diff --git a/src/hawk/client.py b/src/hawk/client.py
index 357046e..6397513 100644
--- a/src/hawk/client.py
+++ b/src/hawk/client.py
@@ -20,6 +20,7 @@
     SessionDetail,
     SessionSummary,
     StatsResponse,
+    ToolResult,
 )
 
 DEFAULT_BASE_URL = "http://127.0.0.1:4590"
@@ -124,6 +125,8 @@ def chat(
         autonomy: str | None = None,
         cwd: str | None = None,
         agent: str | None = None,
+        tools: list[dict[str, Any]] | None = None,
+        tool_results: list[ToolResult] | None = None,
     ) -> ChatResponse:
         """Send a prompt and return the complete response."""
         request = ChatRequest(
@@ -134,6 +137,8 @@ def chat(
             autonomy=autonomy,
             cwd=cwd,
             agent=agent,
+            tools=tools,
+            tool_results=tool_results,
         )
 
         def _do() -> ChatResponse:
@@ -156,6 +161,8 @@ def chat_stream(
         autonomy: str | None = None,
         cwd: str | None = None,
         agent: str | None = None,
+        tools: list[dict[str, Any]] | None = None,
+        tool_results: list[ToolResult] | None = None,
     ) -> StreamReader:
         """Send a prompt and stream the response via SSE."""
         request = ChatRequest(
@@ -166,6 +173,8 @@ def chat_stream(
             autonomy=autonomy,
             cwd=cwd,
             agent=agent,
+            tools=tools,
+            tool_results=tool_results,
         )
 
         def _do() -> StreamReader:
@@ -190,12 +199,18 @@ def _do() -> StreamReader:
     def create_session(
         self,
         name: str | None = None,
+        model: str | None = None,
+        cwd: str | None = None,
         metadata: dict[str, Any] | None = None,
     ) -> SessionDetail:
         """Create a new session."""
         body: dict[str, Any] = {}
         if name is not None:
             body["name"] = name
+        if model is not None:
+            body["model"] = model
+        if cwd is not None:
+            body["cwd"] = cwd
         if metadata is not None:
             body["metadata"] = metadata
 
@@ -333,6 +348,8 @@ async def chat(
         autonomy: str | None = None,
         cwd: str | None = None,
         agent: str | None = None,
+        tools: list[dict[str, Any]] | None = None,
+        tool_results: list[ToolResult] | None = None,
     ) -> ChatResponse:
         """Send a prompt and return the complete response."""
         request = ChatRequest(
@@ -343,6 +360,8 @@ async def chat(
             autonomy=autonomy,
             cwd=cwd,
             agent=agent,
+            tools=tools,
+            tool_results=tool_results,
         )
 
         async def _do() -> ChatResponse:
@@ -365,6 +384,8 @@ async def chat_stream(
         autonomy: str | None = None,
         cwd: str | None = None,
         agent: str | None = None,
+        tools: list[dict[str, Any]] | None = None,
+        tool_results: list[ToolResult] | None = None,
     ) -> AsyncStreamReader:
         """Send a prompt and stream the response via SSE."""
         request = ChatRequest(
@@ -375,6 +396,8 @@ async def chat_stream(
             autonomy=autonomy,
             cwd=cwd,
             agent=agent,
+            tools=tools,
+            tool_results=tool_results,
         )
 
         async def _do() -> AsyncStreamReader:
@@ -399,12 +422,18 @@ async def _do() -> AsyncStreamReader:
     async def create_session(
         self,
         name: str | None = None,
+        model: str | None = None,
+        cwd: str | None = None,
         metadata: dict[str, Any] | None = None,
     ) -> SessionDetail:
         """Create a new session."""
         body: dict[str, Any] = {}
         if name is not None:
             body["name"] = name
+        if model is not None:
+            body["model"] = model
+        if cwd is not None:
+            body["cwd"] = cwd
         if metadata is not None:
             body["metadata"] = metadata
 
diff --git a/src/hawk/sessions.py b/src/hawk/sessions.py
new file mode 100644
index 0000000..ea331f5
--- /dev/null
+++ b/src/hawk/sessions.py
@@ -0,0 +1,174 @@
+"""Phase-attributed session state for multi-phase agent pipelines.
+
+Mirrors hawk-core-contracts/sessions/sessions.go so Python agents can track
+token spend and context snapshots with the same structure as Go agents.
+
+Phase attribution data shows that code review alone consumes 59.4% of tokens
+(Tokenomics paper, arXiv 2601.14470). These dataclasses let callers record
+per-phase usage and surface it in dashboards without coupling to the Go runtime.
+"""
+
+from __future__ import annotations
+
+import time
+from dataclasses import dataclass, field
+from enum import Enum
+
+
+class Phase(str, Enum):
+    """Phase identifies a step in the localize → repair → validate pipeline."""
+
+    LOCALIZE = "localize"
+    REPAIR = "repair"
+    VALIDATE = "validate"
+    REVIEW = "review"
+    PLANNING = "planning"
+    UNKNOWN = ""
+
+    @classmethod
+    def parse(cls, s: str) -> Phase:
+        """Return the Phase matching s, or Phase.UNKNOWN if unrecognised."""
+        for member in cls:
+            if member.value == s:
+                return member
+        return cls.UNKNOWN
+
+
+@dataclass
+class PhaseUsage:
+    """Token usage totals for a single pipeline phase."""
+
+    input_tokens: int = 0
+    output_tokens: int = 0
+    total_tokens: int = 0
+    cost_usd: float = 0.0
+
+
+@dataclass
+class ContextSnapshot:
+    """A point-in-time snapshot of context window state within a phase."""
+
+    session_id: str
+    phase: Phase
+    token_count: int
+    compressed_at: float = field(default_factory=time.time)  # Unix timestamp
+    summary: str = ""
+    anchors: list[str] = field(default_factory=list)
+
+
+@dataclass
+class ToolCallRecord:
+    """A record of a single tool call with phase attribution and token counts."""
+
+    session_id: str
+    phase: Phase
+    tool_name: str
+    input_tokens: int = 0
+    output_tokens: int = 0
+    duration_ms: int = 0
+    error: str = ""
+    timestamp: float = field(default_factory=time.time)  # Unix timestamp
+
+
+@dataclass
+class CostAccumulator:
+    """Accumulates per-phase token usage for a session.
+
+    Usage::
+
+        acc = CostAccumulator(session_id="sess-abc")
+        acc.add(Phase.LOCALIZE, input_tokens=500, output_tokens=100, cost_usd=0.002)
+        acc.add(Phase.REVIEW, input_tokens=3000, output_tokens=600, cost_usd=0.012)
+        print(acc.phase_share(Phase.REVIEW))  # ~0.857
+    """
+
+    session_id: str
+    by_phase: dict[Phase, PhaseUsage] = field(default_factory=dict)
+    total_tokens: int = 0
+    total_cost_usd: float = 0.0
+
+    def add(
+        self,
+        phase: Phase,
+        input_tokens: int,
+        output_tokens: int,
+        cost_usd: float = 0.0,
+    ) -> None:
+        """Record token usage for phase."""
+        if phase not in self.by_phase:
+            self.by_phase[phase] = PhaseUsage()
+        pu = self.by_phase[phase]
+        pu.input_tokens += input_tokens
+        pu.output_tokens += output_tokens
+        pu.total_tokens += input_tokens + output_tokens
+        pu.cost_usd += cost_usd
+        self.total_tokens += input_tokens + output_tokens
+        self.total_cost_usd += cost_usd
+
+    def phase_share(self, phase: Phase) -> float:
+        """Return the fraction of total tokens consumed by phase (0.0-1.0)."""
+        if self.total_tokens == 0:
+            return 0.0
+        usage = self.by_phase.get(phase)
+        if usage is None:
+            return 0.0
+        return usage.total_tokens / self.total_tokens
+
+    def __str__(self) -> str:
+        parts = [
+            f"session={self.session_id} total={self.total_tokens}tok ${self.total_cost_usd:.4f}"
+        ]
+        for phase, pu in sorted(
+            self.by_phase.items(), key=lambda kv: kv[1].total_tokens, reverse=True
+        ):
+            share = self.phase_share(phase)
+            parts.append(
+                f"  {phase.value or 'unknown'}: {pu.total_tokens}tok ({share:.1%}) ${pu.cost_usd:.4f}"
+            )
+        return "\n".join(parts)
+
+
+@dataclass
+class ResolutionRequest:
+    """SDK-level request to run a multi-phase resolution against a repository."""
+
+    session_id: str = ""
+    root_dir: str = ""
+    query: str = ""
+    max_files: int = 0
+    max_symbols: int = 0
+    language: str = ""
+
+
+@dataclass
+class PatchCandidate:
+    """A single proposed code change returned by the repair phase."""
+
+    file_path: str = ""
+    symbol: str = ""
+    original_body: str = ""
+    patched_body: str = ""
+    confidence: float = 0.0
+
+
+@dataclass
+class PhaseMetrics:
+    """Token spend and timing for a single pipeline phase."""
+
+    phase: Phase = Phase.UNKNOWN
+    input_tokens: int = 0
+    output_tokens: int = 0
+    elapsed_ms: int = 0
+
+
+@dataclass
+class ResolutionResult:
+    """Complete output of a multi-phase resolution run."""
+
+    session_id: str = ""
+    candidates: list[PatchCandidate] = field(default_factory=list)
+    validation_passed: bool = False
+    validation_failures: list[str] = field(default_factory=list)
+    phase_metrics: list[PhaseMetrics] = field(default_factory=list)
+    total_input_tokens: int = 0
+    total_output_tokens: int = 0
diff --git a/src/hawk/tools.py b/src/hawk/tools.py
index 760d306..615f695 100644
--- a/src/hawk/tools.py
+++ b/src/hawk/tools.py
@@ -10,6 +10,8 @@
 if TYPE_CHECKING:
     from .types import ChatResponse
 
+from .types import ToolResult
+
 
 @dataclass
 class Tool:
@@ -135,7 +137,7 @@ def chat_with_tools(
             break
 
         # Execute each tool call and collect results.
-        tool_results = []
+        tool_results: list[ToolResult] = []
         for tc in response.tool_calls:
             tool_name = tc.get("name") if isinstance(tc, dict) else getattr(tc, "name", None)
             arguments = (
@@ -146,12 +148,11 @@ def chat_with_tools(
             else:
                 result = json.dumps({"error": f"Unknown tool: {tool_name}"})
             tool_results.append(
-                {
-                    "tool_use_id": tc.get("id", "")
-                    if isinstance(tc, dict)
-                    else getattr(tc, "id", ""),
-                    "content": result,
-                }
+                ToolResult(
+                    tool_use_id=tc.get("id", "") if isinstance(tc, dict) else getattr(tc, "id", ""),
+                    content=result,
+                    is_error=bool(tool_name) and tool_name not in tool_map,
+                )
             )
 
         # Send tool results back to continue the conversation.
@@ -207,7 +208,7 @@ async def chat_with_tools_async(
         if not hasattr(response, "tool_calls") or not response.tool_calls:
             break
 
-        tool_results = []
+        tool_results: list[ToolResult] = []
         for tc in response.tool_calls:
             tool_name = tc.get("name") if isinstance(tc, dict) else getattr(tc, "name", None)
             arguments = (
@@ -218,12 +219,11 @@ async def chat_with_tools_async(
             else:
                 result = json.dumps({"error": f"Unknown tool: {tool_name}"})
             tool_results.append(
-                {
-                    "tool_use_id": tc.get("id", "")
-                    if isinstance(tc, dict)
-                    else getattr(tc, "id", ""),
-                    "content": result,
-                }
+                ToolResult(
+                    tool_use_id=tc.get("id", "") if isinstance(tc, dict) else getattr(tc, "id", ""),
+                    content=result,
+                    is_error=bool(tool_name) and tool_name not in tool_map,
+                )
             )
 
         response = await client.chat(
diff --git a/src/hawk/types.py b/src/hawk/types.py
index 0401004..c664b57 100644
--- a/src/hawk/types.py
+++ b/src/hawk/types.py
@@ -19,6 +19,8 @@ class ChatRequest(BaseModel):
     autonomy: str | None = None
     cwd: str | None = None
     agent: str | None = None
+    tools: list[dict[str, Any]] | None = None
+    tool_results: list[ToolResult] | None = Field(None, alias="tool_results")
 
     model_config = {"populate_by_name": True}
 
@@ -81,8 +83,8 @@ class Message(BaseModel):
 
     role: str
     content: str | None = None
-    tool_use: Any | None = Field(None, alias="tool_use")
-    tool_result: Any | None = Field(None, alias="tool_result")
+    tool_use: list[ToolCall] | None = Field(None, alias="tool_use")
+    tool_result: list[ToolResult] | None = Field(None, alias="tool_result")
 
     model_config = {"populate_by_name": True}
 
@@ -95,6 +97,16 @@ class ToolCall(BaseModel):
     arguments: dict[str, Any] = Field(default_factory=dict)
 
 
+class ToolResult(BaseModel):
+    """Result of executing a tool call."""
+
+    tool_use_id: str = Field(alias="tool_use_id")
+    content: str
+    is_error: bool = Field(False, alias="is_error")
+
+    model_config = {"populate_by_name": True}
+
+
 class Usage(BaseModel):
     """Token usage information."""
 
diff --git a/tests/test_sessions.py b/tests/test_sessions.py
new file mode 100644
index 0000000..dd4ea77
--- /dev/null
+++ b/tests/test_sessions.py
@@ -0,0 +1,96 @@
+"""Tests for hawk.sessions phase-attributed session state."""
+
+from hawk.sessions import ContextSnapshot, CostAccumulator, Phase, ToolCallRecord
+
+
+def test_phase_parse_known():
+    assert Phase.parse("localize") == Phase.LOCALIZE
+    assert Phase.parse("repair") == Phase.REPAIR
+    assert Phase.parse("validate") == Phase.VALIDATE
+    assert Phase.parse("review") == Phase.REVIEW
+    assert Phase.parse("planning") == Phase.PLANNING
+
+
+def test_phase_parse_unknown():
+    assert Phase.parse("nonexistent") == Phase.UNKNOWN
+    assert Phase.parse("") == Phase.UNKNOWN
+
+
+def test_phase_is_string():
+    # Phase values must be plain strings to ensure JSON serialisation parity
+    # with the Go contracts package.
+    assert Phase.LOCALIZE == "localize"
+    assert Phase.REVIEW == "review"
+
+
+def test_cost_accumulator_add_single_phase():
+    acc = CostAccumulator(session_id="test-session")
+    acc.add(Phase.REVIEW, input_tokens=3000, output_tokens=600, cost_usd=0.012)
+
+    assert acc.total_tokens == 3600
+    assert abs(acc.total_cost_usd - 0.012) < 1e-9
+    assert Phase.REVIEW in acc.by_phase
+    pu = acc.by_phase[Phase.REVIEW]
+    assert pu.input_tokens == 3000
+    assert pu.output_tokens == 600
+
+
+def test_cost_accumulator_add_multiple_phases():
+    acc = CostAccumulator(session_id="test-session")
+    acc.add(Phase.LOCALIZE, input_tokens=500, output_tokens=100, cost_usd=0.002)
+    acc.add(Phase.REPAIR, input_tokens=1000, output_tokens=200, cost_usd=0.004)
+    acc.add(Phase.REVIEW, input_tokens=3000, output_tokens=600, cost_usd=0.012)
+
+    assert acc.total_tokens == 500 + 100 + 1000 + 200 + 3000 + 600
+    assert len(acc.by_phase) == 3
+
+
+def test_cost_accumulator_phase_share_dominant():
+    acc = CostAccumulator(session_id="test-session")
+    acc.add(Phase.REVIEW, input_tokens=5940, output_tokens=0)
+    acc.add(Phase.LOCALIZE, input_tokens=1000, output_tokens=0)
+
+    share = acc.phase_share(Phase.REVIEW)
+    assert share > 0.8, f"review share expected > 0.8, got {share}"
+
+
+def test_cost_accumulator_phase_share_zero_total():
+    acc = CostAccumulator(session_id="empty")
+    assert acc.phase_share(Phase.REVIEW) == 0.0
+
+
+def test_cost_accumulator_phase_share_missing_phase():
+    acc = CostAccumulator(session_id="test")
+    acc.add(Phase.REPAIR, input_tokens=100, output_tokens=20)
+    assert acc.phase_share(Phase.REVIEW) == 0.0
+
+
+def test_cost_accumulator_str():
+    acc = CostAccumulator(session_id="sess-abc")
+    acc.add(Phase.REVIEW, input_tokens=3000, output_tokens=600, cost_usd=0.012)
+    s = str(acc)
+    assert "sess-abc" in s
+    assert "review" in s
+
+
+def test_context_snapshot_defaults():
+    snap = ContextSnapshot(session_id="s1", phase=Phase.LOCALIZE, token_count=1024)
+    assert snap.session_id == "s1"
+    assert snap.phase == Phase.LOCALIZE
+    assert snap.token_count == 1024
+    assert snap.anchors == []
+    assert snap.compressed_at > 0
+
+
+def test_tool_call_record_fields():
+    rec = ToolCallRecord(
+        session_id="s1",
+        phase=Phase.REPAIR,
+        tool_name="SearchBySymbol",
+        input_tokens=50,
+        output_tokens=200,
+        duration_ms=42,
+    )
+    assert rec.tool_name == "SearchBySymbol"
+    assert rec.phase == Phase.REPAIR
+    assert rec.duration_ms == 42