Authentication is where "same API, different platform" stops being literally true. Anthropic's first-party API wants an API key in a header. Amazon's platforms want cryptographically signed requests. Google wants a short-lived OAuth token from its credential chain. Azure wants either a resource key or a Microsoft Entra ID token. The good news: the platform-specific SDK clients absorb almost all of it, so the differences live in configuration rather than code.
What each platform expects
| Platform | Primary mechanism | What travels on the wire |
|---|---|---|
| Claude API (1P) | API key, or Workload Identity Federation | x-api-key header (sk-ant-api...) or short-lived bearer token |
| Claude Platform on AWS | AWS IAM + SigV4 (primary); AWS-issued API keys | SigV4 signature, service name aws-external-anthropic |
| Amazon Bedrock (current surface) | IAM service role / assumed role; bearer tokens (least preferred) | SigV4 signature for bedrock-mantle, or a token in x-api-key |
| Google Vertex AI | Application Default Credentials (ADC) | Authorization: Bearer Google access token |
| Microsoft Foundry | Azure API key or Microsoft Entra ID | api-key / x-api-key header, or Authorization: Bearer Entra token |
The four mechanisms in plain English
API key (1P). A long-lived secret created in the Claude Console and sent in the x-api-key header. Keys have no expiry, so Anthropic's guidance is to keep them in a secrets manager, rotate periodically, and revoke on suspected leak. For production cloud workloads, Anthropic recommends Workload Identity Federation instead: your workload exchanges an identity token from its own IdP for a short-lived Claude API token, and the SDK refreshes it automatically.
SigV4 (both AWS routes). SigV4 is AWS's request-signing scheme: rather than sending a secret, the client computes a signature over each request using AWS credentials, and the service verifies it. There is no Anthropic key at all — access is granted by IAM policy. Claude Platform on AWS signs with service name aws-external-anthropic; the current Bedrock surface signs for bedrock-mantle, where the relevant IAM action is bedrock-mantle:CreateInference. Both platforms also offer time-limited bearer tokens (capped at 12 hours) for components that can only speak "API key," but if a process can mint the token locally it already has SigV4 credentials, so plain SigV4 is usually simpler.
Application Default Credentials (Vertex). ADC is Google Cloud's standard credential-resolution chain: locally it is typically set up with gcloud auth application-default login; in production it resolves to the attached service account. The SDK turns whatever ADC finds into a bearer token on each request. Making calls requires the aiplatform.endpoints.predict permission, included in the Vertex AI User role.
Azure API key or Entra ID (Foundry). Every Foundry resource has Azure-issued keys, shown on the deployment's Details tab, passed in the api-key or x-api-key header. The enterprise-grade alternative is Microsoft Entra ID: a bearer token acquired through Azure's identity libraries, which brings Azure RBAC and centralized identity management. A 403 under Entra ID usually means a missing role assignment such as Cognitive Services User. Some gated models (Claude Mythos 5 and Mythos Preview) accept Entra ID only — no API keys.
What the SDK clients do for you
Each provider client hides its platform's mechanics behind the same client.messages.create(...) interface:
from anthropic import (AnthropicAWS, AnthropicBedrockMantle,
AnthropicVertex, AnthropicFoundry)
aws = AnthropicAWS() # SigV4 + workspace header
bedrock = AnthropicBedrockMantle(aws_region="us-east-1") # SigV4, AWS cred chain
vertex = AnthropicVertex(project_id="my-proj", region="global") # ADC
foundry = AnthropicFoundry(api_key="...", resource="example-resource")AnthropicAWS and AnthropicBedrockMantle both walk the standard AWS credential provider chain — environment variables, shared config files (including SSO), web identity, container credentials, instance metadata — and handle signing plus session-token headers automatically. AnthropicAWS additionally injects the required anthropic-workspace-id header and refuses to construct without a region and workspace ID. AnthropicVertex goes through the standard Google auth flow. AnthropicFoundry also accepts an Entra ID token provider instead of a key.
sk-ant-api... key does not work on Claude Platform on AWS — its API keys are generated in the AWS Console, and bearer-token callers there need the aws-external-anthropic:CallWithBearerToken IAM action.Where to go next
Authentication is one of the two request-level deltas between platforms; the other is covered in the request-shape matrix. To wire all four credentials into one deployable config, see the environment-variable matrix.