Terraform Provider Configuration and Authentication

How a provider accepts connection settings and resolves credentials. Poor authentication UX is the first thing every user of a provider hits; a well-designed credential provider chain is what separates a production-grade provider from a demo. The examples use a fictional examplecloud provider and the Plugin Framework.

References (load when needed):

• references/credential-chain.md — complete, compilable credential chain implementation (providers, chain, file profiles, Configure wiring, tests)
• references/case-studies.md — how the AWS provider (aws-sdk-go-base) and smaller providers structure real credential chains

Provider Schema for Authentication

Every authentication attribute must be Optional, never Required — a Required attribute forces users to put credentials in configuration and makes environment-variable and credentials-file resolution impossible. Mark secrets Sensitive so Terraform redacts them in plan output, and state the environment-variable fallback in each description so tfplugindocs publishes the resolution rules.

func (p *examplecloudProvider) Schema(ctx context.Context, req provider.SchemaRequest, resp *provider.SchemaResponse) {
    resp.Schema = schema.Schema{
        Attributes: map[string]schema.Attribute{
            "endpoint": schema.StringAttribute{
                Optional:            true,
                MarkdownDescription: "API endpoint. May also be set via the `EXAMPLECLOUD_ENDPOINT` environment variable.",
            },
            "api_key": schema.StringAttribute{
                Optional:            true,
                MarkdownDescription: "API key. May also be set via the `EXAMPLECLOUD_API_KEY` environment variable, or in a shared credentials file.",
            },
            "api_secret": schema.StringAttribute{
                Optional:            true,
                Sensitive:           true,
                MarkdownDescription: "API secret. May also be set via the `EXAMPLECLOUD_API_SECRET` environment variable, or in a shared credentials file.",
            },
            "profile": schema.StringAttribute{
                Optional:            true,
                MarkdownDescription: "Named profile in the shared credentials file. May also be set via the `EXAMPLECLOUD_PROFILE` environment variable. Defaults to `default`.",
            },
            "skip_credentials_validation": schema.BoolAttribute{
                Optional:            true,
                MarkdownDescription: "Skip the identity check normally performed during provider configuration.",
            },
        },
    }
}

Never add a Default to a credential attribute, and never hardcode a credential anywhere in the provider. Defaults belong in the resolution logic (where environment variables and files can override them), not in the schema.

The Credential Provider Chain

Resolve credentials by consulting an ordered list of sources and taking the first one that produces a complete set. This is the pattern the AWS provider uses via aws-sdk-go-base, and it generalizes to any provider. The canonical precedence, highest first:

1. Static configuration — values set directly in the provider block. Explicit always wins.
2. Environment variables — EXAMPLECLOUD_API_KEY, etc. The CI-friendly path.
3. Shared credentials file — named profiles in ~/.examplecloud/credentials, for humans with multiple accounts.
4. Platform identity — instance metadata, workload identity, or OIDC token exchange, where the platform offers it. Credentials nobody has to store.

Two rules make the chain predictable:

• Resolve secrets as a set, not field-by-field. If the environment supplies an API key but no secret, that source offers nothing — fall through to the next source for both values. Mixing an env-var key with a file-profile secret produces authentication failures that are nearly impossible for users to debug.
• Resolve non-secret connection settings field-by-field. endpoint, profile, or insecure can each independently follow config > env > file > default, because a mismatch there is visible and harmless.

The core abstraction is a single-method interface with a sentinel error that distinguishes "this source has nothing to offer" (fall through) from "this source is misconfigured" (surface it):

// ErrNoCredentials signals a source had nothing to offer. The chain falls
// through to the next source. Any other error means the source was
// configured but unusable (e.g. malformed credentials file) and is
// preserved so the final diagnostics can surface it.
var ErrNoCredentials = errors.New("no credentials found")

type Credentials struct {
    APIKey    string
    APISecret string
    Source    string // which provider supplied them, for logging
}

func (c Credentials) Complete() bool {
    return c.APIKey != "" && c.APISecret != ""
}

type Provider interface {
    Retrieve(ctx context.Context) (Credentials, error)
    Name() string
}

A Chain (itself a Provider, so chains compose) walks the providers in order and returns the first complete set of credentials. Every skipped source is recorded into an aggregate ChainError whose Error() lists each source with the reason it was skipped, and whose Is method makes errors.Is(err, ErrNoCredentials) true only when every source fell through cleanly — so Configure can tell "nothing supplied" from "something supplied but broken" with one check. The full implementation — the chain loop, the static, environment, and file providers, and the NewDefaultChain constructor that owns the canonical order — lives in references/credential-chain.md.

Wiring the Chain into Configure

Configure runs once per Terraform operation, before any resource CRUD. The shape:

func (p *examplecloudProvider) Configure(ctx context.Context, req provider.ConfigureRequest, resp *provider.ConfigureResponse) {
    var config examplecloudProviderModel
    resp.Diagnostics.Append(req.Config.Get(ctx, &config)...)
    if resp.Diagnostics.HasError() {
        return
    }

    // 1. Guard against unknown values (e.g. api_key = some_resource.output).
    if config.APIKey.IsUnknown() {
        resp.Diagnostics.AddAttributeError(
            path.Root("api_key"),
            "Unknown API Key",
            "The provider cannot connect because api_key depends on a value known only after apply. "+
                "Set a static value, or use the EXAMPLECLOUD_API_KEY environment variable.",
        )
    }
    // ... repeat for each auth attribute, then:
    if resp.Diagnostics.HasError() {
        return
    }

    // 2. Resolve credentials through the chain.
    chain := credentials.NewDefaultChain(
        config.APIKey.ValueString(),
        config.APISecret.ValueString(),
        credentials.Options{Profile: config.Profile.ValueString()},
    )
    creds, err := chain.Retrieve(ctx)
    if err != nil {
        if errors.Is(err, credentials.ErrNoCredentials) {
            resp.Diagnostics.AddError(
                "No Valid Credential Sources Found",
                "No examplecloud credentials were found. Sources tried, in order:\n\n"+err.Error()+
                    "\n\nSet api_key and api_secret in the provider block, export "+
                    "EXAMPLECLOUD_API_KEY and EXAMPLECLOUD_API_SECRET, or add a profile to "+
                    "~/.examplecloud/credentials. See https://example.com/docs/auth.",
            )
        } else {
            resp.Diagnostics.AddError("Failed to Resolve Credentials", err.Error())
        }
        return
    }
    tflog.Debug(ctx, "resolved credentials", map[string]any{"source": creds.Source})

    // 3. Build the client once; share it with every resource and data source.
    client := examplecloud.NewClient(endpoint, creds.APIKey, creds.APISecret)
    resp.DataSourceData = client
    resp.ResourceData = client
}

Why each step matters:

• Unknown-value guards. During planning, an attribute wired to another resource's output is unknown, not null. Without the guard the provider silently treats it as empty, falls through the chain, and authenticates as the wrong identity — or fails with a misleading "missing credentials" error. Name the environment-variable workaround in the guard message.
• The sentinel check picks the right message. "You gave me nothing" (actionable list of options) is a different failure from "you gave me something broken" (show the parse error). Collapsing them into one message is how providers end up with users pasting secrets into config to debug.
• Log the source, never the secret. Knowing which source won is the single most useful debugging fact and costs nothing to log.

Diagnostics That Unblock Users

An authentication error message is the provider's most-read documentation. Every credential failure diagnostic should name:

• Every source tried, in order, with why it was skipped — the ChainError provides this. aws-sdk-go-base does the same with its NoValidCredentialSourcesError.
• The exact environment variable names and the credentials file path and profile that were consulted — not "set the appropriate environment variables".
• A documentation URL for the provider's authentication guide.

Use warnings (not errors) for conditions that are suspicious but not fatal, naming what took precedence: a profile set while environment credentials are also present (which wins?), or a credentials file with group/world-read permissions (suggest chmod 0600).

Secret Hygiene

• Give the Credentials type String() and GoString() methods that redact secret fields, so a stray %v, %+v, or error wrap can never leak a secret into logs or diagnostics.
• Never include credential values in diagnostics, log lines, or wrapped errors — log the source name and non-secret identifiers only.
• Warn when a credentials file is readable by other users (info.Mode().Perm()&0o077 != 0); skip this check on Windows, where POSIX permission bits are not meaningful.

Configure-Time Validation

Resolve the chain eagerly in Configure — never lazily on first resource use — so a credentials problem fails one time, at plan, with a good message, instead of failing in the middle of an apply. If the API has a cheap identity endpoint (the equivalent of AWS sts:GetCallerIdentity or a /whoami), call it after resolving credentials so invalid (not just missing) credentials also fail at configure time. Gate it behind a skip_credentials_validation attribute for air-gapped or stubbed environments.

Unit Testing the Chain

The chain is pure logic — test it with unit tests (Test prefix, no TF_ACC), not acceptance tests. Make the environment injectable (a getenv func(string) string field defaulting to os.Getenv, or use t.Setenv) and point the file provider at t.TempDir() fixtures. The tests that matter:

• Per-source: each provider returns its credentials when set and ErrNoCredentials when incomplete (a key with no secret is incomplete).
• Precedence: static beats env; env beats file; chain falls through to the file when nothing above supplies a complete set.
• Failure aggregation: with all sources empty, errors.Is(err, ErrNoCredentials) is true and the message names every source.
• Hard errors: a malformed credentials file or an explicitly requested profile that does not exist surfaces a descriptive error rather than silently falling through (a merely defaulted profile falls through).
• Redaction: fmt.Sprintf("%v") and %+v of a Credentials value never contain the secret.

Full test examples are in references/credential-chain.md.

Checklist

• All auth attributes Optional; secrets marked Sensitive: true
• Attribute descriptions name their environment-variable fallbacks
• Unknown-value guards on every auth attribute in Configure
• Chain precedence: static config > env vars > credentials file > platform identity
• Secrets resolved as a complete set; non-secret settings field-by-field
• Sentinel ErrNoCredentials distinguishes fall-through from hard failure
• Missing-credentials diagnostic lists every source tried + docs URL
• Credentials type redacts secrets in String()/GoString()
• Credentials-file permission warning (non-Windows)
• Eager resolution in Configure; optional identity check with skip_credentials_validation
• Unit tests cover per-source behavior, precedence, aggregation, redaction
• No credential value ever logged or embedded in an error

Related Skills

Use the new-terraform-provider skill (if available) to scaffold the provider this configuration lives in, and the provider-resources skill for consuming the configured client from resources and data sources.