diff --git a/packages/primitives/package.json b/packages/primitives/package.json
index 47d8bf1..0781e87 100644
--- a/packages/primitives/package.json
+++ b/packages/primitives/package.json
@@ -5,7 +5,8 @@
   "type": "module",
   "exports": {
     ".": "./src/index.ts",
-    "./agent-os": "./src/agent-os.ts"
+    "./agent-os": "./src/agent-os.ts",
+    "./signal": "./src/signal.ts"
   },
   "scripts": {
     "check-types": "tsc --noEmit",
diff --git a/packages/primitives/src/index.ts b/packages/primitives/src/index.ts
index 7c8f0e2..7228130 100644
--- a/packages/primitives/src/index.ts
+++ b/packages/primitives/src/index.ts
@@ -1 +1,3 @@
+// oxlint-disable-next-line no-barrel-file -- The package root intentionally exposes its public modules.
 export * from "./agent-os";
+export * from "./signal";
diff --git a/packages/primitives/src/signal.test.ts b/packages/primitives/src/signal.test.ts
new file mode 100644
index 0000000..e767463
--- /dev/null
+++ b/packages/primitives/src/signal.test.ts
@@ -0,0 +1,273 @@
+import { Effect } from "effect";
+import { describe, expect, it } from "vitest";
+
+import { composeSignal, SignalValidationError } from "./signal";
+import type { SignalValidationReason } from "./signal";
+
+type TestSignalScope =
+  | {
+      _tag: "Organization";
+      organizationId: string;
+    }
+  | {
+      _tag: "Project";
+      organizationId: string;
+      projectId: string;
+    };
+
+interface TestSourceMessage {
+  conversationId: string;
+  messageId: string;
+  rawText: string;
+  createdAt: number;
+}
+
+interface TestSignalInput {
+  id: string;
+  scope: TestSignalScope;
+  sourceMessages: [TestSourceMessage, TestSourceMessage];
+  problemStatement: {
+    title: string;
+    summary: string;
+    desiredOutcome: string;
+    constraints: string[];
+  };
+  processedBy: {
+    agentName: string;
+    agentInstanceId: string;
+  };
+  createdAt: number;
+}
+
+const makeSignalInput = (): TestSignalInput => ({
+  createdAt: 300,
+  id: "signal-1",
+  problemStatement: {
+    constraints: ["Keep existing account data"],
+    desiredOutcome: "Users can complete setup.",
+    summary: "The setup flow stops before completion.",
+    title: "Users cannot finish setup",
+  },
+  processedBy: {
+    agentInstanceId: "organization-1",
+    agentName: "zopu",
+  },
+  scope: {
+    _tag: "Organization",
+    organizationId: "organization-1",
+  },
+  sourceMessages: [
+    {
+      conversationId: "conversation-1",
+      createdAt: 100,
+      messageId: "message-1",
+      rawText: "First source",
+    },
+    {
+      conversationId: "conversation-1",
+      createdAt: 200,
+      messageId: "message-2",
+      rawText: "Second source",
+    },
+  ],
+});
+
+const runSignal = (input: unknown) => Effect.runSync(composeSignal(input));
+
+const getValidationError = (
+  input: unknown,
+  reason: SignalValidationReason
+): SignalValidationError => {
+  const error = Effect.runSync(Effect.flip(composeSignal(input)));
+  expect(error).toBeInstanceOf(SignalValidationError);
+  expect(error.reason).toBe(reason);
+  return error;
+};
+
+describe("composeSignal", () => {
+  it("accepts organization scope", () => {
+    const signal = runSignal(makeSignalInput());
+
+    expect(signal.scope).toEqual({
+      _tag: "Organization",
+      organizationId: "organization-1",
+    });
+  });
+
+  it("accepts project scope", () => {
+    const input = makeSignalInput();
+    input.scope = {
+      _tag: "Project",
+      organizationId: "organization-1",
+      projectId: "project-1",
+    };
+
+    const signal = runSignal(input);
+
+    expect(signal.scope).toEqual({
+      _tag: "Project",
+      organizationId: "organization-1",
+      projectId: "project-1",
+    });
+  });
+
+  it("rejects empty IDs and processed-agent identity", () => {
+    const mutations: readonly ((input: TestSignalInput) => void)[] = [
+      (input) => {
+        input.id = "";
+      },
+      (input) => {
+        input.scope.organizationId = "";
+      },
+      (input) => {
+        input.scope = {
+          _tag: "Project",
+          organizationId: "organization-1",
+          projectId: "",
+        };
+      },
+      (input) => {
+        input.sourceMessages[0].conversationId = "";
+      },
+      (input) => {
+        input.sourceMessages[0].messageId = "";
+      },
+      (input) => {
+        input.processedBy.agentName = "";
+      },
+      (input) => {
+        input.processedBy.agentInstanceId = "";
+      },
+    ];
+
+    for (const mutate of mutations) {
+      const input = makeSignalInput();
+      mutate(input);
+      getValidationError(input, "InvalidInput");
+    }
+  });
+
+  it("rejects negative and fractional timestamps", () => {
+    const mutations: readonly ((input: TestSignalInput) => void)[] = [
+      (input) => {
+        input.sourceMessages[0].createdAt = -1;
+      },
+      (input) => {
+        input.sourceMessages[0].createdAt = 1.5;
+      },
+      (input) => {
+        input.createdAt = -1;
+      },
+      (input) => {
+        input.createdAt = 1.5;
+      },
+    ];
+
+    for (const mutate of mutations) {
+      const input = makeSignalInput();
+      mutate(input);
+      getValidationError(input, "InvalidInput");
+    }
+  });
+
+  it("rejects empty problem-statement fields", () => {
+    for (const field of ["title", "summary", "desiredOutcome"] as const) {
+      const input = makeSignalInput();
+      input.problemStatement[field] = "";
+      getValidationError(input, "InvalidInput");
+    }
+  });
+
+  it("rejects whitespace-only problem fields and constraints", () => {
+    const input = makeSignalInput();
+    input.problemStatement.title = "   ";
+    input.problemStatement.constraints = ["\t\n"];
+
+    getValidationError(input, "InvalidInput");
+  });
+
+  it("rejects an empty constraint item", () => {
+    const input = makeSignalInput();
+    input.problemStatement.constraints = [""];
+
+    getValidationError(input, "InvalidInput");
+  });
+
+  it("accepts an empty constraints list", () => {
+    const input = makeSignalInput();
+    input.problemStatement.constraints = [];
+
+    expect(runSignal(input).problemStatement.constraints).toEqual([]);
+  });
+
+  it("rejects an empty source list", () => {
+    const input = {
+      ...makeSignalInput(),
+      sourceMessages: [],
+    };
+
+    getValidationError(input, "InvalidInput");
+  });
+
+  it("rejects duplicate source identities", () => {
+    const input = makeSignalInput();
+    input.sourceMessages[1].messageId = input.sourceMessages[0].messageId;
+
+    getValidationError(input, "DuplicateSource");
+  });
+
+  it("rejects sources from mixed conversations", () => {
+    const input = makeSignalInput();
+    input.sourceMessages[1].conversationId = "conversation-2";
+
+    getValidationError(input, "MixedConversation");
+  });
+
+  it("rejects decreasing source timestamps", () => {
+    const input = makeSignalInput();
+    input.sourceMessages[1].createdAt = 99;
+
+    getValidationError(input, "OutOfOrderSource");
+  });
+
+  it("preserves source order and exact raw whitespace", () => {
+    const input = makeSignalInput();
+    input.sourceMessages[0].rawText = "  first source\n";
+    input.sourceMessages[1].rawText = "\tsecond source  ";
+
+    const signal = runSignal(input);
+
+    expect(signal.sourceMessages.map(({ messageId }) => messageId)).toEqual([
+      "message-1",
+      "message-2",
+    ]);
+    expect(signal.sourceMessages.map(({ rawText }) => rawText)).toEqual([
+      "  first source\n",
+      "\tsecond source  ",
+    ]);
+  });
+
+  it("preserves problem-statement whitespace while validating trimmed non-emptiness", () => {
+    const input = makeSignalInput();
+    input.problemStatement.title = "  Setup blocked  ";
+    input.problemStatement.constraints = ["  Preserve data  "];
+
+    const signal = runSignal(input);
+
+    expect(signal.problemStatement.title).toBe("  Setup blocked  ");
+    expect(signal.problemStatement.constraints).toEqual(["  Preserve data  "]);
+  });
+
+  it("does not include candidate, work-unit, or run fields", () => {
+    const signal = runSignal({
+      ...makeSignalInput(),
+      candidateId: "candidate-1",
+      runId: "run-1",
+      workUnitId: "work-unit-1",
+    });
+
+    expect(signal).not.toHaveProperty("candidateId");
+    expect(signal).not.toHaveProperty("workUnitId");
+    expect(signal).not.toHaveProperty("runId");
+  });
+});
diff --git a/packages/primitives/src/signal.ts b/packages/primitives/src/signal.ts
new file mode 100644
index 0000000..ceb1f30
--- /dev/null
+++ b/packages/primitives/src/signal.ts
@@ -0,0 +1,159 @@
+import { Effect, Schema } from "effect";
+
+const MeaningfulString = Schema.String.check(
+  Schema.makeFilter((value) => value.trim().length > 0, {
+    expected: "a non-empty string",
+  })
+);
+
+export const SignalId = MeaningfulString.pipe(Schema.brand("SignalId"));
+export type SignalId = typeof SignalId.Type;
+
+export const OrganizationId = MeaningfulString.pipe(
+  Schema.brand("OrganizationId")
+);
+export type OrganizationId = typeof OrganizationId.Type;
+
+export const ProjectId = MeaningfulString.pipe(Schema.brand("ProjectId"));
+export type ProjectId = typeof ProjectId.Type;
+
+export const ConversationId = MeaningfulString.pipe(
+  Schema.brand("ConversationId")
+);
+export type ConversationId = typeof ConversationId.Type;
+
+export const ConversationMessageId = MeaningfulString.pipe(
+  Schema.brand("ConversationMessageId")
+);
+export type ConversationMessageId = typeof ConversationMessageId.Type;
+
+export const TimestampMs = Schema.Int.check(
+  Schema.isGreaterThanOrEqualTo(0)
+).pipe(Schema.brand("TimestampMs"));
+export type TimestampMs = typeof TimestampMs.Type;
+
+export const OrganizationSignalScope = Schema.Struct({
+  _tag: Schema.Literal("Organization"),
+  organizationId: OrganizationId,
+});
+export type OrganizationSignalScope = typeof OrganizationSignalScope.Type;
+
+export const ProjectSignalScope = Schema.Struct({
+  _tag: Schema.Literal("Project"),
+  organizationId: OrganizationId,
+  projectId: ProjectId,
+});
+export type ProjectSignalScope = typeof ProjectSignalScope.Type;
+
+export const SignalScope = Schema.Union([
+  OrganizationSignalScope,
+  ProjectSignalScope,
+]);
+export type SignalScope = typeof SignalScope.Type;
+
+export const SignalSourceMessage = Schema.Struct({
+  conversationId: ConversationId,
+  createdAt: TimestampMs,
+  messageId: ConversationMessageId,
+  rawText: Schema.String,
+});
+export type SignalSourceMessage = typeof SignalSourceMessage.Type;
+
+export const ProblemStatement = Schema.Struct({
+  constraints: Schema.Array(MeaningfulString),
+  desiredOutcome: MeaningfulString,
+  summary: MeaningfulString,
+  title: MeaningfulString,
+});
+export type ProblemStatement = typeof ProblemStatement.Type;
+
+export const ProcessedByAgent = Schema.Struct({
+  agentInstanceId: MeaningfulString,
+  agentName: MeaningfulString,
+});
+export type ProcessedByAgent = typeof ProcessedByAgent.Type;
+
+const signalFields = {
+  createdAt: TimestampMs,
+  id: SignalId,
+  problemStatement: ProblemStatement,
+  processedBy: ProcessedByAgent,
+  scope: SignalScope,
+  sourceMessages: Schema.NonEmptyArray(SignalSourceMessage),
+} as const;
+
+export const SignalInput = Schema.Struct(signalFields);
+export type SignalInput = typeof SignalInput.Type;
+
+export const Signal = Schema.Struct(signalFields);
+export type Signal = typeof Signal.Type;
+
+export const SignalValidationReason = Schema.Literals([
+  "InvalidInput",
+  "DuplicateSource",
+  "MixedConversation",
+  "OutOfOrderSource",
+]);
+export type SignalValidationReason = typeof SignalValidationReason.Type;
+
+export class SignalValidationError extends Schema.TaggedErrorClass<SignalValidationError>()(
+  "SignalValidationError",
+  {
+    message: Schema.String,
+    reason: SignalValidationReason,
+  }
+) {}
+
+export const composeSignal = Effect.fn("Signal.compose")(
+  function* composeSignal(input: unknown) {
+    const signal: Signal = yield* Schema.decodeUnknownEffect(SignalInput)(
+      input
+    ).pipe(
+      Effect.mapError(
+        (cause) =>
+          new SignalValidationError({
+            message: cause.message,
+            reason: "InvalidInput",
+          })
+      )
+    );
+
+    const [{ conversationId, createdAt: firstCreatedAt }] =
+      signal.sourceMessages;
+    let previousCreatedAt = firstCreatedAt;
+    const messageIds = new Set<ConversationMessageId>();
+
+    for (const source of signal.sourceMessages) {
+      if (source.conversationId !== conversationId) {
+        return yield* Effect.fail(
+          new SignalValidationError({
+            message: "Signal sources must belong to one conversation",
+            reason: "MixedConversation",
+          })
+        );
+      }
+
+      if (messageIds.has(source.messageId)) {
+        return yield* Effect.fail(
+          new SignalValidationError({
+            message: "Signal sources must have unique message identities",
+            reason: "DuplicateSource",
+          })
+        );
+      }
+      messageIds.add(source.messageId);
+
+      if (source.createdAt < previousCreatedAt) {
+        return yield* Effect.fail(
+          new SignalValidationError({
+            message: "Signal source timestamps must be non-decreasing",
+            reason: "OutOfOrderSource",
+          })
+        );
+      }
+      previousCreatedAt = source.createdAt;
+    }
+
+    return signal;
+  }
+);