diff --git a/runway/core/topickey/BUILD.bazel b/runway/core/topickey/BUILD.bazel
new file mode 100644
index 00000000..9aa38b1a
--- /dev/null
+++ b/runway/core/topickey/BUILD.bazel
@@ -0,0 +1,9 @@
+load("@rules_go//go:def.bzl", "go_library")
+
+go_library(
+    name = "topickey",
+    srcs = ["topickey.go"],
+    importpath = "github.com/uber/submitqueue/runway/core/topickey",
+    visibility = ["//visibility:public"],
+    deps = ["//core/consumer"],
+)
diff --git a/runway/core/topickey/topickey.go b/runway/core/topickey/topickey.go
new file mode 100644
index 00000000..7e919465
--- /dev/null
+++ b/runway/core/topickey/topickey.go
@@ -0,0 +1,46 @@
+// Copyright (c) 2025 Uber Technologies, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Package topickey defines the Runway-owned queue identifiers. Runway owns the
+// merge queues — a dry-run merge-conflict check pair and a committing merge
+// pair, both carrying the shared entity.MergeRequest/MergeResult contract.
+// Other services (e.g. SubmitQueue) import these keys to publish onto / consume
+// from them.
+package topickey
+
+import "github.com/uber/submitqueue/core/consumer"
+
+// TopicKey is the shared pipeline stage identifier type.
+type TopicKey = consumer.TopicKey
+
+const (
+	// TopicKeyMergeConflictCheck is the runway-owned queue that carries dry-run
+	// merge-conflict check requests. A client publishes a full
+	// entity.MergeRequest here; runway consumes it, attempts the merge without
+	// committing, and reports only whether it was mergeable.
+	TopicKeyMergeConflictCheck TopicKey = "merge-conflict-checker"
+	// TopicKeyMergeConflictCheckSignal is the runway-owned queue that carries
+	// merge-conflict check results. Runway publishes a full entity.MergeResult
+	// here (with no produced revisions); the requesting client consumes it.
+	TopicKeyMergeConflictCheckSignal TopicKey = "merge-conflict-checker-signal"
+	// TopicKeyMerge is the runway-owned queue that carries committing merge
+	// requests. A client publishes a full entity.MergeRequest here; runway
+	// consumes it, applies the steps, commits the result, and reports the
+	// revisions it produced.
+	TopicKeyMerge TopicKey = "merger"
+	// TopicKeyMergeSignal is the runway-owned queue that carries committing
+	// merge results. Runway publishes a full entity.MergeResult here (with the
+	// produced revisions populated); the requesting client consumes it.
+	TopicKeyMergeSignal TopicKey = "merger-signal"
+)
diff --git a/runway/entity/BUILD.bazel b/runway/entity/BUILD.bazel
new file mode 100644
index 00000000..a66f9012
--- /dev/null
+++ b/runway/entity/BUILD.bazel
@@ -0,0 +1,24 @@
+load("@rules_go//go:def.bzl", "go_library", "go_test")
+
+go_library(
+    name = "entity",
+    srcs = ["merge.go"],
+    importpath = "github.com/uber/submitqueue/runway/entity",
+    visibility = ["//visibility:public"],
+    deps = [
+        "//entity/change",
+        "//entity/mergestrategy",
+    ],
+)
+
+go_test(
+    name = "entity_test",
+    srcs = ["merge_test.go"],
+    embed = [":entity"],
+    deps = [
+        "//entity/change",
+        "//entity/mergestrategy",
+        "@com_github_stretchr_testify//assert",
+        "@com_github_stretchr_testify//require",
+    ],
+)
diff --git a/runway/entity/merge.go b/runway/entity/merge.go
new file mode 100644
index 00000000..cf2a50f3
--- /dev/null
+++ b/runway/entity/merge.go
@@ -0,0 +1,128 @@
+// Copyright (c) 2025 Uber Technologies, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Package entity holds Runway's domain entities, including the wire contract for
+// the merge queues that Runway owns. The contract crosses a service boundary (a
+// calling service cannot read Runway's storage and vice versa), so these
+// payloads carry the full data needed to perform a merge attempt rather than
+// opaque entity IDs.
+//
+// One contract serves two queue pairs because a merge-conflict check is a dry
+// run of a merge: Runway applies the same ordered Steps onto the same target
+// branch, and the only difference is whether it commits the result and reports
+// the revisions it produced. The queue a request arrives on encodes that choice
+// — the merge-conflict-checker pair for a dry run, the merger pair for a
+// committing merge — so MergeRequest and MergeResult are identical on both.
+package entity
+
+import (
+	"encoding/json"
+
+	"github.com/uber/submitqueue/entity/change"
+	"github.com/uber/submitqueue/entity/mergestrategy"
+)
+
+// MergeStep is one step of an ordered merge: a single set of change(s) applied
+// with a strategy. Runway applies the steps of a request in order on top of the
+// target branch; the ordering encodes the base-layering (earlier steps are the
+// in-flight base, the last step is the candidate).
+type MergeStep struct {
+	// StepID is an opaque, caller-assigned identifier for this step. Runway
+	// treats it as an attribution token only — it echoes it back per-step in
+	// StepResult so a multi-step result is attributable — and never interprets
+	// its contents. (A caller might use, for example, its own request id here.)
+	StepID string `json:"step_id"`
+	// Changes are the code change(s) to apply for this step (provider URIs with
+	// head commit SHAs; see entity/change.Change).
+	Changes []change.Change `json:"changes"`
+	// Strategy is how this step's changes are integrated into the target branch.
+	Strategy mergestrategy.MergeStrategy `json:"strategy"`
+}
+
+// MergeRequest is the payload a client publishes to one of Runway's merge
+// queues: TopicKeyMergeConflictCheck for a dry-run check, TopicKeyMerge for a
+// committing merge. The ID is owned by the client so it can record the
+// in-flight work before publishing and correlate the asynchronous result;
+// runway echoes it back unchanged.
+type MergeRequest struct {
+	// ID is the client-owned correlation id for this request (one per request).
+	// Runway echoes it back on the result unchanged.
+	ID string `json:"id"`
+	// QueueName is the caller-provided queue name the request belongs to. Runway
+	// resolves the target branch and provider config per-queue from this name;
+	// no target ref is passed.
+	QueueName string `json:"queue_name"`
+	// Steps is the ordered application sequence: in-flight steps first, the
+	// candidate last. A single-element slice expresses "candidate vs target
+	// branch".
+	Steps []MergeStep `json:"steps"`
+}
+
+// ToBytes serializes the MergeRequest to JSON bytes for the queue payload.
+func (r MergeRequest) ToBytes() ([]byte, error) {
+	return json.Marshal(r)
+}
+
+// MergeRequestFromBytes deserializes a MergeRequest from JSON bytes.
+func MergeRequestFromBytes(data []byte) (MergeRequest, error) {
+	var req MergeRequest
+	err := json.Unmarshal(data, &req)
+	return req, err
+}
+
+// StepResult reports what happened to a single MergeStep, so a multi-step result
+// is attributable to the step that produced (or failed to produce) it.
+type StepResult struct {
+	// StepID echoes the StepID of the step this result is for (see MergeStep.StepID).
+	StepID string `json:"step_id"`
+	// OutputIDs are the VCS-neutral identifiers of the revisions this step
+	// produced on the target branch — a git commit SHA, a Mercurial changeset
+	// hash, a Subversion revision number, a Perforce changelist, and so on —
+	// opaque to the caller. Empty for a dry-run check (which produces nothing),
+	// for a change already present on the target, or for a step that failed to
+	// apply.
+	OutputIDs []string `json:"output_ids,omitempty"`
+	// Reason is a human-readable explanation when the step failed to apply.
+	// Empty on success.
+	Reason string `json:"reason,omitempty"`
+}
+
+// MergeResult is the payload runway publishes to the corresponding signal queue
+// (TopicKeyMergeConflictCheckSignal for a check, TopicKeyMergeSignal for a
+// merge) once a request completes.
+type MergeResult struct {
+	// ID echoes the client-owned correlation id from the request.
+	ID string `json:"id"`
+	// Success is true if the whole ordered step sequence applied cleanly:
+	// mergeable for a dry-run check, merged for a committing merge.
+	Success bool `json:"success"`
+	// Reason is a human-readable explanation when Success is false. Empty on success.
+	Reason string `json:"reason,omitempty"`
+	// Steps optionally reports per-step outcomes, in request order. A committing
+	// merge populates each step's OutputIDs with the revisions it produced; a
+	// dry-run check leaves them empty.
+	Steps []StepResult `json:"steps,omitempty"`
+}
+
+// ToBytes serializes the MergeResult to JSON bytes for the queue payload.
+func (r MergeResult) ToBytes() ([]byte, error) {
+	return json.Marshal(r)
+}
+
+// MergeResultFromBytes deserializes a MergeResult from JSON bytes.
+func MergeResultFromBytes(data []byte) (MergeResult, error) {
+	var res MergeResult
+	err := json.Unmarshal(data, &res)
+	return res, err
+}
diff --git a/runway/entity/merge_test.go b/runway/entity/merge_test.go
new file mode 100644
index 00000000..19ab9fa4
--- /dev/null
+++ b/runway/entity/merge_test.go
@@ -0,0 +1,88 @@
+// Copyright (c) 2025 Uber Technologies, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package entity
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	"github.com/uber/submitqueue/entity/change"
+	"github.com/uber/submitqueue/entity/mergestrategy"
+)
+
+func TestMergeRequestRoundTrip(t *testing.T) {
+	req := MergeRequest{
+		ID:        "queue-a/42",
+		QueueName: "queue-a",
+		Steps: []MergeStep{
+			{
+				StepID:   "queue-a/1",
+				Changes:  []change.Change{{URIs: []string{"github://uber/repo/pull/1/" + "0123456789abcdef0123456789abcdef01234567"}}},
+				Strategy: mergestrategy.MergeStrategyRebase,
+			},
+			{
+				StepID:   "queue-a/2",
+				Changes:  []change.Change{{URIs: []string{"github://uber/repo/pull/2/" + "89abcdef0123456789abcdef0123456789abcdef"}}},
+				Strategy: mergestrategy.MergeStrategyMerge,
+			},
+		},
+	}
+
+	data, err := req.ToBytes()
+	require.NoError(t, err)
+
+	got, err := MergeRequestFromBytes(data)
+	require.NoError(t, err)
+	assert.Equal(t, req, got)
+}
+
+func TestMergeResultRoundTrip(t *testing.T) {
+	// A committing merge reports the revisions each step produced on the target;
+	// a dry-run check leaves OutputIDs empty and reports a per-step reason on
+	// failure. Both shapes share the one MergeResult contract.
+	t.Run("merged with produced revisions", func(t *testing.T) {
+		res := MergeResult{
+			ID:      "queue-a/42",
+			Success: true,
+			Steps: []StepResult{
+				{StepID: "queue-a/1", OutputIDs: []string{"0123456789abcdef0123456789abcdef01234567"}},
+			},
+		}
+
+		data, err := res.ToBytes()
+		require.NoError(t, err)
+
+		got, err := MergeResultFromBytes(data)
+		require.NoError(t, err)
+		assert.Equal(t, res, got)
+	})
+
+	t.Run("failed with per-step reason", func(t *testing.T) {
+		res := MergeResult{
+			ID:      "queue-a/42",
+			Success: false,
+			Reason:  "conflict in foo.go",
+			Steps:   []StepResult{{StepID: "queue-a/2", Reason: "conflict in foo.go"}},
+		}
+
+		data, err := res.ToBytes()
+		require.NoError(t, err)
+
+		got, err := MergeResultFromBytes(data)
+		require.NoError(t, err)
+		assert.Equal(t, res, got)
+	})
+}