diff --git a/docs/adr/runtime-identity-context.md b/docs/adr/runtime-identity-context.md
new file mode 100644
index 0000000..b058ebb
--- /dev/null
+++ b/docs/adr/runtime-identity-context.md
@@ -0,0 +1,132 @@
+# ADR: Per-Fleet managing identity — binding config, context switch, and verification
+
+- **Status:** Proposed (stub)
+- **Date:** 2026-08-13
+- **Author:** Orca (ecs-claude)
+- **Builds on:** [deployment-control-plane](./deployment-control-plane.md) (ADR-2, read model), [desktop-core-sidecar](./desktop-core-sidecar.md) (credentials resolve in the sidecar from the standard chain)
+- **Related:** desktop console (ADR-3), fleet-store (#18)
+
+> **Y-statement.** In the context of Studio managing multiple Fleets that may
+> each live under a different AWS account / role / project (and later a different
+> k8s context), facing the problem that the *managing* credential is ambient and
+> invisible — so a deploy call can silently run under the wrong principal (a
+> static `[default]` profile shadowed the intended task role, discovered only via
+> a manual `sts get-caller-identity`) — we decided to make the per-Fleet managing
+> identity **explicit, switchable, and verified**: a **FleetBinding config**
+> (Fleet → managing context), an **active context switch** so selecting a Fleet
+> binds subsequent calls to that Fleet's credential, and a **read-only
+> RuntimeContext panel + IdentityMismatch flag** that confirms the effective
+> identity matches the declared binding — **accepting that this is operator-side
+> credential *selection*, not per-caller authz** (authz still deferred per ADR-2).
+
+---
+
+## 1. Context & Problem
+
+Motivating incident (2026-08-13): `deploy_list` returned `AccessDenied` on
+`ecs:ListServices`. The root cause was **not** a missing permission — the SDK
+credential chain resolved a static `[default]` profile (an IAM *user* in a
+different account and region) **before** the container-credentials (task-role)
+provider, so the call executed as the wrong principal entirely. Nothing declared
+which identity *should* manage that target, and nothing surfaced which one
+actually did; it took a manual `aws sts get-caller-identity` to see it.
+
+Studio's north star is managing **multiple Fleets** across runtimes and **different
+AWS accounts / roles / projects** (later **k8s contexts**). Two gaps follow:
+
+1. **No declared binding.** Nothing says "to manage Fleet X, act as identity Y in
+   account Z." The managing credential is whatever the ambient chain resolves —
+   easy to get silently wrong, especially when switching between Fleets.
+2. **No verification.** Even once resolved, the *effective* identity is invisible,
+   so a wrong/misfired binding is undetectable at a glance.
+
+Per ADR-2 credential resolution is deferred to "the AWS-credential ceiling", and
+(ADR-5) resolves lazily inside the sidecar from the standard chain. This ADR does
+**not** introduce per-caller authz. It makes the operator's *own* managing
+identity per Fleet **declarative, actively selected, and observable**.
+
+## 2. Decision
+
+A `declare → switch → observe → reconcile` loop, in three parts.
+
+### 2a. FleetBinding config (declare)
+
+A declarative Studio/control-plane config mapping each managed **Fleet → its
+managing context**. Generic fields; vendor specifics resolved by the
+`RuntimeDriver`:
+
+| field | meaning | AWS driver | k8s driver (later) |
+|---|---|---|---|
+| `fleet` | the managed Fleet | — | — |
+| `principal_source` | how to obtain the managing credential | named profile / assume-role ARN / task-role | kubeconfig context / SA |
+| `scope` | account / project boundary | account id | cluster / namespace |
+| `location` | region / zone | region | context region |
+
+This is the operator/CP's binding, **not** the agent's `Spec.identity` (that is
+the agent's own logical identity). Where it is persisted — Studio config vs
+control-plane state — interacts with fleet-store (#18); see §5.
+
+### 2b. Active context switch (apply)
+
+Selecting / switching to a Fleet in Studio **switches the active managing
+context** to that Fleet's binding: the driver/sidecar resolves credentials
+**parameterized by the binding** (the chosen profile / assumed role), instead of
+falling through to whatever ambient `[default]` the chain finds first. This is
+the *proactive* fix — the wrong-account call never happens because the binding is
+selected before the call, not discovered after it fails.
+
+### 2c. RuntimeContext panel + IdentityMismatch (observe & reconcile)
+
+A read-only **`RuntimeContext`** Status projection (observed, per Instance;
+foldable per Fleet) that the panel renders:
+
+| field | meaning | AWS driver | k8s driver (later) |
+|---|---|---|---|
+| `principal` | effective acting identity | STS caller ARN (role vs user) | context user / SA |
+| `scope` | account / project | account id | cluster / namespace |
+| `location` | region / zone | region | context region |
+| `source` | where the credential came from | container-creds / named profile / static keys / env | kubeconfig / in-cluster SA |
+| `verified_at` | last resolved | STS call ts | `auth whoami` ts |
+
+- **Read-only**, emitted by the driver — the only place vendor calls (STS /
+  `kubectl auth whoami`) live (consistent with ADR-2 §4: state is observed).
+- **`IdentityMismatch`** — a **non-blocking** Condition raised when the effective
+  `principal` ≠ the Fleet's declared binding (2a). It catches a switch that did
+  not take, or an ambient fallback. Today's incident trips it on sight. Stays on
+  the read side: **no enforcement**, consistent with ADR-2's deferred authz.
+
+## 3. Scope (now / later)
+
+- **Now (AWS):** FleetBinding config with a per-Fleet credential source
+  (named profile / assume-role / task-role); switch binds the driver's resolution
+  to it; panel renders `RuntimeContext` per Instance/Fleet; `IdentityMismatch`
+  fires when effective ≠ declared.
+- **Later:** k8s driver mapping (context / SA / namespace); a **deploy-call
+  preflight** that resolves and shows the *caller's own* effective identity before
+  a write, so a mis-resolved CLI/sidecar is caught before it acts; richer `source`
+  provenance.
+
+## 4. Consequences
+
+- ✅ Switching Fleets no longer risks acting under the wrong account — the binding
+  is selected up front (proactive), and the panel + mismatch verify it (detective).
+- ✅ Stays within ADR-2's generic vocabulary and deferred-authz stance: operator
+  credential *selection*, not per-caller *authorization*; vendor terms scoped to
+  the driver.
+- ⚠️ The sidecar (ADR-5) currently resolves lazily from the **ambient** chain;
+  honoring a FleetBinding means Studio must **parameterize** that resolution per
+  Fleet (pass the profile / role in), not rely on the default chain order.
+- ⚠️ The driver must make a live identity call (STS / `whoami`); cache with a
+  `verified_at`, treat failure as `Unknown`, and never render access keys/tokens.
+
+## 5. Open questions
+
+- Where does the FleetBinding live — Studio-local config vs control-plane state
+  (fleet-store #18)? Single source of truth either way.
+- How does `principal_source` map to a concrete credential — named profile,
+  `sts:AssumeRole`, or an expected task-role — and how is that handed to the
+  sidecar's lazy resolution (ADR-5)?
+- Should the deploy-call preflight (caller's own identity) be standing, or
+  on-demand before writes only?
+- Per-caller authz remains deferred (ADR-2) — this ADR deliberately does not
+  touch it; the FleetBinding is *selection*, not *authorization*.