diff --git a/docs/designs/prebuilt-runtime-container-images.md b/docs/designs/prebuilt-runtime-container-images.md
new file mode 100644
index 000000000..3bc6d3ccb
--- /dev/null
+++ b/docs/designs/prebuilt-runtime-container-images.md
@@ -0,0 +1,188 @@
+# Pre-built container images for Runtime deployments
+
+Status: design proposal
+
+Tracking issue: aws/agentcore-cli#503
+
+## Summary
+
+Allow an AgentCore Runtime with `build: "Container"` to reference an image that already exists
+in ECR. When a pre-built image is configured, deployment must create or update the Runtime
+directly from that image without uploading source, creating a managed ECR repository, or running
+CodeBuild.
+
+The image should be modeled as Runtime configuration rather than passed to the generated CDK app
+through an environment variable. This keeps `agentcore build`, `agentcore deploy`, and direct CDK
+synthesis deterministic and gives every Runtime in a multi-Runtime project an unambiguous image
+source.
+
+## Goals
+
+- Deploy a private or public ECR image without invoking CodeBuild.
+- Reject malformed ECR image URIs before CDK synthesis.
+- Preserve the existing Dockerfile-to-CodeBuild deployment path.
+- Support projects containing more than one Runtime.
+- Keep the generated CDK application as a thin `AgentCoreApplication` instantiator.
+- Preserve the Runtime ID, ARN, role ARN, and endpoint output contracts.
+
+## Non-goals
+
+- Building or copying images between registries, accounts, or regions.
+- Verifying that an image or tag exists before deployment.
+- Managing repository policies for a customer-owned cross-account ECR repository.
+- Selecting a different image URI for each deployment target.
+- Introducing a general-purpose deployment override system.
+- Removing the existing CodeBuild-based container workflow.
+
+## Proposed project configuration
+
+Add an optional `containerUri` field to a Runtime:
+
+```json
+{
+  "name": "my_agent",
+  "build": "Container",
+  "entrypoint": "main.py",
+  "codeLocation": "app/my_agent",
+  "containerUri": "123456789012.dkr.ecr.us-west-2.amazonaws.com/my-agent:abc123"
+}
+```
+
+For the initial implementation, `entrypoint` and `codeLocation` remain required by the existing
+Runtime schema for compatibility, but their local files are not build inputs when `containerUri`
+is set.
+
+A Container Runtime has exactly one image source:
+
+1. `containerUri` is set: deploy the referenced image directly.
+2. `containerUri` is not set: retain the existing Dockerfile, CodeBuild, and managed ECR path.
+
+`containerUri` is valid only with `build: "Container"` and is mutually exclusive with:
+
+- `dockerfile`
+- `buildContextPath`
+- `customDockerBuildArgs`
+
+An explicitly configured `containerUri` also prevents Runtime build synchronization from inferring
+a Dockerfile merely because one exists in the code directory.
+
+Because the field belongs to the Runtime specification, the same URI applies to every deployment
+target. The image must therefore be accessible to each target. A target-specific image mapping can
+be designed separately if that becomes a supported requirement.
+
+## CLI behavior
+
+The Runtime schema remains the source of truth. The initial implementation does not add an
+environment-variable override such as `AGENTCORE_CONTAINER_URI`.
+
+The exact `agentcore deploy --container-uri` interface originally proposed by #503 is ambiguous
+for projects with multiple Runtimes. If a deploy-time convenience override is added later, it
+should require a Runtime selector:
+
+```text
+agentcore deploy --runtime my_agent --container-uri <uri>
+```
+
+That override would be separate follow-up work because it must be passed to synthesis without
+silently changing `agentcore.json` or making direct CDK synthesis produce a different template.
+
+## Validation and deployment preparation
+
+The CLI and CDK package maintain mirrored Runtime schemas, so both must enforce the same rules:
+
+- Accept only supported private ECR and `public.ecr.aws` image URI formats.
+- Enforce the service maximum URI length.
+- Reject `containerUri` on a CodeZip Runtime.
+- Reject local container-build fields when `containerUri` is present.
+
+Runtime preparation must skip these CodeBuild-specific checks for a pre-built image:
+
+- Dockerfile existence and path validation.
+- Local container build-context validation.
+- The CodeBuild VPC ID requirement.
+- CodeBuild's five-security-group limit.
+
+Runtime networking validation still applies because it configures the deployed Runtime, not the
+image build.
+
+## CDK design
+
+Branch in `RuntimeEnvironment`, where Runtime artifact construction is already owned:
+
+```text
+build = CodeZip
+  -> existing bundled-code path
+
+build = Container and containerUri is set
+  -> AgentCoreRuntime from the existing image
+
+build = Container and containerUri is not set
+  -> existing source asset -> CodeBuild -> managed ECR path
+```
+
+The pre-built path must not create:
+
+- `ContainerSourceAsset`
+- `ContainerRepository`
+- `ContainerBuildProject`
+- `ContainerImageBuilder`
+
+`AgentCoreRuntime` already supports a container URI, but its ECR access logic currently expects the
+managed repository ARN supplied by the build path. For a private pre-built URI, the construct must
+derive the repository ARN and grant the Runtime execution role:
+
+- `ecr:GetAuthorizationToken`
+- `ecr:BatchGetImage`
+- `ecr:GetDownloadUrlForLayer`
+- `ecr:BatchCheckLayerAvailability`
+
+Public ECR images must not require a private repository ARN. Cross-account private repositories may
+also need a customer-managed repository policy; the CLI does not modify an external repository.
+
+Runtime and endpoint outputs are already emitted after artifact selection, so their logical IDs and
+export names remain unchanged.
+
+## Compatibility
+
+- Existing CodeZip deployments are unchanged.
+- Existing Container deployments without `containerUri` continue through CodeBuild.
+- Existing project files remain valid.
+- The generated CDK stack continues to instantiate `AgentCoreApplication`; branching stays inside
+  `@aws/agentcore-cdk`.
+- The CDK package change must be released before the CLI updates its generated project dependency.
+
+## Test plan
+
+CLI:
+
+- Accept valid private and public ECR image URIs.
+- Reject invalid URI formats and over-length values.
+- Reject incompatible Runtime fields.
+- Do not infer or validate a Dockerfile when `containerUri` is configured.
+- Do not apply CodeBuild-only VPC restrictions to a pre-built image.
+- Preserve the existing Dockerfile synchronization and validation tests.
+
+CDK:
+
+- A pre-built image synthesizes the expected Runtime `ContainerUri`.
+- The pre-built path synthesizes no CodeBuild, managed ECR, source asset, or build custom resource.
+- A private ECR URI produces repository-scoped pull permissions.
+- Public ECR does not require a private repository ARN.
+- Dockerfile-based Container deployment remains unchanged.
+- Runtime ID, ARN, role ARN, endpoint, and output export-name contracts remain unchanged.
+
+End-to-end:
+
+- Push an ARM64 Runtime image to ECR.
+- Deploy a project referencing that image.
+- Confirm no CodeBuild build starts.
+- Wait for the Runtime to become ready and invoke it.
+- Repeat with an ordinary Dockerfile Runtime as the regression case.
+
+## Delivery sequence
+
+1. Add schema, branching, IAM, and tests to `@aws/agentcore-cdk`.
+2. Publish the compatible CDK package.
+3. Add the mirrored CLI schema and Runtime preparation behavior.
+4. Update the generated CDK dependency and command/configuration documentation.
+5. Run the Runtime container end-to-end deployment tests.