunes reordered arrays whose order is semantically meaningful (`extra_search_path`).
+
+## Related Decisions
+
+- [ADR 0009](0009-configuration-schema-and-validation.md): Configuration Schema & Validation — the umbrella charter this decision answers a slice of (default config generation, `@supabase/config` package architecture)
+- [ADR 0006](0006-environment-management.md): Environment Management — remote blocks and branch mapping semantics
+
+## See Also
+
+- [CLI-2155](https://linear.app/supabase/issue/CLI-2155/store-the-config-default-values-and-provide-mapping-function) — this ticket
+- [CLI-2156](https://linear.app/supabase/issue/CLI-2156/add-supabase-config-diff-to-the-cli) — `config diff`, consumer of the subtract core
+- [CLI-2064](https://linear.app/supabase/issue/CLI-2064/add-supabase-config-pull-to-the-cli) — `config pull`

docs/adr/README.md

@@ -41,23 +41,24 @@ When an ADR becomes outdated, mark it as `deprecated` or reference the supersedi
 
 ## ADR index
 
-| ID   | Title                                                                                      | Status   |
-| ---- | ------------------------------------------------------------------------------------------ | -------- |
-| 0000 | [Use ADR to Record Decisions](0000-use-adr-to-record-decisions.md)                         | accepted |
-| 0001 | [CLI DX Architecture: The 7 Pillars](0001-cli-dx-architecture-pillars.md)                  | accepted |
-| 0002 | [CLI Product Metrics](0002-cli-product-metrics.md)                                         | accepted |
-| 0003 | [Self-Documenting CLI & Documentation Strategy](0003-self-documenting-cli.md)              | accepted |
-| 0004 | [CLI Design Goals & Development Workflows](0004-cli-design-goals-and-workflows.md)         | accepted |
-| 0005 | [OpenAPI-Driven Code Generation for CRUD Commands](0005-openapi-driven-code-generation.md) | proposed |
-| 0006 | [Environment Management & Variable Resolution](0006-environment-management.md)             | proposed |
-| 0007 | [Real-time Progress in Command Handlers](0007-realtime-progress-in-command-handlers.md)    | proposed |
-| 0008 | [Authentication & Token Management](0008-authentication-and-token-management.md)           | proposed |
-| 0009 | [Configuration Schema & Validation](0009-configuration-schema-and-validation.md)           | proposed |
-| 0011 | [CLI Release & Distribution Strategy](0011-cli-release-and-distribution-strategy.md)       | proposed |
-| 0013 | [Live E2E Tests Bypass the Replay Server](0013-live-e2e-bypasses-replay-server.md)         | proposed |
-| 0015 | [Managed Stack Contract Fixtures](0015-managed-stack-contract-fixtures.md)                 | superseded |
-| 0016 | [Legacy Port Completion and Go CLI Authority Scope](0016-legacy-port-completion-and-go-cli-authority-scope.md) | proposed |
-| 0017 | [Simplified Managed Stack Architecture](0017-simplified-managed-stack-architecture.md)     | accepted |
+| ID   | Title                                                                                                          | Status     |
+| ---- | -------------------------------------------------------------------------------------------------------------- | ---------- |
+| 0000 | [Use ADR to Record Decisions](0000-use-adr-to-record-decisions.md)                                             | accepted   |
+| 0001 | [CLI DX Architecture: The 7 Pillars](0001-cli-dx-architecture-pillars.md)                                      | accepted   |
+| 0002 | [CLI Product Metrics](0002-cli-product-metrics.md)                                                             | accepted   |
+| 0003 | [Self-Documenting CLI & Documentation Strategy](0003-self-documenting-cli.md)                                  | accepted   |
+| 0004 | [CLI Design Goals & Development Workflows](0004-cli-design-goals-and-workflows.md)                             | accepted   |
+| 0005 | [OpenAPI-Driven Code Generation for CRUD Commands](0005-openapi-driven-code-generation.md)                     | proposed   |
+| 0006 | [Environment Management & Variable Resolution](0006-environment-management.md)                                 | proposed   |
+| 0007 | [Real-time Progress in Command Handlers](0007-realtime-progress-in-command-handlers.md)                        | proposed   |
+| 0008 | [Authentication & Token Management](0008-authentication-and-token-management.md)                               | proposed   |
+| 0009 | [Configuration Schema & Validation](0009-configuration-schema-and-validation.md)                               | proposed   |
+| 0011 | [CLI Release & Distribution Strategy](0011-cli-release-and-distribution-strategy.md)                           | proposed   |
+| 0013 | [Live E2E Tests Bypass the Replay Server](0013-live-e2e-bypasses-replay-server.md)                             | proposed   |
+| 0015 | [Managed Stack Contract Fixtures](0015-managed-stack-contract-fixtures.md)                                     | superseded |
+| 0016 | [Legacy Port Completion and Go CLI Authority Scope](0016-legacy-port-completion-and-go-cli-authority-scope.md) | proposed   |
+| 0017 | [Simplified Managed Stack Architecture](0017-simplified-managed-stack-architecture.md)                         | accepted   |
+| 0018 | [Sparse Config Subtraction](0018-sparse-config-subtraction.md)                                                 | proposed   |
 
 ## Template
 

packages/config/src/index.ts

@@ -49,5 +49,11 @@ export { type ProjectPaths, findProjectPaths, findProjectRoot } from "./paths.ts
 export { projectConfigStoreLayer } from "./project-config.layer.ts";
 export { ProjectConfigStore } from "./project-config.service.ts";
 export { PROJECT_CONFIG_SCHEMA_URL } from "./schema-metadata.ts";
+export {
+  type SparseProjectConfig,
+  getDefaultProjectConfig,
+  omitDefaultValues,
+  subtractProjectConfig,
+} from "./sparse.ts";
 export { KONG_LOCAL_CA_CERT } from "./tls.ts";
 export { ENV_CAPTURE_REGEX } from "./lib/env.ts";

packages/config/src/io.ts

@@ -8,6 +8,7 @@ import {
 } from "./errors.ts";
 import { interpolateEnvReferencesAgainstSchema } from "./lib/env.ts";
 import { findProjectPaths } from "./paths.ts";
+import { getDefaultProjectConfig, subtractValue } from "./sparse.ts";
 import { loadProjectEnvironment, type ProjectEnvironment } from "./project.ts";
 
 const projectConfigSchemaKey = "$schema";
@@ -147,7 +148,7 @@ const decodeRemotesWithoutChecks = Schema.decodeUnknownSync(RemotesSchema, {
   disableChecks: true,
 });
 const encodeProjectConfig = Schema.encodeSync(ProjectConfigSchema);
-const defaultEncodedProjectConfig = encodeProjectConfig(decodeProjectConfig({}));
+const defaultEncodedProjectConfig = encodeProjectConfig(getDefaultProjectConfig());
 const defaultEncodedFunctionConfig = {
   enabled: true,
   verify_jwt: true,
@@ -346,68 +347,6 @@ const applyRemoteOverride = Effect.fnUntraced(function* (
   return { document: merged, appliedRemote: name, remoteLeafPaths };
 });
 
-function isEqualValue(left: unknown, right: unknown): boolean {
-  if (Array.isArray(left) && Array.isArray(right)) {
-    if (left.length !== right.length) {
-      return false;
-    }
-
-    for (let index = 0; index < left.length; index += 1) {
-      if (!isEqualValue(left[index], right[index])) {
-        return false;
-      }
-    }
-
-    return true;
-  }
-
-  if (isObject(left) && isObject(right)) {
-    const leftKeys = Object.keys(left);
-    const rightKeys = Object.keys(right);
-
-    if (leftKeys.length !== rightKeys.length) {
-      return false;
-    }
-
-    for (const key of leftKeys) {
-      if (!(key in right) || !isEqualValue(left[key], right[key])) {
-        return false;
-      }
-    }
-
-    return true;
-  }
-
-  return Object.is(left, right);
-}
-
-function stripDefaults(value: unknown, defaults: unknown): unknown {
-  if (defaults === undefined) {
-    return value;
-  }
-
-  if (Array.isArray(value)) {
-    return isEqualValue(value, defaults) ? undefined : value;
-  }
-
-  if (isObject(value)) {
-    const defaultObject = isObject(defaults) ? defaults : {};
-    const result: Record<string, unknown> = {};
-
-    for (const [key, child] of Object.entries(value)) {
-      const stripped = stripDefaults(child, defaultObject[key]);
-
-      if (stripped !== undefined) {
-        result[key] = stripped;
-      }
-    }
-
-    return Object.keys(result).length === 0 ? undefined : result;
-  }
-
-  return isEqualValue(value, defaults) ? undefined : value;
-}
-
 function stripFunctionRecordDefaults(value: unknown): unknown {
   if (!isObject(value)) {
     return value;
@@ -420,15 +359,15 @@ function stripFunctionRecordDefaults(value: unknown): unknown {
 
   const functions: Record<string, unknown> = {};
   for (const [name, functionConfig] of Object.entries(functionsValue)) {
-    functions[name] = stripDefaults(functionConfig, defaultEncodedFunctionConfig) ?? {};
+    functions[name] = subtractValue(functionConfig, defaultEncodedFunctionConfig) ?? {};
   }
 
   return { ...value, functions };
 }
 
 function encodeMinimalProjectConfig(config: ProjectConfig): Record<string, unknown> {
   const encoded = stripFunctionRecordDefaults(encodeProjectConfig(config));
-  const stripped = stripDefaults(encoded, defaultEncodedProjectConfig);
+  const stripped = subtractValue(encoded, defaultEncodedProjectConfig);
   return isObject(stripped) ? stripped : {};
 }